The present invention relates to a radiographic image information reading method and a radiographic image reading apparatus, and in particular, to correction of an image to be read.
Recently, there has been devised a method for obtaining radiographic image information without using a radiographic film composed of a silver halide light-sensitive material. According to one such method, radiation transmitted through a subject is absorbed in a phosphor of a certain kind, and then the phosphor is excited by light or heat energy, for example, so that radiation energy accumulated in the phosphor through the aforesaid absorption is emitted as a fluorescence which is detected to be an image. To be concrete, for example, U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection No. 12144/1980 (hereinafter referred to as Japanese Patent O.P.I. Publication) disclose methods employing a radiographic image conversion panel and showing a radiographic image conversion method with excitation light of visible light or infrared radiation wherein a radiographic image conversion panel in which a stimurable phosphor layer is formed on a support is used. Radiation transmitted through a subject is projected on the accelerating phosphor layer of the radiographic image conversion panel so that radiation energy corresponding to radiation transmittance on each part of the subject is accumulated to form a latent image. Then, the accelerating phosphor layer is scanned by the aforesaid exciting light so that radiation energy accumulated on each part of the panel is emitted to be converted to light, and then, its intensity is detected by a photoelectric transfer element such as a photomultiplier or a photodiode to obtain a radiographic image.
FIG. 19 is an illustration showing how an image is recorded on a stimurable phosphor like that explained above. In the drawing, X-rays emitted from X-ray source 1 are narrowed down by diaphragm 2, and then are projected on subject 3. X-rays transmitted through the subject 3 enter accelerating phosphor 4 wherein a latent image of an image of the subject 3 is formed.
FIG. 20 is a block diagram showing a structural example of a conventional radiographic image information reading apparatus which reads radiographic images recorded on an accelerating phosphor in the manner explained above. The numeral 101 is a semi-conductor laser light source for generating exciting light, and the semi-conductor laser light source 101 is driven by laser driver circuit 102 in the form of pulses in synchronization with an image clock signal from image clock generator 125. Laser beam LB generated from the semi-conductor laser light source 101 arrives at deflector 107 through monochromatic light filter 103, mirror 104, beam-forming optical system 105 and mirror 106. The deflector 107 is equipped with polygon mirror 109 driven by deflector driver 108, and it deflects the laser beam LB to cover a certain angle in the scanning area. The deflected laser beam LB is adjusted to be at a constant speed on a scanning line, and it scans on radiographic image conversion panel 111 employing a stimurable phosphor as a radiographic image information recording medium in the direction of arrow "a" through mirror 110. The radiographic image conversion panel 111 moves simultaneously in the sub-scanning direction (direction of arrow "b"), and thereby an overall surface can be scanned. Light emitted from the radiographic image conversion panel 111 after scanning by the laser beam LB are converged by converging means 112 and arrive, through filter 113 transmitting only a wavelength range of the light, at light detector 114 provided with a photomultiplier where the light are converted to analog electric signals (image signals).
The numeral 115 is a power supply which supplies high voltage to the light detector 114 (photomultiplier). An image signal outputted as an electric current from the light detector 114 passes through front end amplifier 116 to be voltage-amplified, and further passes through logarithmic amplifier 118 that converts radiation intensity signals into image density signals, filter 119 and sample hold circuit 120 that holds signals for a certain period in synchronization with image clock signals, and then is converted into a digital signal by A/D converter 121 to be sent to an external data processing apparatus through interface 124.
In the case of reading radiographic image information, however, there have been problems such as sensitivity irregularity of radiations and accelerating phosphors (irregularity in both main- and sub-scanning directions), irregularity of exciting light scanning system and light converging system (irregularity in main-scanning direction) and an influence of a change with age of a stimurable phosphor (irregularity in the sub-scanning direction). As a technology for overcoming the aforesaid problems, there has been a correction technology described in Japanese Patent O.P.I. Publication No. 234643/1985.
However, in the technology described in the aforesaid patent, correction data for correcting the data read have been needed to be prepared for all pixels of a radiographic image conversion panel. Therefore, the capacity of a memory for the storage of correction data needs to be increased. When using a plurality of radiographic image conversion panels, in particular, memory capacity is increased in proportion to the number of panels, which is not preferable in practical use.
For solving the problems mentioned above, the present inventor proposed a novel method and a novel apparatus for reading radiographic image information (Japanese Patent O.P.I. Publication No. 158536/1988). According to this invention, correction data in a form of a line in the main-scanning direction and/or the sub-scanning direction are obtained from a solid image obtained through photographing without a subject and stored, and they are used for correcting image data in the case of photographing an image with a subject arranged. It is possible to save memory capacity greatly by having at least one list of correction data in the form of a line or a row without having correction data with total pixels two-dimensionally. Further, when obtaining correction data in the main-scanning direction or in the sub-scanning direction, it is possible to eliminate an influence of noise by averaging data of plural lines or rows.
In the invention, however, when streak-shaped noise (irregularity without position repeatability or an irregularity with poor reproducibility in terms of its position on an image) caused by vibration or irregularity of intervals on a polygon mirror appears on an image without a subject in the case of preparing correction data, the correction data can not be free from this influence. When image data is corrected by the use of correction data having the noise, the noise appears on the corrected image. As a cause of irregularity without position repeatability, there are irregularity in a light converging system and irregularity in an exciting light scanning system (defect and dust) for the main-scanning direction, and there are relative displacement (stationary vibration and shock from the outside) between a radiographic image conversion panel and a reading system, irregularity in an inclination angle of a polygon mirror and irregularity of reflection factor in mirror surfaces for the sub-scanning direction. With regard to such irregularity without repeatability, there have been some cases where the irregularity was increased when it was corrected.
FIGS. 24(a-1) to 24(c-2) are illustrations of some problems in conventional technologies. In the drawing, f1 in FIG. 24(a-1) represents correction data, FIG. 24(a-2) is a frequency spectrum of f1, f2 in FIG. 24(b-1) represents image data, FIG. 24(b-2) is a frequency spectrum of f2, and f3 in FIG. 24(c-1) represents image data after correction and FIG. 24(c-2) is a frequency spectrum of f3. The axis of ordinate for f1-f3 represents a size of image data, the axis of abscissa represents a position, and the axis of ordinate for a frequency spectrum represents intensity, and the axis of abscissa represents a spatial frequency. An example shown in the drawing indicates an occasion wherein irregularity having position repeatability at a relatively gentle change and irregularity having no position repeatability appearing at a constant cycle at a relatively abrupt change are mixed. Since the irregularity having position repeatability appears similarly on correction data f1 and image data f2, effective correction is made on image data f3 after correction. On the other hand, irregularity having no position repeatability appears differently in terms of position between correction data f1 and image data f2. Therefore, effective correction can not be obtained, and irregularity is rather increased on the corrected image data f3. This is clear from the frequency spectrum shown in FIG. 24(c-2), wherein a peak of a specific frequency component is increased.