JP2006058174A - COLLIMATOR FOR gamma RAYS/X RAYS - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a beam profile without making the structure of a collimator for γ rays/X rays complex. <P>SOLUTION: The collimator 10 has an aluminum layer 14 in which an absorption coefficient to energy, such as γ rays, is smaller than that of lead inside a lead layer 12 for shielding γ rays, or the like. More specifically, the collimator 10 has a layer 14 made of aluminum having a small absorption coefficient between a through hole 16 for transmitting γ rays, or the like and the lead layer 12 for shielding γ rays, or the like. Since aluminum has a much smaller absorption coefficient than lead, the aluminum layer 14 having the same length cannot shield γ rays, or the like completely even if the lead layer 12 has a sufficient width for shielding incident γ rays, or the like completely. As a result, the beam profile can be changed as compared with a conventional structure in which a body made of a single material, namely lead, is provided with a through hole. An appropriate profile can be obtained by determining the layer thickness of the aluminum layer 14 appropriately. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a collimator for γ rays or X rays.

  In the medical field such as positron tomography (PET) and cinch scanner, or other fields, measurement of the source distribution of γ-rays or X-rays (hereinafter abbreviated as “γ-rays”) is performed. In such an application, if γ rays or the like are incident on the detector from various directions, the position of the generation source cannot be specified with high accuracy, and the accuracy of the distribution deteriorates. Thus, by providing a collimator in front of the detector, the direction in which γ rays and the like enter the detector is limited.

  A conventional collimator for γ-rays is configured by forming a hole that allows γ-rays to pass through a single material with high absorption such as γ-rays, such as lead or tungsten, which has excellent shielding ability for γ-rays. It had been. The shape of the hole includes a parallel hole shape having a circular or hexagonal cross section (Patent Document 1), a tapered cone shape (Patent Document 2), a fan beam shape as shown in FIG. 1, and a shape as shown in FIG. There is a pinhole shape or the like, and a collimator having a hole shape corresponding to the direction of γ rays or the like to be detected is used. Further, in order to make the intensity and intensity distribution of incident γ rays and the like desired, as shown in FIG. 3, a groove 125 is formed on the inner side surface 120 of the hole 115 provided in the shield 110 made of lead. Therefore, a collimator design is also made so that the density of lead or the like per unit length along the transmission direction of γ-rays in the portion 128 where the groove is formed is smaller than the density of the outer portion. Yes.

JP 2000-28733 A Japanese Patent Laid-Open No. 11-153673

  A collimator constructed by forming parallel holes in lead or the like is easy to manufacture, but the intensity distribution of γ-rays that pass through it is determined by the diameter and length of the holes, so that a desired intensity distribution can be obtained. It can be difficult. On the other hand, if the cross-sectional shape of the hole is devised as shown in FIG. 3, the intensity distribution can be adjusted, but there is a problem that the manufacturing takes time and cost as much as the cross-sectional shape becomes complicated.

  The collimator such as γ-ray according to the present invention has a multilayer structure centered on a hole for transmitting γ-rays or X-rays, and the material forming the first layer in the multilayer structure is the first layer. The absorption of the γ-rays or X-rays is greater than the material of the inner second layer.

  In a preferred aspect of the present invention, the material forming the first layer has a larger absorption coefficient for the energy of the γ-rays or X-rays than the material forming the second layer. .

  In a preferred aspect of the present invention, the first layer is made of lead, and the second layer is made of aluminum.

  In the collimator of the present invention, the inner second layer has a smaller shielding ability against γ rays and the like than the outer first layer, so that an intensity distribution close to the conventional collimator structure illustrated in FIG. 3 can be realized. In addition, there is an advantage that manufacture is easy as compared with the structure of FIG. 3 in which the shape of the inner surface is complicated.

  The best mode for carrying out the present invention (hereinafter referred to as “embodiment”) will be described below with reference to the drawings.

  FIG. 4 is a cross-sectional view for explaining the structure of the collimator 10 for gamma rays or the like according to the present invention.

  As shown in FIG. 4, the collimator 10 has an aluminum layer 14 that has an absorption coefficient smaller than that of lead for energy such as γ rays to be collimated inside the lead layer 12 for shielding γ rays and the like. Yes. That is, the collimator 10 is made of aluminum having a relatively small absorption coefficient for energy such as γ rays to be collimated between the through hole 16 that transmits γ rays and the like and the lead layer 12 that shields γ rays and the like. It has a layer 14.

