JPH06242525A - Radiation image reader - Google Patents

Abstract

PURPOSE:To eliminate the distortion of a radiation image, and to improve S/N. CONSTITUTION:The amplification factor of a photoelectric amplifier 52E is set by an amplification factor controlling part 53G so as to be a level being a supersaturated condition obtained when a laser beam having maximum intensity from a laser beam generating part 52A scans a stimulable phosphor panel, the intensity of the laser beam is controlled to be the maximum intensity by an intensity converting circuit 53E and a laser controlling part 53F till an image signal detected at the photoelectric amplifier 52E approaches the saturated level, and the image signal is attenuated so as not to be saturated in the vicinity of the saturated level.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation image reading apparatus, and more particularly, to a technique for suppressing S / N ratio deterioration of an image due to the influence of image signal distortion and afterglow.

[0002]

2. Description of the Related Art Radiation images such as X-ray images are often used for diagnosing diseases, etc. In order to obtain this X-ray image, X-rays transmitted through a subject are displayed on a fluorescent screen (phosphor layer). Irradiation is performed to generate visible light according to the transmitted dose, and this visible light is applied to a film using a silver salt as in ordinary photography to develop the film. ing.

However, in recent years, a method for directly reading image information from a phosphor layer has been devised without using a film coated with silver salt. As such a method, the radiation transmitted through the subject is absorbed by the stimulable phosphor, and then the stimulable phosphor is excited by, for example, light or thermal energy to absorb the stimulable phosphor. There is a method in which the accumulated radiation energy (radiation image information) is stimulated to emit fluorescence as fluorescence, and the stimulated emission light is photoelectrically converted to obtain an image signal.

Specifically, for example, US Pat. No. 3,859,527 and JP-A-55-12144 disclose a radiation image conversion method using a stimulable phosphor and using visible light or infrared light as stimulated excitation light. ing. This method uses a stimulable phosphor panel having a stimulable phosphor layer formed on a support, and irradiates the stimulable phosphor layer of this stimulable phosphor panel with radiation transmitted through a subject, A latent image is formed by accumulating radiation energy corresponding to the radiation transmittance of each part of the subject, and then the accumulated radiation energy is converted into light by scanning this stimulable phosphor layer with stimulating excitation light. Then, the light signal is photoelectrically converted to obtain a radiation image signal.

[0005]

By the way, in the case of obtaining the image signal by photoelectrically converting the radiation energy as described above, it is necessary to ensure good gradation of the subject image area in order to improve the diagnostic accuracy. Therefore, in the prior art, for example, the one disclosed in Japanese Patent Laid-Open No. 56-11347, the excitation light whose light amount is controlled to be constant prior to reading is guided to a photoelectric converter to be photoelectrically converted so that the value is matched with a reference level. The gain of the photoelectric converter is controlled.

Further, in the technique disclosed in Japanese Patent Laid-Open No. 58-67240, first, scanning with weak excitation light is performed to detect a dynamic range of radiation intensity in a subject image area, and reading is performed according to the dynamic range of the image. The scanning is performed after the dynamic range of is determined. Further, in the one disclosed in Japanese Patent Laid-Open No. 63-189854, the radiation intensity for each image area is first detected by performing scanning with weak excitation light in the same manner as described above, and the excitation light is adjusted so as not to be saturated depending on the intensity. The reading is performed while controlling the intensity of the light and compressing the dynamic range according to the intensity of the excitation light.

However, in the first conventional example, only the amplification factor is set according to the photographing condition, and not the dynamic range of the radiation intensity of the actual image. Therefore, the amplification factor is not always appropriate. It cannot be set completely. If it is attempted to control the amplification factor to match the dynamic range of the radiation intensity of the subject image area, it is necessary to detect the dynamic range by so-called pre-reading by scanning with weak excitation light as in the second and third conventional examples. If the pre-reading function is provided, the device becomes large and complicated.

