Abstract:
Apparutus and methods are described for performing percutancous catheter-based interventional surgery. The apparatus comprises first and second devices that are located in adjacent body cavities, such as adjacent blood vessels, the first device being capable of transmitting a directional signal that can be received by the second device. The direction of the signal is correlated with the facility to direct therapy, such that improved accuracy in therapy placement is thereby achieved. Methods for treating patients utilising the means and apparatus are also provided.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to apparatus and methods for performing percutaneous catheter-based interventional surgery. In particular, the invention relates to apparatus and techniques for transvascular interstitial surgery. 
       BACKGROUND 
       [0002]    Minimally invasive surgery, or ‘key-hole’ surgery, allows for surgical devices to be inserted into a patient&#39;s body cavity through a small aperture cut. This form of surgery has become increasingly popular as it allows patients treated successfully to suffer less surgical discomfort while retaining the benefits of conventional surgery. Patients treated by such techniques are exposed to lower levels of trauma and their recovery times can be significantly reduced compared to conventional surgical procedures. 
         [0003]    Key-hole surgery has been adopted as a favoured route for performing laparoscopic surgery as well as in a number of cardiovascular procedures. In the latter case, a balloon catheter may be used to open a partially occluded coronary artery as an alternative to open heart surgery. This technique is known as balloon angioplasty. The balloon catheter is typically a small, hollow, flexible tube that has a balloon near to its distal tip. The catheter is inserted into an artery (usually near the patient&#39;s groin) and then guided through the body to the patient&#39;s heart. The heart and cardiac arteries are visualized by using X-ray fluoroscopy, and blockages in the heart vessels are identified. A balloon catheter is then inserted in or near the blockage and inflated, thus widening the occluded blood vessel and helping to restore blood flow to the cardiac tissue. 
         [0004]    However, balloon angioplasty is not always a suitable measure, especially in acute cases and in cases where a coronary artery is completely occluded. In these instances the typical treatment is to employ coronary bypass which involves open-heart surgery. Hence, there is a need to provide new and improved methods and apparatus for use in minimally invasive surgical procedures, such the restoration of a blood supply to ischaemic tissue. 
         [0005]    Conventional coronary bypass surgery is not always an option for certain patients. Factors such as age, obesity, diabetes and smoking can exclude a proportion of candidate patients who are in genuine need of such treatment. In these cases it has been postulated that minimally invasive surgery could provide a means for treating a broader range of patients including those currently excluded from standard techniques. Oesterle et al (Catheterization and Cardiovascular Interventions (2003) 58: 212-218) describe a technique they call percutaneous in situ coronary venous arterialization (PICVA) which is a catheter based coronary bypass procedure. In PICVA, the occlusion in the diseased artery is ‘bypassed’ by creation of a channel between the coronary artery and the adjacent coronary vein. In this way the arterial blood is diverted into the venous system and can perfuse the cardiac tissue in a retrograde manner (retroperfusion). The technique of retroperfusion has been known for some time, having first been performed in humans by Beck in the 1940s and 1950s (for review see Keelan et al. Current Interventional Cardiology Reports (2000) 2: 11-19). Apparatus and methods for performing procedures like PICVA are described in WO-A-99/49793 and US-A-2004/0133225. 
         [0006]    However, as the clinical results show in Oesterle et al. (supra), successfully performing a minimally invasive procedure of diverting blood flow from the coronary artery to the adjacent vein has a low success rate. In six out of the 11 cases described this was simply due to an inability to target the adjacent vein from the artery. As such, Oesterle et al&#39;s procedure is too often doomed to failure before it even starts. At present, the means for targeting the catheter consist of a combination of X-ray fluoroscopy and an imaging ultrasound probe located on the distal tip of the catheter (e.g. see US-A-2004/0133225). Indeed, such an arrangement is difficult to navigate and localisation of the adjacent vein requires considerable skill on the part of the clinician. Hence, there is a need for improvements in the means for targeting devices, such as catheters, that are used for procedures such as PICVA and in general transvascular surgery. Indeed, in the absence of such improvement it seems that such techniques will remain peripheral to the conventional surgical procedures of open-heart coronary bypass. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention provides means, methods and apparatus for overcoming the problems identified in the prior art. Most notably, the means, methods and apparatus of the invention allow for greatly improved targeting and localisation of the therapy to be administered. Hence, the invention shows particular advantage in treating patients requiring coronary bypass by enabling minimally invasive surgical techniques to be used more successfully than previously known. 
         [0008]    Accordingly, in a first aspect the invention provides a means for directing therapy within the body of a patient, the means comprising:
       a) a first therapeutic device that is located in a first body cavity, the first therapeutic device comprising signal means for generating a directional signal;   b) a second therapeutic device located in a second body cavity adjacent to the first body cavity, the second therapeutic device comprising receiving means for receiving the directional signal; and   c) therapeutic means for administering therapy to the body of the patient
 
