Patent Document

This is a continuation of Ser. No. 08/285,799 filed on Aug. 4, 1994 now abandoned which is a continuation of application Ser. No. 07/935,528 filed on Aug. 25, 1992 now U.S. Pat. No. 5,355,399. 
    
    
     FIELD OF THE INVENTION 
     The present invention concerns an x-ray source for radiography, more particularly a portable x-ray source and methods for conducting medical, biological and industrial x-ray radiography. 
     BACKGROUND OF THE INVENTION 
     The existing equipment used for medical (and dental) x-ray radiography contains high voltage vacuum tubes and produce x-rays as a result of the bombardment of a target by electrostatically accelerated electrons. The electrical supplies for such tubes are based on high voltage (˜100 kilivolts) transformers. These transformers are very heavy, cumbersome, dangerous, and expensive pieces of equipment. Such conventional x-ray medical radiograph equipment is not portable and thus limits the use of the x-ray radiography in ambulances, distant areas, etc. 
     U.S. Pat. No. 5,323,442, filed Feb. 28, 1992, which is commonly assigned to the assignee of this patent document, the disclosure of which is incorporated herein by reference, describes an x-ray source that is based on an Electron Cyclotron Resonance (ECR) plasma. The ECR x-ray source is quite convenient to be used as a light, compact, safe and inexpensive low-voltage (but high enough photon energy and intensity) x-ray source. However, that ECR x-ray source has a large x-ray emitting surface which makes the resolution of the x-ray image poor and, without modification, not reasonably practicable for x-ray radiography, particularly in the medical field. 
     There remains a continuing need for better sources of x-rays for radiography. There also is a need for economical x-ray sources having sufficient intensity for radiography that are lightweight, portable, and may be operated from conventional energy supplies. 
     SUMMARY OF THE INVENTION 
     The present invention concerns an x-ray source based on an ECR plasma that, in contrast to the above ECR x-ray source of U.S. Pat. No. 5,323,442, possesses acceptable x-ray image resolution features for use as an exceptionally light, compact and safe portable x-ray radiograph. It also concerns an x-ray source which is free of the above deficiencies and provides nearly the same x-ray intensity and energy as the classical high voltage x-ray sources, although it has a drastically smaller volume, weight, electrical consumption and cost. In addition, the x-ray source of the present invention produces an x-ray intensity that is sufficient to produce high quality x-ray images on conventional x-ray sensitive films, with about the same exposure time as conventional high voltage x-ray sources. 
     Broadly, the invention is directed to apparatus and methods for producing x-ray radiation by providing a vacuumated chamber that is filled with a plasma support gas at low pressure and an x-ray emitter, and exposing the chamber to a resonant electrical field and perpendicular magnetic field to generate an Electron Cyclotron Resonance (ECR) plasma inside the chamber. The plasma support gas preferably is a heavy atomic weight gas. The chamber is configured and the magnetic field is established so that the ECR plasma forms a ring of hot electrons which bombard the x-ray emitter. This bombardment, in turn, produces an x-ray emission from the emitter generally directed at a target. As used herein, the term target includes any object to be irradiated. Where the context permits, it also includes a primary target or object which is being studied, and a secondary target or object such as x-ray sensitive film to record an image of the primary. 
     In one preferred embodiment, the chamber is within a microwave resonant cavity and between a pair of magnetic members that generate an axisymmetric magnetic mirror trap inside the cavity and chamber. This produces an ECR plasma occurring on an axisymmetric hyperboloid sheet with a ring of hot electrons in the central part of the magnetic mirror trap. The electron ring provides a steady (or controllable) electron current which is received by the x-ray emitter, and thus produces a continuous x-ray emission on the emitter surface. If both the position and orientation of the emitter surface are appropriately selected, the emission will be outgoing, perpendicular to the magnetic field lines. The emission is at a sufficient intensity to irradiate an object and expose an x-ray sensitive film using conventional exposure times as explained below. 
