JP2981733B2 - Image reading device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] 1. Field of the Invention [0002] The present invention relates to an image reading apparatus which reads out light representing an image obtained from a recording sheet on which an image is recorded to obtain an image signal.

[0002]

2. Description of the Related Art In various fields, reading a recorded image to obtain an image signal, subjecting the image signal to appropriate image processing, and reproducing and recording the image have been performed. For example, an X-ray image is recorded using an X-ray film having a low gamma value designed to be compatible with the subsequent image processing, and the X-ray image is read from the film on which the X-ray image is recorded, and is converted into an electric signal. After conversion, the electric signal (image signal) is subjected to image processing, and then reproduced as a visible image in a copy photograph or the like, a reproduced image with good image quality performance such as contrast, sharpness, and graininess can be obtained. A system has been developed (see Japanese Patent Publication No. 61-5193).

[0003] Further, the applicant of the present invention has proposed that radiation (X-ray, α
Radiation, β-rays, γ-rays, electron beams, ultraviolet rays, etc.), a part of this radiation energy is accumulated, and then irradiation with excitation light, such as visible light, causes a photostimulated emission according to the accumulated energy. Using stimulable phosphor (stimulable phosphor)
A radiation image of a subject such as a human body is once photographed and recorded on a sheet-shaped stimulable phosphor, and the stimulable phosphor sheet is scanned with excitation light such as laser light to generate stimulated emission light. A radiation image recording / reproducing system has been already proposed in which an exhaust signal is photoelectrically read to obtain an image signal, and a radiation image of a subject is output as a visible image to a recording material such as a photographic material or a CRT based on the image signal. (Japanese
55-12429, 56-11395, 55-163472, 56-104
Nos. 645 and 55-116340).

[0004] This system has the practical advantage of being able to record images over a very wide radiation exposure area compared to conventional radiographic systems using silver halide photography. That is, in the case of the stimulable phosphor, 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 fluctuates considerably, the amount of the stimulating 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 materials such as photographic photosensitive materials using
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 a change in radiation exposure.

In the above-mentioned system using a recording sheet such as the X-ray film or the stimulable phosphor sheet, in order to obtain an image signal by reading an image recorded on the recording sheet,
Usually, a recording sheet is irradiated with light using an image reading device, and light (for example, light transmitted through an X-ray film or reflected from the X-ray film, or light reflected from the X-ray film) Stimulable light emitted from the stimulable phosphor sheet) is received by a photodetector, a signal corresponding to the amount of the light is obtained, and the signal is input to a logarithmic amplifier to logarithmically compress the signal. Done.

[0006]

After obtaining an image signal using the image reading device as described above, the image signal is subjected to appropriate image processing by the image processing device, and the image is reproduced by the image reproducing device. Is reproduced and displayed as a visible image, and the visible image is used for observation.

However, light representing an image obtained from a recording sheet has an extremely wide light amount range from weak light to strong light (for example, an intensity ratio of about 1:10 ⁴ ). Even with such a light amount width, if a high-performance photomultiplier tube or the like is used as the photodetector, it can be converted into an electric signal corresponding to each light amount with sufficient accuracy and sufficient speed. However, when this electric signal is input to a logarithmic amplifier for logarithmic conversion, the logarithmic amplifier usually has poor response to a weak input signal. It has been necessary to limit the speed at which an image is read from a recording sheet so that even the weakest signal can be logarithmically converted with sufficient accuracy. For this reason, the speed of the entire apparatus is limited, and the processing capacity per unit time is limited.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides an image reading apparatus which overcomes the condition that the logarithmic amplifier has poor response to a weak input signal and has an improved processing capability per unit time. It is intended to do so.

[0009]

An image reading apparatus according to the present invention irradiates a stimulable phosphor sheet on which a radiation image is stored and recorded with excitation light, and receives stimulating light emitted by the irradiation. In an image reading apparatus having a detector and a current input type logarithmic amplifier for inputting an output signal of the photodetector and logarithmically compressing the output signal, the output signal is converted into a signal within a desired input signal range of the logarithmic amplifier. A bias adding means for adding a bias for conversion to the output signal, and a means for removing distortion generated in the logarithmically compressed signal by the bias addition are provided.

