Patent Publication Number: US-2023149192-A1

Title: Expandable stent and a method for promoting a natural intracranial angiogenesis process, and use of the expandable stent in the method for promoting a natural intracranial angiogenesis process

Description:
The present invention relates to an expandable stent, and in particular, though not limited to an expandable stent for use in treating intracranial atherosclerotic disease (ICAD). The stent provides a method to restore sufficient blood flow to mitigate risks of hypoxia, ischemia and infarction, while being mechanically controlled to prevent lesion snow-ploughing. Furthermore the stents of this invention are configured to perfuse downstream tissue to an oligemic state for promoting natural intracranial angiogenesis. 
     The invention also relates to a method for promoting a natural intracranial angiogenesis process, and further the invention relates to use of the expandable stent in the method for promoting a natural intracranial angiogenesis process. The invention also relates to a method for treating intracranial atherosclerotic disease (ICAD). The invention also relates to use of the expandable stent in the treatment of intracranial atherosclerotic disease. 
     Intracranial atherosclerotic stenosis (ICAS) is a narrowing (stenosis) of an artery, vein, lumen or other blood vessel, hereinafter referred to as vessels, within the brain caused by a build-up of plaque (atheroma) on the internal arterial wall, thereby reducing blood flow to the brain. This can lead to thromboembolic or haemodynamic ischemic stroke, a leading cause worldwide of disability. 
     Intracranial atherosclerotic stenosis has a particularly high prevalence amongst certain ethnic groups, the condition being especially prevalent amongst Asians ( Lancet Neurol.  2013 November; 12(11): 1106-1114). Within Chinese patient populations, the proportion of ischemic stroke which is caused by ICAS can be as high as 50%. 
     During the early stages of atherosclerosis, fatty material collects along the walls of vessels. The fatty material then thickens, hardens with calcium deposits, which initially results in narrowing of the vessel, and eventually obstruction of the vessel, thereby preventing blood flow through the vessel. 
       FIG.  1    illustrates an initial narrowing of a diseased intracranial artery  100 . The artery  100  comprises a wall  101 . A stenosis  102  is formed by plaque on the wall  101 . The stenosis  102  of  FIG.  1    has not completely blocked the artery  100  but rather has a bore  103  extending therethrough. In  FIG.  1    the stenosis  102  is of about 90%; that is to say, about 90% of the healthy diameter of the artery is blocked. This represents a reduction in open cross-sectional area of the artery by a factor of nearly 100. Symptoms can, however, occur at lower levels of stenosis. The plaque may eventually block or fully occlude a vessel or an artery or it may develop a rough surface or thrombus that may repeatedly generate distal emboli. Other plaque characteristics such as intraplaque haemorrhage/haematoma, lipid rich core, thin fibrous cap can also result in plaques which are prone to fracture and lead to clots which block the vessel or repeatedly generate distal emboli. 
     The current recommended approach for management of ICAS is to use medication, such as blood thinning agents, cholesterol-reducing medications and/or blood-pressure regulating medications. However, the rate of ischemic stroke for ICAS patients treated with medication can be as high as 20% at two years (Chimowitz M I, Lynn M J, Howlett-Smith H, et al. Comparison of warfarin and aspirin for symptomatic intracranial arterial stenosis. N Engl J Med 2005; 31:1305-16) (Nahab F, Cotsonis G, Lynnet M, et al. Prevalence and prognosis of coexistent asymptomatic intracranial stenosis. Stroke 2008; 39:1039-41). In the event that an ICAS patient has been treated with optimal medication, yet has still experienced a second stroke, guidelines allow for further interventions. Currently, the recommended intervention is placement of a stent across the stenosis, performing a balloon angioplasty to open the stenosis, or, more commonly, a combination of both, as a salvage therapy. Stenting acts to widen the stenosis thereby increasing blood flow through the vessel. The stent can also provide a scaffold for a smoother plaque surface to develop and prevent the recurrent generation of distal emboli. However, some studies have indicated that the use of metal stents risks pushing plaques into adjacent, previously-unaffected vessels, by a process called “snow-ploughing”. Arterial structures in the brain are highly branched and a recognised problem with existing stents is the snow-ploughing of material from the stenosis into arterial side branches, or so called “perforator” arteries, thereby blocking them and causing strokes in the territories supplied by these side branches. For this reason, the use of stents and/or balloon angioplasty is currently primarily reserved for a follow-up or salvage treatment, once medication has failed. 
     Accordingly, there is a need to provide an alternative approach for management of ICAS. 
     The present invention is directed towards providing an expandable stent, and the invention is also directed towards a method for promoting a natural intracranial angiogenesis process, and further, the invention is directed towards use of an expandable stent in a method for promoting a natural intracranial angiogenesis process. The invention is also directed towards an intracranial vessel comprising the expandable stent. The invention is also directed towards a method for treating intracranial atherosclerotic disease, and to use of the expandable stent in the treatment of intracranial atherosclerotic disease. Further, the invention is directed towards the expandable stent for use in treating intracranial atherosclerotic disease, and to the use of the stent in a method to restore sufficient blood flow to mitigate risks of hypoxia, ischemia and infarction, while at the same time being mechanically controlled to prevent lesion snow-ploughing. The invention is also directed towards a stent, a method and use of the stent to perfuse downstream tissue to an oligemic state for promoting natural intracranial angiogenesis. 
     According to the invention there is provided an expandable stent for stenting a stenosis in an intracranial vessel, the stent in its expanded state being configured to comprise a first end portion having a first bore extending therethrough, a second end portion spaced apart from the first end portion and having a second bore extending therethrough, and a central portion extending between the first and second end portions and having a central bore extending therethrough communicating the first and second bores of the first and second end portions with each other, the central portion being of external transverse cross-section less than the external transverse cross-section of the first and second end portions, wherein the first and second end portions are configured in the expanded state of the stent to engage an arterial wall of the vessel on respective opposite ends of the stenosis to anchor the stent in the vessel, and the central portion is configured in the expanded state of the stent to extend through a bore extending through the stenosis and to bear on the material forming the stenosis with a radial outward pressure to at least prevent further narrowing of the bore through the stenosis. 
     The invention also provides an intracranial vessel comprising a stenosis, and an expandable stent according to the invention wherein the expandable stent in its expanded state is located in the vessel with the central portion of the stent located in a bore extending through the stenosis and bearing on the material forming the stenosis, and with the first and second end portions of the stent engaging a wall of the vessel on respective opposite ends of the stenosis. 
     Preferably, the first and second end portions of the stent in the expanded state engage the wall of the vessel adjacent the respective opposite ends of the stenosis 
     Additionally, the invention provides a method for promoting a natural intracranial angiogenesis process to supply blood to an intracranial site being supplied through a vessel having a stenosis therein, the method comprising:
         providing an expandable stent, the expandable stent comprising a first end portion having a first bore extending therethrough, a second end portion spaced apart from the first end portion and having a second bore extending therethrough, and a central portion extending between the first and second end portions and having a central bore extending therethrough communicating the first and second bores of the first and second end portions with each other, the central portion being of external transverse cross-section less than the external transverse cross-section of the first and second end portions,   placing the expandable stent in an unexpanded state in the stenosis with the central portion of the stent extending through the bore extending through the stenosis and with the first and second end portions on opposite ends of the stenosis, and   expanding the stent with the first and second end portions of the expanded stent engaging the wall of the vessel on the respective opposite ends of the stenosis to anchor the stent in the vessel, and with the central portion of the expanded stent bearing on the material forming the stenosis with an outward radial pressure to at least prevent further narrowing of the bore extending through the stenosis in order to promote the natural intracranial angiogenesis process.       