     Advantageously, because of its small size, low cost, and low power requirements, the x-ray source of the present invention is easily manipulated, can be used in a conventional manner, and can be made portable to make x-ray photographs virtually anywhere. For example, in the case of medical x-ray radiographs, the x-ray source of the present invention can be conventionally used, e.g., in a hospital, doctor&#39;s or dentist&#39;s office. A portable device can be used to obtain x-ray images of injuries at the injury site, before the patient is moved or transported to another location. Thus, civilian and military rescue vehicles, e.g., ambulances, helicopters, fire engines and the like, can be equipped with the portable x-ray source of the present invention for use during emergencies, whether on a city street, in a desert, or in space. Similarly, in the case of x-ray radiography of structures, welds and other physical things, a portable x-ray source in accordance with the present invention can be easily used at the site where the object to be examined is located, e.g., at any time during construction of a structure such as a submarine, nuclear power facility or spacecraft. The present invention also can be used for non-medical radiography, such as for fault analysis and identification of paintings and other works of art in museums and art galleries. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features of the invention, its nature and various advantages will be more apparent from the drawings and the following detailed description of the invention, in which like reference numerals refer to like elements, and in which: 
     FIG. 1 shows a side cross-sectional schematic view of an x-ray source for radiography of the present invention, drawn to the scale indicated; 
     FIG. 2 shows an end cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 shows a schematic view of the azimuthal drift of electrons due to the radial gradient of the magnetic field of the source of FIG. 1; 
     FIG. 4 shows a side schematic view of the hot electron ring formation in an ECR supplemented magnetic mirror configuration in accordance with the present invention; and 
     FIG. 5 is an image of an x-ray photograph taken using a prototype of the invention in accordance with FIGS.  1 - 3 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIGS. 1-4, a preferred embodiment of the x-ray source for radiography in accordance with the present invention includes a microwave resonant cavity  10 , a vacuumated discharge chamber  20 , an x-ray emitter  30 , a microwave energy source  40 , a vacuum window  50 , and a pair of magnetic members  61  and  62 . 
     In the present invention, the x-ray is produced during the bombardment of a solid body, i.e., emitter  30 , by an ECR plasma. The ECR plasma is created in a compact axisymmetric magnetic mirror trap which is formed by two permanent disk magnets, namely magnetic members  61  and  62 . Members  61  and  62  are preferably symmetrically arranged about a midplane of chamber  20  with opposite poles, North N and South S, facing one another, as illustrated in FIG.  1 . 
     If one applies in this magnetic field configuration an oscillating electrical field perpendicular to the magnetic field lines, then the phenomenon of the ECR can occur. The condition to be satisfied for an ECR condition is: 
     
       
         ω=eB/mc (CGS units)  
       
     
     where ω is the circular frequency of the oscillating (microwave) field, m and e are respectively the mass and the charge of an electron, c is the light speed in free space, and B is the magnetic induction. 
     In an axisymmetric magnetic mirror with the field value in the geometric center slightly exceeding the ECR value at the given microwave frequency (which is feasible if strong enough magnetic members  61 ,  62  are used), the ECR phenomenon occurs on a axisymmetric physical surface resembling a hyperboloid of one sheet. This is illustrated in section by the double cross hatched curves labeled  63  on FIGS. 1 and 4. 
     If a gas at low pressure fills the area in discharge chamber  20  between magnets  61  and  62 , then an ECR plasma starts up. The electrons on the ECR surface  63  acquire high energy, ranging from between 50 and 200 kev depending on the microwave power applied. The electrons are accumulated near the midplane of the mirror configuration due to the action of the magnetic mirrors. As a result a hot electron ring  64  is built up in the central part of the magnetic mirror trap. This is illustrated by the black dots labeled  64  on FIG.  4  and the helical strand labeled  64  in FIG.  3 . 
