Patent Publication Number: US-6907106-B1

Title: Method and apparatus for producing radioactive materials for medical treatment using x-rays produced by an electron accelerator

Description:
Priority is claimed from U.S. Provisional Application Ser. No. 60/097,564, filed Aug. 24, 1998, entitled “Method and Apparatus for Producing Radioactive Materials for Medical Treatment Using X-rays Produced by an Electron Accelerator.” 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method and apparatus for imparting radioactive properties to target objects, such as implantable medical devices, by exposure of materials to radiation produced by an electron accelerator. 
     BACKGROUND OF THE INVENTION 
     In medical practice, a variety of apparatus and techniques have been developed for treating stenotic sites within body lumens. A complication of the known treatments is a condition known as restinosis (i.e., re-narrowing) of the stenotic region following treatment. This condition can be alleviated to some degree by the use of drugs and or by implantable medical devices, namely stents. 
     Stents come in a variety of shapes and sizes. Generally speaking, stents provide a structure having an opening, such as a generally hollow open cylinder. Some stents provide relatively thin walls made of metal or other suitable material for in vivo implantation, the walls defining through hole, such as for the flow through of a fluid such as blood or other body fluid. Typical vascular or coronary stents are constructed of an open mesh or lattice structure and are designed to be expandable following placement within a patient&#39;s body lumen, such as an artery, to facilitate increased blood flow at the diseased location. Even with a stent in place, restinosis has been known to occur at treated sites, such as due to the occurrence of excessive tissue growth. 
     It is also known that if the material comprising the stent is pre-processed so that it can provide a therapeutic treatment to the arterial wall that it is in contact with, then the probability of a reoccurrence of stenosis at the location may be reduced. This desired effect has been achieved through the introduction of certain drugs or by the emission of ionizing radiation, by the stent, or by a combination of these agents. 
     Various techniques are known for irradiating stents, such as those described in U.S. Pat. No. 5,059,166 and U.S. Pat. No. 5,213,561. Examples of the known techniques include having a spring coil stent irradiated so that it becomes radioactive, alloying a stent spring wire with a radioactive element, such as phosphorous  32 , forming a stent coil from a radioisotope core material which is formed within an outer covering, and plating a radioisotope coating (such as gold  198 ) onto a stent. 
     One disadvantage of the known manufacturing techniques is the transport time between the site of manufacture and the site of use. Because of the need for transporting stents off-site using these known techniques, at least some of the radioactive dose imparted during the manufacturing process can be lost, especially since it is desirable to use radioactive materials having relatively short half lives. In the known techniques for irradiating stent materials, it is often required to use a reactor or high power charged particle accelerator, which are not understood generally to be readily available and which may not be conveniently located to the site of medical use. In order to compensate for the undesirable transport times and distances using the known techniques, users may need to resort to materials having longer half lives, or to imparting greater radioactive doses to the stent material during manufacture, in order to compensate for the delays between manufacture and use such as in hospitals. This leads to increased inefficiency and cost. 
     From the above, it is apparent that there is a need for systems to handle and transport medical devices so that they are exposed to x-rays of the appropriate energy level required to generate isotopes that are emitted from known and widely available compact industrial and medical high energy x-ray sources that may be located in hospitals at sites proximate to the points of use. 
     Relatively lower power, and more widely available and readily accessible industrial and medical linear accelerators are also known, such as the LINATRON® and the CLINAC® linear accelerators from Varian Associates, 3100 Hansen Way, Palo Alto, Calif. 94304. These linear accelerators have been used in industry for high-energy radiography or in hospitals for clinical radiation treatments. They may provide a directed beam of high energy x-rays at structures to be analyzed or at a diseased site for therapeutic purposes. It is known that these accelerators can generate an electron beam directed at an x-ray generating target, where the energy of the electrons in the beam is converted into x-ray flux. This phenomena is known as a bremstrahlung effect and is well known in atomic and high energy physics. An example of an x-ray generating target for use with the CLINAC® medical linear accelerator is described in commonly assigned U.S. Pat. No. 5,680,433. 
     It is therefore an object of the present invention to provide a more economical system for irradiating target objects for use in medical applications, such as stents, using compact and efficient x-ray sources and material handling systems. It is also an object of the present invention to provide a method of making radioactive stents which can be performed at distributed sites, such as within or close to hospitals or other facilities where they may be used. 
     It is another object of the present invention to provide an apparatus and method for efficient irradiation of materials using available medical linear accelerators or high energy x-ray radiographic accelerators. 
