JPH0620237B2 - Image signal noise detection method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for detecting a noise component included in an image signal obtained by reading radiation image information 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 Problems) A first image signal noise detection method according to the present invention is directed to pixels A ₁ and A ₁ arranged in a one-dimensional direction among the image signals obtained by the radiation image information reading process described above. _2, a ₃ ...... a picture signal relates _{n F 1, F 2, F} 3 ...... F n difference values D _i = F _i -F between each of
_i−1 [i = 2, 3, ... N] is obtained, and two difference values D _{j such} that D _j > T ₁ and D _k <−T ₂ are obtained, where T ₁ and T ₂ are positive constants, respectively. D _k [j <k] is detected, and if k <j + u [u is a positive number], it is considered that the image signals F _{j to} F _k−1 include noise.

In addition, the second noise detection method of the present invention uses k as described above.
The two-dimensional position (s ₁ , t ₁ ) on the stimulable phosphor sheet of the central pixel A ₀ of the pixels A _{j1 to} A _k-1 with <j + u is obtained, and the arrangement direction of the pixels A _{1 to} A _n is also determined. Pixels lined up at a right angle to
_{B 1, B 2, B 3} ... the related B _m image signals _{G 1, G 2, G 3} = ... difference value between each of _{G m E i G i -G i} -1 [i = 2,3 ...... m], and using T ₃ and T ₄ as positive constants, two difference values E _p and E _q [p <q] that satisfy E _p > T ₃ and E _q <−T ₄ are detected, and q The two-dimensional position (s ₂ , t ₂ ) of the central pixel B ₀ of the pixels B _{p to} B _q-1 where <p + v [v is a positive constant] on the sheet is calculated, and | s ₁ −s ₂ | If <α | t ₁ −t ₂ | <β, the image signals F _{j to} F _k-1 or G _{p to} G _q-1 are considered to include noise.

(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 have a fairly high correlation as is well known. Therefore, the above difference value D
_i takes an extremely large positive value and an extremely small (large absolute value) negative value at the edge portion where radiation energy as a noise component is accumulated. Therefore, as described above, the pixels A _j and A _k for which D _j > T ₁ and D _k <−T ₂ are considered to be the pixels of the edge portion (edges on the opposite sides of the pixels A _{1 to} A _{n in} the arrangement direction). Pixel A _j is included in a portion where radiation energy that is a noise component is accumulated, while pixel A _k is adjacent to this portion). The edge portion as described above also exists in a high density portion of normal image information, but such a high density portion covers an extremely wide area as compared with the black dot. Therefore, the constant u
If you set 1 to 2 or at most 6
For a high density portion of normal image information, k> j + u, whereas for an edge portion of a black spot, k> j + u.
Since <j + u, the presence or absence of the noise component can be detected based on this comparison.

On the other hand, in the second method of the present invention, k <
Even if it is j + u, it is not considered that the image signals F _{j to} F _k-1 immediately include noise, and the central pixels of the pixels A _{j to} A _k-1
Pixels that are considered to have accumulated radiation energy that is noise (in different pixel extraction directions) in the same way as _obtaining the two-dimensional position (s ₁ , t ₁ ) of A ₀
The position (s ₂ , t ₂ ) of the central pixel B ₀ of B _{p to} B _q-1 is determined, and when these positions are close to each other, in other words, pixel extraction is performed in two directions to detect noise. As a result, when a certain location on the stimulable phosphor sheet can be considered to have noise in both cases, the image signal at that location is considered to have noise, so the risk of false detection is increased. Get lower.

(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 first method of the present invention. For example, the stimulable phosphor sheet 10 in which the transmitted radiation image information of the subject is accumulated and recorded by irradiating the subject with radiation such as X-rays is excited by the excitation light sub-scan by the 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 a direction of an arrow X substantially perpendicular to the sub-scanning direction Y. To scan. In this way, from the portion of the sheet 10 irradiated with the laser beam 13, 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 collected by the condenser 16. Photomultiplier (photomultiplier tube) as a photodetector that is illuminated
Photoelectrically detected by 17.

