JP3425780B2 - Dynamic range compression method for radiographic images - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compressing a dynamic range of a radiation image, and more particularly, to a dynamic image of a radiation image for processing an original image signal to obtain an image signal carrying an image having a narrower dynamic range than the original image. The present invention relates to a technique for improving a range compression method.

[0002]

2. Description of the Related Art Conventionally, as a method for compressing a density region of a radiation image while ensuring proper observation of fine structures in the image region, there is one disclosed in, for example, Japanese Patent Application Laid-Open No. 3-222577. .

In the compression method disclosed in Japanese Patent Laid-Open No. 3-222577, a non-sharp mask is obtained by averaging the original image signal Sorg in a predetermined mask area including each pixel point corresponding to each pixel point. Signal (blur mask signal) Sus
And f (Sus) is a function that decreases monotonically as the unsharp mask signal Sus increases, the processed image signal Sproc whose dynamic range is compressed is
It is obtained as proc = Sorg + f (Sus).

[0004]

By the way, as mentioned above,
In dynamic range compression, the objective is to obtain the optimum contrast characteristics for image interpretation in the region of interest in the visualization state when a hard copy is finally obtained. Since the correction degree is determined on the basis of the digital image signal (original image signal) thus generated, the following problems occur.

That is, even if a fixed correction function (for example, a linear function) is set with respect to the input digital signal value, it is finally visualized depending on the gradation characteristics of the recording material when obtaining a hard copy. been when may inadvertently changed correction degree of dynamic range compression (disappears by a linear function), the best dynamic range compression processing on the final display state (on hard copy), stably Could not be applied to .

The present invention has been made in view of the above problems, and in a dynamic range compression method using a non-sharp mask signal, a final display state (hard copy)
The optimum dynamic range compression process for
The purpose is to be able to apply it regularly .

[0007]

Therefore , in the method for compressing the dynamic range of a radiation image according to the present invention,
After the image processing for obtaining the processed image signal Sproc in which the Mick range is compressed is performed and then the processed image signal Sproc is output to the display device, the unsharp mask signal Sus is obtained corresponding to each pixel point. , Dorg, the display density of the display device corresponding to the original image signal Sorg, Dproc, the display density of the display device corresponding to the processed image signal Sproc, and the display of the display device corresponding to the non-sharp mask signal Sus. Concentration is D
Let us be the density correction value f which is a function of the display density Dus.
When 2 (Dus) is set, Dproc = Dorg + f2 (Du
s) the processed image signal Spro so as to satisfy the relation
I got c.

In the method of compressing a dynamic range of a radiation image, the processed image signal Sproc is obtained by correcting the original image signal Sorg with a correction value f1 (Sus) corresponding to the unsharp mask signal Sus. The absolute value of the change rate of the correction value f1 (Sus) with respect to the change of Sus gradually increases in accordance with the decrease in the non-sharp mask signal Sus for the correction value f1 (Sus) for increasing and correcting the original image signal Sorg. and, for the correction value corrected by decreasing the original image signal Sorg f1 (Sus), gradually increasing according to an increase of the unsharp mask signal Sus
I made a big change.

[0009]

According to this structure , in the structure in which the processed image signal Sproc subjected to the dynamic range compression processing is output to the display device, the desired dynamic range compression is performed on the display density of the display device.
The dynamic range compression required on the basis of the display density is set, and the processed image signal Sproc corresponding to the request is set.
Was obtained.

The gradation characteristics of the display device are
Generally, the contrast decreases as the density becomes lower and the density becomes higher. Therefore, it is desirable to make the correction characteristic suitable for the gradation characteristic. Therefore, the correction value f1 (at a constant rate with respect to the change of the non-sharp mask signal Sus). Sus) does not change, but the correction value f1 (Sus) is changed at a larger change rate toward the low or high density side .

[0011]

EXAMPLES Examples of the present invention will be described below. FIG. 1 showing an embodiment shows a radiation image reading apparatus including an image processing apparatus to which a method for compressing a dynamic range of a radiation image according to the present invention is applied, and an example of photographing a human body for medical use is shown. Show.

The radiation source 1 is controlled by the radiation control device 2 to irradiate the subject (human chest or the like) M with radiation (generally X-rays). The recording / reading device 3 is provided with a conversion panel 4 on a surface facing the radiation source 1 with the subject M interposed therebetween, and the conversion panel 4 has energy according to a radiation transmittance distribution of the subject M with respect to an irradiation radiation amount from the radiation source 1. Are accumulated in the photostimulation layer, and a latent image of the subject M is formed there.

