JP3426677B2 - X-ray CT system - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an X-ray CT apparatus, and more particularly, an X-ray C which can reduce image deterioration due to scattered X-rays scattered by a subject and can perform more accurate X-ray measurement.
It relates to a T-apparatus.

[0002]

2. Description of the Related Art An X-ray CT apparatus is provided with an X-ray detector provided behind an object to detect attenuation information when an X-ray beam emitted from an X-ray tube is collimated into a fan shape and transmitted through the object. And a device for reconstructing as a tomographic X-ray image by computer processing the information obtained when the X-ray beam and the X-ray detector are rotated around the subject axis while keeping the relative positions of the X-ray beam and the X-ray detector the same. is there.

In order to perform image processing at high speed, a plurality of X-ray detection elements are arranged on a circular arc whose focal point is a normal X-ray tube.
A so-called multi-channel X-ray detector is used.

By the way, when the X-ray beam penetrates into the subject, it is not only absorbed by the tissue and attenuated, but also scattered as a result of interaction with the constituent atoms of the tissue. It is known that X-ray scattering has different mechanisms, that is, coherent Rayleigh scattering and incoherent Compton scattering.

(1) When a photon of Rayleigh scattered X-rays passes in the immediate vicinity of an electron, the electron vibrates in synchronism with the electric field of the X-ray photon to absorb the photon, and then this electron is the same. Radiates photons of frequency. Therefore, the wavelength of the scattered X-rays is the same as the wavelength of the incident X-rays, which causes interference between the incident X-rays and the scattered X-rays. This coherent Rayleigh scattering has a smaller probability of occurring compared to Compton scattering, but has the property of sharply flying forward, and when low-energy X-rays are incident on a high atomic number substance, the proportion of the total scattered dose is growing.

(2) If a photon of Compton scattered X-ray collides with a free electron of a material atom or an outer shell electron having a small bond with a nucleus, the collided electron cannot absorb all the energy of the photon, Are re-emitted as photons, and fly out with the remaining kinetic energy. This phenomenon is called the Compton effect. The photons that are re-emitted by changing the energy with respect to the incident photon are called scattered photons (scattered X-rays), and the electrons that fly out by collision are called Compton electrons or recoil electrons. As the energy of incident X-rays increases, the proportion of this Compton scattering increases with respect to the total scattered dose.

(3) Effect of scattered X-rays on image The detection element measures the X-ray attenuation of the portion of the subject on the line (measurement path) connecting the X-ray tube focus and the center of the detection element. If there are scattered X-rays from other parts of the subject, an error will occur in this measurement. When scattered X-rays are incident, the output of the detection element increases, and the attenuation of the subject on the measurement path is apparently reduced, and the measurement is performed so as to be small.
When such an error increases, the resolution of the CT image reconstructed using these data will decrease. In particular, deterioration of low contrast resolution called density resolution becomes a problem. In addition, clinically, a CT value inside the rib called a rib artifact may sink, a black region may appear on the image, and the CT value in the liver may vary depending on the location. Therefore, in order to obtain a highly accurate X-ray tomographic image, it is necessary to remove the influence of scattered X-rays.

Conventionally, as a method of eliminating the influence of scattered X-rays, (1) the scattered X-rays incident on each channel X-ray detection surface of the X-ray detector are cut. (2) Subtract the output of the scattered X-rays incident from the output of each channel of the X-ray detector. Has been adopted.

As a concrete method of (1), a grid parallel to the X-ray detection surface of each channel is provided (for example, JP-A-62-60539 and JP-A-4-336044).
No.) or providing a filter on the front surface of the detection surface (for example, Japanese Patent Laid-Open No. 63-40405). As a specific method of (2), a method of actually measuring the scattered X-ray dose and subtracting it from the output of each channel is common.
A plurality of scattered X-ray detectors for detecting the scattered X-ray dose,
A structure to be arranged in front of the main X-ray detector (for example, Japanese Patent Laid-Open No. 63-63
No. 305846, JP-A-63-38438, JP-A-63-40534, and JP-A-1-62126). Further, the X-ray detector is only the main detector, and when measuring scattered X-rays, a lead rod for X-ray absorption is arranged in front of each channel to cut off only the signal X-rays (Japanese Patent Laid-Open No. 62-261342). No.) is also disclosed.

