JP2013246156A - Three-dimensional radiation position detector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a three-dimensional radiation position detector capable of identifying a three-dimensional (height, width and depth) position of an interaction position within a scintillator by the centroid calculation which is a simple position calculation method, and also capable of high resolution without making structure complicated.SOLUTION: A three-dimensional radiation position detector includes: a light receiving element 12 optically-connected with at least two opposing parallel surfaces of a scintillator block 22; and at least one layer of an optical discontinuity surface 11a formed on only the surface in parallel with the light receiving element surface inside the scintillator block 22.

Description

  The present invention relates to a three-dimensional radiation position detector, and in particular, a three-dimensional radiation position detector that is suitable for use in a positron emission tomography apparatus (PET apparatus) and can detect a position in the depth direction at which radiation is incident ( Depth of interaction (DOI) detector).

  As a radiation detector used in a PET apparatus, a DOI detector capable of detecting a position in a depth direction where radiation is incident has been developed (see Non-Patent Document 1). In order to detect the position where the radiation is incident on the DOI detector and emits light, generally, a method of calculating the center of gravity by anger calculation and approximating the position of the light emission event (referred to as an event) (the center of gravity calculation method or the anger method) Is known) (see Patent Documents 1-5).

  The concept of the center of gravity calculation method will be described with reference to FIG. As shown in FIGS. 1A and 1B, in the radiation detector composed of the scintillator 10 and a large number of light receiving elements (readout channels) 12, the intensity distribution of the signal changes when the position where the gamma rays interact with each other is different. To do. Therefore, the gamma ray detection position can be estimated from the barycentric point of the signal intensity distribution.

  A practical implementation method of the DOI detector is a method in which the scintillator is divided into small segments of several mm in the vertical, horizontal, and depth directions. If a patterned reflective film is inserted between the scintillator segments, the position of the vertical, horizontal, and depth scintillator segments can be specified by calculating the center of gravity even when receiving light from one surface of the scintillator block (non-patented) Reference 1). Alternatively, if light is received from multiple sides of the scintillator block, the vertical, horizontal, and depth scintillator segments can be identified by center-of-gravity calculation without inserting a reflector between the scintillator segments (non-patented). Reference 2).

  In the method of dividing the scintillator in this way, the position resolution of the detector matches the size of the scintillator segment. Therefore, in order to increase the position resolution, it is necessary to reduce the size of the scintillator segment. However, the smaller the scintillator segment, the more time is required to process and assemble the block.

  In order to solve this problem, as shown in FIG. 2, a method of optically coupling a lump scintillator (referred to as a monolithic scintillator) directly to a light receiving element without being divided into segments has been studied (non-patent document). 3 and 4). 2A is an example in which the light receiving element 12 is coupled to two opposing surfaces of the monolithic scintillator 11, and FIG. 2B is an example in which the light receiving element 12 is coupled to one surface of the monolithic scintillator 11. FIG. Since it is not divided into scintillator segments, there are no physical restrictions on resolution, and the manufacturing process can be simplified.

  The biggest drawback of the monolithic scintillator is that it is difficult to discriminate the position in the depth direction in the center of gravity calculation, even though the position discrimination in the vertical and horizontal directions is possible. FIG. 3 shows the signal distribution on the surface of the light receiving element 12 and the centroid calculation result when the gamma rays are photoelectrically absorbed (emitted) at points A, B, C and D of the monolithic scintillator 11, respectively. Show. The light emitting points A and B have the same position in the horizontal direction (x direction), but have different depths (z). As the interaction (photoelectric absorption) position is farther from the light receiving surface, the signal distribution on the surface of the light receiving element 12 tends to be wider, but the center of gravity is the same regardless of the depths z1 and z2.

  In order to correctly calculate the interaction position in the detector of the monolithic scintillator 11, a position calculation method using a statistical method such as pattern matching or maximum likelihood estimation has been proposed instead of the centroid calculation (see Non-Patent Documents 5 to 8). .