wherein, therapy is directed by aligning the first therapeutic device with the second therapeutic device via the directional signal transmitted by the first therapeutic device being received by the second therapeutic device, and administering therapy at a location that is aligned to the path taken by the directional signal.
       
 
         [0012]    Optionally the therapeutic means is comprised within either the first or the second therapeutic devices. Typically, the first and second medical devices are catheters. In embodiments of the invention where the first therapeutic device comprises the therapeutic means, the first device is also referred to herein as the ‘launching device’. Likewise, where the second therapeutic device does not comprise the therapeutic means it is, thus, also referred to herein as the ‘target device’. 
         [0013]    A second aspect of the invention provides means for aligning a first therapeutic device located in a first body cavity with a second therapeutic device located in a second body cavity adjacent to the first body cavity, the means comprising:
       a) signal means for generating a directional signal, the signal means being located in the first therapeutic device; and   b) receiving means for receiving the directional signal, the receiving means being located in the second therapeutic device;       
 
         [0016]    wherein, alignment of the first therapeutic device and the second therapeutic device is achieved when the directional signal transmitted by the first therapeutic device is received by the second therapeutic device. 
         [0017]    A third aspect of the invention provides apparatus for traversing tissue intervening first and second body cavities comprising:
       a) a launching device suitable for location within the first body cavity, the launching device comprising
           (i) an elongate outer sheath with a distal end and a proximal end, the outer sheath defining and enclosing an interior lumen;   (ii) a signal transducer located at the distal end of the outer sheath, the signal transducer being arranged so as to transmit a directional signal; and   (iii) traversing means for traversing the tissue intervening the first and second body cavities, the traversing means being located within the lumen at the distal end of the outer sheath, wherein in use the traversing means is in a retracted state and can be extended out of the lumen via an aperture in the outer sheath such that it engages and traverses the tissue intervening the first and second body cavities, and wherein extension of the traversing means is along a path that is aligned with the direction of the signal;   
           and,   b) a target device suitable for location within the second body cavity, the target device comprising
           (i) an elongate outer sheath with a distal end and a proximal end, the outer sheath defining and enclosing an interior lumen; and   (ii) a signal receiving transducer located at the distal end of the outer sheath;   
               
 
         [0026]    wherein, in use, the signal transducer on the launching device transmits the directional signal that is capable of being received by the signal receiving transducer on the target device, and 
         [0027]    when the signal is received by the signal receiving transducer on target device it is determined that the devices are located in the correct juxtaposition within their respective body cavities such that the traversing means can be extended out of the launching device and traverses the tissue intervening the first and second body cavities. 
         [0028]    A fourth aspect of the invention provides a method for directing therapy in the body of a patient, comprising:
       a) placing a first therapeutic device into a first body cavity, the first therapeutic device comprising signal means for generating a directional signal, and therapeutic means for administering therapy to the body of the patient; and   b) placing a second therapeutic device into a second body cavity that is adjacent to the first body cavity, the second therapeutic device comprising receiving means for receiving the directional signal;       
 
         [0031]    wherein, therapy is directed by aligning the first therapeutic device with the second therapeutic device via the directional signal transmitted by the first therapeutic device being received by the second therapeutic device, and administering therapy at a location that is aligned to the path taken by the directional signal. 
         [0032]    In a particular embodiment of the invention the step of administering therapy comprises creation of an aperture in tissue between the first and second body cavities, thereby allowing fluid communication between the first and second body cavities. In accordance with the invention, the aperture is created at a position that lies along the path taken by the directional signal. 
         [0033]    All references cited herein are incorporated by reference in their entirety. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. 
     