(3) As another specific method of (2), there is JP-A-4-170942. In this method, the scattered X-ray correction constant is prepared by using the correction constant for air and the correction constant for the water phantom obtained from the regression line y = ax + b of the sum values of all ch data at the first projection angle, and after the Log conversion in the actual correction. All measurement data of (air calibration data, phantom calibration data,
A method of correcting the scattered dose by multiplying (measurement data of the subject) by a correction coefficient for correction after Log conversion is used. That is, all are corrections to post-measurement Log value data.

[0011]

In the above-mentioned conventional technique, the X-ray incident on the detector is cut off (1).
Method, the incident intensity of the signal X-ray also decreases, so S
There is a problem in that the / N ratio becomes worse and eventually a good CT image cannot be obtained. On the other hand, of the methods (2) for measuring the scattered X-ray dose and removing it from the data, the method using a detector dedicated to scattered X-rays has the following problems.

If the scattered X-ray detector is installed in front of the main X-ray detector, the width of the X-ray detector becomes thick and the size becomes large, which makes mounting difficult. In order to eliminate this, a small scattered X-ray detector is arranged adjacent to the main X-ray detector in the slice direction of the channel in the central region of the main X-ray detector to measure the scattered X-ray dose in the central region. There is also disclosed a method of calculating the scattered X-ray dose in the channel of the peripheral region by using a spread function.

However, the main detector (mainly the ionization chamber structure) and the scattered X-ray detector (mainly the solid-state detector) have a difference in the detection sensitivity of the amount of scattering due to the difference in the structure, and are practically difficult to use. is there. In addition, in an apparatus equipped with a scattered X-ray detector in addition to the main X-ray detector, the expensive detector and the detection circuit are duplicated regardless of whether the scattered X-ray detectors are arranged in the front or in the same row. However, there is a problem that it leads to a significant cost increase.

On the other hand, the method (2) of using only the inner main X-ray detector and using the shield for blocking the signal X-ray during the scattered X-ray measurement is operated while securing the positional accuracy of the shield. Since it is necessary to add complicated functions to the device, cost increase is inevitable.

In the method (3), all processing is Log
After conversion, there are the following problems. (A), processing for obtaining scattering constants C _a , C _w , C _s , scattering correction processing for water phantom data using the scattering constants C _a , C _w and other data, scattering constants C _a , C _s And the scatter correction processing of raw data using other data is required. Therefore, the processing content increases and the load on the computer becomes heavy. Although the effects of scattered radiation may be removed by performing various treatments such as (b) and (a), the generation of scattered radiation and its effect are extremely complicated. In many cases, it does not depend on the structure, and complete correction of scattered radiation is difficult with the correction method that corresponds to the measured value (proportional to the amount of attenuation) like this method.

The object of the present invention is to obtain the scattered dose as the total amount, measure the accuracy of the scattered dose as the total amount without increasing the cost, and enable highly accurate scattered X-ray processing. The present invention provides an X-ray CT apparatus.

Incidentally, the applicant of the present patent claims that the prior application (Japanese Patent Application No.
No. 274171) proposed an X-ray CT apparatus that performs scattered X-ray correction by a correction function that gives a scattered X-ray correction amount corresponding to the added value of all channels. The common points and differences between this prior application and the present invention will be listed below. (1) Common point. The point that the scattered radiation correction amount in the linear region is obtained is common. Moreover, the added value Sj for obtaining this correction amount is Log
It is common in that it is a region after conversion (region of absorption coefficient).

(2) Difference in added value. The earlier application uses the added value of all channels. The present application uses the addition value of one channel or a plurality of channels less than all channels.

(3) Difference between addition targets. The prior application is the first projection data (data at the first projection angle)
Calculate the added value for all channels. The present application calculates, for each projection angle, an addition value of one channel of the projection angle or a plurality of channels less than all channels. Therefore, in each one measurement section corresponding to 360 ° scan or half scan, the prior application has one added value, but this application adds the number corresponding to the total projection angle number in the one measurement section. The value is obtained.

