JPH0772254A - Method for reading radiation-image signal and radiation detector used therefor - Google Patents

Abstract

PURPOSE:To improve S/N by decreasing dark current noises in the image signal obtained with a radiation detector without narrowing the freedom of image pickup. CONSTITUTION:Image signals E are read out of solid-state photodetector elements P constituting a solid-state photodetector 31 at the specified period, respectively. The signal E is compared with a specified threshold value K stored in the inside beforehand. When the signal E is more than the threshold value K, the image signal E is outputted. When the signal E is less than the threshold value K, the image signal E is not outputted and dissipated in the inside. This is controlled with a data judging and controlling part 33. The image signals E outputted from the controlling part 33 are added in an adder 35. The operated sum values are stored in an image memory 36. These parts are provided. In the image memory 36, the noise-signal component caused by the dark current accumulated in the solid-state photodetector 31 during the period before the start of the irradiation of radiation and after the end of the irradiation is not stored. Only the image signal component carrying the radiation image information of the object accumulated by irradiation with the radiation is stored.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and a radiation detector for reading a radiation image signal from a radiation detector.
More specifically, the present invention relates to a method of removing a dark current noise accumulated in a radiation detector and reading a signal representing a radiation image, and an improvement of the radiation detector.

[0002]

2. Description of the Related Art Conventionally, in radiography in various fields such as medical radiography for radiography for medical diagnosis, industrial radiography for nondestructive inspection of substances, etc. A so-called radiographic method combined with a radiographic film is used. According to this method, when radiation such as X-rays transmitted through the subject enters the intensifying screen, the phosphor contained in the intensifying screen absorbs the energy of this radiation and emits fluorescence (instantaneous emission). Due to this light emission, the radiographic films superposed so as to be in close contact with the intensifying screen are exposed to light, and a radiographic image is formed on the radiographic film. In this way, the radiation image can be directly obtained as an image visualized on the radiation film.

On the other hand, in various fields, a radiographic image recorded on a radiographic film is photoelectrically read to obtain an image signal, the image signal is subjected to appropriate image processing, and then the image is reproduced and recorded. It is being appreciated. For example, an X-ray image is recorded using a film having a low gamma value designed so as to be suitable for later image processing, and the X-ray image is read from the film on which the X-ray image is recorded and converted into an electric signal. By performing image processing on this electric signal (image signal) and reproducing it as a visible image on a copy photograph or the like, a reproduced image with good image quality performance such as contrast, sharpness, and graininess is obtained. (See Japanese Patent Publication No. 61-5193).

In addition, the applicant of the present invention has conducted radiation (X-ray, α
Radiation, β rays, γ rays, electron rays, ultraviolet rays, etc.), a part of this radiation energy is accumulated, and when excitation light such as visible light is then irradiated, stimulated emission is shown according to the accumulated energy. Using a stimulable phosphor (stimulable phosphor), the radiation image information of a subject such as a human body is once recorded in a sheet-shaped stimulable phosphor, and this stimulable phosphor sheet is excited by a laser beam or the like. To generate stimulated emission light, photoelectrically read the obtained stimulated emission light to obtain an image signal, and based on this image data, the radiation image of the subject is a recording material such as a photographic photosensitive material, CRT, etc. A radiation image recording / reproducing system for outputting a visible image to the computer has already been proposed (Japanese Patent Laid-Open No. 55-55).
-12429, 56-11395, 55-163472, 56-10464
No. 5, No. 55-116340, etc.).

This system has the practical advantage of being able to record images over a very wide radiation exposure area as compared to conventional radiographic systems using silver halide photography. That is, in the stimulable phosphor, it has been recognized that the amount of emitted light stimulated by excitation after storage is proportional to the radiation exposure dose over a very wide range, and therefore the radiation exposure dose varies depending on various imaging conditions. Even if it fluctuates significantly, the amount of stimulated emission 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. Recording material such as photographic light-sensitive material,
By outputting a radiation image as a visible image on a display device such as a CRT, it is possible to obtain a radiation image that is not affected by variations in radiation exposure dose.

However, in order to obtain a radiographic image with such a radiographic system, when the above-mentioned radiographic image is directly visualized, the radiographic film used for radiography and the intensifying screen are made to coincide in sensitivity region. Need to do.