    
     
         [0034]    The invention is further illustrated by reference to the accompanying drawings in which: 
           [0035]      FIG. 1  is a representation of an embodiment of the invention in which the launching device directs a signal from a first body cavity to the target device located in an adjacent second body cavity; 
           [0036]      FIG. 2  is a cross sectional representation along the line of BB in  FIG. 1 ; 
           [0037]      FIG. 3  is a representation of a specific embodiment of the launching device of the invention; 
           [0038]      FIG. 4  is a representation of a specific embodiment of the target device of the invention. Arrow A shows the reversed direction of blood flow after an arterial-venous stenosis (also called PICVA) has been effected; 
           [0039]      FIG. 5  is a representation of a specific embodiment of the launching device of the invention in which the signal transducer is comprised of an array of signal transducer elements; 
           [0040]      FIG. 6  is a representation of an embodiment of the invention wherein the launching and target devices comprise centring means; 
           [0041]      FIG. 7  is a representation of a stent in place following a procedure such as arterial-venous stenosis. Interrupted arrow A shows the direction of blood flow through the stent between the first and second body cavities. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0042]    In the embodiment of the invention as shown in  FIG. 1 , there is provided a launching device ( 10 ), which comprises a signal transmitter ( 12 ). The launching device ( 10 ) is typically a catheter that consists of an elongate flexible rod-like portion and a tip portion, and which provides a conduit for administering therapy within the body of a patient. Hence, the launching device ( 10 ) is suitable for location and movement through a first cavity or vessel ( 30 ) within a patient&#39;s body. The elongate portion of the launching device ( 10 ) comprises an outer sheath ( 11 ) that encloses a space, defining a lumen ( 13 ). The space within the lumen ( 13 ) may be suitably partitioned or subdivided as necessary so as to define channels for administering therapy or controlling the positioning of the launching device ( 10 ). Such subdivision may, for instance, be achieved either longitudinally or concentrically in an axial fashion. 
         [0043]    A signal transducer ( 12 ) is located on the launching device ( 10 ). The signal transducer ( 12 ) provides a signal ( 40 ) that is directed outwards from the first launching device ( 10 ). In the embodiment shown in  FIG. 1  the signal ( 40 ) is directed radially outward from the launching device ( 10 ) in a direction that is perpendicular to the longitudinal axis of the launching device ( 10 ). As mentioned in greater detail below, in alternative embodiments of the invention the direction of the signal ( 40 ) need not be perpendicular and can be directed at an angle to that of the axis of the launching device ( 10 ). The signal transducer ( 12 ) is, thus, comprised within the signal generating means of the apparatus of the invention. 
         [0044]    The signal transducer ( 12 ) is connected to signal transmitter ( 50 ). The signal transmitted can be suitably selected from ultrasound or appropriate electromagnetic sources such as a laser, microwave radiation or via radio waves. In a specific embodiment of the invention described in further detail below, the signal transmitter ( 50 ) generates an ultrasound signal, which is relayed to the signal transducer ( 12 ), which in turn directs the signal ( 40 ) out of the body cavity ( 30 ) into the surrounding tissue. 
         [0045]    According to the invention, a second device is located within an adjacent second body cavity or vessel ( 32 ). The first and second body cavities ( 30  and  32  respectively) are separated by intervening tissue ( 34 ), sometimes referred to as interstitial tissue or a septum. The first and second body cavities ( 30 ,  32 ) are located next to each other in a parallel fashion for at least a portion of their respective lengths. For example, many of the veins and arteries of the body are known to run in parallel with each other for at least a portion of their overall length. 
         [0046]    The second device is the target device ( 20 ), which assumes a similar arrangement to that of the first device ( 10 ). The target device ( 20 ) can also be a catheter that consists of an elongate flexible rod-like portion and a tip portion, such that fine movement and positioning of the target device ( 20 ) within the body cavity ( 32 ) can be achieved. In common with the launching device ( 10 ) the target device ( 20 ) comprises an outer sheath ( 21 ) that encloses a space, defining a lumen ( 23 ). The lumen ( 23 ) can be suitably partitioned as with the launching device ( 10 ). 