(4) Difference in scatter correction. The prior application is to correct scattered X-rays by subtracting only one added value from the projection data for all projection angles and all channels in each measurement section corresponding to 360 ° scan or half scan. To do. This application is 3
In each measurement section corresponding to 60 ° scan or half scan, scattered X-rays are corrected by subtracting one added value from the projection data over all channels for each projection angle.

[0021]

The present invention is a multi-channel X-ray detector for detecting the intensity distribution of X-rays transmitted through an object, and means for performing logarithmic conversion on the detected signal intensity.
For each projection angle j of the subject after the logarithmic conversion, a specific one-channel value or a value obtained by adding projection data pro (i, j) of a plurality of channels (hereinafter, these are defined as an addition value Sj). ) Is calculated by the following equation,

[Equation 5] pro (i, j): projection data (i: channel, j:
Projection angle) n1, n2: specific channel, n1 <n2 <N
(All channels) Coefficients A and B related to the scattered X-ray correction amount determined in advance from an X-ray transmission image of a material close to the X-ray absorption coefficient of the subject such as water, polyethylene, and acrylic having different sizes are added. Means for obtaining the scattered X-ray correction amount C _s j in the linear region from the value of the value _S j by the following equation,

[Equation 6] Scattered ray correction means for subtracting the scattered X-ray correction amount C _s j from projection data after antilogarithmic conversion obtained by performing antilogarithmic conversion of projection data after logarithmic conversion of all channels for each projection angle j, An X-ray CT apparatus including a unit that performs image reconstruction by logarithmically converting the corrected measurement data is disclosed.

A multi-channel X-ray detector for detecting the intensity distribution of X-rays transmitted through the object, means for performing logarithmic conversion to the detected signal intensity, and each projection angle j of the object after the logarithmic conversion. A means for calculating a value Sj obtained by adding a specific one-channel value or projection data pro (i, j) of a plurality of channels by the following equation:

[Equation 7] pro (i, j): projection data (i: channel, j:
Projection angle) n1, n2: specific channel, n1 <n2 <N
(All channels) Coefficients A and B related to the scattered X-ray correction amount determined in advance from an X-ray transmission image of a material close to the X-ray absorption coefficient of the subject such as water, polyethylene, and acrylic having different sizes are added. Means for obtaining the scattered X-ray correction amount C _s j in the linear region by the following formula based on the value of the value _S j,

[Equation 8] Scattered ray correction means for subtracting the scattered X-ray correction amount C _s j from projection data after antilogarithmic conversion obtained by performing antilogarithmic conversion of projection data after logarithmic conversion of all channels for each projection angle j, An X-ray CT apparatus including a unit that performs image reconstruction by logarithmically converting the corrected measurement data is disclosed.

[0023]

According to the present invention, scattering X performed in consideration of the fact that the measurement path length of the object differs depending on the projection angle as described above.
The line correction recognizes the outer shape of the subject by a simple method and provides an optimal correction amount for each projection angle.

[0024]

Embodiments of the present invention will be described below with reference to FIGS. FIG. 1 is a processing flow chart of an embodiment of the X-ray CT apparatus of the present invention. The processing flow from X-ray measurement to image display will be described. First, the measurement conditions of the X-ray CT apparatus are set (flow F1), and then X-rays are exposed to the subject and measurement is performed by the X-ray detector. (Flow F2). Next, as preprocessing, sensitivity correction (Flow F
3), logarithmic conversion (flow F4) is performed. This gives the absorption coefficient. Further, phantom calibration (flow F5) for non-linear correction for the attenuation amount of each channel is performed. By the processes up to this phantom calibration, the calibration of the sensitivity difference and the non-linear amount due to the channel variation of the X-ray detector is completed. Further, the measurement data is subjected to inverse logarithmic conversion (flow F6) to return to the original measurement data, and scattered X-ray correction (flow F9) is performed. In order to perform this scattered X-ray correction, it is necessary to calculate the correction amount Cs. For this, the detector central 100-channel addition value S is calculated for each projection angle after phantom calibration (flow F5) (flow F7). ), And the correction amount Cs in the linear region corresponding to the added value S is obtained from the correction function (flow F8). The correction amount Cs is subtracted from the inversely logarithmically converted measurement data (flow F9) to correct the scattered X-ray. Logarithmic conversion (Flow F10)
Then, the absorption coefficient is converted again, and the image reconstruction (Flow F
After performing 11), CT value correction (flow F12) is performed, and then image display (flow F13) is performed.