     It is a further object of the present invention to provide increased efficiency in irradiating materials. 
     It is another object of the present invention to provide an apparatus and method of making radioactive stents in a manner that could be done within the hospital or facility on an as-needed basis. 
     SUMMARY OF THE INVENTION 
     The present invention alleviates to a great extent the disadvantages of the known systems for manufacturing radioactive materials, such as stents for in vivo implantation, by providing a method and apparatus for irradiating target objects using x-rays alone. This description covers preferred apparatus and methods with which objects for use in medical treatment, such as stents, are processed to become radioactive, so as to be capable of emitting ionizing radiation having characteristics for effective therapy. In particular, an x-ray source is provided for generating high energy x-rays. The x-rays impinge upon and are received by a target object. The target object is either held stationary while being irradiated, or is translated by a translation assembly. 
     Various methods are described in further detail below by which stent devices may be efficiently activated using an accelerated beam of electrons to produce x-rays, which subsequently induce the gamma-neutron reaction in the stent material. The effectiveness of inducing radioactivity in the stent depends on several factors. For instance, the gamma-neutron reaction cross-section has a maximum between 15 and 20 MeV for most materials appropriate for use in this application. Thus, the accelerator used to produce the x-rays preferably produces electrons with energies adjustable to maximize the production of x-rays within this energy range. This preferably is in a range from approximately 20 MeV to 25 MeV. 
     In a preferred embodiment, a medical or industrial linear accelerator is used to generate a beam of high energy electrons. The beam impinges upon and is received by a primary x-ray conversion target, which generates an x-ray flux in a predominantly forward direction downstream of the electron beam source. One or more secondary target objects, such as pre-formed medical stents, are positioned downstream of the primary target, in a position to efficiently intercept the x-ray flux generated by the primary x-ray conversion target. 
     Other x-ray sources may be used as well, provided they produce x-rays of the appropriate energy level to generate radioisotopes. 
     The target objects may be stationary while being irradiated, or alternatively, may be translated in some fashion. If the target objects are held stationary, the radioactive dose imparted to them may be localized, depending on their orientation with respect to the x-ray flux. Alternatively, the electron beam and consequent x-ray flux produced by the primary target may be controlled to impart a distributed x-ray dose on the secondary target objects, which in turn results in a distributed and more uniform level of radioactivity in the target objects. 
     If the secondary target objects are translated during irradiation, the distribution of the x-ray dose may be controlled by controlling the movement of the target objects. For example, the target objects may be translated linearly to provide a longitudinal distribution of x-ray dose, and may also be rotated to impart a circumferentially distributed x-ray dose. The target objects also may be positioned on a rotating carousel, allowing a designated number of target objects to receive the bulk of the x-ray flux at any given time and also to promote cooling of the target objects by alternating target objects exposed to the x-ray flux at any given time. In another embodiment, the primary x-ray conversion target is incorporated in the secondary target object translation assembly. For example, the x-ray conversion target is formed within a rotating carousel, between an electron beam source and the target object. This embodiment also promotes cooling of the x-ray conversion target by alternating the area of the x-ray conversion target exposed to the electron beam at any given time. 
