JPH0778826B2 - Image processing method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image processing method applied to a digital image signal, and more particularly to an image processing method for changing the sharpness of an image.

(Prior Art) It is performed in various fields to read a recorded image, obtain an image signal, perform appropriate image processing on the image signal, and then reproduce and record the image. For example, an X-ray image is recorded using an X-ray 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. It is possible to obtain a reproduced image with good image quality performance such as contrast, sharpness, and graininess by converting the electric signal (image signal) and subjecting it to image processing and then reproducing it as a visible image on a copy photograph or the like. A system has been developed (see Japanese Patent Publication No. 61-5193).

In addition, the applicant of the present application, radiation (X-ray, α-ray, β-ray, γ
Ray, electron beam, ultraviolet ray, etc.) causes a part of this radiation energy to be accumulated, and then irradiation with excitation light such as visible light causes stimulated emission depending on the accumulated energy. Radiation image of a subject such as a human body is temporarily photographed and recorded on a sheet-shaped stimulable phosphor by using a fluorescent phosphor, and this stimulable phosphor sheet is scanned with excitation light such as laser light to stimulate emission. An image signal is generated by photoelectrically reading the resulting stimulated emission light that produces light, and a radiation image of the subject is recorded on the basis of this image signal, a recording material such as a photographic light-sensitive material,
A radiation image recording / reproducing system for outputting a visible image on a CRT or the like has already been proposed (JP-A-55-12429,
56-11395, 55-163472, 56-104645, 5
5-116340 etc.).

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 stimulable phosphor, it has been recognized that the amount of emitted light stimulated by excitation after storage is proportional to the radiation exposure amount over a very wide range,
Therefore, even if the radiation exposure amount fluctuates considerably due to various photographing conditions, the amount of stimulated emission light emitted from the stimulable phosphor sheet is read by the photoelectric conversion means by setting the reading gain to an appropriate value. By converting this into an electric signal and using this electric signal to output a radiation image as a visible image on a recording material such as a photographic photosensitive material or a display device such as a CRT, a radiation reproduction image that is not affected by fluctuations in the radiation exposure amount can be obtained. Obtainable.

In the system using the X-ray film or the stimulable phosphor, the recording medium on which an image is recorded is normally main-scanned in a predetermined direction and sub-scanned in a direction substantially perpendicular to the main-scanning direction. Thus, the recording medium is two-dimensionally scanned, the analog signal representing the image obtained from the recording medium is logarithmically compressed by a logarithmic amplifier, and the logarithmically compressed analog signal is sampled to obtain the image. After obtaining the digital image signal corresponding to each of a large number of pixels of
Image processing is performed on the image signal to obtain a reproduced image with improved image quality.

Here, the analog signal representing the image obtained from the recording medium is logarithmically compressed by the logarithmic amplifier, because the analog signal obtained from the recording medium is from a very small signal to a very large signal (for example, a minimum signal). On the other hand, the maximum image signal often has an enormous range of 10 ⁴ to 10 ⁶ times. This is converted to a range of about 1: 4 to 6 by using a logarithmic conversion circuit. This is because the compression facilitates the subsequent handling of the signal.

(Problems to be Solved by the Invention) As described above, various signals within a huge range from a very small signal to a very large signal are input to the logarithmic amplifier. On the other hand, the value of the maximum signal that can be input to the logarithmic amplifier is limited by the power supply voltage and the logarithmic amplifier itself, and even if the maximum signal is adjusted to this upper limit, it is a signal in a huge range. Will be input to the logarithmic amplifier as a small signal.

However, the logarithmic amplifier has a characteristic that the response is slow to a minute input signal and the gain is reduced. Therefore, when logarithmic conversion is performed ignoring this characteristic, the main scanning direction in which a continuous image signal is obtained depends on the magnitude of the input signal of the logarithmic amplifier (that is, the system using the above-mentioned stimulable phosphor). In, in accordance with the amount of stimulated emission light, that is, according to the amount of radiation of each pixel irradiated to the stimulable phosphor), the smaller the input signal, the sharper the image in the main scanning direction is. On the other hand, it does not decrease in the sub-scanning direction, resulting in a difference in sharpness between the main scanning direction and the sub-scanning direction.

