With a conventional radiographic apparatus, an X-ray source projects X-rays to an object (e.g., a medical patient). X-ray beams transmitted through the object are detected to radiograph the object by a screen film cassette, film autochanger, CR (Computed Radiography), FPD (Flat Panel Detector), or the like.
In the field of radiography, a high-resolution solid X-ray detector which uses an FPD is proposed. This solid X-ray detector has an X-ray sensor including a two-dimensional array in which arrays of 3,000 to 4,000 photoelectric conversion devices (e.g., photodiodes) are arrayed two-dimensionally. Each photoelectric conversion device generates an electrical signal corresponding to the X-ray dose projected to the X-ray sensor. The X-ray image of an object is obtained by arranging the object between an X-ray source and X-ray sensor and converting the X-ray dose which has been transmitted through the object into an electrical signal. A signal from each photoelectric conversion device is read out individually and digitized, and thereafter image-processed, stored, or displayed.
FIG. 9 is a conceptual view showing the structure of a system which includes a conventional cassette type radiographic apparatus. As shown in FIG. 9, a radiographic apparatus 801 incorporates an X-ray detector 802. X-rays generated by an X-ray generator 803 irradiate an object 804. The X-rays transmitted through the object 804 are detected by photoelectric conversion devices (not shown) which are arrayed like a matrix on the X-ray detector 802. Electrical signals output from the photoelectric conversion devices are image-processed by an image processor 805 to display the X-ray image of the object 804 on a display 806 such as a monitor.
In recent years, a low-profile, higher-density mounting technique has improved, and a compact, low-profile solid X-ray detector which uses an FPD is becoming possible, e.g., an X-ray screen film cassette (see Japanese Patent Laid-Open Nos. 2003-057352 and 2002-186604).
FIG. 10 is a side sectional view showing an example of a cassette type radiographic apparatus which uses an FPD. As shown in FIG. 10, an electronic cassette used for radiography or the like has a particle phosphor 131, e.g., GOS, which converts X-rays into visible light, a MIS photosensor portion 115 which uses amorphous silicon and photoelectric conversion devices 109 which are arranged like a matrix to convert the visible light into electrical signals, a TFT switching portion 116, a base 110, a circuit board 111 which supports the base 110, a circuit board 113 on which electronic components for processing the photoelectrically converted electrical signals are mounted, wiring lines 114, case lid 101 which is used to house the above members, and a case main body 117. A buffer member 102 serving as a relaxation portion that relaxes a force from outside the housing is arranged between the case lid 101 and particle phosphor 131. A resin 130 made of PET or the like is arranged between the buffer member 102 and particle phosphor 131.
The particle phosphor 131 is adhered to the photoelectric conversion devices 109 by an adhesion layer 106 through a second protection layer 107 made of an organic substance such as PI and a first protection layer 108 made of a nitride or the like. The circuit board 111 on which the electronic components 113 for processing the electrical signals photoelectrically converted by the photoelectric conversion devices 109 are mounted is planarly attached to the lower surface of the base 110 in tight contact through an insulating sheet 112.
Conventionally, in the electronic cassette which uses the particle phosphor 131 such as GOS, the photoelectric conversion devices 109 which are made of a component such as glass are more vulnerable to an external force than the particle phosphor 131 and protection layers 107 and 108. Therefore, a cassette type radiographic apparatus which uses an FPD has been designed with reference to the strength of the photoelectric conversion devices 109.
In the conventional cassette type radiographic apparatus which uses the FPD, problems occur when a columnar crystal phosphor such as CsI (cesium iodide crystal) is used in place of the particle phosphor 131. The reasons of the problems are roughly classified into two. According to the first reason, the columnar crystal phosphor is fractured by a weaker external force than the photoelectric conversion devices made of a component such as glass. As the columnar crystal phosphor is more vulnerable to the external force, the conventional technique for protecting the photoelectrical conversion devices cannot sufficiently protect the columnar crystal phosphor. According to the second reason, as the columnar crystal phosphor has stricter demands for an external force acting on a small area than a granular phosphor used in the conventional cassette type radiographic apparatus using the FPD, the crystals of the columnar phosphor may be broken. When a stress is applied, holes may be formed in the protection film by steps formed by variations in crystal length.
According to the prior art, a standing- or lying-position radiographic apparatus is available which uses a columnar crystal phosphor and an FPD. When this apparatus is used as a portable cassette type radiographic apparatus, problems occur. The reasons of the problems are roughly classified into two. According to the first reason, unlike the standing- or lying-position portable cassette type radiographic apparatus, the portable cassette type radiographic apparatus is used in various applications, and accordingly the weight of the object may be applied to it. Therefore, a stress absorbing portion which is not necessary in the standing- or lying-position radiographic apparatus is necessary in the portable cassette type radiographic apparatus in case various types of external forces are applied when, e.g., the operator erroneously hits the apparatus with something from above or places his or her elbow on the apparatus. According to the second reason, the cassette type radiographic apparatus is not provided with a grid space. The standing- or lying-position radiographic apparatus usually uses a scattered ray removing mechanism (grid), and the scattered ray removing mechanism (grid) serves as a stress absorbing portion. In the cassette type radiographic apparatus, however, the radiographic target is often a body portion, e.g., a limb, which does produce many scattered rays, and accordingly a scattered ray removing mechanism (grid) is not usually mounted due to the requirements for a lower profile and lighter weight. Therefore, a structure is necessary which protects the columnar crystal phosphor from the external force.
In the cassette type radiographic apparatus, for achieving a lower profile., a gap must be minimized as small as possible in the direction of thickness of the cassette. With a small gap, however, if something is erroneously dropped on the case to deform it elastically, the phosphor may be broken at a high possibility. In particular, in the case of a columnar crystal phosphor such as CsI, when the stress acts on the crystals to break the phosphor crystals, the directional characteristics of light in the phosphor change, and a scar may be undesirably present in an image obtained by X-ray irradiation.
FIGS. 11A to 11D show images obtained when a stress acts on the columnar crystal phosphor to break the phosphor crystals. After an experiment (FIG. 11A) of dropping a screwdriver onto the columnar crystal phosphor (CsI: TI+) was conducted, the columnar crystal phosphor was adhered to photoelectric conversion devices, and radiography was performed (FIG. 11B). As shown in FIG. 11B, a large hitting mark ranging for a diameter of 3 mm appears on the image. FIG. 11C is a graph showing the sectional profile of an output value of a photodetector taken along the line A–A′ of FIG. 11B. As shown in FIG. 11C, a portion where the optical output increases by about 10% and a portion where the optical output decreases by about 10% are present. FIG. 11D Is a graph showing the sectional profile of the output value of the photodetector taken along the line B–B′ of FIG. 11B. As shown in FIG. 11D, many portions are obviously present where the optical output increases by about 10%. In this manner, with reference to FIGS. 11C and 11D, portions where the output value Increases or decreases are obviously present, and a difference in output value is as large as ±10%. The output value fluctuates probably because the phosphor crystals are broken and a light-outputting portion changes. As is apparent from the experimental images of. FIGS. 11A to 11D, since the columnar crystal phosphor has a low strength, even when only a lightweight material such as a screwdriver is erroneously dropped, it forms a scar In the image to cause a clinical problem.