FIG. 2 shows an X-ray phantom assuming an abdominal cross section.
It is a schematic front view of the state measured with the line CT device. In FIG. 2, the i-th projection angle of the X-ray tube 21 (projection angle 0 °)
In order to explain how to measure the X-rays transmitted through the phantom 22 in the central 1-channel of the multi-channel X-ray detector 23 assuming the abdominal cross section of the X-ray beam at the jth projection angle (projection angle 90 °). Figure 3 shows a schematic drawing of only the central 1-channel part.

FIG. 3 is a graph of the measurement data of FIG.
In FIG. 3, 1 in the center before logarithmic conversion of the measurement result of FIG.
The graph which showed the measured data of a channel in the projection angle direction is shown. According to FIG. 3, in the normal measurement data including the scattered X-rays, the data curve has a large undulation in accordance with the elliptical shape of the outer shape of the phantom 22, and it is assumed that the spine and ribs existing in the phantom are on the undulation. It can be seen that there are small irregularities corresponding to the attenuation by the Teflon rod. However, on the other hand, the scattered X-ray data curve obtained by measuring only the scattered X-rays of the phantom 22 is
It draws a gentle sine waveform that follows the elliptical shape of 2.
There are no small irregularities due to the influence of the Teflon rod, which is assumed to be the spine or ribs.

The phenomenon observed in the measurement data of FIG. 3 is due to the following reason. Generally, when X-rays are incident on a subject (in this case, the phantom 22), scattering occurs at each part, so that scattered rays enter the detector from all angles at which the subject is seen from the incident portion of the detection element. However, the scattered X-rays that actually enter the detection element have the property that the scattered X-rays generated near the measurement path line account for a large proportion of the total scattered X-ray dose that enters the detection element. It has an elliptical shape such as the abdomen, and the measurement path length of the object depends on the projection angle (the length through which the object passes through the measurement path).
When the value changes, the phenomenon that the scattered X-ray dose changes according to the object measurement path length is observed. That is, when the subject has an elliptical shape or the like, the scattered X-ray dose generated differs between the major axis direction and the minor axis direction of the ellipse, and therefore the scattered X-ray dose changes depending on the projection angle.

FIG. 4 shows an X-ray C of a phantom with a head cross section assumed.
It is a schematic front view of the state measured by the T apparatus. In FIG. 4, multi-channel X-ray detection of X-rays transmitted through the phantom 22 assuming the head cross section at the i-th projection angle (projection angle 0 °) and the j-th projection angle (projection angle 45 °) of the X-ray tube 21. Bowl 2
3 is a schematic diagram in which only the central 1-channel portion of 3 is extracted and drawn.

FIG. 5 is a graph of the measurement data of FIG.
FIG. 5 is a graph showing the measurement data in the central one channel before logarithmic conversion of the measurement result of FIG. 4 in the projection angle direction. According to FIG. 5, in the normal measurement data including the scattered X-rays, the influence of the Teflon rod in the phantom 22 is seen (jth projection angle), but in the scattered X-ray data, this influence is seen. Has become. In this way, the scattered X-ray dose that changes according to the measurement path length of the object (in this case, the phantom 22) is not affected by internal structural factors such as bones in the object existing on the object measurement path at all. Can also be seen with the phenomenon of FIG.

From the two measurement examples shown in FIGS. 2 to 5, the scattered X-rays do not depend on the structural factors inside the object, but depend on the external shape of the object, that is, the path of the X-ray object measurement path. I understand. In consideration of such a phenomenon, the method for obtaining the scattered X-ray dose is to remove the influence of the internal structure factor of the subject from the projection data for each projection angle acquired by the measurement and determine the subject measurement path length. The scattered X-ray dose can be estimated from this data, corrected to the only dependent data. Then, the scattered X-ray correction can be performed by subtracting this scattered X-ray dose as the correction amount from the projection data detected from the subject. Here, as the method of estimating the measurement path length of the object, for example, the detection value at the detector central channel is used as the reference position of the measurement path, and as the method of removing the internal structural factors of the object, the central channel shown in FIG. By performing the summation processing of the detected values in the plural channels before and after, the smoothing shown in FIG. 7 can be achieved and the influence of the internal structure factor of the subject can be removed.

