Patent Publication Number: US-6661875-B2

Title: Catheter tip x-ray source

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to a catheter, and more particularly, to a catheter having a miniature x-ray generator unit at its tip for providing a biologically effective dose of x-ray radiation. 
     X-ray radiation having a biologically effective spectrum, for example, having an energy in a range of about 10 keV to about 40 keV, can be utilized for a variety of medical treatments. For example, such x-ray radiation can be employed for preventing restenosis in blood vessels that have undergone angioplasty and/or it can be employed in some oncological procedures, such as interstitial radiosurgery of tumors. 
     Miniature x-ray generators that produce biologically effective radiation, and that can be deployed in proximity of a target tissue by utilizing flexible catheters, are known. Such x-ray generators provide advantages over radioactive isotopes, such as,  90 Sr,  32 P, or  192 Ir, as sources of x-ray radiation. For example, unlike the radioactive isotopes, the energy spectrum and/or the dose rate provided by these x-ray generators can be varied over a relatively wide range. 
     A conventional approach to powering a miniature x-ray generator disposed in a catheter utilizes an external power supply to generate a high voltage, e.g., in a range of about 10 to 40 kV, required by the x-ray generator, and transmits the voltage to the generator via a small-diameter electrical cable that extends from the power supply through the catheter to the generator. 
     This approach has a number of disadvantages. For example, the thickness of an insulation layer needed to insulate the electrical cable can adversely affect the flexibility of the cable, and consequently maneuverability of the catheter. Further, there is a danger of insulation breakdown, particularly, during a medical procedure. Such insulation failure can at the least require the withdrawal of the catheter and the x-ray tube, or more ominously, it can expose the patient or medical personnel to electrical shock. 
     Accordingly, there is a need for a catheter having an x-ray generator which provides enhanced operational safety, ease of construction, and better flexibility. 
     SUMMARY OF THE INVENTION 
     The present invention provides a catheter having a flexible body that extends from its proximal end to its distal end. The catheter further includes an x-ray generator disposed in the distal end region of the flexible body for generating biologically effective x-ray radiation, for example, x-ray radiation having an intensity in a wavelength range that is effective for treating a patient&#39;s tissue. A miniature transformer is also disposed in the distal end region of the flexible body, and is electrically coupled to the x-ray generator, to power the generator. More particularly, the transformer includes a primary winding that receives an AC input voltage having a root-mean-square (rms) amplitude, for example, in a range of about 100 V to about 4 kV, and further includes a secondary winding that applies an AC output voltage having an rms amplitude, for example, in a range of about 10 kV to about 40 kV, to the x-ray generator. The incorporation of the miniature transformer at the distal end of the catheter eliminates the need for a high voltage transmission line along the entire length of the instrument and greatly reduces the insulation needed within the body of the catheter. 
     In one aspect, the x-ray generator has a length that is less than about 30 millimeters, and a maximum cross-sectional dimension, for example, a diameter when the cross-section is circular, that is equal or less than approximately 3 millimeters. Further, the transformer can have a length that is less than about 50 millimeters and a maximum cross-sectional dimension that is equal or less than approximately 3 millimeters. The small dimensions of the x-ray generator and the transformer advantageously allow their coupling to the tip of a catheter having an outer diameter with a dimension of a few millimeters to be deployed, for example, in a patient&#39;s artery to irradiate a selected tissue target. 
     The x-ray generator and the transformer can be formed as a monolithic device. Alternatively, the x-ray generator and the transformer can be formed as separate devices that are mechanically and electrically coupled to one another. 
     In further aspects, a catheter of the invention includes a flexible electrical cable, having a diameter in a range of about one millimeter to about two millimeters, that extends from the proximal end to the distal end of the catheter body. The electrical cable transmits an AC input voltage from an AC source, for example, an AC power converter, to the primary winding of the transformer. The electrical cable can be, for example, in the form of a coaxial cable having a pair of elongated coaxial conductors, one of which is electrically grounded and the other carries an AC electrical voltage to the primary winding of the transformer. An insulating inner layer having a thickness in a range of about 0.01 mm to about 0.2 mm insulates the two conductors from one another. In addition, an outer insulating layer having a thickness in a range of about 0.001 mm to about 0.2 mm provides an insulating cover for the cable. The insulting layers can be formed of a variety of materials, such as, polyethylene or Teflon™. The inner insulating layer is preferably selected to be able to withstand a voltage differential in a range of about 100 V to about 4 kV. 
