WO2016059830A1 - 放射線検出器、放射線撮像装置、コンピュータ断層撮影装置および放射線検出方法 - Google Patents
放射線検出器、放射線撮像装置、コンピュータ断層撮影装置および放射線検出方法 Download PDFInfo
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- G01T—MEASUREMENT OF NUCLEAR OR X-RADIATION
- G01T1/00—Measuring X-radiation, gamma radiation, corpuscular radiation, or cosmic radiation
- G01T1/29—Measurement performed on radiation beams, e.g. position or section of the beam; Measurement of spatial distribution of radiation
- G01T1/2914—Measurement of spatial distribution of radiation
- G01T1/2985—In depth localisation, e.g. using positron emitters; Tomographic imaging (longitudinal and transverse section imaging; apparatus for radiation diagnosis sequentially in different planes, steroscopic radiation diagnosis)
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- G—PHYSICS
- G01—MEASURING; TESTING
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B6/00—Apparatus or devices for radiation diagnosis; Apparatus or devices for radiation diagnosis combined with radiation therapy equipment
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- A61B6/03—Computed tomography [CT]
- A61B6/032—Transmission computed tomography [CT]
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- A—HUMAN NECESSITIES
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- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
- H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
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- H—ELECTRICITY
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- H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
- H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
- H01L31/10—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors characterised by potential barriers, e.g. phototransistors
- H01L31/115—Devices sensitive to very short wavelength, e.g. X-rays, gamma-rays or corpuscular radiation
Definitions
- the present invention relates to a radiation detector, a radiation imaging apparatus, a computed tomography apparatus, and a radiation detection method.
- Patent Documents 1 and 2 disclose detectors provided with sub-pixels having different sizes.
- the detector In the pulse mode, there is a problem that the detector is saturated when the incidence rate of radiation is high.
- the above document discloses one provided with a small square subpixel at the center of a square pixel and one provided with a small rectangular subpixel at the end of the square pixel. .
- the number of subpixels is the main factor in consideration of the fact that increasing the number of subpixels results in mounting and cost difficulties due to miniaturization and increased processing circuit density.
- the cases 2 and 3 have been studied.
- the inventors have reached the following issues.
- the smallest subpixel In order not to saturate the detector even when the incidence rate is high, the smallest subpixel needs to be sufficiently small.
- it is necessary to prevent the area difference between pixels from becoming too large.
- the size of the pixel itself In order to measure with high accuracy even when the number of incidents is low, the size of the pixel itself cannot be made too small.
- an error is likely to occur or the influence of the shadow of the collimator is increased.
- the present invention has been made in view of the above problems, and an object thereof is to provide a radiation detector capable of performing radiation measurement with high accuracy in a wide incidence rate region.
- the radiation detector of the present invention is configured by arranging a plurality of planar pixels for detecting radiation, and each of the pixels has at least two sub-areas having different effective areas.
- the sub-pixels are divided into pixels, and the sub-pixels are removed from the pixels in the descending order of effective area by an arbitrary number smaller than the number of sub-pixels that divide the pixels.
- the pixel is divided so that the center of gravity is the same as that of the pixel and the length of each side is located in the half of the similar shape region of the pixel.
- the present invention it is possible to advantageously provide a radiation detector capable of accurately performing radiation measurement in a wide incidence rate region, and further a CT using the radiation detector in terms of mounting and cost.
- the total amount of electrical signals generated during a certain time span, for example, 1 millisecond, is a measured value, and each X-ray photon is not decomposed. Therefore, for example, when one X-ray photon with an energy of 100 keV is detected and when two X-ray photons with an energy of 50 keV are detected, the same measurement result is obtained.
- the major problem is that the incidence rate of X-ray photons is extremely high.
- X-ray photons on the order of 10 9 (10 9 cps / mm 2 ) per second per square millimeter of the detector may be detected.
- the typical detector pixel size in CT is on the order of 1 mm square, for example, if the detector system takes 50 nanoseconds to process the signal of one X-ray photon.