     The electron beam may be translated or shaped in any desired fashion onto the x-ray generating target. For example, multiple target objects may be irradiated by translating the electron beam or the x-rays relative to the target objects and to impinge upon and be received by one or more of the target objects at any one time. A feedback control system may also be provided in which the amount of x-ray radiation is monitored and the intensity, duration or other characteristics of the electron beam are controlled so as to control the amount of x-ray radiation applied to the target objects. 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings in which like reference characters refer to like parts throughout. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram of an exemplary apparatus in accordance with the present invention; 
         FIG. 2  is a diagram of an alternative exemplary apparatus in accordance with the present invention; 
         FIG. 3  is a diagram of an exemplary apparatus in accordance with the present invention including multiple translational devices and feedback control systems; 
         FIG. 4  is a cross section taken along line  4 — 4  of the exemplary apparatus illustrated in  FIG. 3 ; 
         FIG. 5  is a diagram of an exemplary apparatus in accordance with the present invention including an electron beam distribution apparatus; 
         FIG. 6  is a diagram of an alternative exemplary apparatus in accordance with the present invention; 
         FIG. 7  is a diagram of an exemplary apparatus in accordance with the present invention including a carousel assembly for positioning target objects; 
         FIG. 8  is a diagram of an alternative exemplary apparatus in accordance with the present invention including a translation assembly for positioning target objects; 
         FIG. 9  is a diagram of an exemplary apparatus in accordance with the present invention including a carousel translation assembly incorporating an x-ray conversion target; 
         FIG. 10  is a detailed view of an exemplary apparatus in accordance with the present invention; 
         FIG. 11  is a diagram of an exemplary apparatus in accordance with the present invention including a linear assembly incorporating an x-ray conversion target; 
         FIG. 12A  is an illustration of an x-ray conversion target and translation assembly in accordance with an embodiment of the present invention; 
         FIG. 12B  is a cross-sectional view of the apparatus illustrated in  FIG. 12A , taken along line B—B; 
         FIG. 13  is an illustration of a coil stent in accordance with the present invention; 
         FIG. 14  is an illustration of a mesh stent in accordance with the present invention; and 
         FIG. 15  is an illustration of a tubular stent in accordance with the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a radioactive object or a radioactive medical device such as a stent for in vivo implantation is produced. Referring to  FIG. 1 , an accelerated beam or stream of electrons  20 , such as provided by a high energy electron beam source  10  (for example, an electron linear accelerator), is used to generate high energy x-rays  40  by an x-ray conversion target  30 . These emitted x-rays  40  also will be characterized as an x-ray beam or x-ray flux. The emitted x-rays  40  operate to impart radioactive properties to the ultimate target object  50 . 
     Any ultimate target object  50  may be used. By way of illustration, metals and non-metals may be used, including stainless steel, aluminum, tungsten, tantalum, strontium, titanium, metal alloys, plated materials, multi-layer materials, composites, plastics, rubber and other polymers, and ceramic materials. In the preferred embodiment, the target object is a pre-formed medical device such as a stent. 
     As illustrated in  FIGS. 1-4 , an electron beam source  10  is used to generate and output a beam of electrons  20 . Any device capable of achieving adequate beam intensity and appropriate energy levels may be used to create the beam of electrons, although it is preferred that a medical or industrial linear accelerator is used, for example the CLINAC® linear accelerator or the LINATRON® radiographic accelerator from Varian Associates. The x-ray conversion target  30 , which includes an x-ray generating material or materials  32 , receives the electron beam  20 , such as by generally directing the electron beam  20  towards the x-ray conversion target  30 . The design optimization of an appropriate x-ray conversion target  30  is well known in the art (for example, in radiation therapy devices). The x-ray conversion target  30  may be mounted in a stationary fashion in relation to the electron beam source  10  or may be movable in the path of the electron beam  20 . The desired effect is to cause the electron beam  20  to impinge upon the x-ray conversion target  30 . In this system, the x-rays  40  are emitted in a dispersed field, with the field being the strongest in the general direction of travel of the electron beam  20 . 
     As is well known in the art, the x-ray generating material  32  in the x-ray conversion target  30  may be made of any material or group of materials suitable for emitting x-rays when receiving an electron beam  20  of a particular energy level. 
     In a preferred embodiment, the x-ray conversion target  30  includes plural layers, for example layers  32  and  34 , as illustrated in  FIGS. 1 and 3  or layers  32  and  38  as illustrated in  FIG. 2 , although other layer arrangements or a single layer x-ray conversion target  30  also may be used. These layers preferably are selected to optimize the x-ray production efficiency of the x-ray conversion target  30  and most effectively absorb the power of the incident electron beam  20 . 
     An electron absorption layer  34  optionally is included downstream of the x-ray generating material  32 , i.e., between the x-ray generating material  32  and the ultimate target object  50 . After passing through the x-ray generating material  32 , all, or a substantial portion of, the remaining electrons are absorbed in the absorption layer  34 . This absorption layer  34  may be constructed of any suitable material for absorbing the excess electrons. Preferably a relatively thick layer of a relatively low-atomic number material, for example copper or aluminum, is used. 
     Heat due to the electron power deposition in the conversion target  30  is conducted away using a cooling system  36 , well known in the art. 
     A metering circuit  39  optionally may be included to monitor the electron beam current incident upon the x-ray conversion target. Any apparatus suitable for measuring electric current may be used. The metering circuit  39  optionally may be electrically connected to a control circuit  120 ,  130 ,  140 ,  150  (shown in  FIG. 3 ) to control the electron beam output of electron beam source  10 . 
