X-ray analysis apparatus, especially computer tomography apparatus

Abstract: An X-ray analysis apparatus has a charged particle beam generator (1,2), an X-ray generating target (3) bombarded by the beam and a detector (8) for X-rays from said target (3) transmitted by a test piece (6). To improve the spatial resolution of the apparatus, the beam size is reduced at the target by locating the target (3) outside a vacuum chamber of the beam generator (1,2) by providing the target (3) as a non-circular narrow track of X-ray generating material exposed at both surfaces of the target, and by providing the beam generator with a beam accelerator and means (2) for reducing the beam diameter between the beam accelerator and the target (3).

Description: This invention relates to X-ray analysis apparatus, particularly but not exclusively apparatus for computer tomography using X-rays. The principles of computer tomography using X-rays are well known, and need not be described here. "Computer tomography" will be abbreviated to CT, in this description. Recently CT systems have been developed for the testing of products for industrial uses. Since such products, e.g. metal products, are generally high in density, testing radiations such as X-rays, gamma-rays or light rays have too low transmittance when their energy is low. Therefore, there is a need to raise the energy level of the testing radiation. Moreover, the spatial resolution sought is as high as 0.2 mm. However, when the testing radiation has high energy, problems discussed below arise, and especially high resolution is difficult of realization. These demands are high particularly for an X-ray CT system. It is now proposed to employ a charged particle beam, such as an electron beam. As described in "Recent Medical Diagnostic Systems" (1988), pages 86-89, the typical X-ray generator of a CT system is one in which a high voltage is applied across a cathode and an anode which are arranged in the interior of a high vacuum tube, and electrodes accelerating from the cathode impinge against the X-ray generating target of the anode (in this description simply called "target") in order to generate X-rays. US-A-4607380 shows a rotating anode, which has customarily been used in order to reduce damage of the anode by the impinging electron beam. The beam is focussed, passes through an aperture plate and is then focussed again by focussing magnets to converge it onto the rotating anode. The anode is in a vacuum chamber connected through the aperture to the vacuum chamber containing the electron source. The X-rays are emitted from the surface of the anode which is bombarded by the electron beam. This apparatus is described as useful for the industrial inspection of objects, such as thick metal parts and achieves a small focal spot with high power output, at the anode. US-A-4573185 shows a rotating anode, without mentioning any particular use of it. A focal track of tungsten is arranged on a substrate and has a width narrower than that of the electron beam from the cathode, so that misalignment between the beam and the track will not affect the effective focal spot size or location. In the Journal of Nuclear Science and Technology (Japan) 26 (1989), pages 826-832, there is described a prototype high energy X-ray CT system for use in imaging dense and large objects. A linear accelerator is employed to accelerate the electron beam and to produce X-ray photons with a maximum energy of 12 MeV. When using an X-ray source of such high energy, however, there is a problem that even a rotating anode cannot be sufficiently cooled in order to avoid damage to the surface bombarded by the energy beam. EP-A-239 882 shows a target for an electron linear accelerator in which a liquid, such as mercury, is retained between parallel plates connected by bellows. The mercury is cooled by passing it through a heat exchanger. The target is located outside the vacuum chamber of the accelerator. First, problems of an X-ray generator using an electron beam will be discussed. The high energy of the X-rays signifies that the energy of the electrons is also high. The first problem of the X-ray generator is that the high energy of the electrons enlarges the size of the X-ray generation region and thereby worsens the spatial resolution. When the accelerated electrons are of at most about 200 keV, the energy levels of the generated electrons are comparatively uniform. Therefore, the electrons defocus little on the target, and the X-ray generation region of 0.2 mm has been realizable without using any special convergence means. However, when the energy of the electrons becomes 1 MeV or above, the dispersion of the energy of the electrons widens, and also the defocussing of the electrons on the target increases, so that the X-ray generation region is enlarged. A second problem is that the vacuum performance of the X-ray generator degrades, so the necessary spatial resolution fails to be achieved. In the prior art, the entire target is made up of an X-ray generating target element of insufficient heat transfer coefficient. Besides, in the prior art, the target is installed in the same vacuum as that of the device for generating the electrons. Consequently, the large amounts of gases produced from the bombarded part (namely, X-ray generating region) of the target, when the electrons have high energy, degrades the vacuum performance of the X-ray generator. When the vacuum is degraded, the generated electrons are not sufficiently accelerated on account of the gases, and the necessary energy cannot be reached. As a result, it becomes impossible to obtain X-rays having sufficient energy and to attain the necessary spatial resolution. Moreover, when the vacuum is degraded, the electrons undergo scattering due to the gases, and the X-ray generating region enlarges. In consequence of this also, spatial resolution deteriorates. A third problem is that the efficiency of maintenance worsens. When bombarded with electrons of high energy, the target is damaged more, so that the frequency of replacement of the target