In order to avoid this, it is conceivable to lower the frequency band of the signal to the extent that it can respond to the minimum signal and input it to the logarithmic amplifier, for example, by slowly reading the image. However, if this method is adopted, it is necessary to read the image slowly, resulting in a reduction in the processing capacity per unit time in this system.

In view of the above circumstances, it is an object of the present invention to provide an image processing method in which the sharpness in the main scanning direction and the sharpness in the sub scanning direction are substantially the same without reducing the processing capacity per unit time. .

(Means for Solving the Problem) An image processing method of the present invention is to perform main scanning on a recording medium on which an image is recorded in a predetermined direction and sub-scan in a direction substantially perpendicular to the main scanning direction. The recording medium is two-dimensionally scanned, an analog signal representing the image obtained from the recording medium is logarithmically compressed by a logarithmic amplifier, and the logarithmically compressed analog signal is sampled to obtain a large number of images of the image. An image processing method for subjecting an image signal to image processing after obtaining a digital image signal corresponding to each pixel, wherein the main scanning caused by the response delay of the logarithmic amplifier of the image carried by the image signal And a difference in sharpness in both directions of the sub-scanning is corrected, and the sharpness conversion processing according to the magnitude of the image signal corresponding to each pixel is performed so that the sharpness in both directions becomes substantially the same. It is characterized in that applied to the item.

Here, the sharpness in the main scanning direction and the sharpness in the sub-scanning direction may be substantially the same in the vicinity of each pixel of the image, and it is not always necessary that the sharpness of the entire image is substantially the same.

Further, the "sharpness conversion processing according to the size of the image signal corresponding to each pixel" means the sharpness directly corresponding to the size of the image signal before processing of the pixel which is the target of the sharpness conversion processing. The size of the image signal of the pixels around the pixel that is the target of the sharpness conversion process may be changed in order to perform a smooth operation over each pixel adjacent to each other. The sharpness conversion processing may be performed according to the magnitude of the image signal after taking into consideration and averaging.

(Operation) The image processing method of the present invention performs sharpness conversion processing according to the magnitude of the analog signal of each pixel input to the logarithmic amplifier so that the sharpness in the main scanning direction and the sharpness in the sub-scanning direction are substantially the same. Since it is applied to the image signal, it is not necessary to limit the processing capability per unit time to limit the frequency of the input signal to the extent that the logarithmic amplifier can sufficiently respond to a minute signal input to the logarithmic amplifier. Moreover, the sharpness in each of the pixels of the image in the main scanning direction and the sub scanning direction is substantially the same.

The sharpness may be different for each pixel of the image. That is, as described above, it is not always necessary that the sharpness is substantially the same over the entire image.
For example, when the original image is, for example, an X-ray image obtained by irradiating the recording medium with X-rays, the region of the image with a small X-ray irradiation amount is affected by quantum noise due to X-ray fluctuations. There is a phenomenon that the image is rough and rough, that is, the so-called poor graininess occurs. Therefore, it may be a better image to reduce the sharpness of such an area compared to other areas. It is known.
Therefore, for example, in a system in which the dose of X-rays is proportional to the magnitude of the input signal of the logarithmic amplifier, the input signal is very small (that is, the dose of X-rays is small), and the sharpness is lowered in the main scanning direction. At this time, the image processing is performed in the main scanning direction in which the sharpness is lowered so as not to match the sharpness in the main scanning direction with the sharpness in the sub-scanning direction with good sharpness, but rather to match the sharpness in the main scanning direction. In some cases, it may be better to deteriorate the sharpness in the sub-scanning direction.

In addition, of course, sharpness enhancement processing according to the degree of deterioration of each pixel is performed so that the sharpness in the main scanning direction, which has deteriorated so that the image as a whole has substantially the same sharpness, is matched with the sharpness in the sub-scanning direction. If it is desirable to obtain a sharpness intermediate between the sharpness in the main scanning direction and the sharpness in the sub-scanning direction for each pixel, it is possible to determine the sharpness in the main scanning direction and the sharpness in the sub-scanning direction. Both may be adjusted.

(Example) Hereinafter, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a perspective view showing an example of an image reading processing apparatus using the image processing method of the present invention. This image reading / processing device is a device using the above-mentioned stimulable phosphor.