JP 2005-43104 A U.S. Pat. No. 3,010,157 JP 7-325156 A JP 2008-51701 A Japanese translation of PCT publication No. 2008-523381

Mutayama H, Ishimashi H, Omura T: Depth encoding multicrystal detectors for PET.IEEE Trans Nucl Sci 45: 1152-1157, 1998. Yujiro Yazaki, et al, “Preliminary Study on a New DOI PET Detector with Limited Number of Photo-Detectors”, The 5th KOREA-JAPAN Joint Meeting on Medical Physics, YI-R2-3, 2008. R. Pani, M.N.Cinti, R. Pellegrini, et al., "LaBr3: Ce scintillation gamma camera prototype for X and gamma ray imaging," Nucl Instr Meth A. 576, pp. 15-18, 2007. Eiji Yoshida, Naoko Inadama, Hiroto Osada, et al., "Basic performance of a large area PET detector with a monolithic scintillator," Radiol. Phys. Technol., Vol. 4, pp. 134-139, 2011. T.D.Milster, et al, “DIGITAL POSITION ESTIMATION FOR THE MODULAR SCINTILLATION CAMERA”, IEEE Transactions on Nuclear Science, Vol. NS-32, No. 1, February 1985. Marnix C. Maas, DJ van der Laan, Dennis R. Schaart, et al., “Experimental Characterization of Monolithic-Crystal Small Animal PET Detectors Read Out by APD Arrays,” IEEE Trans. Nucl. Sci., 53, pp. 1071 -1077, 2006. Marnix C Maas, Dennis R Schaart, D J (Jan) van der Laan, et al., "Monolithic scintillator PET detectors with intrinsic depth-of-interaction correction," Phys. Med. Biol., 54, 1893-1908, 2009. Dennis R Schaart, Herman T van Dam, Stefan Seifert, et al., "A novel, SiPM-array-based, monolithic scintillator detector for PET," Phys. Med. Biol., 54, pp. 3501-3512, 2009. Takahiro Moriya, et al., “Crystal Cube: Laser Processing Application to Scintillator and Its Detector Performance,” 2009 Next Generation PET Research Report, pp. 44-47, 2010.

  However, the method based on the pattern matching or statistical method is not only complicated in the first place, but also needs to be measured in advance for all interaction positions. Acquisition of output distribution pattern (response function) is not easy. In order to create the response function, as shown in FIG. 4, the collimator 20 is used to limit the portion that is incident on the monolithic scintillator 11 so that one point on the surface of the monolithic scintillator 11 is irradiated with gamma rays. . It is possible to create the distribution of the light receiving element output at each position, move the irradiation position by the required number of positions, and create the response function at each position, but in reality, the interaction position However, it is impossible to irradiate gamma rays so as to concentrate on a certain point of the monolithic scintillator 11.

  The present invention has been made to solve the above-mentioned conventional problems, and it is possible to specify the three-dimensional position of the interaction position in the scintillator by the center of gravity calculation, and to complicate the structure. It is an object of the present invention to provide a three-dimensional radiation position detector that can increase the resolution without causing it to occur.

  The present invention includes a light receiving element optically coupled to at least two opposing parallel surfaces of the scintillator block, and at least one optical discontinuous surface formed only on a surface parallel to the light receiving element surface inside the scintillator block. The above-mentioned problem is solved by a three-dimensional radiation position detector.

  Here, the scintillator block can be formed from a plate-like scintillator laminated in two or more.

  Further, a thin air layer can be formed between the plate scintillators without optical adhesion.

  Further, the plate scintillators can be optically bonded.

  Further, a cover that covers a surface of the scintillator where the light receiving element is not optically coupled can be provided.

  Further, the cover can be a light absorbing material or a reflecting material.

  The light receiving element on the radiation incident surface side can be a semiconductor light receiving element.

  Further, at least one of the light receiving elements may be a position discrimination type.

  Further, the position discriminating type light receiving elements may be a plurality of non-position discriminating light receiving elements arranged in a matrix.

  In addition, a light guide can be disposed between the scintillator block and the position discrimination type light receiving element.

  Further, the scintillator block can be formed by stacking a plurality of monolithic scintillators in which scintillator layers partitioned by optical discontinuous surfaces are stacked.

  Further, the thickness of the plate scintillator on the radiation incident surface side or the interval between the optical discontinuous surfaces can be made thinner than the thickness of the plate scintillator on the opposite side or the interval between the optical discontinuous surfaces.

  According to the present invention, the center-of-gravity calculation, which is a simple position calculation method, can specify the three-dimensional position of the interaction position in the scintillator in the vertical, horizontal, and depth directions, and can be performed without complicating the structure. A three-dimensional radiation position detector capable of resolution can be realized.