  As is well known, interactions between γ rays and substances include photoelectric effect, Compton effect, and electron pair generation, and attenuation (absorption) by substances such as γ rays is caused by the sum of these interactions. Thus, the effect of the sum is as follows: William J. Price original, supervised by Nishino Osamu, Satoshi Sekiguchi Translated, “Radiation Measurement”, 11th edition, Corona, July 10, 1978, p. As shown in 22-31, it is expressed by the absorption coefficient of the substance. This absorption coefficient depends on both the type of substance and the energy of the target γ-ray, as shown in the same document. As shown in the same book, as an overall trend, the absorption coefficient has a larger value as a substance having a larger atomic number, and a smaller value as the energy of the target γ-ray increases. However, the magnitude relationship between the absorption coefficients of the two substances may be reversed depending on the energy range such as gamma rays.

  The basic idea of this embodiment is that the collimator has a multi-layer structure, and a layer made of a material having a large absorption (attenuation) ability for gamma rays to be collimated is arranged on the outside and a layer made of a small material is arranged on the inside. However, the absorption coefficient indicating the absorption ability of each material in this case depends not only on the material but also on energy such as γ rays to be collimated. For this reason, in the above description, the absorption coefficient for the γ-rays of the material is referred to as “absorption coefficient for the energy of γ-rays to be collimated”. However, in the following, in order to avoid complexity, this is simply referred to as “absorption coefficient”.

  Aluminum has a much smaller absorption coefficient than lead, and thus has a shielding ability against γ rays and the like that is smaller than that of lead. For this reason, even if the lead layer 12 has a sufficient width (that is, a length along the transmission direction) to completely shield incident γ rays and the like, in the aluminum layer 14 having the same length as that, Gamma rays and the like are not completely shielded, and a part thereof reaches the detector through the aluminum layer 14. For this reason, the intensity | strength and intensity distribution of the permeation | transmission gamma ray etc. can be changed with the conventional collimator which provided the through-hole with respect to the main body of the single material which consists of lead.

  The results of simulation for confirming the difference between the effects of the structure of the present embodiment and the conventional structure will be described below.

  FIG. 5A shows the structure of a collimator 20 having a conventional structure as a comparison target. This collimator 20 is formed by forming a through hole 24 having a circular cross section in the center of a cylindrical shield 22 made of lead. The diameter of the through hole 24 is 5 mm, and the length along the transmission direction of the shield 22 is long. Suppose that it is 10 cm.

  The structure of the collimator 30 of this embodiment compared with this is shown in FIG. The collimator 30 has a structure in which an aluminum layer 34 and a lead layer 32 are arranged in this order from the inside around a through hole 36. The length of the collimator 30 is 10 cm as in the conventional structure. As a result, the lead layer 32 functions as a shield against incident γ rays or X rays. The diameter of the through hole 36 is 5 mm as in the conventional structure. The thickness of the aluminum layer 34 is 2.5 mm.

  In the simulation, as shown in FIG. 6, the detection surface of the detector 50 is brought into close contact with one end surface of the collimator 40 so that one opening of the through hole 42 is completely covered with the detection surface. Then, the point source 60 that emits γ rays is moved along the diameter of the other end face 44 of the collimator 40, and the counting efficiency of the detector 50 when the point source 60 is located at each position on the diameter is determined. Calculated. A γ-ray of 511 keV (γ-ray energy generated when the positron disappears) is emitted from the point source 60 with a probability equal to all directions. The number of γ rays emitted from the point source 60 at each position is a sufficient number from a statistical viewpoint. Further, the energy range of γ rays measured by the detector 50 is set to 450 to 550 keV in order to remove noise. Since it is a simulation, the number of γ rays emitted from the point source 60 is known, and the number of γ rays reaching the detector 50 through the collimator 40 can also be calculated by simulation, so that the counting efficiency can be calculated.

  The distribution of counting efficiency obtained by such a simulation is shown in FIG. The horizontal axis of the graph of FIG. 7 indicates the position of the point source 60 on the diameter of the end face of the collimator (the center of the through hole is the origin position (0 mm)), and the vertical axis indicates the counting efficiency. In FIG. 7, a curve 72 shows the distribution of counting efficiency in the conventional structure of FIG. 5 (a), and a curve 74 shows the distribution of counting efficiency in the structure of this embodiment of FIG. 5 (b).

  As can be seen from the comparison of these curves 72 and 74, it can be seen that the structure of this embodiment has a higher counting efficiency than the conventional structure, starting with the peak at the center. Thus, according to the collimator structure of the present embodiment, it can be seen that the peak intensity of γ rays can be increased if the through holes have the same diameter.

