JPH0677571B2 - Medical image capturing posture determination method - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for automatically discriminating a photographing position of a human body in a medical image such as a radiographic image.

(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 a stimulable phosphor sheet, and the stimulable phosphor sheet is scanned with excitation light such as laser light to stimulate emission. Generates light, photoelectrically reads the resulting stimulated emission light to obtain an image signal, and based on this image signal, a radiation image of the subject is visible 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.) This system is practical because it can record an image over an extremely wide radiation exposure area as compared with a conventional radiographic system using silver salt photography. Have advantages. 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. A radiation image that is not affected by fluctuations in radiation exposure is obtained by converting it into an electrical signal and using this electrical 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. be able to.

By the way, in the above system, in order to eliminate the influence of fluctuations in imaging conditions or to obtain a radiographic image with excellent observation and interpretation suitability, the recording state of the radiographic image information accumulated and recorded in the stimulable phosphor sheet, or the chest , A part of the subject such as the abdomen, a recording pattern determined by an imaging method such as simple imaging or contrast imaging (hereinafter, these are collectively referred to as “accumulated recording information”) for observation and interpretation. Grasping is performed prior to visual image output, the reading gain is adjusted to an appropriate value based on this accumulated recording information, and the recording scale factor is set so that the resolution is optimized according to the contrast of the recording pattern. When the image processing is determined and further image processing such as gradation processing is performed on the read image signal, it is desirable to optimally set the image processing conditions.

A method disclosed in Japanese Patent Laid-Open No. 58-67240 is known as a method of grasping the accumulated record information of the radiation image prior to the output of the visible image. This method uses excitation light of a lower level than the excitation light to be emitted during a reading operation for obtaining a visible image for observation / interpretation (hereinafter referred to as “main reading”), in advance of the main reading. A reading operation (hereinafter referred to as "pre-reading") for grasping the accumulated record information of the radiation image accumulated and recorded on the stimulable phosphor sheet is performed, the outline of the accumulated record of the radiation image is grasped, and the main reading is performed. At this time, the read gain is appropriately adjusted based on the preread information, the recording scale factor is determined, or the image processing condition is determined.

According to the above method, since the recording state and recording pattern of the radiation image information accumulated and recorded on the stimulable phosphor sheet can be grasped in advance before the main reading, a reading system having a particularly wide dynamic range can be provided. Even if it is not used, the reading gain is adjusted appropriately based on this recorded information, the recording scale factor is determined, and the signal processing according to this recording pattern is performed on the electrical signal after reading, thereby making observation and interpretation. It is possible to obtain a radiation image with excellent suitability.

(Problems to be Solved by the Invention) However, when the reading condition and / or the image processing condition of the radiation image information is determined as described above, when the same subject is photographed in different photographing positions, the respective reproductions are performed. The density of the region of interest in the subject may change in the image. Hereinafter, this will be described in detail. For example, consider a case where the chest is photographed from the front as shown in FIG. 2A and a case where the chest is photographed from the side as shown in FIG. 2B in order to diagnose the thoracic spine. In the case of frontal imaging, the thoracic spine K, which is the region of interest, overlaps the mediastinum where it is difficult for radiation to pass therethrough. On the other hand, in lateral imaging, the thoracic vertebra K overlaps with the lung field P through which radiation easily penetrates. Therefore, in the stimulable phosphor sheet, the thoracic vertebra portion has a high accumulated radiation dose, and this portion becomes a high light emission amount portion. Then, in both frontal and side imaging, the maximum value S _max and the minimum value S _{min of the} image signal read from the stimulable phosphor sheet are used.
Since it does not change much, the reading condition and / or the image processing condition determined based on the maximum value S _max and the minimum value S _min as is conventionally done are almost the same in both cases. Therefore, when an image is read under such a reading condition and / or an image processing condition to obtain a reproduced image, the thoracic vertebra part has a relatively low density in the front image, while it has a relatively low density in the lateral image. It becomes a high concentration.