         [0047]    The target device ( 20 ) comprises a receiving transducer ( 22 ) for receiving the signal ( 40 ). The receiving transducer ( 22 ) is comprised within the signal detection means of the apparatus of the invention. In use, when the receiving transducer ( 22 ) receives the signal ( 40 ) transmitted from signal transducer ( 12 ), it transmits the received signal to signal detector ( 60 ). The signal detector ( 60 ) provides an output reading to the user of the apparatus via output display ( 61 ). 
         [0048]    In this way, the transmission and detection of the directed signal ( 40 ) allows for the navigation and positioning of the launching device ( 10 ) relative to the target device ( 20 ). In use, the launching device ( 10 ) and target device ( 20 ) can be maneuvered by the user of the apparatus until the output display ( 61 ) indicates that signal ( 40 ) is being received by the target device ( 40 ). 
         [0049]    In a specific embodiment of the invention, the signal ( 40 ) is an ultrasound signal. The signal ( 40 ) is directional and is emitted by the signal transducer ( 12 ) in shape of a narrow cone or arc—i.e. with the width of the signal band increasing as the distance from the signal transducer ( 12 ) increases. Hence, the precision of alignment between the launching device ( 10 ) and the target device ( 20 ) depends not only upon signal detection but also upon the distance between the two devices, as at greater distances the signal bandwidth is also greater. This level of error is referred to as ‘positional uncertainty’. It will be appreciated that a certain level of tolerance exists for positional uncertainty, however, if therapy is to be directed with precision the amount of uncertainty should be minimised. For example, if the diameter (d) of the signal transducer ( 12 ) is 1 mm and the frequency of the ultrasound signal is 30 MHz, then the positional uncertainty (x)—i.e. the margin of error on either side of a centre line—will be 1 mm at a perpendicular separation of 5 mm between the launching and target devices ( 10 ,  20 ). For clinical applications of the invention, it is preferred that the positional uncertainty does not exceed around +/−5 mm (that is a total signal bandwidth of 10 mm at the point reception). More preferably, the positional uncertainty should be between around +/−0.01 mm and around +/−4.50 mm. Even more preferably, the positional uncertainty should be between around +/−0.1 mm and around ±2 mm. Ideally, the positional uncertainty does not exceed around +/−1 mm. 
         [0050]    The strength of the signal ( 40 ) will also be a determining factor and it will be appreciated that signal strength will diminish significantly as the distance between the launching device ( 10 ) and the target device ( 20 ) increases. This distance is in part determined by the amount of intervening tissue ( 34 ) between the two devices. By way of example, if the signal ( 40 ) is an ultrasound signal, it can be expected that significant deterioration of signal will occur where the launching device ( 10 ) and the target device ( 20 ) a separated by more than around  20  mm of solid tissue. Obviously, the density of the intervening tissue ( 34 ) will also have an effect upon the deterioration of signal ( 40 ) over distance. 
         [0051]    The frequency of the desired ultrasound signal also determines the thickness of the signal transducer, which for a standard ultrasound ceramic transducer—such as a PZT—will be 0.075 mm at 30 MHz. 
         [0052]      FIG. 2  shows a cross sectional view of the arrangement in  FIG. 1 , along the line BB. The correct orientation of the launching device relative to the target device is an important factor as it is this line of orientation ( 41 ) that determines where the therapy is to be applied. It will be understood by the skilled addressee that the clinical need for precisional placing of therapy in a patient necessitates a requirement for a directional signal ( 40 ) that is linked to the means for delivering therapy. In this way, the user of the apparatus of the invention can administer therapy to the correct location by ensuring that the launching device ( 10 ) and the target device ( 20 ) are correctly positioned via transmission and reception of the signal ( 40 ). Hence, the orientation line ( 41 ) denotes not only the direction of signal travel but also the path along which therapy can be administered to the patient. 
         [0053]    An embodiment of the invention is shown in  FIG. 3  in which the signal transducer ( 120 ) is oriented at an oblique angle relative to the longitudinal axis of the launching device ( 10 ). Hence, the signal ( 40 ) is transmitted at an angle that is in the direction of forward travel of the launching device ( 10 ) as it enters a body cavity ( 30 ). The preferred signal beam angle is between around 20° and around 60° to the perpendicular, more preferably between around 30° and around 50° to the perpendicular, and most preferably around 45° to the perpendicular, when 0° corresponds to the longitudinal axis of the launching device in the direction of travel. 