rises. Nevertheless, replacement is troublesome because the target is installed within the vacuum region. It is mentioned that FR-A-2 184 238 shows a target in X-ray apparatus having elements of X-ray generating material mounted in apertures in a plate. Accordingly, a first object of the present invention is to provide an X-ray analysis apparatus, particularly a CT system, of high resolution. A second object of the present invention is to provide an X-ray analysis apparatus of high maintenance efficiency, and high target performance. The invention provides an X-ray analysis apparatus as set out in claim 1. Preferably an electron beam generator is used which may be capable of generating an electron beam having an average energy of at least 1 MeV. The X-ray analysis apparatus may use a detector generating electrical signals or the detector may be an X-ray sensitive film, e.g. for radiography. Preferably means for cooling the target, e.g. by air or water are provided. Since a conventional fast rotating anode may be unsuitable for high energy beams, preferably the target is displaceable relative to the beam, thereby to present to the charged particle beam a fresh portion of target not previously bombarded by the particle beam. Thus the target can be displaceable relative to the charged particle beam so that analysis operations can continue even when the beam causes perforation of the target. Suitably the X-ray generating region of the target has a maximum width of 0.2 mm, or the X-ray generating target material has a width less than the diameter of the charged particle beam which bombards the target. The target may comprise a matrix material, e.g. of highly thermally conductive material, in which the track is embedded and the track may have a non-circular shape, e.g. selected from spiral and continuous rectangular. Alternatively the target material is provided by a plurality of wires supported in said target. The spacing of the target and the detector may be variable. This permits adaptation of the apparatus according to the width of the test piece. This apparatus may have a collimator for the X-rays transmitted by a test piece, arranged in front of the detector. The collimator has a plurality of collimating slits having axes extending in the direction of the transmitted X-rays, and further comprises means for changing the relative directions of the axes of the collimator slits. The collimator may have a plurality of movable parts providing side walls of the collimator slits and means for moving the movable parts in conjunction, so as to vary the relative directions of the slit axes. Alternatively a collimator for the transmitted X-rays transmitted by a test piece is arranged in front of the detector and has a plurality of exchangeable sets of collimating slits, the angle between the axes of two adjacent slits within each of the sets of slits being a constant angle, while being different as between the sets of slits. Suitably the collimator comprises a collimator body having a plurality of collimating slits extending continuously through said body, said slits having a slit width which is changeable. This allows selection of slit width according to a desired level of spatial resolution. The slit width may be changeable by exchanging, in the collimating position in front of the detector, at least two exchangeable sets of slits having respectively different slit widths, so that the effective slit entrance openings are changed in width and each slit has a constant width over its whole length. Alternatively, the collimator body has two parts which are movable relatively so as to vary the slit width in such a manner that the effective slit entrance openings change in width and each slit has a constant width over its whole length. In this case the collimator preferably has means for providing a fluid shield against passage of X-rays in undesired gaps in the collimator body caused by movement of the relatively movable parts. Means for reducing the diameter of the charged particle beam may be interposed between the accelerator and the target. Preferably means for selecting the energy of the charged particle beam are interposed between the accelerator and the target. The target for of the X-ray generating apparatus of the invention may have a first side at which the charged particle beam is incoming and opposite the first side a second side at which X-rays are emitted. This target further has an element made of X-ray generating material exposed at both the first side and the second side, the element having narrow elongate shape with its direction of elongation extending parallel to the first and second surfaces. Preferably the element has a width of less than 1 mm in the direction transverse to its direction of elongation. Embodiments of the invention will now be described by way of non-limitative example with reference to the accompanying generally schematic drawings, in which:- Fig. 1 is a plan view showing an embodiment of a CT system according to the present invention; Fig. 2(a) is a side view showing an embodiment of the target device of the system of Fig. 1, and Fig. 2(b) is a face view of a target used in the present invention; Fig. 3 is a view showing another target used in the present invention and a target device; Figs. 4(a), 4(b) and 4(c) are views showing other embodiments of targets used in the invention. Fig. 5 is a view for explaining factors which contribute to the spatial resolution of the present invention; and Fig. 6 is a general perspective view of an industrial CT system embodying the invention. First, factors contributing to spatial resolution will be described in conjunction with an X-ray CT system embodying the present invention and shown in Fig. 5 which schematically shows the relationships between an electric beam source 1 including an accelerator, a target 3 which is an X-ray source in the CT system, a test piece 6, a collimator 7 and a detector 8. X-rays are generally emitted in the shape of a fan beam, and Fig. 5 illustrates the X-ray beam entering one collimator slit. Since the X-rays impinging on the side