FIG. 6 is a schematic front view of the measurement state at the central 100 channels for explaining the X-ray detector central multi-channel addition method as a method for removing the internal structure factor of the object. In FIG. 6, the i-th projection angle (projection angle 0 °) and the j-th projection angle (projection angle 90 °) of the X-ray tube 21
Center 10 of the multi-channel X-ray detector 23 for X-rays transmitted through the phantom 22 assuming the abdominal section of the X-ray beam at
In order to explain how the total value is measured on channel 0, a schematic diagram is shown in which only the central channel 100 is extracted.

FIG. 7 is a graph of the measurement data of FIG.
FIG. 7 is a graph showing the total value data of the detector central plural 100 channels before logarithmic conversion of the measurement result of FIG. 6 in the projection angle direction. According to FIG. 7, the normal measurement data including the scattered X-rays are smoothed to remove the influence of the internal structure factor of the subject, and thus show the correlation with the scattered X-ray data curve. Although such a correction method is not accurate in a strict sense, it is considered to be considerably effective in removing artifacts from ribs in the subject and improving CT value uniformity from the liver. As described above, the present method is a scattered ray correction method mainly based on the projection angle direction in consideration of the scattered X-ray change in the projection angle direction, and the scattered X-ray is performed in consideration of the fact that the object measurement path length varies depending on the projection angle. The correction recognizes the outer shape of the subject and provides an optimal correction amount for each projection angle. The apparatus configuration and each part of the X-ray CT apparatus will be described below with reference to FIG.
This will be described with reference to FIG.

FIG. 8 is a perspective view showing the configuration of the entire X-ray CT apparatus including the image diagnostic apparatus of one embodiment of the X-ray CT apparatus of the present invention. In FIG. 8, this X-ray CT apparatus is composed of a high voltage generator 1, a gantry 2, a patient table 3 and an image diagnostic apparatus 4, and inside the gantry 2 a predetermined part of a subject (patient) as shown in FIG. An X-ray tube 21 and a multi-channel X-ray detector 23, which face each other with respect to each other, are integrally rotatably incorporated.

FIG. 9 is a block diagram showing the internal structure of the image diagnostic apparatus of FIG. In FIG. 9, the image diagnostic device 4 includes a magnetic disk 5, a preprocessing device 6, a central control device 7,
It is composed of a main memory 8, a reconfiguration processing device 10, a display device 11, and a high-speed internal bus 12. With this configuration, the magnetic disk 5 is a storage device that stores various reconstructed image data and raw data after various correction processes, and the preprocessing device 6 corrects the detection element sensitivity variation of the detector output data obtained by measurement. Preprocessing such as (FIG. 1, F3), logarithmic transformation (F4) or inverse logarithmic transformation (F6) is performed.

The correction processing device 9 first stores the measurement data that has undergone sensitivity correction and logarithmic conversion in the projection data storage memory 14, and stores the measurement data that has undergone antilogarithmic conversion in the pre-correction data memory 13. The projection data storage memory 14
From the above, data over a plurality of specific channels is read out, the sum is calculated in the adder 15, and this value is set in the table reader 16 as a parameter. The table reader 16 reads the correction amount in the linear region of the scattered X-rays from the correction table (table in which the function of FIG. 11 is set) memory 17 according to the set value and sets it in the subtracter 18. The subtracter 18 subtracts the correction amount in the set linear region from the measurement data stored in the pre-correction data memory 13 (F
9), and stores the result in the corrected data memory 19.
The corrected data is logarithmically converted by the preprocessor 6 (F10) and then stored in the main memory 8 or the magnetic disk 5. This corrected data, which has been corrected, is subjected to image reconstruction by the image reconstruction processing device 10 (F
11), after CT value correction (F12), a final tomographic image is created and displayed on the display device 11 (F1).
3). The central control unit 7 controls the devices connected to the high-speed internal bus 12 such as the preprocessing device 6, the correction device 9 and the image reconstructing device 10 according to the program stored in the main memory 8 and the inter-device control. Manages data transfer. Next, a method of calculating the correction amount for the scattered X-ray correction (F8) will be described.