Further, the stimulable phosphor has a property of emitting radiation energy accumulated in the phosphor as stimulated emission light when irradiated with excitation light as described above. Emits so-called afterglow for a response time peculiar to the phosphor even after the irradiation of the excitation light ends. Therefore, when the stimulable phosphor panel is scanned with the excitation light and the stimulated emission light emitted from the stimulable phosphor is photoelectrically sequentially read to obtain an image signal on a pixel-by-pixel basis, the pixels being irradiated with the excitation light are The information component of the pixel during the irradiation of the excitation light is detected in addition to the light emission component of the excitation light, including the afterglow component from the pixel that has already been irradiated with the excitation light. For this reason, the signals between pixels are not completely separated, the contrast resolution is lowered, and the sharpness of the reproduced image is lowered. In an extreme case, a false image such as tailing may appear. There is.

To solve the problem of afterglow, even when the dynamic range of reading is determined in accordance with the dynamic range of the radiation intensity of the subject image area as in the second conventional example, the radiation outside the subject passes through. In a region (hereinafter referred to as a “pass-through region”), the emitted light intensity becomes significantly higher than that in the subject image region, and accordingly, the afterglow also becomes extremely large, and the S / N ratio of the entire image deteriorates significantly. This is also the case with the first conventional example, which is a problem that cannot be solved unless the excitation light intensity is controlled.

In the third conventional example, since the intensity of the excitation light is controlled so that the image signal is not saturated, the influence of the afterglow is improved. However, in order to variably control the intensity of the excitation light, the reading dynamic range is compressed in proportion to the intensity of the excitation light, but the intensity of the excitation light and the amount of the emitted light do not always have linearity. Not, especially, the linearity is considerably destroyed at a level where the intensity of the excitation light and the amount of the emitted light are large, so that the image is distorted and the image is not a true radiation image and normal processing cannot be performed when performing image processing. There is a problem.

The present invention has been made in view of such conventional problems, and by appropriately controlling the amplification factor of photoelectric conversion and the excitation light intensity, there is no image distortion and the influence of afterglow. It is an object of the present invention to provide a radiation reading device that can be avoided as much as possible.

[0012]

Therefore, according to the present invention, as shown in FIG. 1, a stimulable phosphor absorbs radiation emitted from a radiation source that has passed through an object to store and record radiation image information. Stimulable fluorescent panel, an excitation means for stimulating the radiation image information accumulated and recorded in the stimulable phosphor by scanning the stimulable fluorescent panel with excitation light, and the stimulable luminescent light is photoelectrically converted. In a radiation image reading apparatus configured to include a photoelectric conversion unit that obtains an image signal in a pixel unit by statically reading, the amplification factor of the photoelectric conversion unit is the radiation that passes through the outside of the subject of the stimulable fluorescent panel. Amplification rate control means for controlling the image signal level photoelectrically converted to a value at which the image signal level photoelectrically converted becomes supersaturated when the absorption portion of the image signal level is saturated with the image signal level. Until near is controlled to a predetermined level of the maximum intensity near the image signal is configured to include a light intensity control means for attenuating so as not to exceed the saturation level reaches the saturation level neighborhood.

Further, as shown by a dotted line in FIG. 1, an afterglow removing means for removing an afterglow component from the image signal may be included. Further, as shown by a chain line in FIG. 1, a signal compensating means for compensating the image signal according to the intensity of the excitation light so as to match the level when the excitation light of the reference intensity is irradiated may be included.

[0014]

The amplification factor control means controls the image signal level to be in a supersaturated state when the radiation passing area of the stimulable fluorescent panel storing and recording the radiation image information is scanned with the excitation light of maximum intensity. Since the radiation absorption amount in the subject image region is incomparably smaller than that in the pass-through region, it is not saturated even if the light intensity control means raises it to a predetermined level near the maximum intensity. On the other hand, since the excitation light intensity is attenuated so as not to be saturated when the image signal increases to near the saturation level, the light amount of the emitted light is suppressed to a predetermined level or less even in the radiation passing area.

As a result, good image quality is obtained in which the effect of afterglow is effectively avoided while maintaining good gradation of the subject image area, and deterioration of the S / N ratio is suppressed. Further, in the one provided with the afterglow component removing means, for example, the effect of the afterglow is coupled with the attenuation control of the excitation light intensity by jointly using afterglow modeling by exponentially modeling the afterglow intensity. It can be avoided as much as possible.