Explanatory drawing showing the concept of the centroid calculation method Perspective view showing an example of a conventional detector using a monolithic scintillator Explanatory diagram showing the problem of the center of gravity calculation method Explanatory diagram showing the problem of response function creation method The (a) exploded perspective view and (b) perspective view which show the composition of the 1st embodiment of the DOI detector concerning the present invention. The figure which shows the state which has arrange | positioned the DOI detector in the PET detector ring. Sectional view of the first embodiment Sectional drawing which shows the structure of 2nd Embodiment of this invention. Sectional drawing which similarly shows the structure of 3rd Embodiment The perspective view which similarly shows the manufacturing method of 4th Embodiment The perspective view which similarly shows the manufacturing method of 5th Embodiment The perspective view which similarly shows the manufacturing method of 6th Embodiment The perspective view which similarly shows the manufacturing method of 7th Embodiment The perspective view which similarly shows the structure of 8th Embodiment The perspective view which similarly shows the structure of 9th Embodiment The figure which shows the modification of a light receiving element The figure which shows various examples of arrangement | positioning of a light receiving element 1 is an exploded perspective view showing a configuration of an embodiment of the present invention. The figure which shows the measurement result of an Example The figure which compares and shows the measurement result of this invention and a prior art example

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  As shown in FIG. 5 (a) (an exploded perspective view showing a state before assembly) and FIG. 5 (b) (a perspective view showing a state after assembly), the first embodiment of the present invention includes a plurality (6 in the drawing). A scintillator block 22 in which plate scintillators 24 are stacked, and optically coupled to two opposing surfaces (upper surface and lower surface in the drawing) on both sides in the stacking direction (vertical direction in the figure) of the plate scintillator 24, A light receiving element 12 capable of discriminating the light receiving position, and a cover 26 covering the front, rear, left and right side surfaces to which the light receiving element 12 of the scintillator block 22 is not optically coupled are provided.

  The limit value of the resolution in the depth (z) direction of the DOI detector 20 is physically limited by the plate thickness of the plate scintillator 24, but the resolution is limited in the vertical and horizontal directions (xy plane). Since there are no physically limiting elements, the positional resolution of the xy plane can be increased without complicating the structure.

  In particular, as shown in FIG. 6, in the case of a PET apparatus using a PET detector ring 30 in which DOI detectors 20 are arranged in a ring shape, the body axis direction and the circumferential direction of the PET detector ring 30 have the greatest influence on the PET image. Even if the radial resolution of the PET detector ring 30 is somewhat limited, the influence on the PET image is small. Therefore, as shown in FIG. 6A, the body axis of the subject 8 is made parallel to the y-axis (or x-axis) of the DOI detector 20, and radiation is incident mainly from the surface of the light receiving element 12a. It is preferable to do so.

  Note that the main incident direction of radiation is not limited to the surface where the light receiving element 12a is present, but may be a surface where the DOI detector 20 is lateral and there is no light receiving element.

  FIG. 6B shows an example in which the DOI detector 20 is arranged in a ring shape so that the body axis is parallel to the z-axis. This is suitable when the radiation position resolution in the body axis direction is not so required.

  Similarly, the body axis is made parallel to the y-axis, but radiation can also be incident from a surface without the light receiving element 12a such as the yz surface.

  The arrangement of the detectors is not necessarily a ring shape, and may be a polygonal shape, a slit or an opening, or an opposing plate shape.

  The front, rear, left, and right side surfaces of the laminated scintillator block 22 where the light receiving element 12 is not optically coupled are provided with a cover 26 to absorb light with black paper or the like, or reflect with a reflective material or white paper or the like. Can do. When emphasizing energy resolution, the loss of scintillation light is reduced by using a reflective material or white paper. On the other hand, when importance is attached to the uniformity of the resolution in the xy direction (when it is desired to increase the position resolution at the end in the xy direction to the same extent as the central portion), black paper is desirable.

  FIG. 7 is a cross-sectional view showing details of the first embodiment.

  The number of plate scintillators 24 may be arbitrary, and can be determined by the type of scintillator and the resolution required in the depth direction. The total thickness T is an important factor that determines the sensitivity of the DOI detector 20, and when a general PET scintillator such as LSO, LYSO, LGSO, GSO, BGO is used, the total thickness T is about 10 mm to 30 mm. It is good to do. For example, when T = 30 mm and the resolution in the DOI direction (depth direction) is 5 mm, six plate scintillators 24 having a thickness of 5 mm are required.

  The interlayer of the plate scintillator 24 (SS in FIG. 7) allows light to be transmitted, but in order to give an optical discontinuity, the refractive index of the interlayer material is smaller than the refractive index of the scintillator. It is desirable to be Specifically, an optical adhesive or grease, or nothing can be inserted between the layers, and an air layer can be formed. The SP between the upper and lower surfaces of the laminated scintillator block 22 and the light receiving element 12 is preferably configured to transmit light as smoothly as possible, and is desirably optically coupled with an optical adhesive or grease.