     In another aspect, the x-ray generator can generate radiation having an energy in a range of about 10 keV to about 40 keV, and provides an x-ray output power in a range of about 1 mW to about 100 mW. The x-ray generator can include an evacuated housing and a cathode that is disposed in that housing. The cathode is preferably formed of a metal, such as tungsten, and is electrically coupled to the secondary winding of the transformer to receive a voltage in a range of about 10 kV to about 40 kV therefrom. Other methods, such as, carbon nanotube or micro-machined silicon pyramid, can also be utilized for forming the cathode. The x-ray generator can further include an anode separated from the cathode by a selected distance. The anode is preferably formed of a high-Z refractory metal, such as tungsten, and can be electrically grounded so as to create an electrical potential difference between the cathode and the anode, thereby generating an electric field therebetween. The cathode emits electrons in response to application of a voltage thereto, for example, during a negative portion of each cycle of an AC voltage. The electric field between the cathode and the anode accelerates these electrons to the anode, and the impact of the electrons with the anode effects production of x-ray radiation. 
     In further aspects, the x-ray generator can include a window that is substantially transparent to the x-ray radiation to facilitate transmission of the generated radiation to the outside environment. The window has preferably a transmission coefficient of approximately 99% or higher for x-ray radiation having an energy in a range of about 10 keV to about 40 keV. In one embodiment, the window is formed of beryllium and has a thickness in a range of approximately 10 microns to approximately 100 microns. The window can be formed of a sheet of a material having a substantially uniform thickness, and can be directly coupled to the housing of the x-ray generator, for example, in an opening provided in the housing. Alternatively, the window can be supported by a mesh, which is in turn mechanically coupled to the x-ray generator&#39;s housing. 
     In a related aspect, the x-ray generator can include an extraction electrode disposed in the housing between the cathode and the anode, and maintained at an electrical potential intermediate the potential difference between the cathode and the anode. The extraction electrode can advantageously control emission of electrons from the cathode, and can further focus the emitted electrons onto the anode. 
     In other aspects, the transformer can include a core that has preferably a cylindrical shape, and is formed of a ferromagnetic material, such as iron or a ferrite composed of oxides of Fe, Ni, Mn, and Zn. The cylindrical core can have a diameter, for example, in a range of about 0.1 mm to about 2 mm, and a length in a range of about 5 mm to about 30 mm. Further, the primary and the secondary windings of the transformer can be formed of coils, constructed of a conductive metal, such as copper wire. The low current, e.g., in a range of about 10 to 100 microamperes, flowing through the secondary windings, and proportionally higher through the primary winding, while the transformer is operational allows utilizing small-diameter wire for the construction of the windings. For example, in one embodiment, copper wire having a diameter in a range of approximately 0.025 mm to approximately 0.1 mm is employed for constructing the secondary winding, and copper wire having a diameter in a range of approximately 0.1 mm to 0.6 mm is employed for constructing the primary winding. Those skilled in the art will appreciate that conductive wires formed of other materials and/or having other diameters or shapes other than circular (e.g., oval or rectangular) can also be utilized for forming the transformer windings so long as the electrical resistance of the wiring is sufficiently low to allow the passage of the requisite currents through the windings without a high degree of heat generation and/or unduly increasing the size of the transformer. 
     In a related aspect, the transformer can include two secondary windings, one of which applies an AC voltage to the cathode of the x-ray generator, and the other applies an AC voltage to the extraction electrode. Alternatively, the transformer includes one secondary winding having a primary tap for applying a voltage to the cathode, and a secondary tap for applying a voltage to the extraction electrode. 
     In another aspect, the core, the primary and the secondary windings of the transformer can be “potted” in an insulating material, such as, polyethylene, silicone, epoxy, or polyurethane that electrically insulates these transformer components from the housing. The insulation layer covering these transformer components can preferably withstand voltage differences of approximately 40 kV or higher. 
     In another aspect, a catheter of the invention includes a miniature x-ray generator having a plurality of cathodes and anodes. The multiple cathodes and anodes can be utilized, for example, to ensure that at least one cathode emits electrons during each of the negative and positive swings of the AC voltage of the secondary winding of the transformer, thereby enhancing the efficiency of x-ray generation. 