In the system for photoelectrically reading a radiation image using the above-described radiographic film and stimulable phosphor sheet, image processing is performed on the radiation image to adjust the density and contrast according to the purpose. Or, the radiation image must be converted into an electric signal once, and it is necessary to perform reading scanning using an image reading device for that purpose, which makes the operation for obtaining the radiation image complicated and obtains the radiation image. It takes time to get to.

Therefore, in order to solve the above problems caused by the conventional system, a radiation detector has been proposed (for example, Japanese Patent Laid-Open Nos. 59-211263 and 2-164067).
Publication, PCT International Publication Number WO92 / 06501, Signal, n
oise, and read out considerations in the developmen
t of amorphous silicon photodiode arrays for radio
therapy and diagnostic x-ray imaging, LE Antonuk
et.al, University of Michigan, RAStreet Xerox,
PARC, SPIE Vol.1443 Medical Imaging V; Image Physic
s (1991), p.108-119).

This radiation detector comprises a plurality of solid-state photodetector elements arranged in a matrix of transparent conductive films and conductive films sandwiching an amorphous semiconductor film on a substrate made of quartz glass having a thickness of 3 mm, and orthogonal to each other. As described above, a scintillator that converts radiation into visible light is stacked on a solid-state photodetector that is composed of a plurality of signal lines and scanning lines that are patterned in a matrix.

By arranging this radiation detector so that the scintillator faces the surface on the radiation incident side, and irradiating the radiation detector with the radiation transmitted through the subject, the radiation is directly incident on the scintillator and converted into visible light. The converted visible light is detected by the photoelectric conversion unit of the solid-state light detection element and photoelectrically converted into an image signal carrying radiation image information. This image signal is read out and output from a transfer unit provided in each solid-state light detecting element of the radiation detector by a predetermined reading means.

On the other hand, a radiation detector which does not require a scintillator has also been proposed. This radiation detector has the scintillator removed in the above-mentioned radiation detector, and (i) radiation of The solid-state photodetector (MATE
RIAL PARAMETERS IN THICK HYDROGENATED AMORPHOUS SI
LICON RADIATION DETECTORS, Lawrence Berkeley Labora
tory.University of California, Berkeley.CA 94720 Xe
rox Parc.Palo Alto.CA 94304), or (ii) a solid-state photodetector (Metal / Amorphous Silicon Multilayer Radiat) in which two or more layers are stacked with a metal plate in the radiation transmission direction.
ion Detectors, IEE TRANSACTIONS ONNUCLEAR SCIENCE.V
OL.36.NO.2.APRIL 1989), or (iii) CdTe
A radiation detector having a configuration using a semiconductor radiation detector (Japanese Patent Laid-Open No. 1-216290) such as, for example, directly detecting radiation without passing through visible light and converting it into an electric signal or the like. The signals are read-out signals input to the scanning lines in the same manner as the radiation detectors described above, and the solid-state photodetection elements arranged in a matrix form (a number of elements constituting the radiation detectors (i) to (iii) above). ) Is read and output separately.

The image signal output in this way is subjected to various signal processing by a signal processing device in the subsequent stage, and then CR.
It is reproduced as visible information or the like by a reproducing means such as T.

By using the above radiation detector, the radiation image of the subject can be obtained in real time without complicated operations, and the radiation image information can be immediately reproduced. It can be resolved.

[0014]

By the way, a noise component due to a so-called dark current is always stored in the solid-state photodetector even when it is not irradiated with radiation, and this noise component lowers the S / N of the image signal. Therefore, there is a problem that the image quality of the radiation image reproduced based on this image signal is deteriorated.

In order to prevent the deterioration of the image quality, usually, the dark current noise accumulated in the radiation detector is read out from the radiation detector immediately before the irradiation of the radiation to reset the storage of the solid-state photodetection element to zero. Then, a method of reading out a radiation image signal is performed in synchronization with the completion of irradiation of the subsequently applied radiation.

However, this synchronization method is usually performed manually, and it is difficult to achieve perfect synchronization.
In order to achieve perfect synchronization by using a synchronizer, it is necessary to connect the radiation irradiation device and the radiation detector to this synchronizer, and in that case, the radiation detector and the radiation detector must be handled integrally. Therefore, the degree of freedom of shooting (detection) is reduced.

