JP2717649B2 - Image signal noise reduction method and apparatus - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for reducing noise of an image signal obtained by subjecting a stimulable phosphor sheet to a reading process, and an apparatus for implementing the method. (Technical Background of the Invention and Prior Art) Certain phosphors are exposed to radiation (X-ray, α-ray, β-ray, γ-ray,
(Electron beams, ultraviolet rays, etc.), a part of this radiation energy is accumulated in the phosphor, and when this phosphor is irradiated with excitation light such as visible light, the phosphor is phosphorated according to the accumulated energy. Is known to exhibit stimulable luminescence, and a phosphor exhibiting such properties is called an accumulating phosphor (stimulable phosphor). Using this stimulable phosphor, radiation image information of a subject such as a human body is temporarily recorded on a stimulable phosphor sheet, and the stimulable phosphor sheet is irradiated with excitation light to emit photostimulated light. The photostimulated emission light obtained is photoelectrically read by a photodetector to obtain an image signal. Based on the image signal, a radiation image of the subject is recorded on a recording material such as a photographic photosensitive material or a display device such as a CRT. A radiation image information recording / reproducing system for outputting as a visible image has already been proposed by the present applicant.
(Japanese Patent Application Laid-Open Nos. 55-12429 and 56-11395) This system can record images over an extremely wide radiation exposure area as compared with a conventional radiographic system using silver halide photography. Has advantages. That is, in the case of stimulable phosphors, it has been recognized that the amount of emitted light that is stimulated by excitation after accumulation is proportional to the radiation exposure amount over an extremely wide range. Even if the amount fluctuates considerably, the amount of photostimulated light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means with the reading gain set to an appropriate value and converted into an electric signal. By outputting a radiation image as a visible image on a recording material such as a photographic photosensitive material or a display device such as a CRT using an electric signal, it is possible to obtain a radiation image which is not largely affected by a change in radiation exposure. Therefore, in this radiation image information recording / reproducing system, high-sensitivity recording (photographing) may be performed by setting the radiation exposure amount particularly low. In the recording of a general transmitted radiation image, a portion where the radiation dose is relatively high and a portion where the radiation dose is relatively low occur on the stimulable phosphor sheet in accordance with the difference in the radiation absorptivity of each part of the subject. However, when the radiation exposure amount is particularly low as described above, the accumulated radiation amount in the portion of the stimulable phosphor sheet where the radiation irradiation amount is relatively low becomes extremely small. The amount of the stimulated emission light emitted from such a portion is of course extremely small. In the above-mentioned radiation image information recording / reproducing system, such a minute level of photostimulated light can be read, but still, an image signal obtained by detecting such a minute level of photostimulated light is The noise component becomes relatively large, and in the reconstructed radiation image, the granularity of the portion carried by the image signal deteriorates. The problem of the deterioration in graininess can be solved by processing such as removing a noise component in a high frequency range by passing the image signal through a low-pass filter. In such a case, however, the high frequency of the image signal indicating the detail component of the image is reduced. The components are also reduced over the entire band, and the sharpness of the reproduced image is reduced, including the part where the radiation dose is relatively high. Since the part where the radiation dose is relatively high is often the part where the detail component is originally desired to be observed in detail, the reconstructed radiation image in which the sharpness of this part is reduced as described above is not suitable for observation interpretation. Will be. (Object of the Invention) Accordingly, the present invention reliably removes a noise component of an image signal obtained by subjecting a stimulable phosphor sheet to a reading process, and on the other hand, reduces the sharpness of a portion where a detail component is to be observed in detail. It is an object of the present invention to provide a method for reducing noise of an image signal which does not invite, and an apparatus capable of implementing the method. (Constitution of the Invention) As described above, the method for reducing the noise of an image signal according to the present invention provides a stimulable phosphor sheet on which radiation image information is accumulated and recorded, when the sheet is irradiated with excitation light. Obtained by photoelectrically detecting the emitted light, the image signal carrying the radiation image information, after correcting based on the standardization information, and input to high-frequency component reduction means capable of arbitrarily reducing high-frequency components, The level of the image signal is continuously detected in one image, the level is corrected based on the standardized information, and the high frequency component of the image signal is reduced by the high frequency component reducing means so that the detection level after the level correction becomes higher. It is characterized in that a lower signal portion is reduced with a greater degree of reduction. The above-described method is formed so that the high-frequency component of the input signal can be arbitrarily reduced,
A high-frequency component reduction unit to which an image signal carrying radiation image information after being corrected based on the standardization information, obtained by subjecting the stimulable phosphor sheet to the reading process as described above, is input; The level is continuously detected in one image, and the level is corrected based on the standardized information.
