Abstract:
Medical devices or components thereof, and particularly intracorporeal devices for therapeutic or diagnostic uses, which are formed at least in part of a polymeric material and a ferromagnetic or paramagnetic material, so that the medical device or component thereof is visible on magnetic resonance imaging (MRI) scans. In one embodiment, the medical device is a balloon catheter having an MRI visible balloon. In a presently preferred embodiment, there is an insufficient amount of the ferromagnetic or paramagnetic material within a wall of the balloon or coated onto a wall of the balloon to make the balloon radiopaque.

Description:
BACKGROUND OF THE INVENTION 
   This invention generally relates to medical devices, and particularly to intracorporeal devices for therapeutic or diagnostic uses such as balloon catheters, and vascular grafts. 
   In percutaneous transluminal coronary angioplasty (PTCA) procedures, a guiding catheter is advanced until the distal tip of the guiding catheter is seated in the ostium of a desired coronary artery. A guidewire, positioned within an inner lumen of a dilatation catheter, is first advanced out of the distal end of the guiding catheter into the patient&#39;s coronary artery until the distal end of the guidewire crosses a lesion to be dilated. Then the dilatation catheter having an inflatable balloon on the distal portion thereof is advanced into the patient&#39;s coronary anatomy, over the previously introduced guidewire, until the balloon of the dilatation catheter is properly positioned across the lesion. Once properly positioned, the dilatation balloon is inflated with fluid one or more times to a predetermined size at relatively high pressures (e.g. greater than 8 atmospheres) so that the stenosis is compressed against the arterial wall and the wall expanded to open up the passageway. Generally, the inflated diameter of the balloon is approximately the same diameter as the native diameter of the body lumen being dilated so as to complete the dilatation but not overexpand the artery wall. Substantial, uncontrolled expansion of the balloon against the vessel wall can cause trauma to the vessel wall. After the balloon is finally deflated, blood flow resumes through the dilated artery and the dilatation catheter can be removed therefrom. 
   In such angioplasty procedures, there may be restenosis of the artery, i.e. reformation of the arterial blockage, which necessitates either another angioplasty procedure, or some other method of repairing or strengthening the dilated area. To reduce the restenosis rate and to strengthen the dilated area, physicians frequently implant a stent inside the artery at the site of the lesion. Stents may also be used to repair vessels having an intimal flap or dissection or to generally strengthen a weakened section of a vessel. Stents are usually delivered to a desired location within a coronary artery in a contracted condition on a balloon of a catheter which is similar in many respects to a balloon angioplasty catheter, and expanded to a larger diameter by expansion of the balloon. The balloon is deflated to remove the catheter and the stent left in place within the artery at the site of the dilated lesion. Stent covers on an inner or an outer surface of the stent have been used in, for example, the treatment of pseudo-aneurysms and perforated arteries, and to prevent prolapse of plaque. Similarly, vascular grafts comprising cylindrical tubes made from tissue or synthetic materials such as polyester, expanded polytetrafluoroethylene, and DACRON may be implanted in vessels to strengthen or repair the vessel, or used in an anastomosis procedure to connect vessels segments together. 
   To facilitate placement of the catheter at the desired location in the patient&#39;s vasculature, X-ray opaque (i.e., radiopaque) material is generally provided on conventional angioplasty catheters so that the physician can view the catheter under fluoroscopy. The radiopaque material is typically a metal marker band on the catheter shaft. For example, two marker bands on the inner tubular member of the shaft are typically provided, to indicate the proximal end and distal end of the working length of the balloon. Blending radiopaque material into the polymer matrix of the catheter components has been suggested as an alternative to radiopaque marker bands on the catheter shaft. Additionally, catheters visible to magnetic resonance imaging (MRI), also known as nuclear magnetic resonance (NMR) imaging systems have been suggested for use during MRI scans of a patient. MRI scans are used provide two-dimensional sectional images of a patient&#39;s internal body structures without exposing the patient to harmful radiation. 
