Patent Number: 062722077
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a high-resolution x-ray or gamma ray imaging apparatus 100 is exemplified in FIGS. 1-7b. In particular, FIG. 1 is a schematic diagram illustrating a view of a side of the imaging apparatus 100 lying in the x-z plane. The imaging apparatus 100 includes a substrate 102, which can be a silicon or glass substrate or any other appropriate material as described in the Background section above, a detector pixel array 103 with detector pixels 104 which are disposed on the substrate 102, and a scintillator 106. The active area of the detector pixels 104 can be any type of pixel as described in the Background section above. In this embodiment, the scintillator 106 converts x-rays or gamma rays to electron-hole pairs or visible photons. The electron hole pairs or visible photons are converted to electrical charge, current or voltage collected on the active radiation detector area of the pixel 104. In the typical digital x-ray or gamma ray detectors and visible imagers, the active area of the detector pixels 104 each measure the amount of charge collected per pixel. In general, the active area of the detector pixel 104 measures the change of electrical properties, material properties, physical properties, and so on, produced by the variation of the electromagnetic radiation intensity on the active area of the detector pixel 104. A mask or mask/antiscatter grid 108 (hereinafter "mask 108") having aperture openings 110 therein is disposed on the upper surface of the scintillator 106. Each aperture opening 110 is aligned with a corresponding active area of the detector pixel 104 as shown. For many applications, the mask 108 can be rigidly attached to the scintillator 106, or can be directly attached to the active area of the detector pixels 104. The mask 108 must be opaque enough to substantially block the penetration of the electromagnetic radiation except through the aperture openings 110. The active area of each detector pixel 104 is larger than its respective aperture opening 110, and detects the electromagnetic radiation (x-rays or gamma rays) passing through its respective aperture opening 110. As discussed below, the size of the aperture openings 110 and the number of images taken, not the detector pixel pitch, determines the image resolution. The detector shown in FIG. 1 can be used to image objects that radiate x-rays or gamma rays. For example, the detector can be used for x-ray astronomy. FIG. 2 is a schematic drawing illustrating a side view of the embodiment of the imaging apparatus 100 shown in FIG. 1 being used in an x-ray radiography application to image the interior of an object 112, which can be, for example, a human body (or a portion thereof) or any other object. An x-ray source 114 is also illustrated schematically. Also, the source 114 could be a gamma ray source, or any energy source. As shown, the object 112 to be imaged is positioned between the x-ray source 114 and the x-ray mask 108 of the imaging apparatus 100. After the x-ray source 114 emits a pulse of x-rays and the x-rays penetrate the object 112, the x-rays reach the mask 108. The mask 108 blocks all the x-rays from hitting the scintillator 106 except at the mask openings 110. The scintillator 106 can be a phosphor screen, which converts the x-rays to optical radiation, and the photodiodes on each detector 104 covert the optical radiation to electrical charge. Alternatively, the scintillator 106 can be of the type that converts the x-rays directly to charge, such as a photoconductor, photocathode, or the like. The geometry and dimensions of the active area of the detector pixels 104 and x-ray mask openings 110 are such that the x-rays passing through a single mask or mask/antiscatter grid opening 110 will strike preferably only a single detector pixel 104. Preferably, the active detector area of one pixel 104 captures the charges created by one x-ray beamlet. The charge collected per pixel is then output via data lines (see FIG. 6), and processed in a manner known in the art. The arrangement of the imaging apparatus 100 will improve the detector system MTF and increase the Nyquist frequency of even the existing best known detector pixels arrays to obtain a resolution much higher than that obtained by the same detector without a mask and without motion. The detector system MTF is the product of MTF associated with various component of the detector. Two MTF will be discussed: MTF associated with detector geometry and MTF associated with x-ray conversion. As will now be explained, the operation of the imaging apparatus 100 