     Further, the invention provides use of an expandable stent in a method for promoting a natural intracranial angiogenesis process to supply blood to a site being supplied through a vessel having a stenosis therein, the expandable stent comprising a first end portion having a first bore extending therethrough, a second end portion spaced apart from the first end portion and having a second bore extending therethrough, and a central portion extending between the first and second end portions and having a central bore extending therethrough and communicating the first and second end portions with each other, the central portion being of external transverse cross-section less than the external transverse cross-section of the first and second end portions, wherein the expandable stent in an unexpanded state is placed in the stenosis with the central portion of the stent extending through the bore extending through the stenosis and with the first and second end portions on opposite ends of the stenosis, and the stent is expanded to an expanded state with the first and second end portions of the expanded stent engaging the wall of the vessel on respective opposite ends of the stenosis to anchor the stent in the vessel, and with the central portion of the expanded stent bearing on the material forming the stenosis with an outward radial pressure to at least prevent further narrowing of the bore extending through the stenosis in order to promote the natural intracranial angiogenesis process. 
     In one aspect of the invention the central portion is configured in the expanded state of the stent to bear solely on the material of the stenosis and not on the wall of the vessel. 
     In another aspect of the invention the first and second end portions are configured in the expanded state of the stent to engage the wall of the vessel adjacent the respective opposite ends of the stenosis. 
     In another aspect of the invention, the first and second end portions in the expanded state of the stent are of substantially circular transverse cross-section. 
     In a further aspect of the invention the first and second end portions in the expanded state of the stent are of substantially cylindrical shape. 
     In another aspect of the invention the first and second end portions are configured in the expanded state of the stent to adapt to the shape of the adjacent part of the vessel. 
     In another aspect of the invention the central portion in the expanded state of the stent is of substantially circular transverse cross-section. 
     In a further aspect of the invention the central portion in the expanded state of the stent is of substantially cylindrical shape. 
     In another aspect of the invention the central portion in the expanded state of the stent is of substantially hourglass shape. 
     Preferably, the central portion is configured in the expanded state of the stent to adapt to the shape of at least a part of the stenosis. 
     In one aspect of the invention the central portion is configured to expand to a predefined maximum transverse cross-sectional area in the expanded state of the stent. 
     Preferably, the predefined maximum transverse cross-sectional area of the central portion is less than the transverse cross-sectional area of a non-diseased part of the vessel adjacent the stenosis, in order to avoid contact of the central portion with the wall of the vessel. 
     In one aspect of the invention the external diameters of the first and second end portions in the expanded state of the stent do not exceed 5 mm. 
     Preferably, the external diameters of the first and second end portions in the expanded state of the stent lie in the range of 2 mm to 5 mm. 
     Advantageously, the external diameters of the first and second end portions in the expanded state of the stent lie in the range of 2.5 mm to 4.5 mm. 
     In one aspect of the invention the first and second end portions in the expanded state of the stent are configured to expand to a diameter in the range of 100% to 125% of the internal diameter of a non-diseased part of the vessel adjacent the stenosis. 
     Preferably the first and second end portions in the expanded state of the stent are configured to expand to a diameter in the range of 100% to 110% of the internal diameter of a non-diseased part of the vessel adjacent the stenosis. 
     In another aspect of the invention the maximum external diameter of the stent in the unexpanded state thereof does not exceed 0.6 mm, and preferably does not exceed 0.5 mm, and advantageously, does not exceed 0.4 mm. 
     Preferably, the maximum external diameter of the stent in the unexpanded state thereof does not exceed 0.3 mm. 
     In one aspect of the invention the external diameter of the central portion in the expanded state of the stent lies in the range of 25% to 70% of the external diameters of the first and second end portions. 
     Preferably, the external diameter of the central portion in the expanded state of the stent lies in the range of 40% to 75% of the external diameters of the first and second end portions. 
     Advantageously, the external diameter of the central portion in the expanded state of the stent lies in the range of approximately 50% of the external diameters of the first and second end portions. 
     In one aspect of the invention the external diameters of the first and second end portions in the expanded state of the stent are substantially equal to each other. 
     In another aspect of the invention the internal diameter of the central portion in the expanded state of the stent lies in the range of 25% to 70% of the internal diameters of the first and second end portions. 
     Preferably, the internal diameter of the central portion in the expanded state of the stent lies in the range of 40% to 75% of the internal diameters of the first and second end portions. 
     Advantageously, the internal diameter of the central portion in the expanded state of the stent lies in the range of approximately 50% of the internal diameters of the first and second end portions. 
     In one aspect of the invention the internal diameters of the first and second end portions in the expanded state of the stent are substantially equal to each other. 
     In another aspect of the invention the wall thickness of the stent lies in the range of 0.02 mm to 0.15 mm. 
     Preferably, the wall thickness of the stent lies in the range of 0.05 mm to 0.1 mm. 
     In one aspect of the invention the central portion in the expanded state of the stent is configured to apply a greater radial outward pressure to the material forming the stenosis than the radial outward pressure applied by the first and second end portions to the wall of the vessel. 
     In another aspect of the invention the radial outward pressure applied by the central portion in the expanded state of the stent is such as to minimise squashing of the material forming the stenosis to thereby minimise urging the material forming the stenosis longitudinally along the wall of the vessel. 
     In a further aspect of the invention the first and second end portions in the expanded state of the stent are configured to bear on the wall of the vessel with a pressure sufficient to prevent the material forming the stenosis being urged between the corresponding one of the first and second end portions and the wall of the vessel. 
     In another aspect of the invention the central portion in the expanded state of the stent is configured to bear on the material forming the stenosis with the radial outward pressure to increase the diameter of the bore extending through the stenosis to a diameter of at least 25% of the non-diseased part of the vessel adjacent the stenosis. 
     Preferably, the central portion in the expanded state of the stent is configured to bear on the material forming the stenosis with the radial outward pressure to increase the diameter of the bore extending through the stenosis to a diameter of at least 30% of the non-diseased part of the vessel adjacent the stenosis. 
     Advantageously, the central portion in the expanded state of the stent is configured to bear on the material forming the stenosis with the radial outward pressure to increase the diameter of the bore extending through the stenosis to a diameter of at least 40% of the non-diseased part of the vessel adjacent the stenosis. 
     Preferably, the central portion in the expanded state of the stent is configured to bear on the material forming the stenosis with the radial outward pressure to increase the diameter of the bore extending through the stenosis to a diameter of at least 50% of the non-diseased part of the vessel adjacent the stenosis, and may increase the diameter of the bore extending through the stenosis to a diameter of at least 70% and even 80% of the non-diseased part of the vessel adjacent the stenosis. 
     Advantageously, the central portion in the expanded state of the stent is configured to bear on the material forming the stenosis with the radial outward pressure to increase the diameter of the bore extending through the stenosis to a diameter in the order of 60% of the non-diseased part of the vessel adjacent the stenosis. 
     In one aspect of the invention the central portion terminates at its opposite ends in respective transition portions, and the central portion is connected to and communicates with the first and second end portions through the transition portions. 
     In another aspect of the invention at least one of the transition portions in the expanded state of the stent extends substantially perpendicularly to the central portion and to the corresponding one of the first and second end portions. 
     In another aspect of the invention the two transition portions in the expanded state of the stent extend substantially perpendicularly to the central portion and to the corresponding one of the first and second end portions. 
     In a further aspect of the invention at least one of the transition portions in the expanded state of the stent is of frusto-conical shape. 
     Preferably, the two transition portions in the expanded state of the stent are of frusto-conical shape. 
     In one aspect of the invention the frusto-conical portion of each transition portion in the expanded state of the stent defines a cone angle in the range of 30° to 160°. 
     Preferably, the frusto-conical portion of each transition portion in the expanded state of the stent defines a cone angle in the range of 30° to 110°. 
     Advantageously, the frusto-conical portion of each transition portion in the expanded state of the stent defines a cone angle in the range of 60° to 110°. 
     In one aspect of the invention the transition portions in the expanded state of the stent are configured to engage the material forming the stenosis adjacent the respective opposite ends thereof. 
     Preferably, the first and second end portions and the central portion are of one of cage like construction, braided construction and perforated construction defining interstices therein. 
     In another aspect of the invention the interstices in the central portion in the expanded state of the stent are of smaller area than the interstices in the first and second end portions. 
     Preferably, the interstices in the central portion in the expanded state of the stent are of sufficiently small area to one of minimise and prevent the material forming the stenosis passing therethrough. Advantageously, the interstices in the transition portions in the expanded state of the stent are of smaller area than the area of the interstices in the first and second end portions. 
     In another aspect of the invention the interstices in the transition portions in the expanded state of the stent are of sufficiently small area to one of minimise and prevent the material forming the stenosis passing therethrough. 
     In one embodiment of the invention the central portion of the stent defines a longitudinally extending main central axis, the first end portion defines a longitudinally extending first central axis, and the second end portion defines a longitudinally extending second central axis. 
     In another embodiment of the invention the main central axis, the first central axis and the second central axis coincide with each other. 
     In another aspect of the invention the main central axis is offset from the first and second central axes. 
     In a further aspect of the invention the first and second central axes coincide with each other. 
     In another aspect of the invention the main central axis extends parallel to the first and second central axes. 
     In a further aspect of the invention the main central axis extends at an angle greater than 0° to the first and second central axes. 
     In one aspect of the invention the stent is configured to expand at blood temperature of a human or animal subject. 
     In another aspect of the invention a biocompatible film is provided on the surface of the stent. 
     In a further aspect of the invention the biocompatible film is implanted with a medication to prevent further growth of atherosclerotic plaque. 
     In another aspect of the invention the stent is implanted at a molecular level or coated with one of a medication to reduce the thrombotic potential of the stent and a material to promote angiogenesis. 
     In one aspect of the invention the stent is coated with a therapeutically active material. Preferably, the therapeutically active material comprises an angiogenesis promoting material. 
     Advantageously, the angiogenesis promoting material comprises methacrylic acid-ecoisodecyl acrylate (MAA-co-IDA; 40% MAA). 
     In another aspect of the invention the stent is configured to minimise endothelial shear stress resulting from the stent. 
     Preferably, the stent is configured so that the endothelial shear stress resulting from the stent does not exceed 250 Pa. 
     In one aspect of the invention the stent comprises a biocompatible material. 
     In another aspect of the invention the stent comprises a biocompatible biodegradable material. 
     In a further aspect of the invention the stent comprises a polymer material. 
     In another aspect of the invention the stent comprises a self-expanding material. 
     In a further aspect of the invention the stent comprises a memory material. 
     In another aspect of the invention the stent comprises an alloy selected from one or more of the following metals:
         Nickel, titanium, cobalt, stainless steel.       

     In a further aspect of the invention the stent comprises a non-self-expanding material. 
     In one aspect of the invention the first and second end portions in the expanded state of the stent engage the wall of the vessel adjacent the respective opposite ends of the stenosis. 
     In another aspect of the invention the first and second end portions in the expanded state of the stent are of substantially circular transverse cross-section. 
     In a further aspect of the invention the first and second end portions in the expanded state of the stent are of substantially cylindrical shape. 
     In another aspect of the invention the first and second end portions in the expanded state of the stent adapt to the shape of the adjacent part of the vessel. 
     In one aspect of the invention the stent is expanded to the extent that the central portion increases the diameter of the bore extending through the stenosis to an extent to increase the rate of the blood flow flowing through the stenosis to lie in the range of 25% to 80% of the normal blood flow rate through the vessel without the stenosis. 
     In another aspect of the invention the stent is expanded to the extent that the central portion increases the diameter of the bore extending through the stenosis to an extent to increase the rate of the blood flow flowing through the stenosis to lie in the range of 30% to 70% of the normal blood flow rate through the vessel without the stenosis. 
     In a further aspect of the invention the stent is expanded to the extent that the central portion increases the diameter of the bore extending through the stenosis to an extent to increase the rate of the blood flow flowing through the stenosis to lie in the range of 40% to 60% of the normal blood flow rate through the vessel without the stenosis. 
     Preferably, the stent is expanded to the extent that the central portion increases the diameter of the bore extending through the stenosis to an extent to increase the rate of the blood flow flowing through the stenosis to lie in the range of approximately 50% of the normal blood flow rate through the vessel without the stenosis. 
     Further the invention provides a method of treating intracranial atherosclerosis disease in a subject, the method comprising the steps of scaffolding the lesion to control migration of atherosclerotic material, dilating the bore of the lesion such that downstream tissue is maintained in an oligemic state, and preferably, the method comprises medicating the patient to promote the development of collateral vessels to support said oligemic tissue. 
     Further the invention provides a method for treating a patient that has an intracranial stenosis, the method comprising the steps of measuring the length of the stenosis, measuring the diameter of a bore extending through the stenosis, and measuring the diameters of the vessel adjacent the distal and proximal ends of the stenosis, selecting a stent with a central portion having a diameter that is at least 50% of the diameter of the vessel adjacent one of the proximal and distal ends of the stenosis, and placing the stent in the vessel with the central portion of the stent extending through the bore of the stenosis, and configuring the stent to remodel the stenosis to increase the diameter of the bore extending through the stenosis to a diameter preferably of at least 50% of the diameter of the vessel adjacent the proximal end of the stenosis. Preferably, the stent is configured to remodel the stenosis within thirty days of placing the stent in the vessel. 
     The invention also provides a method for preventing a secondary infarction when treating a stenosis in a vessel of a subject with a branch vessel adjacent the stenosis, the method comprising providing a stent that is configured to under perfuse downstream tissue, positioning the stent relative to the stenosis, and expanding the stent to an hourglass shape to conform to the stenosis, and confirming the preservation of said branch vessel before implanting the stent. 
     Further, the invention provides a method for treating a human or animal subject to restore sufficient blood flow through a vessel in an intracranial vascular system to mitigate the risk of one or more of hypoxia, ischemia and infarction, the method comprising provided the expandable stent according to the invention, loading the expandable stent in a collapsed state into a proximal end of a lumen extending through a delivery microcatheter, the distal end of the delivery microcatheter being positioned across the stenosis, advancing the stent in the collapsed state through the lumen of the delivery microcatheter and positioning the stent in its collapsed state within the lumen of the delivery microcatheter adjacent the distal end thereof, such that the central portion of the stent in its collapsed is located within the bore of the stenosis. Preferably, the method further comprises withdrawing the delivery microcatheter while holding the stent with the central portion thereof located within the bore of the stenosis to expose the second end portion of the stent. Preferably, the axial position of the delivery microcatheter is adjusted along with the axial position of the stent so that one of the first end portion and the second end portion of the stent is located in the vessel adjacent a distal end of the stenosis. 
     Preferably, the method further comprises further withdrawing the delivery microcatheter to expose the entire stent. 
     Preferably, the method further comprises expanding the stent to scaffold the stenosis. 
     In one aspect of the invention, the method comprises confirming the position of the stent by fluoroscopy. 
     Preferably, the method comprises decoupling the stent from the delivery microcatheter or other element of a delivery system. 
     In another embodiment of the invention, the method comprises capturing an angiographic image of the stent in the stenosis to confirm the position of the stent. 
     In another aspect of the invention the method further comprises removing the delivery catheter from the subject. 
     In another aspect of the invention the method comprises providing the expandable stent with a central portion of hourglass shape. 
     In a further aspect of the invention the expandable stent comprises a radiopaque marker, and preferably, the radiopaque marker is located on one or both of the first end portion and the second end portion of the stent. 
     In another aspect of the invention the radiopaque markers are located adjacent the portion of the central portion of hourglass shape of minimum diameter. 
     In another aspect of the invention the method comprises visualising the stenosis using fluoroscopy and dimensioning key features of the stenosis prior to treatment. 
     Preferably, the method comprises computing dimensions of the most suitable shape of the central portion of the stent of hourglass shape for the stenosis. 
     In another aspect of the invention the method comprises positioning the stent in the collapsed state under fluoroscopic guidance, such that the central portion of the stent of hourglass shape is located in the stenosis. 
     Preferably, the stent is expanded with the central portion of hourglass shape located in the stenosis for dilating the bore extending through the stenosis to perfuse distal tissue to an oligemic state. 
     Preferably, the transition portions are configured into an undulating tubular body. 
     Advantageously, the first and second end portions, the transition portions and the central portion are configured into a monolith structure. 
     In one embodiment of the invention the transverse cross-sectional area of the bore through the stenosis is increased by 3080% while the vessel adjacent the stenosis is dilated by less than 18%. 
     In another embodiment of the invention the transverse cross-sectional area of the bore through the stenosis is increased by 2000% while the vessel adjacent the stenosis is dilated by less than 14%. 
     In another embodiment of the invention the transverse cross-sectional area of the bore through the stenosis is increased by 1000% while the vessel adjacent the stenosis is dilated by less than 16%. 
     In another embodiment of the invention the transverse cross-sectional area of the bore through the stenosis is increased by 3000% while the vessel adjacent the stenosis is dilated by less than 12%. 
     Preferably, the transverse cross-sectional area of the bore through the stenosis is increased in the range of 1000% to 3000% while the vessel adjacent the stenosis is dilated in the range of 10% to 18%. 
     The advantages of the invention are many. Essentially, the expandable stent according to the invention when located in a stenosis in a vessel within the intracranial vascular branched system is not intended to fully reinstate a partially occluded vessel to its fully normal state nor to its original internal diameter, but rather, to maintain blood flow through the vessel at a partially restricted level, or to increase the rate of the blood flow through the vessel to a flow rate anywhere in the range of 25% to 80% of the normal flow rate through that vessel prior to the formation of the stenosis, and preferably, to increase the rate of blood flow to approximately 50% of the normal flow rate through the vessel prior to the formation of the stenosis. This allows sufficient time to allow natural intracranial angiogenesis within the intracranial vascular system proceed, whereby collateral blood vessels grow in order to allow the site or sites being supplied with blood through the diseased vessel to be supplied through one or more alternative vessels which may or may not already supply the site which is supplied by the diseased vessel. Thus, on completion of the natural intracranial angiogenesis process the site being supplied by the diseased vessel is fully supplied with a blood supply from the newly grown collateral vessels resulting from the natural intracranial angiogenesis process. 
     The stents according to the invention are also configured to treat brain artery lesions that carry significant risk for patients. These lesions have a high level of restriction (stenosis) and the shear forces of high velocity blood flowing through these lesions puts the patient at high risk of a stroke. At the same time intracranial vessels in human subjects are thinner than in other parts of the body and are thus more fragile than vessels of similar size in the heart or the kidneys or elsewhere. Aggressive stenting in these vessels has been shown to be inferior to medical therapy. In one embodiment this invention provides an entirely new strategy for treating these patients. The method of partially dilating these intracranial stenoses with the devices of this invention has the effect of restoring sufficient blood flow to ensure that the downstream tissue is not hypoxic while at the same time preserving an oligemic state in the vascular bed. This oligemic state in the intracranial vascular bed promotes the development of collateral vessels (new vessels) in the brain in an angiogenesis process and this therapeutic strategy allows the brain to evolve its own circulatory redundancy over time. This has the effect of reducing the risk that the stenosis presents to the patient. Even an acute occlusion at the site of the original lesion may not significantly harm the patient in situation where collateral vessels have developed. 
     Another aspect of the expandable stent of this invention is the way in which it manages the atherosclerotic material that makes up the body of the restriction (stenosis). In a conventional stenting procedure this material is aggressively displaced and it causes the native artery to expand to accommodate. The expandable stents of this invention avoid this aggressive action and displace atherosclerotic material to very minimal levels. 
     Accordingly, the expandable stent according to the invention, while in general it is used to increase blood flow through the diseased vessel, it is not used to reinstate normal blood flow through the diseased vessel, since to increase the diameter of the bore extending through the stenosis to reinstate normal blood flow through the diseased vessel would, in general, result in the material forming the stenosis being squashed and spread longitudinally along the vessel wall. Due to the large number of branched vessels in the intracranial vascular system and the closeness of the branches extending from a main vessel or artery this would result in the material forming the stenosis being spread past one or more branched vessels adjacent the stenosis thereby blocking the branched vessels. The squashing and spreading of material forming the stenosis beyond a stent, generally is referred to as “snow-ploughing”. The expandable stent according to the invention and its use avoids the danger of snow-ploughing, and thus blocking branched vessels or arteries adjacent to the stenosis, while at the same time maintaining a flow of blood through the diseased vessel or artery sufficient to promote and support the natural intracranial angiogenesis process until it has been completed, and new or collateral blood vessels have grown in order to provide an alternative blood supply to the site or sites supplied by the diseased vessel. 
     A further advantage of the invention is that by only partly opening the bore through the stenosis, the risk of intracranial hyperperfusion injury is reduced, and in general avoided. Intracranial hyperperfusion injury can result from rapid restoration of normal perfusion pressure, and may potentially lead to intracranial haemorrhage. Intracranial hyperperfusion may also involve dysregulation of the intracranial vascular system and hypertension, in the setting of an increase in the intracranial blood flow. 
     Accordingly, the advantages of the invention are many. Firstly, the expandable stent avoids snow-ploughing of material forming a stenosis which would otherwise block an adjacent branched vessel or branched vessels adjacent the stenosis. In the neurovascular circulation some of these branched vessels are called perforators because they originate from vessels that sit in the folds of the brain (ex middle intracranial artery) and they perforate into intracranial tissue of the brain. It is the case that some of these perforator vessels are associated with very high level functions such as speech and so an occlusion of one of these tiny vessels can have devastating consequences for the patient. The expandable stent of this invention is configured to protect these perforator vessels from being occluded by snow-ploughing while restoring blood flow to an oligemic level. The expandable stent while avoiding snow-ploughing of the material forming the stenosis at the same time maintains, or increases the diameter of the bore extending through the stenosis to an extent that blood flow through the diseased vessel is maintained at a value of greater than 30% of normal and preferably approximately 50% of normal, thereby permitting natural intracranial angiogenesis processes to proceed. The expandable stent according to the invention maintains the flow of blood at a sufficient flow rate at least until the natural intracranial angiogenesis processes have been completed and new or collateral blood vessels have been grown in order to supply the sites which had originally been supplied by the diseased vessel or artery. Due to the fact that the blood flow is maintained at a value of 50% of the normal blood flow rate intracranial hyperperfusion is avoided, and the risk of stroke is also low. Although in some cases the expandable stent may be configured to increase the flow rate of blood to 70% of normal, and in some limited cases to 80% of normal. 
    