    
    
     Exemplary embodiments of the invention will be described below with reference to the following drawings to provide further understanding of the invention. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a catheter according to an exemplary embodiment of the invention having an x-ray generation unit at its tip which includes a miniature x-ray generator that is powered by a miniature transformer, 
     FIG. 2 is a partial perspective view of a coaxial cable that can be utilized for transmitting an input AC voltage from a power supply to a primary winding of the miniature transformer of the x-ray generation unit of the previous figure, 
     FIG. 3 schematically illustrates a miniature transformer coupled to a miniature x-ray generator to form the x-ray generation unit of FIG. 1 as a monolithic device, 
     FIG. 4A is a partial perspective view of a catheter according to the teachings of the invention illustrating two of a plurality of beryllium window sections that form an x-ray transmissive window for coupling the generated x-ray radiation to the outside environment, 
     FIG. 4B is a perspective view of one of the window sections depicted in the previous figure which has a curved structure with a radius of curvature commensurate to that of the housing, 
     FIG. 4C schematically depicts a mesh that can be utilized in some embodiments for providing support for any one of the window sections depicted in FIG. 4A, 
     FIG. 4D is an end sectional view of the mesh of FIG. 4C, 
     FIG. 5A is a diagram illustrating electrical interconnections of various components of the x-ray generation unit of FIG. 1, 
     FIG. 5B is another diagram illustrating electrical interconnections of another embodiment of an x-ray generator unit according to the teachings of the invention having two secondary windings, one of which applies a voltage to the cathode and the other applies a voltage to an extraction electrode of the x-ray generator, 
     FIG. 6 schematically illustrates an x-ray generator for use in a catheter of the invention having two cathodes and two anodes, and 
     FIG. 7 is a schematic diagram of another x-ray generator with multiple cathodes and anodes that is suitable for use in a catheter of the invention. 
    
    
     DETAILED DESCRIPTION 
     The present invention provides a flexible catheter having at its tip a miniature transformer coupled to an x-ray generator for powering the x-ray generator. The x-ray generator and the transformer can be formed as a monolithic device, or alternatively, they can be formed as separate devices that are mechanically and electrically coupled to one another. The x-ray generator produces x-ray radiation in a biologically effective wavelength range, for example, in a range of about 10 keV to about 40 keV. Further, the transformer includes a primary winding that receives an input AC voltage having a root-mean-square amplitude in a range of about 100 V to 4 kV, and a secondary winding that up-converts the input voltage to generate an output voltage having an rms amplitude in a range of about 10 kV to about 40 kV for powering the x-ray generator. The use of the transformer in proximity of the x-ray generator, and more preferably, as a portion of a monolithic x-ray generation unit formed of the transformer and the x-ray generator, advantageously allows a more flexible electrical cable to be utilized for transmission of an input voltage across the catheter, and also allows a safer operation of the catheter, as described in more detail below. 
     With reference to FIG. 1, an exemplary catheter  10  according to the present invention includes a flexible body  12  having a lumen  14  that extends from a proximal end  16  of the flexible body to a distal end  18  thereof. The flexible body has an outer diameter in range of about 1 to about 4 millimeters, and is sufficiently flexible to be readily guided through a patient&#39;s lumen, e.g., a patient&#39;s artery, to be deployed in proximity of a target region that needs to be treated with x-ray radiation. A variety of materials can be employed for manufacturing the flexible body  12 . These materials can include, but are not limited to, polyethylene and Teflon™. 
     An x-ray generator unit  20  is coupled to the catheter&#39;s body in a distal end region thereof, and more preferably, at the tip of the catheter by one or more support elements  22 . The exemplary x-ray generator unit  20  includes an x-ray generator  24 , and a transformer  26  that provides power for the x-ray generator  24 . In this embodiment, the x-ray generator  24  and the transformer  26  are formed as a monolithic device, as discussed in detail below. In other embodiments, the x-ray generator  24  and the transformer  26  can be formed separately and be mechanically and electrically coupled to one another. 
