Housing of Radiation Detector

The present invention relates to a housing which enables radiation detectors to be accommodated within cassette cases in a reliable manner when they are used in a positron computed tomographic (CT) apparatus or a single-photon emission CT apparatus.

FIG. 5 is a schematic representation of a recently proposed positron CT apparatus capable of high resolution. As shown, this apparatus comprises basically a ring-shaped gantry 1 and a holder 3 such as a bed which holds the subject 2 in position.

The gantry 1 comprises a plurality of generally fan-shaped cassettes 4 for detecting gamma-rays that are arranged in annular form. Each of these cassettes accommodates four rows of a radiation detector 14 consisting of a scintillator 5 and a photomultiplier tube 6. These cassettes are juxtaposed in five stacks and a shield collimator 7 is provided on both the inside and the outside of the five stacks of cassettes 4.

Further, a slice collimator (not shown) is provided as required between adjacent stacks of scintillator 5 to reduce the entrance of background noise (i.e., scattering radiation and single gamma-rays).

Each of the radiation detecting cassettes 4 is accommodated in a cassette case 8 as shown in FIG. 4. This cassette case 8 is made of a material such as aluminum or a plastic resin that is transmissive of radiation (gamma-rays) but which is opaque to light. The cassette case 8 is provided with spaced partitions 9 that are formed as integral parts of this case. A radiation detector 14 consisting of a scintillator 5 and a photomultiplier tube 6 and which is fixed by a bleeder socket 12 is accommodated in each of grooves 10 which are delineated by side walls 11 and adjacent partitions 9. A circuit board 13 containing a bleeder circuit and any other necessary circuits is provided in the cassette case 8 along its outer circumference.

The photomultiplier tube 6 in each radiation detector 14 is usually encased within a glass tube having variations in its outside dimensions. Hence, each of the grooves 10 in the cassette case 8 is made considerably larger than the dimensions of the glass tube. However, this creates an unavoidable gap between the glass tube and each accommodating groove 10 and on account of nonuniformity in the size of this gap, the photomultiplier tube 6 and scintillator 5 of which each radiation detector 14 is composed will rattle or move about to make it impossible to perform precise measurements. Further, the rattling photomultiplier tube or scintillator can damage themselves. In order to avoid this problem, one may measure the nonuniform gap and insert a spacer of commensurate size. However, because of the thermal expansion mismatch between the aluminum cassette case 8 and the glass tube the radiation detector 14 might be damaged resulting from a temperature elevation that occurs after the spacer is fitted into the gap. Further, the gap must be covered with an aluminum foil or some other light-shielding material in order to avoid the noise problem due to entrance of ambient light.

An object, therefore, of the present invention is to provide a housing that allows radiation detectors to be accommodated within grooves in a cassette in a safe and reliable manner and that insures effective light shielding.

The housing of the present invention is generally of a type that comprises a radiation detecting cassette which comprises a light-shielding case having accommodating grooves delineated by partitions integral with said cassette case, a radiation detector accommodated in each of said grooves and which consists essentially of a scintillator and a photomultiplier tube, and a cover that closes said cassette case. The present invention attains the above-stated object by injecting a flowable and light-shielding filler material into the space between each of said grooves and the radiation detector and curing the injected filler material after closing the cassette case with said cover.

In the accompanying drawings:

FIG. 1 is a front view of a radiation detector housing according to an embodiment of the present invention; FIG. 2 is a cross section taken on line II--II of FIG. 1;

FIG. 3 is a cross section taken on line III--III of FIG. 1;

FIG. 4 is a perspective view of a cassette case to which the invention is applied; and

FIG. 5 is a front view of a gantry in a position CT apparatus.

An embodiment of the present invention is described hereinafter with reference to FIGS. 1-3. Shown by 8 is a cassette case which is made of aluminum or a plastic resin that is transmissive of gamma-rays but which is opaque to light. This cassette case is generally fan-shaped and a specified number, say, fifteen, of these cassette cases are arranged in annular form. Each cassette case is open on the top and has four grooves 10 for accommodating a corresponding number of radiation detectors 14 that are formed inwardly of an area 15 for accommodating a circuit board 13 for a bleeder circuit and some other necessary circuits. The grooves 10 are delineated by side walls 11 and adjacent partitions 9.

The side walls 11 and partitions 9 are in the form of a triangle that is tapered toward the center of the gantry in a direction parallel to the center line 16. A light-shielding step 18 is formed on the top surface of each side wall 11 and the front wall 17.

Semicircular light-shielding grooves 20 are formed on the bottom plate 19, side walls 11 and partitions 9 of the cassette case 8 along its entire circumference in areas close to the base end of each groove 10 and where a bleeder socket 12 is inserted.

The radiation detectors 14 accommodated in the cassette case 8 are then covered with a cover 21. A step 22 that engages the step 18 on the cassette case 8 is formed on three sides of the bottom surface of the cover 21, and light-shielding grooves 23 are also formed in registry with the grooves 20.

Four units of radiation detector 14 are fitted into the respective four grooves 10 in the cassette case 8 having the construction described above. A filler material 24 such as rubber is injected into each groove 10 and cured after the cover 21 is secured by a suitable fastening means such as screws. In order to insure satisfactory light-shielding effects, a highly flowable filler material 24 is injected into the area of groove 10 where the scintillator 5 is accommodated, whereas a less flowable filler material 24 is injected into the areas where the photomultiplier tube 6 and the bleeder socke 12 are accommodated for the purpose of insuring their complete fixation. Particular care should be taken to inject the filler material 24 into light-shielding grooves in the area where bleeder socket 12 is positioned. The cover 21 on the cassette case 8 is secured by screws or some other suitable fastening means and subsequently, the injected filler material 24 is cured.

In the embodiment described above, four scintillators 5 are provided in such a way as to divide the photocathode of a single photomultiplier tube 6 into four parts. If desired, the photocathode need not be divided and a single scintillator may be combined with a single photomultiplier tube.

Having the construction described on the foregoing pages, the housing of the present invention eliminates rattling of radiation detectors in the cassette and thus effectively prevents them from being damaged during use. Further, the housing provides exact directionality to insure precise measurements. The filler material injected into the grooves where the detectors are accommodated insure effective light shielding. The areas of the grooves where the scintillators are accommodated are provided with improved light-shielding effects since every part of them is injected with a highly flowable filler material. On the other hand, the areas of the grooves where photomultiplier tubes and bleeder sockets are accommodated are injected with a less flowable filler material to improve the fixation of these components. Further, the steps provided in the area where the cassette case and the cover are joined and the light-shielding grooves that are provided along the entire circumference of the areas where bleeder sockets are to be fitted in and that are injected with a filler material insure further improved light shielding and fixation.