surfaces of a slit can be neglected, the X-rays entering the detector are those emitted from a width W t on the target. The spatial resolution becomes higher with a smaller width W of the beam at the test piece. Effective measures for narrowing the width W are (a) to reduce the width W t , namely, to make small the X-ray generating region on the target, (b) to reduce the width of the collimator slit, (c) to shorten the distance between the radiation source and the test piece, and (d) to lengthen the distance between the test piece and the collimator (detector). The present invention in its various aspects provides techniques providing solutions for achieving at least some of these measures. Methods for achieving the measure (a) in which the X-ray generating region is made small are broadly classified into three. Charged particles such as electrons impinge on a target element, thereby to generate the X-rays. One aim therefore is to reduce the diameter, on the target, of the charged particles which are caused to collide against the target element. A second aim is that the width of the target element for generating the X-rays is made less than the diameter of the charged particle beam which is defined on the target. This is proposed in the prior art for a rotating anode, as mentioned. A third aim is that the vacuum of a charged particle generator is maintained in order that charged particles generated in the charged particle generator should not diverge due to their collisions with gases at a stage at which they are accelerated. Effective for this purpose is the arrangement wherein the target which is the production source of the gases is disposed outside the vacuum, namely, outside the charged particle generator. The charged particles pass outside the vacuum, but this has little effect at high energy levels. Moreover, since the target is outside the vacuum, the replacement of the target and the maintenance of a target driving device are greatly facilitated. Now, the X-ray CT system shown in Fig. 1 embodying the present invention will be described. This system has a generator and accelerator 1 which is a charged particle generator (electron beam generator) for generating and accelerating electrons and a charged particle converger 2 by which an accelerated electron beam 24 from the generator and accelerator 1 is converged on a target 3. A target moving mechanism 4 controls the impinging position of the electron beam 24 on the target 3. A CT scanner 5 scans a test piece 6 by translating and rotating it. A collimator 7 selects an X-ray beam 10 generated by the target and transmitted by the test piece 6. A detector 8 for the collimated X-ray beam 10 transmitted through the test piece 6 is located behind the collimator. The detector 8 may be a conventional one of the CdWO 4 scintillator type. A controller 9 controls the operations of these components. The X-ray CT system obtains the tomogram of any desired plane of the test piece 6. For this purpose the test piece 6 is scanned for translation and rotation by the CT scanner 5, the transmission of the X-ray beam through the test piece 6 in all directions is detected by the detector 8 and converted into data signals, and the transmission data items are processed according to the principle of computer tomography by data processing apparatus (not shown). The charged particle converger 2 reduces the size of the X-ray generating region on the target 3. The charged particle converger 2 is interposed between the accelerator 1 and the target 3, and includes a plurality of convergence magnet coils 21 and a diaphragm or aperture 22 located between the convergence coils. Fig. 1 illustrates a case of two convergence magnet coils. The charged particle converger 2 is connected with the accelerator 1 so as to be kept under the same vacuum, and this vacuum is maintained by a film 23 providing an exit window on the target side thereof. Since the electron beam 24 has high energy, it is converged on the target 3 with very little attenuation even when transmitted through the film 23. The energy of the electron beam accelerated by the accelerator 1 involves dispersion, and for example an electron beam 25 whose energy is higher or lower than specified energy is present besides the electron beam 24 of the specified energy. When the beam energy differs, a focal distance based on the convergence coil differs. That is, the electron beam of higher energy is focussed behind a target focal position, whereas the electron beam of lower energy is focussed before the target focal position. For this reason, the electron beam 25 of the unspecified energy can be eliminated by disposing the diaphragm 22 at the focal position of the electron beam 24 of the specified energy. The electron beam 24 of the specified energy is converged again by the succeeding convergence coil 21, and is focussed on the target 3. An electron beam of very small diameter can be obtained by providing a plurality of sets of the combination of the convergence coil and the diaphragm. Thus, according to this embodiment, the diameter of the electron beam to be defined on the target 3 can be reduced, so that the diameter of the X-ray beam to be generated by the target can be reduced. As a result, the test piece can be analysed at a high resolution. Although the above embodiment is provided with the plurality of convergence coils and diaphragms, the electron beam diameter can be made smaller than in the prior art even when the electron beam is converged by the convergence coils only. Alternatively, a similar effect may be achieved when the convergence magnet coils are substituted by electrostatic lenses. As shown in Fig. 1, the target 3 is installed in the atmospheric air outside the vacuum region of the charged particle converger 2. Accordingly, even when a large number of gas particles are produced by the collisions between the electron beam 24 and the target 3, the gases do not intrude into the generator and accelerator 1 or the charged particle converger 2, which can therefore maintain the vacuum. As a result, the electron beam 24 can