will improve MTF associated with the detector system geometry for detectors which perform either direct or indirect conversion of the x-rays or gamma rays as discussed above. FIGS. 3a and 3b are schematic diagrams illustrating the manner in which phosphor screens scatter the light generated by the x-rays during indirect x-ray conversion. As shown, the light scatter is proportional to the thickness of the phosphor screen. A thicker phosphor screen will provide a greater light scatter. FIG. 4 is a schematic diagrams illustrating that for direct conversion of x-rays, charge smear is minimal when the x-ray incidence angle is zero degrees, and increases as the x-ray incidence angle increases. For both of these situations, an active pixel detector area much larger than the x-ray mask aperture will reduce conversion blurring and improve conversion MTF. The active area of the detector pixels 104 and mask 108 can have a wide range of pattern or layout. For example, FIG. 5 is a schematic diagram of mask 108 of the imaging apparatus, with apertures 110 viewed in the x-y plane in FIG. 1. The apertures 110 are square or essentially square, and each have a length and width equal to d1. The area of each aperture is d1.times.d1, and the pitch of the aperture is equal to the pixel pitch D1 in both directions. The arrangement of the apertures 110 forms a uniform grid of openings in the mask 108. As discussed above, the electromagnetic radiation to be detected has to be completely blocked by the mask 108 except at apertures 110 in the mask 108. The apertures 110 are used to control the area and position at which the electromagnetic radiation hits the detector pixels. In this embodiment, the pixel pitch D1 is an integer multiple of d1. To enable the object to be 112 imaged without missing any areas and without double-exposing any areas, the imaging apparatus 100 is configured and operated so that the beamlets will each "fit" into a respective active area of the detector pixel 104 an exact number of times. In other words, D1=nd1, and n is an integer equal to or greater than 2. FIG. 5 shown an aperture arrangement where D1=2d1. FIG. 6 is a generalized schematic illustration of a top view of a possible layout of the detector pixel array 103 and the active area of the detector pixels 104 for the imaging apparatus 100 as shown in FIGS. 1 and 2. The active radiation detector areas of the pixels 104 are shown shaded with hatched lines. It is noted that the dimensions of the active area of the detector pixels 104 vary greatly from one manufacturer to another, and that the shapes of the active radiation detector areas of the pixels 104 can vary widely and are represented as squares only for illustration purposes. Row control (selection) lines 116, which are disposed on the substrate 102 (see FIGS. 1 and 2), are spaced uniformly from each other at the distance D1 as shown. Column data lines 118, which are also disposed on substrate 102, are also spaced uniformly from each other at the distance D1. Typically, data is read out one row at a time (but could be more than one row at a time) through the column data lines 118 to a processing device, such as a computer 119 or the like, as controlled by the row control lines 116. FIG. 7a is a schematic representation of the radiation beamlets 120 that pass through the apertures 110 of the mask 108 which has been superimposed over the active area of the detector pixels 104. Specifically, the electromagnetic radiation beamlets 120 are illustrated as white squares on the pixels 104, with each white square having a dimension d1.times.d1, which is equal to or essentially equal to the dimension of the aperture 110 through which the beamlet 120 has passed. In summary, as shown in FIG. 7a, the radiation beamlets 120 hit the scintillator above the active area of the detector pixels 104 with dimension d1.times.d1. The distance between the centers of adjacent apertures 110 is equal to D1, which is the pitch of the active area of the detector pixels 104. The relationship between the dimensions of each active area of the detector pixel and the dimensions of the radiation beamlets when they hit the detector pixel is D1=nd1, where n=2 in this example. Also, the x-rays are only allowed to impact the detector during the x-ray exposure time, but not during the data read out time or while the mask or detector is being moved. To assure that the entire object 112 (FIG. 2) is imaged, a conveying device 124 (see FIG. 1), such as a stepper motor, servo motor, motorized table, or any other suitable device, is configured to move the imaging apparatus 100 in a controlled manner. The