JP2013057554A - Radiation detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a radiation detector which detects radiation with a high dose and concurrently collects energy information, accurately acquiring the energy information.SOLUTION: A plurality of detection elements 11,..., 13 which generate electric charges due to energy imparted by incident radiation are arranged on an incident line of radiation at positions with a different distance to each other from the incident end. Absorbers 14 are placed at one or more points between the detection elements in the row of the detection elements. All the detection elements have the same structure using the same detection medium. Three to six detection elements are arranged in a row. Examples of the absorber for use include tin.

Description

  The present invention relates to a radiation detector having a structure in which a plurality of detection elements are arranged along the incident direction of radiation. More specifically, the present invention relates to radiation having a high count rate by attaching an absorber to a part of the detection elements. It is related with the radiation detector which enabled it to perform simultaneously detection of this and collection of highly accurate energy information. This technique is useful, for example, for radiation transmission imaging.

  As is well known, X-rays and gamma rays, which are electromagnetic radiation (hereinafter simply referred to as “radiation”), have a high transmission capability, so that they can be diagnosed in the medical field, non-destructive inspection and crystal structure analysis in the industrial field. Widely used. In recent years, as a radiation transmission imaging method using the radiation transmission capability, instead of X-ray imaging using a photosensitive action on a photographic plate, the radiation excitation action on a detection medium is electrically detected, and the digital result is obtained based on the detection result. Image acquisition methods have become mainstream. When radiation energy is applied to the inside of the radiation detector, depending on the type of detection medium, for example, an electron / hole pair for a semiconductor, an electron / ion pair for a gas, and a fluorescence / superconductivity for a scintillator If it is a body, excitons such as quasiparticles are generated. In the radiation detector, a voltage proportional to the applied energy is induced by the movement of these excitons to the electrode, whereby the energy of the radiation transmitted through the human body or the like can be measured.

  However, since such a radiation detector measures the induced voltage every time the radiation is incident, when the next radiation is incident while the exciton is moving to the electrode, There is a risk of recognizing it as radiation. As a result, the dose of radiation incident per unit time (hereinafter also simply referred to as “dose”) is limited, and it is extremely high in a short time as in a radiation transmission imaging method (such as an X-ray CT scan). When measuring a dose of radiation, there is a problem that the energy information of the radiation cannot be used. On the other hand, when the dose is limited to a low dose, a radiographic image cannot be obtained in a short time. For example, in a medical X-ray CT scan or the like, a sharp image of a fast-moving organ such as the heart is obtained. It was extremely difficult to obtain. Therefore, the radiation transmission imaging apparatus employs a method of measuring the amount of X-rays transmitted per unit time through the subject by detecting the current flowing through the electrodes without performing X-ray energy measurement. (See Patent Document 1).

  On the other hand, when an image for determining the presence or absence of cancer tissue is obtained using an iodine contrast agent, only a small part of the irradiated X-rays are absorbed by iodine, so it is necessary to irradiate a large amount of X-rays on the human body. There was a problem of increasing the risk of radiation exposure. In view of this, a low-exposure type X-ray transmission imaging apparatus has been developed that employs an energy difference method that utilizes the difference in X-ray absorption between the human body and iodine. In this type of X-ray transmission imaging device, a device that irradiates white X-rays by sandwiching a metal plate such as copper between two X-ray films, or a device that uses two types of monochromatic X-rays, is common. In these, X-rays are measured as current. In addition, using a method for measuring the energy of each X-ray, a human body is irradiated with a filter X-ray whose high energy portion is cut by a lanthanum (La) filter. A technique has also been proposed in which the number of X-rays in the upper and lower energy ranges sandwiching the K absorption edge is measured, and two types of energy information obtained thereby are subtracted to obtain an image in which only the contrast agent is emphasized ( Patent Document 2).

  However, the conventional radiation detector as described above can cope with a high dose, but information on energy given from individual radiation is lost. Only information on whether the amount of X-rays that have passed through a certain tissue is large or small can be obtained. Therefore, it has been difficult to apply such a radiation detector to the energy difference method as shown in Patent Document 2.