A noise reduction device for an image signal according to the present invention, comprising: a control means for operating the high-frequency component reducing means so as to reduce the high-frequency component with a greater degree of reduction in a signal portion having a lower detection level after the level correction. Can be implemented. As described above, if the high-frequency component is further reduced in accordance with the level decrease of the image signal, a noise component having a relatively high level with respect to the signal component is removed. Since the signal is not or only slightly subjected to the high-frequency component reduction processing, the sharpness of the portion of the reproduced image carried by the high-level image signal is not reduced, and observation is desired. Detail components are left. (Embodiments) Hereinafter, the present invention will be described in detail based on embodiments shown in the drawings. Here, the basic mechanism of the noise reduction method according to the present invention will be described first with reference to FIG. This first
The illustrated noise reduction device is combined with the above-described device for reading radiation image information from a stimulable phosphor sheet. First, the radiation image information reading device will be described. For example, the stimulable phosphor sheet 10 on which the transmitted radiation image information of the subject is accumulated by irradiating radiation such as X-rays transmitted through the subject, is subjected to sub-scanning by sheet conveying means 11 such as an endless belt. Is transported in the arrow Y direction. A laser beam 13 as excitation light emitted from a laser light source 12 is deflected by an optical deflector 14 such as a galvanometer mirror, and moves the stimulable phosphor sheet 10 in an arrow X direction substantially perpendicular to the sub-scanning direction Y. Main scan.
From the portion of the sheet 10 irradiated with the laser beam 13 in this manner, the stimulated emission light 15 of an amount corresponding to the accumulated and recorded radiation beam image information is diverged, and the stimulated emission light 15 is
The light is condensed by 16 and is photoelectrically detected by a photomultiplier (photomultiplier tube) 17 as a photodetector. The light collector 16 is formed by molding a light-guiding material such as an acrylic plate, so that a linear incident end face 16a extends along a beam scanning line on the stimulable phosphor sheet 10. The light receiving surface of the photomultiplier 17 is coupled to an emission end face 16b formed in an annular shape. Stimulated luminescence light 15 entering the light collector 16 from the incident end face 16a
Travels through the interior of the light collector 16 by repeating total reflection, exits from the exit end face 16b, is received by the photomultiplier 17, and the amount of the stimulating light 15 carrying the radiation image information is Detected by pliers 17. The analog output signal of the photomultiplier 17 is logarithmically converted by the logarithmic converter 20, and the readout image signal S carrying the light intensity of the stimulating luminescence light 15, that is, the radiation image information, is used as the frequency characteristic variable amplifier 21 and the low-pass filter 25.
Is input to The read image signal S ′ amplified by the amplifier 21 is digitized in the A / D converter 22. The digital read image signal Sd thus obtained is input to an image reproducing device 24 such as a CRT or an optical scanning recording device through an image processing circuit 23, and the stimulable phosphor sheet 10 is stored in the image reproducing device 24. The recorded radiation image is reproduced. The frequency characteristic variable amplifier 21 has a configuration as shown in FIG. 2, and the read image signal S input to the operational amplifier 21a is changed to the value of the control voltage Vc input to the multiplier 21b as control means. Accordingly, the signal is amplified under the characteristics shown in FIG. 3 and output as a read image signal S ′. The multiplier 21b includes the low-pass filter 25 described above.