[0010]

[0011]

As a general property of the logarithmic amplifier, if the magnitude of the input signal is within a predetermined range, the logarithmic conversion is performed with high accuracy up to a predetermined high frequency. As the input signal becomes weaker, the logarithmic conversion is performed with higher accuracy only up to a frequency lower than the predetermined high frequency. Also, if the input signal is too large and exceeds the specification of the logarithmic amplifier, the logarithmic amplifier does not operate normally.

Here, when a bias is added to the input signal of the logarithmic amplifier, the output signal of the logarithmic amplifier is distorted. Therefore, no bias has been added to the input signal of the logarithmic amplifier.

However, in the radiation image reading and reproducing system, since the signal output from the logarithmic amplifier is usually subjected to analog and / or digital image processing thereafter, it is necessary to remove the distortion. Focusing on the fact that it can be removed at that time if necessary, and by adding a bias to the input signal of the logarithmic amplifier, the signal range of the input signal of the logarithmic amplifier can be reduced to the above-mentioned predetermined range with good responsiveness of the logarithmic amplifier. The present invention has been conceived by paying attention to the fact that it can be kept within the range.

The image reading apparatus of the present invention includes a bias adding means for adding a bias for converting the output signal of the photodetector to a signal within a desired input signal range of the logarithmic amplifier to the output signal. Only a good range of the response characteristics of the logarithmic amplifier is used, so that the logarithmic conversion is performed with sufficient accuracy even when the reading speed is increased. In addition, since a means for removing distortion generated in the signal after logarithmic compression by the addition of the bias is provided, distortion of the output signal of the logarithmic amplifier can be appropriately removed.

[0015]

As described in detail above, the image reading apparatus of the present invention adds a bias to the output signal to convert the output signal of the photodetector into a signal within a desired input signal range of the logarithmic amplifier. Since the bias applying means is provided, the image can be read from the recording sheet at a high speed, and the processing capability of the image reading apparatus per unit time is improved.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of an image reading apparatus according to the present invention will be described with reference to the drawings.

FIG. 2 is a perspective view showing an embodiment of the image reading apparatus of the present invention.

This image reading apparatus is an apparatus using the above-described stimulable phosphor sheet.

The stimulable phosphor sheet 11 on which the radiation image is recorded is set at a predetermined position of the image reading device 100.
The stimulable phosphor sheet 11 set at this predetermined position is
The sheet is conveyed (sub-scanning) in the direction of arrow Y by a sheet conveying means 13 such as an endless belt driven by a driving means (not shown).

On the other hand, a light beam 15 emitted from a laser light source 14 is reflected and deflected by a rotary polygon mirror 16 driven by a motor 23 and rotated at a high speed in the direction of the arrow, and after passing through a focusing lens 17 such as an fθ lens, a mirror 18 The main scanning is repeated in the direction of arrow X substantially perpendicular to the sub-scanning direction (direction of arrow Y). This excitation light
From the portion of the sheet 11 irradiated with 15, stimulating luminescence light 19 of an amount corresponding to the image information stored and recorded is diverged,
The stimulated emission light 19 is guided by a light guide 20 and is photoelectrically detected by a photomultiplier (photomultiplier tube) 21.

The light guide 20 is formed by molding a light-guiding material such as an acrylic plate, and a linear incident end face 20a extends along the main scanning line on the stimulable phosphor sheet 11. The light receiving surface of the photomultiplier 21 is coupled to the emission end face 20b formed in such a manner as to be annular. Stimulated luminescence light 19 entering the light guide 20 from the incidence end face 20a
Travels through the interior of the light guide 20 by repeating total reflection,
The photomultiplier 21 emits the light emitted from the emission end face 20b, receives the photomultiplier 21, and converts the amount of the photostimulated light 19 representing an image into an electric signal.