The light collector 16 is formed by molding a light guide material such as an acrylic plate, and the linear incident end face 16a extends along the beam scanning line on the stimulable phosphor sheet 10. The light receiving surface of the photomultiplier 17 is coupled to the exit end surface 16b which is arranged and formed in an annular shape. The photostimulated luminescent light 15 that has entered the light collector 16 from the incident end face 16a proceeds by repeating total reflection inside the light collector 16 and is emitted from the emission end face 16b to be received by the photomultiplier 17. The photomultiplier 17 detects the amount of stimulated emission light 15 that carries the radiation image information.

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 21 as a read image signal S that carries the light amount of the stimulated emission light 15, that is, the radiation image information. The read image signal S is digitized by the 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, The image is reproduced by inputting it to an image reproducing device 24 such as a CRT or an optical scanning recording device.
In 4, the radiation image accumulated and recorded by the stimulable phosphor sheet 10 is reproduced.

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 the stimulable phosphor sheet 10 thus formed is subjected to radiation image information reading as described above, the read image signal S
Will contain a noise component due to this radiation energy, and the image 30 reproduced by the image reproducing device 24 will be displayed.
Then, a small black dot N is generated as shown in FIG. The noise removing circuit 22 is provided to prevent the occurrence of such black dots N. Hereinafter, the processing by the noise removing circuit 22 will be described in detail.

The noise removal circuit 22 including a microprocessor is 1
Receiving a digital image signal Sd that carries one image, pixels A ₁ , A ₂ , A ₃ ... Of the signal Sd arranged in the excitation light main scanning direction
... Image signals F ₁ , F ₂ , F _{3 for} A _n (see Fig. 3) ... F _n
And the difference value D _i = F _i −F _i−1 [i = 2, 3 ... N] between each of these image signals F _{1 to} F _n is obtained. The image signals F _{1 to} F _n are, for example, as shown in FIG. 4 (1), and the difference values D ₂ , D ₃ ... D _n between them are as shown in FIG. 4 (2). Will be things.

Next, the noise removing circuit 22 sets D _j > T ₁ and D _k <− among the difference values D _{2 to} D _n with T ₁ and T ₂ as positive constants.
_Two difference values D _j and D _k [j <k] that are T ₂ are detected. Note that T ₁ and T ₂ may be the same or different. When two such difference values D _j and D _k are detected,
The noise removal circuit 22 checks whether k <j + 6, that is, whether the pixel A _k is more than 6 pixels away from the pixel A _j . If k <j + 6, that is, pixel A _k is a pixel
When it is separated from A _{j by no} more than 5 pixels, the noise removal circuit 22 considers that the image signals F _{j to} F _k-1 include the noise components as described above, and determines that these image signals F _j ~ F
_k-1 is, for example, the minimum value of the image signals F _{1 to} F _j-1 and F _{k to} F _n ,
Convert to maximum, average, median, etc. The above processing is
The lines for extracting the pixels A _{1 to} A _n are shifted by one pixel column in the sub-scanning direction, and are performed over the entire area of the stimulable phosphor sheet 10.

As described above, it is possible to detect whether or not each image signal includes a noise component due to environmental radiation or the like, as described above.

In the image signal Sd obtained from the stimulable phosphor sheet 10 in which an extremely high density portion is recorded, the image signals A _{1 to} A _n have a distribution state as shown in FIG. 5 (1), As a result, as shown in FIG. 5 (2), D _j > T ₁ , D _k <
There will be two difference values D _j , D _k that are −T ₂ .
However, in this case, the high-density portion, which is correct image information, is extremely larger than the black point N, and therefore k> j + 6.

When it is considered that the image signal includes a noise component as described above, the noise component due to environmental radiation or the like is substantially removed by converting the image signal as described above. Therefore, the noise removing circuit 22
When the radiation image is reproduced on the basis of the image signal Sd ′ that has been subjected to the above processing in (1), it is possible to prevent the black dot N from being generated in the reproduced image.

In the above embodiment, the value of the constant u set to 6 is set to any other suitable value based on the size of the black spot N, that is, the accumulated size of the environmental radiation on the stimulable phosphor sheet 10. Good. Further, the conversion value of the image signal F when it is considered that the noise component is included is the minimum value and the average value of the image signals F _{1 to} F _j-1 and F _{k to} F _n as described above, and The value generated based on the average value and the variance of the image signal may be used.