         [0054]    The launching device ( 10 ) in  FIG. 2 , also shows an embodiment of the invention in which one means for administering therapy is provided. Launching device ( 10 ) comprises a hollow needle or cannula ( 17 ). The hollow needle ( 17 ) is located in an undeployed or retracted state within the lumen ( 13 ) of launching device ( 10 ). The hollow needle ( 17 ) may be deployed/extended from the launching device ( 10 ) at a time deemed appropriate by the user of the apparatus, via an aperture ( 16 ) in the outer sheath ( 11 ), The aperture ( 16 ), thus, can allow communication between the lumen ( 13 ) and the body cavity ( 30 ). It should be noted that the hollow needle ( 17 ) preferably travels along a path that is parallel to the direction of the signal ( 40 ) and is used to pierce the intervening tissue ( 34 ). In a preferred embodiment of the invention, the hollow needle makes a transit across the entirety of the intervening tissue ( 34 ) and in doing so allows the launching device ( 10 ) to access the second body cavity ( 32 ). If desired, the pathway made by the hollow needle ( 17 ) through the intervening tissue ( 34 ) can be subsequently widened to allow fluid communication between the first body cavity ( 30 ) and the second body cavity ( 32 ). 
         [0055]    Therapeutic means suitable for use in the invention can comprise devices or instruments selected from the group consisting of a cannula; a laser; a radiation-emitting device; a probe; a drill; a blade; a wire; a needle and appropriate combinations thereof. 
         [0056]    In a specific embodiment of the invention, the hollow needle ( 17 ) comprises a sensor ( 19 ) so as to assist further in determining positional information of the tip of the hollow needle relative to the launching device. In another specific embodiment of the invention the sensor ( 19 ) is capable of detecting changes in hydrostatic pressure. Other sensors that are suitable for use in the apparatus and methods of the invention can include temperature sensors, oxygenation sensors and/or colour sensors 
         [0057]    Optionally, the hollow needle can further comprise an additional signal transducer ( 122 ). In the embodiment shown in  FIG. 3  the signal transducer ( 122 ) is located near the tip of the hollow needle on the end of a guide wire ( 14 ). However, the signal transducer ( 122 ) can easily be located on the hollow needle if this is preferred. In use, the signal transducer ( 122 ) is driven with a short transmit pulse which produces a non-directional signal pulse. The signal pulse can be detected by the receiving transducer ( 23 ) mounted on the target device ( 20 ). From the time delay between the transmit pulse to the receipt of the signal pulse on the receiving transducer ( 23 ) the distance from the incoming guide wire ( 14 ) or hollow needle ( 17 ) to the receiving transducer ( 23 ) and hence the target device ( 20 ), can be determined. 
         [0058]    As mentioned above, the target device ( 20 ) comprises a receiving transducer ( 22 ) for receiving the signal ( 40 ). The receiving transducer ( 22 ) can be unidirectional—i.e. capable of receiving a signal from one direction only—or omnidirectional—i.e. capable of receiving a signal from any direction. In the embodiment of the invention shown in  FIG. 4 , a target device ( 20 ) is located within a body cavity ( 32 ). The target device ( 20 ) comprises an omnidirectional ultrasound signal receiving transducer. A reflecting cone ( 601 ) directs the signal ( 40 ) onto a disc-shaped receiving transducer ( 60 ). An acoustically transparent window ( 602 ) separates the reflecting cone ( 601 ) from the receiving transducer ( 60 ). In an alternative embodiment, an omnidirectional ultrasound signal receiving transducer can be obtained by locating cylinder of a flexible piezoelectric material such as PVDF (polyvinyldifluoride) around the outer sheath of the target device ( 20 ). In such a way the cylinder acts in an equivalent manner to the receiving transducer ( 60 ). 
         [0059]      FIG. 4  also shows an embodiment of the invention in which the target device ( 20 ) comprises a channel ( 25 ) for administering an agent, such as a therapeutic agent, to a patient. In a specific embodiment, the channel ( 25 ) functions as a conduit to allow application of a blocking material ( 251 ) that serves to obstruct or occlude the body cavity ( 32 ). The blocking material ( 251 ) can be suitably selected from a gel based substance. The placement of the blocking material ( 251 ) can be directed by movement of the target device ( 20 ). The presence of a guide member ( 24 ) within the lumen ( 23 ) of the target device ( 20 ) allows the user of the apparatus to precisely manipulate the position of the target device ( 20 ) as required. Alternative blocking materials ( 251 ) can include embolisation members (such as balloons) and self-expanding stents, for example. 
