Radiant energy – Source with recording detector – Using a stimulable phosphor
Reexamination Certificate
2001-11-14
2004-07-20
Hannaher, Constantine (Department: 2878)
Radiant energy
Source with recording detector
Using a stimulable phosphor
C250S581000
Reexamination Certificate
active
06765226
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a radiation image readout apparatus for reading out, by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, a radiation image stored on a stimulable phosphor sheet or the like.
2. Description of the Related Art
There are known stimulable phosphors, which upon the irradiation thereof by radiation store a portion of the radiation energy, and emit upon the subsequent irradiation thereof with a visible or other excitation light a stimulated emission corresponding to the stored radiation energy. Radiation image readout apparatuses employing, for example, stimulable phosphor sheets, which utilize these stimulable phosphors in a layer containing stimulable phosphors which has been formed on a substrate, to temporarily record the radiation image data of the radiation that has passed through a human body or other subject of photographing, upon the irradiation thereof by a radiation, are in wide use as CR (Computed Radiography) apparatuses.
An image signal obtained by the system described above is subjected to image processing such as gradation processing, frequency processing, etc. appropriate to diagnostic reading, after which the processed image signal is recorded on a film as a visible diagnostic image (final image), or displayed on a high-resolution CRT monitor for diagnostic reading by a doctor.
Further, in accordance with the radiation image readout apparatus utilizing the stimulable phosphor sheet described above and with a view to making the apparatus compact and of reduced cost, a line light source is used as an excitation light source for irradiating the sheet with an excitation light in a line form, together with a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line along the lengthwise direction of the line-shaped portions of a stimulable phosphor sheet that has been irradiated with an excitation light emitted from the line light source (hereinafter referred to as the main scanning direction); a scanning means for moving the line light source and line sensor from one end of the stimulable phosphor sheet to the other end in the direction substantially perpendicular to the lengthwise direction of the line-shaped portions (hereinafter referred to as the sub-scanning direction) (refer to Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540).
However, when a stimulated emission based on a radiation image is received by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, because the photoelectric converting elements are arranged at a uniform pitch, the stimulated emission is sampled at every pixel; accordingly, at this time, for cases in which the signal component of the stimulated emission received by the line sensor, based on a radiation image scanned by the line light source, includes a signal component having a frequency higher than ½ the sampling frequency (hereinafter referred to as the Nyquist frequency), a Moiré effect is caused in the final image, whereby the image quality thereof is deteriorated.
SUMMARY OF THE INVENTION
The radiation image readout apparatus according to the present invention has been developed in consideration of the circumstances described above, and it is a primary objective of the present invention to provide a radiation image readout apparatus for reading out, by use of a line sensor formed of a plurality of photoelectric converting elements arranged in a straight line, a radiation image stored on a stimulable phosphor sheet or the like, wherein the occurrence of a Moiré effect in the final image is controlled by preventing distortion due to feedback.
The first radiation image readout apparatus according to the present invention comprises: a illuminating means for illuminating with an excitation light in a line form a portion of the surface of a stimulable phosphor sheet on which a radiation image has been stored; a focusing optical system provided with a focusing lens for focusing the stimulated emission emitted from a portion of the stimulable phosphor sheet upon the illumination thereof by the excitation light emitted from the illuminating means in a line form and a detecting means provided with a line sensor, which is formed of a plurality of photoelectric converting elements arranged in a straight line and at a uniform pitch, for receiving and photoelectrically converting a stimulated emission focused by the focusing optical system; a scanning means for moving the illuminating means and the detecting means relatively from one end of the stimulable phosphor sheet to another in a direction differing from that of the lengthwise direction of the illuminated portions, wherein the MTF of the focusing optical system is 50% or less of the Nyquist frequency determined by the aforementioned uniform pitch.
Here, the referent of “the focusing lens of the focusing optical system” is not limited to being a single lens, but includes systems provided with a plurality of focusing lenses, as well as systems provided with elements for performing an optical function other than focusing.
Further, the referent of “the MTF of the focusing optical system” as shown in
FIG. 4
, for example, falls in inverse proportion to the rising of the frequency of the signal component of a stimulated emission based on a radiation image. Further, the phrase “the Nyquist frequency determined by the aforementioned uniform pitch” refers to the frequency equivalent to ½ the sampling frequency, which is determined by the pitch at which the photoelectric converting elements of the line sensor are disposed. Generally, if there is a signal component having a frequency higher than the Nyquist frequency, distortion due to feedback is produced. The expression “the MTF of the focusing optical system is 50% or less of the Nyquist frequency determined by the aforementioned uniform pitch” means that the signal component occurring in the Nyquist frequency is caused to become 50% or less by the MTF of the focusing optical system.
Still further, the MTF of the focusing optical system can also be caused to be 20% or less of the Nyquist frequency determined by the aforementioned uniform pitch, as shown in FIG.
5
.
The second radiation image readout apparatus according to the present invention comprises: a illuminating means for illuminating with an excitation light in a line form a portion of the surface of a stimulable phosphor sheet on which a radiation image has been stored; a focusing optical system provided with a focusing lens for focusing the stimulated emission emitted from a portion of the stimulable phosphor sheet upon the illumination thereof by the excitation light emitted from the illuminating means in a line form; a detecting means provided with a line sensor, which is formed of a plurality of photoelectric converting elements arranged in a straight line and at a uniform pitch, for receiving and photoelectrically converting a stimulated emission focused by the focusing optical system; a scanning means for moving the illuminating means and the detecting means relatively from one end of the stimulable phosphor sheet to another in a direction differing from that of the lengthwise direction of the illuminated portions; and a readout means for reading out the output of the line sensor in the order corresponding to the movement thereof, and obtaining the data forming the final image; wherein the aforementioned uniform pitch is less than the width of the pixels of the final image; further comprising a pixel-density converting filter for converting the image signal of the pixel density output from the line sensor based on the aforementioned uniform pitch to the pixel density of the final image; wherein the frequency characteristic of the pixel-density converting filter is caused to be 50% or less of the Nyquist frequency determined by the pixel density of the final image.
Here, the expression
Fuji Photo Film Co. , Ltd.
Hannaher Constantine
Moran Timothy
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