The stimulable phosphor sheet 1 on which the radiation image information of the subject is stored and recorded is conveyed (sub-scanned) in the arrow Y direction by the sheet conveying means 3 such as an endless belt driven by a motor 2. On the other hand, the excitation light 5 emitted from the laser light source 4 is reflected and deflected by a rotary polygon mirror 6 driven by a motor 13 and rotating at a high speed in the direction of the arrow, and after passing through a focusing lens 7 such as an fθ lens, the optical path is changed by a mirror 8. Instead, the light is incident on the sheet 1 and the main scanning is performed in the arrow X direction substantially perpendicular to the sub-scanning direction (arrow Y direction). From the portion of the sheet 1 irradiated with the excitation light 5, the stimulated emission light 9 having a light amount corresponding to the stored and recorded radiation image information is emitted, and the stimulated emission light 9 is guided by the light guide 10. , Photomultiplier (photomultiplier tube) 11 for photoelectric detection. The light guide 10 is made by molding a light guide material such as an acrylic plate, and has a linear incident end face 10.
a is disposed so as to extend along the main scanning line on the stimulable phosphor sheet 1, and the light receiving surface of the photomultiplier 11 is coupled to the emission end surface 10b formed in a ring shape. The stimulated emission light 9 that has entered the light guide 10 from the incident end face 10a travels inside the light guide 10 by repeating total reflection,
The photomultiplier 11 detects the amount of stimulated emission light 9 emitted from the emission end face 10b and received by the photomultiplier 11 and carrying the radiation image information. To change the number of processed sheets 1 per unit time, the sub-scanning speed and the main-scanning speed are changed in synchronization. Therefore, the control unit 12 for controlling the speed of the motor 2 for sub-scanning and the control unit 14 for controlling the speed of the motor 13 for driving the rotary polygon mirror 6 are connected by a signal line 15 so that the above synchronization is performed. Controlled. The signal line 15 is also connected to the arithmetic unit 18 in order to obtain information on the speeds of the main scanning and the sub-scanning necessary for the arithmetic unit 18 to perform the arithmetic operation described later.

The analog output signal S output from the photomultiplier 11 is logarithmically compressed by the logarithmic amplifier 16.

FIG. 2 is a diagram showing frequency characteristics of the logarithmic amplifier 16 shown in FIG. The horizontal axis of the figure shows the frequency of the input signal of the logarithmic amplifier 16, and the vertical axis of the figure shows the attenuation amount (dB) of the output signal. In addition, × mark, △ mark, ○ mark, □ mark, ● shown in the figure
The graphs connecting the marked points indicate that the magnitude of the input signal to the logarithmic amplifier 16 is 0.1 μA, 1.0 μA, 10 μA, 100 μA, 1
It is a graph at the time of mA.

As shown in this figure, the higher the input signal is, the more the input signal is attenuated, and even if the input signal has the same frequency, the smaller the input signal is, the more the input signal is attenuated. The frequency of the input signal of the logarithmic amplifier 16 corresponds to the spatial frequency of the radiographic image stored on the sheet 1 shown in FIG. 1, and the attenuation of the input signal with a small current means that the sheet 1 emits light. The smaller the stimulated emission light, that is, the smaller the amount of radiation applied to the sheet 1, the lower the sharpness in the main scanning direction. In the sub-scanning direction, since the signals are not continuous in time, the sharpness does not change depending on the characteristics of the logarithmic amplifier 16. Therefore, depending on the dose of radiation applied to each pixel of the radiation image recorded on the sheet 1, the lower the dose is, the larger the difference in sharpness between the main scanning direction and the sub-scanning direction is.

As shown in FIG. 1, the analog signal S'logarithmically compressed by the logarithmic amplifier 16 is sampled by the A / D converter 17, and the digital image signal D for each pixel of the radiation pixels accumulated and recorded on the sheet 1 is recorded. Is obtained, and the image signal D is input to the calculation unit 18.

In the calculation unit 18, the difference in sharpness between the main scanning direction and the sub-scanning direction caused by the response delay of the logarithmic amplifier 16 is corrected, and the sharpness in both directions is input to the logarithmic amplifier 16 so that the sharpness becomes substantially the same. The image signal D is subjected to sharpness conversion processing according to the magnitude of the analog signal S.