Further, it is conceivable to appropriately set the image processing condition based on the read image signal obtained by the main reading without performing the pre-reading as described above, but in such a case, the above problem similarly occurs. .

In order to solve the above-mentioned problems, conventionally, when reading radiation image information from a stimulable phosphor sheet, it is necessary to determine in what position the subject is photographed on the sheet one by one. The reading conditions and / or the image processing conditions are input to the image processing apparatus, and the above-mentioned reading conditions and / or image processing conditions are set according to the input photographing body position information.

However, it is very troublesome to input the above-mentioned image-taking position information every time each stimulable phosphor sheet is read, and it is easy to mistakenly input the image-taking position information.

Therefore, it is an object of the present invention to provide a method capable of automatically discriminating a photographing position of a medical image recorded on the above-mentioned stimulable phosphor sheet or the like.

(Means for Solving Problems) A medical image capturing position determining method according to the present invention is an image signal obtained by a reading process from the above-mentioned stimulable phosphor sheet, that is, an image signal for transmitting a human body. A histogram is created, the degree of separation of this histogram, that is, the degree of separation defined by one of the class separation, the correlation criterion, and the least-squares criterion is obtained, and the imaged posture of the image is determined based on the value of the separation factor. It is characterized by doing. The class separation degree, the correlation criterion, and the least squares criterion have been conventionally used for discriminant analysis in statistical processing, and are mathematically equivalent to each other.

(Operation) For example, the histogram of the image signal that carries the radiographic image of the chest of the human body is roughly as shown in FIG. 3A in the front photographed image and roughly as shown in FIG. 3B in the side photographed image. Looking at these histogram patterns, the histogram in Figure 3A shows that the signal minimum S
Whereas high frequency portions occur at positions relatively close to _min and the maximum value S _max , the histogram of FIG. 3B shows high frequency portions near the signal intermediate value S _mid . Therefore, comparing the resolution of these histograms,
The resolution of the histogram in Figure 3A is greater. Therefore, in this chest image, it is possible to discriminate a front image when the degree of separation is relatively large and a side image when the degree of separation is relatively small.

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

FIG. 1 shows an example of a radiation image information recording / reproducing system configured to determine the imaged posture of a human body by the method of the present invention. This radiation image information recording / reproducing system basically includes a radiation image capturing unit 20 and a read-ahead reading unit 3
0, a main reading reading unit 40, and an image reproducing unit 50. In the radiation image capturing unit 20, radiation 102 is emitted from a radiation source 100 such as an X-ray tube toward a subject (examinee) 101. At the position where the radiation 102 transmitted through the subject 101 is irradiated, the stimulable phosphor sheet 103 for accumulating the radiation energy is arranged as described above, and the transmitted radiation of the subject 101 is placed on the stimulable phosphor sheet 103. Image information is accumulated and recorded.

The stimulable phosphor sheet 103 on which the radiation image information of the subject 101 is recorded in this manner is sent to the prereading reading unit 30 by the sheet transfer means 110 such as a transfer roller. The laser light 202 emitted from the laser light source 201 for pre-reading in the pre-reading reading section 30 passes through the filter 203 that cuts the wavelength region of the stimulated emission light emitted from the stimulable phosphor sheet 103 by the excitation of this laser light 202. After that, it is linearly deflected by an optical deflector 204 such as a galvanometer mirror, and is incident on the stimulable phosphor sheet 103 via a plane reflecting mirror 205. Here, the laser light source 201 is selected so that the wavelength range of the laser light 202 as the excitation light does not overlap with the wavelength range of the stimulated emission light emitted by the stimulable phosphor sheet 103. On the other hand, the phosphor sheet 103 is transferred in the direction of arrow 206 by the sheet transfer means 210 such as a transfer roller and is sub-scanned, and as a result, the entire surface of the phosphor sheet 103 is irradiated with the laser beam 202. Here, the emission intensity of the laser light source 201, the beam diameter of the laser light 202, the scanning speed of the laser light 202, and the transfer speed of the stimulable phosphor sheet 103 are the energy of the pre-read excitation light (laser light 202), which will be described later. It is selected to be smaller than that of the main reading performed by the main reading reading unit 40.

