JPH0620236B2 - Image signal noise detection method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for detecting a noise component contained in an image signal obtained by reading a radiation image signal stored and recorded in a stimulable phosphor sheet. Is.

(Prior Art) When a certain kind of phosphor is irradiated with radiation (X-ray, α-ray, β-ray, γ-ray, electron beam, ultraviolet ray, etc.), a part of this radiation energy is accumulated in the phosphor, It is known that when a phosphor is irradiated with excitation light such as visible light, the phosphor exhibits stimulated emission depending on the stored energy, and a phosphor having such a property is a stimulable phosphor (luminescent material). Exhaustible phosphor).

Using this stimulable phosphor, the radiation image information of a subject such as a human body is once recorded on the stimulable phosphor sheet, and this stimulable phosphor sheet is irradiated with excitation light to generate stimulated emission light. The obtained stimulated emission light is photoelectrically read by a photodetector to obtain an image signal, and on the basis of this image signal, a radiation image of the subject is visualized on a recording material such as a photographic photosensitive material or a display device such as a CRT. The present applicant has already proposed a radiation image information recording / reproducing system for outputting as.
(JP-A-55-12429, JP-A-56-11395, etc.) The above-mentioned stimulable phosphor sheet is, for example, JP-A-56-11392.
No. 56-12599, the radiation energy remaining after reading the radiation image information can be repeatedly used by irradiating it with light or heat to release it.

(Problems to be solved by the invention) However, since this stimulable phosphor sheet has a very high sensitivity, ²²⁶ Ra which is mixed in a small amount in its own phosphor and
Energy of environmental radiation such as radiation emitted from radioactive isotopes such as ⁴⁰ K, or radiation emitted from radioactive isotopes contained in cosmic rays and paints on interior wall surfaces will also be accumulated. When the above-mentioned radiation image information recording / reproduction is performed using the stimulable phosphor sheet that has accumulated such radiation energy, small black dots are generated in the reproduced image due to the radiation energy. Naturally, such black dots impair the quality of the reproduced image.

From the image signal obtained by the radiation image information reading process described above, if a noise component that causes a black dot as described above can be detected, various processes for removing this noise component are applied to the image signal to obtain a black dot in the reproduced radiation image. Occurrence can be prevented.

Therefore, it is an object of the present invention to provide a method capable of accurately detecting the noise component from an image signal obtained by subjecting a stimulable phosphor sheet to a reading process.

(Means for Solving the Problems) In the noise detection method of the image signal according to the present invention, the image signal obtained by the above-mentioned radiation image information reading process is stored in each block obtained by dividing the area on the stimulable phosphor sheet. Of the image signals Fi [i = 1, 2,
3 ... n], an average value m is obtained, each image signal Fi in each block is sequentially compared with the value of m + k (k is a constant), and if Fi> m + k, the image signal Fi contains noise. It is characterized by being regarded as being

In addition, as the average value m, the image signal Fi [i
= 1, 2, 3, ... N] can be used.

(Operation) According to the research conducted by the present inventors, the radiation energy accumulated in the stimulable phosphor sheet as a noise component as described above becomes the radiation energy around the radiation energy, that is, the radiation energy responsible for a correct radiation image. Specifically higher than
It has been found that, in an image generally used for diagnosis, it is usually accumulated within a range of about 9 pixels, and particularly concentrated in a range of about 1 pixel or about 2 pixels. On the other hand, the image signals of pixels that are close to each other show a fairly high correlation as is well known. That is, the image signal Fi [i = 1, 2, 3 ... For each pixel in the block as described above.
n] are normally highly correlated with each other.
Therefore, if the value of the constant k is appropriately determined, the image signal F
When i does not include the noise component, Fi <m
On the other hand, when the image signal Fi has a specific high value including the noise component, Fi> m + k. Therefore, the presence or absence of the noise component can be detected based on such comparison.

(Example) Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows a radiation image information reading apparatus for detecting noise by the method of the present invention. For example, a stimulable phosphor sheet 10 in which transmission radiation image information of the subject is accumulated and recorded by irradiating radiation such as X-rays that have passed through the subject is excited by an excitation light sub-scan by a sheet conveying means 11 such as an endless belt. Is transported in the direction of the arrow Y. A laser beam 13 as excitation light emitted from the laser light source 12 is deflected by an optical deflector 14 such as a galvanometer mirror, and the stimulable phosphor sheet 10 is mainly moved in an arrow X direction substantially perpendicular to the sub-scanning direction Y. To scan. From the position of the sheet 10 irradiated with the laser beam 13 in this way,
The stimulated emission light 15 having a light amount corresponding to the accumulated and recorded radiation image information is diverged, and the stimulated emission light 15 is condensed by the condensing body 16 and used as a photomultiplier (photomultiplier) as a photodetector. Tube 17 to detect photoelectrically.