     In the midplane of an axisymmetric magnetic mirror trap the magnetic field strength decreases when moving from the axis to the periphery. Consequently, a well known phenomenon of the “gradient drift” occurs, as described in, for example, F. F. Chen  Introduction to Plasma Physics and Controlled Fusion,  Plenum Press, New York and London, 1984. Due to this phenomenon, the electron Larmor orbit in the hot electron ring  64  drifts azimuthally so that every electron participates in two rotations: first one around the field line and second one around the axis passing azimuthally from one field line to another. This is illustrated in FIG.  3 . This azimuthal drift allows a small body, i.e., emitter  30 , intersecting the ring  64 , to “catch” all the hot electrons. Since the period of the azimuthal rotation is very short, i.e., 0.1 to 3 μs, most if not all of the electrons are received by (i.e., bombard) the emitter  30 , rather than being pushed to the periphery due to the flute instability. The latter phenomenon occurs in the ECR x-ray source described in U.S. Pat. No. 5,323,442, where there is no emitter body interposed in the electron current flow. 
     Thus, in the present invention, emitter  30  receives a permanent, i.e., continuous, current of very energetic electrons once the plasma is ignited and maintained. As a result, a permanent, i.e., continuous, x-ray emission is produced on the surface of emitter  30 . The emission is outgoing perpendicular to the magnetic lines, as illustrated by the arrows labeled x on FIGS. 1-3, if both the position and orientation of the emitter surface are appropriately chosen. Preferably, emitter  30  is inside the hot electron ring. The optimal orientation is empirically obtained to provide the desired direction of the x-ray beam emission. 
     In one embodiment, the microwave resonant cavity  10  and the vacuumated discharge chamber  20  are formed as a unitary composite structure, namely a vacuumated microwave resonant cavity which also serves as a discharge chamber filled with the plasma support gas at low pressure. Alternatively, the chamber  20  may be enveloped by cavity  10 , in which case cavity  10  need not be vacuumated. Advantageously, in either embodiment, the gas and emitter  30  may be sealed inside either chamber  20  or a combined cavity  10 /chamber  20  and provided as a replaceable cartridge for the x-ray source that has a useful life, and which can be easily replaced when its usefulness is consumed. 
     Referring now to FIGS. 1 and 2, one embodiment of the present invention is described in which microwave resonant cavity  10  is vacuumated and also serves as discharge chamber  20 . Cavity  10  is preferably a metallic cylinder (other shapes are also possible) having an axis A inside of which a metallic emitter  30  is fixed in the midplane between axis A and the wall. The axis A is shown on FIG.  4 . Emitter  30  is securely suspended from support  31 , which preferably lies in the midplane of cavity  10 , and is oriented at an angle α (see FIG. 3) of between 15 and 75 degrees, preferably between 70 and 75 degrees, relative to the tangent of electron ring  54 , and in a plane perpendicular to the plane of electron ring  54 . Supports  31  and  32  are transparent to the microwave energy and the magnetic field and are made of, e.g., quartz, quartz glass, or a ceramic. Supports  31  and  32  also may be made of non-magnetic metals, e.g., tantalum, molybdenum, and stainless steel, arranged perpendicular to the electric field lines. 
     Cavity  10 /chamber  20  is filled with a gas at a low pressure and is placed between two magnetic members  61  and  62 . Members  61  and  62  are preferably permanent magnets, aligned coaxially with and spaced equidistantly about the midplane of the cavity on axis A. Members  61  and  62  also may be made of electromagnets or solenoids. Permanent magnets are preferred because they are compact, light in weight, and do not consume electrical energy to generate the magnetic mirror. 