FIG. 10 is a scattered X-ray intensity distribution chart of the circular water phantom measurement used to calculate the correction amount of the scattered X-ray correction.
The distribution of the data before logarithmic conversion of the scattered X-ray intensity incident on the X-ray detector when circular water phantoms having different diameters of ψ230 mm to ψ380 mm are placed at the center of the gantry aperture is shown. In FIG. 10, the horizontal axis represents the position in the channel direction on the X-ray detector, and the vertical axis represents the scattered X-ray intensity at that position, and the scattered X-ray intensity is small in the channel center part in each phantom. The larger the area, the larger it gets. The larger the phantom diameter, the weaker the average strength as a whole. This is because the intensity of scattered X-rays also decreases inside the phantom, and even in the phantom having a small diameter or the same phantom, the scattered X-ray intensity becomes stronger in the peripheral portion having a shorter measurement path length. Therefore, in order to determine the estimated correction amount of the scattered X-ray dose, such a property must be taken into consideration. Here, in order to perform accurate scattered X-ray correction, the correction amount should be determined according to the distribution of scattered X-ray intensity in the channel direction, but even if the scattered X-ray correction is performed with the same correction amount for all channels, the correction amount is corrected. The effect hardly deteriorates. This is because the scattered X-ray correction is strongly applied to the central portion of the subject whose detector output becomes small at the time of measurement, and the influence of the correction is weakened in the periphery. Therefore, assuming that the correction amount in the channel direction is constant, the correction is performed by considering only the scattered X-ray change in the projection angle direction as described above. As described above, in this embodiment, the correction amount in the channel direction is constant, and the scattered X-ray amount measured in the central 100 channels is used as the correction amount. Next, a function (correction function) for determining the correction amount for calculating this correction amount for each projection angle will be described.

FIG. 11 is a curve diagram of a function (correction function) for determining the scattered X-ray correction amount by the functional formula used for calculating the scattered X-ray correction amount for each projection angle. The detector center 10 after logarithmic conversion is shown in FIG.
FIG. 11 is a curve diagram showing a relationship between the 0-channel added value S and an appropriate correction area for scattered X-ray correction in a linear area in the circular water phantoms of various sizes shown in FIG. 10. Here, the linear region is a region obtained by logarithmic transformation (F4 in FIG. 1) and further inverse logarithmic transformation (F6 in FIG. 1), which is so-called measurement data (sensitivity correction or phantom calibration. (Which means the measurement data after correction) is treated. The reason why the relationship between the central 100-channel addition value S and the correction amount is used in the figure is to perform the smoothing process for removing the internal structure factor of the subject as described above. As can be seen, the normal side measurement data that includes scattered X-rays that have not been smoothed does not easily correlate with the scattered X-ray data, but as shown in FIG. 6, the normal side that includes scattered X-rays This means that if the measurement data is smoothed (100 channels in the center are added), the correlation with the scattered X-ray data can be expected. Although the number of addition channels is 100 in this embodiment, it is not limited to 100.

Here, the relationship between the two is not a simple linear relationship, but there is a correlation, and by using an appropriate function, the relationship between the central 100-channel addition value S after logarithmic conversion and the appropriate scattered X-ray correction amount. A curve is obtained that represents As this functional expression, an exponential function or a power function is suitable, and specifically,