         [0060]    The launching device ( 10 ) comprises a signal transducer ( 12 ) that is optionally oriented so that the signal ( 40 ) is transmitted at an angle as shown in  FIG. 2 . In an alternative embodiment of the invention, shown in  FIG. 5 , the signal transducer is in the form of a signal transducer array ( 123 ). The signal transducer array ( 123 ) comprises a plurality of signal transducer elements ( 124 ) which can be oriented collectively and thereby define the signal beam width and angle relative to the launching device ( 10 ). A further advantage of the embodiment shown in  FIG. 5 , is that the smaller size of the elements ( 124 ) means that the signal transducer does not occupy a significant proportion the lumen ( 13 ) of the launching device ( 10 ). 
         [0061]    The embodiment in  FIG. 5  is particularly suited to ultrasound beam-forming signalling.  FIG. 5  shows an array of signal transducer elements ( 124 ) that are separately connected to the transmitter ( 50 ) via delays ( 51 ) so that the signals to each element are delayed relative to each other. The delays ensure that the ultrasound wavefronts from each element are aligned to produce a beam of ultrasound ( 40 ) at the requisite angle. In an alternative embodiment where the signal ( 40 ) is in the form of visible light, an array of LEDs can be used. 
         [0062]    To assist in the process of alignment between the launching device ( 10 ) in the first body cavity ( 30 ) and the target device ( 20 ) in the second body cavity ( 32 ), a further embodiment of the invention provides for the devices to comprise means for centring the respective devices within the body cavities. In one embodiment the centring means comprises an inflatable bladder or balloon ( 111 ) which is located in the lumen ( 13 ,  23 ) when in an undeployed state and, when the device ( 10 ,  20 ) reaches the desired location within the patient, can be inflated. The balloon ( 111 ) can be annular in shape such that is surrounds the device ( 10 ,  20 ) in a doughnut-like fashion. The balloon ( 111 ) can also be arranged such that it inflates on only one or on two opposite sides of the device. In  FIG. 6 , an embodiment of the invention is shown where the balloon ( 111 ) is shown deploying on one side of the launching device ( 10 ). 
         [0063]    Alternatively, in a further embodiment, the centring means is comprised of one or more loop structures ( 211 ). In this embodiment, the one or more loop structures ( 211 ) are located either in the lumen ( 13 ,  23 ) or within recesses made in the outer sheath ( 11 ,  21 ) when in an undeployed or retracted state. Hence, when the device ( 10 ,  20 ) reaches the desired location within the patient, the one or more loop structures ( 211 ) can be expanded outwardly from the device ( 10 ,  20 ), thereby, centring the device ( 10 ,  20 ) within the body cavity ( 30 ,  32 ). Outward expansion of the loop structures ( 211 ) can be suitably effected by compression of a length of wire, for example, such that it bows outwardly from the outer sheath ( 11 ,  21 ). A centring device that adopts this conformation typically comprises a plurality of compressible lengths of wire, or other suitable flexible material, arranged in parallel at radially spaced intervals around the periphery of the outer sheath ( 11 ,  21 ). Compression of the plurality of wires can be induced by way of a sliding member (not shown) located proximally and/or distally near to the ends of the plurality of wires. The sliding member is capable of translational movement along the longitudinal axis of the device ( 10 ,  20 ). 
         [0064]    In  FIG. 6 , an embodiment of the invention is shown where the target device ( 20 ) comprises fully deployed centring means ( 211 ) that has allowed the target device ( 20 ) to be centred within the body cavity ( 32 ). Arrangements for centring the devices within the body cavities include, but are not limited to, expandable Chinese-lantern type devices, reversibly expandable stents, coils, helices and retractable probes or legs. 
         [0065]    The invention is further illustrated by the following non-limiting example. 
       EXAMPLE 
       [0066]    The methods and apparatus of the present invention demonstrate particular utility in cardio-vascular surgery. In the present example the apparatus of the invention is used by a clinician to perform the procedure of arterial-venous stenosis (PICVA) so as to enable retroperfusion of cardiac tissue following occlusion of a coronary artery. 
         [0067]    The launching catheter ( 10 ) is inserted into the occluded coronary artery by standard keyhole surgical techniques. Likewise, the target catheter ( 20 ) is inserted into the coronary vein that runs parallel to the coronary artery. The coronary vein is not occluded and, therefore, provides an alternative channel for blood flow to the cardiac muscle effectively allowing the occlusion in the coronary artery to be bypassed. 