  In order to solve such a problem, the detection medium that generates electric charge by the energy applied from the incident radiation and the detection medium at a position where the distance from the incident end of the radiation is different from each other are installed on the detection medium. A radiation detector (see Patent Document 3) including a plurality of electrodes, and a radiation detector in which a plurality of detection elements including CsI (Tl) scintillators and photodiodes are arranged along the incident direction of radiation (Non-Patent Document) 1), or a radiation detector (see Non-Patent Document 2) in which a plurality of detection elements made of Si (Li) semiconductor detectors are arranged along the radiation incident direction.

  Although these radiation detectors are arranged in the same manner, the configuration is simplified and the detection of high doses of radiation and the collection of energy information can be performed simultaneously. Since the response characteristics with respect to the energy of the incident radiation are similar, there is a problem that a large amount of calculation is required to obtain the energy information with high accuracy. Further, in order to accurately obtain the iodine thickness on the X-ray passing line as the analysis result, it is necessary to prepare a large number of initial estimated X-ray energy distributions and input them to the unfolding code. Furthermore, it is necessary to use the response function obtained for the distance that the X-rays have passed through the subject for analysis. For this reason, the X-rays are detected at each measurement point using the CT image reconstructed from the measured current values. It was necessary to calculate the distance that passed through the subject.

JP-A-2005-77152 JP 2004-223158 A JP 2007-71602 A

I. Kanno, et al., "A Current-Mode Detector for Unfolding X-ray Energy Distribution", Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 45, No. 11, p.1165-1170 (2008). R. Imamura, et al., "Unfolding Method with X-ray Path Length-Dependent Response Functions for Computed Tomography Using X-ray Energy Information", Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol.47, No.11, p.1075 -1082 (2010).

  The problem to be solved by the present invention is to enable detection of a high dose of radiation and collection of radiation energy information at the same time, and to obtain energy information with high accuracy. Another problem to be solved by the present invention is to provide a radiation detector that can be manufactured at low cost without complicating the structure.

  According to the present invention, a plurality of detection elements that generate charges by energy applied from incident radiation are arranged on the radiation incident line at positions different from each other from the incident end, and the detection elements in the row of the detection elements The radiation detector is characterized in that an absorber is installed at one or more locations in between.

  The plurality of detection elements all have the same structure using the same detection medium, and it is preferable that 3 to 6 of them are arranged in a line. As the absorber, a material containing an element from Al of atomic number 13 to Bi of atomic number 83 (excluding Tc of atomic number 43 and Pm of atomic number 61) is used.

  As a typical example, there is a radiation detector for measuring X-rays and measuring an iodine contrast medium in a subject from X-ray energy information. In this case, all the detection elements are the same. The same structure using a detection medium, three of which are arranged in a line on the incident line of radiation, the absorber is made of tin, and the absorber is a detection element in the middle of the row of detection elements In addition, there is a configuration that is arranged on the radiation incident end side of the last detection element.

  Here, the detection medium of the detection element is made of a semiconductor, a scintillator, a gas, a superconductor, or an insulator.

  According to the radiation detector of the present invention, it is possible to detect a high dose of radiation and collect energy information of the radiation at the same time, and further, energy information can be obtained without requiring much calculation by arranging the absorber. Can be obtained with high accuracy. In addition, since detection elements having the same structure using the same detection medium can be used, the configuration is simplified and the detection element can be manufactured at low cost.

1 is a schematic configuration diagram of an X-ray transmission imaging apparatus using a radiation detector according to the present invention. Schematic which shows one Example of the radiation detector which concerns on this invention. The graph which shows an example of an X-ray energy spectrum. Explanatory drawing which shows the usefulness of this invention. Schematic which shows the measurement form by a radiation detector. A cross-sectional profile of a subject obtained by a radiation detector. Graph of _{I 2 / I 1 -I 3 /} I 1 in the case of observing the iodine contrast agents. Graph of _{I 2 / I 1 -I 3 /} I 1 in the case of observing the barium contrast agent. Graph of _{I 2 / I 1 -I 3 /} I 1 in the case of observing a gold contrast medium.

  In the radiation detector according to the present invention, a plurality of detection elements that generate charges by energy applied from incident radiation are opposed to the radiation source at positions where the distance from the incident end is different from each other on the radiation incident line. Are arranged in a line, and an absorber is installed at one or more positions between the detection elements in the detection element line. All the detection elements have the same structure using the same detection medium, and 3 to 6 of them are arranged.

