1. Field of the Invention
This invention relates to a radiation image read-out apparatus for performing a radiation image read-out operation by use of a photomultiplier.
2. Description of the Related Art
It has heretofore been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a light, which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored on the stimulable phosphor sheet during exposure of the stimulable phosphor sheet to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal by use of image read-out means, such as a photomultiplier (hereinbelow referred to as the PMT). The image signal is then processed and used for the reproduction of the radiation image of the object as a visible image on a recording material.
As for the PMT utilized in the radiation image recording and reproducing systems described above, the sensitivity (i.e., the level of an output signal with respect to a light intensity of incident light) of the PMT is determined by the level of an applied high voltage. However, even though PMT's are of an identical type, the characteristics vary for respective PMT's. The level of the high voltage required to obtain a desired sensitivity varies for the respective PMT's. Therefore, at the time of delivery of each of the radiation image recording and reproducing systems to a user, the level of the high voltage has heretofore been set appropriately for each of the radiation image recording and reproducing systems such that a desired sensitivity of the PMT is capable of being obtained. (The setting of the level of the high voltage is described in, for example, Japanese Unexamined Patent Publication Nos. 3(1991)-132644 and 2001-100340.)
However, recently, it has been found that the sensitivities of the PMT's become low due to the use of the PMT's. In particular, as for the PMT's utilized in the radiation image recording and reproducing systems described above, it has been found that, since a large anode current flows through each of the PMT's, the sensitivities of the PMT's become markedly low. FIG. 4 is a graph showing a relationship between a cumulative dose of radiation having been delivered to a stimulable phosphor sheet, i.e. a cumulative addition value of anode currents flowing through a PMT, and a sensitivity of the PMT. It has been experimentally found that, as illustrated in FIG. 4, in cases where the cumulative dose of the radiation becomes large, the sensitivity of the PMT becomes as low as approximately one tenth of the original sensitivity.
Therefore, in cases where the voltage applied across the PMT is set appropriately at the time of the delivery of the PMT to the user, the sensitivity of the PMT becomes low due to the use of the PMT. Accordingly, particularly, appropriate detection is not capable of being made with respect to the light having a low light intensity, which light is emitted from a site of the stimulable phosphor sheet having been exposed to a small dose of the radiation. Also, in cases where a visible image is reproduced from an image signal having been obtained with the PMT whose sensitivity has become low as described above, it is not always possible to discriminate only through the seeing of the reproduced visible image whether the image quality of the reproduced visible image is due to the lowering of the sensitivity of the PMT or is due to the original quality of the radiation image having been stored on the stimulable phosphor sheet. In such cases, there is the risk that an appropriate image diagnosis will not capable of being made.
Further, it is considered that the lowering of the sensitivity of the PMT depends upon the cumulative value of anode currents, which have flowed through the PMT. Therefore, the extent of the lowering of the sensitivity of the PMT is not capable of being detected accurately in accordance with only the period, for which the PMT has been used, or the number of times of the radiation image read-out operations, which have been performed by use of the PMT. (The aforesaid problems with regard to the detection of the extent of the lowering of the sensitivity of the PMT are described in, for example, Japanese Patent Publication No. 7(1995)-031373.)