    
     
       The invention and its many advantages will be more clearly understood from the following description of some preferred embodiments thereof which are given by way of example only with reference to the accompanying drawings, in which: 
         FIG.  1    is a cross-sectional side elevational view of a disease intracranial artery, 
         FIG.  2    is a side elevational view of an expandable stent according to the invention for use in maintaining blood flow through a stenosis in the diseased artery of  FIG.  1   , 
         FIG.  3    is an end elevational view of the stent of  FIG.  2   , 
         FIG.  4    is a cross-sectional side elevational view of the stent of  FIG.  2   , 
         FIG.  5    is a cross-sectional side elevational view of the stent of  FIG.  2    in an unexpanded state, 
         FIG.  6    is a cross-sectional side elevational view of the stent of  FIG.  2    in use in the artery of  FIG.  1   , 
         FIG.  7    is a cross-sectional side elevational view of a delivery system for delivering the stent of  FIG.  2    into the stenosis of the diseased artery, 
         FIG.  8    is a cross-sectional side elevational view of a portion of the artery of  FIG.  7    illustrating the stent of  FIG.  2    being positioned in the stenosis of the diseased artery, 
         FIG.  9    is a view similar to  FIG.  8    further illustrating the placing of the stent of  FIG.  2    in the diseased artery, 
         FIG.  10    is a side elevational view of an expandable stent according to another embodiment of the invention, 
         FIG.  11    is a cross-sectional side elevational view of a stent according to another embodiment of the invention, 
         FIG.  12    is a cross-sectional side elevational view of a stent according to a further embodiment of the invention illustrated in a stenosis in a diseased artery, 
         FIG.  13    is a cross-sectional side elevational view of a stent according to a further embodiment of the invention illustrated being placed in a stenosis in an artery, 
         FIGS.  14   a, b  and  c    are side elevational, cross-sectional side elevational and end elevational views of a stent according to another embodiment of the invention, 
         FIGS.  15   a, b  and  c    are side elevational, cross-sectional side elevational and end elevational views of a stent according to another embodiment of the invention, 
         FIGS.  16   a, b  and  c    are side elevational, cross-sectional side elevational and end elevational views of a stent according to another embodiment of the invention, 
         FIGS.  17   a, b  and  c    are side elevational, cross-sectional side elevational and end elevational views of a stent according to another embodiment of the invention, and 
         FIG.  18    is a perspective view of a stent according to another embodiment of the invention. 
     
    
    