imaging apparatus 100 is moved with respect to the object 112 in increments equal to d1 along the pattern shown in FIG. 7b. That is, after one exposure of the object 112 to the x-rays, a x-ray image of a respective portion of the object 112 is obtained by each pixel 104. The data produced by the pixels 104 is output through the column data lines 118. The imaging apparatus 100 is then moved in the x-y plane by a distance d1 along an arrow in FIG. 7b. This process is repeated n.sup.2 times with the imaging apparatus 100 (i.e., the detector pixels grid 103, scintillator 106 and mask 108) moved systematically in the x-y plane, for example, in the directions along arrows 126, 128, 130 and 132 for each exposure and reading, so that every part of the object 112 is imaged. After all four x-ray image patterns (n.sup.2 =4 in this example) have been obtained and stored, they are reconstructed by a processing device, such as the computer 119 or the like into a complete image representative of the entire object 112. The reconstructed image has higher resolution than any single x-ray image pattern obtained with or without the mask 106. The principle of improvement of image resolution is explained first assuming no x-ray conversion blurring and then expanded to include x-ray conversion blurring. For the fill factor of the active area of the detector is 100%, EQU MTF.sub.geometry =sin(.pi.fD)/(.pi.fD), PA0 Where MTF.sub.geometry is the MTF associated with the geometry of the detector system in one direction, D is the dimension of the pixel pitch, and f is the spatial frequency. The Nyquist frequency is 1/2D. PA0 When the linear dimension of the active area of the detector pixel is reduced to d1, for D=2d1, EQU MTF.sub.geometry =sin(.pi.f(d1))/(.pi.f(d1)), PA0 and the Nyquist frequency is still 1/2D. PA0 When the linear dimension of the active area of the detector is d1 and D=2(d1), and the detector is moved as shown in FIG. 7b and D=2(d1), then EQU MTF.sub.geometry =sin(.pi.f(d1))/ (.pi.f(d1)), PA0 and the Nyquist frequency is increased to 1/4D. This technique is used to reduce aliasing and improve image resolution for infrared cameras. The technique is called microscanning, dithering and microdithering, as described in the following publications: J. C. Gillette, T. M. Stadtmiller and R. C. Hardie, "Aliasing reduction in staring infrared imagers utilizing subpixel techniques," Optical Engineering 34, 3130-3137 (1995); R. C. Hardie, K. J. Barnard, J. G. Bognar, E. E. Armstrong and E. A. Watson, "High-resolution image reconstruction from a sequence of rotated and translated frames and its application to an infrared imaging system," Optical Engineering 37, 247-260 (1998), the entire contents of each being incorporated by reference herein. For x-ray and gamma ray imaging, there is conversion blurring. Conversion blurring can eliminate the benefits of microscan without mask and significantly reduce the signal. For example, for a TFT digital x-ray detector having an active area of the pixel with a dimension d1.times.d1, if If N number of x-rays impinges on this active area of the pixel and M number of electrons are created per x-ray, then the total number of electrons created per pixel would be MN. When there is no conversion blurring, the total number of charge collected by this pixel would be MN. Due to conversion blurring, the percentage of charge collected by this pixel decreases as the pixel dimension decreases, and the remaining charges are spread to the neighboring pixels. In the detector system of the present invention as shown, for example, in FIGS. 1-2, the aperture size of the mask determines the Nyquist frequency and the MTF associated with the pixel, while the active area of the pixel is kept large to increase the percentage of charge collected as the aperture of the mask decreases. The small aperature of the mask and large detector pixel size also improves the MTF associated with the conversion blurring, MTF.sub.conversion. The detector system MTF, MTF.sub.system, is the product of the MTF associated with the various aspects of the system, EQU MTF.sub.system =MTF.sub.geometry *MTF.sub.conversion *MTF.sub.others, Where MTF.sub.others is the MTF associated with other component of the detector system. The detector system described in FIGS. 1-2 and 5-7b with a mask and motion has a higher Nyquist frequency, larger values for the MTF within the Nyquist frequency and improve signal as compared to imaging without the mask and motion. As explained above, the detector pixel array 103 and mask 108 arrangement can have a wide variation of patterns and dimensions. For example, FIG. 