     The distance d between magnets  61  and  62  is adjustable and is chosen in such a manner that the ECR surface  63  becomes a one-sheet hyperboloid and emitter  30  is effectively positioned to enter and intersect the ECR surface  63  from inner side, as illustrated in FIG.  1 . In this regard, selecting the distance d controls the magnetic mirror field profile and, hence, the relative location and shape of ECR surface  63  inside chamber  20 , and controls the optimum conditions to ignite the plasma on start up and to maintain the plasma and x-ray emission during continued operation. Adjustment may be achieved, for example, rotating magnets  61  and  62  in cooperating threaded recesses  67  and  68  on opposite sides of chamber  20  (FIG.  1 ). However, in as much as most radiographic procedures have exposure times on the order of seconds, once an x-ray source is tuned for a sustained plasma, no adjustment may be required during continued operation. 
     The x-ray coming from emitter  30  outgoes through a vacuum window  50 . Window  50  may be mechanically protected by a rigid protective cover  52 . Window  50  is presented facing the target or object to be irradiated, e.g., the patient during a medical radiographic procedure. Both window  50  and any cover  52  are transparent for the x-ray. 
     Cavity  10  has a conventional electrically conductive material on its inside surface and is fed microwave energy through a vacuum window  42  using any conventional technique. FIG. 1 illustrates one coupling using a coaxial cable  44  and an electrical field antenna  45  introduced in the volume of cavity  10  without deterioration of the vacuum conditions. Since the exposure time is quite short (on the order of seconds) there is no appreciable concern of heating window  42  or any related difficulties. In this regard, window  42  is made of a microwave transparent material that is capable of sustaining the low pressure inside chamber  20 , e.g., quartz, quartz glass or a ceramic. In an alternate embodiment, where cavity  10  is not vacuumated, window  42  may be omitted. 
     Chamber  20  may be filled with the heavy, chemical-passive gas in a well-known manner, for example, by evacuating chamber  20  on a commercially available vacuum pump, at an elevated temperature, to out gas any impurities in the chamber material. The chamber is then filled with the gas and the tubulation used for out-gassing and filling is sealed. If chamber  20  is not a part of cavity  10 , it may be made of a dielectric material that is transparent to microwave energy, magnetic fields and x-ray radiation, e.g., quartz, quartz glass or a ceramic. 
     Cavity  10 , when also serving as discharge chamber  20 , has to be made of a highly conductive metal which, after a conventional treatment during fabrication, is not outgasing during a long time. Another requirement, whether or not it also serves as discharge chamber  20 , is that it provide good protection for the operator against the x-ray radiation, which can penetrate through the resonant cavity walls. Accordingly, the conductive metal is coated with a 2 mm thick copper layer which is in turn covered by a 2 mm thick lead layer. The copper provides good thermal conductivity to minimize localized heating, and the lead provides x-ray absorption. 
     To ignite and maintain a hot electron plasma, cavity  10  has to be fed sufficient microwave energy. Since the minimum diameter of cavity  10  is of the order of the microwave wavelength, the latter should be chosen in the range of 10 cm in order to have a portable device which is convenient to handle physically, and may be handheld. A large choice of inexpensive microwave power sources in the frequency band of 2.45 GHz (corresponding to a wavelength of 12.2 cm) are available and may be used as the working frequency. 
     The needed microwave power from source  40  is based upon the sensitivity of the available medical x-ray film. Standard x-ray film sensitivity is typically 1.0 milliwatt per cm 2  per second. To obtain a photograph of 100 cm 2  one needs 0.1 watt of x-ray during 1.0 second. To obtain such an x-ray power emitted by emitter  30  made of tungsten, at the electron energy of 100 keV, one has to dissipate on the surface of emitter  30  an electron flux power of to 15 watts (W. J. Price,  Nuclear radiation detection,  McGraw Hill Book Company Inc., N.Y.,Toronto, London, 1958, p. 19). 