  When detecting an iodine contrast medium in a subject, for example, tin is used as an absorber, and this tin absorber is disposed in front of one detection element. Typically, three detection elements having the same structure, all using the same detection medium, are arranged in a line on the radiation incident line, and the tin absorber is disposed on the radiation of the last detection element in the detection element line. It is arranged on the incident end side. When the same plurality of detection elements are used, the respective response functions are similar to each other. The slight difference in the response function is due to the change in the energy spectrum of the X-ray due to the X-ray being absorbed by the detection element in front of a certain detection element. If only the same detection element is used, the change in the response function is slight. However, if the absorber is placed in front of one detection element, the energy spectrum of the X-ray changes greatly, and the detection element immediately after the absorber The response function is greatly different from the response function of the detection element in front of the absorber. Therefore, it becomes possible to acquire energy information with high accuracy.

  The position where the absorber is installed may be other than the front of the leading detection element. This is because if the absorber is installed in front of the first detection element, the response functions of all three detection elements are the same, and the meaning of the absorber installation is lost. The absorber may be installed not only in front of the last detection element but also in front of an intermediate detection element. In that case, the response functions of the first detection element and the response functions of the middle and last detection elements are different. Further, the absorber can be installed both in front of the last detection element and in front of the intermediate detection element. When a plurality of absorbers are installed, it is possible to change the material of the absorber.

  The absorber which can be used in the present invention is a material containing an element from Al of atomic number 13 to Bi of atomic number 83. Although it is conceivable to use a material made of an element having an atomic number smaller than that of aluminum, in that case, the thickness of the absorber increases, which is not practical. In addition, a material containing Tc of atomic number 43, Pm of atomic number 61, and an element of atomic number 84 or more is not suitable as an absorber used in the present invention because it has a radioactive isotope.

  When measuring an iodine contrast medium in a subject, tin is typically used as an absorber material, but is not limited to tin, and aluminum, iron, iodine, bismuth, tantalum, etc. may be used. it can. For example, when tin is used as the absorber, it is typically better to use tin foil (metal foil). However, not only metal foil, but also oxide powder filled in a thin container made of acrylic resin is used. Is possible.

  In addition, for example, when measuring a barium contrast agent in a subject, aluminum, tin, silver, bismuth, or the like is preferable as an absorber. When measuring a gold contrast agent in a subject, aluminum, gold, tantalum, bismuth, tungsten, or the like is preferable as an absorber.

  The detection medium may be a semiconductor, a scintillator, a gas, a superconductor, or an insulator. The number of detection elements is not particularly limited as long as it is plural. However, since the energy information can be obtained more accurately as the number of detection elements is larger, it is preferable to set the number to three or more. On the other hand, if the number of detection elements is excessively large, the cost for manufacturing the radiation detector becomes high.

  In the following examples, a case where a CT image indicating the thickness of an iodine contrast agent is obtained using a transmission X-ray image will be specifically described. However, the radiation detector of the present invention is not limited to such an application. Needless to say, the present invention can be similarly applied to various radiation inspection apparatuses using gamma rays or the like. FIG. 1 is a schematic configuration diagram of an X-ray inspection apparatus using a radiation detector according to the present invention, and FIG. 2 is a schematic diagram showing an embodiment of the radiation detector.

<Configuration of X-ray inspection apparatus and radiation detector>
As shown in FIG. 1, an X-ray examination apparatus 1 includes an X-ray tube 2 that irradiates X-rays toward a subject S, and a number of radiation detectors (X-ray detection) that detect X-rays transmitted through the subject S. Detector) 3 arranged vertically and horizontally, amplifier 5 for amplifying the output from each radiation detector, current reading device 6 for reading each output current, contrast agent thickness calculating device 7, image And the like.

  The X-ray irradiated from the X-ray tube 2 is not particularly limited. For example, an electron accelerated to 120 kVp collides with a tungsten target, and a high-energy portion (38.9 keV or more) is emitted from the emitted white X-ray by a La filter. A filter X-ray or the like obtained by removing is preferable. When X-rays are irradiated from the X-ray tube 2 toward the subject S, X-rays that have passed through the subject S enter the radiation detector 3 in the radiation detector array 4.