     Referring to the drawings and initially to  FIGS.  2  to  6    thereof, there is illustrated an expandable stent according to the invention indicated generally by the reference numeral  1 . The stent  1  is expandable from an unexpanded state illustrated in  FIG.  5    to an expanded state illustrated in  FIGS.  2 , 3 , 4  and  6   . The stent  1  is of dimensions in the unexpanded state which are suitable for urging the stent through the intracranial vascular system (not shown) in a delivery microcatheter, as will be discussed below, to a stenosis, for example, the stenosis  102  in an intracranial artery, for example, the diseased intracranial artery  100  of the intracranial vascular system (not shown). In the expanded state the dimensions of the stent  1  are such that the stent  1  expands in the stenosis  102  in order to maintain the bore  103  through the stenosis  102  open and/or to increase the transverse cross-sectional area of the bore  103  through the stenosis  102  in order to maintain and/or increase the rate of blood flow through the stenosis  102 . Maintaining or increasing the rate of blood flow through the stenosis  102  protects downstream tissue, namely, tissue supplied with blood through the artery  100 , from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state. This thus promotes a natural intracranial angiogenesis process and allows the natural intracranial angiogenesis process to be completed, whereby new collateral blood vessels are grown to supply the downstream tissue which is being supplied through the diseased artery  100 . The dimensions of the stent  1  in the expanded state and the unexpanded state are discussed in more detail below. 
     The stent  1  in its expanded state comprises a first end portion  3  and a second end portion  4 , both of circular transverse cross-section and of the same external and internal diameters. A central portion  5  also of circular transverse cross-section extends between the first and second end portions  3  and  4  and terminates at its opposite ends in respective transition portions  7  which connect the central portion  5  to the corresponding ones of the first and second end portions  3  and  4 . In this embodiment of the invention when the stent  1  is in its expanded state the transition portions  7  extend substantially perpendicularly from the central portion  5 , and perpendicularly from the corresponding one of the first and second portions  3  and  4 . However, as discussed below with reference to  FIGS.  11  to  13   , the transition portions  7  may be, for example, frusto-conical of progressively increasing transverse cross-sectional area from the central portion  5  to the first and second end portions  3  and  4 , and indeed in other embodiments of the invention, such stents as the stents described with reference to  FIGS.  11  to  13    may be preferable to the stent  1 . 
     The first and second end portions  3  and  4  are formed by expandable first and second walls  10  and  11 , respectively, having first and second bores  12  and  13  extending therethrough. The central portion  5  comprises an expandable central wall  15  having a bore  16  extending therethrough communicating the first and second bores  12  and  13  of the first and second end portions  3  and  4 , respectively. In this embodiment of the invention in the expanded state of the stent  1 , the first, second, and central walls  10 ,  11  and  15 , respectively, are cylindrical. In this embodiment of the invention the transition portions  7  are formed by respective transition walls  17 , which are expandable, and in the expanded state of the stent  1  expand outwardly so that the transition walls  17  extend perpendicularly from the central wall  15  and from the first and second walls  10  and  11 . The central wall  15  of the central portion  5  defines a longitudinally extending main central axis  18 , and the first and second walls  10  and  11  of the first and second end portions  3  and  4  defined respective longitudinally extending first and second central axes  21  and  22 , respectively. In this embodiment of the invention the first and second end portions  3  and  4  are axially aligned with each other, and the central portion  5  is axially aligned with the first and second end portions  3  and  4 . Accordingly, in this embodiment of the invention the first and second central axes  21  and  22  and the main central axis  18  coincide with each other. 
     It is envisaged, and indeed will be appreciated by those skilled in the art that the first and second walls  10  and  11  and the central wall  15  need not necessarily be cylindrical. For example, it is envisaged that in the expanded state of the stent  1 , the central wall  15  may diverge outwardly from a central point intermediate the transition walls  17  towards the transition walls  17 , in other words, the central wall  15  may be of “hourglass” shape, whereby the diameter of the central wall  15  adjacent the transition walls  17  would be greater than the diameter of the central wall  15  at the central point midway between the transition walls  17 , in which case, the diameter of the diverging portions of the cylindrical wall  15  would increase gradually from the central point of the central wall  15  towards the transition walls  17 . Indeed, it is envisaged that when the stent  1  is expanded in situ, depending on the shape of the bore  103  extending through the stenosis  102 , the central wall  15  may take up a shape corresponding to the profile of the bore  103  extending through the stenosis  102 . 
     It is also envisaged that the first and second walls  10  and  11  of the first and second end portions  3  and  4  may be of shape other than cylindrical, and in some embodiments of the invention, it is envisaged that the first and second walls  10  and  11  in the expanded state of the stent  1  may take up a shape whereby the first and second walls  10  and  11  converge in a longitudinal direction from the transition wall  17 , or may converge from a central point midway between the corresponding transition wall  17  and a corresponding free end  19  of the end walls  10  and  11  towards the corresponding transition wall  17  and the corresponding free end  19 . Indeed, when in situ, it is envisaged that the first and second walls  10  and  11  of the first and second end portions  3  and  4  may take up a shape which would correspond to the longitudinal profile of respective proximal and distal non-diseased portions  104  and  106 , respectively, of the artery adjacent a proximal end  105  and a distal end  107 , respectively, of the stenosis  102 . 
     Needless to say, the central wall  15  of the central portion  5  and the first and second walls  10  and  11  of the first and second end portions  3  and  4  may take up any suitable shape in the expanded state of the stent  1  and also when the stent  1  is located in situ in the artery  100 , for example, the external surfaces of the first and second end walls  10  and  11  and the central wall  15  may be concave or convex. 
     It will also be appreciated that in some embodiments of the invention the first and second end portions may not be axially aligned with each other, and furthermore, the central portion  15  may not be axially aligned with the first and second end portions  2  and  3 . In which case, only the central axes  18 ,  21  and  22  of those portions of the central portion  18  and the first and second end portions  3  and  4  which are axially aligned with each other will coincide. For example, in cases where the first and second end portions  3  and  4  are axially aligned, the first and second central axes  21  and  22  will coincide with each other, and in cases where the central portion  5  is not axially aligned with the first and second end portions  3  and  4 , the main central axis  18  of the central portion  15  will be offset from the first and second central axes  21  and  22  of the first and second end portions  3  and  4 . In general, in such a case, the main central axis  18  would extend parallel to the first and second central axes  21  and  22  of the first and second end portions  3  and  4 . Although, it is envisaged that in some cases the main central axis  18  of the central portion  5  may extend at an angle to the first and second central axes  21  and  22  of the first and second end portions  3  and  4 . Indeed, in use, depending on the shape and angle of the bore  103  extending through the stenosis  102 , the central portion  5  may take up an orientation which would extend at an angle to the first and second end portions  3  and  4 . In which case, it is envisaged that the first and second central axes of the first and second end portions  3  and  4  may not be axially aligned, but would be offset from each other, and the main central axis  18  of the central portion  5  would extend at an angle to the first and second central axes  21  and  22  of the first and second end portions  3  and  4 . Additionally, in the event of the stenosis  102  being located in a curved or angled artery, the first and second central axes  21  and  22  of the first and second end portions  3  and  4  may extend at an angle to each other, and also at respective different angles to the main central axis  18  of the central portion  5 . 
     The dimensions of the stent  1  in the expanded state in general, are chosen based on the diameter of the stenosis  102 , and the length of the stenosis  102 , and also on the diameter of the non-diseased portion of the artery  100  adjacent the stenosis  102 . In general, the stents  1  will be provided with the first and second end portions  3  and  4  and the central portion  5 , as well as the transition portions  7  in a range of different diameters and lengths in order to accommodate arteries or vessels of different diameters and stenoses of different lengths, and having bores extending therethrough of different diameters. The selection of a stent for a particular stenosis in a particular artery or vessel in general, will be a two-step process. Firstly, a surgeon, interventionist or other surgical or medical personnel would measure the length of the stenosis, and the diameter of the bore extending through the stenosis, as well as the diameter of the non-diseased part  104  and  106  of the artery adjacent the respective opposite ends of the stenosis. A stent  1  according to the invention of suitable dimensions would then be selected. In the absence of a suitably dimensioned stent  1 , a stent  1  according to the invention of suitable dimensions would be manufactured. 
     In this embodiment of the invention the stenosis  102  is formed by plaque on the arterial wall  101  of the artery  100 . The diameter d of a bore  103  extending through the stenosis  102  is approximately 0.6 mm. 
     The length L of the stenosis  102  is approximately 5 mm. The diameter D of the proximal and distal non-diseased parts  104  and  106 , respectively, of the artery  100  adjacent respective proximal and distal ends  105  and  107 , respectively, of the stenosis  102  is approximately 3 mm. 
     Accordingly, in this embodiment of the invention the stent  1  is configured to expand when placed in situ in the stenosis  102  so that the external diameter d 1  of the central portion  5  is approximately 1.5 mm and the external diameters D 1  of the first and second end portions  3  and  4  are equal to each other and are approximate 3.5 mm. The axial length L 1  of the central portion  5  in the expanded state of the stent  1  is approximately 5 mm, while the axial length L 2  of the first and second end portions  3  and  4  in the expanded state of the stent  1  in this embodiment of the invention are equal to each other and in this embodiment of the invention are approximately 2 mm. It will be appreciated that the axial lengths of the first and second end portions  3  and  4  of the stent  1  may be the same or different. 
     As mentioned above the dimensions of the stent  1  in the expanded state are largely dependent on the dimensions of the artery and the stenosis thereof and are chosen to be suitable for the particular stenosis. However, typically, in the expanded state the external diameter d 1  of the central portion  5  of the stent  1  would range between 1 mm and 3 mm, and more typically would lie within the range of 1.2 mm to 2.5 mm. The external diameter D 1  of the first and second end portions  3  and  4  of the stent  1  in the expanded state, in general, would lie within the range of 2 mm to 5 mm, and more typically would lie in the range of 2.5 mm to 4.5 mm. Additionally, the length L 1  of the central portion  5  of the stent  1  in the expanded state of the stent would typically lie in the range of 5 mm to 20 mm, and more typically, would lie in the range of 5 mm to 15 mm. The axial length L 2  of each of the first and second end portions  3  and  4  of the stent  1  in the expanded state thereof typically would lie in the range of 2 mm to 4 mm, and more typically would lie in the range of 2 mm to 3 mm, and may be of the same or different axial lengths. 
     In order that the stent  1  in the unexpanded state can be manoeuvred through the intracranial vascular system to the stenosis  102  in the artery  100 , in general, in the unexpanded state, the stent  1  would be of substantially constant external diameter d 3  along its entire axial length L 3 , of not more than 0.6 mm, and the axial length L 3  of the stent  1  in the unexpanded state, in general, would not exceed 30 mm, see  FIG.  5   . Although, depending on the location of the artery or vessel and the stenosis thereof in the intracranial vascular system, the maximum external diameter d 3  of the stent  1  in the unexpanded state would not exceed 0.5 mm. Ideally, the stent  1  in the unexpanded state would be of maximum external diameter d 3  not exceeding 0.4 mm, and of length L 3  not exceeding 25 mm. 
     The relationship between the external diameter d 1  of the central portion  5  and the external diameter D 1  of the first and second end portions  3  and  4  in the expanded state of the stent  1  are chosen so that the radial outward pressure exerted by the central portion  5  on the stenosis  102  is sufficient in order to maintain the diameter of the bore  103  extending through the stenosis  102  to be at least 25% of the diameter of the proximal and distal non-diseased parts  104  and  106  of the artery  100  adjacent the proximal and distal ends  105  and  107  of the stenosis  102 , and the radial outward pressure exerted by the first and second end portions  3  and  4  on the proximal and distal non-diseased parts  104  and  106  of the wall  101  of the artery  100  adjacent the respective proximal and distal ends  105  and  107  of the stenosis  102  is sufficient to anchor the stent  1  in the artery  100  against the arterial wall  101  thereof. The pressure exerted by the first and second end portions  3  and  4  on the arterial wall  101  may also be sufficient in many cases to prevent the material forming the stenosis  102  being squeezed longitudinally along the wall  101  of the artery  100  between the first and second end portions  3  and  4  and the wall  101  of the artery  100  from the stenosis  102 . 
     The external diameter d i  of the central portion  5  of the stent  1  in the expanded state is chosen to apply a radial outward pressure to the stenosis to increase the diameter of the bore  103  through the stenosis  102  to a diameter in the range of 25% to 60%, and in some cases up to 75% and even 80% of the diameter of the proximal and distal non-diseased parts  104  and  106  of the artery  100  adjacent the proximal and distal ends  105  and  107 , respectively, of the stenosis  102 . Although ideally, the external diameter of the central portion  5  in the expanded state is such as to apply a sufficient radial outward pressure to the stenosis  102  to increase the diameter of the bore  103  therethrough to approximately 50% of the diameter of the proximal and distal non-diseased parts  104  and  106  of the artery  100  adjacent the proximal and distal ends  105  and  107 , respectively, of the stenosis  102 . 
     Additionally, and in general, the external diameter d 1  of the central portion  5  in the expanded state of the stent  1  is chosen so that the radial outward pressure applied by the central portion  5  of the stent  1  in its expanded state to the stenosis  102  is sufficient to increase the diameter of the bore  103  extending through the stenosis  102  to a diameter, such that the blood flow rate through the artery  100  is increased to a value greater than 50% of the normal blood flow rate through the artery prior to the formation of the stenosis. Since resistance to flow is inversely related to the fourth power of the internal diameter of a conduit, the external diameter of the central portion  5  in the expanded state of the stent  1  is selected based on this relationship in order to restore the blood flow rate through the bore  103  of the stenosis  102  to 50% or greater than that of the normal blood flow rate. This, it has been found, is sufficient to maintain the blood flow to the downstream tissue being supplied by the diseased artery  100  to protect the downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, and is also sufficient to promote the natural intracranial angiogenesis process until it has been completed and new collateral blood vessels have been grown to supply the sites supplied by the diseased artery. However, in some embodiments of the invention it is envisaged that the external diameter of the central portion  5  of the expanded stent  1  may be such as to lightly bear on the stenosis  102  in order to maintain the blood flow through the artery at the current flow rate or slightly above the current flow rate, depending on the current flow rate, which may be sufficient to protect the downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state. 
     In general, the radial outward pressure applied to the stenosis  102  by the central portion  5  is greater than the radial outward pressure applied by the first and second end portions  3  and  4  to the wall  101  adjacent the proximal and distal non-diseased parts  104  and  106  of the artery  100  in the expanded state of the stent  1 . However, the central portion  5  is configured so that the maximum transverse cross-sectional area to which the central wall  15  can expand is limited to a predefined maximum transverse cross-sectional area, which is chosen to be less than the transverse cross-sectional area of the proximal and distal non-diseased parts  104  and  106  of the artery  100 , adjacent the stenosis  102 . This is to ensure that the central wall  15  of the central portion  5  in the expanded state of the stent  1  is spaced apart from the arterial wall  101  of the artery  100 , and does not come in contact with the arterial wall  101  of the non-diseased part of the artery  100  adjacent the stenosis  102 . 
     The stent  1  in this embodiment of the invention is formed from a perforated material which is perforated to form struts  23  suitably connected to form interstices  20  therebetween extending through the material. In this embodiment of the invention the material of the stent  1  comprises a memory metal, so that the stent  1  is self-expanding and preferably, expands at the normal blood temperature in the human or animal body. While many metals and other materials may be used to provide such a self-expanding stent, in this embodiment of the invention the material of the stent is a nickel titanium alloy known as Nitinol. In the unexpanded state of the stent  1 , the stent  1  is radially compressed so that the struts  23  lie adjacent each other, and depending on the pattern of the perforations which form the stent  1 , the struts  23  in the compressed unexpanded state of the stent  1  may extend in an axial direction. 
