Patent Number: 051986809
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, one embodiment of a method for manufacturing a single focus collimator according to the present invention will be described. First, as shown in FIG. 1A, a bulk block member 1 made of an aluminum plate is prepared in thickness T=60 mm, length L=400 mm, and width W=200 mm, for example. Then, as shown in FIG. 1B, fine grooves G of approximately 0.2 mm in width and 40 mm in depth each are formed on an upper surface of the bulk block member 1. The width of each groove G is selected from the preferable range of 0.1 to 1 mm. Here, the grooves G in the length L direction are cut perpendicularly and arranged to be parallel with each other, while the grooves G in the width W direction are cut in a fan shape pattern such that they are oriented toward a common focal line F. This cutting of the grooves G can be achieved by using a wire cut electric spark manufacturing process under the numerical control, although the use of the wire cut electric spark manufacturing process can be time consuming and therefore expensive. More preferably, the cutting of the grooves G can be achieved by using a disk cutter device under the numerical control. Here, the use of the presently available disk cutter device is possible because the grooves G are to be formed on the aluminum bulk block member 1, and preferable because of the cheaper cost required for such a manufacturing by the disk cutter device compared with the use of the wire cut electric spark manufacturing process. Note also that this disk cutter device cannot be used in the conventional method using the tungsten plates since the presently available disk cutter device is unable to cut the tungsten plates because of the very high rigidity of the tungsten. Next, the lead is casted into the grooves G formed on the bulk block member 1. In this step, in order to secure the ample casting of the lead into the fine grooves G, the melt lead may be pressurized, or the chemical such as antimony may be added into the lead. After this lead casting, the whole bulk block member 1 with the lead casted into the grooves G are immersed into a solvent made of strong acid such as hydrochloric acid, sulfuric acid, or nitric acid, or else strong alkali such as sodium hydroxide which can dissolve the aluminum bulk block member 1 but not the lead cast. As a result, as shown in FIG. 1C, a desired single focus collimator body 2 with all the holes 3 arranged in an array oriented toward the predetermined single focal line F can be obtained from the lead cast which is not dissolvable by the solvent. Note here that the relatively low rigidity of the lead does not cause any trouble in this embodiment because the entire collimator body 2 is formed integrally in a lattice shape and therefore no assembling process is necessary. In the practical collimator, the collimator body 2 so manufactured will be mounted on a metallic frame for supporting the collimator body 2, and then covered by a cover member for protecting the collimator body 2. In this procedure of this embodiment, it becomes possible to achieve the precision of approximately 0.05 mm required for the single focus collimator. Here, the wire cut electric spark manufacturing process is capable of achieving the precision as high as 0.001 mm, but the precision of the collimator body 2 after the casting becomes approximately 0.05 mm after the casting. It is to be noted that, instead of using the bulk block member 1 made of aluminum and the solvent made of hydrochloric acid or sodium hydroxide, the bulk block member 1 made of sodium chloride may be used. In such a case, the solvent made of water can be used for dissolving the bulk block member 1, so that the cost for manufacturing can be reduced considerably. Now, just as in the conventional collimator, when all the holes 3 are made in the same size, the sensitivity of the collimator body 2 becomes higher in a central region compared with peripheral regions. In order to make the sensitivity of the collimator body 2 substantially uniform over the entire view field, the size of the holes 3 should be made narrower toward the center, as shown in FIG. 2. For example, the width of the hole 3 at the center can be set to be 1 mm, while the width of the hole 3 at the periphery can be set to be 1.4 mm. In addition, the further treatment for making the sensitivity of the collimator body 2 uniform such as the shaping of the surface of the collimator body 2 may additionally be applied. It is to be noted here that in the conventional method using pins, manufacturing of such a configuration in which the size of the holes 3 is made to be narrower toward the center would be extremely time consuming and costly because a large number of pins of different sizes must be prepared in advance. Such a single focus collimator according to the present invention is intended to be useful primarily in the SPECT apparatus. More specifically, the SPECT apparatus in which the single focus collimator according to the present invention is to be used has a schematic configuration as shown in FIG. 3. This SPECT apparatus of FIG. 3 comprises: a frame 101 placed around the head portion of the patient P; three .gamma. ray detector devices 106 (each including a scintillator and a photoelectric converter) for detecting the .gamma. rays emitted from radioactive materials deposited inside the patient P and outputting the electric signals corresponding to the detected .gamma. rays, which are mounted on the frame 101 and arranged in a form of an equilateral triangle with the head portion of the patient P located inside; three single focus collimators 105 detachably mounted on the front sides of these .gamma. ray detector devices 106 facing toward the patient P; a data collection unit 102 for collecting the .gamma. rays signals outputted from the .gamma. ray detector devices 106; an image reconstruction unit 103 for carrying out the image reconstruction process by using the detected .gamma. ray signals collected by the data collection unit 102 as the projection image data in order to obtain an image of a distribution of the radioactive materials inside the patient P; and a display unit 104 for displaying the obtained image of a distribution of the radioactive materials inside the patient P for the sake of the diagnosis of a cancer and a tumor. In this SPECT apparatus of FIG. 3, when the single focus collimators 105 of the type in which all the holes are made in the same size throughout the effective view field of the .gamma. ray detector devices 106 are used, the sensitivity of the .gamma. ray detection is not uniform over the entire effective view field and becomes higher in a central region compared with peripheral regions, as shown in FIG. 4. On the other hand, when the single focus collimators of the type shown in FIG. 2 described above in which the size of the holes are made narrower toward the center are used, the sensitivity of the .gamma. ray detection can be made substantially uniform over the entire effective view field as shown in FIG. 5, so that the complicated correction of the detector output becomes unnecessary in the SPECT apparatus. As described, according to the present invention, it becomes possible to manufacture a single focus collimator in a high precision, without increasing a cost for manufacturing, because the tedious preparatory and manual works as well as the source for the deterioration of the precision involved in the conventional method using pins are totally absent in the method of the present invention, while at the same time the use of the very expensive tungsten and the very time consuming wire cut electric spark manufacturing process involved in the conventional method using tungsten plates can be avoided in the method of the present invention. Consequently, by using the method of the present invention, it becomes possible to provide a single focus collimator which can be manufactured in a high precision inexpensively. Furthermore, according to the present invention, it also becomes readily possible to provide a single focus collimator with a uniform sensitivity which can be manufactured in a high precision inexpensively. It is to be noted that, contrary to the configuration of FIG. 2, the size of the holes 3 may be made wider toward the center, to make the collimator body 2 with a sharply concentrated high sensitivity region around the center, which may be useful in a case of examining a small size target. It is also to be noted that a so called septa thickness determined by the width of the grooves G may be varied in different sections of the collimator body 2 in order to realize the different .gamma. ray shielding properties at different sections of the collimator. Besides these, many modifications and variations of the above embodiments may be made without departing from the novel and advantageous features of the present invention. Accordingly, all such modifications and variations are intended to be included within the scope of the appended claims.