Radiation image information read-out method and system

Details Radiation image information read-out method and system Radiation image information read-out method and system

2002-05-22

2003-10-21

Gagliardi, Albert (Department: 2878)

Radiant energy

Source with recording detector

Using a stimulable phosphor

C250S585000, C250S586000

Reexamination Certificate

active

06635897

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method of and a system for reading out radiation image information stored on a stimulable phosphor sheet in which the stimulable phosphor sheet is exposed to stimulating rays, thereby causing it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation and the light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal representing the radiation image information, and more particularly to an improvement for preventing saturation of a photodetector having a photoelectric surface.
2. Description of the Related Art
When certain kinds of phosphors are exposed to radiation such as X-rays, &agr;-rays, &bgr;-rays, &ggr;-rays, cathode rays or ultraviolet rays, they store a part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is referred to as “a stimulable phosphor”. It has been known to use stimulable phosphors in radiation image recording and reproducing systems (sometimes referred to as “computed radiography”). Specifically, a radiation image of an object, such as a human body, is recorded on a stimulable phosphor sheet (a recording medium provided with a layer of the stimulable phosphor). The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is used for reproducing the radiation image of the object as a visible image on a recording medium such as a photosensitive material or a display such as a CRT. See Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, 56(1981)-11397 and the like.
The radiation image recording and reproducing system is practically advantageous in that as compared with conventional radiographies using silver halide film, an image can be recorded over an extremely wide radiation exposure range.
When reading out radiation image information from the stimulable phosphor sheet in the radiation image recording and reproducing system, for instance, a light beam such as a laser beam is caused to two-dimensionally scan the stimulable phosphor sheet storing thereon a radiation image, and the light emitted from the stimulable phosphor sheet upon stimulation thereof is transmitted to a photodetector through an optical guide having a light inlet end face extending along the main scanning line. The photodetector photoelectrically detects in time the light emitted from the stimulable phosphor sheet upon stimulation thereof and an image signal made up of image signal components for respective picture elements is obtained.
The photodetectors generally employed in such systems include those utilizing an internal photoelectric effect, e.g., a phototransistor, a photodiode and the like, and those having a photoelectric surface and utilizing a photoemissive effect on the photoelectric surface, e.g., a photomultiplier. When those having a photoelectric surface such as a photomultiplier are employed in the above system, the following provision is generally made in order to improve sensitivity of the photodetector.
That is, the photodetector of this type is generally provided with an optical guide for collecting the light, emitted from the stimulable phosphor sheet upon stimulation thereof, to the photoelectric surface. The provision involves increasing light collecting efficiency to the photoelectric surface by improving the light collecting performance of the optical guide and/or employment of a photoelectric surface made of a material such as bialkali, e.g., Sb—K—Cs, which is high in quantum efficiency.
However, increasing the sensitivity of a photodetector with a photoelectric surface gives rise to another problem that a saturation phenomenon, that the sensitivity of the photodetector deteriorates for a while after detection of a large amount of light, is apt to occur at the photoelectric surface and when a visible image is reproduced on the basis of an image signal obtained from a saturated photodetector, a ghost image appears and the image quality deteriorates.
The system may be employed in general industrial field as well as a medical field. In the medical field, the object is the human body and since the irradiation dose does not greatly vary depending on the part whose radiation image is to be taken, the amount of light emitted from the stimulable phosphor sheet upon stimulation thereof does not greatly fluctuate.
To the contrast, in the general industrial field, where the system is used in non-destructive inspection of products such as a cast iron block, an iron block and the like, various kinds of materials can be the object. Accordingly, in order to obtain radiation image information suitable for the respective kinds of materials, the irradiation dose varies depending on the kind of the object over a very wide range (two to three figures in terms of dose ratio).
Thus in the radiation image recording and reproducing system for use in the general industrial field, the aforesaid saturation phenomenon is apt to occur, and accordingly, there has been a demand for a radiation image information read-out system in which the saturation phenomenon at the photodetector is suppressed with the sensitivity of the photodetector kept high.
The intensity of light emitted from the stimulable phosphor sheet upon stimulation thereof rapidly increases from initiation of exposure to the stimulating light and is maximized in a short time (e.g., in several ns) and then is gradually weakened, with the stimulable phosphor sheet keeping emitting light for a time unique to the phosphors on the stimulable phosphor sheet after termination of exposure to the stimulating light. The light emitted from the stimulable phosphor sheet after termination of exposure to the stimulating light is generally referred to as “afterglow”. Accordingly when the stimulable phosphor sheet is scanned by the stimulating light and the light emitted from the stimulable phosphor sheet is photoelectrically read out in time series, the afterglow component of picture elements precedingly exposed to the stimulating light is read in addition to the light emitted from a given picture element upon stimulation thereof as the radiation image information component for the picture element, which results in incomplete separation of image signal components for the picture elements and deterioration in sharpness of the reproduced image. Accordingly, when the stimulable phosphors on the stimulable phosphor sheet exhibit long afterglow, the sharpness of the image deteriorates to an unacceptable level.
Such a phenomenon occurs substantially in proportion to the irradiation dose (radiation energy) of the stimulating light per unit area of the stimulable phosphor sheet. Accordingly, for example, when a picture element where the amount of light emitted upon stimulation is relatively small exists just behind a picture element where the amount of light emitted upon stimulation is relatively large, the afterglow component from the large emission picture element is superposed on the light emitted from the small emission picture element upon stimulation thereof, and the amount of light read out as that emitted from the small emission picture element is increased by the amount of the afterglow. This reduces the difference between the image signal components for the large emission picture element and the small emission picture element as compared with the actual difference therebetween, and accordingly the reproduced image deteriorat

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