     In one embodiment, a transport apparatus  60  receives the material  50  being irradiated and positions it as desired to efficiently receive the emitted x-ray flux  40 . Any open or enclosed form of transport apparatus  60  may be used as long as it positions the target object  50  in the desired positions. For example, as illustrated in  FIGS. 1 and 2 , the transport apparatus  60  may include a filament  62  upon which the target object slides or is pushed, such as using push member  63 . Alternatively, the transport apparatus may include a slider or gripper mechanism  64  ( FIG. 2 ) or a conveyor belt  67  (FIG.  3 ). In another embodiment, the transport apparatus  60  includes a tube assembly  66   a ,  66   b  (FIGS.  3 - 5 ). The tube assembly includes at least one tube  66   a ,  66   b  receiving the target object  50 ,  52  within its interior portion, and a lateral and/or rotational positioning assembly  68  ( FIG. 3 ) moving the target object  50  (or objects) within the tube  66   a  or  66   b  in a desired location to situate the target object, or objects, within the tube to receive the x-rays  40 . Positioning assembly  68  may include any suitable apparatus so long as it can orient the target object  50  in a desired position within the tube assembly, for example, conveyor  67 , filament  62 , push member  63  or slider or gripper mechanism  64 . The tubes  66   a ,  66   b  define any suitable cross section, including a circle, oval, square or other polygonal shape. The target object can be rotated as indicated by arrow  75  or linearly translated, as indicated by arrow  70  using the positioning assembly  68  (FIG.  3 ). Other motions also can be achieved as desired. Alternatively, any of the tubes  66   a ,  66   b  may be angled so that the target object  50  moves using gravitational force. When it reaches the desired position the angle may reduced so as to hold the target object  50  in place or to move slowly. 
     Tubes  66   a ,  66   b  preferably have an appropriate thickness for maximizing the x-ray intensity flux in the target, for the tube material selected. This effect, known as the build-up effect, is well known in the art. This x-ray generating material is in addition to the x-ray emitting x-ray conversion target  30 . Alternatively, the x-ray conversion target  30  can be eliminated and replaced by the x-ray generator material incorporated in the tube  66   a ,  66   b . In the latter embodiment, when the electron beam  20  impinges upon the tube  66   a ,  66   b , x-rays are emitted into the interior of the tube and are received by any target object  50  in the path of this x-ray flux. In a preferred embodiment, tube  66   a  or  66   b  is as thin as possible to provide the required structural integrity, while maximizing photon flux to target object  50 . 
     It should be understood that the positioning assembly  60  may include any structure orienting the target object  50  in the path of the emitted x-rays  40  and/or the electron beam  20 . For example, the positioning member may retain the target object  50  in a fixed position and the irradiating apparatus may translate in relation to the target. 
     The target object  50  preferably is positioned within the portion of the x-ray beam  40  that has the greatest intensity. Likewise, the transport apparatus  60  and enclosed target object  50  may preferably be placed in close proximity to the x-ray conversion target so as to maximize the fluence of x-rays through the target object  50 . It is preferred that the target object  50  be generally immobile in relation to the transport apparatus allowing for more precise locating of the target object  50  within the emitted x-rays  40 . In the embodiment in which the transport apparatus  60  includes a tube, the target object  50  preferably is constrained from moving relative to the tube. 
     In the preferred embodiment, the material being irradiated  50  is a medical stent, although any other target objects may be irradiated as well. For example, material for constructing stents may be irradiated. Likewise, other implantable medical devices may be irradiated. 
     In the embodiment in which the target object  50  is a stent, the stent can be constructed with a generally cylindrical cross-section allowing it to be supported and also snugly fit within a tube shaped transport apparatus  60 . In this embodiment, any suitable transport tube may be used. Preferably it is constructed with relatively thin walls. For example, the walls may have a thickness of generally 0.01 inches, and the transport apparatus preferably is constructed of a substance selected to minimize attenuation of the x-rays while not being subject to degradation of its material properties by exposure to the x-rays. Such a substance has a low atomic number and low density, for example, aluminum or carbon. Alloys of such substances also may be used. 
     In operation, the target object  50  within the transport apparatus  60 , or the target object  50  and transports apparatus  60  together can be translated in the axial direction, as indicated by arrow  70 , and about the axis, as indicated by arrow  75  while being irradiated to provide greater uniformity of the radioactivation within the target object  50 . Alternatively, the transport apparatus  60  may dwell at a particular location so as to create an uneven radioactivation within the target object  50 . In one embodiment, both the transport  60  and the target object  50  are independently movable. Alternatively, the target object  50  may be fixed in reaction to the transport  60 . 