The present invention has been made in view of the above circumstances and obtains an image signal for reproducing a high S / N radiation image by reducing dark current noise without narrowing the freedom of photographing. An object of the present invention is to provide a radiation image signal reading method and a radiation detector.

[0018]

It is generally recognized that the signal value stored by dark current noise is smaller than the signal value stored by radiation irradiation, and the radiation image signal reading method of the present invention. Uses the difference between these signal values, and during the period from before irradiation to after irradiation, the signal detected by the radiation detector is read multiple times at short intervals, and each read time is read. Depending on whether the signal value of is greater than or less than the threshold
Alternatively, depending on the proportion of solid-state photodetector elements that output a signal value equal to or higher than a threshold value, the read signal may be stored due to dark current noise, or may be irradiated with radiation carrying radiation image information. If it is determined that it is one that carries image information, the signal value is added for each solid-state photodetection element and stored in the memory, and when it is determined that it is due to dark current noise, ,
The feature is that the signal value is read and discarded.

That is, according to the first radiation image signal reading method of the present invention, as described in claim 1, the radiation carrying image information is detected and converted into a two-dimensional array for converting into a signal representing a radiation image as a whole. In a radiation image signal reading method for reading the signal from a radiation detector having a large number of solid-state photodetection elements, the signal is read at a predetermined cycle from the radiation detector, and the signal read for each cycle. And a predetermined threshold value set in advance are compared for each of the solid-state photodetecting elements, and for the solid-state photodetecting elements having a value of the signal or more, the value of the signal is stored in a memory. It is characterized in that the solid-state photodetecting element which is cumulatively accumulated and the value of the signal is equal to or less than the threshold value is not cumulatively accumulated in the memory.

Here, the above-mentioned predetermined cycle means a time sufficiently shorter than the irradiation time of radiation, and the above-mentioned predetermined threshold value is the solid-state light detection due to dark current noise at every above-mentioned predetermined cycle. It is a value that is set within a range that is larger than the signal value generated in the element and smaller than the signal value generated by irradiation with radiation.

When the signal value is equal to the threshold value, it is possible to selectively take one of the operation when the signal value is equal to or more than the threshold value and the operation when the signal value is equal to or less than the threshold value.

A second radiographic image signal reading method of the present invention, as described in claim 2, is a two-dimensional method for detecting a radiation carrying image information and converting it into a signal representing a radiographic image as a whole. In a radiation image signal reading method for reading the signal from a radiation detector having a large number of arranged solid-state light detecting elements, the signal is read at a predetermined cycle from the radiation detector, and the signal is read for each cycle. The value of the signal and a preset predetermined threshold value are compared for each of the solid-state photodetecting elements, and the number of solid-state photodetecting elements whose signal value is equal to or more than the threshold value is all the solid-state photodetecting elements. When the ratio is equal to or more than a predetermined ratio with respect to the number of detection elements, the value of the signal is added and accumulated in a memory from all the solid-state light detection elements, and the value of the signal is equal to or higher than the threshold value. Is the number of all solid-state photodetectors When the predetermined ratio or less with respect to the number, is characterized in that no perform addition storage to the memory of the value of the signal.

Further, the radiation detector of the present invention is the radiation detector used in the first or second method of the present invention, and detects radiation carrying image information as described in claim 3. In a radiation detector having a large number of solid-state photodetector elements arranged in a two-dimensional array for converting into a signal representing a radiation image as a whole, a signal reading means for reading out the signal at a predetermined cycle from the radiation detector; The value of the signal read by the signal reading means is compared with a preset predetermined threshold value, and if the value of the signal is equal to or more than the threshold value as a result of the comparison, the value of the signal is output. However, when the value of the signal is less than or equal to the threshold value, the comparison means for controlling not to output the value of the signal and the value of the signal output from the comparison means are added for each solid-state photodetection element. And an adding means for It is characterized in that a memory for storing the summed calculated value.