  The light receiving element 12 may be of any type such as a photomultiplier tube or a semiconductor light receiving element, but at least the light receiving element 12a covering the radiation incident surface is preferably a thin semiconductor light receiving element. As the semiconductor light receiving element, Avalanche Photodiode (APD) and Geiger mode APD (also referred to as SiPM) are suitable.

  The light receiving elements 12 and 12a (represented by 12 except for the case where distinction is particularly necessary) are arranged in a 2 × 2 or more matrix of position discrimination type or non-position discrimination type. In general, the finer the resolution of the light receiving element 12, the better. However, when the resolution is small, such as 2 × 2, or even when the resolution is high (the number of elements is large), there is a gap (insensitive area) between them. If it is wide, a light guide 28 may be inserted between the scintillator block 22 and the light receiving element 12 as in the second embodiment shown in FIG. In this case, it is desirable to optically couple SL between the scintillator block 22 and the ride guide 28 and / or LP between the light guide 28 and the light receiving element 12 with an optical adhesive or grease.

  The vertical and horizontal sizes of the plate scintillator 24 are arbitrary, but may be equal to or smaller than the size of the light receiving element 12. When the size of the plate scintillator 24 and the size of the light receiving element 12 are different, the difference in size can be absorbed by the light guide 28 as in the third embodiment shown in FIG.

  FIG. 10 shows a fourth embodiment of the present invention in which instead of laminating the plate scintillator 24, the monolithic scintillator 11 is subjected to laser processing from the outside to form an optical discontinuous surface 11a similar to the laminating plate scintillator. It is a perspective view which shows the manufacturing method of a form. When laser light in the visible to near-infrared wavelength band is condensed inside the monolithic scintillator 11 using a condensing lens, nonlinear absorption such as multiphoton absorption is caused, and microcracks having a diameter of about several tens of μm are formed. It can be introduced into the monolithic scintillator 11 (see Non-Patent Document 9). Here, the optical discontinuous surface 11a is formed along a surface parallel to the laser incident surface (a surface perpendicular to the laser irradiation axis). At that time, the optical discontinuous surface 11a is sequentially formed from the surface far from the laser incident surface. However, when the thickness of the monolithic scintillator 11 hinders the laser focusing, as shown in FIG. When the processing is formed and one side is finished, the processing on the opposite side may be performed by turning it over.

  FIG. 11 is a perspective view showing the manufacturing method of the fifth embodiment in which the optical discontinuous surface 11a is formed along a surface perpendicular to the laser incident surface.

  Also in this embodiment, when the thickness of the monolithic scintillator 11 hinders laser focusing, as shown in FIG. 11, the processing is formed from the center, and when one side is finished, it is turned over and the processing on the opposite side is performed. Can be done.

  FIG. 12 and FIG. 13 are perspective views showing the manufacturing methods of the sixth and seventh embodiments in which a part of laser processing is incorporated by taking a six-layer scintillator block as an example. In the sixth embodiment shown in FIG. 12, when two monolithic scintillators 11 subjected to three-layer laser processing are combined, the seventh embodiment shown in FIG. 13 performs two-layer laser processing on the monolithic scintillator 11. This is a case where three of the applied ones are combined.

  Note that the resolution in the z direction is not necessarily uniform, and the thickness of the plate scintillator 24 may be changed. In particular, when radiation is incident mainly from a surface on which the light receiving element 12a is present, the closer the surface is to the surface where the radiation is incident, the higher the probability that the radiation will interact. Good.

  FIG. 14 shows a manufacturing method of the eighth embodiment in which the z-direction resolution is gradually changed from high resolution to low resolution, and FIG. 15 shows a ninth method divided into a high resolution layer and a low resolution layer. The manufacturing method of embodiment is shown.

  In the above-described embodiment, the resolution of the light receiving elements 12 and 12a facing each other is the same. However, the resolution of the light receiving elements is not necessarily the same between the two facing surfaces.

  As shown in FIG. 16, the light receiving element 12a on one side has a resolution of 4 × 4, but the light receiving element 12b on the opposite side has a resolution of 2 × 2 as shown in FIG. 16 (a), or FIG. 1 × 1, that is, a light receiving element that is not a position discrimination type may be used.