     A flexible electrical cable  28  extends from the proximal end  16  of the catheter body  12  to its distal end  18 , and is electrically coupled to the transformer  26 . The electrical cable  28  can be, for example, a flexible coaxial cable that is connected at one end, via a connector  30  and a cable  32 , to a power supply  34 . As shown in FIG. 2, the coaxial cable  28  can include an inner conductor  36 , and an inner insulation layer  38  having a thickness in a range of about 0.01 mm to 0.2 mm that covers the inner conductor  36 . The exemplary coaxial cable  28  further includes an outer conductor  40 , for example, in the form of braided copper wiring, that is covered by an outer insulation layer  42  having a thickness in a range of about 0.001 mm to about 0.2 mm. In operation, the outer conductor  40  is grounded, and the inner conductor  36  is coupled to a primary winding of the transformer  26 , as described in more detail below, to transmit an input AC voltage from the power supply  34  to the primary winding. The input AC voltage can have a root means square (rms) amplitude in a range of about 100 V to about 4 kV, and a frequency in a range of about 60 Hz to about 10 MHz. 
     Although in this embodiment, the catheter  10  includes a lumen, in another embodiment, the flexible body of the catheter can be constructed without a lumen, and the x-ray generator unit  20  still be coupled to the distal region of the catheter. Alternatively, the flexible body of such a catheter can include a housing in its distal region for accommodating the x-ray generator unit  20 . Further, the flexible body of such a catheter can include a channel extending from its proximal end to its distal end in which the flexible cable  28  can be disposed. Alternatively, the flexible cable can be disposed on the external wall of the flexible body of the catheter. Those skilled in the art will appreciate that structures for the flexible body other than those described above, and other modes of coupling the generator unit and the flexible cable to the flexible body of the catheter, can also be utilized. 
     With reference to FIG. 3, the x-ray generating unit  20  includes a housing  44  in which the miniature transformer  26  and the miniature x-ray generator  24  are disposed. The exemplary housing  44  has a circular cross-section, although other cross-sectional shapes can also be utilized, with a diameter that is preferably less than about 4 millimeters, for example, in range of about 1 to about 3 millimeters, and a length that is in a range of about 20 to 50 millimeters. A portion  46  of the housing  44  is evacuated for accommodating various components of the x-ray generator  24 . The housing  44  is preferably formed of a metal, such as, stainless steel, and can be electrically grounded via electrical and mechanical coupling to the outer conductor  40  of the flexible cable  28  (or a third ground wire). 
     The exemplary x-ray generator  24  includes a cathode  48  separated from an anode  50  along an axial direction A of the housing  44 . In this embodiment, the cathode  48  is a field emitter cathode, formed for example of tungsten, that emits electrons during a negative portion of each cycle of an AC voltage applied thereto by a secondary winding of the transformer  26 , as described in more detail below. A support element  52 , in the form of an annular ring that is mechanically coupled to the housing  44 , can be used to position the cathode  48  centrally within the housing  44 , and provide a vacuum-tight coupling with the cathode  48  to ensure that the portion  46  of the housing  44  is maintained at a sufficiently low pressure, e.g., a pressure in a range of about 10 −7  torr to about 10 −5  torr, to allow operation of the x-ray generator  24 . 
     The miniature x-ray generator  24  can further include a heat sink  54 , for example, in the form of a solid cylinder formed of a metal, such as copper, that is coupled mechanically and electrically at one end to the housing  44 , and at its other end to the anode  50 . The heat sink  54  can be connected to the housing and/or the anode by utilizing any suitable technique. For example, the heat sink  54  can be soldered or braised to the housing and/or the anode. Alternatively, the heat sink and the housing can be machined as a monolithic structure. The heat sink  54  mechanically supports the anode  50  within the housing  44 , and removes heat from the anode  50  during the operation of the x-ray generator  24 . Further, the heat sink  54  maintains the anode  50  at the ground electrical potential via its coupling to the housing  44 , which, as described above, can be maintained at the ground electrical potential. It is well known that metals with high atomic numbers, so-called high Z metals, are particularly suitable for efficient production of x-ray radiation. Accordingly, the anode  50  is preferably formed of a high-Z refractory metal, such as tungsten. 