The stimulable luminescent light 19 is substantially proportional to the dose of radiation applied to each part of the stimulable phosphor sheet 11 over a wide range, and the stimulable luminescent light 19 represents an image stored and recorded on the sheet 11. Is in the range of about 4 digits (where the intensity of the weakest light among the stimulating light to be read is 1.0, the light having the largest light amount among the stimulating light to be read is about 1.0 × 10 ^4). Range having the strength of In addition, since the output of the photomultiplier 21 is proportional to the amount of the stimulating luminescence light input, the output of the photomultiplier 21 is also about four digits (about 10 ⁻⁷ amperes in this embodiment).
About 10 ^-3 amps).

Although a current actually flows into the photomultiplier 21, it is referred to as an output or an output current of the photomultiplier 21 because the signal follows the flow of the signal.

The analog output signal S (t) output from the photomultiplier 21 is input to an adder 25 which constitutes an example of the bias applying means of the present invention.

A constant current S '( ^{9.0.times.10.sup.-7} amps in this embodiment) is also input to the adder 25, and these S (t),
S 'is added, and the added signal S (t) + S' is output and input to the logarithmic amplifier 26. The added signal S (t)
+ S 'is logarithmically amplified by a logarithmic amplifier 26, and digitized by the A / D converter 27, the image signal S _Q is obtained. After the obtained image signal _SQ is temporarily stored in the storage unit 28,
It is transmitted to the image processing device 200. In the image processing apparatus 200, appropriate image processing is performed on the image signal S _Q received.

The image signal S _R subjected to the image processing is transmitted to the image reproducing device 300, and the image reproducing device 300 reproduces and records a radiation image based on the image signal S _R.

Here, the characteristics of the logarithmic amplifier 26 will be described.

FIG. 1 is a graph showing an example of the characteristics of the logarithmic amplifier.

This logarithmic amplifier has an input signal (input current).
It responds to a frequency of about 200 KHz between 10 ^-5 amps and 10 ^-3 amps. However, as the input signal becomes weaker than this range, the highest frequency of a good response decreases as shown in the figure, and the response speed decreases to about 100 KHz at 10 ^-6 amps and about 25 KHz at 10 ^-7 amps.

Here, if the current signal in the range of 10 ^-7 amps to 10 ^-3 amps output from the photomultiplier 21 is to be logarithmized with high accuracy, the response of the logarithmic amplifier 26 at 10 ^-7 amps is required. Is up to 25 KHz, so that the conveying speed (sub-scanning speed) of the sheet 11 by the sheet conveying means 13 and the rotating polygon mirror 16 are adjusted so that the information representing the radiation image carried by the stimulating light 19 is 25 KHz or less. It is necessary to determine the rotation speed (the speed of main scanning).

In this case, however, since a certain amount of bias S '(9.0.times.10.sup.- ⁷ amperes) is added to the output signal S (t) from the photomultiplier 21 by the adder 25, it is inputted to the logarithmic amplifier. The lower limit of the signal S (t) + S 'is about 10
^-7 amperes +9.0 × 10 ^-7 amperes = 10 ^-6 amperes, the upper limit is about 3 orders of magnitude ranging from about 10 ^-3 amps +9.0 × 10 ^-7 amperes = about ^10-3 amps. Therefore logarithmic amplifier 26 is approximately
The main scanning and sub-scanning speeds may be determined so that the response is good up to 100 KHz (see FIG. 1) and the information representing the radiation image carried by the stimulating light 19 is 100 KHz or less, and the bias S 'is added. The reading speed is 100KHz /
25KHz = 4 times higher.

FIG. 3A shows the state of the photomultiplier 21.
And the signal S (t) output from the logarithmic amplifier 26
Signal S_PFIG. The horizontal axis is Photomulti
The logarithm log S (t) of the output signal S (t) of the chipplier 21, the vertical axis
Is the output signal S of the logarithmic amplifier 26 _PIs represented.