     The same translation motion of the target object  50  is also suitable for inserting and extracting the target object  50  from the transport  60 . In the embodiment described above in which the target object  50  is a stent or stent material and the transport  60  is tubular, a continuous line of stents can be processed, i.e., stents are inserted into the transport tube  60  and are translated in direction  70  from one end of the tube to the other end of the tube  60 . Alternatively, plural stents may be placed on the transport  60 , and the transport  60  may be translated to irradiate the stents being transported. 
     The radioactivation produced in the target object  50  generally is dependent upon the energy and intensity of the x-ray beam  40  and the length of time the target object  50  is irradiated, i.e., placed within the a path of the x-rays  40 , although other factors may influence irradiation as well. 
     A thermal shield  80  optionally is placed between the x-ray conversion target  30  and the transport apparatus  60  to diminish the amount of thermal radiation reaching the target object  50  from the x-ray conversion target  30 . The use of a thermal shield is particularly appropriate in applications in which the target object  50  or the transport apparatus  60  will degrade if heated excessively. 
     Further cooling of the target object  50  or transport apparatus  60  is achieved by optionally providing a heat transfer fluid  90  within the interior of transport apparatus  60 . This form of cooling is particularly suited to the embodiment in which the transport apparatus  60  includes a tubular structure and the fluid  90  is directed into the interior of the tube of the transport  60 . Any suitable gas or liquid may be used, which can achieve a sufficient degree of heat transfer so as to maintain the material within a desired temperature range. Preferably the fluid  90  is selected to minimize corrosion of the apparatus, including the transport  60  and the target object  50 . For example, gases such as helium or nitrogen are suitable as such a coolant. 
     A temperature monitoring device  100  may optionally be included to provide cooling feedback. Any form of thermostatic control may be used to maintain the required temperature of target object  50 . 
     A radiation detector  110  optionally may be used. Any suitable detector may be used that can measure the flux of x-rays passing through the target object  50  and attendant apparatus, if any. One suitable radiation detector has an ionization chamber. The radiation detector  110  monitors the irradiation process and preferably provides information suitable for controlling the exposure of the target object  50  to the x-rays  40 . This information provided by the radiation detector  110  also assists in maintaining a stable electron beam  20  energy level since the ratio of the x-ray flux  40  to the incident electron beam  20  current typically is proportional to the amount of energy. Thus, a feedback system is used in which the electron current in the x-ray conversion target  30  (such as measured by the metering circuit  39 ) is compared to the output of the radiation detector  110  so as to control the electron beam source  10  and stabilize the energy level of the electron beam  20 . Any appropriate electronic or digital control known in the art may be used to provide this feedback system. Such a control system is illustrated in  FIG. 3  in which the output of the radiation detector  110  is provided to controller  120  as illustrated with line  130 . The output of metering circuit  39  also is provided to controller  120 , as illustrated with line  140 . Based on this output, controller  120  regulates the operational parameters of electron beam source  10  so as to control the energy level of electron beam  20 . The connection between the controller  120  and electron beam source  10  is illustrated with line  150 . It should be understood that the electron beam source optionally may provide feedback to controller  120  as well. 
     Optionally, the output of temperature monitoring device  100  can be provided to controller  120 , as indicated by line  145 . In this optional embodiment, the controller  120  controls the cooling system to maintain the desired temperature. Alternatively, a second controller (not shown) receives the output of the temperature monitoring device  100  and controls the cooling system. 
     In an alternate embodiment, plural transport apparatus  60  are used for transporting the target object  50  in the path of the x-ray beam  40 . As illustrated in  FIGS. 3 and 4 , two tube assemblies  66   a  and  66   b , are provided as part of the transport apparatus  60 . An additional target object being irradiated  52  also is shown. In one embodiment, the multiple transports can include additional tubes; however, it should be understood that any form of transport apparatus may be used which can position the additional target object  52  in a desired location. The additional target object  52  can be any material suitable for irradiation, including for example a stent or other implantable medical device. In the embodiment illustrated in  FIGS. 3 and 4 , the additional tube assembly  66   b  and additional object  52  is irradiated by x-rays which pass through the upstream tube assembly and target object  66   a ,  50 . Any number of transports (and transport tubes as illustrated) may be used. In this manner different sizes and types of transports or associated tubes can be used to accommodate a variety of target objects  50 ,  52  or target object shapes. Transport parameters, such as motion (indicated by arrows  70 ,  75 ) can be varied for each of the arrangements so that each target object  50  and  52  attains the desired radioactivity. 