8 is a schematic of a top view of a mask 134 which can be used in the imaging apparatus 100 shown in FIGS. 1 and 2 instead of mask 108. Mask 134 includes apertures 136 which are square or essentially square and have a dimension d2.times.d2, such that the pixel pitch D1 of detector pixels 104 is equal to 3(d2), (D1=3(d2)) in both directions. FIG. 9a is a schematic view showing the electromagnetic radiation that has passed through the mask 134 and has impacted on the scintillator above detector pixels 104. That is, the x-ray beamlets 138 pass through respective apertures 136 in the mask 134 and strike the center of the active radiation detection area of the respective pixel 104. To obtain an entire x-ray image of the object 112 with an imaging apparatus 100 including mask 134, the imaging apparatus 100 is moved along a pattern as shown, for example, in FIG. 9b. That is, as discussed above with regard to FIGS. 7a and 7b, after each exposure of the object 112 to x-rays and, generation of an x-ray image sub-pattern by the pixels 104, and read-out of the pixel data through column data lines 118, the imaging apparatus 100 is moved to a new location. The imaging apparatus 100 is moved sequentially each time an x-ray image is taken, and is moved in a possible pattern shown in FIG. 9b with each arrow representing one successive movement(d2=D/3). This process is repeated n.sup.2 =9 times with the detector 103, scintillator 106 and mask 134 moved in unison so that every part of the object 112 will be imaged. After all of the x-ray image sub-patterns have been obtained and stored, they are combined by a processor such as a computer or the like to provide an x-ray image representative of the entire object 112. In addition, aliasing can be further minimized and MTF improved by oversampling and applying appropriate mathematical algorithms. That is, returning to the example discussed with regard to FIGS. 7a and 7b, instead of moving the imaging apparatus 100 including detector 108 by a distance d1 between successive x-ray or gamma ray exposures, the imaging apparatus 100 is moved by a distance of (d1)/2=(D1)/2n, so the total number of sub-frames required is (2n).sup.2. The value (d1)/2=(D1)/2n. The arrows shown in the diagram of FIG. 10 suggest a possible sequence of movements for imaging apparatus 100 including detector pixels array 103, scintillator 106 and mask 108 for a detector motion of (d1)/2 between exposures, with the distance d1 being equal to one-half the pixel pitch D1 (i.e., D1/d1=2). An example of sampling variation by increasing the size of the apertures in the mask without changing the detector size or the distance between exposures is exemplified in FIGS. 11, 12a and 12b. FIG. 11 shows a mask 140 with apertures 142 each having a dimension d3.times.d3, where D1/(n-1)&gt;d3&gt;D1/n. In this example, n=2. FIG. 12a shows the spot size of the radiation beamlets 144 formed by mask 140 on the scintillator above the detector pixels 104. After each x-ray exposure and data readout operation is performed in the manner discussed above, the detector is moved a distance D1/n along the arrows shown in FIG. 12b. This process is repeated n.sup.2 times with the detector 103, scintillator 106 and mask 140 moving in unison so that every part of the object 112 is imaged. The suggested motion is similar to that of the example showing FIG. 10 to reduce aliasing. The aliasing reduction is dependent on the amount of overlapping image. It is noted that the periodicity of the detector pixel pitch need not be square. For example, as shown in FIG. 13 shows a detector pixel array 146 having the active area of the detector pixels 148 within the D1.times.0.75(D1) pixel pitch. For some applications, a rectangular area of the detector pixel layout is more effective than a layout of square detector pixels. FIG. 14 is a schematic illustration of a mask 150 having apertures 152 appropriate for the detector pixels 148 shown in FIG. 13. In this example, n=D1/d4=3. FIG. 15a is a schematic diagram illustrating the location of the radiation beamlets 154 passing through the apertures 152 of the mask 150 onto the scintillator above the detector pixels 148. Preferably, the x-ray beams that pass through each aperture 152 in the mask 150 are centered on the active radiation detection area of a respective pixel 148. After each x-ray exposure to the object 112 and data readout is performed in the manner discussed above, the imaging apparatus 100 including detector pixel array 146 and mask 150 is moved a distance (D1)/4 along the arrows shown in FIG. 15b. This process is repeated 6 times with the detector grid 146 