Further, in the case where the signal compensating means is provided, since the image signal is compensated so as to match the level when the excitation light of the reference intensity (for example, the predetermined level) is irradiated, the light emitted from the lung region or the like is compensated. It is possible to obtain a reproduced image without distortion even in an image area where the linearity between the intensity of the excitation light and the light intensity of the emitted light collapses near the saturation level, and the concentration of the radiation passing area is also true radiation absorption. The density becomes darker in proportion to the amount, and the distinction from the subject image area becomes clear, and an easy-to-see image can be obtained.

[0017]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 2 is a schematic diagram showing the hardware configuration of an embodiment of the radiation image reading apparatus according to the present invention, and this embodiment shows a case where it is applied to chest radiography of a human body for medical use.

A series of radiation irradiation and image reading operations will be described. Imaging order data is input to the operator console 1 from a computer, and based on the input data, the patient name, age, and other information, as well as order information such as an imaging region is displayed. The technician confirms the reservation list of imaging orders on the operator console 1, sets the imaging conditions, and sets the X-ray operator console 2
Is operated to operate the X-ray generation controller 3 to irradiate the subject M with X-rays as radiation from the X-ray tube 4 to photograph on the stimulable fluorescent panel 51 fixed inside the reader 5. Here, the photographing on the stimulable phosphor panel 51 is performed on the stimulable phosphor layer in which the energy according to the X-ray transmittance distribution of the subject M with respect to the irradiation X-ray dose from the X-ray tube 4 is provided. Accumulate,
This is performed by forming a latent image of the subject M on the stimulable phosphor layer. Examples of the material for the stimulable phosphor layer include, for example, JP-A-61-27201, or JP-A-5-72091.
Materials such as those disclosed in 9-75200 are used.

The reading device 5 starts reading based on the imaged region and conditions immediately after the image is taken in association with the X-ray console 2. Reading is performed by scanning while moving the optical system 52 on the stimulable fluorescent panel 51. Such control is performed by the control unit 53 in the device 5 as described later.

When the reading is completed, the erasing lamp 54 refreshes the entire surface of the stimulable fluorescent panel 51 (removes the remaining X-ray energy), thereby enabling the next photographing. The image signal read by the reading device 5 is transmitted to the image processing device 6, automatically subjected to the most preferable process for the imaged region, and displayed on the display device 7 such as a CRT. If the process does not suit you, release the automatic process mode and change the process manually.

Finally, if there is no problem, the data is transmitted to the film printer 8 and reproduced as a film hard copy. Since the image processing device 6 has a hard disk built therein as an image memory, it is possible to independently perform shooting as a process or perform the process in batch. The configuration and function of the control unit 53 of the reading device 5 will be described with reference to FIG.

To explain from the outline of the basic configuration, the reading is performed by irradiating the photostimulable fluorescent panel 51 with laser light as excitation light, and photoelectrically converting the emitted light emitted from the photostimulable fluorescent panel 51. An image signal is obtained, and the features of the present invention are the setting of the amplification factor of photoelectric conversion before the reading of the subject image and the feedback control of the laser light intensity based on the detected image signal level at the time of reading.

Next, the function of each circuit will be described.
Laser light source (semiconductor laser is used in this embodiment, but gas laser used together with appropriate modulation means may be used) 52A
Emits laser light whose emission intensity is controlled, and the laser light is deflected by the optical system 52 and guided to the stimulable fluorescent panel 4 as scanning light (excitation light) for exciting excitation. Therefore, the excitation means in this embodiment is the optical system
52 and a laser light source 52A.

The photoelectric amplifier 52E receives the photostimulated luminescence having a luminescence intensity proportional to the latent image energy emitted from the stimulable fluorescent panel 51 by the scanning of the excitation light through a condenser and photoelectrically converts it. Is. The amplification factor of the photoelectric amplifier 52E is controlled by the signal from the amplification factor control unit 53G as the amplification factor control means. Here, although the value of the appropriate amplification factor changes to some extent depending on the imaging site and conditions, the stimulable fluorescent panel 51 under the imaging conditions is the laser beam with the maximum intensity.
When the X-ray passing region is irradiated with, the setting is made so as to exceed the upper limit of the A / D converter 53D, which will be described later, and become supersaturated.
Specifically, it is set to 3 to 20 times the upper limit. Therefore, shoot under normal shooting conditions with no subject,
The intensity of the laser light is adjusted to 1 / maximum of the maximum intensity by the intensity adjustment signal
It is fixed at about 8 and read. Then, while performing the reading, the amplification factor of the photoelectric amplifier 52E is increased from a low level,
The amplification factor at the time when the output of the detected image signal reaches the saturation level is stored and used when photographing the subject (patient).