Is connected, and the output of the low-pass filter 25 is input as the control voltage Vc. As described above, the read image signal S is input to the low-pass filter 25. The read image signal S has, for example, a waveform as shown in FIG. 4, and as described above, in the low-level region corresponding to the weak stimulating light, many high-frequency noise components are contained. I have. By passing the read image signal S through the low-pass filter 25, a low-frequency component (a component indicated by a broken line in FIG. 4) indicating the basic level of the read image signal S is extracted, and this signal is used as the control voltage Vc. Multiplier
Entered in 21b. Therefore, as is clear from the characteristic diagram of FIG. 3, the read image signal S input to the frequency characteristic variable amplifier 21 is output with its high frequency component further reduced as its level decreases. That is, the read image signal S shown in FIG.
Is obtained as shown in FIG. The control voltage of the frequency characteristic variable amplifier 21
As Vc, the read image signal S may be directly input without passing through the low-pass filter 25. If the high-frequency component of the read image signal S is reduced as described above, the high-frequency noise component is removed, so that the radiation image reproduced by the image reproducing device 24 based on the read image signal S ′ has a granularity. And becomes suitable for observation and interpretation. By performing such processing, the sharpness of the image portion carried by the read image signal S in the low-level region is reduced. However, this image portion has a lower irradiation dose and a lower sharpness than the original, and the detail is low. Since component observation is an undesired region, it does not pose a problem in observation interpretation. Further, the processing for reducing the high frequency component is performed by the read image signal S
The sharpness of the image portion carried by the read image signal S in the high-level range is not reduced, or is small, if any. Sharpness is kept high. Next, an embodiment of the noise reduction device of the present invention will be described with reference to FIG. In FIG. 6, elements that are the same as the elements in FIG. 1 are given the same reference numerals, and descriptions thereof will be omitted unless otherwise necessary. This embodiment apparatus performs pre-reading for reading the outline of the radiation image information recorded on the stimulable phosphor sheet 10, and based on the information obtained by the pre-reading, performs main reading, that is, reproduces a reproduced image for observation reading. It is applied to a radiation image information reading apparatus that performs reading for obtaining. This radiation image information reading apparatus includes a sheet conveying means 11, a laser light source 12, an optical deflector 14, and a condenser 16 as in the apparatus shown in FIG.
And a reading unit 30 for book reading comprising a photomultiplier 17 and, similarly to the reading unit for book reading 30, sheet conveying means 41, a laser light source 42 for emitting a laser beam 43, an optical deflector 44, and a stimulating emission light 45. Reading unit consisting of a light collector 46 for collecting light and a photomultiplier 47
Has 60. The stimulable phosphor sheet 10 is read by the main-reading reading section 30 before reading radiation image information (main reading) for reproducing a radiation image to be used for observation reading.
In the pre-reading section 60, pre-reading is performed to grasp the outline of the stored record information of the sheet 10. In this pre-reading, the excitation light (laser beam 43) energy is set to be lower than that of the excitation light (laser beam 13) in the main reading. The output of the photomultiplier 47 obtained by the pre-reading is amplified by a pre-reading log amplifier 50 with a constant reading gain to be a pre-reading image signal Sp, and the pre-reading A / D converter 51
By this, it is digitized with a fixed recording scale factor (latitude). The digitized preread image signal Spd is input to the main read control circuit 52. Based on the pre-read image number Spd indicating the outline of the radiation image information accumulated and recorded on the sheet 10, the main reading control circuit 52 sets the optimum reading gain and the set values Sk and Gp (which indicate the recording scale faster, respectively). (Referred to as standardized information). After the above-mentioned pre-reading, the stimulable phosphor sheet 10 is sent to the main-reading reading section 30, and is subjected to the main reading. At the time of the main reading, the reading conditions are optimally set based on the information obtained by the pre-reading. That is, the above set value Sk corresponds to the radiation dose actually applied to the stimulable phosphor sheet 10, and as an example, the high voltage of the photomultiplier 17 for main reading is changed according to the set value Sk. As a result, the reading gain is set optimally. Further, a multiplier 29 is provided between the logarithmic converter 20 and the frequency characteristic variable amplifier 21, and an A / D converter is provided by changing a multiplication coefficient in the multiplier 29 according to the set value Gp. twenty two