and mask 150 moved systematically so that every part of the object 112 will be imaged. FIG. 16 is a schematic of a top view of a variation in the layout of the detector pixels for the imaging apparatus 100 shown in FIGS. 1 and 2. In the pixel array 156, the active areas for radiation detection of the pixels 158 are shown shaded with hatched lines. The shape of each pixel 158 is shown as a square for schematic purpose only. In general, the pixel shape can vary from one product to another and from one manufacturer to another. As shown, the detector pixels 158 are staggered in formation. The periodicity of the pixel is 2D1 in the horizontal direction and D1 in the vertical direction. The arrangement further includes column data lines 160, which are similar to the column data lines 118 discussed above and are spaced uniformly a distance D1 apart. Each data line will be connected to all the pixels 158 in a respective column of pixels. Control lines 162 run in a staggered zigzag pattern from left to right in this embodiment, and are spaced uniformly a distance D1 apart. FIG. 17 is a schematic illustration of the aperture layout of the mask 164 employed in the imaging apparatus 100 shown in FIGS. 1 and 2 having a detector pixel layout as shown in FIG. 16. The apertures 166 are arranged in a staggered fashion as shown, and D1/(d5)=2. FIG. 18a is a schematic illustration showing the locations at which the radiation beamlets 168 pass thought the apertures 166 overlaying the detector pixel array 156. FIG. 18b is a diagram showing an example of movement of the mask 164 and detector pixel array 156 for four x-ray exposures and returning to its original position and image readings which occur in the manner discussed above. As shown, the mask 164 and detector pixel array 156 move along the arrows by a distance d5 between each exposure and image reading. The minimum number of exposures is n.sup.2, and n=2 in this example. In general, there are many variations in direction and distance in which the detector pixel array 156 and mask 164 can be moved. For instance, D1/(d5) can be any number greater than or equal to 2, and various image data sampling algorithms can be implemented. Also, the pixel pitch does not have to be square. For example, FIG. 19 is another schematic illustration of a top view of a detector pixel array 170 which can be employed in imaging apparatus 100 shown in FIGS. 1 and 2 in place of detector pixel array 103. This figure is similar to FIG. 16, except the periodicity of the pixel detectors 172 is 3(D1) in the x direction. FIG. 20 is a schematic illustration of a mask 174 which can be employed in an imaging apparatus 100 which includes detector pixel array 170 shown in FIG. 19. The apertures 176 of the mask 174 are arranged in a staggered fashion along the x direction, and D1=3(d6). FIG. 21a is a schematic illustration of the positions at which the radiation beamlets 178 which pass through the aperture of the x-ray mask 174 overlaying the detector pixel array 170 strike the detector pixels 172 of the grid 170. FIG. 21b is a diagram of an example of the manner in which the detector pixel array 170 and mask 174 are moved for nine exposures by a distance d6 between exposures and returning to its original position. As can be appreciated from FIG. 21b, the staggered formation of the detector pixels grid 170 and mask 174 enable the entire object to be imaged by moving the grid 170 and mask 174 in one direction (i.e., the x direction), as opposed to in the x and y directions as from a non-staggered grid discussed above. Another mask variation is that the apertures are not squares. For some applications, other x-ray aperature shapes might be more appropriate. Although only several examples of masks and detector pixel array arrangements are described above, various types of mask having various apertures patterns can be used in the imaging system 100 to provide a wide variety of possible image system configurations. Also, as discussed below, the masks need not be attached to the scintillator, but rather, could be positioned at any appropriate location between the x-ray or gamma ray source and the detector pixel array. For example, FIG. 22 is a schematic illustrating an embodiment of an imaging apparatus 180 which includes a substrate 182, a detector pixel array 184 including detector pixels 186, a scintillator 188, and a mask 190 having apertures 191 therein similar to those described above. The imaging apparatus 180 can also include an antiscatter grid 192 which is disposed over the scintillator as shown. An example of an antiscatter grid is disclosed in related copending