When the laser light 202 is irradiated as described above, the stimulable phosphor sheet 103 emits stimulated emission light of a light amount corresponding to the radiation energy stored and recorded in the stimulable phosphor sheet 103. Incident on. The stimulated emission light is guided through the light guide 207, emitted from the emission surface, and received by a photodetector 208 such as a photomultiplier.
On the light receiving surface of the photodetector 208, a filter that transmits only light in the wavelength range of stimulated emission light and cuts light in the wavelength range of excitation light is attached, and detects only stimulated emission light. Is ready to go. The detected stimulated emission light is converted into an electric signal carrying the accumulated record information and amplified by the amplifier 209. The signal output from the amplifier 209 is the A / D converter 211.
Is digitized by and is input to the main reading control circuit 314 of the main reading reading unit 40 as the preread image signal Sp. The main reading control circuit 314, based on the accumulated record information indicated by the preread image signal Sp, for example, by histogram analysis,
The reading gain setting value a, the recording scale factor setting value b, and the reproduced image processing condition setting value c are determined.

The stimulable phosphor sheet that has been read ahead as described above
103 is transferred to the reading unit 40 for main reading. Book reader 4
The laser light 302 emitted from the main reading laser light source 301 at 0 passes through the filter 303 that cuts the wavelength region of the stimulated emission light emitted from the stimulable phosphor sheet 103 by the excitation of the laser light 302, and then the beam The beam diameter is strictly adjusted by the expander 304, is linearly deflected by the optical deflector 305 such as a galvanometer mirror, and the stimulable phosphor sheet 1 is passed through the plane reflecting mirror 306.
03 incident on. An fθ lens 307 is arranged between the light deflector 305 and the plane reflecting mirror 306 so that the beam diameter of the laser light 302 scanning the stimulable phosphor sheet 103 becomes uniform. On the other hand, the stimulable phosphor sheet 103 is transferred in the direction of arrow 308 by the sheet transfer means 320 such as a transfer roller to be sub-scanned, and as a result, the stimulable phosphor sheet 10 is transferred.
The entire surface of 3 is irradiated with laser light. When the laser light 302 is thus irradiated, the stimulable phosphor sheet 103 emits stimulated emission light in an amount corresponding to the radiation energy stored and recorded therein, and this emission light is incident on the main reading light guide 309. To do. The stimulated emission light guided by repeating the total reflection in the main reading light guide 309 is emitted from the emission surface and is received by the photodetector 310 such as a photomultiplier. A filter that selectively transmits only the wavelength region of the stimulated emission light is attached to the light receiving surface of the photodetector 310,
The photodetector 310 is adapted to detect only stimulated emission light.

The output of the photodetector 310 that photoelectrically detects stimulated emission light indicating the radiation image recorded on the stimulable phosphor sheet 103 is the read gain based on the read gain set value a determined by the control circuit 314. Is amplified to an appropriate level electric signal by the amplifier 311 in which is set. The amplified electric signal is input to the A / D converter 312, converted into a digital signal with a recording scale factor suitable for the signal fluctuation width based on the recording scale factor setting value b, and the signal processing circuit 31.
Entered in 3. In the signal processing circuit 313, the digital signal is subjected to image processing (signal processing) such as gradation processing based on the reproduction image processing condition set value c so that a radiographic image excellent in observation and interpretation suitability is obtained, and output. To be done.

The read image signal (main read image signal) So output from the signal processing circuit 313 is input to the optical modulator 401 of the image reproducing unit 50. In the image reproducing section 50, the recording laser light source
The laser light 403 from 402 is modulated by the optical modulator 401 based on the main reading image signal So input from the signal processing circuit 313, deflected by the scanning mirror 404, and scanned on the photosensitive material 405 such as photographic film. . And photosensitive material 4
Reference numeral 05 is transferred in a direction (arrow 406 direction) orthogonal to the scanning direction in synchronization with the scanning, and a radiation image based on the main reading image signal So is output onto the photosensitive material 405. As a method for reproducing the radiation image, various methods such as the above-described CRT display can be adopted in addition to such a method.