The light collector 16 is formed by molding a light guide material such as an acrylic plate, and the linear incident end face 16a is extended along the beam scanning line on the stimulable phosphor sheet 10. The light receiving surface of the photomultiplier 17 is coupled to the emitting end surface 16b which is arranged and has an annular shape. The stimulated emission light 15 that has entered the condenser 16 from the incident end face 16a.
Is propagated through the inside of the light collector 16 by repeating total reflection, is emitted from the emission end face 16b, is received by the photomultiplier 17, and the light amount of the stimulated emission light 15 carrying the radiation image information is the photomultiplier. Detected by pliers 17.

The analog output signal of the photomultiplier 17 is logarithmically converted and amplified by the log amplifier 20, and is input to the A / D converter 12 as a read image signal S that carries the light amount of the stimulated emission light 15, that is, radiation image information. The read image signal S is
It is digitized by this A / D converter 21. The digital read image signal Sd thus obtained is then subjected to the below-mentioned processing in the noise removing circuit 22 to be a converted signal Sd ′, and after being subjected to image processing such as gradation processing in the image processing device 23, for example, It is input to an image reproducing device 24 such as a CRT or an optical scanning recording device, and the image reproducing device 24 reproduces the radiation image accumulated and recorded by the stimulable phosphor sheet 10.

As described above, the stimulable phosphor sheet 10 may accumulate energy of radiation emitted by radioisotopes in the phosphor and environmental radiation such as cosmic rays.
When such a stimulable phosphor sheet 10 is subjected to radiation image information reading as described above, the read image signal S contains a noise component due to this radiation energy and is reproduced by the image reproducing device 24. In the image 30, small black spots N occur as shown in FIG. The noise removing circuit 22 is provided to prevent the occurrence of such a black dot N. Hereinafter, the processing by the noise removing circuit 22 will be described in detail.

The noise removing circuit 22 including a microprocessor receives the digital image signal Sd that carries one image, and outputs the signal S
d is extracted for each signal in each block obtained by dividing the area on the stimulable phosphor sheet 10. The state of this block division is shown in FIG. 3 for easy understanding. In FIG. 3, the stimulable phosphor sheet 10 is divided into M × N blocks B. Here, each block B is assumed to include n pixels. The noise removing circuit 22 outputs the image signal Fi [i = 1, 2, 3 ... For each block thus extracted.
The average value m of [n] and the variance σ are calculated. Next, the noise removing circuit 22 outputs the image signals F ₁ , F ₂ , F in each block B.
₃ and _{...... F n, m + k 0} σ (k 0 is a constant) sequentially compares the value of (the mean value m and variance sigma is the value of each block B). Then, for a certain image signal Fi, if Fi> m + k ₀ σ, the noise removal circuit 22 converts the image signal Fi into the average value m, for example. The above processing is performed for the image signals Fi [i = 1, 2, 3, ... N] in all blocks B. As described above, Fi> m + k (k = k ₀ σ in this example)
If so, it can be considered that the image signal Fi contains a noise component due to environmental radiation or the like, as described above. At this time, as described above, the image signal Fi
Is converted into the average value m, the noise component is almost removed. Therefore, if the radiation image is reproduced on the basis of the image signal Sd 'which has been subjected to the above processing in the noise removing circuit 22, it is possible to prevent the black dot N from being generated in the reproduced image.

Note that the value of the constant k may be a value that is not particularly related to the variance σ, and may depend on the accumulated energy level of environmental radiation or the like on the stimulable phosphor sheet 10, the distribution state of the value of the image signal Fi for each block B, and the like. Can be appropriately determined. Further, instead of the average value m, the median value of n image signals in the block B may be used. Further, as the conversion value of the image signal Fi, in addition to the average value m, the median value, the minimum value of n image signals in the block B, and further the random value generated based on the average value m and the variance σ. Etc. may be used.

If the magnitude of the noise component due to environmental radiation or the like is substantially constant, the noise component can be detected and removed by the method described above, but this noise component is gradually reduced toward a peripheral pixel at a certain pixel. When it has a distribution, depending on the value of the constant k, a relatively low noise component in the peripheral pixel portion may not be detected. Hereinafter, another embodiment will be described in which such a problem does not occur. Also in this case, the noise removing circuit 22 of FIG.
The same processing as described above is performed until the image signal Fi for which Fi> m + k ₀ σ is found in each block B. Then, the noise removing circuit 22 further sets Fi> m + k ₀
Eight pixels C ₁ , C ₂ , which are adjacent to the pixel Ai that has become σ,
For each of the image signals of C ₃ , C ₄ , C ₅ , C ₆ , C ₇ , and C ₈ (see FIG. 4), it is checked whether Fi> m + k ₁ σ is a component as described above. Here, the constant k ₁ is set to a value smaller than the constant k ₀ described above. If Fi> m + k ₁ σ, the noise removal circuit 22 determines that the image signal Fi
Is converted into the average value m or the like as in the case described above. By doing so, it becomes possible to detect and remove a relatively low level noise component that cannot be detected only by checking whether Fi> m + k ₀ σ. Environmental radiation, etc. is adjacent 8 pixels C ₁
In the case where it is considered that the pixels are accumulated in the distribution state as described above over a range further outside than C ₈ to C ₈ , the image signal Fi regarding the adjacent pixels of these pixels C _{1 to} C ₈ is also Fi. It suffices to check whether or not> m + k ₂ σ (k ₂ <k ₁ ).