In the method of the present invention, the difference value between the image signals F _{1 to} F _n is defined as D _i = F _i −F _i−1 , but the difference value D _i = F _i−1 −F _i It is possible to take a value. However, the method of obtaining such a difference value is the same as that in the method of the present invention after all if the concept of the numbers of the pixels A _{1 to} A _n is set in the opposite direction. It is to be included in the inventive method. This is the second aspect of the invention described below.
The same applies to the method of.

Next, an example of the second method of the present invention will be described. This second method is based on the noise removing circuit 22 shown in FIG.
Can be implemented by changing the configuration of In this case the noise elimination circuit 22, as in the first method described above, receives the digital image signal Sd, the image signal F ₁ to F _n regarding pixels A ₁ to A _n arranged in the excitation light main scanning direction Is extracted, the difference value D _i = F _i −F _i-1 [i = 2, 3 ... n] is obtained, and T ₁ and T ₂ are respectively positive constants from the difference values D _{2 to} D _n. And two difference values D _j and D _k for which D _j > T ₁ and D _k <−T ₂
[J <k] is detected. Then, the noise removing circuit 22
When k <j + 6, the position of the central pixel A ₀ of the pixels A _{j to} A _k-1 is obtained. This position is 2 on the stimulable phosphor sheet 10.
It is a dimensional position, and is defined as (s ₁ , t ₁ ) in the Cartesian coordinate system set on the sheet. This position (s ₁ , t ₁ ) is stored in the internal memory of the noise removing circuit 22. The above processing is performed for all pixel rows in the main scanning direction on the stimulable phosphor sheet 10.

Then the noise elimination circuit 22, the pixel A ₁ to A _n arrangement direction a direction substantially perpendicular, i.e. image signals G ₁ ~ about pixel B arranged in the excitation light sub-scanning direction ₁ .about.B _m (see FIG. 6) G _m is extracted, the difference value E _i = G _i −G _i-1 [i = 2, 3 ... n] is obtained, and T ₃ and T ₄ are respectively set to be positive from the difference values E _{2 to} E _m. Two difference values E _p and E _{q with} E _p > T ₃ and E _q <−T ₄ as constants of
[P <q] is detected. Then, the noise removing circuit 22
When q <p + 6, the position of the central pixel B ₀ of the pixels B _{p to} B _q-1 is obtained. This position is of course a two-dimensional position on the stimulable phosphor sheet 10 and is defined as (s ₂ , t ₂ ), and each position is stored in the internal memory of the noise removing circuit 22. The processing described above is also performed for all the pixel rows in the sub-scanning direction on the stimulable phosphor sheet 10.

Next, the noise removing circuit 22 determines that each pixel column A ₁ to
For A _n , it is checked whether the position (s ₁ , t ₁ ) of the central pixel A ₀ is stored, and if the position (s ₁ , t ₁ ) is stored, as shown in FIG. Pixel column B ₁ above s = s ₁
It is checked whether the above-mentioned position (s ₂ , t ₂ ) is stored for B _m . If such a position (s ₂ , t ₂ ) is not stored, the noise removing circuit 22 determines the image signals F _j to F _j.
It is assumed that _k-1 does not contain noise. On the other hand, when the position (s ₂ , t ₂ ) as described above is stored, the noise removal circuit 22 calculates whether or not | t ₁ −t ₂ | <β with β as a positive constant. . Then, as shown in Fig. 7 (1), | t ₁
If −t ₂ | ≧ β, the noise removing circuit 22 determines that the image signal F _j
It is assumed that ~ F _k-1 does not contain noise, while Fig. 7
If | t ₁ −t ₂ | <β as in (2), the image signals F _{j to} F _k-1
Is considered to contain noise. That is, as described above, if k <j + 6, it can be considered that the image signals F _{j to} F _k-1 include noise, but in the second method, such noise detection is extracted in the sub-scanning direction. Selected pixel row B _{1 to} B _m
Finally, in both cases, only the portions that are considered to have noise are finally considered to have noise. By doing so, the accuracy of noise detection is improved. The conversion of the image signal regarded as having noise is performed in the same manner as in the above-mentioned first method.

It should be noted that a plurality of central pixel positions (s ₂ , t ₂ ) may be stored for one pixel row B _{1 to} B _m , and in that case, the noise elimination circuit 22 will set all the positions (s ₂ , t ₂ ). For | t ₁ −t ₂ | <β, and if there is at least one central position (s ₂ , t ₂ ) satisfying this condition, it is considered to be noise.