[0034] Since the horizontal axis is logarithmic, if the bias S 'by the adder 25 is not added if proportional to the log S (t) and S _P, as shown in the graph 31 indicated by a broken line in FIG. is there. However, since the bias S 'has been added,
Signal actually input to the logarithmic amplifier 26 is biased in the summed S (t) + S ', thus S _P is log (S
(t) + S '), but not proportional to log S (t).
Particularly, in the minute input signal region D, distortion occurs as shown by a solid line graph 32 in the figure.

The graph 33 of FIG. 3 (B) is a diagram showing a correction curve for correcting the signal S _P output from the logarithmic amplifier 26, the distortion due to the bias S 'is added. This graph 33 is stored in the form of a table in the image processing device 200, and the digitized image signal S _Q
Is corrected when image processing is performed based on.

In this embodiment, since a bias of 9.0 × 10 ^-7 amps is added to a 4-digit signal of about 10 ^-7 amps to 10 ^-3 amps, the output signal S _P from the logarithmic amplifier 26 is added.
When the minimum signal is set to 1.0, the maximum signal becomes 4.0. Therefore, it is only necessary to A / D convert a signal having a width of 4.0−1.0 = 3.0, so that the resolution is improved as a whole. However, the resolution is reduced in the minute input signal area D.

However, since this area has a somewhat lower importance in a normal radiographic image, a slight decrease in resolution is not usually a problem. When the amount of the bias S 'added to the signal S (t) by the adder 25 is increased, the range of the signal input to the logarithmic amplifier 26 is narrowed and the reading speed can be further improved. The distortion in the region D increases, resulting in a decrease in resolution when A / D conversion is performed. Therefore, the amount of the bias S ′ is appropriately determined in consideration of the reading speed, the magnitude of the distortion (the degree of reduction in the resolution of the signal in the small signal input area D), and the like.

In the above embodiment, the distortion caused by the addition of the bias S 'and the logarithmic amplification is removed by the image processing apparatus 200. However, this removal need not be performed during image processing. output signal S _P output amplifier 26 is a /
The processing may be performed in an analog manner before being input to the D converter 27, or may be performed when the image reproducing apparatus 300 generates an image signal for reproduction in consideration of the characteristics of the photographic film for image reproduction. . If this distortion is not a problem due to the nature of the radiographic image to be taken, the purpose of taking the image, the degree of the distortion, and the like, there is no need to correct this distortion.

Further, in a radiation image reading apparatus using a stimulable phosphor sheet, an image signal is obtained by reading under an optimal reading condition according to the dose of radiation applied to the stimulable phosphor sheet and the like. The stimulable phosphor sheet is scanned in advance by a low-level light beam to perform a pre-read for reading an outline of the radiation image recorded on the sheet, and a pre-read image signal obtained by the pre-read is analyzed, and then the sheet is read. There is also an apparatus configured to perform scanning by irradiating a scan with a light beam having a higher level than the light beam at the time of the pre-reading, and reading the radiation image under an optimum reading condition to obtain an image signal (Japanese Patent Application Laid-Open (JP-A) No. 2002-110630). 58-67240, 58-67241, 58
-67242). It goes without saying that the radiation image reading method of the present invention can be applied to both pre-reading and main reading in such a pre-reading apparatus.

Here, since it is sufficient to know the outline of the image at the time of pre-reading, the amount of the bias S 'is increased to increase the reading speed, and at the time of the main reading, the amount of the bias S' is reduced to perform accurate reading. The amount of the bias S 'may be changed between the pre-reading and the main reading, for example, by sacrificing the speed somewhat.

The present invention can be used not only for an apparatus using a stimulable phosphor sheet but also for an apparatus using a conventional X-ray film.

FIG. 4 is a perspective view of an embodiment of an X-ray image reading apparatus for reading an X-ray image recorded on an X-ray film.

An X-ray film 40 set at a predetermined position and having an X-ray image recorded thereon is conveyed (sub-scanning) in the direction of arrow Y 'shown in FIG.

