Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2001-06-29
2003-05-27
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S586000, C250S584000
Reexamination Certificate
active
06570178
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a method of and apparatus for reading out radiation image data, and more particularly to a method of and apparatus for reading out the radiation image data stored on a stimulable phosphor sheet by use of a line sensor formed of one or more CCD sensors.
2. Description of the Related Art
There are in wide use today technologies for obtaining an image signal of a radiation image by use of a stimulable phosphor sheet. Such stimulable phosphor sheets have a layer containing stimulable phosphor, which are supported on a sheet-type substrate, that cumulatively store a portion of the radiation passing through the body of a subject, such as the body of a human being, and emit a stimulated emission corresponding to the radiation energy stored thereon upon irradiation thereof by a stimulating light such as visible light or a laser beam. In such technologies, radiation image energy that has been once cumulatively stored on a stimulable phosphor sheet is caused to be emitted as a stimulated emission by scanning each pixel of the stimulable phosphor sheet in order with a deflected laser beam or other stimulating light; the stimulable emission is then photoelectrically read out in order by a photoelectrical readout means and an image signal is obtained. On the other hand, after the radiation image signal has been readout, the stimulable phosphor sheet is irradiated with an erasing-light that causes the radiation energy remaining thereon to be dissipated, and the sheet can be reused for the same operation of storing and reading out a radiation image (refer to Japanese Unexamined Patent Publication Nos. 55(1980)-12429, 55(1980)-116340, 56(1981)-104645, and etc.).
In addition, a system has been proposed in which, in order to raise the efficiency of quantum detection upon formation of the radiation image, that is, the absorption rate of the radiation energy, the efficiency of the output of the stimulated emission, and the efficiency of detecting the stimulated emission, the radiation absorbing function and the energy accumulating function occurring in a conventional stimulable phosphor are separated, and a phosphor having superior radiation absorption properties and a phosphor showing superior response to stimulable emission are each used separately for absorbing radiation and accumulating radiation image data, respectively: using the phosphor superior in absorbing radiation (radiation absorption use phosphor), the radiation is absorbed and light in the visible to UV spectra is emitted; this emitted light is absorbed and the energy therefrom accumulated using the phosphor showing superior response to stimulated emission (dedicated accumulation-use phosphor); this accumulated energy is stimulated by the irradiation thereof with a stimulating light in the visible to infrared range, causing a stimulated emission to be emitted; and the light of the stimulated emission is photoelectrically readout sequentially by a photoelectrical readout means and a image signal is obtained (refer to Japanese Patent Application No. 11(1999)-372978).
The image signal obtained by these systems is subjected to gradation processing, frequency processing, and etc. image processing applied in diagnostic reading, and after being subjected to such processing, the image signal is recorded on film as a diagnostic image (final image), or displayed on a high-resolution CRT display for diagnostic use by a doctor, etc. On the other hand, if the stimulable phosphor sheet is irradiated by an erasing-light, the energy remaining on the stimulable phosphor sheet after the radiation image data once stored thereon has been readout is erased, and it becomes possible to again cumulatively record radiation image data on the sheet, and it possible to repeatedly use the sheet for such operations.
Here, according to the radiation image data readout apparatus occurring in the radiation image recording and reproducing systems described above, a configuration has been proposed with a view to shortening the readout time for the stimulated emission, reducing the cast and the size of the apparatus: a line-light source is used as the stimulating-light source for irradiating the stimulable phosphor sheet with a line-shaped stimulating-light; a line sensor formed of a plurality of photoelectric converting elements arranged along the lengthwise direction (hereinafter referred to as the main-scanning direction) of the line-shaped portion of the stimulable phosphor sheet irradiated by the stimulating-light emitted from the line-light source; and a scanning apparatus moving in a direction substantially perpendicular to the lengthwise direction of aforementioned line-shaped portion (hereinafter referred to as the sub-scanning direction), from one side to another of the sheet relative to the line-light source and the line sensor (refer to Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, 1(1989)-101540, etc.).
However, because of remarkable performance advances such as increased pixel density, improved sensitivity, noise reduction, reduction in image size, and etc. attainable due to innovations in micro-processing used in the manufacture of semiconductors in recent years, CCD sensors are frequently used as the photoelectric converting elements that form the line sensor described above.
However, there is a limit inherent in a line sensor formed of one or more CCD sensors regarding the quantity of charge that can be accumulated before the charge saturation quantity is reached, and because each CCD element of each CCD sensor can not detect quantities of light above a fixed quantity, for portions of a stimulable phosphor sheet where a large amount of radiation data (radiation energy) have been accumulated (high-radiation zones), a precise measurement thereof cannot be obtained, and a problem arises in that the range with respect to the quantity of light possible of being read out (dynamic range) when a series of CCD sensors are used as a line sensor is said to be narrow.
Therefore, as described in PCT International Publication No. WO99/28765, a method for expanding the dynamic range of the line sensor has been proposed, wherein, readout of a quantity of light below the saturated charge quantity is performed a plurality of times, and by adding together the data obtained by this plurality of readout operations, each CCD element does not become saturated.
However, in the method proposed in PCT International Publication No. WO99/28765, because the actual quantity of radiation energy stored on each portion of a stimulable phosphor sheet is not grasped and readout of low-quantity of light is simply performed repeatedly for high-radiation zones, each CCD element may become substantially saturated, and there is a possibility that it will not be possible to read out accurate data. Further, because the noise occurring when readout is performed n times is n times as large as the noise occurring when readout is performed only once, for portions of a stimulable phosphor sheet which have accumulated only a small quantity of radiation energy (low-radiation zones), the higher the number of times readout is performed, the lower the S/N ratio becomes, and a problem arises in that the quality of the image is thereby deteriorated.
SUMMARY OF THE INVENTION
The present invention has been developed in consideration of the circumstances described above, and it is a primary object of the present invention to provide a method and apparatus for always performing read out of radiation image data at a detected light quantity range below the charge saturation quantity of the each CCD element of CCD sensors forming a line sensor, which controls the increase of noise due to performing readout a plurality of times and expands the dynamic range of the line sensor.
The radiation image data readout method according to the present invention comprises: irradiating with a stimulating-light a portion of the face of a stimulable phosphor sheet on which radiation image data has
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