[Equation 9] Where S is the total value of 100 channels in the center of the detector, and Cs is the correction amount of scattered X-rays in the linear region. FIG. 11 shows this relationship. In order to obtain the proper correction amount Cs for the arbitrary S, the coefficients A and B in the equation must be determined. In determining this coefficient, we can experimentally measure several circular water phantoms with different diameters to obtain an appropriate correction amount at that point, and then determine the coefficient so that it passes through these points or its vicinity. it can. In this embodiment, as shown in FIG. 11, ψ160 mm, ψ
Four types of water phantoms of 230 mm, ψ305 mm and ψ380 mm are used. In this determination, the numerical value may be logarithmically converted and then automatically determined from the data obtained by using the least square method or the like. In this embodiment, the water phantom is used to obtain the appropriate correction amount, but polyethylene or acrylic other than water may be used. In the description here, the inverse logarithmic conversion process (F6)
→ The scattered radiation correction processing (F9) → logarithmic conversion processing (F10) is performed on the data after the phantom calibration processing (F5), but the phantom calibration processing (F5) and the sensitivity correction processing (F3) are performed. It is also possible to do it in the previous stage. In this case, the data after the scattered radiation correction process is used as the data for the sensitivity correction and the phantom calibration. Further, in the present embodiment, the number of addition channels is set to 100 at the center, but it is not limited to this. That is, if generalized, it will be as follows.

The scattered radiation correction amount Cs in the relational expression of the scattered radiation correction processing is defined by the addition value Sj of a certain one channel or a plurality of channels for each projection angle j of the object as a variable of the equation. The correction amount C _s j is obtained by the formula and applied to all channels for each projection angle data j of the object measurement data.

[Equation 10] Or

[Equation 11] However,

[Equation 12] pro (i, j): projection data (i: channel, j:
Projection angle) n1, n2: specific channel, n1 <n2 <N
(All channels) A and B are values determined from images of phantoms made of materials having different X-ray absorption coefficients, such as water, polyethylene, and acrylic, which have different sizes.

(Equation 10), (Equation 11), (Equation 1)
2) will be specifically described. (A), projection data for all channels at projection angle j (output of F5 in FIG. 1)
, Pro (1, j), pro (2, j), ... pr
o (n, j) (where n is the total number of channels).
From these n data, the added value S of the projection data pro (n1, j), ... Pro (n2, j) of channels n1 to n2
ask for j. Sj = pro (n1, j) + ...... + pro (n2, j) calculates the correction amount C _s j in the linear region corresponding to the addition value Sj by (Expression 11), or the memory 17 is read out (FIG. 9) (FIG. F8 of 1.) On the other hand, the projection data (output from F5 in FIG. 1) is subjected to inverse logarithmic conversion (F6 in FIG. 1) to be returned to the data in the linear region shown below. D (1, j), D (2, j), ... D (n, j) The correction amount C _s j is subtracted from the data of this linear region as follows to perform scattered radiation correction (see FIG. 1). F9). _{D (1, j) -C s} j D (2, j) -C s j:: D (n, j) -C s j The data obtained by the scattered radiation correction again logarithmic conversion (F in Figure 1
10) Do.

Next, the same processing is performed for the projection angle j + 1, and the following scattered ray correction is performed. D (1, j + 1) -C _s j + 1 D (2, j + 1) -C _s j + 1 :: D (n, j + 1) -C _s j + 1 The data obtained by this scattered ray correction is logarithmically converted again.

Image reconstruction is performed based on the logarithmically converted data obtained for each projection angle (F11 in FIG. 1).

Although a plurality of channels n1 to n2 are used, the data itself of any one channel may be used (in the mathematical expression, n1 = n2 as shown in (Equation 12)).
Is applicable). Further, it may be addition of consecutive channels between n1 and n2. There is also an example of using a deviation value or an average value other than addition. Hereinafter, the same process is executed for all projection angles.

[0044]

According to the present invention, the influence of scattered X-rays captured in the measurement data is eliminated by correcting the scattered X-ray correction amount in the proper linear region for the object according to the above-mentioned correction processing. The image quality can be improved by performing image reconstruction using the corrected data.
This has the effect of obtaining a T tomographic image.

[Brief description of drawings]

FIG. 1 is a process flow chart of the present embodiment.

FIG. 2 is a schematic front view of a state in which a phantom assuming an abdominal cross section is being measured.

FIG. 3 is a graph of the measurement data of FIG.

FIG. 4 is a schematic front view of a state in which a phantom assuming an abdominal cross section is being measured.

5 is a graph of the measurement data of FIG.

FIG. 6 is a schematic view of a state in which measurement is performed on 100 channels at the center of the detector for removing internal structural factors of the object.

7 is a measurement result graph of FIG.

FIG. 8 is a perspective view of the entire X-ray CT apparatus according to the present embodiment.