As the radiation detector, for example,
A plurality of solid-state photo-detecting elements arranged in a matrix of a transparent conductive film and a conductive film with an amorphous semiconductor film sandwiched between them and a pattern formed in a matrix so as to be orthogonal to each other on a substrate made of quartz glass having a predetermined thickness. A solid-state photodetector composed of a plurality of signal lines and scanning lines, the solid-state photodetector having (i) the thickness in the radiation transmission direction set to be about 10 times thicker than a normal one. Alternatively, (ii) a solid-state photodetector in which two or more layers are stacked via a metal plate in the radiation transmission direction, and (iii) a semiconductor radiation detector such as CdTe can be used. Further, a solid-state photodetector composed of a solid-state photodetection element such as an ordinary photodiode can be used. In this case, Gd ₂ O ₂ which receives irradiation of radiation and converts it into visible light having an intensity corresponding to the intensity of the radiation
A configuration in which a scintillator made of a phosphor such as S or CsI is laminated on the solid-state photodetector can also be adopted.

The predetermined thickness means a thickness at which the amount of absorbed radiation is not so large as to deteriorate the quality of a radiation image, but more specifically, to a certain degree for supporting the solid-state photodetector. It is said that it is about several hundreds of microns because the strength of is required.

[0026]

According to the radiation image signal reading method and the radiation detector used therefor of the present invention, the signal detected by the radiation detector in the state where the radiation is not applied is stored by dark current noise. Signal is smaller than the threshold value,
Therefore, this signal is discarded. The radiation detector resets the stored signal value to zero by reading the detected and stored signal, so it resets to zero each time this signal value is read multiple times in a short cycle, and the next The signal value due to dark current noise accumulated during the reading period is discarded.

Next, when the irradiation of radiation is started, a signal due to irradiation of radiation is stored from the state where the radiation detector is reset to zero. During this radiation irradiation period, the value of the signal detected becomes equal to or higher than the threshold value, and therefore the signal value detected during this period is added for each solid-state photodetector element. It is stored in a memory or the like as a signal value corresponding to the total amount of radiation emitted from the start of irradiation to the time when the signal is read.

When the irradiation of the radiation is completed, the detected signal value becomes less than the threshold value again, and this signal value is discarded.

The signal value accumulated and stored in the memory by the above-mentioned operation is the signal value detected by the irradiation of the radiation, and is the signal due to the dark current noise accumulated before the irradiation of the radiation and during each period after the completion of the irradiation of the radiation. The values have not been added. Therefore, the S / N of the reproduced radiation image can be improved and a high-quality radiation image can be obtained without synchronizing with the radiation irradiation device.

In the comparison between the threshold value and the signal value, the subsequent action depends on whether the signal values from all the solid-state photodetection elements forming the radiation detector are above or below the threshold value. It may be set to proceed, or after all the solid-state photodetector elements, the signal value from a predetermined proportion of the solid-state photodetector elements exceeds or falls below the threshold value, and the subsequent action proceeds. May be set as follows.

[0031]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a radiation detector used for carrying out the radiation image signal reading method according to the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of a radiation detector of the present invention, and FIG. 2 is a system diagram showing a mode of detection of radiation image information using the radiation detector of this embodiment. . The radiation detector shown in the figure is a radiation R emitted from a radiation source 20 and transmitted through an object 10 as shown in FIG.
A planar scintillator 32 that receives the light and converts it into visible light;
A large number of two-dimensional solid-state photodetectors P, which are laminated on the scintillator 32, detect the visible light converted by each part of the scintillator 32 and convert it into an image signal E representing a radiation image as a whole are arranged. And a solid-state photodetector 31.

FIG. 3 shows the detailed structure of the solid-state photodetector P that constitutes the solid-state photodetector 31. Solid-state photodetector 31
Has signal lines 31B and 31H made of a conductive film patterned on a substrate 31A made of a resin sheet, and is provided in a photodiode 31E made of amorphous silicon 31C and a transparent electrode 31D, amorphous silicon 31F and amorphous silicon 31F. Transfer electrode as a gate
A large number of solid-state photo-detecting elements P are formed by the thin film transistor 31G composed of 31J. Here, the thickness of the resin sheet 31A of the solid-state photodetector 31 is about several hundreds of microns, and the radiation absorption rate is low. The thickness of the amorphous silicon 31C is about 1 micron.