     At the electron energy of 100 keV, an electron current of only 150 micro-amperes on the surface of emitter  30  produces a power of 15 watts. This amount of electron current is usually produced in ECR plasmas without requiring any special operating conditions. Supposing that one-half of the energy stored in the ECR plasma discharge is accumulated in the electron ring  64  and that the other half of the microwave energy is absorbed by the ECR discharge plasma, a microwave power of 100 watts is sufficient for a normal operation of the portable medical x-ray imaging apparatus of the present invention. A power range of 50 to 1,000 watts is believed suitable for most medical x-ray imaging for exposing standard film sizes of 100 to 1,000 cm 2 . One such power supply may provide an adjustable range, e.g., between 200-500 watts, or between 50 and 300 watts, etc. 
     The discharge chamber  20  (i.e., the interior microwave cavity  10 ) has to be filled by a plasma support gas in order to produce an ECR plasma providing energetic electrons. The requirements are that the support gas not interact with the walls of chamber  20 , have a large atomic mass to reduce plasma losses, and have a low ionization potential to ignite and sustain easily an ECR plasma. Suitable gases are the heavy noble gases, such as argon, krypton or xenon gases. The gas is preferably sealed inside chamber  20  at a desired low pressure in the range of 10 −3  to 10 −6  Torr, preferably 1×10 −5  to 4×10 −4  Torr, and more preferably 9×10 −5  to 4×10 −4  Torr. It is to be understood that the interior conductive layer of cavity  10  may be coated with a material that will not react with the plasma support gas, and permit the forming of ECR plasma, if necessary. 
     In the case that the magnetic members  61  and  62  are permanent magnets, they are secured in parallel about cavity  10  separated by a distance d along axis A. Accordingly, their magnetic field strength should be sufficient to produce in the central point of the cavity a magnetic induction value |B| exceeding the ECR value for the selected microwave frequency. 
     If a frequency of 2.45 GHz is used, the magnetic induction |B| in the central point is preferably not lower than 1 kG (the ECR value is 0.865 kG). At a typical distance d of 10 cm, magnetic members  61  and  62  each may be made in the form of a disk of 5 cm diameter and 2 cm thick, from such widely used and inexpensive magnetic materials as samarium-cobalt or neodymium-ferrum-boron. Such magnetic disks  61  and  62  produce the needed magnetic induction without difficulty or adverse consequences. 
     Emitter  30  is preferably a solid body, more preferably a metallic plate for receiving energetic electrons and converting some of their energy into the x-ray. The choice of the emitter material is determined by two requirements: the conversion rate has to be maximal and the non-converted energy (thermal) should not damage emitter  30 . To satisfy both conditions the material chosen must have a relatively large atomic number and high melting temperature. Preferred metals for emitter  30  are tungsten and tantalum. Any other material that satisfies these conditions may be used. Thus, a tungsten or tantalum plate emitter  30  electrode that is 5 mm×5 mm and 1 mm thick will in practice satisfy these requirements. 
     Window  50  plays a double role. First, it allows x-ray radiation to pass to the target. Second, it preserves the vacuum in chamber  20 . To accomplish both functions, the material of the window must have as low an atomic number as possible, be rigid mechanically, and be a good vacuum material. Suitable materials for window  50  include light element metals, quartz, aluminum, and plastics, preferably beryllium or aluminum. Cover  52 , when used, may be any rigid x-ray transparent material, such as plastic, plexiglass, or polyethylene. Cover  52  may be spaced a distance from window  50  that is selected to correspond to the area of the target to be irradiated by the x-rays and placed in touching contact with the target. This provides for accurate alignment of the area of target to be exposed with the x-ray. The distance between window  50  and cover  52  also may be selected to provide a spacing in the nature of a focal length (or plane) for irradiating the target with a controlled x-ray beam area and intensity. 
     As shown in FIGS. 1 and 2, window  50  is a round cross-sectional area that is in a flat plane spaced a distance of about 1.0 cm from the circumference of chamber  20  and cover  52  is secured about 1.0 cm from window  50  in a parallel flat plane. Other shapes, spacings, and contoured planes for window  50  and cover  52  may be used. 