- signal processing dozens of other X-ray photon signals are received, or two or more X-ray photon signals are mistaken as one X-ray photon signal, and signal processing is performed. (So-called pile-up). This is a state in which the detector is saturated. When the detector is saturated, X-ray photons cannot be counted correctly, and energy information cannot be obtained correctly.
- One method is a method in which one pixel is divided into a plurality of smaller subpixels, and an independent signal processing circuit is connected to each subpixel. For example, if a 1 mm square pixel is divided into 16 subpixels of 0.25 mm square, the count rate characteristic can be expected to be simply improved by 16 times. If the number of divisions is increased, the count rate characteristics per area will be improved by that amount. However, since the detector is miniaturized and the density of the signal processing circuit is increased, excessive division is accompanied by difficulty in mounting and cost. It will be.
- Patent Documents 1 and 2 disclose a method of providing subpixels having different sizes. In the region where the incidence rate is low, X-ray photons are counted in all subpixels. In a region with a high incidence rate, subpixels with a large area are saturated, but subpixels with a small area can continue to be counted, so X-ray counting is possible even with a higher incidence rate.
- Patent Document 1 and Patent Document 2 disclose a method as shown in FIGS. 1A to 1D as a state of division into sub-pixels having different areas.
- FIG. 1A shows a division method into subpixels disclosed in FIG. 5A of Patent Document 1 and FIG. 19 of Patent Document 2.
- Patent Document 1 discloses that the centroids of two subpixels may be the same. In this case, it is taught that the symmetry is good and useful for rebinning. Further, Patent Document 2 teaches that it is believed that the arrangement of small subpixels in the center is less likely to cause crosstalk between subpixels.
- FIG. 1B shows a division method into sub-pixels disclosed in FIG. Patent Document 1 discloses that the centroids of two subpixels may be different.
- the smallest subpixel 22 is located near the outer periphery of the pixel.
- the incidence rate increases, subpixels with a large area saturate and cannot be counted, and only the smallest subpixel 22 counts X-ray photons in the region with the highest incidence rate. That is, only the X-ray photons that have arrived near the outer periphery of the pixel are detected. In this case, the following problem occurs.
- the collimator shadow may be applied to the sub-pixel 22 when the production accuracy of the collimator is not ideal. Since the sub-pixel 22 having a small area is located in the periphery that is susceptible to the influence of shadows, it can be considered that the count of X-ray photons is greatly affected. In this case, image quality is also deteriorated.
- a problem occurs only by dividing a pixel into two or three subpixels.
- the size of the pixel 20 is not so large in order to accurately count even in a low incidence rate region. I can't make it smaller.
- the area of the smallest subpixel 22 in FIG. the area ratio with the largest sub-pixel 21 becomes large, and the number of X-ray photons to be counted greatly differs for each sub-pixel.
- X-ray photons are counted by the subpixels 21 and 22 at a low incidence rate, but when the incidence rate gradually increases and the maximum subpixel 21 is saturated, the area of the detector that contributes to the X-ray photon count is reduced. It decreases rapidly, the accuracy of counting rapidly deteriorates, the statistical error increases discontinuously, and adversely affects image quality. That is, in order to suppress the adverse effect on the image quality, it is necessary to make the size of one pixel sufficiently large, make the smallest subpixel sufficiently small, and prevent the area difference of each subpixel from becoming too large. For this purpose, the number of subpixels must be increased to some extent, and at least the number of subpixels is four or more.
- CT a schematic diagram of CT 100 using a plurality of radiation detectors 150 using the present invention is shown in FIG.
- the subject 200 lies on the bed 140 and is arranged near the center of the apparatus from an opening near the center of the gantry 110.
- An X-ray tube is preferably used as the X-ray source 120, and an X-ray photon 130 is emitted from the X-ray tube, and a part of the X-ray photon is absorbed by the subject 200 according to the substance distribution in the body. Some of them pass through the subject 200 and are detected by a plurality of radiation detectors 150.
- the detected X-ray photons are counted by the signal processing circuit 160 in the pulse mode.
- the counting here includes not only counting the detected X-ray photons but also acquiring energy information.
- the X-ray source 120 and the plurality of radiation detectors 150 acquire data while rotating around the subject 200.