Further, the radiation detector of this embodiment reads the image signal E from each solid-state photodetector P at a predetermined cycle, and the read image signal E and a predetermined signal stored in advance. The threshold value K is compared, and as a result of the comparison, if the image signal E is above the threshold value K, this image signal E is output, and if the image signal E is below the threshold value K, the image signal E is reached.
A data judgment control unit 33 for controlling so as not to output and internally erasing, an A / D converter 34 for A / D converting the image signal E output from the data judgment control unit 33, and an A / D converter
An adder 35 that adds the D-converted image signal E for each solid-state photodetector P and an image memory 36 that stores the calculated value added by the adder 35 are provided.

Here, the threshold value K is set within a range that is larger than the noise signal generated in the solid-state photodetection element P by the dark current in each of the predetermined cycles and smaller than the signal generated by the irradiation of radiation. Is the value to be set.

Next, the operation of the radiation detector of this embodiment will be described.

Before the irradiation of the radiation R, a noise signal due to a dark current is generated in each solid-state light detecting element P, and the noise signal is generated by the photodiode 31E in the image signal E (K>K>).
E) is stored. Here, the data judgment control unit 33
For example, a transfer pulse is sent to each solid-state photodetector P at a cycle of 1 msec, and each time the transfer pulse is sent, the switch of each solid-state photodetector P is in the “on” state (voltage is applied to the transfer electrode 31J of the solid-state photodetector P). Then, the image signal E accumulated in the photodiode 31E is output to the data judgment control unit 33 through the thin film transistor 31G.

Each time the solid-state photodetector P outputs the image signal E, the amount of accumulated image signal is reset to zero to eliminate the influence of dark current, and the transfer pulse is sent next from the moment of resetting. Until then, the action of accumulating the noise signal due to the dark current is repeated again.

On the other hand, the image signal E input to the data judgment control unit 33 every 1 msec cycle is the data judgment control unit 33.
The value is compared with a threshold value K stored in advance, and since the noise signal is smaller than the threshold value K, it is erased internally. The image memory 36 is reset to zero in synchronization with the first input of the image signal E to the data judgment control unit 33.

Next, as shown in FIG. 2, the radiation R is irradiated through the subject 10, and the radiation detector 30 that has received the radiation R converts the radiation R into visible light by the scintillator 32. This visible light carries the radiation image information of the subject 10 and is incident on the amorphous silicon 31C of each solid-state photodetector P constituting the solid-state photodetector 31, and this visible light is the same as in the case of the dark current noise signal. An image signal E '(E'> K) carrying radiation image information is photoelectrically converted and stored. The data judgment control unit 33 determines this image signal E ′ by a predetermined value (1 mse) as in the case of the dark current noise signal.
It is read in the cycle of c), and the magnitude is compared with the internally stored threshold value K in each cycle.

Since the image signal E'carrying the radiation image information of the subject 10 is larger than the threshold value K, it is output to the A / D converter 34, converted into a digital signal, and input to the adder 35. Whenever the digital image signal E'is input, the adder 35 adds the input image signal E'to the image signal previously stored in the image memory 36, and adds the image signal to the image memory 36. To enter. Image memory 36
Is initially set to zero by the above-mentioned reset action, and the solid state photodetector P is set in the image memory 36 between the time when the radiation R is irradiated and the moment when the radiation R is actually irradiated. The sum total of the read image signals will be stored.

When the irradiation of the radiation R is completed, the signal accumulated in each solid-state light detecting element P is lowered and only the noise signal due to the dark current before the irradiation of the radiation is detected. Since this signal is smaller than the threshold value K, it is erased inside the data judgment control unit 33.

As described above, the noise signal component due to the dark current accumulated in the solid-state photodetector 31 is not stored in the image memory 36 before the irradiation of the radiation R is started and after the irradiation is terminated, and the radiation R Only the image signal component carrying the radiation image information of the subject 10 accumulated by the irradiation is stored. Therefore, the image signal stored in the image memory 36 is
The radiation image information of the subject 10 is carried at a high S / N, and the image signal stored in the image memory 36 can be output in the latter stage to be visualized as a radiation image by an image reproducing device or the like.

As described above, according to the radiation detector of this embodiment, since it is not necessary to synchronize with the irradiation of the radiation R from the radiation source 20, the freedom of photographing is not narrowed and the dark current noise is reduced. Thus, an image signal for reproducing a high S / N radiation image can be obtained.

