Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
1999-03-30
2001-11-06
Kim, Robert H. (Department: 2882)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S584000, C250S586000
Reexamination Certificate
active
06313477
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an improvement in a method of acquiring shading and/or fading correction data for a radiation image reading apparatus which reads image signals from both sides of a stimulable phosphor sheet storing a radiation image.
Description of the Related Art
Radiation image recording reproducing systems using stimulable phosphor which emits light, upon exposure to a stimulating ray such as visible light, in accordance with radiation energy stored therein originated form radioactive rays (such as X rays, &agr; rays, &bgr; rays, &ggr; rays, electron rays, and ultraviolet rays) have been irradiated thereon have been known (see Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 56(1981)-11395, and 56(1981)-11397, for example). A radiation image recording reproducing system records radiation image information of a subject such as a human body on a stimulable phosphor sheet and causes the sheet to emit light by scanning the sheet with a laser beam or the like. The emitted light is read photoelectrically and an image signal is thereby obtained. Based on the image signal, the radiation image recording reproducing system causes the radiation image of the subject to be output as a visual image on a recording medium such as a photosensitive material or on a radiation image display apparatus. A system of this kind is practically advantageous in terms of capability of recording an image in an extremely wide radiation exposure range compared to radiography systems using conventional silver halide film photographs.
However, in some cases, image information reading apparatuses used in the radiation image recording reproducing systems described above have fluctuations of image signals obtained by their photoelectric reading means, caused by unevenness in strength of a scanning beam due to a fluctuation in reflectance of a reflection plane of a photodeflector (such as a polygon mirror) for scanning with stimulating rays, or caused by a scanning speed fluctuation due to variance of deflection speed of the photodeflector, or caused by uneven delectability due to variance in sensitivity along the main scan direction of photoelectric reading means such as a photomultiplier (photomultiplier tube) having an elongated shape and placed along the main scan direction. Once a partial reduction (shading) in photodetection efficiency due to such unevenness occurs, it becomes impossible to properly detect image information recorded on the scanned surface.
The applicant has already proposed a shading correction method and the like (see Japanese Unexamined Patent Publication Nos. 61(1986)-189763, 62(1987)-47259, 62(1987)-47261, 64(1989)-86759, and 2 (1990)-58973, for example) which avoidthe effects of shading by correcting image signals or the sensitivity of photoelectric reading means corresponding to main scan position of a beam, for example, based on a shading characteristic (shading correction data) pre-detected by using a stimulable phosphon sheet on which radioactive rays have been irradiated uniformly.
Furthermore, the stimulable phosphor sheets used in such radiation image recording reproducing systems have a phenomenon (fading phenomenon) such that the amount of light emitted therefrom decreases when the stimulable phosphor sheets are stimulated by stimulating rays, due to gradual loss in the radiation energy stored thereon, in accordance with time elapsed after recording of a radiation image on a stimulable phosphor sheet, and progresses with time elapsed thereafter. In the system described above, the phenomenon causes lower sensitivity of the image signal read by the system, in the case where there is a time-lag between radiation image recording and image signal reading by exposure of the sheet to the stimulating ray. Furthermore, in a system carrying out pre-reading, a main reading condition determined by pre-reading becomes inappropriate due to the fading phenomenon if there is a time-lag between pre-reading and main reading.
It has been known that the loss of radiation energy stored on a stimulable phosphor sheet reaches an equilibrium state after a certain amount of time has elapsed after irradiating radiation on the sheet. Therefore, after the certain amount of time has elapsed, no fluctuation with time due to the fading effect is observed when the image signal representing a radiation image is obtained from the stimulable phosphor sheet. Consequently, if image signals are obtained upon exposure of the stimulable phosphor sheets to a stimulating ray after fluctuation with time due to the fading effect has been diminished, no fluctuation in image signal levels due to the elapsed time is observed and images having temporarily uniform quality can be obtained. However, since some amount of time is necessary for the fading phenomenon to reach the equilibrium state, efficient reading has not been carried out in the case where images have temporarily uniform quality are sought.
In order to efficiently obtain images having temporally uniform quality by promoting radiation energy loss caused by fading, a radiation image reading method wherein a stimulable phosphor sheet is heated to promote radiation energy loss due to fading, before exposure to a stimulating ray and after irradiation of a radioactive ray thereon, so that radiation energy loss from the sheet after the radioactive ray has been irradiated thereon can promptly reach the equilibrium state has been proposed (Japanese Unexamined Patent Publication No. 59(1984)-72079).
Furthermore, another radiation image reading method wherein a stimulable phosphor sheet storing a radiation image is chilled immediately after radiation has been irradiated thereon so that loss of the radiation energy stored on the sheet is prevented and image reproducibility is improved by suppressing the fading effect, has also been proposed (Japanese Unexamined Patent Publication No. 63(1988)-165842).
Moreover, taking fading into consideration, a main reading condition correcting method wherein a main reading condition temporarily determined through pre-reading is correted so that an appropriate reading condition is always output has been proposed (Japanese Unexamined Patent Publication No. 4(1992)-157440). In this method, a fading characteristic in accordance with time elapsed after photographing is pre-stored. The amount of fading at pre-reading is thus found by measuring the time elapsed from photographing to pre-reading and the amount of fading at main reading is also found by measuring the time elapsed from photographing to mainreading. The sensitivity at mainreading temporarily set based on a pre-read signal is corrected based on the fading amount difference between pre-reading and main reading. In other words, a fading characteristic in accordance with time Elapsed after photographing is pre-stored, and the amount of fading at pre-reading (F
1
) is found from time (T
1
) elapsed between photographing and pre-reading. The amount of fading at main reading (F
2
) is also found from time (T
2
) elapsed between photographing and reading. The sensitivity (Sk) temporarily set based on a signal at pre-reading is then corrected based on the fading amount difference (F
1
-F
2
) between pre-reading and main reading.
Meanwhile, as a method of photoelectrically reading the emitted light, a radiation image reading apparatus which comprises photoelectric reading means separately located on the front and back sides of a stimulable phosphor sheet and respectively reads the light emitted from both sides of the sheet by using the photoelectric reading means on both sides while irradiating a stimulating ray either on one side or on both sides to scan the sheet, has been proposed (see Japanese Unexamined Patent Publication No. 55(1980)-87970, for example). Such a radiation image reading apparatus has an improved light collection efficiency and thus an improved S/N ratio, since one stimulable phosphor sheet has recorded one radiation image and the light emitted therefrom can be collected by the photoelectric reading means on both
Iwakiri Naoto
Yasuda Hiroaki
Fuji Photo Film Co. , Ltd.
Ho Allen C.
Kim Robert H.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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