- the speed of rotation is typically 1 to 4 revolutions per second.
- the time for accumulating data for acquiring projection data (one view) from one direction is typically on the order of 0.1 to 1 millisecond.
- a method in which the X-ray source 120 covering the entire subject 200 and the plurality of radiation detectors 150 rotate around the subject 200 as in this embodiment may be referred to as a third generation CT.
- the present invention can also be applied to other CTs.
- the plurality of radiation detectors 150 are arranged so that the pitch is shifted by 1/4 pixel with respect to the center of rotation when the detector is at a position of 0 degrees and when the detector is at a position of 180 degrees. (So-called quarter offset).
- the rotation operation of the X-ray source 120, the emission of the X-ray photon 130, the movement of the bed 140, and the like are controlled by signals from the CT controller 170.
- the control device 170 also serves to process a signal from the signal processing circuit 160 and transfer it to the computer 180.
- the computer 180 reconstructs a tomographic image based on the obtained projection data group from each direction.
- the tomographic image is finally output from the output device 191 and used for diagnosis.
- Parameters necessary for data collection such as the value of voltage applied to the X-ray tube from a high-voltage power supply (not shown), the tube current, the rotational speed of the X-ray source 120, and the like are input from the input device 192.
- the state can be confirmed from the output device 191.
- FIG. 1 A state of the plurality of radiation detectors 150 is shown in FIG.
- the pixel 20 as one unit of the radiation detector is arranged in a two-dimensional manner.
- the number of pixels is, for example, 892 in the longitudinal direction and 64 in the lateral direction.
- the pixels 20 are depicted as being approximately curvedly arranged, but generally the pixel surface is often a flat surface with no curvature, so that the detectors are arranged in a polygonal shape.
- X-ray photons 130 that have passed through the subject 200 enter each pixel 20 and are counted.
- a collimator (not shown) is disposed in front of the pixel 20 for the purpose of removing X-ray photons scattered by the subject 200.
- This collimator may be a two-dimensional rectangular collimator whose pitch and shape coincide with those of the pixels 20 or a one-dimensional slit collimator.
- FIG. 4 is a conceptual diagram when one pixel 20 is viewed from the direction in which X-ray photons are incident.
- the size of the pixel 20 is 1 mm square, and is divided into 16 sub-pixels.
- the detector material can be a scintillator (indirect radiation detection material) optically coupled with an optical device, but it is easy to microfabricate and can directly read electrical signals. It is preferable to use a direct radiation detection material such as cadmium, zinc cadmium telluride, thallium bromide, mercury iodide, and the like.
- the 16 sub-pixels have various effective areas, and those having a large effective area are arranged on the outer peripheral side of the pixel 20 and are arranged so as to fill small and medium sub-pixels in the gaps.
- the larger subpixels are arranged on the outer periphery, and the smaller subpixels are arranged on the inner periphery. This means that this arrangement is not strict, and consideration is given to filling the pixel 20 with sub-pixels without a large gap.
- FIG. 5 is a cross-sectional view of the pixel 20 when the direct radiation detection material 40 is used as the material of the detector.
- the thickness of the direct radiation detection material 40 is preferably about 0.5 mm to 3 mm.
- a common electrode 41 that covers the entire pixel is formed on the upper surface of the direct radiation detection material that is the incident surface of the X-ray photon 130.
- a voltage of ⁇ 600 V, for example, is applied to the common electrode 41 by a high voltage power source (not shown).
- sub-pixel electrodes 42 are formed on the lower surface for each sub-pixel, and further, individual channels 165 of the signal processing circuit are connected to the respective sub-pixel electrodes 42 to read out signals and to acquire energy information. Photon counting is performed.
- the X-ray photons are not attenuated in the common electrode 41 and the sub-pixel electrode 42, it is known that these electrodes are sufficiently thinner than the direct radiation detection material 40 and can have a thickness of 1 micrometer or less. ing.
- areas corresponding to the subpixel electrodes 42 form the respective subpixels 23 in the direct radiation detection material 40.
- the subpixel boundary as depicted in FIG. 4 is not physically visible when viewed from the top surface of the pixel 20.