   It would be a significant advance to provide catheter balloon or other medical device or component thereof with improved visibility within the patient. 
   SUMMARY OF THE INVENTION 
   This invention is directed to medical devices or components thereof, and particularly intracorporeal devices for therapeutic or diagnostic uses, which are formed at least in part of a polymeric material and a ferromagnetic or paramagnetic material, so that the medical device or component thereof is visible on magnetic resonance imaging (MRI) scans. In one embodiment, the medical device is a balloon catheter having an MRI visible balloon. While discussed below primarily in terms of a catheter balloon, it should be understood that the invention includes additional MRI visible medical devices or components thereof, and particularly expandable or inflatable members. 
   In a presently preferred embodiment, the MRI visible material is a ferromagnetic material, and a presently preferred ferromagnetic material is iron oxide, due to the ease of compounding iron oxide in the polymeric balloon material, the relative ease of dispersing in many types of balloon materials, and the lack of a magnetic field orientation effect in which the MRI image varies depending on the orientation of the medical device or component thereof. Preferably, a background is provided by a contrast solution within and/or around the balloon, such as the MRI visible bright/white background of a Gadolinium solution, to facilitate viewing the ferromagnetic containing balloon. A balloon with iron oxide present in the balloon wall in a concentration of about 5% is readily visible as a dark image in a bright background of a 1:10 Gadolinium contrast solution. A variety of suitable ferromagnetic materials can be used including iron, nickel and cobalt, and compounds thereof such as iron oxide, typically in the form of a fine powder. In one embodiment, the preferred MRI visible materials have hydrating power (i.e., they are present in a hydrated state), which facilitates MRI visibility. For example, in one embodiment, a proton donating fluid at the device or component is not required in order to produce the MRI image. In an alternative embodiment, the MRI visible material may be a paramagnetic material, preferably provided that magnetic field orientation effects are minimal or nonexistent. In one embodiment, the paramagnetic material is selected from the group consisting of dysprosium, gadolinium, chromium, copper, manganese, and vanadium, and compounds thereof such as dysprosium oxide. 
   The MRI visible materials suitable for use in the invention may be radiopaque in addition to being MRI visible. However, in a presently preferred embodiment, there is an insufficient amount of the ferromagnetic or paramagnetic material in or on the balloon wall to make the balloon radiopaque. The ferromagnetic or paramagnetic material does not have a disadvantageous effect on the strength and compliance of the balloon, unlike prior art catheter balloons in which it was proposed to make the balloon radiopaque by forming the balloon of a blend of polymeric and radiopaque materials. Specifically, such prior art catheter balloons would require a relatively large amount of radiopaque material to make the balloon radiopaque, which would consequently reduce the strength and effect the compliance of the balloon. Thus, the balloon of the invention, unlike prior art balloons, has an amount of ferromagnetic or paramagnetic material which is sufficient to make the balloon MRI visible but insufficient to make the balloon radiopaque, and does not have a radiopaque material (either the ferromagnetic or paramagnetic material, or a separate radiopaque material) in sufficient amounts to make the balloon radiopaque. Consequently, the balloon is MRI visible and is not radiopaque in use, and the balloon has excellent performance characteristics such as a relatively high rupture pressure. 
   The MRI visible ferromagnetic or paramagnetic material is a solid, typically with a particle size of about 0.01 to about 50 μm. In one embodiment of the invention, the ferromagnetic or paramagnetic material is dispersed in the polymeric material, preferably by compounding the polymeric material with the ferromagnetic or paramagnetic material. The term compounding should be understood to refer to a process in which a high concentration of ingredient(s) is mixed with a specific polymer to form a master batch. This master batch is then added to resin during extrusion to obtain a uniform dispersion of a desired concentration. As a result, the ferromagnetic material is typically located uniformly throughout the polymeric wall of the balloon as discrete particles of material. Alternatively, in another embodiment, the ferromagnetic or paramagnetic material is a coating on a surface of the polymeric wall of the balloon. The balloon may be a single layered balloon, or alternatively, may comprise multiple layers, at least one of which has the ferromagnetic or paramagnetic material therein or thereon. The multiple layers are preferably coextruded, although they may alternatively be separately extruded and then placed together. In one embodiment, the balloon has a first polymeric layer, and a second polymeric layer coextruded with the first layer and containing the ferromagnetic or paramagnetic material therein. In one embodiment, the second layer is an inner layer, to minimize any microbiological or other effects of the ferromagnetic or paramagnetic material on the patient. 