Here, an example will be described in which the sharpness in the sub-scanning direction having substantially the same sharpness over the entire surface of the radiation image is matched with the sharpness in the main-scanning direction for each pixel.

FIG. 3 shows a representative pixel (the central pixel (i, i, i) of the pixels of the radiation image accumulated and recorded on the sheet 1 shown in FIG.
FIG. 9 is a diagram showing j)) and its surrounding pixels, and the image signal values (D _{i, j,} etc.) corresponding to each of the pixels of the digital image signal D input to the arithmetic unit 18. Arrow shown in the figure
X and Y correspond to the main scanning direction and the sub scanning direction (X and Y directions in FIG. 1), respectively. Pixel in the center of the figure (value of image signal
A case of _performing image processing on D _ij ) will be described.

First, using this image signal D, the magnitude of the analog signal input to the logarithmic amplifier is determined. Here, the average value of the image data of the pixel (i, j) of interest and a total of nine pixels surrounding the pixel is compared with a predetermined value α, , That is, when the radiation dose in the vicinity of the pixel (i, j) is small (that is, the amount of stimulated emission light is small and the input current to the logarithmic amplifier 16 is small), the next sharpness conversion is performed. Perform processing. still, In this case, it is determined that the difference in sharpness between the main scanning direction and the sub-scanning direction is not so large that correction is necessary, and the next sharpness conversion process is not performed for this pixel (i, j).

When the expression (1) is satisfied, the pixel (i,
If the value of the image signal of j) is represented by D ′ _{i, j} , The sharpness conversion processing is performed on each pixel (i, j) according to the equation. Here, k is a value of 0 <k <1 and is a value determined by the characteristics of the logarithmic amplifier 16, the main scanning speed, and the like.

Qualitatively explaining the meaning of the above equation (3),
A boundary point for determining whether or not to perform sharpness conversion processing, that is, In the case of, Then, from the equation (3), D ′ _{i, j} = D _{i, j} (6) That is, no processing is performed. When the above equation (1) is established, Then, if this is set as ε, the above equation (3) becomes Becomes That is, the value D _{i, j of} the pixel (i, j) and two pixels (i−1, j) and (i + 1, j) adjacent to the pixel (i, j) in the sub-scanning direction It shows that D _{i-1, j} and D _{i + 1, j} are weighted by kε and added. This weighting is , Which means that the sharpness in the sub-scanning direction of each pixel is reduced according to the degree of deterioration of the sharpness in the main-scanning direction of each pixel, and the sharpness in the main-scanning direction of each pixel is substantially reduced. This means that adjustments are made to be the same.

The above calculation is performed in order to correct the image signal value D _{i, j} of each pixel (i, j) of interest to obtain D ′ _{i, j.}
The value of the image signal of each pixel is considered, but this is only an example of the image processing method, and the value of the image signal of each surrounding pixel is not taken into consideration, and the value of each pixel (i, j) Whether or not the correction is performed and the correction amount in the case of performing the correction may be determined based only on the value, and the value of the image signal of each pixel (for example, (i-2, i + 2), etc.) farther from the above-described embodiment is determined. You may consider.

Also, when correcting the value of D _{i, j}
Image signal value D of one pixel (i−1, j), (i + 1, j)
_{Although i-1, j} and D _{i + 1, j} are weighted and added, pixels further apart in the sub-scanning direction (for example, (i-2 j), (i + 2, j))
Etc.) may be taken into consideration.

The above embodiment is an example of adjusting the sharpness of each pixel in the sub-scanning direction to the sharpness in the main scanning direction. Next, an example of an image processing method of adjusting the sharpness in the main scanning direction to the sharpness in the sub-scanning direction will be shown. .