- the radiation detector is divided into sub-pixels.
- the saturated sub-pixel data is removed, and only the non-saturated sub-pixel data is collected. Can be the output of the pixel.
- the signal processing circuit 160 can be provided with a mechanism for detecting saturation, and data output from the saturated subpixel can be suppressed in real time. In this case, since the amount of data transferred to the computer 180 is reduced, the burden of data transfer is reduced.
- the incidence rate of each pixel in each view can be predicted, and data output from a saturated sub-pixel can be suppressed.
- the incidence rate of each pixel in each view is predicted based on the height and weight of the subject 200 input from the input device 192, and data output from the saturated sub-pixel is suppressed. Can do.
- the smallest sub-pixel 22 is 0.05 mm square. Smaller subpixel sizes can accommodate higher incidence rates. However, in order to perform a desirable operation as a radiation detector, it is necessary to maintain an outer periphery-to-area ratio of a certain level or less for the following reason, and the subpixel cannot be made as small as possible.
- the detector detects an X-ray photon, high-energy primary electrons are generated in the pixel 20, and secondary carriers are generated by ionizing the surroundings while moving. The travel distance of the primary electrons is finite and can escape to the adjacent subpixel. Also, characteristic X-rays that may be generated with primary electrons can also escape to the adjacent subpixel.
- the so-called crosstalk effect becomes prominent, and the counting accuracy deteriorates. This is the reason why it is necessary to maintain a certain ratio of the outer circumference to the area.
- the effective area of the subpixel is 0.05 mm square, even if the X-ray photon has an incidence rate of 10 9 cps / mm 2 , the subpixel count rate is 2.5 ⁇ 10 6 cps. If the signal processing circuit has a response time of 50 nanoseconds and is multiplied by the detection efficiency (for example, 99%), counting can be performed without saturation.
- the radiation detector of the present embodiment has variations in the size of the subpixels and effectively arranges them, so that it can cope with a high incidence rate while suppressing the number of subpixel divisions to 16. It has become.
- the shape that gives the minimum perimeter pair area is a circle, but it is impossible to fill the pixel with circular sub-pixels. Therefore, the shape of the subpixel is desirably a square or a rectangle having an aspect ratio close to 1, and preferably has a substantial shape from the same viewpoint.
- the solid shape means a shape having no hollow portion or concave portion such as a donut shape.
- all of the 16 subpixels are square or rectangular and have a solid shape, and the aspect ratio is 0.5 or more and 2 or less.
- the movement locus 52 of the center of gravity of the subpixels contributing to data acquisition is as shown in FIG. 7B.
- the center of gravity of the sub-pixel that is not saturated and contributes to data acquisition remains in the region 30 near the center of the pixel 20. This is partly because the outer periphery of the pixel 20 is occupied by six subpixels from the largest, and the ten subpixels from the smallest do not share sides with the pixel 20. Since the center of gravity of the sub-pixel that contributes to data acquisition is near the center, it is possible to acquire more representative values in the pixel as transmission data for X-ray photons than when it is near the outer periphery. There is also a positive effect from the viewpoint of spatial resolution and artifact reduction.
- the centroid of the subpixel contributing to data acquisition is preferably closer to the centroid of the pixel, but considering that the effective sampling density is doubled by the quarter offset method, the subpixel contributing to data acquisition It is desirable that the center of gravity always stays within a similar shape region that is at least as large as the original pixel and has a size (the length of each side) that is half that of the original pixel.
- the position of the center of gravity of the sub-pixel that contributes to data acquisition is known, rebinning and correction processing in the tomographic image reconstruction processing are performed in consideration of this.
- the division is performed so that sampling is not performed such that the reconstruction process is limited due to the difference in the centroid position of the subpixels.
- the center of gravity of the sub-pixels contributing to data acquisition is near the center of the pixel, the sub-pixels are less likely to be a shadow of the collimator, and the image quality is less likely to be adversely affected.
- Statistic error is given as a typical factor that determines the accuracy of X-ray photon counting.
- the statistical error is 10% obtained by dividing 10 which is the square root of 100 by the original 100.