The logarithmic amplifier 53C converts the image signal photoelectrically converted and amplified by the photoelectric amplifier 52E so as to be linear with respect to the logarithm. This is to reduce the quantization error of low level signals. That is, logarithmic amplifier 53C
The analog signal from is converted into a digital signal by the A / D converter 53D, but the dynamic range of the A / D converter 53D is about 3 digits, and if it exceeds it, it will be saturated and the photoelectrically converted image will be generated. When the signal is A / D converted as it is, the low signal level is hardly quantized, and the gradation of the most important subject image area is not good, resulting in an image unfavorable for diagnosis. Therefore, the logarithmic transformation is performed to quantize the low signal level sufficiently finely to reduce the error.

The digital image signal thus A / D converted is output to the intensity conversion circuit 53E as a detection signal for controlling the intensity of the laser light and remains on the stimulable fluorescent panel 51. Afterglow removal unit 53 that removes afterglow
It is output to H (the antilog conversion circuit 53H _{1 at the} first stage). The intensity conversion circuit 53E is a conversion circuit that outputs a light intensity control signal according to the detected image signal level. As shown in FIG. 4, the conversion characteristic is such that the light intensity control signal is fixed to a certain maximum level until the detected image signal approaches the saturation level, and the light intensity is attenuated when the saturation level approaches. There is. The light intensity is at the maximum level.
It can attenuate two orders of magnitude at 200mW on the surface. Also,
When an intensity adjustment signal to be described later is input from the control circuit 53O, it is forcibly fixed to the light intensity set by the intensity adjustment signal. The image signal is substantially close to the saturation level in the area outside the subject of the stimulable fluorescent panel 51, that is, in the pass-through area or the area near the cavity such as the lung area. Therefore, for example, when performing a chest radiography, as shown in FIG. 5, the subject region except the lung region of the stimulable fluorescent panel 51 is irradiated with the maximum level of laser light, and the intensity is attenuated from the maximum level in the lung region. Laser light is emitted,
In the pass-through region, the laser beam whose intensity is greatly attenuated is irradiated. Further, the image signal detected by the photoelectric amplifier 52E at this time is as shown in FIG. 5, and as described above, the intensity of the laser beam is rapidly attenuated in the vicinity of the saturation region of the detection signal level. It approaches saturation, but does not reach saturation.

The laser control section 53F includes the intensity conversion circuit 53.
The laser light intensity is controlled based on the light intensity control signal from E. Further, a signal monitoring the light quantity of the laser light generated from the laser light source 52A is input, and the intensity is feedback-controlled so that the monitored light quantity matches the control light quantity of the light intensity control signal. However, in order to eliminate the influence of scattered light outside the effective area for main scanning, the laser light is turned off by the main scanning effective signal. The intensity conversion circuit 53E
The laser control section 53F constitutes a light intensity control means.

An afterglow removing section 53H as the afterglow removing means.
Is an inverse logarithmic conversion circuit 53H ₁ and an afterglow removal filter circuit 53H.
₂ and a logarithmic conversion circuit 53H ₂ . Since the inverse logarithmic conversion circuit 53H ₁ needs to accumulate and calculate afterglow, the characteristic converted into logarithm is once converted into linear characteristic for calculation, and the afterglow removal filter circuit 53H ₂ causes afterglow component. after removal of, it converted to a logarithmic characteristic by the logarithmic converter 53H ₂ again. Here, considering the effect of afterglow, when the photostimulable fluorescent panel 51 is irradiated with laser light for a moment,
The emitted light emits afterglow even after the irradiation of laser light is completed (Fig. 6).
See). The amount of afterglow L is calculated by the following equation.