U.S. patent application Ser. No. 08/879,258, cited above. An x-ray source 194 and object 196 being imaged are also illustrated in relation to the apparatus 180. Unlike imaging apparatus 100, in this embodiment the object to be imaged 196 is positioned between x-ray mask 190 and the detector pixel array 184. As shown, the x-ray energy propagates out of a point x-ray source in a cone shape. FIG. 23a shows the mask 190 as viewed in the x-y plane. The apertures 191 are shown as having a square shape, but could have any suitable shape as discussed above for the other masks configurations. Primarily, the size and arrangement of the apertures 191 on the mask 190 should be such that they permit uniform sized and equally spaced beamlets to form on the detector pixels 186. The periodicity of the square digital detector pixels is defined to be D1.times.D1. The dimension of each x-ray beamlet as it hits the detector pixel (the "x-ray spot size") is equal to d7.times.d7, where d7&lt;D. Using Euclidean geometry, if the x-ray source 194 is considered to be a vertex of a triangle, the x-ray beamlet on the detector pixels 186 is the base of the triangle, and the distance between the x-ray source 194 and the detector pixel is L (distance measured orthogonally), then if the x-ray mask 190 is placed a distance .alpha.L from the x-ray source where .alpha. is a fraction less than 1, the dimensions of the apertures 191 in the x-ray mask 190 must be equal to .alpha.(d7).times..alpha.(d7). Also, as with the variations discussed above, the apertures of the mask and the detector pixels can vary in size and shape depending on the need. The operation of the imaging apparatus 180 will now be described. When the x-ray source 194 emits a pulse of x-ray energy which strikes the x-ray mask 190, the mask blocks all of the x-rays from striking the object except at the mask apertures 191. The x-ray beamlets which pass through the apertures of the mask penetrate the object 196 and propagate toward the antiscatter grid 192. The antiscatter grid 192 eliminates the scattered radiation, so that only the primary radiation impacts the scintillator 188. As in the imaging apparatus 100 shown in FIGS. 1 and 2, the scintillator 188 can be a phosphor screen, which converts the x-rays to optical radiation. A photodiode on each detector pixel coverts the optical radiation to electrical charge. Alternatively, the scintillator 188 can be of the type that converts the x-rays directly to electrical charge, such as photoconductor, photocathodes, and so on. The geometry and dimensions of the detector pixels 186 and x-ray mask openings 191 are such that each x-ray beamlet passing through a respective aperture in the mask and a respective aperture in the antiscatter grid 192 will strike within a single detector pixel 186. Preferably, the active detector area of one pixel 186 captures the charges created by the impacting x-ray beamlet. After each exposure, the x-ray source is turned off or x-ray shutter is closed. The charges collected by the pixels 186 are then output via data lines in a manner similar to that described above for imaging apparatus 100. For this example, n=D1/d7=2. After one exposure and data read out, the detector grid 184 (and hence the substrate 182, scintillator 188 and antiscatter grid 192) is moved a distance D1/2 for n=2 in a sequence as shown in FIG. 12b and the x-ray mask 190 is moved by a distance .alpha.d7 in the same sequence as shown in FIG. 23b while the object 196 (patient) remains stationary, to expose a different portion of the object 196. This process is repeated n.sup.2 times with the detector and mask moved in unison so that every part of the object will be imaged. After all the necessary sub-images have been output and stored, the data is processed to produce one image in a manner similar to that described above. Even though n.sup.2 exposures are taken, the tissue is exposed to the same dose of x-ray as in one exposure without the mask, because each exposure is 1/n.sup.2 the area of an exposure without the mask. The data is then reconstructed digitally to produce the high-resolution image. Variations of the embodiments for the mask and the detector grid layout are the same as those exemplified in FIGS. 8 through 21, except that each aperture of the mask is reduced in size by the factor a and the motion of the mask is reduced by the same factor. FIG. 24a is a schematic diagram illustrating that the image filtering concept can be obtained by moving the location of the x-ray source 194 without moving the mask 190. For the detector shown in FIG. 6 and D1/d7=n=2, the detector motion is shown in FIG. 12b, the corresponding x-ray source displacement is shown in FIG. 24b, where the distance between displacement is d8 and d8.apprxeq.