- the saturation of a certain subpixel causes the total area of the subpixels contributing to data acquisition to decrease discontinuously, and the statistical error increases discontinuously. From the viewpoint of image quality, it can be said that sub-pixel division is desirable so that the statistical error does not vary greatly depending on the incidence rate.
- the i + 1th smallest subpixel is saturated at a certain incidence rate, and only i subpixels contribute to data acquisition, and the incidence rate is further increased to increase the ith smallest subpixel. It is only necessary that the statistical error of the entire pixel, that is, the count or the count rate is the same in a situation where the pixel is saturated and only i ⁇ 1 subpixels contribute to data acquisition. However, i> k + 1.
- Equation (2) n .
- ⁇ can be obtained using Equation (3) for an arbitrary k.
- FIG. 8 shows a schematic diagram of the ratio of the standard deviation of the count with respect to the incidence rate for the radiation detector in the present embodiment shown in FIG.
- the incidence rate increases, there are a plurality of points where the standard deviation increases discontinuously. These are discontinuous points due to the saturation of the subpixels.
- the increase in standard deviation at these discontinuities is suppressed and a substantially constant behavior is realized in a wide incidence rate region.
- Patent Document 1 the ratio of the area when dividing into four or more sub-pixels is taught that “it can be in the range of about 1: 4: 8 to about 2: 4: 8”. Yes.
- the sum of the effective area of subpixels may not exactly match the geometric area of the pixel.
- the effective area of the subpixel is a value obtained by dividing the count rate of the subpixel by the incidence rate of X-ray photons per unit area and the detection efficiency.
- there is a gap serving as a sub-pixel boundary between the sub-pixel electrodes 42 and there is a possibility that X-ray photons incident thereon may not be counted correctly.
- a factor is that X-ray photons incident near the edge of the pixel may not be counted correctly due to the crosstalk effect.
- the geometric area of the subpixel electrode may not exactly match the effective area of the subpixel, but the effect is small and does not affect the gist of the present embodiment.
- detection of X-ray photons has been described.
- the present invention can also be applied to gamma-ray photons, ultraviolet photons, and charged particle beam detectors.
- application examples to whole body CT have been described, but for dental CT, CT for non-human subjects, imaging devices using X-rays for homeland security, nuclear medicine diagnostic devices such as SPECT / PET, etc. It can also be applied to.
- the sub-pixel division is performed by providing the common electrode on the upper surface of the direct radiation detection material and the sub-pixel electrode on the lower surface, but the common electrode is not provided, and the upper surface is also provided for each sub-pixel. May be provided.
- adjacent radiation detector pixels 20 may share a common electrode on the upper surface, or may have electrodes individually.
- a detector material not a direct radiation detection material but a scintillator (indirect radiation detection material) optically coupled to an optical device can be used.
- a scintillator whose periphery is covered with a light shielding agent may be provided for each subpixel, or a method of generating a microcrack by a laser between subpixels for one scintillator.
- Sub-pixel division may be performed.
- a photomultiplier tube (PMT), a photodiode (PD), an avalanche photodiode (APD), a silicon photomultiplier tube (SiPM), or the like can be used.
- the signal from each subpixel is processed by the individual channel 165 of the signal processing circuit.
- the signals from a plurality of subpixels can be connected to the channel of one signal processing circuit.
- a simple switch may be provided.
- multiple subpixels can be effectively integrated into one large subpixel, and the influence of crosstalk can be suppressed in a region where the incidence rate is low, and the number of channels of the signal processing circuit to be used Can be reduced.