     The interstices  20   a  extending through the central wall  15  of the central portion  5  in the expanded state of the stent  1  are of area less than the area of the interstices  20   b  extending through the first and second walls  10  and  11  of the first and second end portions  3  and  4 . The interstices  20   c  extending through the transition walls  17  of the transition portions  7  are of area substantially similar to the area of the interstices  20   a  extending through the central wall  15  of the central portion  5 . In this embodiment of the invention the interstices  20   a  and  20   c  extending through the central wall  15  of the central portion  5  and the transition walls  17  of the transition portions  7  in the expanded state of the stent  1  are of sufficiently small area in order to at least minimise and preferably prevent the material forming the stenosis  102  penetrating through the interstices  20   a  and  20   c  in the central portion  5  and the transition portions  7 . Additionally, by providing the interstices  20   a  and  20   c  of the central portion  5  and the transition portions  7  to be of area smaller than the interstices  20   b  of the first and second end portions  3  and  4 , results in the radial outward pressure exerted on the stenosis  102  by the central portion  5  being greater than the radial outward pressure exerted on the proximal and distal parts  104  and  106  of the artery  100  by the first and second end portions  3  and  4 . 
     The wall thickness of the first and second walls  10  and  11  of the first and second end portions  3  and  4  is similar to the wall thickness of the central wall  15  of the central portion  5 . Accordingly, in the expanded state of the stent  1  the relationship between the internal diameter d 2  of the central wall  15  of the central portion  5  and the internal diameter D 2  of the first and second walls  10  and  11  of the first and second end portions  3  and  4  is substantially similar to the relationship between the external diameter d 1  of the central wall  15  of the central portion  5  and the external diameter D 1  of the first and second walls  10  and  11  of the first and second end portions  3  and  4 . In this embodiment of the invention the wall thickness of the first and second end walls  10  and  11 , the central wall  15  and the transition walls  17  is approximately 0.1 mm. However, it is envisaged that the thickness of the first and second end walls, the central wall and the transition walls may lie in the range between 0.02 mm and 0.1 5 mm, although preferably, the thickness of the first and second end walls, the central wall and transition walls will ideally lie in the range of 0.05 mm to 0.1 mm. The selection of the thickness of the first and second end walls, the central wall and the transition walls will be chosen to optimise between the strength and function of the stent on the one hand, and minimising endothelial shear stress resulting from the use of the stent on the other hand. In general, the wall thickness of the stent will be selected so that the endothelial shear stress does not exceed 250 Pa. 
     In this embodiment of the invention the stent is configured so that in the expanded state of the stent the external diameter of the first and second end portions  3  and  4  of the stent is approximately 110% of the diameter of the proximal and distal non-diseased parts  104  and  106  of the artery adjacent the proximal and distal ends  105  and  107 , respectively, of the stenosis  102 . However, it is envisaged that the external diameter of the first and second end portions  3  and  4  of the stent  1  may be configured to expand in the expanded state of the stent to a diameter lying in the range of 100% to 125% of the diameter of the proximal and distal non-diseased parts  104  and  106  of the artery adjacent the proximal and distal ends  105  and  107 , respectively, of the stenosis  102 , and preferably, in the range of 100% to 110% of the diameter of the artery in the proximal and distal non-diseased parts  104  and  106  thereof. 
     The stent  1  is coated with a therapeutically active material, and in this embodiment of the invention is coated with two therapeutically active materials, one of which comprises an angiogenesis promoting material, and the other of which prevents further growth of plaque. Typically, the angiogenesis promoting material comprises methacrylic acid-ecoisodecyl acrylate (MAA-co-IDA; 40% MAA). The therapeutically active material for preventing further growth of plaque may comprise any one or more of the sirolimus and paclitaxel. 
     Referring now to  FIGS.  7  to  9    there is illustrated a delivery system  30  for delivering the stent  1  to the stenosis  102  in the artery  100 . The delivery system  30  comprises a delivery microcatheter  31  extending between a proximal end  32  and a distal end  33 . An elongated bore  34  extends through the delivery microcatheter  31  from the proximal end  32  to the distal end  33  for accommodating delivery of the stent  1  therethrough to the stenosis  102  when the distal end  33  of the delivery microcatheter  31  is located in the stenosis  102  and projects distally from the stenosis, see  FIG.  7   . The bore  34  is of diameter such that the stent  1  in its unexpanded state is freely slideable through the bore  34 , and will vary depending on the diameter of the stent in the unexpanded state, and typically will be in the range of 0.68 mm to 0.84 mm. The outer diameter of the delivery microcatheter  31  is such that the delivery microcatheter  31  is urgeable through the intracranial vascular system to and through the bore  103  of the stenosis  102  from a suitable entry point, which typically, may be the femoral, radial, brachial or carotid arteries. Additionally, the delivery microcatheter  31  is of sufficient flexibility in order to facilitate negotiating the delivery microcatheter  31  through the intracranial vascular system to the stenosis  102  in the artery  100 . 
     An elongated positioning member  40  of diameter less than the diameter of the bore  34  of the delivery microcatheter  31  is provided for delivering the stent  1  through the delivery bore  34  of the delivery microcatheter  31  to the stenosis  102  in the artery  100 . The positioning member  40  extends between a proximal end  41  and a distal end  42  and terminates at its distal end  42  in a releasable coupling mechanism  44  for releasably coupling the positioning member  40  to the stent  1  adjacent the free end  19  of the first wall  10  of the first end portion  3 . The first end portion  3  of the stent  1  is configured for locating in the proximal non-diseased part  104  of the artery  100  adjacent a proximal end  105  of the stenosis  102 , while the second end portion  4  is configured for locating in the distal non-diseased part  106  of the artery  100  adjacent a distal end  107  of the stenosis  102 . In this embodiment of the invention the coupling mechanism  44  for coupling the free end  19  of the first wall  10  of the first end portion  3  to the positioning member  40  is a conventional coupling mechanism, which will be known to those skilled in the art, and is operable from the proximal end  41  of the position member  40  for releasing the stent  1  from the positioning member  40  when the stent  1  has been correctly positioned in the stenosis  102 . Alternatively, the coupling mechanism may be releasably coupled to the free end  19  of the second end wall  11  of the second end portion  4 . 
     In use, typically, the procedure for delivering the stent  1  to the stenosis  102  in the artery  100 , commences with a guide wire (not shown) being entered into the intracranial vascular system, typically, through the femoral, radial, brachial or carotid arteries. The guide wire is urged from the entry point into the intracranial vascular system, and is urged through the intracranial vascular system to the stenosis  102  in the artery  100 . The guide wire is urged through the bore  103  in the stenosis  102  so that a distal end of the guide wire extends into the artery  100  distally of the distal end  107  of the stenosis  102 . The delivery microcatheter  31  is then urged over the guide wire through the intracranial vascular system until the distal end  33  of the delivery microcatheter  31  extends through the bore  103  extending through the stenosis  102  to the distal non-diseased part  106  of the artery  100  adjacent the distal end  107  of the stenosis  102 . The guide wire is then withdrawn through the bore  34  of the delivery microcatheter  3 . 
     The stent  1  in its unexpanded state is coupled to the positioning member  40  adjacent the distal end  42  by the coupling mechanism  44 . With the stent  1  in its unexpanded state coupled to the distal end  42  of the positioning member  40 , the positioning member  40  urges the stent  1  into the bore  34  of the delivery microcatheter  31  adjacent the proximal end  32  thereof with the second end portion  4  leading the stent  1 . The stent  1  is then urged through the bore  34  of the delivery microcatheter  31  to the distal end  33  thereof by the positioning member  40 . When the stent  1  is within the bore  34  of the delivery microcatheter  31  adjacent the distal end  33  thereof, the leading portion of the stent  1 , namely, the second end wall  10  of the second end portion  4  is exposed by slightly withdrawing the delivery microcatheter  31  proximally. The second end wall  11  of the second end portion  4  of the stent  1  on being exposed and coming in contact with the blood at the normal body temperature of the subject commences to expand in the distal non-diseased part  106  of the artery adjacent the distal end  107  of the stenosis  102 , see  FIG.  8   . 
     The position of the stent  1  is adjusted by appropriately manoeuvring the delivery microcatheter  31  and the positioning member  40  simultaneously in order to correctly position the second wall  11  of the second end portion  4  in its correct position in the distal non-diseased part  106  of the artery  100  adjacent the distal end  107  of the stenosis  102 . Once the second wall  11  of the second end portion  4  is correctly positioned in the distal non-diseased part  106  of the artery  100  adjacent the distal end  107  of the stenosis  102 , the delivery microcatheter  31  is further proximally withdrawn in order to completely expose the stent  1 . 
     The remainder of the stent  1  on coming in contact with the blood in the artery  100  at body temperature expands with the central wall  15  of the central portion  5  located in the bore  103  of the stenosis  102  bearing on the stenosis  102 , and the first wall  10  of the first end portion  3  bearing on the proximal non-diseased part  104  of the artery  100  adjacent the proximal end  105  of the stenosis  102 . The material forming the stenosis  102  is completely contained within an annulus  108  defined by the central wall  15  of the central portion  5 , the transition walls  17  of the transition portions  7  and the portion of the wall  101  of the artery  100  adjacent the stenosis  102 . 
     The positioning member  40  is decoupled from the stent  1  by decoupling the coupling mechanism  44  from the stent  1 . The positioning member  40  and the delivery microcatheter  31  are withdrawn through the intracranial vascular system and in turn through the femoral, radial, brachial or carotid arteries as the case may be. 
     With the stent  1  in its expanded state in the stenosis  102  the material forming the stenosis  102  is retained in the annulus  108  by the central wall  15  of the central portion  5  and the transition walls  17  of the transition portions  7  and by the action of the first and second walls  10  and  11  of the first and second end portions  3  and  4  bearing on the proximal and distal non-diseased parts  104  and  106  of the artery  100  adjacent the proximal and distal ends  105  and  107  of the stenosis  102 . The action of the first and second walls  10  and  11  of the first and second end portions  3  and  4  on the proximal and distal non-diseased parts  104  and  106  retain the stent  1  firmly anchored in the artery  100 , and also prevent the material forming the stenosis  102  being urged between the wall  101  of the artery  100  and the first and second walls  10  and  11  of the first and second end portions  3  and  4 . The central wall  15  of the central portion  5  bears on the stenosis  102  to either maintain the diameter of the bore  103  extending through the stenosis  102  at its diameter prior to placing the stent  1  in the stenosis  102 , or to increase the diameter of the bore  103  extending through the stenosis  102  to a diameter sufficient to maintain the flow rate of blood through the artery  100  at or greater than 50% of the normal flow rate of the blood therethrough prior to the formation of the stenosis  102  therein. By maintaining the flow rate of the blood through the artery  100  at or greater than 50% of the normal flow rate, the downstream tissue is protected from hypoxia, ischemia and infarction while maintaining the downstream tissue oligemic state. Thus, the natural intracranial angiogenesis process is promoted whereby new and collateral blood vessels are grown to provide a blood supply to the downstream tissue supplied by the diseased artery  100 . 
     The provision of the therapeutically active intracranial angiogenesis material coated onto the stent further enhances the natural intracranial angiogenesis process, and the therapeutically active material coated onto the stent for preventing further growth of plaque, prevents or at least minimises any further growth of plaque. 
     Referring now to  FIG.  10    there is illustrated a stent according to another embodiment of the invention indicated generally by the reference numeral  50 . The stent  50  is substantially similar to the stent  1 , and similar components are identified by the same reference numerals. The stent  50  in this embodiment of the invention is also particularly suitable for stenting a intracranial artery, such as the artery  100  having a stenosis  102  therein, for maintaining or increasing the rate of blood flow through the stenosis  102  to protect downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, for in turn promoting natural intracranial angiogenesis. The dimensions of the stent  50  are within the dimensional ranges discussed with reference to the stent  1 . The main difference between the stent  50  and the stent  1  lies in the transition portions  7 . In this embodiment of the invention instead of the transition portions  7  being formed by a transition wall  17  extending perpendicularly from the central wall  15  of the central portion  5  and extending perpendicularly from the first and second walls  10  and  11  of the first and second end portions  3  and  4 , the transition portions  7  of the stent  50  are formed by frusto-conical wall portions  52 , which in turn are formed by transition walls  53  which diverge outwardly from the central wall  15  of the central portion  5  to the first and second walls  10  and  11  of the first and second end portions  3  and  4 , respectively. 
     The wall thickness of the transition walls  53  is substantially similar to the wall thickness of the first and second walls  10  and  11  and the central wall  15  of the first and second end portions  3  and  4  and the central portion  5 , respectively. The external diameter of the transition walls  53  adjacent the central wall  15  of the central portion  5  is similar to the external diameter d 1  of the central wall  15 . The internal diameter of the transition walls  53  adjacent the central wall  15  of the central portion  5  is similar to the internal diameter d 2  of the central wall  15  of the central portion  5 . The external diameter of the transition walls  53  adjacent the first and second walls  10  and  11  of the first and second end portions  3  and  4  is similar to the external diameter D 1  of the first and second walls  10  and  11  of the first and second end portions  3  and  4 , while the internal diameter of the transition walls  53  adjacent the first and second walls  10  and  11  of the first and second end portions  3  and  4  is similar to the internal diameter D 2  of the first and second walls  10  and  11  of the first and second end portion  3  and  4 . 
     In this embodiment of the invention the cone angle defined by the frusto-conical transition walls  53  is approximately 60°. This, thus, results in the external diameter of each transition wall  53  increasing from the central wall  15  to the corresponding one of the first and second walls  10  and  11  at a rate of approximately 1.72 mm per 1 mm of length from the central wall  15 . 
     In this embodiment of the invention the stent  50  also comprises a memory alloy know as Nitinol, and is formed of perforated construction having interstices  20 . The interstices  20   a  and  20   c  extending through the central wall  15  and the transition walls  53  are of area less than the area of the interstices  20   b  extending through the first and second walls  10  and  11  of the stent  50 . In this embodiment of the invention the area of the interstices  20   a  and  20   c  extending through the central wall  15  and the transition walls  53  is such as to minimise, and in general, prevent the material forming the stenosis  102  penetrating through the central wall  15  and the transition walls  53 . 
     In this embodiment of the invention the stent  50  is also coated with a therapeutically active intracranial angiogenesis promoting material, and a therapeutically active material for preventing the growth of plaque. 
     Use of the stent  50  and the delivery of the stent  50  to the stenosis  102  of the artery  100  is similar to that already described with reference to the use and delivery of the stent  1  to the stenosis  102  of the artery  100 . However, in this embodiment of the invention when the stent  50  has been urged through the bore  34  of the delivery microcatheter  31  by the positioning member  40 , the delivery microcatheter  31  is urged proximally to expose both the second end wall  11  and the adjacent transition wall  53  thereby permitting the second wall  11  and the adjacent transition wall  53  to expand on coming into contact with the blood in the artery  100  of the subject, so that the second wall  11  of the stent  50  engages and bears on the distal non-diseased part  106  of the artery  100  distally of the stenosis  102 . However, thereafter the manoeuvring of the delivery microcatheter  31  and the positioning member  40  to accurately position the stent  50  in the stenosis  102  is similar to that described with reference to delivery and positioning of the stent  1 . 