The conversion panel 4 is provided with a stimulable layer on a support by vapor deposition of a stimulable phosphor or coating of a stimulable phosphor coating. The stimulable layer has a negative effect on the environment and It is shielded or covered by a protective member to block damage. Examples of the stimulable phosphor material include:
JP-A-61-72091 or JP-A-59-
A material as disclosed in Japanese Patent No. 75200 is used.

The light beam generator (gas laser, solid-state laser, semiconductor laser, etc.) 5 generates a light beam whose emission intensity is controlled, and the light beam reaches the scanner 6 via various optical systems. , Where it is deflected, and the reflector 7
The optical path is deflected by and is guided to the conversion panel 4 as stimulated excitation scanning light.

The condensing body 8 is located at the condensing end which is an optical fiber in the vicinity of the conversion panel 4 which is scanned by the stimulated excitation light, and the latent image energy from the conversion panel 4 which is scanned by the light beam is detected. The stimulated luminescence having a luminescence intensity proportional to is received.

Reference numeral 9 is a filter that allows only light in the stimulated emission wavelength range from the light introduced from the light collector 8 to pass through. The light that has passed through the filter 9 is incident on the photomultiplier 10,
It is photoelectrically converted into a current signal corresponding to the incident light.

An output current from the photomultiplier 10 is converted into a voltage signal by a current / voltage converter 11, amplified by an amplifier 12, and then a radiation image signal composed of digital data for each pixel by an A / D converter 13. Is converted to. Then, this digital radiation image signal (original image signal Sorg) is
The data is sequentially output to the image processing device 14 having a built-in microcomputer.

Reference numeral 15 is an image memory (magnetic disk device) for storing image signals. Reference numeral 16 is an interface for transmitting a radiation image signal read from the image processing device 14 directly or from the image memory 15 to the printer 17.

Reference numeral 18 is a read gain adjusting circuit. The read gain adjusting circuit 18 adjusts the light beam intensity of the light beam generating section 5, the gain adjustment of the photomultiplier 10 by adjusting the power supply voltage of the high voltage power supply 19 for photomultiplier, and the current / current. The gain of the voltage converter 11 and the amplifier 12 is adjusted, and the input dynamic range of the A / D converter 13 is adjusted, so that the read gain of the radiation image signal is comprehensively adjusted.

The method of obtaining the original radiation image signal Sorg to be output to the image processing device 14 is limited to the method of photoelectrically converting the stimulated luminescence emitted by scanning the stimulable phosphor with excitation light to emit light. However, for example, a method of reading an image on a radiation film by photoelectric conversion, a method of irradiating a phosphor with radiation passing through an object to convert it into fluorescence, and photoelectrically converting the fluorescence to read good.

The original radiation image signal Sorg may be in proportion to the intensity of the detected radiation or may be in proportion to the logarithm of the intensity of the detected radiation, but the latter is preferable.

Here, the image processing apparatus 14 compresses the dynamic range of the input original image signal Sorg to obtain a processed image signal Sproc carrying an image having a narrower dynamic range than the original image. A processing function is provided, and image processing for such dynamic range compression is performed according to the following equation.

Sproc = Sorg + f1 (Sus)

In the above equation, Sus is the original image signal S corresponding to each pixel point within the predetermined mask area including each pixel point.
It is a non-sharp mask signal obtained by averaging org. Further, f1 (Sus) added to the original image signal Sorg is a correction value obtained as a function of the unsharp mask signal Sus, and monotonically decreases as the unsharp mask signal Sus increases.

The "monotonic decrease" does not necessarily mean that f1 (Sus) decreases with an increase in the non-sharp mask signal Sus, and f1 (Sus) may partially change even if Sus changes.
There may be areas where us) does not change .

By the way, when compressing the dynamic range
In some cases, it is desirable to reproduce the optimum density for image interpretation.
Is, in the case where the density reproduction characteristics according conducted independently of dynamic range compression may desired compression effect can not be obtained when allowed finally perform display.

[0027] Therefore, an embodiment of performing dynamic range compression in consideration of the reproducibility characteristic of the density will be described below with reference to the flowchart of FIG.

First, a histogram analysis of a region of interest in an image is performed based on the original image signal Sorg, and a standard gradation conversion table LUT set in advance is rotated or moved in parallel according to the result of the histogram analysis. Thus, the optimum gradation conversion table LUT is determined for each radiation image (S1).