     With continuing reference to FIG. 3, the exemplary miniature x-ray generator  24  further includes an x-ray transmissive window  56  that allows transmission of the generated x-ray radiation to the outside environment. The window  56  has preferably a transmission coefficient of approximately 99% or higher for x-ray radiation having an energy in a range of about 10 keV to about 40 keV. With reference to FIG. 4A, in this exemplary embodiment, the window  56  can be formed of a plurality of beryllium window sections, such as sections  58   a ,  58   b , herein collectively referred to as window sections  58 , disposed in the metal housing  44 , and spanning a circumference of a portion of the cylindrical housing to provide an approximately 360 degree field of view, or a smaller fraction thereof, depending on the application of the x-ray radiation, for transmission of the generated x-rays to the outside environment. Each beryllium window section  58  can be formed, for example, as a curved structure, such as that shown in FIG. 4B, having the same radius of curvature as that of the cylindrical housing and a thickness in a range of about 10 to about 100 microns. 
     With reference to FIGS. 4C and 4D, in some embodiments, each beryllium window section  58  can be supported by a mesh  60 , herein referred to as “waffle” support formed, for example, of a metal such as stainless steel. The mesh  60  can be mechanically coupled to the housing  44 , and can include protrusions  60   a  in the form of metal “ribs” for contact with the beryllium window section that it supports. 
     Those having ordinary skill in the art will appreciate that materials, such as, diamond-like carbon or diamond, can also be employed to construct x-ray transmissive windows for use in a catheter of the invention. 
     Referring again to FIG. 3, the x-ray generator  24  can optionally include an extraction electrode  62  coupled to the housing  44  via a support element  64  to be positioned in proximity of the cathode  48 . The support element  64  can be, for example, an annular ring, formed of an insulating material, which is coupled to the inner surface of the housing  44 . The support elements  52  and  64  can be formed as a monolithic unit, as shown in this embodiment, or alternatively as separate structures. The extraction electrode  62  is maintained at an electric potential that is intermediate that of the cathode  48  and the anode  50 , as described in detail below, to control electron emission from the cathode  48 , and to focus the electrons emitted from the cathode onto the anode. 
     With continuing reference to FIG. 3, the exemplary miniature transformer  26  includes a core  66 , which is preferably formed of a ferromagnetic material, such as iron or a ferrite composed of oxides of Fe, Ni, Mn and Zn. The exemplary core  66  is in the form of an elongate cylinder having a diameter in a range of about 0.1 millimeters to about 2 millimeters, and a length in a range of about 5 millimeters to about 30 millimeters. The core  66  can be electrically isolated or can be connected electrically to the inner conductor  36  of the coaxial cable  28 , whose outer conductor  40  is electrically coupled to the metal housing  44  in order to maintain the housing at a ground electrical potential. 
     The transformer  26  further includes a primary winding  68  in the form of a coil having a number of turns in a range of about 5 to about 60 turns that are wound on the ferrite core  66 . The primary winding  68  is connected electrically at one end  68   a  to the inner (non-grounded) conductor of the input power cable, and at its other end  68   b  to the housing  44 . Thus, a voltage differential between the inner and the outer conductors ( 36 ,  40 ) of the cable  28  is applied across the primary winding to induce a current therein, and consequently a magnetic flux within the core  66 , as discussed in more detail below. The primary winding coil can be formed, for example, of copper wire having a diameter in a range of approximately 0.16 millimeters (#34 gauge wire) to approximately 0.57 millimeters (#23 gauge). Alternatively, a ribbon can be utilized to form the primary winding  68 . A coil forming the primary winding is preferably wound tightly around the ferrite core  66  so as to produce negligible effect on the overall diameter of the transformer. 
     The transformer  26  further includes a secondary winding  70  in the form of a coil having a number of turns in a range about 100 to about 1000 turns that are wound on the ferrite core  66 . The coil  70  can be formed, for example, of copper wire having a diameter of approximately 0.080 millimeters and a resistance of about 3.85 ohm/meter, e.g., #40 gauge copper wire. 
     The secondary winding  70  is coupled at one end  70   a  to the housing  44 , and is coupled, mechanically and electrically, at its other end  70   b , herein referred to as primary tap, to the cathode  48  to apply a voltage in a range of about 10 kV to about 40 kV thereto. The secondary winding  70  is connected electrically at a secondary tap  70   c  to the extraction electrode  62  in order to apply a voltage, which is less than the voltage applied to the cathode  48 , thereto. 
     The exemplary ferrite core  66 , and the primary and secondary windings  68  and  70  can be “potted” in an insulator  72 , such as, silicone, epoxy, polyethylene, polyimide or Teflon™, that electrically insulates these components from the housing  44 . The insulator  72  is preferably selected so as to withstand an electrical potential difference of at least 40 kV. 