     Referring now to  FIG.  11    there is illustrated a stent according to another embodiment of the invention indicated generally by the reference numeral  60 . The stent  60  is substantially similar to the stent  1 , and similar components are identified by the same reference numerals. The stent  60  in this embodiment of the invention is also particularly suitable for stenting a intracranial artery, such as the intracranial artery  100  having a stenosis  102  therein for maintaining or increasing the rate of blood flow through the stenosis  102  to protect downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, for in turn promoting natural intracranial angiogenesis. The dimensions of the stent  60  are within the dimensional ranges discussed with reference to the stent  1 . The only difference between the stent  60  and the stent  1 , is that in the stent  60 , the central portion  5  is of “hourglass” shape having a minimum external diameter d 1 . Transition portions  7  extending from the central portion  3  are of frusto-conical shape, similar to the transition portions  7  of the stent  50 . In this embodiment of the invention the area of the interstices  20   a  and  20   c  of the central portion  5  and the transition portions  7  are of area smaller than the area of the interstices  20   b  of the first and second end portions  3  and  4 . Additionally, in this embodiment of the invention the area of the interstices  20   a  and  20   c  of the central portion  5  and the transition portions  7  is such as to at least minimise, and preferably, prevent material forming the stenosis  102  extending through the interstices  20   a  and  20   c  in the central portion  5  and the transition portions  7 . 
     Otherwise the stent  60  and its use is substantially similar to that described with reference to the stent  1 , and is of dimensions within the ranges discussed in connection with the stent  1 . 
     Referring now to  FIG.  12    there is illustrated a stent according to a further embodiment of the invention indicated generally by the reference numeral  70 . The stent  70  is substantially similar to the stent  1 , and similar components are identified by the same reference numerals. The stent  70  in this embodiment of the invention is also particularly suitable for stenting a diseased artery, for example, the artery  100  having a stenosis  102  therein, for maintaining or increasing the rate of blood flow through the stenosis  102  to protect downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, for in turn promoting natural angiogenesis. The diameter dimensions of the stent  70  are within the diameter dimensional ranges discussed with reference to the stent  1 . However, in this case a perforator artery  109  is branched off from the artery  100  at a location adjacent the stenosis  102 . In this embodiment of the invention the length L 1  of the stent  70  between the first and second end portions  3  and  4 , including the central portion  5  and the transition portions  7  is of sufficient length so that the first and second end portions  3  and  4  engage the proximal and distal non-diseased parts  104  and  106  of the artery such that both the stenosis  102  and the connection of the perforator artery  109  are contained within the annulus  108  defined between the central portion  5 , the transition portions  7  and the wall  101  of the artery  100  adjacent the stenosis  102 . 
     In this embodiment of the invention the radial outward pressure exerted by the central portion  5  on the stenosis  102  is such as to maintain the rate of the blood flow through the artery  100  at approximately 50% of the normal blood flow rate through the artery  100  prior to the formation of the stenosis  102 . However, the radial outward pressure exerted by the central portion  5  on the stenosis  102  is such as to prevent excessive squashing of the material forming the stenosis  102  between the central portion  5  and the arterial wall  101  to the extent that the material of the stenosis would extend longitudinally along the arterial wall  101 , and would have resulted in blocking of the perforator artery  109 . In this embodiment of the invention while the area of the interstices  20   a  of the central portion  5  and the interstices  20   c  of the transition portions  7  is such as to prevent material forming the stenosis  102  extending therethrough, the area of the interstices  20   a  and  20   c  extending through the central portion  5  and the transition portions  7  is such as to permit blood flow through the central portion  5  and the transition portions  7  to the perforator artery  109 . 
     Referring now to  FIG.  13    there is illustrated a stent according to another embodiment of the invention indicated generally by the reference numeral  80 , which is also suitable for urging through an intracranial vascular system to a stenosis  102  in an intracranial artery  100  in order to maintain the diameter or to increase the diameter of the bore  103  through the stenosis  102  for maintaining or increasing the rate of blood flow through the stenosis  102  to protect downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, for in turn promoting and supporting a natural intracranial angiogenesis process. In this embodiment of the invention the stent  80  is not of a self-expanding material, and accordingly, is configured for delivery to the stenosis  102  by a balloon microcatheter  82 . The balloon microcatheter  82  comprises an elongated microcatheter  83  terminating at its distal end  84  in a balloon  85 . The balloon  85  is appropriately shaped so that when inflated it inflates to take up the approximate shape of the stenosis  102  and the proximal and distal non-diseased parts  104  and  106  of the artery  100 . The stent  80  when expanded by the balloon  85  in the stenosis  102  is configured to take up a shape substantially similar to that of the stent  50 , whereby the stent  80  when expanded by the balloon  85  comprises first and second end portions  3  and  4 , a central portion  5  of diameter less than the diameter of the first and second end portions  3  and  4 , and respective frusto-conical transition portions  7  extending from the central portion  5  to the first and second end portions  3  and  4 . The material of the stent  80 , is such that once expanded by the balloon  85  in the stenosis  102 , the stent  80  retains its expanded shape, thereby maintaining the bore  103  extending through the stenosis  102  at a diameter in order to provide a blood flow rate through the stenosis  102  of approximately 50% of the normal flow rate through the artery  100  prior to the formation of the stenosis  102 . The material of the stent is of perforated construction having interstices  20  extending through the first and second end portions  3  and  4 , the central portion  5  and the transition portions  7 . The interstices  20   a  and  20   c  extending through the central portion  5  and the transition portions  7  are smaller than the interstices extending through the first and second end portions  3  and  4 , as already described with reference to the stent  1 . 
     Delivery of the stent  80  to the stenosis  102  is substantially similar to the delivery of the stent  1  to the stenosis  102  as already described with reference to  FIGS.  7  to  9   , with the exception that instead of delivering the stent  80  through the delivery microcatheter  31  by a positioning member  40 , the stent  1  in the unexpanded state is delivered through the delivery microcatheter  31  on the balloon microcatheter  82 . The stent  80  in its unexpanded state is located on the balloon  85  in its deflated state, and is delivered through the delivery microcatheter  31  to the stenosis  102 . On the balloon  85  with the stent  80  thereon in its unexpanded state being located in the stenosis  102 , the balloon  85  is inflated to in turn expand the stent  80  in the stenosis  102 , with the central portion  5  of the stent  80  bearing on the stenosis  102 , and the first and second end portions  3  and  4  of the stent  80  bearing on and engaging the proximal and distal non-diseased parts  104  and  106  of the artery  100 . On completion of the positioning of the stent  80  in the stenosis  102 , the balloon  85  is deflated and withdrawn along with the delivery microcatheter as already described with reference to  FIGS.  7  to  9   . 
     Referring now to  FIGS.  14   a, b  and  c    to  FIGS.  17   a, b  and  c   , there is illustrated four expandable stents according to further embodiments of the invention indicated generally by the reference numerals  90  to  93 , respectively, for use in a human or animal subject, for opening, or at least maintaining a stenosis open in a diseased artery in the intracranial vascular system, for maintaining or increasing the rate of blood flow through the stenosis  102  to protect downstream tissue from hypoxia, ischemia and/or infarction while maintaining the downstream tissue in an oligemic state, for in turn promoting a natural intracranial angiogenesis process. The expandable stents  90  to  93  are substantially similar to the expandable stent  1  described with reference to  FIGS.  2  to  6   , and similar components are identified by the same reference numerals. The main difference between the stents  90  to  93  and the stent  1  is that, in the expanded state of each of the stents  90  to  93  the central portion  5  of each of the stents  90  to  93  is eccentrically located relative to the first and second end portions  3  and  4  thereof. In other words, a main central axis  95  defined by the central portion  5  of each stent  90  to  93  is offset from first and second central axes  96  and  97  defined by the first and second end portions  3  and  4  of the corresponding one of the stents  90  to  93 . In this embodiment of the invention the first and second end portions  3  and  4  of the stents  90  to  93  are axially aligned with each other, and therefore the first and second central axes  96  and  97  coincide with each other. However the main central axis  95  of the central portion  5  of each stent  90  to  93  extends parallel to the first and second central axes  96  and  97  of the first and second end portions  3  and  4  of the corresponding one of the stents  90  to  93 , and is transversely offset from the first and second central axes  96  and  97  thereof. Although, it is envisaged that the main central axis  95  of the central portion  5  in some cases may extend at an angle relative to the first and second central axes  96  and  97  of the first and second end portions  3  and  4  of the corresponding stent  90  to  93 . The stents  90  to  93  are particularly suitable for use in a diseased intracranial artery in which the stenosis is located asymmetrically in the diseased artery. 
     In the stents  90  and  91 , the transition portions  7  extend perpendicularly from the central portion  5  and also extend perpendicularly from the first and second end portions  3  and  4 . In the stents  92  and  93 , the transition portions  7  are formed by transition walls  17 , which are of asymmetric frusto-conical shape. Additionally, in the stents  90  and  92  of  FIGS.  14   a, b  and  c   , and  16   a, b  and  c , respectively, the central wall  15  of the central portion  5  and the first and second end walls  10  and  11  of each of the stents  90  and  92  run tangentially to each other along one side of the stent. 
     The dimensions of the stents  90  to  93  lie within ranges similar to the dimensional ranges discussed with reference to the stent  1  described with reference to  FIGS.  2  to  6   . Additionally, the stents  90  to  93  may or may not be coated with a therapeutically active material as described with reference to the stent  1 . 
     Use and delivery of the stents  90  to  93  to a stenosis in the intracranial vascular system is similar to that already described with reference to the stent  1  with reference to  FIGS.  2  to  9   . Otherwise, the stents  90  to  93 , their use and delivery are similar to the stent  1 . 
     Referring now to  FIG.  18    there is illustrated a stent according to another embodiment of the invention indicated generally by the reference numeral  200  which is also suitable for advancement through a delivery microcatheter placed in the intracranial vascular system to a stenosis similar to the stenosis  102  in an intracranial artery similar to the intracranial artery  100  in order to achieve a threshold level of dilation to the bore  103  through the stenosis  102  to maintain blood flow through the stenosis  102  in order to prevent hypoxia, ischemia of infarction and to promote and support a natural intracranial angiogenesis process. The stent  200  is substantially similar to the stent  1 , with the exception that the transition portions  7  are formed by frusto-conical portions, and for convenience similar components to those of the components of the stent  1  are identified by the same reference numerals. In  FIG.  18    the stent  200  is illustrated with a support element extending through the stent  200 . The support element does not form a part of the stent  200 . 
     In this embodiment of the invention the stent  200  is of a self-expanding stent made from a nitinol material comprising struts  23  connected at crowns  201  which define interstices  20  therebetween, the stent  200  comprises a highly polished surface finish. It will be noted with reference to  FIG.  18    that the stent comprises a first portion  3 , a second portion  4  and a central portion  5  similar to the embodiments described with reference to the stents of  1 ,  50 ,  60 ,  70  and  80 . In this embodiment the stent  200  comprises transition portions which are of frusto-conical shape defining a cone angle of approximately 110°. The crowns  201  comprise V or U shaped elements in the stent pattern and the struts  23  interconnect the crowns  201 . 
     The stent  200  further comprises a plurality of ring structures, in this embodiment of the invention seven ring structures. Each ring structure comprises a plurality of interconnected struts  23  and crowns  201  organised to form a tubular ring around the main and first and second central axes  18 ,  21  and  22 , which in this embodiment of the invention coincide. Adjacent rings are connected together with connectors. In one embodiment the connectors are configured such that adjacent rings are spaced apart from each other. In another embodiment the crowns  201  of a first ring interpenetrate with the crowns  201  of an adjacent ring. 
     The stent  200  has an expanded state and a collapsed state. In the fully expanded state the stent  200  is stress free. The stent  200  is compressed, and thus stressed, in order for it to assume the collapsed state for delivery. In the expanded state the V angle Θ bf the crowns  201  is large and in the collapsed state the V angle Θ bf the crowns  201  is small. Indeed the V angle Θ of the crowns  201  is less than 5° in the collapsed state. In contrast the V angle Θ bf the crowns  201  is relatively larger in the expanded state. Preferably the V angle Θ s greater than 40° in the expanded state. More preferably the V angle Θ s greater than 60° and even more preferably the V angle Θ is greater than 80°. 
     The stent  200  of this embodiment is designed to be conformable and to protect flow to perforators and branch vessels, such as the perforator vessel  109  in  FIG.  12   . The larger V angle in the expanded configuration is configured to be large to provide flow orifices formed by the interstices  20  between the struts  23 . The struts  23  are configured to be short and narrow. Short and narrow struts while challenging to manufacture make the stent  200  more conformable and the narrow struts  23  maintain flow through the flow orifices formed by the interstices  20 . In one embodiment the struts  23  are configured such that the length of at least one ring in the stent is less than 2 mm. In another embodiment the struts  23  are configured such that the length of at least one ring in the stent is less than 1.5 mm. In yet another embodiment the struts  23  are configured such that the length of at least one ring in the stent is less than 1.0 mm. In one embodiment the struts  23  are configured to be less than 80 micrometres wide. In another embodiment the struts  23  are configured to be less than 60 micrometres wide. In yet another embodiment the struts  23  are configured to be less than 40 micrometres wide. 
     In one embodiment at least one ring of the stent structure comprises six struts  23  with three pairs of opposing crowns  201 . In another embodiment at least one ring of the stent structure comprises eight struts  23  with four pairs of opposing crowns  201 . In another embodiment at least one ring of the stent structure comprises ten struts  23  with five pairs of opposing crowns  201 . In another embodiment at least one ring of the stent structure comprises twelve struts  23  with six pairs of opposing crowns  201 . In another embodiment at least one ring of the stent structure comprises fourteen struts  23  with seven pairs of opposing crowns  201 . In another embodiment at least one ring of the stent structure comprises sixteen struts  23  with eight pairs of opposing crowns  201 . In the embodiment of the stent  200  illustrated in  FIG.  18    at least one ring of the stent structure comprises eighteen struts  23  with nine pairs of opposing crowns  201 . In one embodiment the number of struts in a first ring is greater than the number of struts in an adjacent ring. In one embodiment the number of struts in a first ring is equal to the number of struts in an adjacent ring. In one embodiment the length of a first ring is greater than the length of a second adjacent ring. 
     The stent  200  of  FIG.  18    is shape set to create an undulating tubular structure with a waisted section, namely, the central portion  5 . In one embodiment the stent is cut from a nitinol tube using a laser process, deburred and grit blasted. The shape setting of the stent  200  comprises heating the stent to a temperature in excess of 400° centigrade for a number of minutes until the desired shape is programmed into the stent  200 . The stent is cooled and the stent is electropolished to produce a surface with a highly polished finish. 
     In one embodiment the stent comprises a completely connected structure. In such an embodiment every crown of the stent is connected to an adjacent crown either directly or indirectly. The exception to this interconnectivity are those crowns that define the very distal and the very proximal ends of the stent. These crowns are not connected to adjacent crowns. 
     In another embodiment the stent  200  is configured for visualisation on fluoroscopy. In this embodiment a highly radiopaque metal is incorporated into the structure. The incorporation of the radiopaque metal allows the interventionalist to visualise the distal end of the stent which may be the first end portion  3  or the second end portion  4 , the proximal end of the stent, which is the other one of the first and second end portions  3  or  4 , the central portion  5  and the transition portions  7  between the central portion  5  and the first end portion  3  and second end portion  4 . 
     In one embodiment the radiopaque element comprises gold, platinum, tantalum or tungsten. In another embodiment the incorporation of the radiopaque element comprises an alloy of Nitinol and the radiopaque element. In another embodiment the incorporation of the radiopaque element comprises the inclusion of a radiopaque marker into the structure of the stent. In one embodiment the radiopaque marker comprises a cylindrical slug of a radiopaque metal pressed into a plurality of laser drilled holes in the structure of the stent. Suitable radiopaque metals for use in said cylindrical slug include gold, platinum, tantalum, tungsten or alloys containing a substantial portion of one or more of these elements. 
     In one embodiment the central portion  5  of the stent  200  is configured to dilate the bore  103  of the stenosis  102  significantly without inducing a corresponding dilation in the vessel wall  100  in the region of the stenosis  102 . The following examples of embodiments of stents similar to the stent  200  but of various dimensions and placed in stenoses  102  of various dimensions highlight this advantage of the invention. 
     EXAMPLE 1 
     Inputs 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Internal diameter D of the vessel 100 
                    3 mm 
               