Next, the reference value Sus1 of the unsharp mask signal Sus for setting the area of dynamic range compression based on the histogram analysis result (for example, the maximum value, the minimum value, the median value in the area of interest). Is determined (S2). Then, the display density Dus1 corresponding to the reference value Sus1 is obtained using the gradation conversion table LUT (S3).

Next, using the display density Dus1 as a reference,
Display density correction function f2 (Du represented by the following linear function)
s) determining (S4 and see FIG. 3 (b)).

F2 (Dus) = β (Dus1-Dus) ... (D
us ≦ Dus1), f2 (Dus) = 0 ... (Dus> Dus1)

When the display density corresponding to the original image signal Sorg is Dorg and the display density corresponding to the processed image signal Sproc is Dproc, Dproc = Dor
g + f2 determine the relationship between the display density Dorg and display density Dproc to satisfy (Dus) (third quadrant in Figure 3 (a)). The display density Dproc is data in which the display density Dorg corresponding to the original image is compressed on the low density side by the density correction value f2 (Dus) on the basis of the display density.

Finally, when the processed image signal Sproc whose dynamic range is compressed is converted by the gradation conversion table LUT and displayed, the display density D is displayed.
It would be good if the characteristics match the proc.

Therefore, the display density Dproc corresponding to the processed image Sproc and the display density Dorg corresponding to the original image Sorg are respectively converted into the gradation conversion table LU.
It is converted into a signal value by inverse conversion by T, and Dproc = Dorg + f2 (D
determining the relationship between the original image signal Sorg to satisfy us) the relationship between the processed image signal Sproc (FIGS. 3 (a)
In the first quadrant).

Then, a signal correction function f1 (Sus) satisfying Sproc = Sorg + f1 (Sus) is calculated in the relationship of the signals obtained by inverse conversion from the display density (first quadrant in FIG. 3 (a)) ( See S5 and FIG. 3 (c)).

Next, the unsharp mask signal Sus for each pixel point is calculated from the original image signal Sorg (S6), and the calculated unsharp mask signal Sus is calculated by the correction function f1 (Sus) obtained as described above. The processed image signal Sproc is obtained by converting to the correction value and correcting the original image signal Sorg by the correction value f1 (Sus) (S
7).

The processed image signal Sproc whose dynamic range is compressed by the correction function f1 (Sus) is
After conversion by the gradation conversion table LUT (S
8) The data is output to the printer (display device) to create a hard copy (S9).

On the display density, a correction characteristic represented by a linear function as shown in FIG. 3 (b) is set, and gradations as shown in the second and fourth quadrants of FIG. 3 (a) are set. conversion in the case of using the table LUT, as shown in FIG. 3 (c), the original image signal correction function for correcting the low concentration side of Sorg f
As 1 (Sus), the non-sharp mask signal Sus is the reference point S.
The characteristic is such that the slope increases as it decreases from us1. This is because the gradation conversion table LUT generally has a characteristic that the inclination gradually decreases on the low density side and the high density side.

As in the above embodiment, the processed image signal S
If dynamic range compression is performed in consideration of the density reproduction characteristics of a display device such as a printer or a CRT that outputs proc, a desired compression effect is achieved on the display image, although the desired compression is performed on the image signal. Therefore, the dynamic range compression that is optimal in the final display state can be performed.

In the above embodiment, the relationship of the display density is inversely converted into the relationship of the image signal by using the gradation conversion table LUT,
Although the signal correction function f1 (Sus) is set so that the desired dynamic range compression is performed on the display density, the signal correction function f1 (Sus) is simply set in consideration of the characteristics of the gradation conversion table LUT. You may make it the structure made to let.

[0041] That is, the characteristics of a general gradation conversion table LUT, in order to realize the correction of the characteristic expressed by a linear function on the display density, as shown in FIG. 3 (c), the signal correction The function f1 (Sus) is required to have a characteristic that the slope increases as the non-sharp mask signal Sus changes to the high density side or the low density side.

[0042] Thus, in the configuration in which the dynamic range compression performed Sproc = Sorg + f1 (Sus) becomes operational, in anticipation of advance above requirements, such as, as shown in FIG. 4, for example, to compress the high density side In this case, the correction function f1 (Sus) may be set so as to increase the decrease rate as the non-sharp mask signal Sus increases. The correction function f1 (Sus) shown in FIG. 4 is expressed as follows.

F1 (Sus) = β (Sus1-Sus) ² ...
(Sus ≧ Sus1), f1 (Sus) = 0 ... (Sus <Sus
1)

Here, the reference value Sus1 is preferably determined based on the histogram analysis of the original image, and is, for example, the average value of the maximum signal value and the minimum signal value in the region of interest.