       FIG. 4  illustrates a cross-sectional view taken along the axis of the tubes and enclosed materials  50 ,  52 , illustrated in FIG.  3 . This figure illustrates an embodiment in which the tubes (labeled  66 ( a ) and  66 ( b ) in the illustrations) of the transport apparatus  60  may be of different diameters and each preferably provides access for the respective enclosed target object  50 ,  52  to the portion of the x-ray beam of greatest intensity. 
       FIG. 5  illustrates another alternative embodiment of the invention. In this embodiment, an electron beam  20  may be applied to the x-ray conversion target in a variety of ways. In one example, the electron beam  20  can be provided in a static manner, in a particular shape. In another example, the electron beam  20  can be provided in a dynamic manner over a distributed region of the x-ray conversion target. For example, the electron beam  20  can be directed along a single line or over any other region using electron beam directing apparatus  210 . Any such electron beam directing apparatus  210  may be used as long at it distributes the beam over the desired area. Examples of suitable electron beam directing apparatus  210  include beam optics, comprised of focusing magnets with static fields or alternatively magnets with time-varying fields. In one embodiment, the electron beam is directed by the electron beam directing apparatus  210  along a line which is oriented along the axis of the target object being irradiated  50 , achieving a uniform flux of x-rays  40  from the x-ray conversion target  30  along the length of the target object  50 . Any other distribution also may be generated. In one alternative embodiment, the target object  50  remains in a static position and is irradiated by directing the electron beam  20  with the electron beam directing apparatus  210  to cover the area to be irradiated. Any apparatus or component of the transport apparatus may be used to retain the target object  50  in a generally stable position, for example a pin or bar barrier. Alternatively, the transport apparatus may be controlled so as to retain the target object in a stable position, such as using any form of electronic control and motor or other motion imparting means. 
     Using such electron beam distributing apparatus typically can result in multiple target objects  50 , such as stents, being irradiated simultaneously, with or without motion of the target objects  50  during irradiation, resulting in an increased efficiency of utilization of the electron beam  20 . One example is illustrated in  FIG. 6 , which includes the optional electron beam distributing apparatus  210 . 
     An alternate embodiment of the transport apparatus  60  is illustrated in  FIG. 7. A  turret or carousel  310  is used to position a collection of target objects  50 , such as stents or other medical devices. The carousel includes a plurality of target mounts  315  capable of receiving and retaining in place at least one of the target objects  50 . The target mounts may include any apparatus that can retain a target object  50  in relation to the carousel, such as an aperture, gripper or other pressure holder, recess, snap, clip and so on. The target objects  50  are positionable within the path of the x-rays emitted from the x-ray conversion target  30  by the rotation of the carousel. A rotational motion of the carousel  310  is indicated by arrow  78 , indicating rotation about the axis indicated by reference numeral  317 . In operation, the rotational motion  78  of the carousel translates the target objects  50  positioned on it to promote uniformity of irradiation. The electron beam source  10  preferably is positioned to provide the electron beam in the axial direction, although any position can be selected as long as the electron beam  20  and x-rays  40  are received by the target objects  50 . Furthermore, while one target object  50  is positioned to receive the x-ray beam  40 , another target object also positioned on the carousel  310 , but away from the path of the x-ray beam  40  can be removed from the carousel  310 , or otherwise processed. If an irradiated target object  50  is removed from the carousel  310  in this fashion, its place on the carousel  310  can subsequently be filled by another unirradiated target object  50 . This lends itself well to continuous processing of target objects. The orientation of the carousel  310  with respect to the incident x-ray beam  40  and the orientation of the target objects  50  placed upon the carousel  310  preferably are optimized to maximize the utilization efficiency of the x-rays  40 . 