The recording scale factor (latitude) is set optimally when digitizing. In the configuration described above, the high voltage of the photomultiplier 17 for main reading is changed in accordance with the irradiation radiation dose to the stimulable phosphor sheet 10, so that the output from the photomultiplier 17 is the irradiation irradiation dose. They will not always match. Therefore, in the present apparatus, the above-described set value Sk is added to the read image signal S output from the logarithmic converter 20 and passed through the low-pass filter 25, and the added signal is used as a control voltage Vc as a frequency characteristic variable amplifier 21. Is input to the multiplier 21b. In this way, the read image signal S input to the frequency characteristic variable amplifier 21 has a reduced high-frequency component corresponding to the decrease in the radiation dose applied to the sheet 10 (of course, according to the decrease in the read image signal S). As in the apparatus of FIG. 1, the effect of improving the granularity of the reproduced radiographic image and maintaining the sharpness of the high-dose irradiation part can be obtained. In the embodiment described above, the frequency characteristic variable amplifier 21 is used as the high frequency component reduction means, and the high frequency component reduction range of the read image signal S input to the frequency characteristic variable amplifier 21 is continuously changed according to the level. However, the reduction range of the high frequency component may be fixed. That is, instead of the frequency characteristic variable amplifier 21 as a high frequency component reducing means, a low-pass filter having a constant characteristic and its bypass circuit are provided, and the read image signal S
The signal S may be passed through a low-pass filter only when the level of the signal S falls below a predetermined value. (Effects of the Invention) As described above in detail, according to the present invention, the high-frequency noise component of the image signal obtained by subjecting the stimulable phosphor sheet to the reading process is surely removed, while the high-level region of the image signal is removed. Can maintain a high sharpness of the image portion carried by the camera and leave the detail components contained therein, so that a high-quality image can be obtained. As a result, the radiation image information reading system can reproduce a radiation image excellent in observation and interpretation suitability, which greatly contributes to improvement of medical diagnosis technology.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram showing a radiation image information reading device provided with a noise reduction device for explaining a basic mechanism of the method of the present invention, and FIG. FIG. 3 is a graph showing characteristics of the noise reduction device, FIG. 4 is a graph showing an example of an image signal according to the present invention, and FIG. FIG. 6 is a schematic diagram showing a radiation image information reading apparatus provided with an apparatus according to an embodiment of the present invention. 10: stimulable phosphor sheets 11, 41: sheet conveying means 12, 42: laser light sources 13, 43: laser beams 14, 44: optical deflectors 15, 45: stimulating light 17, 47 photomultiplier 20 logarithmic converter 21 frequency-variable amplifiers 21b, 29 multiplier 25 low-pass filter S read image signal S 'noise-reduced image signal

Claims (1)

(57) [Claims] An image carrying the radiation image information, obtained by photoelectrically detecting stimulable emission light emitted from the stimulable phosphor sheet on which the radiation image information is accumulated and irradiated when the sheet is irradiated with excitation light. A method for reducing signal noise, wherein after correcting the image signal based on standardized information,
A high-frequency component reducing unit that can arbitrarily reduce high-frequency components, continuously detects the level of the image signal in one image, and corrects the level based on standardized information; A method of reducing a high-frequency component of the image signal with a higher degree of reduction in a signal portion having a lower detection level after the level correction. 2. An image carrying the radiation image information, obtained by photoelectrically detecting stimulable emission light emitted from the stimulable phosphor sheet on which the radiation image information is accumulated and irradiated when the sheet is irradiated with excitation light. A method for reducing noise of a signal, comprising: a high-frequency component reduction unit configured to arbitrarily reduce a high-frequency component of an input signal and receiving the image signal after being corrected based on standardized information; Upon receiving the image signal, the level is continuously detected in one image, and the level is corrected based on the standardization information. A noise reduction device for an image signal, comprising: a control unit that operates to reduce the high-frequency component with a greater degree of reduction.