  Further, a simulation was performed by increasing the diameter of the through-hole of the conventional structure so that the peak intensity (peak counting efficiency) of the γ-ray is substantially equal to that of the structure of the present embodiment in FIG. In this case, the diameter of the through hole of the conventional structure is 7.7 mm, and the other dimensions are the same as those in the example of FIG. The distribution of counting efficiency of the conventional structure obtained by this simulation is a curve 76 in FIG.

  Comparing curves 74 and 76, the peak heights are approximately the same, while the “shoulder” portions 75 and 77 of the distribution are clearly lower in the shoulder portion 75 of the curve 74 (this embodiment). I understand that This means that the spatial resolution is higher in the structure of this embodiment if the peak height is the same. Therefore, according to the structure of the present embodiment, it can be seen that the beam profile in the space can be improved without reducing the counting efficiency.

  As described above, according to the collimator structure of the present embodiment, the beam profile can be improved as compared with a conventional collimator made of a single material having a simple cylindrical hole, so that an improvement in the position resolution of the detector can be expected. In addition, the structure of this embodiment has an advantage that it is easier to manufacture than the conventional structure having a complicated cross-sectional shape illustrated in FIG.

  In the above, a two-layer structure of aluminum and lead has been exemplified, but this is only an example. For example, the material of the inner layer may be copper or iron instead of aluminum. The material of the outer layer may be tungsten instead of lead. In any case, as a material of the inner layer, a material having a small absorption coefficient may be used so long as the collimator length (length along the transmission direction) does not completely block γ rays or the like. On the other hand, as the outer layer, a material having a high absorption coefficient that can completely shield γ rays and the like by its length may be used. By appropriately selecting the material and thickness (layer thickness) of the inner layer, various beam profiles (counting efficiency distribution of the detector) can be realized.

  In the above, a single element metal material such as lead or aluminum is exemplified as the material of each layer of the collimator, but an alloy can be similarly used. In the case of using an alloy as well, in the same way as in the case of a single element metal, the absorption capability of γ rays and the like can be determined by the absorption coefficient, so that material selection may be performed according to the absorption coefficient.

  Moreover, although the case where the dense metal substance was used as a material of each layer of a collimator was illustrated above, a non-dense material such as a porous metal material can also be used. Such a non-dense material has a lower ability to absorb γ rays or the like than a dense material made of the same substance, and therefore may be used as a material for an inner layer or the like according to the absorption ability. Of course, materials other than metal can be used in the same manner.

  It is also possible to make a multilayer structure of two or more layers. In this case, if the absorption coefficient of the material of each layer is increased in order from the inner side (that is, the side close to the through hole) to the outer side, a mountain-shaped beam profile can be obtained as in the illustrated two-layer structure. . Under this restriction, various beam profiles can be realized by adjusting the material and thickness of each layer.

  In the above description, the cross-sectional shape of the through-hole is circular. However, the structure of the present embodiment can be applied to a through-hole other than the circular cross-section.

  In the above, a collimator having only one through-hole that transmits γ-rays and the like has been exemplified, but the same structure can be adopted in the case of a porous collimator having a plurality of through-holes.

  In addition, although the simulation result in the case of γ rays has been described above, since the X-rays are equivalent to γ rays as light rays, it is considered that the same effects can be obtained for X-rays.

It is a perspective view which shows an example of the structure of the conventional collimator. It is a perspective view which shows another example of the structure of the conventional collimator. It is sectional drawing which shows another example of the structure of the conventional collimator. It is sectional drawing which shows the structure of the collimator of embodiment. It is a figure which shows the collimator structure of the prior art used for simulation, and this embodiment. It is a figure for demonstrating simulation. It is a figure which shows the count efficiency distribution obtained by simulation.

Explanation of symbols

  10 collimators, 12 lead layers, 14 aluminum layers, 16 through holes.

Claims (3)

  The material for forming the first layer in the multilayer structure is more than the material of the second layer inside the first layer, with a multilayer structure centered on the hole for transmitting γ-rays or X-rays. A γ-ray / X-ray collimator characterized by having a large absorption with respect to the γ-ray or X-ray.   2. The γ-ray / X according to claim 1, wherein the material forming the first layer has a larger absorption coefficient for the energy of the γ-rays or X-rays than the material forming the second layer. Line collimator.   2. The γ-ray / X-ray collimator according to claim 1, wherein the first layer is made of lead, and the second layer is made of aluminum.

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