   The balloon may be detectable when viewed by magnetic resonance imaging as a dark image or alternatively as a bright image, depending on the nature of the MRI visible material forming the balloon. In a presently preferred embodiment, the MRI visible layer extends the entire length of the balloon. However, the ferromagnetic or paramagnetic material may be present in sections of the balloon covering less than the entire area of the balloon, including sections spaced apart along the length or around the circumference of the balloon. 
   In a presently preferred embodiment, the MRI visible medical device or component thereof is configured to be expandable or inflatable. It is particularly important to avoid disadvantageous effects on the strength of expandable or inflatable members, such as catheter balloons and vascular grafts, because the members are required to not tear or burst during expansion thereof. Thus, the relatively low loading of ferromagnetic or paramagnetic material, in accordance with the invention, is particularly advantageous in expandable or inflatable members. 
   The medical device or component thereof may comprise a variety of devices, including a vascular graft, a stent cover, and an intravascular catheter component, for a variety of clinical applications including coronary, peripheral, and neurological applications. Stent covers and vascular grafts of the invention generally comprise a tubular body formed at least in part of polymeric material and the ferromagnetic or paramagnetic material. The terminology vascular graft as used herein should be understood to include grafts and endoluminal prostheses which are surgically attached to vessels in procedures such as vascular bypass or anastomosis, or which are implanted within vessels, as for example in aneurysm repair or at the site of a balloon angioplasty or stent deployment. A balloon catheter of the invention, such as an angioplasty dilatation catheter or a stent delivery catheter, generally comprises an elongated shaft with at least one lumen and balloon on a distal shaft section with an interior in fluid communication with the at least one lumen. A wall of the catheter balloon, or a separate sheath member on an outer surface of the balloon, may be MRI visible in accordance with the invention. 
   The balloon, or other medical device or component thereof, of the invention has improved MRI visibility due to the ferromagnetic or paramagnetic material, without a disadvantageous effect on strength or compliance of the balloon. Additionally, radiopaque marker bands are not required on the balloon catheter of the invention for visualization of the balloon location in the patient. As a result, the distal section of the balloon catheter is more flexible and has a smaller profile for improved tracking compared to conventional balloon catheters. These and other advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying exemplary drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an elevational view, partially in section, of a stent delivery balloon catheter having a covered stent on the catheter balloon, which embodies features of the invention. 
       FIG. 2  is a transverse cross-section of the catheter shown in  FIG. 1  taken at line  2 — 2 . 
       FIG. 3  is a transverse cross-section of the catheter shown in  FIG. 1  taken at line  3 — 3 , showing the covered stent disposed over the inflatable balloon. 
       FIG. 4  is an elevational view, partially in section, of a vascular graft or stent cover which embodies features of the invention. 