       FIG. 8  shows an another embodiment in which a target object  50  is mounted on a positioning apparatus, such as a translation armature  320 . The translation armature  320  is movable to position the target object  50  to impinge upon and receive the emitted x-rays  40 . In other words, the translation armature  320  can act to suspend the target object  50  in a desired position for irradiation. Any form of translation armature  320  may be used, and any material also may be used as long as the form and material adequately support the target object  50 , or target objects, positioned on the translation armature  320  and serve to position them for irradiation. For example, the translation armature may include a rod, wire, or other assembly suitable for retaining and translating a target object. It is preferred that the portions of the target armature  320  placed within the path of the x-ray flux  40  are constructed primarily of a low atomic number and low density material, such as aluminum, carbon or graphite to minimize x-ray attenuation. The translation armature  320  and mounted target objects  50  preferably can be translated axially, as indicated by arrow  70 , and rotated, as indicated by arrow  75 , to promote uniform exposure to the x-rays  40 . The irradiation takes place in a chamber  330  into which a heat transfer fluid can be introduced to transfer heat from said chamber and/or prevent corrosion of the target object  50  during irradiation. The translation armature  320  may be introduced into the chamber  330  through an entry port  340 . This port  340  assists positioning the target object  50  at a known and predetermined location within the x-ray emissions  40 . Additional ports  340  optionally are provided to accommodate different size target objects  50  and to provide for insertion of plural target objects  50  within the chamber  330  for irradiation. A radiation detector  110  optionally is situated inside or outside of the chamber  330 . The radiation chamber  330  optionally is mounted to or formed integrally with the x-ray conversion target  30 . Alternatively, the radiation chamber  330  may be separated from the x-ray conversion target  30 . 
     It should be understood that the above embodiments summarized in this description are exemplary and that other embodiments of the present invention are also envisioned. For example, as illustrated in  FIGS. 9-11 , an alternative embodiment of the present invention includes an x-ray conversion target  30  that is translated in a path corresponding to the path of travel of the target objects  50 . In this embodiment, a transport mechanism  410  translates both the x-ray conversion target  30  and the target objects  50  at the same time, or alternatively separate transport mechanisms are used to translate each of the x-ray conversion target  30  and the target objects  50 . Target  30  generates an x-ray flux  40  towards the one or more target objects  50  on the transport mechanism  410 . Likewise, a plurality of targets  30  may be provided, generating an x-ray flux received by the respective target objects  50 . 
     Any apparatus may be used to translate the target objects  50  and the source target  30 . As illustrated in  FIGS. 9 and 10 , in one embodiment, the transport mechanism includes a carousel  420 . The carousel  420  includes source target mounts  430  receiving target objects  50  for translation in the desired fashion. Various appropriate target mounts may be used, which retain the target objects  50  on the carousel  420 , such as grippers, snaps, clips or other pressure holders and apertures (as illustrated). The carousel  420  is rotatable in the directions indicated by arrows  440 . The carousel  420  optionally includes a cooling mechanism such a fluid cooling via pipes  450 . Likewise, the carousel may be rotably mounted on an axis corresponding to pipes  450 . Preferably the pipes  450  include a set of concentric pipes, one of which carries a cooling fluid, preferably a gas, which cools the target objects  50  within the carousel  420  and the other of which carries a cooling fluid to cool the carousel  420  itself. To cool the carousel  420 , it is preferred that channels be constructed within the carousel  420  to increase the surface area exposed to the cooling fluid thereby increasing the heat transfer to the cooling fluid. Channels also are included in the carousel  420  giving the target objects  50  cooling fluid access to the target mounts  430 , giving access to the target objects  50  retained in them. It is preferred that the cooling gas be helium because it is understood to have a relatively high thermal conductivity. Another cooling gas is argon which is also favored, because it is inert and is understood to have a relatively high density, such as compared to helium. 
     In operation, an electron beam source  10  generates an electron beam  20 , which optionally is directed using electron beam directing apparatus  460 . Any form of beam optics well known in the art may be used to form the beam  20  to the desired shape or size, or optionally for translating the beam as desired. The beam  20  may be formed for example into an oval, or elongated in order to control the irradication and uniformity of irradiation of the target object. The beam  20  impinges on the carousel from any angle. It may impinge upon the carousel from the side, as illustrated in  FIGS. 9 and 10 , or alternatively, from any other direction, such as the top, as illustrated in  FIG. 7 , as long as the source target  30  is situated between the beam  20  and the target object  50  at the desired time. 
     For example, the carousel  420  itself or the circumferential outer edge of the carousel may be formed of a suitable material that generates an x-ray flux  40  upon receiving an appropriate electron beam  20 . In this example, illustrated in  FIG. 9 , the carousel itself serves as the x-ray conversion or source target  30 , generating an x-ray flux which is received by the target object  50  within the carousel. The electron beam from source  10  and optical beam directing apparatus  460  is directed into the radial edge of carousel  420  so as to optimally irradiate target objects  50 , and so is not limited to being only normal to the carousel rotational axis. 