assume a stable path without any influence from gases inside the generator and accelerator 1 and the charged particle converger 2. The electron beam diameter on the target can be reduced, and can be consequently made smaller than in the prior art system wherein the target is disposed within the vacuum. In addition, this embodiment is equipped with the target moving mechanism 4 by which an X-ray generating target having an elongate spiral element 32 as shown in Fig. 2 is moved so as to lie in the electron beam at all times. Owing to the installation of the target outside the vacuum, the target moving mechanism 4 can be constructed, operated and maintained easily. As a result, the replacement of the target 3 and the maintenance of the target moving mechanism 4 are facilitated, and a CT system of high reliability can be provided. The targets of Figs. 2 to 4 are designed for operation using the accelerated electron beams of high energy preferably employed in this invention. They differ from the rapidly rotating anode targets of the prior art in that they are moved only slowly or intermittently, because the target material is designed to be consumed. Although the target may be cooled, cooling is insufficient to prevent such consumption. Secondly, the target has a first face on which the charged particle beam impinges and an opposite, second face from which the desired X-rays are emitted towards the test piece. The target material, which in these embodiments is arranged on a narrow track which is not of continuous circular shape, is thus exposed at both faces of the target. An embodiment where the electron energy is so high as to perforate the target will be described in more detail with reference to Fig. 3. When the diameter of the electron beam impinging on the target is small, the target is damaged. By way of example, in case of an electron beam diameter of 0.2 mm, the target material reaches a high temperature much exceeding its melting point, and it vaporizes. In view of this, the target elements 32 (e.g. of tungsten) are formed in a rectangular pattern within a target holding member 31 of high thermal conductivity, and X-rays are generated in such a manner that the target is perforated by the electron beam while being scanned vertically and laterally by the target driver 4. The case of Fig. 3 is an example in which the moving speed of the target is set at about 1 - 2 mm per pulse of the electron beam, and in which apertures having diameters nearly equal to the electron beam diameter of 0.2 mm are successively provided at intervals of about 1 - 2 mm. Thus, when the target is located outside the vacuum as stated before, the path of the electron beam is not affected even by the production of large amounts of gases due to the perforation of the target, so that the X-ray generating region can be made small to achieve a higher spatial resolution. Next, there will be described other embodiments of the target used in the present invention for making the X-ray generating region small. Fig. 2 shows one of the embodiments. The target 3 consists of a target holding member 31 made of copper or the like having a high thermal conductivity, and the spiral target element 32 made of a metal such as tungsten. The width of the target element 32 is less than the diameter of the electron beam, so that the X-ray generating region can be made small irrespective of the electron beam diameter. Moreover, since the target element 32 is held by the target holding member 31 of high thermal conductivity, heat loss from the element is facilitated to diminish the emission of gases. As a result, the consumption of the target element 32 can be suppressed to prolong the lifetime of the target, to extend the replacement term and to enhance the maintenance efficiency. Besides, owing to the suppression of the emission gases, the target 3 may sometimes be permitted to be installed within vacuum. Further, since the target element 32 is formed as a spiral track, its total length can be increased to prolong the target lifetime. An embodiment of the target used in the present invention for making the X-ray generating region still smaller will be described with reference to Fig. 4. In this embodiment, in order to further reduce the diameter or width of the region, the target element 32 has the shape of a thread or filament as shown in Fig. 4(a) or is a spot having the shape of a small ball or disc carried by a thread as shown in Fig. 4(b), or the whole target has the form of a net shown in Fig. 4(c). Thus, when the target width is set at, for example, about 0.15 mm, a high spatial resolution of 0.2 mm can be achieved, even considering the degradation of the spatial resolution attributed to the degradation of the signal processing of the detector, etc. Of course, a similar result can be achieved also by a method in which the electron beam diameter is sufficiently reduced to about 0.15 mm by the charged particle converger 2. Fig. 6 is a perspective view of an industrial X-ray CT imaging apparatus. The test piece 6 to be analysed by CT scanning is mounted on a rotating turntable 5a which is in turn carried on a transversely movable carriage 5. A chuck 5b is shown locating the test piece 6. The electron beam generator and accelerator 1 and electron beam converger 2, together with the target 3 are mounted in a box carried on a carriage 11 which is movable vertically and horizontally towards and away from the test piece 6, in a frame 12. Likewise the detector 8 including a collimator is movable vertically in a frame 82, in order to retain its vertical position relative to the X-ray generator 1, 2, 3. Lead screws 13, 83 move the X-ray generator 1, 2, 3 and the detector 8 vertically in the frames 12,82. Although X-ray CT systems have thus far been described, the present invention is also applicable to other X-ray systems, e.g. transmission systems for radiography etc., wherein X-ray sensitive film is used as the detector. Furthermore, charged particle beams other than electron beams may be used to generate the X-rays, e.g. proton beams and alpha-particle beams.