       FIG. 5  is a transverse cross-section of the graft or cover shown in  FIG. 4 , taken along lines  5 — 5 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3  illustrate an over-the-wire type stent delivery balloon catheter  10  embodying features of the invention. Catheter  10  generally comprises an elongated catheter shaft  12  having an outer tubular member  14  and an inner tubular member  16 . Inner tubular member  16  defines a guidewire lumen  18  adapted to slidingly receive a guidewire  20 , and the coaxial relationship between outer tubular member  14  and inner tubular member  16  defines annular inflation lumen  22  (see  FIGS. 2 and 3 , illustrating transverse cross sections of the catheter  10  of  FIG. 1 , taken along lines  2 — 2  and  3 — 3  respectively). An inflatable balloon  24  is disposed on a distal section of catheter shaft  12 . Balloon  24  has a proximal shaft section sealingly secured to the distal end of outer tubular member  14  and a distal shaft section sealingly secured to the distal end of inner tubular member  16 , so that its interior is in fluid communication with inflation lumen  22 . An adapter  26  at the proximal end of catheter shaft  12  is configured to provide access to guidewire lumen  18 , and to direct inflation fluid through arm  28  into inflation lumen  22 . Balloon  24  has an inflatable working length located between tapered sections of the balloon. An expandable stent  30  is mounted on the balloon working length, with a stent cover  40  on an outer surface of the stent  30 .  FIG. 1  illustrates the balloon  24  in an uninflated configuration prior to deployment of the stent  30 . The distal end of the catheter may be advanced in a conventional manner to a desired region of a patient&#39;s vessel  32  defining a body lumen, and balloon  24  inflated to expand stent  30 , thereby implanting the stent in the body lumen. 
   The balloon  24  is formed of a polymeric material and an amount of ferromagnetic or paramagnetic material which is sufficient to make the balloon MRI visible and insufficient to make the balloon radiopaque within the patient. The ferromagnetic or paramagnetic material is preferably dispersed in the polymeric material forming the balloon wall, and in a presently preferred embodiment, the dispersed material is a ferromagnetic material. In the embodiment illustrated in  FIG. 1 , the balloon comprises an outer layer  33  and an inner layer  34 , at least one of which is formed of the polymeric material/ferromagnetic or paramagnetic dispersion. The outer and inner layers  33 / 34  may be formed of the same polymeric material or different polymeric materials. A variety of suitable polymeric materials may be used to form the balloon, conventional in medical device balloon construction, including polyamides such as nylon 11 or nylon 12, copolyamides such as polyether block amide (PEBAX), copolyesters such as HYTREL or ARNITEL. 
   In a presently preferred embodiment, the amount of ferromagnetic or paramagnetic material is about 1% to about 30%, preferably about 5% to about 20%, by weight of the polymeric material/ferromagnetic or paramagnetic material dispersion, depending on the magnetic field strength, gradient field strength, and pulse sequences of the MRI system being used, as well as the clinical application of the catheter. The preferred percentages are for a multilayered balloon with a first layer formed of the MRI visible material dispersed in a polymer (i.e., the MRI visible layer), and a second layer free of the ferromagnetic or paramagnetic material. In an alternative embodiment in which the balloon is a single layered balloon (not shown) formed of the MRI visible material dispersed in a polymer, the concentration of ferromagnetic or paramagnetic material is typically lower, as for example about 50% lower than the above values for a single layered balloon having a wall thickness about 50% greater than the wall thickness of the MRI visible layer of the multilayered balloon. Applied as a coating, the ferromagnetic or paramagnetic material is preferably about 10% to about 20% or more by weight of the balloon. 
   The balloon  24  has a rupture pressure of about 200 to about 390 psi, preferably about 270 to about 330 psi. The rupture pressure is preferably the same as the rupture pressure of a balloon otherwise identical to the balloon but without the ferromagnetic or paramagnetic material. 
   The balloon catheter  10  can be used, for example in a balloon angioplasty procedure or stent deployment to treat a stenosed region of the patient&#39;s vasculature. The catheter  10  is introduced into the vessel  32  defining the body lumen, and advanced therein. The balloon is visualized under MRI to position the balloon at the desired location in the body lumen. The balloon is then inflated by introduction of inflation fluid into the balloon interior via the inflation lumen. A contrast solution is typically introduced into the balloon which doubles as the inflation fluid, and around the balloon through the guiding catheter, to enhance visibility of the balloon. A presently preferred contrast solution for a ferromagnetic containing balloon is a paramagnetic containing contrast solution. Because the wall of the balloon can be visualized during inflation thereof, the balloon  24  can be inflated at the site of a lesion in the body lumen to determine information about the lesion as part of a MRI diagnostic procedure. Specifically, for example, the compliance of the lesion to the inflated balloon can be determine by observing the balloon inflate against the lesion. Following the procedure, the balloon is deflated, and the catheter repositioned or removed from the patient. 