     Window  50  also may be provided with a shutter that absorbs the x-ray radiation and when open, permits x-ray transmission (not shown). This may be used to absorb x-ray emissions until the plasma has reached a steady state condition after startup. The shutter also may be used for time lapse exposure for a sequence of x-ray images are desired, e.g., to prepare a motion picture of some event or activity, or to obtain a large number of images in rapid succession. 
     EXAMPLE 
     A prototype x-ray source for medical radiographic procedures in accordance with the source illustrated in FIGS. 1-4 was built and tested. The parameters for one construction of the prototype were as follows. The microwave resonant cavity  10 , which also served as discharge chamber  20 , was vacuumated. It had a diameter of 13 cm, a height of 9 cm (measured along axis A). The cavity  10 /chamber  20  was a composite unitary structure made of a layer of aluminum 5 mm thick and an outer layer of either stainless steel 5.0 mm thick or lead 2.5 mm thick. It was filled with argon gas at a pressure of 2×10 −5  Torr. The window  50  was 40 mm in diameter and 12 mm thick and made of a commercial PLEXIGLASS material. The emitter  30  was a 4 mm×4 mm×1 mm tantalum plate. It was positioned at an angle of 15 degrees relative to the direction of the radius passing through the center of the emitter plate and was spaced 10 mm from axis A in the midplane of cavity  10 /chamber  20 . The microwave source  40  was a magnetron at 2.45 GHz and produced 150 watts. An image of an x-ray (70 cm 2  having a diameter of about 9.4 cm) of a rat taken using the prototype at an exposure time of 2 seconds is illustrated in FIG.  5 . No light amplifier was used. 
     Another prototype x-ray source has the following construction parameters. The cavity  10 /chamber  20  of the same dimensions was a unitary structure having a layer of aluminum 10 mm thick and filled with argon gas at 2×10 −5  Torr. The window was made of a commercial PLEXIGLASS material that was 85 mm in diameter. The emitter was a 4 mm×4 mm×1 mm tantalum plate positioned at an angle of 45° relative to the window axis and was spaced 15 mm from axis A in the midplane of cavity  10 /chamber  20 . The same microwave source and power is used. 
     Another aspect of the invention is directed to a source and a method for irradiating body tissue with x-rays at a dosage level and for a time sufficient for medical or dental diagnostic or therapeutic purposes. This includes fluoroscopy and exposing x-ray film. Such methods include generating an ECR plasma to product x-rays in a given direction, for example, in a given solid angle, to expose a film for x-ray evaluation of tissue, bone and other physical structures. These exposure methods include mammography and computer aided tomography (CAT scans). Such methods also include generating an ECR plasma to produce x-rays for medical therapeutics, for example, cancer therapy, diathermy, and activating x-ray responsive drugs. In this regard, the x-ray dosages to be used are those generally used in medical and dental diagnostic and therapeutic practices. Advantageously, the small and light weight of the x-ray source of the present invention, together with a lead shield that covers all of the cavity except suitably shaped window  50 , provide easy maneuverability to locate the source proximate to the subject and easy portability of the apparatus, for example, for a mobile medical clinic. In addition, the small size, simplicity of operation, and low power requirements permit providing emergency service vehicles such as ambulances, fire rescue vehicles and the like with portable x-ray machines, which may be hand held and battery powered, for obtaining x-ray images of injured patients prior to moving them. In this regard, the x-ray source may include a battery power supply or be powered by the alternator of a vehicle or a generator or line current (110 volt). A suitable rechargeable battery would require a 12 volt and 10 amp-hour capacity which could provide approximately fifty x-ray film exposures before requiring a recharge. A 24-volt battery having a 50 amp-hour charge would provide a longer useful life before requiring a recharge and higher power output levels. 
     One skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments which are presented for purposes of illustration and not of limitation.

Technology Category: h