Next, the method of the present invention for automatically discriminating the photographing position of the subject 101 will be described. The pre-reading image signal Sp output from the A / D converter 211 is input to the main reading control circuit 314 as described above and also to the imaging position determining circuit 500. FIG. 4 shows the configuration of the photographing body position determination circuit 500 in detail, which will be described below with reference to FIG.
The histogram creation unit 511 of the photographing position determination circuit 500 receives the prefetch image signal Sp and creates a histogram of the image signal Sp. When the image recorded on the stimulable phosphor sheet 103 is a chest image, the histogram created in this way is as shown in FIGS. 3A and 3B for the front image and the side image, respectively, as described above. Will be things. Hereinafter, this chest image will be described as an example. The information H indicating the histogram is sent to the histogram analysis unit 513. The histogram analysis unit 513 obtains the maximum value R _max of the class separation degree R used in the discriminant analysis as the separation degree of the histogram indicated by the information H. As shown in FIG. 5, the maximum value R _max of the class separation R is set to a point Sm that divides all the signals into the large value side and the small value side in each histogram,
The ratio of the frequency cumulative value in each divided area A ₁ , A ₂ is W ₁ , W ₂
(W ₁ + W ₂ = 1), assuming that the weighted average values of the signals in the respective regions A ₁ and A ₂ are μ ₁ and μ ₂ , and the variance of the entire histogram is δ ² , it changes according to the setting of m R = It is specified by the maximum value of W ₁ W ₂ (μ _1- μ ₂ ) ² / δ ² . The signal mean values μ ₁ , μ ₂
Where S _i is the signal value and f _i is the frequency (frequency), Is. When creating the above histogram, histograms of all the pre-read image signals Sp may be created, or signals that are not directly related to the subject, that is, image signals for the direct radiation irradiation area (so-called blank portion), etc. Except for the above, histograms for other image signals may be created.
After the histograms of all Sp are created, the histogram area corresponding to the image signal such as the above-mentioned blank portion may be excluded. That is, the histogram created by the method of the present invention may be for all of the image signals or for some of them.

The information Sr indicating the maximum value R _max of the class separation degree R is sent to the determination unit 514. The determination unit 514 is a reference value setting unit 515.
The maximum value R of the standard value Th and the above class separation degree R sent from
_Max is compared, and if R _max ≦ Th, it is determined that the image carried by the prefetch image signal Sp is a side face photographed image, and a correction signal T is output. If R _max > Th, it is a front face photographed image. It is determined that the correction signal T is not output. This correction signal T is sent to the gain correction circuit 507 shown in FIG. When the correction circuit 507 receives the correction signal T, the main reading control circuit 314
The read gain setting value a determined as described above is corrected so as to reduce the read gain. As described above, when the image reading condition and the image processing condition are constant, the density of the portion of the thoracic vertebra K in the reproduced image of the lateral chest image becomes higher than that in the front image. Therefore, when the value of the class separation degree R is relatively small as described above, that is, when the reading gain is reduced when reading the side face image, the main reading image signal So becomes a low level as a whole, and the reproduction recorded on the photosensitive material 405 is performed. The density of the radiation image is generally low. As a result, the density of the portion of the thoracic vertebra K in the reproduced image of the side surface of the chest becomes equal to the density of the thoracic vertebra in the reproduced image of frontal photography. The proper correction amount of the reading gain can be obtained based on experiments or experience.

The separability of the histogram can be defined by a correlation criterion or a least squares criterion in addition to the class separability described above.

