Stimulable phosphor sheet

Details Stimulable phosphor sheet Stimulable phosphor sheet

2000-09-29

2003-09-23

Hannaher, Constantine (Department: 2878)

Radiant energy

Source with recording detector

Using a stimulable phosphor

C250S484400

Reexamination Certificate

active

06624436

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a stimulable phosphor sheet. This invention particularly relates to a stimulable phosphor sheet for use in an energy subtraction processing technique for radiation images, wherein an image signal is obtained which represents an image of a specific structure or part of an object represented by the radiation images.
2. Description of the Related Art
It has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is firstly exposed to radiation, which carries image information of an object, such as a human body. In this manner, a radiation image of the object is stored on the stimulable phosphor sheet. The stimulable phosphor sheet, on which the radiation image has been stored, is then exposed to stimulating rays, which cause the stimulable phosphor sheet 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, when it is exposed to the stimulating rays, is photoelectrically detected and converted into an electric image signal. The electric image signal is then processed, and the processed image signal is utilized for reproducing a visible image, which has good image quality and can serve as an effective tool in, particularly, the efficient and accurate diagnosis of an illness. The visible image finally obtained may be reproduced in the form of a hard copy or may be displayed on a display device, such as a cathode ray tube (CRT) display device. In the radiation image recording and reproducing systems, the stimulable phosphor sheet temporarily stores the radiation image, such that a final visible image can be reproduced on a final recording medium. For the sake of economy, therefore, it is desirable that the stimulable phosphor sheet be used repeatedly.
In order that the stimulable phosphor sheet may be reused in the manner described above, any energy remaining on the stimulable phosphor sheet after the image signal has been detected from the stimulable phosphor sheet may be erased. For such purposes, as disclosed in, for example, U.S. Pat. No. 4,400,619 or Japanese Unexamined Patent Publication No. 56(1981)-12599, the stimulable phosphor sheet may be exposed to erasing light or heat, and energy remaining on the stimulable phosphor sheet may thereby be released. The erased stimulable phosphor sheet can then be reused to record a next radiation image.
Also, subtraction processing techniques for radiation images have heretofore been known. With the subtraction processing techniques for radiation images, an image corresponding to a difference between a plurality of radiation images of an object, which have been recorded under different conditions, is obtained. Specifically, a plurality of the radiation images, which have been recorded under different conditions, are read out at predetermined sampling intervals, and a plurality of image signals thus detected are converted into digital image signals which represent the radiation images. The image signal components of the digital image signals, which image signal components represent the image information recorded at corresponding sampling points in the radiation images, are then subtracted from each other. A difference signal is thereby obtained which represents the image of a specific structure or part of the object represented by the radiation images.
Basically, subtraction processing is carried out with either the so-called temporal (time difference) subtraction processing technique or the so-called energy subtraction processing technique. In the temporal subtraction processing technique, in order for the image of a specific structure of an object (for example, in cases where the object is a human body, the image of a blood vessel) to be extracted from the image of the whole object, the image signal representing a radiation image, which has been obtained without injection of contrast media, is subtracted from the image signal representing a radiation image, in which the image of the specific structure (for example, the blood vessel) of the object has been enhanced by the injection of contrast media. In the energy subtraction processing technique, such characteristics are utilized that a specific structure of an object exhibits different levels of radiation absorptivity with respect to radiation having different energy distributions. Specifically, an object is exposed to several kinds of radiation with different energy distributions. Alternatively, the energy distribution of the radiation, which carries image information of an object, is changed after the radiation has been irradiated onto one of a plurality of stimulable phosphor sheets, after which the radiation impinges upon the second stimulable phosphor sheet. In this manner, a plurality of radiation images, in which different images of a specific structure are embedded, are formed with the radiation having different energy distributions. Thereafter, the image signals representing the plurality of the radiation images are weighted appropriately and subjected to a subtraction process in order to extract the image of the specific structure.
The subtraction processing is extremely effective, particularly for medical diagnosis, and electronics research has continued to develop improved subtraction processing techniques.
In the aforesaid radiation image recording and reproducing systems utilizing a stimulable phosphor sheet, the radiation image stored on the stimulable phosphor sheet is read out directly as an electric image signal. Therefore, with the radiation image recording and reproducing systems, the subtraction processing described above can readily be carried out. In cases where the energy subtraction processing is to be carried out, radiation images may be stored on two stimulable phosphor sheets such that the parts of the radiation images, which parts correspond to a specific structure, are different in the two radiation images. For such purposes, a technique may be employed, wherein the operation for recording a radiation image is performed two times with two kinds of radiation having different energy distributions. Alternatively, a technique may be employed, wherein a filter, which is constituted of a metal, or the like, and which absorbs low energy components of radiation, is located between two stimulable phosphor sheets, and the two stimulable phosphor sheets are simultaneously exposed to radiation, which carries the image information of the object. However, with the former technique, the problems occur in that the operation for recording the radiation image must be performed two times, and an artifact is apt to occur due to movement of the object between the two radiation image recording operations. Also, with the former technique, radiation irradiating means must be adjusted for each of the radiation image recording operations, and therefore the operation characteristics are bad. With the latter technique, the problems occur in that the filter must be located between the stimulable phosphor sheets, and therefore the processing of the stimulable phosphor sheets at the time of the image recording and the image readout is not easy to perform.
In order for the aforesaid problems to be eliminated, there has been proposed a stimulable phosphor sheet comprising a substrate constituted of a substance, which absorbs low energy components of radiation, and stimulable phosphor layers, which are overlaid respectively on the two surfaces of the substrate. Such a stimulable phosphor sheet is disclosed in, for example, U.S. Pat. No. 4,855,598. In cases where the proposed stimulable phosphor sheet is exposed to radiation carrying image information of an object, radiation images of the object are stored respectively on the two stimulable phosphor layers of the stimulable phosphor sheet. Scanning with the stimulating rays is performed successi

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