The above processing is performed for all pixel rows A _{1 to} A _n extending in the main scanning direction.
The noise detection can be performed on the image signal regarding the entire area on the stimulable phosphor sheet 10.

In the above example, whether or not the image signals F _{j to} F _k-1 contain noise is confirmed based on whether or not | t ₁ −t ₂ | <β. On the contrary, the image signal G _p
~G to _q-1 whether noise is _{included, | s 1 -s 2 | <} α
It may be confirmed based on whether or not (α is a positive constant).

Further, in the above example, for the pixel columns A _{1 to} A _n , s ₁ =
The pixel columns B _{1 to} B _m that are s ₂ are selected, but one pixel column
For the central pixel position (s ₁ , t ₁ ) in A _{1 to} A _n , for example, the central pixel position (s ₂ , t ₂ ) of each of a plurality of pixel columns B _{1 to} B _m near s = s ₁ is stored in the memory. If there is at least one central pixel position (s ₂ , t ₂ ) such that | s ₁ −s ₂ | <α | t ₁ −t ₂ | <β read from
It may be considered that the image signals F _{j to} F _k-1 include noise.

(Effect of the invention) As described in detail above, according to the noise detection method of the present invention, the radiation of the radioactive isotope in the stimulable phosphor and the energy of the environmental radiation are accumulated in the stimulable phosphor sheet. Also, the noise component in the read image signal can be accurately detected, and this noise component can be accurately removed. 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 diagram for explaining pixel extraction according to the first method of the present invention, and FIGS. 4 and 5 are examples of image signal distribution states and image signal difference value distribution states according to the present invention. FIG. 6 is an explanatory diagram for explaining pixel extraction according to the second method of the present invention, and FIG. 7 is an explanatory diagram for explaining the second method of the present invention. 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 ₁ ~A _n ...... pixels B ₁ ~B _m ...... pixels, F ₁ ~F _n ...... image signal D ₂ to D _n ... … Differential value

Claims (2)

[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. read by obtained, said radiographic responsible for image information to a method for detecting a noise included in the image signal, the pixel a ₁ arranged in one-dimensional direction, a _2, a ₃ ... the related a _n image signal F Difference values between ₁ , F ₂ , F ₃ ... F _n D _i = F _i −F _i-1 [i =
2, 3 ... n], and two difference values D _j and D _k [j <k] satisfying D _j > T ₁ and D _k <−T ₂ with T ₁ and T ₂ as positive constants, respectively. Is detected, and if k <j + u [u is a positive constant], it is considered that the image signals F _{j to} F _k-1 include noise. 2. 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. read by obtained, said radiographic responsible for image information to a method for detecting a noise included in the image signal, the pixel a ₁ arranged in one-dimensional direction, a _2, a ₃ ... the related a _n image signal F Difference values between ₁ , F ₂ , F ₃ ... F _n D _i = F _i −F _i-1 [i =
2, 3 ... n], and two difference values D _j and D _k [j <k] satisfying D _j > T ₁ and D _k <−T ₂ with T ₁ and T ₂ as positive constants, respectively. Detecting the two-dimensional position (s ₁ , t ₁ ) on the sheet of the central pixel A ₀ of the pixels A _{j to} A _{k−1 where} k <j + u [u is a positive constant], and Pixels lined up in a direction substantially perpendicular to the direction in which pixels A _{1 to} A _n are lined up
Difference values E _i = G _i −G _i−1 [i = ₂ , ₃ ... B ₁ , B ₂ , B ₃ ... B _m between the image signals G ₁ , G ₂ , G ₃ ... G _m with respect to B _m. m], and using T ₃ and T ₄ as positive constants, two difference values E _p and E _q [p <q] that satisfy E _p > T ₃ and E _q <−T ₄ are detected, and q The two-dimensional position (s ₂ , t ₂ ) of the central pixel B ₀ of the pixels B _{p to} B _q-1 where <p + v [v is a positive constant] on the sheet is calculated, and | s ₁ −s ₂ | If <α | t ₁ −t ₂ | <β, it is considered that the image signals F _{j to} F _k-1 or G _{p to} G _q-1 contain noise. .