L = L ₀ · β · exp (-t / τ) β: initial attenuation coefficient, τ: attenuation time constant When the laser light is scanned, the above-mentioned afterglow component appears with respect to the light emission amount of the scanning locus. Since the light collector 52D collects all the light emitted in the main scanning direction, the afterglow other than the laser light irradiation position is detected at the same time. In particular, in the X-ray passing region, the amount of light emission is larger than that of the subject region by one digit or more, and the afterglow is also large (see FIG. 5).

The afterglow removing filter circuit 53H ₂ for removing such afterglow components is constructed as shown in FIG. 7, for example. The arithmetic function of this circuit is shown by the following equation. S ′ (n) = S (n) −β · Σ [S (k) · δ ^n-1-k ] [where δ = exp (-Δt / τ) when the sampling period of the image signal is Δt] When the afterglow component is removed, the image signal does not reach saturation as described above, so that the removal can be correctly performed even in a region having a large light emission amount. Further, such removal is performed on the afterglow modeled to change exponentially, but in reality, the afterglow has a random photon noise component, and such a random component Is impossible to remove. Since the random component becomes larger in a region where the afterglow is stronger, a problem such as an X-ray passing region becomes the most problematic. However, in the present invention, the intensity of the laser light is rapidly attenuated in the region where the afterglow becomes strong. As a result, the amount of afterglow generated can be suppressed to a sufficiently small level, the generation of random components can also be suppressed, the deterioration of the S / N ratio of the image can be prevented, and the image quality can be improved. That is, the effect of afterglow can be eliminated as much as possible by performing the afterglow removal, but the afterglow removal is not performed, and the laser beam in the vicinity of the saturation region according to the present invention is removed. The effect of afterglow can be largely avoided only by performing the intensity attenuation control of.

The image signal from which the afterglow is removed by the afterglow remover 53H is output to the compensation circuit 53I. Compensation circuit
In addition to the image signal, the compensation signal from the compensation characteristic conversion circuit 53J is input to 53I. The compensation characteristic conversion circuit 53J is
The image signal is compensated according to the change of the light intensity control signal.
That is, in the present invention, since the laser light intensity is changed in the region where the image signal level detected as described above approaches saturation,
If the detection signal of the area is directly output as an image signal, the density of the reproduced image of the area will be abnormally lowered.
Further, since the linearity of the light emission amount with respect to the laser light intensity is broken in the region where the intensity of the laser light is large as described above, the intensity of the light to be irradiated is increased as disclosed in the prior art. According to the method, a system in which the dynamic range of photoelectric conversion is linearly compressed causes distortion of an image. Therefore, the same intensity is obtained when the laser light intensity is changed.
A compensating signal for compensating is formed so as to match the signal obtained when the laser beam of the maximum intensity is irradiated in the embodiment. Specifically, the characteristic of the light emission amount with respect to the laser light intensity shown in FIG. 8 is stored in advance, and the compensation signal log (L / L ₀ ) as shown in FIG. 9 is formed. Here, L ₀ is the amount of light emitted when the laser beam with the maximum intensity is irradiated with the light intensity control signal being the maximum, and L is the amount of light emitted when the laser beam with the light intensity control signal of an arbitrary level is emitted.

The compensating circuit 53I is composed of an adding circuit for inputting the image signal from the afterglow removing section 53H and the compensating signal and adding them as described above. That is, in the present embodiment, since both the image signal and the compensation signal have logarithmic characteristics, the image signal is compensated L / L ₀ times by adding both signals. In the case of the chest radiography, L =
Since it is L _0, it is as it is, but in the lung region and the X-ray passing region, it is compensated by L / L ₀ times (addition in logarithm), so that the laser beam of the maximum level is irradiated even in these regions. The signal level is compensated. As a result of such compensation, it is possible to obtain a distortion-free reproduced image even in a region where the intensity of laser light is large and the amount of emitted light with respect to the laser light intensity is not linear, such as in the lung region.
The density of the line-passing region also becomes a high density corresponding to the true X-ray absorption amount, and the image is clearly distinguished from the subject image region and an easy-to-see image is obtained. Further, the range of the image signal thus compensated is expanded from the level of the photoelectrically converted image signal before compensation, whereby the gradation is enhanced. The compensation characteristic conversion circuit 53J and the compensation circuit 53I constitute a signal compensation means.