The coefficient β was calculated as β = k / (Sus1−Smax), where Smax is the maximum signal value in the region of interest. The above k is a constant, and it is preferable that 0.5 ≦ k ≦ 1.5. However, the reference value used when calculating β is not limited to the maximum value Smax.

As described above, the gradation conversion table L is actually used.
If the display density is not inversely converted from the UT and the signal correction function is set, but the function is set in consideration of the characteristics of the gradation conversion table LUT, the signal correction function f1 ( Sus) can be easily set.

[0047] In the above description, the example of setting so as to substantially conform to the characteristics of the high density side tone conversion Teberu LUT compression can be set in the same manner even in a low density side, S
In the configuration in which the calculation of proc = Sorg + f1 (Sus) is performed to compress the dynamic range, the non-sharp mask signal S
A function may be set so that the rate of increase of the correction value f1 (Sus) increases with the decrease of us.

That is, the absolute value of the change rate of the correction value f1 (Sus) with respect to the change of the unsharp mask signal Sus is
Correction value f1 for increasing and correcting the original image signal Sorg
(Sus) gradually increases as the non-sharp mask signal Sus decreases, and the correction value f1 (Sus) that decreases and corrects the original image signal Sorg gradually increases as the non-sharp mask signal Sus increases. A function that increases and changes
You can set it.

[0049]

As described above, according to the method of compressing the dynamic range of a radiation image according to the present invention, the display density is increased.
Since the desired dynamic range compression characteristic was set, and the compressed image signal was obtained from this,
Finally, when the radiation image whose dynamic range is compressed is displayed, the desired compression effect can be surely obtained.

[0050] Further, in accordance with the tendency of the gradation conversion characteristic in the display, by so as to set the function of the correction value, so that can be performed easily to the dynamic range compression commensurate with the requirements on substantially the displayed image become.

[Brief description of drawings]

FIG. 1 is a block diagram showing a radiation image reading processing apparatus to which the present invention is applied.

FIG. 2 is a flow showing a flow of compression processing according to display density .
chart.

FIG. 3 is a diagram showing characteristics of compression processing according to display density.

FIG. 4 shows a functional form of a correction value in consideration of a display density characteristic .
Diagram.

[Explanation of symbols]

1 ... Radiation source 3 ... Recording / reading device 14 ... Image processing device 15 ... Image memory 16 ... Interface 17 ... Printer

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI H04N 7/18 H04N 1/40 101E (58) Fields investigated (Int.Cl. ⁷ , DB name) A61B 6/00-6 / 14 H04N 1/407 H04N 1/41

Claims (2)

(57) [Claims] 1. Image processing for processing an original image signal Sorg representing an original image based on radiation image information transmitted through an object to obtain a processed image signal Sproc carrying an image having a narrower dynamic range than the original image. A method for compressing a dynamic range of a radiation image that outputs the processed image signal Sproc to a display device after obtaining the unsharp mask signal Sus corresponding to each pixel point, while corresponding to the original image signal Sorg. The display density of the display device is Dorg, and the display density of the display device corresponding to the processed image signal Sproc is Dpr.
oc, Dproc = Dorg + f2 (Dus), where Dus is the display density corresponding to the non-sharp mask signal Sus in the display device and f2 (Dus) is a density correction value that is a function of the display density Dus. A method of compressing a dynamic range of a radiation image, characterized in that the processed image signal Sproc is obtained so as to satisfy the relationship. 2. A dynamic image of a radiation image for processing an original image signal Sorg representing an original image based on radiation image information transmitted through an object to obtain a processed image signal Sproc carrying an image having a narrower dynamic range than the original image. In the range compression method, the non-sharp mask signal Sus is obtained corresponding to each pixel point, and the original image signal Sorg is corrected by the correction value f1 (Sus) which is a function of the non-sharp mask signal Sus. A method of compressing a dynamic range of a radiographic image for obtaining an image signal Sproc, comprising the correction value f with respect to a change in the unsharp mask signal Sus.
The absolute value of the change rate of 1 (Sus) is the correction value f1 (Sus) for increasing and correcting the original image signal Sorg,
A correction value f that gradually increases as the non-sharp mask signal Sus decreases and decreases the original image signal Sorg.
1 (Sus), the dynamic range compression method of the radiation image is characterized in that the increase is gradually changed according to the increase of the non-sharp mask signal Sus.