     The primary and the secondary windings of the exemplary transformer  26  exhibit an inductance to resistance ratio (L/R) that is significantly higher than the inverse of the voltage frequency. For example, at a frequency (f) of 1 MHz, the inductance to resistance ratio of the secondary winding  70  having 600 turns (and a diameter of 2.1 mm including insulation for 40 kV potential difference, wound on a ferrite core having a magnetic permeability of  100 ) is approximately 3×10 −4  s, which is approximately three orders of magnitude greater than (2πf) −1 =1.6×10 −7  s. 
     With reference to FIG. 5A, in operation, the primary winding  68  of the transformer  26  (FIG. 3) receives an input AC voltage from the power supply  34  (FIG.  1 ), via the inner conductor  36  of the electrical cable  28  (FIG.  3 ). The power supply  34  can have a transformer  74  that receives the AC voltage at a primary winding  74   a , and generates an output voltage having an rms amplitude in a range of about 100 V to about 4 kV at a secondary winding  74   b . This output voltage is applied, via the inner conductor  36  of the cable  38 , to the primary winding  68  of the miniature transformer  26 , and generates an AC current in a range of about 200 microamperes to about 5 milliamperes therein. The AC current through the primary winding  68  in turn generates a time-varying magnetic flux in the core  66  (FIG.  3 ), and consequently in the secondary winding  70 . 
     This time-varying magnetic flux induces a voltage across the secondary winding  70 . Because the number of turns of the coil forming the secondary winding is approximately 40 to 100 times higher than that of the primary winding, the voltage induced in the secondary winding has an rms value in a range of about 10 to 40 kV, and more preferably, in a range of about 20 to about 30 kV (rms). The induced voltage at the primary tap  70   a , which equals the voltage generated across the entire secondary winding  70 , is applied to the cathode  48 , thereby generating a time-varying electric field between the cathode  48  and the anode  50 , which is maintained at a ground electric potential. The AC voltage applied to the cathode further causes emission of electrons therefrom during each negative swing of the voltage. Further, the voltage induced at the secondary tap  70   c , which equals a selected fraction of the voltage induced across the entire secondary winding, is applied to the extraction electrode  62 . 
     The electric field established between the cathode  48  and the anode  50  accelerates these electrons to energies in a range of about 10 keV to about 40 keV upon impact with the anode. The impact of the electrons with the anode causes generation of x-ray radiation. As discussed above, the anode is preferably formed of a high-Z metal to enhance the x-ray production. Electrons with an energy of about 30 keV typically convert between about 0.05% to 0.25% of their kinetic energy into x-ray energy when they impinge on a high-Z metal, such as tungsten. A portion of the x-ray radiation that escapes to the outside environment through the window  56  can be utilized for a variety of different applications, as discussed in more detail below. 
     With reference to FIG. 5B, in another embodiment, the exemplary transformer  26  also includes another secondary winding  76  that is formed of a coil having approximately  20  to 200 turns, and is wound on the ferrite core  66  shown in FIG.  3 . This secondary winding  76  is mechanically and electrically coupled at one end to the extraction electrode  62  to apply a voltage thereto, which is intermediate the voltage difference between the cathode and the anode. 
     Further, the time-varying magnetic flux generated by the current in the primary winding  68  induces a voltage in the second secondary winding  76 , which is applied to the extraction electrode  62 . Because the number of turns of the coil forming the winding  76  is less than that of the winding  70 , e.g., 120 turns compared to 600 turns, the voltage induced across the winding  76  is less than that induced in the winding  70 , e.g., in range of about 2 to 8 kV. The application of this intermediary voltage to the extraction electrode helps guide the electrons emitted from the cathode  48  to the anode  50 . 
     An x-ray generator unit in accordance with the teachings of the invention, such as the exemplary unit  20  described above, provides a number of distinct advantages. For example, it is sufficiently small to be coupled to the tip of a catheter having an outer diameter of a few millimeters, thereby permitting its use, for example, within a patient&#39;s artery. Further, the close proximity of the step-up transformer to the x-ray generator obviates the need for transmission of high voltages, e.g., in the range of 10 to 40 kV, from an external power supply, along an electrical cable that extends from the proximal end to the distal end of the catheter, to the x-ray generator. Rather, the electrical cable in a catheter of the invention carries much lower voltages, e.g., in a range of about 100 V to about 4 kV. This allows the use of a more flexible cable in a catheter of the invention, and further enhances operational safety of the catheter. The term “proximity” as used herein to describe the spatial relationship of the transformer and x-ray generator is extended to describe separation distances less than about 50 millimeters, typically on the order of a few millimeters or less. 
