Stimulable phosphor sheet

Details Stimulable phosphor sheet Stimulable phosphor sheet

2000-03-15

2002-03-19

Dang, Hung Xuan (Department: 2873)

Radiant energy

Invisible radiation responsive nonelectric signalling

Luminescent device

C250S473100, C250S581000, C250S582000

Reexamination Certificate

active

06359285

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a stimulable phosphor sheet employable in a radiation image recording and reproducing method utilizing stimulated emission produced by a stimulable phosphor.
BACKGROUND OF THE INVENTION
As a method replacing a conventional radiography using a combination of a radiographic film and radiographic intensifying screens, a radiation image recording and reproducing method utilizing a stimulable phosphor was proposed and is practically employed. The method employs a stimulable phosphor sheet comprising a stimulable phosphor layer (i.e., radiation image storage panel) provided on a support, and the procedure of the method comprises the steps of causing the stimulable phosphor of the stimulable phosphor sheet to absorb radiation energy having passed through an object or having radiated from an object; sequentially exciting the stimulable phosphor with an electromagnetic wave such as visible light or infrared rays (hereinafter referred to as “stimulating rays”) to release the radiation energy stored in the phosphor as light emission (i.e., stimulated emission); photoelectrically detecting the emitted light to obtain electric signals; and reproducing the radiation image of the object as a visible image from the electric signals. The stimulable phosphor sheet thus processed is subjected to a step for erasing a radiation image remaining therein, and then stored for the next recording and reproducing procedure. Thus, the stimulable phosphor sheet can be repeatedly employed.
In general, a support and a protective film are provided on the top and bottom surfaces of the stimulable phosphor sheet, respectively. The stimulable phosphor sheet generally comprises a binder and stimulable phosphor particles dispersed therein, but it may consist of agglomerated phosphor without binder. The stimulable phosphor sheet containing no binder can be formed by the deposition process or firing process. Further, the stimulable phosphor sheet comprising agglomerated phosphor soaked with a polymer is also known. For the aforementioned method, any types of the stimulable phosphor sheets are employable.
The radiation image recording and reproducing method is often used in radiography for medical diagnosis. In that case, it is especially desired to obtain a radiation image of high quality (particularly, high sharpness for high resolution) by applying a small dose of radiation. Therefore, the stimulable phosphor sheet is required to have high sensitivity and to provide a radiation image of high quality.
The sharpness of tradiation image is mainly affected by diffusion or scattering of the stimulating rays in the stimulable phosphor sheet. The procedure for reading the latent image comprises the steps of sequentially scanning a beam of the stimulating rays on the surface of the stimulable phosphor sheet to induce the stimulated emission, and successively detecting the stimulated emission. If the stimulating rays diffuse or scatter (horizontally in particular) on the plane of the phosphor sheet, it excites the phosphor not only at the target spot but also in its periphery. Consequently, the stimulated emission emitted from the target position is collected together with that from the periphery. Such contamination of the emissions impairs the sharpness of the resulting radiation image.
For avoiding the diffusion or scattering of the stimulating rays, it has been proposed to divide the plane of the stimulable phosphor sheet into small sections (cells) with a partition capable of reflecting the stimulating rays.
Japanese Patent Provisional Publication No. 59-202100 discloses a stimulable phosphor sheet having a honey-comb structure consisting of many small cells filled with a stimulable phosphor. The phosphor sheet comprises a substrate and a stimulable phosphor layer provided thereon, and the honey-comb structure sectioned with a partition is further provided on the phosphor layer.
Japanese Patent Provisional Publication No. 62-36599 discloses a stimulable phosphor sheet employing a support provided with many hollows regularly arranged on one surface. The hollows are filled with a stimulable phosphor, and the ratio of depth to diameter of each hollow is 3.5 or more.
Japanese Patent Provisional Publication No. 2-129600 discloses a stimulable phosphor sheet (i.e., radiation image storage panel) employing a support plate having many holes vertically bored and filled with a stimulable phosphor.
In these known stimulable phosphor sheets employing a support or substrate provided with many cells (i.e., holes or hollows) incorporated with a stimulable phosphor, a part of the support or substrate serves as a partition for reflecting the stimulating rays to prevent diffusion of the stimulating rays.
In the stimulable phosphor sheet having the cells, the partition preferably has a small thickness so that the stimulable phosphor-incorporated area is formed as large as possible. However, it is necessary to incorporate a light-absorbing material into a partition if the partition is made to have a small thickness, maintaining the light-intercepting property. The incorporation of the light-absorbing material into the partition decreases efficiency of recovery of the stimulated emission. If the thickness of the partition is increased, the amount of stimulable phosphor which can be incorporated into the stimulable phosphor sheet decreases and hence an enough amount of radiation energy is absorbed by the phosphor sheet. It can be considered to increase the thickness of the stimulable phosphor layer so as to incorporate an enough amount of the stimulable phosphor. However, the radiation energy hardly reaches the stimulable phosphor in the bottom portion in the phosphor layer having the increased thickness. Moreover, the stimulated emission produced in the bottom portion is not efficiently recovered from the surface of the stimulable phosphor sheet. For these reasons, there is not known a stimulable phosphor sheet that has a cell structure and gives a reproduced radiation image of satisfactorily high quality.
Accordingly, the present invention has an object to provide a stimulable phosphor sheet which gives a reproduced radiation image of high quality, particularly, in its sharpness.
The invention further has an object to provide a stimulable phosphor sheet that has high sensitivity and gives a reproduced radiation image of high quality, particularly, in its sharpness.
SUMMARY OF THE INVENTION
The present inventors have made studies on the cell structure of the stimulable phosphor sheet and discovered that the partition should have a high reflectance to the stimulating rays by shortening its light-scattering length and should have a low absorbance for the stimulated emission by prolonging its light-absorption length for the stimulated emission, so that the diffusion of the stimulating rays on the plane of the phosphor sheet can be efficiently reduced to increase sharpness of the obtained radiation image and the decrease of emission efficiency can be effectively obviated by suppressing absorption of the stimulating rays as well as the stimulated emission. The inventors have further discovered that the stimulable phosphor-incorporated area should show a high transmittance for the stimulating rays by prolonging its light-scattering length and should have a low light-absorbance for the stimulated emission by prolonging the light-absorption length for the stimulated emission, so that the stimulating rays can reach the deep area of the stimulable phosphor sheet and the stimulated emission produced in the deep area can be efficiently recovered. Thus designed stimulable phosphor sheet gives a reproduced radiation image of high quality. Moreover, the stimulable phosphor sheet which has such characteristics and a large thickness gives a reproduced radiation image of increased high quality, increasing its sensitivity, namely, a ratio of absorption of radiation energy.
The invention resides in a stimulable phosphor sheet for a radiation image recording and reproducing method comprising the steps of

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