               
                   
                 External diameter d 1  of the central portion 5  
                  1.4 mm 
               
               
                   
                 of the stent 200 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  7.07 mm 
               
               
                   
                 vessel 100 adjacent the stenosis 102 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  1.54 mm 2   
               
               
                   
                 central portion 5 of the stent 200 
                   
               
               
                   
                 Diameter d of the bore 103 extending through  
                  0.25 mm 
               
               
                   
                 the stenosis 102 
                   
               
               
                   
                 Transverse cross-sectional area of the bore 103 
                 0.046 mm 2   
               
               
                   
                 extending through the stenosis 102 
                   
               
               
                   
                 % occlusion by the stenosis 102 
                 92% 
               
               
                   
                   
               
            
           
         
       
     
     Calculations 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Increase in transverse cross-sectional area of the bore  
                 3036% 
               
               
                   
                 103 extending through the stenosis 102 
                   
               
               
                   
                 Transverse cross-sectional area of the stenosis annulus 
                 1.490 mm 2   
               
               
                   
                 formed by the displaced material of the stenosis 
                   
               
               
                   
                 Diameter of the dilated vessel adjacent the stenosis 
                  3.30 mm 
               
               
                   
                 Increase in the vessel diameter adjacent the stenosis 
                  10% 
               
               
                   
                   
               
            
           
         
       
     
     Conclusion 
     With, a 92% stenosis the stent  200  of this example can increase the cross sectional area of the bore of the stenosis by over 3000%, while only dilating the vessel diameter by 10%. 
     EXAMPLE 2 
     Inputs 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Internal diameter D of the vessel 100 
                  2.5 mm 
               
               
                   
                 External diameter d 1  of the central portion 5  
                  1.2 mm 
               
               
                   
                 of the stent 200 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  4.91 mm 2   
               
               
                   
                 vessel 100 adjacent the stenosis 102 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  1.13 mm 2   
               
               
                   
                 central portion 5 of the stent 200 
                   
               
               
                   
                 Diameter d of the bore 103 extending through  
                  0.3 mm 
               
               
                   
                 the stenosis 102 
                   
               
               
                   
                 Transverse cross-sectional area of the bore 103 
                 0.071 mm 2   
               
               
                   
                 extending through the stenosis 102 
                   
               
               
                   
                 % occlusion by the stenosis 102 
                 88% 
               
               
                   
                   
               
            
           
         
       
     
     Calculations 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Increase in transverse cross-sectional area of the bore  
                  1500% 
               
               
                   
                 103 extending through the stenosis 102 
                   
               
               
                   
                 Transverse cross-sectional area of the stenosis annulus 
                 1.060 mm 2   
               
               
                   
                 formed by the displaced material of the stenosis 
                   
               
               
                   
                 Diameter of the dilated vessel adjacent the stenosis 
                  2.76 mm 
               
               
                   
                 Increase in the vessel diameter adjacent the stenosis 
                  10.3% 
               
               
                   
                   
               
            
           
         
       
     
     Conclusion 
     With an 88% stenosis the stent  200  of this example can increase the cross sectional area of the bore of the stenosis by over 1500%, while only dilating the vessel diameter by 10.3%. 
     EXAMPLE 3 
     Inputs 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Internal diameter D of the vessel 100 
                    2 mm 
               
               
                   
                 External diameter d 1  of the central portion 5  
                  1.1 mm 
               
               
                   
                 of the stent 200 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  3.14 mm 2   
               
               
                   
                 vessel 100 adjacent the stenosis 102 
                   
               
               
                   
                 External transverse cross-sectional area of the 
                  0.95 mm 2   
               
               
                   
                 central portion 5 of the stent 200 
                   
               
               
                   
                 Diameter d of the bore 103 extending through the 
                  0.5 mm 
               
               
                   
                 stenosis 102 
                   
               
               
                   
                 Transverse cross sectional area of the bore 103 
                 0.196 mm 2   
               
               
                   
                 extending through the stenosis 102 
                   
               
               
                   
                 % occlusion by the stenosis 102 
                 75% 
               
               
                   
                   
               
            
           
         
       
     
     Calculations 
     
       
         
           
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Increase in transverse cross-sectional area of the bore  
                  384% 
               
               
                   
                 103 extending through the stenosis 102 
                   
               
               
                   
                 Transverse cross-sectional area of the stenosis annulus 
                 0.754 mm 2   
               
               
                   
                 formed by the displaced material of the stenosis 
                   
               
               
                   
                 Diameter of the dilated vessel adjacent the stenosis 
                  2.23 mm 
               
               
                   
                 Increase in the vessel diameter adjacent the stenosis 
                 11.4% 
               
               
                   
                   
               
            
           
         
       