         [0068]    The launching catheter ( 10 ) comprises a PZT ultrasound transducer ( 12 ) (CTS Piezoelectric Products, Albuquerque, N. Mex.) that is oriented such that a directional ultrasound beam is transmitted at a 45° angle (relative to the longitudinal axis of the launching device) in the direction of blood flow in the artery. The ultrasound transducer ( 12 ) is activated and a 30 MHz directional ultrasound signal ( 40 ) is transmitted from the launching catheter ( 10 ). The target catheter ( 20 ) comprises an omnidirectional ultrasound receiving transducer ( 60 ). To assist with localisation of both the launching and target catheters ( 10 ,  20 ), both catheters comprise centring means in the form of an annular inflatable balloon ( 111 ). The centring means on the launching catheter ( 10 ) is deployed by the clinician when the launching catheter ( 10 ) is deemed to be in an appropriate location close to the site of the occlusion within the coronary artery. This is typically determined via standard fluoroscopic imaging techniques. The target catheter ( 20 ) is then moved within the adjacent coronary vein until the directed ultrasound signal ( 40 ) is detected by the signal receiving transducer ( 60 ). To enable more precise alignment between the launching and target catheters ( 10 ,  20 ) the centring means ( 111 ) on the target catheter ( 20 ) can be deployed either before or after the signal ( 40 ) is detected. 
         [0069]    On reception of the transmitted signal ( 40 ) the clinician can be certain that the launching and target catheters ( 10 ,  20 ) are correctly located within their respective blood vessels to allow for the arterial-venous stenosis procedure to commence. The target catheter ( 20 ) is used to block blood flow within the coronary vein via administration of a gel blocking material ( 251 ) though a channel ( 25 ) in the target catheter ( 10 ). The blocking material ( 251 ) is administered at a position downstream in terms of the venous blood flow relative to the location of the receiving signal transducer ( 60 ). 
         [0070]    The clinician is then able to initiate arterial-venous stenosis by deploying a hollow needle ( 17 ) from the launching catheter ( 10 ) substantially along a path that is parallel and close to that taken by the ultrasound signal ( 40 ) though the intervening tissue ( 34 ) between the coronary artery and the coronary vein. The hollow needle ( 17 ) comprises a sensor means ( 19 ) near its tip that detects changes in hydrostatic pressure. Hence, the clinician is able to monitor the transition from arterial pressure to venous pressure as the hollow needle passes between the two vessels. The hollow needle ( 17 ) further comprises a guide member ( 14 ) in the form of a wire located in the bore of the needle. Once the hollow needle has been passed across the intervening tissue ( 34 ) it is retracted leaving the guide wire ( 14 ) in place. Alternatively, once the hollow needle ( 17 ) has made the transition across the intervening tissue ( 34 ) the clinician is able to pass the guide wire ( 14 ) through the bore of the needle and then retract the needle ( 17 ) into the launching catheter ( 10 ). 
         [0071]    The clinician withdraws the launching catheter ( 10 ) from the patient leaving the guide wire ( 14 ) in place. A further catheter device is then slid along the guide wire ( 14 ) and an expandable stent ( 26 ) is deployed in order to widen the perforation in the intervening tissue ( 34 ) between the coronary artery and vein (see  FIG. 7 ). The target catheter ( 20 ) is withdrawn from the patient leaving the blocking material ( 251 ) in position. Optionally, a further block or suture may be inserted into the coronary vein prevent reversal of arterial blood flow. 
         [0072]    Hence, arterial blood is thereby diverted into the venous system and is enabled to retroperfuse the cardiac muscle tissue. 
         [0073]    Whilst the specific example described above is restricted to the field of cardio-vascular surgery, it is envisaged that the present method and apparatus could have far reaching applications in other forms of surgery. For example, any surgery involving the need to direct therapy from one body cavity towards another adjacent body cavity could be considered. Hence, the present invention finds ready applications in the fields of neurosurgery, urology and general vascular surgery. In addition the type of therapy need not be restricted to formation of channels between body cavities. For instance, the apparatus and methods described herein are also of use in directing techniques such as catheter ablation, non-contact mapping of heart chambers and the delivery of medicaments to precise areas of the body. 
         [0074]    Although particular embodiments of the invention have been disclosed herein in detail, this has been done by way of example and for the purposes of illustration only. The aforementioned embodiments are not intended to be limiting with respect to the scope of the appended claims, which follow. It is contemplated by the inventors that various substitutions, alterations, and modifications may be made to the invention without departing from the spirit and scope of the invention as defined by the claims. 
       NUMERALS USED IN THE FIGURES 
       [0075]      
         [0000]    
       