If the blocks B are set so that they do not overlap each other,
As shown in FIG. 5, the pixel Ai in which the highest level of environmental radiation is accumulated and the adjacent pixels C ₃ , C ₅ , C _{8 in} which lower levels of environmental radiation and the like are accumulated are It may happen that the blocks are divided into different blocks Bn and B _{n + 1} . In this case, when the high-level noise component is detected and the lower-level noise components are sequentially detected as described above, the block B _{n + 1} is
Image signals are input to the adjacent pixels C ₃ , C ₅ , and C ₈ as Fi> m + k.
_It cannot be picked up as a target for checking whether it is ₁ σ. In order to prevent such a problem from occurring, it is preferable to divide the blocks so that the peripheral portions of adjacent blocks slightly overlap each other, as shown in FIG. Although FIG. 6 shows only the positional relationship of blocks in the left-right direction, it goes without saying that the peripheral portions of the blocks also overlap in the vertical direction.

If the image signal Fi for each pixel in the block B is checked from the beginning whether Fi> m + k ₂ σ, it is possible to detect a relatively low-level noise component as described above. . However, in that case, there is a risk that a signal component that carries normal image information and has a particularly high level is regarded as a noise component. On the other hand,
As mentioned above, first detect the highest level noise component,
Next, if the peripheral pixels of the pixel which is determined to have the noise component are examined, the above-mentioned problem can be prevented.

(Effects of the Invention) As described in detail above, according to the noise detection method of the present invention, the radiation emitted by the radioactive isotope in the stimulable phosphor and the energy of environmental radiation are accumulated in the stimulable phosphor sheet. However, it is possible to accurately detect the noise component in the read image signal, and thereby to accurately remove this noise component. Therefore, it is possible to prevent the above-described black spots from occurring in the reproduced radiation image, so that the image quality of the reproduced radiation image is improved and the diagnostic performance thereof is greatly improved. In addition, since it is not necessary to pay great attention to the management of the stimulable phosphor sheet in order to avoid the accumulation of radiation energy, which is a noise source, the susceptibility to accumulation of stimulable phosphor sheets, especially in large hospitals, etc. The management of the phosphor sheet is facilitated.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a radiation image information reading apparatus equipped with a means for detecting noise by the method of the present invention, and FIG. 2 is a schematic diagram showing an example of a reproduction radiation image showing black dots according to the present invention, FIG. 3 is an explanatory view for explaining the block division of the area on the stimulable phosphor sheet in the method of the present invention, and FIG. 4 is an explanatory view for showing the relationship between a pixel considered to have a noise component and its adjacent pixel. FIG. 6 is an explanatory diagram showing the above block division in detail, and FIG. 6 is an explanatory diagram showing another example of block division. 10 ... Accumulative phosphor sheet, 11 ... Sheet conveying means 12 ... Laser light source, 13 ... Laser beam 14 ... Optical deflector, 15 ... Photostimulated emission light 17 ... Photomultiplier 20 ... Log amplifier, 21 ...... A / D converter 22 ...... noise removing circuit, A ...... target pixel _Ai, C 1 ~C ₈ ...... pixels, B ...... block

Claims (4)

[Claims] 1. A stimulable phosphor sheet on which radiation image information is stored and recorded is irradiated with excitation light, and stimulated emission light generated from a portion of the sheet irradiated with this excitation light is photoelectrically detected by a photodetector. A method for detecting noise included in an image signal carrying the radiation image information, obtained by reading the image signal, wherein the image signal is a signal for n pixels in each block obtained by dividing an area on the sheet. Image signals Fi [i = 1, 2,
3 ... n], an average value m is obtained, each image signal Fi in each block is sequentially compared with the value of m + k (k is a constant), and if Fi> m + k, the image signal Fi contains noise. A method for detecting noise in an image signal, which is characterized in that it is regarded as being detected. 2. An image signal Fi as the average value m
The image signal noise detection method according to claim 1, wherein an average value of [i = 1, 2, 3 ... N] is used. 3. An image signal Fi as the average value m
The image signal noise detection method according to claim 1, wherein a median value of [i = 1, 2, 3, ... N] is used. 4. The constant k is an image signal Fi [i = 1, 2,
3. The noise detection method for an image signal according to any one of claims 1 to 3, wherein the variance σ of 3 ... N] is k ₀ times (k ₀ is a constant).