- the radiation detector of this embodiment can be arranged in a plurality of layers in a direction parallel to the incident direction of radiation. Thereby, the radiation which permeate
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Abstract
Description
一般に、X線を利用した医療用の診断装置、例えばコンピュータ断層撮影装置(CT)では、X線源から発生させたX線を被検体に照射し、このX線を放射線検出器で検出する。これにより、X線が被検体によってどのくらいの減衰を受けたかの情報を得ることで、被検体の体内の様子を画像化し、診断に供する。
実施例として、本発明を利用した複数の放射線検出器150を用いたCT100の模式図を図2に示す。被検体200は、寝台140の上に横たわり、ガントリ110の中央付近にある開口部より装置中央付近に配置される。X線源120としてはX線管を用いるのが好適であり、X線管からはX線フォトン130が放出され、これの一部は被検体200によって体内の物質分布に応じて吸収され、また一部は被検体200を透過して複数の放射線検出器150で検出される。検出されたX線フォトンは、パルスモードで信号処理回路160によって計数される。なお、ここでいう計数とは、検出したX線フォトンを数えることに加え、エネルギー情報を取得することも含んでいる。
複数の放射線検出器150の様子を図3に示す。放射線検出器の一単位であるピクセル20が2次元状に配置された構成となっている。ピクセルの数は、例えば長手方向に892、短手方向に64である。図3ではピクセル20が近似的に曲面状に配置されているように描写されているが、一般にはピクセル表面は曲率の無い平面であることが多いので、検出器の配置は多角形状になることもある。それぞれのピクセル20には、被検体200を透過したX線フォトン130が入射し、計数される。なお、被検体200で散乱されたX線フォトンを除去する目的で、ピクセル20の手前にはコリメータ(図示せず)が配置されている。このコリメータは、ピクセル20とピッチおよび形状が一致するような二次元矩形コリメータであっても良いし、一次元のスリットコリメータであっても良い。
X線フォトンの入射率が低い領域では、全てのサブピクセルは飽和していないため、入射するX線フォトンを正しく計数することができ、データ取得に寄与できる。入射率が高くなっていくと、最初に最大のサブピクセル21が飽和し、X線フォトンを正しく計数できなくなる。この場合、最大のサブピクセル21以外の15のサブピクセルは飽和していないので、これらの計数データを用いて正しくX線フォトンの計数を実施することができる。さらに入射率が高くなると、2番目に大きなサブピクセルが飽和するので、それ以外の14のサブピクセルの計数データを用いてX線フォトンを計数する。このようにして、入射率に応じて飽和していないサブピクセルのみを利用することとし、最も入射率が高い領域では、飽和していない最小のサブピクセル22のみを用いてX線フォトンの計数を実施する。断層画像の再構成処理の際には、どのサブピクセルがデータ取得に寄与していたかを考慮することでピクセルの大きさの補正処理を実施する。
図4で、最小のサブピクセル22は0.05ミリ四方としている。サブピクセルのサイズが細かい方がより高い入射率まで対応できる。ただし、放射線検出器として望ましい動作をするためには、以下の理由から一定以下の外周対面積比を保つ必要があり、サブピクセルをいくらでも小さくできるわけではない。検出器がX線フォトンを検出すると、ピクセル20内では高エネルギーの一次電子が生成され、これが移動しながら周囲を電離させることで二次キャリアが生成される。この一次電子の移動距離は有限であり、隣のサブピクセルにエスケープし得る。また、一次電子と共に発生することのある特性X線も、やはり隣のサブピクセルにエスケープし得る。これらのことから、いわゆるクロストーク効果が顕著になってしまい、計数の精度が劣化する。これが一定以下の外周対面積比を保つことが必要な理由である。サブピクセルの実効面積が0.05ミリ四方の場合、X線フォトンが109cps/mm2の入射率であったとしても、サブピクセルの計数率は2.5×106cpsに検出器の検出効率(例えば99%)を乗じたものとなり、応答時間が50ナノ秒の信号処理回路であれば飽和せずに計数が可能である。