   Co-extruded balloon tubing, formed of a 20 wt % dispersion of iron oxide in a PEBAX 72D or Nylon 12 polymeric material as the inner layer of the multilayered balloon with a PEBAX 72D outer layer, was blow molded to form a balloon. The iron oxide particles had a particle size of about 0.01 μm. The balloon had a dual wall thickness of about 40 μm, and a burst pressure of about 250 psi to about 300 psi. The balloon was inflated at an inflation pressure of about 116 psi to about 150 psi to an inflated diameter of 3 mm, and MRI images of the inflated balloon were obtained at a field strength of 1.5 Tesla. A 1% to 10% Gadolinium solution in water is preferably used as a contrast solution within and/or around the balloon to enhance the visibility of the iron oxide containing balloon. 
   To the extent not discussed herein, the various catheter components can be formed conventionally of materials commonly used in catheter construction. The balloon  24  is typically secured to the catheter shaft as is conventionally known by adhesive or fusion bonding. 
   The dimensions of catheter  10  are determined largely by the size of the guidewires to be employed and the size of the artery or other body lumen through which the catheter must pass or the size of the stent being delivered. The outer tubular member  14  typically has an inner diameter of about 0.015 to about 0.035 inch (0.038 to 0.089 cm), usually about 0.03 inch (0.076 cm). The inner tubular member  16  typically has an outer diameter of about 0.012 to about 0.016 inch (0.030 to 0.041 cm), usually about 0.014 inch (0.036 cm). The overall working length of the catheter  10  may range from about 100 to about 150 cm, and is typically about 135 cm. Preferably, balloon  24  has a length about 0.5 cm to about 6 cm and typically about 2 cm, and an inflated working diameter of about 1 to about 8 mm, typically about 3 mm. 
     FIGS. 4 and 5  illustrate another embodiment of the invention, in which the expandable MRI visible medical device is a vascular graft  50 . The vascular graft  50  generally comprises a tubular body  51  formed at least in part of a polymeric material and a ferromagnetic or paramagnetic material in accordance with the invention, having a lumen  52  therein and ports  53 ,  54  at either end of the graft  50 . The graft  50  is configured for being implanted in the patient, and it may be expanded into place within a vessel, or surgically attached to a vessel such as to a free end or a side wall of a vessel. The graft  50  length is generally about 4 to about 80 mm, and more specifically about 10 to about 50 mm, depending on the application, and single wall thickness is typically about 40 μm to about 2000 μm, preferably about 100 μm to about 1000 μm. The diameter is generally about 1 to about 35 mm, preferably about 3 to about 12 mm, depending on the application. Stent cover  40  is similar to vascular graft  50 , except it is on a stent as illustrated in FIG.  1 . 
   While the present invention is described herein in terms of certain preferred embodiments, those skilled in the art will recognize that various modifications and improvements may be made to the invention without departing from the scope thereof. For example, in the embodiment illustrated in  FIG. 1 , the catheter is over-the-wire stent delivery catheter. However, one of skill in the art will readily recognize that other types of intravascular catheters may be used, such as rapid exchange balloon catheters having a distal guidewire port and a proximal guidewire port and a short guidewire lumen extending between the proximal and distal guidewire ports in a distal section of the catheter. Additionally, although the balloon catheter illustrated in  FIG. 1  is a stent deploying catheter, a variety of balloon catheters may be used including dilatation balloon catheters. Moreover, although individual features of one embodiment of the invention may be discussed herein or shown in the drawings of the one embodiment and not in other embodiments, it should be apparent that individual features of one embodiment may be combined with one or more features of another embodiment or features from a plurality of embodiments.