As a determination as to whether or not to perform image processing, here, three pixels (i, j−1), (i, j) arranged in the main scanning direction are arranged.
j), (i, j + 1) image signal values D _{i, j-1} , D _{i, j} , D
The average value of _{i, j + 1} is used. That is, when the predetermined threshold value is α ′, At this time, the sharpness of the pixel (i, j) in the main scanning direction is emphasized to match the sharpness in the sub-scanning direction. That is, the above (9)
When the value of the image signal after processing the pixel (i, j) is D ″ _{i, j} when the expression is satisfied, Where β is a constant. The above equation (10) is the three-pixel average value, that is, Is equal to α ′, that is, When is satisfied, the equation (10) becomes D ″ _ij = D _ij (12), and when the equation (9) is satisfied, Putting that, equation (10) becomes Becomes Ie Represents the difference of the image signal value D _ij of pixel (i, j) from the three-pixel average value. Multiplying this difference by βε ′ yields the original value.
By adding to D _ij , the value is corrected so as to emphasize the difference, and it is shown that the sharpness in the main scanning direction of each pixel is emphasized so as to have substantially the same sharpness over the entire image. Needless to say, also in this embodiment, various methods other than the three-pixel averaging can be used as in the above-described embodiments. Further, it is also possible to perform a process in which both the processing for emphasizing the sharpness in the main scanning direction as in this embodiment and the processing for reducing the sharpness in the sub-scanning direction as in the above-described embodiment are combined. Of course. The sharpness of each pixel is preferably determined in consideration of the graininess described above.

Since the frequency of the analog signal S is changed depending on the speeds of the main scanning and the sub-scanning, the speeds of the main scanning and the sub-scanning from the control units 14 and 12 are required when the image processing is performed by the arithmetic unit 18 shown in FIG. The values of α, α ′, β and the like are changed in response to the information of.

The image signal D'which has been subjected to the image processing by the calculation section 18 is temporarily stored in the memory 19 and then, if necessary, the image display device 20.
And the image based on the image signal D ′ is reproduced and displayed.

The above embodiment is an example in which the image processing method of the present invention is applied to an apparatus using a stimulable phosphor sheet, but the present invention is an apparatus using a stimulable phosphor sheet and an apparatus using a conventional X-ray film. Etc., and further radiation images (X-ray images, etc.)
The same applies to other devices that handle general images.

In addition, the two-dimensional scanning on the recording medium on which the image is recorded is
As a matter of course, in addition to the two-dimensional scanning with the light beam as in the above-described embodiment, it may be electrically scanning using, for example, a CCD or a MOS sensor.

(Effects of the Invention) As described in detail above, the image processing method of the present invention is
The sharpness according to the magnitude of the image signal corresponding to each pixel is corrected so that the difference in sharpness in both the main scanning direction and the sub-scanning direction caused by the response delay of the logarithmic amplifier is corrected so that the sharpness in both directions becomes substantially the same. Since the degree conversion processing is applied to the image signal, the processing capacity per unit time is not limited in order to limit the frequency of the input signal to the extent that the logarithmic amplifier can sufficiently respond even to a minute input signal. , The sharpness of each pixel point of each image in the main scanning direction and the sub scanning direction can be made substantially the same.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of an image reading processing apparatus using the image processing method of the present invention, FIG. 2 is a view showing frequency characteristics of logarithmic amplification shown in FIG. 1, and FIG. FIG. 2 is a diagram showing some pixels of a radiation image accumulated and recorded on the sheet shown in FIG. 1. DESCRIPTION OF SYMBOLS 1 ... Accumulative phosphor sheet, 2, 13 ... Motor 3 ... Sheet conveying means, 4 ... Laser 6 ... Rotating polygon mirror, 9 ... Excited emission light 10 ... Optical guide 11 ... Photomultiplier 12, 14 ... Control part, 15 ... Signal line 16 ... Amplifier, 17 ... A / D converter 18 ... Calculator, 19 ... Memory 20 ... Image display device

Claims (1)

[Claims] 1. A recording medium on which an image is recorded is main-scanned in a predetermined direction and sub-scanning is performed in a direction substantially perpendicular to the main-scanning direction so that the recording medium is two-dimensionally scanned.
An analog signal representing the image obtained from the recording medium is logarithmically compressed by a logarithmic amplifier, and the logarithmically compressed analog signal is sampled to obtain a digital image signal corresponding to each of a large number of pixels of the image. An image processing method for subjecting the image signal to image processing, wherein the image carried by the image signal has a difference in sharpness in both directions of the main scanning and the sub scanning caused by a response delay of the logarithmic amplifier. An image processing method, wherein the image signal is subjected to a sharpness conversion process according to the magnitude of an image signal corresponding to each pixel so that the sharpness in both directions is substantially the same.