1. Field of the Invention
The present invention relates to a luminescent storage screen of the type suitable for latently storing an x-ray image in an x-ray diagnostics installation, and an apparatus for reading-out the stored image from the screen.
2. Description of the Prior Art
An x-ray diagnostics installation is described in German OS No. 35 29 306 which includes a luminescent storage screen having a photo stimulable phosphor which is irradiated by x-rays attenuated by an examination subject. The phosphor serves as a radiation-sensitive transducer in which holes (in the electronic sense) are generated, and are stored in potential traps or wells. The entire surface of this storage screen is scanned pixel-by-pixel during read-out by a different radiation source, for example a laser, so that the holes stored in the traps are excited, and then fall back into a stable energy level. The energy difference is emitted in the form of light quanta. The x-ray image latently stored in the screen can thus be read-out of the screen at a later time.
The storage screen in this known installation consists of two phosphors separated by a light-impermeable layer. These phosphor layers are scanned surface-wide by a beam deflected by a mirror, for example, in the vertical direction, with the beam being divided by a beam splitter so that both phosphor layers are simultaneously scanned. All picture elements in a line of the storage screen are successively scanned by this deflection. The entire screen surface is scanned by moving the storage screen perpendicularly relative to the line scanning, advancing the screen line-by-line. The light emitted by the stimulable phosphor layers as a result of the interaction with the read-out beam is acquired by two optical collectors, and is supplied to the light-sensitive input faces of two separate light detectors. The respective output signals of the detectors are supplied to a video chain for reproduction on a display, with the output signals of the two detectors being combined.
Instead of dividing the read-out beam, however, it is also possible to use two separate radiation generators, each undertaking a separate scanning of one of the phosphor layers. In this case, however, it is necessary to synchronize the deflection of both scan beams, so that the picture elements of the two phosphor layers which correspond to each other are simultaneously read-out. Synchronization need not be undertaken at the time of read-out; it can alternatively be undertaken electronically in a processing circuit following the light detectors.
The two storage layers in combination function as a single storage layer having an increased thickness. It is known that the efficiency of a phosphor layer increases with increasing thickness, since more x-ray radiation is absorbed in a thicker layer. Image sharpness, however, decreases with increasing layer thickness, so that fine details of the image can no longer be adequately portrayed. By using two separate phosphor layers, with a separate read-out of each layer, the efficiency of a layer having a thickness corresponding substantially to the combined thickness of the two layers is achieved, while a better image sharpness is obtained than would be possible using a single layer of the same combined thickness. A disadvantage is the complexity of the read-out system which is required for such a screen, which not only requires two separate read-out beams, but also structure for guiding and synchronizing the two beams so that the pixels separately read-out from each phosphor layer can be correlated to form a single image. Two synchronized scan systems, or at least two synchronized scan beams, are required.
Another x-ray diagnostics installation is described in OS No. 29 51 501 which includes a luminescent storage screen having a phosphor layer scanned by a read-out beam, with detectors being disposed on both sides of the storage screen. These detectors respectively collect the light emitted from the phosphor layer in an upward direction and the light emitted in a downward direction. A higher light yield and thus an improved bright-dark contrast are thereby achieved. Because only a single phosphor layer is used, however, the competing problems of efficiency of x-ray radiation absorption and image sharpness are still present.