In the above example, the front view image is read with the read gain determined by the main reading control circuit 314 as it is, and the read gain is corrected to a low value when reading the side view image. The captured image may be read with the read gain determined by the main reading control circuit 314 as it is, and the read gain may be corrected to be high when the front captured image is read. In addition, in order to adjust the density of the reproduced image, in addition to changing the reading gain as described above,
The condition of the recording scale factor in the A / D converter 312 may be changed, or the condition of the gradation processing in the signal processing circuit 313 may be changed. Further, these concentration adjusting methods may be used together. Although the embodiment for discriminating between the front photographed image and the side photographed image of the chest has been described above, the present invention can be applied to discriminate other regions, and further other photographing positions. That is, if the imaged body positions are different in the image of a certain common region, the degree of separation of the histogram of the image signal that carries the image of each imaged body position, that is, the degree of class separation, the correlation criterion, or the least squares criterion is often greatly different. It is possible to correctly discriminate the photographing position based on the magnitude of the degree of separation.

Further, in the above embodiments, the pre-reading image signal Sp is used to determine the photographing position, but the pre-reading as described above is not performed, and the signal processing circuit 31 is based on the main reading image signal So.
When the image processing condition in 3 is set, the photographing position may be determined by using the main reading image signal So. Further, in the above-mentioned embodiment, the density of the reproduced image is corrected according to the discriminated photographing posture, but the present invention is of course applicable to discriminating the photographing posture for other purposes. .

Furthermore, in the above embodiment, the stimulable phosphor sheet 103
However, the present invention is applied to determine not only the radiation image recorded on the stimulable phosphor sheet 103 but also other medical images. Of course, it is also possible.

(Effects of the Invention) As described in detail above, according to the method for discriminating the medical image capturing body position of the present invention, the medical image capturing body posture can be automatically and accurately determined. Therefore, if this method is applied to the radiation image information recording / reproducing system as described above, the densities of the regions of interest in the reproduced image can be made uniform even if the imaged posture of the subject is different. It is possible to greatly improve the diagnostic performance.

[Brief description of drawings]

FIG. 1 is a schematic diagram showing an example of a radiation image information recording / reproducing system for discriminating an imaged body position by the method of the present invention, and FIGS. 2A and 2B are schematic diagrams showing examples of a radiation image in which an imaged body position of a subject is different, 3A and 3B are graphs showing an example of a histogram of a read image signal from a stimulable phosphor sheet photographed by changing a photographing position of a subject, and FIG. 4 shows an apparatus for carrying out the method of the present invention. FIG. 5 is a block diagram showing an example, and FIG. 5 is an explanatory diagram for explaining the class separation degree according to the present invention. 20 ... Radiation image photographing unit, 30 ... Pre-reading reading unit 40 ... Main-reading reading unit, 100 ... Radiation source 101 ... Subject, 102 ... Radiation 103 ... Accumulable phosphor sheet 201 ... Pre-reading laser light source 202 ... Pre-reading laser light 204 ... Pre-reading light deflector 208 ... Pre-reading light detector 210 ... Pre-reading sheet transport means 301 ... Main reading laser light source 302 ... Main reading laser light 305 ... Main reading optical deflector 310 ... Main reading optical detector 311 ... Amplifier 312 ... A / D converter, 313 ... Signal processing circuit 314 ... Control circuit 320 ... Main reading sheet transfer means 500 ... Imaging position determination circuit 507 ... Reading gain correction circuit 511 ... Histogram creation unit 513 ... Histogram analysis unit, 514 ... Discriminating unit 515 ... Reference value setting unit a ... Reading gain setting value b ... Recording scale factor setting value c ... Image processing condition setting value H ... Histogram information Sp ... Pre-reading image signal, Sr ... Separation degree information So ... Main reading image signal, T … Positive signal Th ... reference value

Claims (1)

[Claims] 1. A histogram of an image signal which carries a transparent image of a human body is created, and a separation degree defined by any one of a class separation degree, a correlation criterion and a least squares criterion is calculated, and this separation degree is calculated. A method for discriminating the imaged body position of the image based on the value of 1.