(D1/n)(.alpha./(1-.alpha.)). The direction of motion for the source, shown in FIG. 24b, is opposite to the direction of motion for the detector, shown in FIG. 12b. The range for .alpha. is between 0 and 1, and the optimal values for a are near 0.5. The positions for the x-ray source 194 are such that every part of the object will be imaged. Variations of the embodiments for the mask and the detector grid layout are the same as those exemplified in FIGS. 8 through 21, except that each aperture is reduced in size by the factor .alpha.. Another variation of FIG. 24a is to move the location of the x-ray source 194 and the x-ray mask 190, but not move the detector 184, the scintillator 188 or the antiscatter grid 192. As discussed above, the x-ray mask 190 should be made of high atomic number materials 191 on x-ray transparent substrate 192, so that the x-rays can be substantially completely blocked with even a thin mask. The desirable thickness will dependent on the allowable transmitted x-rays and the x-ray energy. Gold is most commonly used as x-ray lithography masks. The attenuation factor of gold over the density, .mu./.rho., varies with x-ray energy. For example, at x-ray energy of 22.16 keV, .mu./.rho.=59.7 cm.sup.2 /g and at x-ray energy of 30 keV, .mu./.rho.=25.55 cm.sup.2 /g, where .rho.=19.3 g/cm3 is the density of gold. The amount of x-ray that penetrates the mask is equal to exp(-.mu.L), where L is the thickness of the mask. Typically gold masks of can produce apertures with dimensions of 75 .mu.m to 100 .mu.m and vertical walls are routinely used to block x-rays in the 5-20 keV range. The mask needs to be thicker as the x-ray energy increases. The aperture walls of the mask should ideally be slanted along the direction at which the x-rays are received. If the x-ray source is from a point, then the mask should have the configuration shown schematically in FIGS. 25a, 25b or 25c, in which the slant angles increase with distance from the center of the mask. The top layer of the mask in FIG. 25c does not have to have the same thickness as the bottom layer. On the other hand, if the x-ray source is a parallel beam, the mask should have a configuration like that shown schematically in FIG. 25d, in which the aperture walls are all substantially vertical. The photoresist used in making the x-ray mask 193 does not have to be removed if it is x-ray transparent material, as shown in FIG. 25e. This is also true for a mask focused to a point x-ray source. In an imaging apparatus 100 as shown in FIGS. 1 and 2, x-ray scatter can be reduced if the mask is thick and configured as an antiscatter grid. However, in the imaging apparatus 180 as shown in FIG. 22, x-ray scatter can be reduced even without the use of an antiscatter grid. That is, when the x-ray sensitive area .epsilon. of the detector pixels is small compared to the area associated with the detector pitch E, the scatter is reduced by approximately the ratio .epsilon./E. Alternatively, a thin mask 200 with aperture d9.times.d9 can be used in the imaging apparatus 180 in place of the antiscatter grid 192, as shown schematically in FIG. 26, to reduce x-ray scatter by the ratio of (d9/D1).sup.2. FIG. 27 is a schematic illustration of another embodiment of an imaging apparatus according to the present invention. Imaging apparatus 202 includes a substrate 204, a digital detector pixel array 206 comprising detector pixels 208, a scintillator 210, and an x-ray mask 212 having apertures d10.times.d10. However, in this embodiment, the mask is placed a distance .lambda.1 above the scintillator, and the object (not shown) to be imaged is placed above the x-ray mask 212. The mask wall thickness and the distance x can act as an antiscatter grid. Alternatively, a properly aligned double mask 214, having apertures d11.times.d11 and individual mask portions separated by an appropriate distance .lambda.2, can be used to reduce scatter as shown schematically in FIG. 28. The invention as described with regard to FIGS. 1-28 employs a detector having a detector pixel pitch that is larger than the x-ray mask opening. The following embodiment of the invention employs detectors that have small pixels to obtain high-resolution images. A schematic of a CCD is shown in FIG. 29. The pixel sizes of the CCD can have dimensions d12.times.d12, with d12 being less than 10 .mu.m. However the resolution of the conventional x-ray image is degraded by the phosphor so that the small pixels of the CCD still cannot