Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2001-04-05
2002-12-10
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S581000, C250S585000
Reexamination Certificate
active
06492655
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for reading out image information, by scanning an image recording sheet having a stimulable phosphor layer with stimulating light, and detecting photostimulated luminescent light emitted from the sheet with an image detector that has a semiconductor as its primary portion.
2. Description of the Related Art
It is well known in the image reading field to utilize a stimulable phosphor sheet to obtain image information. The stimulable phosphor stores part of radiation energy when exposed to radiation, and exhibits photostimulated luminescence (PSL) according to the stored energy when exposed to stimulating light, such as visible light, etc. The radiation image information of a subject, such as a human body, etc., is temporarily recorded on the stimulable phosphor sheet having a stimulable phosphor layer. The stimulable phosphor sheet emits photostimulated luminescent light when scanned with a stimulating light beam such as a laser light beam. The photostimulated luminescent light is detected photoelectrically and converted to an image signal carrying the radiation image information. As the image information reading apparatus, there have been proposed a wide variety of apparatuses that vary in manner of scanning a stimulating light beam and converting photostimulated luminescent light to an image signal.
For example, as disclosed in Japanese Unexamined Patent Publication Nos. 55(1980)-12492 and 56(1981)-11395, a photomultiplier has been widely employed as a photoelectric converter, which detects photostimulated luminescent light emitted from a stimulable phosphor sheet when scanned with a stimulating light beam. The photomultiplier, however, has the following problems:
1) The photomultiplier is low in shock resistance, large in size, and high in cost, because it is constructed of a hollow glass tube.
2) The photomultiplier is fairly difficult to make thin, as it uses a complex multistage dynode to double photons. The cost of manufacturing a long photomultiplier 17 inches in width would become unduly high.
3) The quantum efficiency of the photocathode utilizing an external photoelectric effect is low. The quantum efficiency with respect to photostimulated luminescent light of wavelength 300 to 500 nm (blue light band) is typically as low as about 10 to 20%, whereas the quantum efficiency with respect to photostimulated luminescent light of wavelength 600 to 700 nm (red light band) is relatively great, typically about 0.1 to 2%. For this reason, a special stimulating-light cut filter becomes necessary to obtain a satisfactory signal-to-noise ratio (S/N ratio) and results in an increase in the manufacturing cost.
Hence, in order to solve the problems associated with the photomultiplier, Japanese Unexamined Patent Publication Nos. 58(1983)-121874, 60(1985)-111568, 60(1985)-236354, “Radiographic Process Utilizing a Photoconductive Solid-State Imager (772/Research disclosure, Oct. 1992/34264),” and Japanese Unexamined Patent Publication No. 7(1995)-76800, disclose that a semiconductor sensor (hereinafter referred to as a solid-state image detector) is used instead of the photomultiplier. The solid-state image detector employs a semiconductor (such as ZnS, ZnSe, CdS, TiO
2
, ZnO, &agr;-SiH, CdS(Cu), ZnS(Al), CdSe, PbO, &agr;-Se, etc.) as a photoconductor.
The solid-state image detectors consisting of the above-mentioned semiconductors, however, have the following problems: photostimulated luminescent light is feeble, so a photoconductor to be used is required to have extremely high dark resistance, but the disclosed photoconductors are all low in dark resistance; and since image reading is performed under a relatively strong electric field, the dark current increases and is superposed on the current component carrying an image signal and develops as dark latent image noise in a reproduced image. Particularly, &agr;-SiH, CdS(Cu), ZnS(Al), CdSe, and PbO, whose bandgap is small, have a large dark current resulting from thermal excitation, so it is extremely difficult to obtain a satisfactory S/N ratio.
&agr;-Se has relatively high dark resistance and can be employed with 10
15
&OHgr;·cm (at 10V/&mgr;m). However, in the case where &agr;-Se is used for detecting photostimulated luminescent light, the dark resistance thereof is insufficient to obtain an image with a satisfactory S/N ratio and is therefore desired to be greater than 10
15
&OHgr;·cm. Furthermore, the solid-state image detector has not only the problem of dark current but also the problem that, because a pseudo signal occurs because of residual electric charge left unread, an image with a satisfactory S/N ratio cannot be obtained.
SUMMARY OF THE INVENTION
The present invention has been made in view of the aforementioned circumstances. Accordingly, it is the primary object of the present invention to provide an image information reading method and an image information reading apparatus that are capable of obtaining an image whose S/N ratio is satisfactory, without being subjected to the influence of dark current or residual electric charge, when reading out image information from an image recording sheet by use of an solid-state image detector.
To achieve the objects mentioned above and in accordance with the present invention, there is provided a first method of obtaining an image signal, comprising the steps of: preparing an image recording sheet having a stimulable phosphor layer which emits photostimulated luminescent light by a quantity corresponding to stored energy when irradiated with stimulating light; scanning the image recording sheet in horizontal and vertical scanning directions with the stimulating light; and obtaining the image signal by detecting the photostimulated luminescent light emitted by the scanning, with a solid-state image detector having a photoconductor; wherein a signal, output from the solid-state image detector when scanning a non-image region other than an image recording region on the image recording sheet with the stimulating light, is detected as a correction signal; and a non-image component, contained in a signal output from the solid-state image detector when scanning the image recording region with the stimulating light, is suppressed with the correction signal.
In the case where a light beam is used as the stimulating light, the aforementioned horizontal scanning direction refers to a direction in which the stimulating light and the scanning speed are relatively high, while the aforementioned vertical scanning direction refers to a direction in which the stimulating light and the scanning speed are relatively low. On the other hand, in the case where long line light is used as the stimulating light, the horizontal scanning direction refers to a direction in which the line light extends, while the vertical scanning direction refers to a direction in which the line light is moved with respect to the image recording sheet.
The expression “non-image region other than an image recording region on the image recording sheet” is intended to mean a region where photostimulated luminescent light carrying image information is not incident on the solid-state image detector even if the stimulating light is scanned on the region. In connection with this meaning, it does not matter whether or not image information has been recorded on the stimulable phosphor sheet. The non-image region may be a non-image region, formed so that photostimulated luminescent light carrying image information is not emitted, such as a region having no stimulable phosphor layer and a region formed so that radiation energy is not stored by intercepting radiation with a lead plate at the time of photographing. Also, it maybe a region on the stimulable phosphor sheet which corresponds to a portion formed so that photostimulated luminescent light is not incident on the detector with a member for intercepting photostimulated luminescent light, regardless of whether or not image information has been actually recorded. Fur
Fuji Photo Film Co. , Ltd.
Gagliardi Albert
Hannaher Constantine
Sughrue & Mion, PLLC
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