  Here, as shown in FIG. 2, the radiation detector 3 includes three detection elements 11, 12, and 13 that generate charges by energy applied from incident radiation, arranged in order along the incident direction of X-rays. This is a structure in which an absorber 14 made of tin foil is disposed closer to the radiation incident end side than the detection element 13 located at the end. In this example, Si (Li) semiconductor is used as the detection medium of each detection element, but other types of semiconductors such as CdTe can be used, and detection media of general radiation detectors such as various scintillators are used. You can also The current output from each detection element 11,..., 13 is sent to the number of amplifiers 5 corresponding to each detection element 11,.

When X-rays enter the radiation detector 3, the first detection element 11 to the third detection element 13 output currents I _{1 to} I ₃ , respectively, according to energy applied from the incident X-rays. The detection currents I _{1 to} I ₃ are amplified by the amplifier 5, measured by the current reading device 6, and output to the contrast agent thickness calculation device 7. In the contrast agent thickness calculation device 7, the thickness of the iodine contrast agent in the subject S is calculated, and the imaging device 8 generates a transmission X-ray image based on the calculation result.

When X-rays are irradiated to the subject S in which the injected iodine contrast medium stays in a lesion or blood vessel such as cancer, as shown in FIG. 3, the energy level (33.2 keV) around the K absorption edge of iodine is found A discontinuous X-ray energy spectrum is obtained. FIG. 3 is a graph of an X-ray energy spectrum obtained by irradiating filter X-rays on a 10 cm thick water layer imitating a human body and containing iodine imitating an iodine contrast agent. shows three energy range E ₁ to E ₃ lines and the number Y ₁ to Y ₃ of X-rays contained in these energy range E ₁ to E _3.

<Structure and characteristic evaluation of prototype radiation detector>
The prototype radiation detector uses three Si (Li) detection elements. As shown in FIG. 2, a tin absorber having a thickness of 58 μm was installed between the second and third Si (Li) detection elements of this radiation detector. The case where this tin absorber was installed was compared with the case where it was not installed, and the effect of the tin absorber was determined. The thickness of the tin absorber is not limited to 58 μm and may be changed as appropriate.

The characteristics of this radiation detector were evaluated as follows. Before the radiation detector of FIG. 2, acrylic and iodine having a known thickness are arranged by changing from 7 mm to 47 mm every 10 mm, and iodine from 0 μm to 60 μm every 15 μm. The current measurement was performed. Here, the acrylic resin is used to simulate soft tissue or soft tissue and bone. FIG. 4 shows a graph in which the current values obtained by the detection elements 11 to 13 are I _{1 to} I ₃ , I ₂ / I ₁ is taken on the x axis, and I ₃ / I ₁ is taken on the y axis. When there was no tin absorber (a), the graph of I ₂ / I ₁ -I ₃ / I ₁ was distributed on one line with the acrylic thickness and iodine thickness being aggregated. On the other hand, when there is a tin absorber (b), the graph of I ₂ / I ₁ -I ₃ / I ₁ is decomposed for each acrylic thickness, and according to two parameters of iodine thickness-acrylic thickness. A two-dimensional map was obtained.

Next, as shown in FIG. 5, an X-ray source, a subject, and a radiation detector are installed, irradiated with X-rays, and each X-ray that has passed through the subject is incident on the radiation detector. The current generated from the element (detection element numbers 1 to 3) was measured. The subject is a cylinder made of acrylic resin, and a hole provided in the center of the cylinder is filled with an iodine solution. By marking the current value ratios I ₂ / I ₁ and I ₃ / I ₁ at each measurement point (x, θ) on the graph of I ₂ / I ₁ −I ₃ / I ₁ , the measurement point ( In x, θ), the acrylic thickness and iodine thickness on the X-ray passing line can be obtained. On the other hand, since the average absorption coefficient of acrylic resin and iodine is known in the literature, etc., the information on the acrylic thickness and iodine thickness obtained as described above can be used without using the conventional unfolding method. The number of X-rays Y _{1 to} Y ₃ can be directly and accurately obtained using Equation 1 below.

FIG. 6 shows a cross-sectional profile of CT values obtained by reconstructing a CT image using Y ₂ among Y ₁ to Y ₃ thus obtained. The horizontal axis in FIG. 6 indicates the position where the center of the subject is 0 mm and transmits X-rays, and the vertical axis indicates the CT value. For reference, a CT value using a current value, which is a conventional method, is shown as a negative value for easy viewing. The result obtained using the radiation detector according to the present invention shows the same CT value as the theoretical value, and a CT value with a larger iodine than the current CT value which is the conventional method is obtained. On the other hand, when the tin absorber is not used, the CT value becomes small and is about the same as the current CT value.