     The carousel  420  or that part of it constructed as an x-ray conversion target, may be fabricated of any material capable of efficiently generating an x-ray flux. For example, it may be constructed of a carbon-carbon fiber substrate that has embedded therein a suitable material for efficiently generating an x-ray flux while also providing for effective cooling of the target. Examples of target substrates doped with a high atomic number materials (i.e., a high Z material) are found in commonly assigned U.S. Pat. No. 5,825,848, entitled “X-ray Target Having Big Z Particles Imbedded in a Matrix.” Alternatively, the x-ray conversion target may be comprised of a conventional high Z material such as tungsten, as generally known in the art. 
     An alternative example is illustrated in FIG.  10 . The x-ray conversion target  30  is retained within the carousel  420  and surrounds at least a portion of the target mount  430 . The x-ray conversion target  30  may have any shape preferably sufficient to ensure efficient generation of x-rays and corresponding coverage of target object  50  by the generated x-ray flux. 
     Other arrangements of the carousel  420  and x-ray conversion target  30  also may be used. By way of example, the x-ray conversion target  30  may surround the target mount  430 , or the x-ray conversion target  30  may be generally planar, but also embedded in the carousel  420 . 
     Another example of this embodiment of the invention is illustrated in FIG.  11 . An x-ray conversion target  30  is mounted to translation assembly  410 . The translation assembly  410  is movable to position the x-ray conversion target  30  to receive the electron beam  20 , resulting in the generation of x-ray flux  40 . The target object  50  is also positioned on the assembly  410 , downstream of the x-ray conversion target  30 , so as to receive the x-ray flux  40  emitted from the x-ray conversion target  30 . Any form of translation assembly  410  may be used, and any materials also may be used to construct the translation assembly  410  so long as the form and material adequately support the x-ray conversion target  30  and target object or objects  50 . X-ray conversion target  30  is constructed so as to allow ready access to the target object, also allowing the possibility that the target object  50  is rotated as indicated by  75  in FIG.  11 . In this case, the x-ray conversion target  30  may be rotatably mounted to the translation assembly  410  and may be of a hollow cylindrical shape, so that it maintains its x-ray production efficiency when rotated. For example, the x-ray conversion target  30  may be mounted to the translation assembly  410  on bearings which enable the target object  50  to be rotated by the translation assembly  410 , while the x-ray conversion target  30  maintains its x-ray flux output. 
     An illustrative example of an x-ray conversion target partially or fully surrounding the target object  50  is illustrated in  FIGS. 12A and 12B . As illustrated therein, translation armature  410  is connected to motion assembly  420 , which provides translational and/or rotational motion to the armature  410  for translating the target object as desired. In this embodiment, the direction of linear travel of the armature  410  is understood to be an axial direction and any direction at right angles to that axial direction is understood to be a radial direction. Any type of motion assembly may be used, such as any type of motor, gear and linkage apparatus, stepper motor, electric motor and so on. The x-ray conversion target  30  is shaped to ensure efficient irradiation of target object  50 . Access to the target objection may be provided in any means, including partial disassembly of the source target  30 , or by removal of  30  from the armature assembly  20 . The target object  50  is retained to the translation armature  410  by any means, for example gripper, tongs, magnetic attraction, fingers, mandrel etc. As illustrated, a gripper device  440  having receiving fingers  450  can be used. A mounting core  460  is also illustrated. 
     The above-described features of the present invention can be combined together in any fashion. For example, the embodiments illustrated in  FIGS. 8 ,  11 ,  12 A and  12 B can be combined and the embodiments illustrated in  FIGS. 7 and 9  can be combined. 
     In the preferred embodiment, the target objects  50  are implantable medical devices, preferably stents. Any form of stent may be irradiated using the apparatus and process of the present invention, so long as the stent can perform the function of placement within a body lumen and retaining a required profile for a sufficient period as required for the desired treatment. Examples of suitable stent structures include a coil stent  52 , illustrated in  FIG. 13 , a mesh or lattice stent  54 , such as illustrated in FIG  14  and a tubular stent  56 , illustrated in FIG.  15 . The target object may be any other shape or size as well so long as it is compatible with the apparatus used for irradiating the target material. 
     Thus, it is seen that an apparatus and method for efficiently irradiating target objects, such as stents or other objects suitable for medical application is provided. One skilled in the art will appreciate that the present invention can be practiced by other than the preferred and other embodiments, all of which are presented in this description for purposes of illustration and not of limitation, and the present invention is limited only by the claims that follow. It is noted that equivalents of the particular embodiments discussed in this description may practice the invention as well.