     Thus, in a catheter of the invention, high electrical voltages are confined to the step-up transformer and its associated x-ray generator. This advantageously enhances the operational safety of the catheter, especially in medical applications in which the catheter is inserted in a patient&#39;s lumen. Further, the lowering of the voltage transmitted by the flexible cable allows utilizing a thinner insulation layer for the cable, thereby resulting in a more flexible cable that enhances the overall flexibility of the catheter. In addition, in a catheter of the invention, the transformer can be advantageously powered by a low-cost AC frequency converter. Hence, a catheter of the invention not only exhibits enhanced safety and flexibility but it can also be constructed in a cost efficient manner. In fact, a catheter of the invention can be constructed as a single-use item. 
     A catheter of the invention can be utilized in a number of different applications. In one such application, it is employed in medical procedures to deliver highly localized, biologically effective doses of ionizing radiation to a patient&#39;s tissue. For example, a catheter of the invention can be utilized to deliver an x-ray dose in a range of about 8 Gray to about 20 Gray (800 to 2000 rads) to a localized portion of a patient&#39;s tissue. The x-ray radiation can be effective, for example, in preventing restenosis. In addition, the x-ray radiation can be employed for interstitial radiosurgery of tumors. It is clear to those skilled in the art that the catheter of the invention can find numerous other medical applications. 
     Although in the embodiments described above, the x-ray generator  24  includes one cathode and one anode, in other embodiments of the invention, the x-ray generator can include a plurality of cathodes and anodes to more efficiently utilize an AC voltage for generating x-ray radiation. For example, FIG. 6 depicts an x-ray generator  78  according to the invention that includes two cathodes  80  and  82 , which can be field emission cathodes formed of tungsten, and two anodes  84  and  86  each of which is in the form of an annular ring formed, for example, of tungsten. The cathode  80  and the anode  84  are coupled to the secondary winding  70  of the transformer  26  such that the positive and negative swings of the voltage across the secondary winding  70  are applied to the cathode  80  and the anode  84 . In contrast, the cathode  82  and the anode  86  are grounded. During a negative swing of the voltage across the secondary winding  70 , the cathode  80  emits electrons that are accelerated to the anode  86  to impact that anode, thereby generating x-ray radiation. Further, during a positive swing of the voltage induced across the secondary winding  70 , the cathode  82  emits electrons which are accelerated towards the anode  84 , which during a positive voltage swing is at a higher electrical potential, and cause generation of x-ray radiation upon impact with that anode. Hence, in this manner, x-ray radiation is generated during the entire period of each cycle of an AC voltage induced in the secondary winding of the transformer  26 , rather than during only one-half of each cycle. 
     Miniature x-ray generators, having multiple cathodes and anodes, which can be utilized in a catheter of the invention as x-ray sources are not limited to the x-ray generator  78 , described above. For example, FIG. 7 depicts another x-ray generator  88  according to the teachings of the invention which also includes two cathodes  90  and  92  and two anodes  94  and  96 . In contrast to the x-ray generator  78 , the cathode  90  and the anode  94 , and similary, the cathode  92  and the anode  96 , are not held at the same electrical potential. Rather, while the cathode  90  is connected across the entire secondary winding at a primary tap  70   b , the anode  94  is connected across a portion of the secondary winding at a secondary tap  70   c . Further, while the cathode  92  is grounded, the anode  96  is connected across a portion of the secondary winding of the transformer at a tertiary tap  70   d . This arrangement allows the anodes  94  and  96  to function not only as anodes but also as extraction electrodes in alternative positive and negative voltage swings of the voltage induced across the secondary winding  70 . 
     The embodiments of the invention described above are intended to be interpreted as illustrative and not in a limiting sense. Those skilled in the art shall be able to make numerous variations and modifications to the above embodiments without departing from the scope of the invention. For example, materials other than those described above can be utilized to form the cathode and the anode of the x-ray generator. In addition, those skilled in the art can readily utilize the teachings of the invention for constructing a miniature x-ray generator that includes more than two cathodes and two anodes.