When the image signal level reaches near saturation like an image of the lung, the above-described compensation has an effect of eliminating the distortion of the image.
Since the amount of X-ray absorption is large in the entire area of the subject image, X
It is only necessary to keep the line constant at the maximum level. Therefore, there is a problem that the density of the X-ray passing region becomes abnormally low without performing the compensation, but a good image can be obtained in the subject image region. Compensation can be omitted.

The image signal thus compensated is output to the fixed pattern removing circuit 53K. The fixed pattern removal circuit 53K removes the fixed pattern component stored in the fixed pattern memory 53L and peculiar to the scanning of the reading device 5 from the image signal, and then outputs the image signal to the image processing device 6. The control of each of these circuits is performed by the control circuit 53O while exchanging control data.

[0035]

As described above, according to the radiation image reading apparatus of the present invention, the photoelectric conversion amplification factor is appropriately set, and the photoelectric conversion image signal is converted into a signal other than near the saturation of the image signal based on the image signal. Since the intensity of the excitation light is kept largely constant and the intensity of the excitation light is attenuated so as not to be saturated in the vicinity of the saturation region, it is possible to suppress the light amount of the emitted light to a predetermined level or less even in the radiation passing region. The effect of afterglow is effectively avoided while maintaining good gradation of the subject image area, and good image quality is obtained in which deterioration of the S / N ratio is suppressed.

By using the afterglow component removal in combination,
The influence of afterglow can be avoided as much as possible. Furthermore, by compensating the image signal according to the intensity of the excitation light, it is possible to obtain a reconstructed image without distortion even in an image region having a large radiation transmission amount such as the lungs, and the concentration of the radiation passing region is high, and the subject image region is also high. An image that is easy to see can be obtained with a clear distinction between and.

[Brief description of drawings]

FIG. 1 is a block diagram showing basic components of a radiation image reading apparatus of the present invention.

FIG. 2 is a system external view of an embodiment of a radiation image reading apparatus according to the present invention.

FIG. 3 is a block diagram of a control unit in the embodiment.

FIG. 4 is a diagram showing characteristics of a light intensity control signal according to the above-mentioned embodiment.

FIG. 5 is a diagram showing a signal state of each part in chest imaging according to the embodiment.

FIG. 6 is a diagram showing a state of occurrence of afterglow in the same example.

FIG. 7 is a circuit diagram showing an example of an afterglow removing circuit according to the above embodiment.

FIG. 8 is a diagram showing the relationship between the laser light intensity and the amount of emitted light in the same example.

FIG. 9 is a diagram showing a relationship between a light intensity signal and a compensation signal according to the embodiment.

[Explanation of symbols]

 5 Radiation reader 51 Photostimulable fluorescent panel 52A Laser light generator 52E Photoelectric amplifier 53 Controller 53E Intensity conversion circuit 53F Laser controller 53H Afterglow eliminator 53I Compensation circuit 53J Compensation characteristic conversion circuit

Claims (3)

[Claims] 1. A stimulable fluorescent panel that stores and records radiation image information by absorbing radiation from a radiation source that has passed through an object into a stimulable fluorescent material, and scans the radiation image conversion panel with excitation light. Excitation means for stimulating the radiation image information accumulated and recorded in the stimulable phosphor by stimulating, and photoelectric conversion means for photoelectrically reading the stimulating luminescence light to obtain an image signal of a pixel unit, In the radiation image reading device configured to include, the amplification factor of the photoelectric conversion unit is photoelectrically converted when the absorption portion of the radiation that has passed through the outside of the subject of the stimulable fluorescent panel is scanned with excitation light of maximum intensity. And an amplification factor control means for controlling the image signal level to a value at which the image signal level becomes supersaturated, and a predetermined level near the maximum intensity until the image signal level reaches the saturation level near the image signal level. Controlling the radiation image reading apparatus characterized by being configured to include a light intensity control means for attenuating so as not to exceed the saturation level reaches the saturation level near the. 2. The radiation image reading apparatus according to claim 1, further comprising afterglow removing means for removing an afterglow component from the image signal. 3. A signal compensating means for compensating the image signal according to the intensity of the excitation light so as to match the level when the excitation light of the reference intensity is irradiated. Or the radiographic image reading device described in 2.