9 is a configuration block diagram of the image diagnostic apparatus of FIG.

FIG. 10 is a scattered X-ray intensity distribution chart of circular water phantom measurement.

FIG. 11 is a function curve diagram for determining a scattered X-ray correction amount according to the present embodiment.

[Explanation of symbols]

1 High voltage generator 2 gantry 3 patient table 4 Image diagnostic equipment 5 magnetic disk 6 Pretreatment device 7 Central control unit 8 main memory 9 Correction processing device 10 Reconstruction processing device 11 Display 12 high-speed internal bus 13 Data memory before correction 14 Projection data storage memory 15 adder 16 table reader 17 Correction table memory 18 Subtractor 19 Correction data memory 21 X-ray tube 22 Phantom 23 Multi-channel X-ray detector

─────────────────────────────────────────────────── ─── Continuation of front page (56) References JP-A-4-170942 (JP, A) JP-A 64-62126 (JP, A) JP-A 63-40535 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) A61B 6/03

Claims (3)

(57) [Claims] 1. A multi-channel X-ray detector for detecting an intensity distribution of X-rays transmitted through an object, means for performing logarithmic conversion to the detected signal intensity, and each projection of the object after the logarithmic conversion. A means for calculating a value of a certain specific one channel or a value obtained by adding projection data of a plurality of channels for each angle; and X-ray transmission data of a material close to the X-ray absorption coefficient of an object of different size in advance and the above A means for obtaining the scattered X-ray correction amount in the linear region based on the added value, and projection data after antilogarithmic conversion obtained by performing antilogarithmic conversion of projection data after logarithmic conversion of all channels for each projection angle, An X-ray CT apparatus comprising: scattered X-ray correction means for subtracting the scattered X-ray correction amount; and means for performing image reconstruction by logarithmically converting the corrected measurement data. 2. A multi-channel X-ray detector for detecting the intensity distribution of X-rays transmitted through an object, means for performing logarithmic conversion to the detected signal intensity, and each projection of the object after the logarithmic conversion. A means for calculating, for each angle j, a value of a certain specific one channel or a value obtained by adding projection data pro (i, j) of a plurality of channels (hereinafter, these are defined as an addition value S _j ) , [Equation 1] pro (i, j): projection data (i: channel, j:
Projection angles) n ₁ , n ₂ : specific channels, n ₁ <n ₂ <N
(All channels) Coefficients A and B related to the scattered X-ray correction amount determined in advance from an X-ray transmission image of a material close to the X-ray absorption coefficient of the subject such as water, polyethylene, and acrylic having different sizes are added. A means for obtaining the scattered X-ray correction amount C _sj in the linear region from the value of the value S _j by the following equation, and Scattered ray correction means for subtracting the scattered X-ray correction amount C _sj from the _{antilogarithmically} converted projection data obtained by inversely logarithmically converting the logarithmically converted projection data of all channels for each projection angle j, and An X-ray CT apparatus comprising: means for performing image reconstruction by logarithmically converting the corrected measurement data. 3. A multi-channel X-ray detector for detecting the intensity distribution of X-rays transmitted through a subject, means for performing logarithmic conversion to the detected signal intensity, and each projection of the subject after the logarithmic conversion. A unit for calculating a value S _j obtained by adding a certain one-channel value or projection data pro (i, j) of a plurality of channels for each angle j by the following equation: pro (i, j): projection data (i: channel, j:
Projection angles) n ₁ , n ₂ : specific channels, n ₁ <n ₂ <N
(All channels) Coefficients A and B related to the scattered X-ray correction amount determined in advance from an X-ray transmission image of a material close to the X-ray absorption coefficient of the subject such as water, polyethylene, and acrylic having different sizes are added. A means for obtaining the scattered X-ray correction amount C _sj in the linear region from the value of the value S _j by the following equation, Scattered ray correction means for subtracting the scattered X-ray correction amount C _sj from the _{antilogarithmically} converted projection data obtained by inversely logarithmically converting the logarithmically converted projection data of all channels for each projection angle j, and An X-ray CT apparatus comprising: means for performing image reconstruction by logarithmically converting the corrected measurement data.