Claims: An X-ray analysis apparatus having a charged particle beam generator (1,2) having a vacuum chamber and means (1) for generating a charged particle beam (24) in said vacuum chamber, an X-ray generating target (3) bombarded by the beam and a detector (8) for X-rays from said target (3), characterised in that said vacuum chamber has a vacuum-retaining exit window (23) for exit of said charged particle beam (24) and said target (3) is made of solid metal material and is located outside said vacuum chamber of said beam generator (1,2) such that the spatial resolution of the apparatus is improved by reducing the beam size at the target (32). An X-ray analysis apparatus according to claim 1 wherein said charged particle beam generator (1,2) is an electron beam generator capable of generating an electron beam having an average energy of at least 1 MeV. An X-ray analysis apparatus according to claim 1 or claim 2, for use in computer tomography, further comprising a collimator (7) located in front of said detector for collimation of X-rays from said target which are transmitted by a test-piece, scanning means (5) for rotating a said test piece relatively to said target and said detector, and data processing means for processing data in signals from said detector. An X-ray analysis apparatus according to claim 1 or claim 2 wherein said detector is an X-ray sensitive film. An X-ray analysis apparatus according to any one of claims 1 to 4 further comprising means for cooling said target. An X-ray analysis apparatus according to any one of claims 1 to 5 wherein said target (3) is displaceable relative to the beam so as to present to the beam a fresh portion of said target not previously bombarded. An X-ray analysis apparatus according to any one of claims 1 to 6 wherein an X-ray generating target element (32) of the target has a maximum width of 0.2 mm. An X-ray analysis apparatus according to any one of claims 1 to 9 wherein said target (7) comprises a matrix material in which a non-circular track (32) of x-ray generating material is embedded. An X-ray analysis apparatus according to any one of claims 1 to 7 wherein said target material is provided by a plurality of wires supported in said target. An X-ray analysis apparatus according to any one of claims 1 to 9 having means (22) for selecting the energy of said charged particle beam, interposed between said means (1) for generating said charged particle beam and said target (3). An X-ray analysis apparatus according to claim 10 wherein said beam generator (1,2) includes means for reducing the beam diameter comprising a plurality of beam-converging magnets (21) or electrostatic lenses spaced apart along the beam direction and a beam energy selecting aperture (22) between said magnets or lenses. An x-ray analysis apparatus according to any one of claims 1 to 11 wherein said beam generator (1,2) has a beam convergence coil (21) for focussing the charged particle beam onto the target (3). An X-ray analysis apparatus according to claim 1 wherein said target (3) has a first side at which said charged particle beam is incoming and opposite said first side a second side at which X-rays are emitted, said target having an element (32) made of X-ray generating material exposed at both said first side and said second side, said element (32) having a narrow elongate shape with its direction of elongation extending parallel to said first and second surfaces. An X-ray analysis apparatus according to claim 13 wherein said element of said target is at least one wire (32) of X-ray generating material. An X-ray analysis apparatus according to claim 13 wherein said element (32) of said target is a wire carrying a body of X-ray generating material.

Inventors: IZUMI SHIGERU, KAMATA SHOJI, IZUMI, SHIGERU, KAMATA, SHOJI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1991
Main classification: H01J35
IPC codes: G21K5, G21K1, H01J35, G01T1, G01N23, H04N5, A61B6