A reading light 43 emitted from a one-dimensionally elongated light source 42 is converged by a cylindrical lens 44, and X 'is substantially perpendicular to the direction of arrow Y' on the X-ray film.
Irradiate linearly in the direction. Below the X-ray film 40 irradiated with the reading light 43, the X-ray is located at a position where the reading light 43 which has passed through the X-ray film 40 and is intensity-modulated by the X-ray image recorded on the X-ray film 40 is received. A MOS sensor 45 is provided in which a number of solid-state photoelectric conversion elements corresponding to each pixel interval in the X 'direction of the line image are linearly arranged. This MOS
While the X-ray film is conveyed in the direction of the arrow Y 'while being irradiated with the reading light 43, the sensor 45 transmits the reading light transmitted through the X-ray film 40 to a predetermined position corresponding to each pixel interval in the Y' direction of the X-ray image. At the time intervals of. The continuous light reception in the X 'direction is the main scanning.

FIG. 5 is a circuit diagram showing an equivalent circuit of the MOS sensor 45.

Signals generated by the photocarriers when the reading light 43 strikes a large number of solid-state photoelectric conversion elements 46 are stored in capacitors Ci (i = 1, 2,... N) in the solid-state photoelectric conversion elements 46. The accumulated photocarrier signals are sequentially read out (main-scanned) by sequentially opening and closing the switch unit 48 controlled by the shift register 47, whereby time-sequential image signals are obtained. This image signal is then amplified by the amplifier 49 and output from its output terminal 50.

The output analog image signal is logarithmically converted by a logarithmic amplifier as in the embodiment shown in FIG. To use the good range of the responsiveness of this log amp,
As in the case described with reference to FIG. 2, after a signal is output from the output terminal 50, a bias is added to the signal, thereby improving the reading speed.

In this embodiment, the MOS sensor 44
It is needless to say that CCD, CPD (Charge Priming Device) or the like can be used in place of the above. In reading an X-ray film, it is a matter of course that reading may be performed by two-dimensionally scanning with a light beam similarly to the above-described reading of the stimulable phosphor sheet. Furthermore, in the present embodiment, the light transmitted through the X-ray film 40 is received. However, it is needless to say that the configuration can be such that the light reflected from the X-ray film 40 is received.

As described above, according to the image reading apparatus of the present invention, a photodetector that receives light representing an image obtained from a recording sheet on which the image is recorded, and an output signal of the photodetector are input. The present invention can be generally applied to an image reading apparatus having a logarithmic amplifier for performing logarithmic compression.

[Brief description of the drawings]

FIG. 1 is a graph showing an example of a characteristic of a logarithmic amplifier.

FIG. 2 is a perspective view showing an embodiment of the image reading apparatus of the present invention.

FIG. 3A is a diagram showing a relationship between an output signal of a photomultiplier and an output signal of a logarithmic amplifier, and FIG.
Diagram showing correction curve of logarithmic amplifier output

FIG. 4 is a perspective view of an embodiment of an X-ray image reading apparatus that reads an X-ray image recorded on an X-ray film.

FIG. 5 is a circuit diagram showing an equivalent circuit of the MOS sensor 45;

[Explanation of symbols]

 11 Storable phosphor sheet 19 Stimulated luminescence 21 Photomultiplier 25 Adder 26 Logarithmic amplifier 27 A / D converter 28 Storage means 40 X-ray film 45 MOS sensor 100 Image reading device 200 Image processing device 300 Image reproducing device

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁶ , DB name) H04N 1/40-1/409 H04N 1/46 H04N 1/60 H04N 1/04-1/207

Claims (1)

(57) [Claims] 1. A photodetector for irradiating a stimulable phosphor sheet on which a radiation image is stored and recorded with excitation light, and receiving photostimulated light emitted by the irradiation, and an output signal of the photodetector. An image reading apparatus comprising a current input type logarithmic amplifier for inputting and logarithmically compressing, wherein a bias for converting the output signal into a signal within a desired input signal range of the logarithmic amplifier is added to the output signal. An image reading apparatus comprising: a bias applying unit; and a unit configured to remove distortion generated in the logarithmically compressed signal by the bias addition.