In the radiation detector according to the present invention, as in this embodiment, the data judgment control unit 33 causes the signals from all the solid-state photodetection elements P constituting the radiation detector to exceed or fall below the threshold value. It may be set so as to control the subsequent action depending on whether or not the signals from a predetermined proportion of all the solid-state photodetector elements are above or below the threshold value. May be set so as to control the subsequent action. By setting the predetermined ratio in this way, for example, the subject has a portion with a very low radiation transmittance, and the image signal due to the slight radiation transmitted through the portion with a low radiation transmittance falls below the threshold value. Also in this case, if the image signal due to the radiation that has passed through another portion of a predetermined ratio or more exceeds the threshold value, the image signal can be additionally stored in the memory, and the radiation irradiation start time can be recognized more accurately. And its action can be controlled.

Further, the radiation detector according to the present invention is not necessarily limited to the one using the scintillator as in the above embodiment, and in the radiation detector of the above embodiment, instead of the solid-state photodetector, for example, the above-mentioned (I) The solid-state photodetector whose thickness in the radiation transmission direction is set to be about 10 times thicker than the normal one, or (ii) in the radiation transmission direction,
If two or more solid-state photodetectors stacked with a metal plate interposed therebetween or (iii) a semiconductor radiation detector such as CdTe are used, there is no need to provide a scintillator.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of a radiation detector of the present invention.

FIG. 2 is a system diagram showing an aspect of detection of radiation image information using the radiation detector shown in FIG.

FIG. 3 is a detailed configuration diagram showing a detailed structure of a solid-state photodetector.

[Explanation of symbols]

 10 Subject 20 Radiation source 30 Radiation detector 31 Solid-state photodetector 32 Scintillator 33 Data judgment control unit 34 A / D converter 35 Adder 36 Image memory

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI Technical display location G03B 42/02 Z H01L 27/14 H04N 5/32

Claims (3)

[Claims] 1. A radiation detector having a large number of solid-state photodetection elements arranged in a two-dimensional array, which detects radiation carrying image information, converts it into a signal representing a radiation image as a whole, and accumulates the signal. In the method of reading out a radiation image signal, the signal is read out from the radiation detector at a predetermined cycle,
The value of the read signal and a predetermined threshold value set in advance are compared for each of the periods for each of the solid-state photodetection elements, and the value of the signal is equal to or more than the threshold value. With respect to the above, the value of the signal is added and stored in the memory, and for the solid-state photodetector whose value of the signal is less than or equal to the threshold value, the value of the signal is not added and stored in the memory. Radiation image signal reading method. 2. Radiation for reading out a signal from a radiation detector having a large number of two-dimensionally arranged solid-state photodetecting elements for detecting the radiation carrying image information and converting it into a signal representing a radiation image as a whole. In the image signal reading method, the signal is read from the radiation detector at a predetermined cycle,
In each of the cycles, the value of the read signal is compared with a predetermined threshold value set in advance for each of the solid-state photodetection elements, and the solid-state photodetection in which the value of the signal is equal to or more than the threshold value When the number of elements is a predetermined ratio or more with respect to all the solid-state photodetection elements, the value of the signal is added and accumulated in the memory from all the solid-state photodetection elements, and the value of the signal is When the number of solid-state light detecting elements having a threshold value or more is less than or equal to a predetermined ratio with respect to the total number of solid-state light detecting elements, addition and accumulation of the value of the signal in the memory is not performed. Radiation image signal reading method. 3. A radiation detector having a large number of two-dimensionally arranged solid-state photodetecting elements for detecting radiation carrying image information and converting it into a signal representing a radiation image as a whole, the radiation detector comprising: A signal reading unit that reads out the signal at a predetermined cycle is compared with a value of the signal read by the signal reading unit and a preset predetermined threshold value, and as a result of the comparison, the value of the signal is If the threshold value or more, output the value of the signal,
When the value of the signal is less than or equal to the threshold value, comparison means for controlling not to output the value of the signal, and addition for adding the value of the signal output by the comparison means for each solid-state photodetection element The radiation detector used in the radiation image signal reading method according to claim 1 or 2, further comprising: means and a memory for storing the calculated value added by the adding means.