  Further, the surface of the scintillator block 22 to which the light receiving element is coupled is basically two opposing surfaces as shown in FIG. 17A. However, as shown in the modified examples shown in FIGS. By increasing the surface of the element 12, more scintillation light can be acquired, and the position resolution and energy resolution can be further increased.

  FIG. 18 shows details of an example of the first embodiment. Eight LYSO plate scintillators 24 of 18 mm × 18 mm × 1 mm were stacked. No optical adhesive or grease is sandwiched between the plate scintillators 24, and a very thin air layer is formed. The light receiving elements 12 were optically coupled with grease on the upper and lower sides of the scintillator block 22. The light receiving element 12 is an array of Multi Pixel Photon counter (MPPC) elements (manufactured by Hamamatsu Photonics) having a sensitive area of 3 mm × 3 mm arranged in a 4 × 4 matrix.

  FIG. 19 is an experimental result showing the position discrimination performance of the example. As shown in FIG. 19A, 511 keV gamma rays collimated to a diameter of 2 mm were irradiated to the center of the DOI detector 20 and a position shifted by 3 mm from the center. FIG. 19B is a projection of the result of the three-dimensional centroid calculation on the z-axis and the x-axis, and shows that position discrimination is possible in the plane direction and the depth direction.

  FIG. 19 shows the position calculation results for individual gamma rays superimposed on about 160,000 counts of gamma ray irradiation. This indicates that when only the position calculation result of the gamma ray that caused the interaction in a specific plate scintillator (here, the layer near the center) is displayed, it becomes as shown in FIG. This means that the position information of is specified. On the other hand, in the conventional case using the monolithic scintillator 11 that is not layered, a sharp peak is formed as shown in FIG. 20B even if only the position calculation result of the gamma rays interacting at a specific position is displayed. In other words, position discrimination in the depth direction is difficult.

  The DOI detector according to the present invention can perform three-dimensional radiation position calculation even with simple centroid calculation, but statistical methods such as pattern matching and maximum likelihood estimation as shown in Non-Patent Documents 5 to 8. If is applied, the accuracy of position calculation can be improved.

  Moreover, although the DOI detector according to the present invention is suitable for use in a PET apparatus, the application target is not limited to this, and it can be used not only for medical purposes but also for environmental measurements and high energy experiments. .

DESCRIPTION OF SYMBOLS 8 ... Subject 10 ... Scintillator 11 ... Monolithic scintillator 11a ... Optical discontinuous surface 12, 12a, 12b ... Light receiving element 20 ... DOI detector 22 ... Scintillator block 24 ... Plate scintillator 26 ... Cover 28 ... Light guide 30 ... PET detector ring

Claims (13)

A light receiving element optically coupled to at least two opposing parallel surfaces of the scintillator block;
At least one optical discontinuous surface formed only on a surface parallel to the light receiving element surface inside the scintillator block;
A three-dimensional radiation position detector.   2. The three-dimensional radiation position detector according to claim 1, wherein the scintillator block is formed of a plate-like scintillator laminated in two or more sheets.   The three-dimensional radiation position detector according to claim 2, wherein the plate-like scintillators are not optically bonded but form a thin air layer.   The three-dimensional radiation position detector according to claim 2, wherein the plate scintillators are optically bonded.   The three-dimensional radiation position detector according to any one of claims 1 to 4, further comprising a cover that covers a surface of the scintillator where a light receiving element is not optically coupled.   The three-dimensional radiation position detector according to claim 5, wherein the cover is a light absorbing material.   The three-dimensional radiation position detector according to claim 5, wherein the cover is a reflective material.   The three-dimensional radiation position detector according to any one of claims 1 to 7, wherein the light receiving element on the radiation incident surface side is a semiconductor light receiving element.   The three-dimensional radiation position detector according to claim 1, wherein at least one of the light receiving elements is a position discrimination type.   The three-dimensional radiation position detector according to claim 9, wherein the position discrimination type light receiving element is a plurality of light receiving elements that are not position discrimination type arranged in a matrix.   The three-dimensional radiation position detector according to claim 9 or 10, wherein a light guide is disposed between the scintillator block and the position discrimination type light receiving element.   12. The scintillator block according to claim 1, wherein the scintillator block is formed by stacking a plurality of monolithic scintillators in which scintillator layers partitioned by optical discontinuous surfaces are stacked. Dimensional radiation position detector.   The thickness of the plate scintillator on the radiation incident surface side or the interval between the optical discontinuous surfaces is made thinner than the thickness of the plate scintillator on the opposite side or the interval between the optical discontinuous surfaces. The three-dimensional radiation position detector in any one of thru | or 12.