  By the way, when calcium is deposited in a blood vessel, conventional current measurement CT may not be distinguished from an iodine contrast agent. Therefore, in conventional current measurement CT, the X-ray tube voltage is changed and irradiation is performed twice. For example, energy information is obtained by performing two X-ray irradiations of 70 kVp and 140 kVp, and whether it is iodine or calcium is distinguished from this.

  However, this method has a problem in that the exposure dose increases because the X-ray irradiation is performed twice. On the other hand, if measurement is performed with the radiation detector according to the present invention, it is possible to distinguish between iodine and calcium by making a CT image using the number of X-rays in several energy ranges in one irradiation. That is, in the case of calcium, the K absorption edge is at a low energy, so at 20 keV or more, the lower the energy, the larger the absorption, and the higher the X-ray energy, the less the absorption. On the other hand, since iodine has a K absorption edge at 33.2 keV, the absorption of X-rays with energy higher than 33.2 keV is increased. Therefore, as the X-ray energy increases, the CT value monotonously decreases with calcium, and with the energy higher than 33.2 keV, the CT value increases with iodine. Therefore, using the radiation detector of the present invention, a CT image using the number of X-rays in a plurality of energy ranges is formed by a single X-ray irradiation measurement, and the pattern of change in CT value with respect to X-ray energy is used. X-ray examination can be performed to distinguish calcium and iodine in the subject.

In the above embodiment, an example using a tin absorber mainly for observing an iodine contrast agent has been described. However, the absorber is not limited to tin, and aluminum, iron, and the like having an atomic number smaller than iodine are possible, and tantalum having an atomic number larger than iodine can also be used. As an example, graphs of I ₂ / I ₁ -I ₃ / I ₁ using aluminum (Al), iodine (I), and bismuth (Bi) absorbers are shown in FIGS.

Similarly, in the case of the barium contrast agent, a graph of I ₂ / I ₁ -I ₃ / I ₁ using aluminum (Al), silver (Ag), and bismuth (Bi) absorbers is shown in FIG. Shown in (c).

When observing a gold contrast agent, an Al filter having a thickness of 3 cm is installed at the exit of the X-ray tube, and the peak energy of the X-ray energy spectrum incident on the subject and the radiation detector is at the gold K absorption edge. Make the value higher than the energy value. A graph of I ₂ / I ₁ -I ₃ / I ₁ when aluminum (Al), gold (Au), and bismuth (Bi) absorbers are installed in front of the third detection element is shown in FIG. ) To (c).

  The radiation detector according to the present invention can be suitably used for a medical X-ray inspection apparatus using an energy difference method, an industrial X-ray inspection apparatus, a gamma ray inspection apparatus, or the like. In particular, in a medical X-ray CT scan, it becomes easy to detect an iodine contrast agent, which can contribute to the discovery of cancer. In addition to CT measurement, X-ray transmission imaging methods such as chest X-ray imaging can also be used to identify iodine thickness, soft tissue thickness, and bone thickness on a line through which X-rays have passed. Furthermore, it can be used for imaging of a catheter inside a human body in catheter surgery.

11, 12, 13 Detection element 14 Absorber

Claims (5)

  A plurality of detection elements that generate charges by energy applied from incident radiation are arranged in a line on the radiation incident line at different distances from the incident end, and between the detection elements in the detection element array The radiation detector characterized by the absorber being installed in one or more places.   2. The radiation detector according to claim 1, wherein all of the plurality of detection elements have the same structure using the same detection medium, and 3 to 6 are arranged in a line.   3. The radiation detector according to claim 2, wherein the absorber is made of a material containing an element from Al of atomic number 13 to Bi of atomic number 83 (excluding Tc of atomic number 43 and Pm of atomic number 61).   A radiation detector for measuring an iodine contrast medium in a subject, wherein each of the detection elements has the same structure using the same detection medium, and three of them are arranged in a line on the radiation incident line. The radiation according to claim 1, wherein the absorber is made of tin, and the absorber is disposed on a radiation incident end side of an intermediate detection element or a last detection element in the row of detection elements. Detector.   The radiation detector according to claim 1, wherein a detection medium of the detection element is made of any one of a semiconductor, a scintillator, a gas, a superconductor, and an insulator.

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