本発明の各実施例では、飽和していないサブピクセルの計数データのみを使用することから、ピクセル内でデータ取得に寄与する領域が入射率に応じて変化し得る。入射率が低い領域では、全てのサブピクセルは飽和せず正しくX線フォトンを計数することから、ピクセル全体がデータ取得に寄与しており、基本的には飽和していないサブピクセルの重心は図7(a)に示すようにピクセルの重心50と一致する。入射率が高くなると、最大のサブピクセル21が飽和し、データ取得に寄与しなくなる。この場合、図7(a)ではピクセルの右下の領域が機能しなくなることから、データ取得に寄与しているサブピクセルの重心51はピクセルの重心50から左上に移動することになる。
N個に分割したサブピクセルを小さい順に並べ、それぞれの実効面積をa1、a2、・・・、aNとした場合に、これらをどのような値に設定しているかについて述べる。最小のサブピクセルをk個設けるとすると、a1=a2=・・・=ak<ak+1<・・・<aNである。ここで、前述したようにサブピクセルの大きさは3種類では不十分であることから、N≧4、1≦k≦N-3である。なお、本実施例はN=16、k=3の場合に該当する。
(1+α)=4/α:4:4(1+α)であり、4/αを約1~2の範囲に収めるにはαは約2~4であることが必要だが、この場合は4(1+α)が約12~20となって特許文献1に開示された8を大きく超える。すなわち、特許文献1に開示された範囲は本実施例と異なっており、特許文献1で開示された思想が、本実施例とは異なっていることが分かる。
以上、好適な実施例について述べてきたが、本発明の要旨を逸脱しない範囲で種々の変更・追加を考えることができる。
21 最大のサブピクセル
22 最小のサブピクセル
23 サブピクセル
30 ピクセルの中心付近の領域
40 直接型放射線検出素材
41 共通電極
42 サブピクセル電極
50 ピクセルの重心
51 最大のサブピクセルが飽和した際のデータ取得に寄与しているサブピクセルの重心
52 実効面積の大きなサブピクセルが順に飽和した際のデータ取得に寄与しているサブピクセルの重心の移動軌跡
100 コンピュータ断層撮影装置(CT)
110 ガントリ
120 X線源
130 X線フォトン
140 寝台
150 複数の放射線検出器
160 信号処理回路
165 信号処理回路の個別のチャンネル
170 制御装置
180 コンピュータ
190 インターフェース
191 出力装置
192 入力装置
200 被検体
Claims (12)
- 放射線を検出する平板状のピクセルが複数配置されて構成され、
前記ピクセルは、それぞれ少なくとも2つが異なる実効面積を持つ4つ以上のサブピクセルに分割され、
前記サブピクセルは、前記ピクセルから実効面積の大きい順に、前記ピクセルを分割するサブピクセルの数よりも小さい任意の数だけ除去されても、残ったサブピクセルの全体の実効面積の重心が、前記ピクセルと重心が同じで各辺の長さが前記ピクセルの半分の相似形領域の内部に位置するように、前記ピクセルを分割したものであることを特徴とする放射線検出器。 - 前記サブピクセルは充実した形状であることを特徴とする請求項1に記載の放射線検出器。
- 前記サブピクセルが、正方形またはアスペクト比が0.5以上2以下の長方形であることを特徴とする請求項2に記載の放射線検出器。
- 前記ピクセルにおいて、最小の実効面積を持つサブピクセルが、前記ピクセルと辺を共有しない位置に設けられていることを特徴とする請求項1に記載の放射線検出器。
- 前記ピクセルは直接型放射線検出素材からなることを特徴とする請求項1に記載の放射線検出器。
- 前記ピクセルには、最小の実効面積を持つサブピクセルが複数設けられていることを特徴とする請求項1に記載の放射線検出器。
- 前記ピクセルにおいて、同一の実効面積を持つサブピクセルが前記ピクセル内の離れた位置に配置されていることを特徴とする請求項1に記載の放射線検出器。
- 前記サブピクセルはパルスモードで動作することを特徴とする請求項1に記載の放射線検出器。
- 前記サブピクセルに信号処理チャンネルが接続された請求項1に記載の放射線検出器を備え、前記信号処理チャンネルから画像構成のための信号を出力すること特徴とする放射線撮像装置。
- 請求項1に記載の放射線検出器を用いて構成されることを特徴とするコンピュータ断層撮影装置(CT)。
- 請求項1に記載の放射線検出器によって検出される放射線検出データを処理するコンピュータが、
前記ピクセルを構成するサブピクセルのうちその一部のサブピクセルが飽和した場合、前記飽和したサブピクセル以外のサブピクセルによって検出される放射線検出データを取得し、
前記放射線検出データがいずれのサブピクセルから取得されたかにより前記取得した放射線検出データを補正すること
を特徴とする放射線検出方法。
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