         
               
               
             
           
               
                   
               
             
             
               
                 10 
                 Launching device 
               
               
                 11 
                 Outer sheath 
               
               
                 111 
                 Centring device 
               
               
                 12 
                 Signal transducer 
               
               
                 120 
                 Angled signal transducer 
               
               
                 122 
                 Needle mounted signal transducer 
               
               
                 123 
                 Signal transducer array 
               
               
                 124 
                 Signal transducer element 
               
               
                 13 
                 Lumen 
               
               
                 14 
                 Guide means 
               
               
                 16 
                 Aperture 
               
               
                 17 
                 Hollow needle 
               
               
                 19 
                 Pressure sensor 
               
               
                 20 
                 Target device 
               
               
                 21 
                 Outer sheath 
               
               
                 211 
                 Centring device 
               
               
                 22 
                 Receiving transducer 
               
               
                 23 
                 Lumen 
               
               
                 24 
                 Guide member 
               
               
                 25 
                 Channel 
               
               
                 251 
                 Blocking material 
               
               
                 26 
                 Stent 
               
               
                 30 
                 First body cavity 
               
               
                 32 
                 Second body cavity 
               
               
                 34 
                 Intervening tissue 
               
               
                 40 
                 Signal 
               
               
                 41 
                 Orientation direction 
               
               
                 50 
                 Signal transmitter 
               
               
                 51 
                 Transmitter delay 
               
               
                 60 
                 Signal detector 
               
               
                 601 
                 Reflecting cone 
               
               
                 602 
                 window 
               
               
                 61 
                 Output display