     
     Conclusion 
     With a 75% stenosis the stent  200  of this example can increase the cross sectional area of the bore of the stenosis by over 384%, while only dilating the vessel diameter by 11.4%. 
     Accordingly, it can be seen from the above three examples that the stent  200  according to the invention of different dimensions when used in stenoses of different bore diameters can produce a significant increase in the stenosis bore transverse cross-section area for a relatively small increase in the diameter of the vessel resulting from the radial outward action of the central portion  5  of the stent  200  on the material forming the stenosis. Typically the increase in the transverse cross-sectional area of the bore extending through a stenosis can range from 300% to over 3000% for an increase in the diameter of the vessel adjacent the stenosis in the range of 10% to 12%. 
     In the embodiments of the invention where the stents have not been described as being coated with a therapeutically active material, in general, although not necessarily, but preferably, the stents according to the invention will be coated with a therapeutically active material. A preferred therapeutically active material with which the stents according to these embodiments of the invention will be coated is an angiogenesis promoting material. The stents according to the invention may be coated with any suitable angiogenesis promoting material, for example, methacrylic acid-ecoisodecyl acrylate (MAA-co-IDA; 40% MAA). 
     While the stents according to the invention have been described as comprising Nitinol, it is envisaged that the stents may comprise any suitable biocompatible material, such as stainless steel or cobalt alloy. By providing the stents to be of perforated construction, the quantity of metal in the stents is minimised, which has the advantage of reducing the thrombotic potential of the stents, and notably permits flow of blood from the stented artery to a perforator artery through the central wall, the transition walls and the first and second walls of the central, transition and first and second end portions, respectively, of the stent, as described in connection with the stent  80  of  FIG.  13   , in the case of a perforator artery branching from an artery adjacent a stenosis. The less material used in the stents, the lower the potential for material to encourage formation of thrombosis once implanted in an artery. 
     While the stents according to the invention have been described as comprising a memory metal, the stents may be of a non-memory metal, and furthermore, may be of a non-self-expanding material. For example, it is envisaged that the stents may be of a material which is a non-self-expanding material as in the case of the stent  80 . Furthermore, it will be readily apparent to those skilled in the art that the stent  80  may be made of a self-expanding memory material. 
     While the stents according to the invention have been described for stenting a stenosis in an intracranial artery, the stents according to the invention may be used for stenting a stenosis in any intracranial vessel, such as a vein, lumen or other such intracranial blood vessel. 
     In other embodiments of the stents, a biocompatible film or membrane may be provided to at least one surface of the stents. The film or membrane may or may not be perforated. 
     The film or membrane may be a non-woven fabric made of plastic fibrils. It is bonded to, or forms a bond with, the walls of the stents. The film or membrane may also be made into a thin strand which can be woven into a lattice which is layered onto the stent. The membrane and/or the lattice may be deposited on the stents in a manner that is aligned with the struts of the stents, or alternatively in a non-aligned manner. In certain embodiments of the invention, the membrane may have a porous structure capable of accommodating pharmaceutical materials and releasing these materials into the surrounding vessels and plaque, under physiological conditions. 
     The membrane may optionally be implanted or coated with one or more pharmaceutical compositions or other compositions having a desirable effect on the properties of the stents. This addresses the challenge of coating the stents surfaces with pharmaceutical materials. These compositions can release medication over time into the surrounding tissues, vascular surface or plaques. Proliferation-inhibiting substances such as paclitaxel and rapamycin, for example, can be beneficial. Other examples are substances that prevent thrombosis, or prevent liquid embolic agents or glue from adhering to the membrane and/or the stents. 
     The film or membrane may be suitably made of a polymeric material, such as polyurethane, polytetrafluoroethylene, polyester, polyamide or polyolefin. These polymeric membranes are not easily attached to a stent. Conventionally, stent membranes are held mechanically against a vessel wall or plaque by the radial force of an expanding stent. However, in the present invention, the central portion of the stents do not contact the arterial wall. The stents according to the invention having a membrane made from a polymeric material which is not easily attached to the walls of the stent are provided with a suitable connection of the membrane to the stents, typically a mechanical connection. Connections of this type are also useful if the stents require re-sheathing into a microcatheter for adjustment or relocation of the stent to another area. 
     The membranes of the stents may be conveniently made by spraying onto the stent walls, provided the stent walls are compatible with the polymer as regards formation of suitable bonds, or by electrospinning of the non-woven fabric around the struts of the stents; by applying an electric current, the fibrils of the membrane are separated from a polymer solution and deposited on a substrate. The technique of electrospinning is well known in the art and will not be described in further detail here. The deposition causes the fibrils to agglutinate into a non-woven fabric. It can be formed into strands that can be woven, if so chosen. The fibrils generally have a diameter of from 100 to 1000 nm. Membranes produced by electrospinning are very thin and uniform in thickness and can easily form a bond with the stent walls. Such membranes are strong enough to withstand mechanical stress during compression of the stents into a deliverable state, and during maneuvering of the stents along and into tortuous vessels. Such membranes can be pierced easily, mechanically, as required, without creating an opening that gives rise to fractures or cracks. The thickness and length of the fibrils can be controlled by the electrospinning process. Examples of such membranes include poly(lactide-co-caprolactone) (PLCL) which can have a degradation time of 6-18 months; poly(caprolactone) (PCL) which can have a degradation time of 2-3 years; or stiffer materials such as polylactides (PLA), poly(lactide-co-glycosides) (PLGA), polyacrylonitrile (PAN) or polyurethane (PU). 
     The membrane can be composed of a single layer or multiple layers. Multiple layers may be produced by different methods, for example a first layer by electrospinning, a second by spray coating and a third by electrospinning. Active pharmaceutical or other agents can be implanted on one or several of these layers. The agent can be released by diffusion or by degradation or erosion of the layers of the membrane. Radiopaque substances may also be implanted in the membrane so that it is more easily visible to the techniques used during stent implantation. In particular, in order to further promote and support angiogenesis, the stent may be coated with methacrylic acid-ecoisodecyl acrylate (MAA-co-IDA; 40% MAA) or similar substances known to promote angiogenesis. 
     The membrane may be laced with graphene to improve its strength and flexibility. Additionally, or alternatively, the membrane may comprise a thin film Nitinol. 
     The stents may be made of a biodegradable material such that it has a limited lifespan. Polymer-based stents are conventionally based on poly(L-lactide) (PLLA), chosen because it is able to maintain a radially strong scaffold that breaks down over time into lactic acid, a naturally occurring molecule that the body can use for metabolism. Other polymers in development include tyrosine polycarbonate and salicylic acid. 
     While a specific delivery system has been described for delivering the stent  1  to a stenosis in a intracranial artery, other suitable delivery means may be used, for example, via a balloon catheter. The stents according to the invention are constructed to be inserted without requiring pre-dilatation of the vessel. However, in certain conditions, pre-dilation of the ICAS may be indicated in order to navigate a delivery microcatheter or balloon microcatheter past the stenotic focus. Similarly, the stents may be constructed so that post-dilatation (dilatation of the stent with a balloon after insertion) is not required. However, in certain instances, such as where there is heavily calcified atherosclerotic plaque, post-dilation may be advantageous. Therefore, the stents, whilst not of themselves requiring pre-dilation or post-dilation, is compatible with pre-dilatation and post-dilatation. Where the stent is not self-expanding, a further aspect of the invention is the provision of a shaped dilation balloon, which will cause the outer ends of the stent to be dilated to a greater extent than the inner section, so as to achieve a dilated stent shape with dimensions as previously described for the self-expanding stent, as described with reference to  FIGS.  2  to  12  and  14  to  18   . 
     The stents according to the invention have been described as comprising perforated expandable walls, such that they may be folded-up in the unexpanded state to form a low profile for delivery into the arterial system. A low profile configuration allows for delivery into the smaller intracranial vessels and arteries via smaller microcatheters. The configuration of the stents can be of an open, closed or woven stent design. 
     The structure of the stents is illustrated as a closed cell type. The cell construction of the stents cause the stent to be auto-expandable once inserted into an artery. The stents according to the invention assumes a dumbbell shape upon expansion. 
     The stents may be formed with a reattachment system so that they may be re-sheathed even when fully delivered, such that an operator may re-adjust the deployment until fully satisfied with the positioning of the stents, relative to the stenosis. Conventional electrolytic severance of an intravascular stent implant involves using an electrolytically corrodible structure on the end of a delivery wire at the connection between the delivery wire and the intravascular stent implant. Such a device can make use of the voltage applied to the intravascular implant serving as an anode for electro-thrombosis. Alternatively, a mechanical reattachment system may be employed. 
     Accurate deployment of stents may be obtained via appropriately positioned radio-opaque markers which are visible under x-ray fluoroscopy, as is known in the art. Optionally, markers may be provided at the proximal and distal ends of the central portion of the stents. Alternatively or additionally, the central portion of stents may be made more radio-opaque by the application of a gold coating to the struts of the stents. Likewise, the whole of the stents can be substantially coated with a radio-opaque marker such as gold. Needless to say, any other radio-opaque markers may be provided on the stents. 
     Where the stents are made of a biodegradable material, such that it has a limited lifespan, the stents remain in place until they dissolve. Anti-platelet therapy is frequently recommended for patients that have received a stent. In the case that the stents are biodegradable, anti-platelet medication can be discontinued after dissolution of the stent. 
     Where the stents are not biodegradable, they are left in place and provide a scaffolding over which endothelial cells will grow over time. 
     The stents are deployed over a stenotic area (as described above). A narrower central portion is located in the bore of the stenosis in order to maintain or increase the diameter of the bore of the stenosis to an extent that restores blood flow distally. The diameter of the central portion is sufficient to open the stenosis to restore blood flow but it does not open the stenosis fully, rather it achieves around 50% of the diameter of the healthy artery. 
     The relatively small diameter of the central portion of the stents according to the invention acts to minimise the amount of material from the stenosis that is pushed into adjacent unoccupied areas of the artery. Furthermore, the sub-maximal angioplasty achieved by the stents minimises the likelihood of fracturing the plaque of the stenosis, and therefore minimises the chance of distal vessel stenosis. 
     Conventional self-expanding stents are anchored in place by the outward force of the stent on vessel walls along the whole length of the stent. This outward force causes a significant amount of atherosclerotic debris to be pushed to the walls of the blood vessel. Atherosclerotic debris that is pushed against walls of the blood vessel can be forced into adjacent side branches or small vessels (snow-ploughing). The arterial structure in the brain is highly branched and the presence of many side branches can readily lead to snow-ploughing in the case where significant amounts of atherosclerotic debris are forced against arterial walls. 
     In use of the stents according to the invention, the two larger diameter first and second end portions bear against the arterial wall of the artery to stabilise and support the narrower central portion the function of which is to open the stenosis. The construction minimises the amount of plaque that is pushed along the arterial walls by the stents, since it is only the first and second end portions that contact the arterial walls. 
     The central portion of the stents is constructed to exert a higher outward radial force than that exerted by the first and second end portions. 
     As discussed above, the provision of the central portion and the transition portions of the stents according to the invention being formed with relatively small interstices, minimises, and in general, prevents material from the stenosis being forced through the interstices in the central portion and the transition portions of the stents according to the invention. If material from the stenosis is forced through the interstices of the stents, small particles of plaque can be released from the stents back into the blood vessel; this is the so-called “cheese-grating” effect. These small plaque particles can float into the distal circulation, moving deeper into the brain vasculature, potentially causing distal emboli. By minimising the area of the interstices in the central and transition portions of the stents according to the invention, the “cheese-grating” effect is minimised. 
     Similarly, a dual layer construction for the stents according to the invention can be used to further decrease the “cheese-grating” effect of atherosclerotic plaque. 
     A biocompatible film or membrane can be provided to a surface of the stents, to further reduce the incidence of the “cheese-grating” effect, thereby minimising the release of distal emboli into the intracranial circulation. 
     The stents of the present invention provide an early stage, non-medical treatment for ICAS and reduce the complications associated with conventional stent treatments of ICAS by, for example, minimising plaque forced into interstices of the stent (‘cheese-grating’), reducing the plaque pushed into arterial side branches by the stent (snow-ploughing), reducing the risk of intracranial hyperperfusion injury, providing low shear stress so as to reduce the occurrence of restenosis, and minimising plaque occluding the stent, ultimately reducing morbidity and mortality. 
     It is also envisaged that the stents according to the invention may comprise a composite structure, for example, Drawn Filled Tube Nitinol (DFT). 
     It is also envisaged that the stents according to the invention may be coated with graphene. 
     While the stents according to the invention have been described and illustrated with the axial length of the first and second walls of the first and second end portions being of equal length, it is envisaged that in some embodiments of the invention the axial length of the first and second walls of the first and second end portions of the stents may be different. For example, it is envisaged that the first walls of the first end portions, which would be located at the proximal end of the stenosis may be of axial length shorter than the axial length of the second wall of the second end portion. Alternatively, the axial length of the second wall of the second end portion of the stent may be of axial length shorter than the axial length of the first wall of the first end portion of the stent. In general, in cases where a perforator vessel is located relatively close to either the proximal or distal end of the stenosis, the axial length of the first or second wall of the first or second end portion, as the case may be of the stent may be shorter than the axial length of the other one of the first and second walls of the first and second end portions of the stent, in order to engage the non-diseased part of the artery adjacent the stenosis between the stenosis and the perforator vessel without blocking or covering the perforator vessel. For example, in the case of a perforator vessel being relatively close to a stenosis on the distal end of the stenosis, the second wall of the second end portion of the stent would be of axial length shorter than the axial length of the first wall of the first end portion of the stent, and the axial length of the second wall of the second end portion would be such that the second end portion of the stent would engage the non-diseased part of the artery adjacent the stenosis and between the stenosis and the adjacent perforator vessel. 
     Therefore, it is envisaged that the axial length of the first and second walls of the first and second end portions of the stents according to the invention may be the same or different. 
     Additionally, it will be appreciated that the external and internal diameters of the first and second end portions may be different. For example, in a case where the diameters of the vessel at the respective opposite ends of the stenosis are different, the diameters of the first and second end portions would be appropriately selected. 
     It is also envisaged that the stent  70  described with reference to  FIG.  12    when being positioned in the stenosis, instead of the second end portion  4  of the stent being located distally of the perforator  109 , the stent may be urged proximally in order to locate the second end portion  4  of the stent between the stenosis and the perforator  109 . It is further envisaged that in cases where a perforator  109  is branched from an artery or vessel adjacent a stenosis, either on the proximal or distal end of the stenosis, and there is no space between the stenosis and the branched perforator vessel, it is envisaged that the adjacent one of the first and second end portions of the stent may be located to extend over the perforator vessel  19 , and a blood supply would be accommodated through the interstices  20   b  through the corresponding one of the first and second walls of the relevant one of the first and second end portions  3  or  4  of the stent into the perforator vessel  103 .