produce high-resolution images. The concept described above is also applicable to the CCD detector. A group of the CCD can be configured together to collect data for one x-ray image pixel, where d12 is the pixel pitch of the CCD. The CCD arrays can be used in configurations shown in FIGS. 1, 2, 22, 24, 26, 27 and 28. FIG. 30 is a schematic illustration showing the pattern of x-rays which passes through the mask overlaying the active area of the detector pixels of the detector pixel array shown in FIG. 29. The example shown in FIG. 30 utilizes 3.times.3 CCD pixels to collect the information relating to x-ray intensity for one x-ray image pixel, i.e., 3(d12)=D2, and d13 is the x-ray spot size overlapping the CCD. The signal collected by each group of CCD pixels with dimension D2.times.D2 under an x-ray beamlet will be grouped together to form the signal for the x-ray beamlet. Each D2.times.D2 group of pixels is effectively a macro pixel analogous to a single pixel of D1.times.D1 as shown, for example, in FIG. 6. For illustration purposes, nine CCD pixels form a macro pixel in FIG. 30. If the CCD pixels are much smaller than D2, then slight misalignment of the CCD array with respect to the mask can be tolerated by redistributing the signal of the CCD pixel to different macro pixels using software algorithms. The amount of misalignment may be on the order d11 over a distance of tens of D2. When CCD detectors are used and d13/d12 is greater than or equal to one, only the mask, and not the detector, needs to move for configurations shown in FIGS. 1, 2, 22, 27 and 28. Neither the mask nor the detector are required to move for the configuration shown in FIG. 24a. The high-resolution x-ray imaging apparatus discussed above according to the present invention has many applications. In addition to medical applications (e.g., mammography), such imaging apparatus can be used in scientific research, defense and security environments, biotechnology, x-ray microscopy, x-ray astronomy, three-dimensional x-ray tomography and various industrial applications such as those in which non-destructive testing is required. For example, radiographic testing is used in industry in process control to detect manufacturing flaws and is increasingly integrated as a crucial component on the manufacturing floor. The trend of non-destructive testing is moving toward the use of real-time, non-film radioscopic systems over traditional film-based systems. Digital non-destructive evaluation offers all the traditional benefits of detecting microscopic flaws and providing permanent inspection records. It enables new capabilities such as computer-based inspection methods and cost reduction. The electronics and automotive industries have moved fastest to adopt radioscopy; many other industries are following this trend. The spatial filtering which is performed by the present invention to obtain high-resolution digital x-ray or gamma ray images provides several advantages. The imaging apparatus can use either direct or indirect x-ray or gamma ray conversion to generate signals representative of the image. The invention provides an improvement of the MTF beyond the limitation of the pixel pitch of the detector pixel array. Image degradation by conversion blurring caused by phosphor screens can be minimized, and image degradation by oblique x-ray incidence can be minimized, thus providing improved image resolution as well as more spatially uniform image resolution. In medical applications, the method and apparatus of the present invention also allow for x-ray detection efficiency beyond the limitation of the fill factor of the imager, without the need for increasing the x-ray or gamma ray dosage to a patient. In addition, a wide range of image resolutions can be achieved using the present invention, with digital x-ray or gamma ray images having a resolution as small as 1 .mu.m. This concept of using mask to select the resolution is independent of the dimensions. Typically, the pixel size of gamma cameras are large while the pixel size of the CCDs are typically small. The pixel size depends on the energy of the radiation to be detected, the application and availability of detectors. Similarly, the mask thickness and the aperature size depends on the application's needs, the x-ray energy and the ability to fabricate the aperture size with the appropriate mask thickness. Although only a limited number of exemplary embodiments of the invention have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the novel teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of the invention as defined in the following claims.