JPH09248301A - Characteristic improving method for x-ray detector and x-ray ct system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent conflict from being generated in sensitivity correction by correcting data collected by real scan by generating correction data from data provided by the calibration scan of plural rotations at the same rotating speed as the real scan. SOLUTION: The fantom scan of plural rotations at the same rotating speed as the real scan is executed, row data are stored in a row data storage part, these row data are averaged by an averaging processing part 41, and the correction data are generated by a correction data calculation part 42 and stored in a correction data storage part 43. Then, an instruction is applied from a system control part 22 to a scan control part 23, the rotating part of a gantry 10 performs prescribed scan and collects data, and the row data are stored in the row data storage part. Further, the correction data are read out of the correction data storage part 43, the row data of the real scan are corrected by a preprocessing part 32, and the image display of the image reconstituted by an image reconstituting part 33 is performed on a display device 34.

Description

Detailed Description of the Invention

[0001]

The present invention relates to an X-ray CT (Computed
The present invention relates to a method for improving the characteristics of the X-ray detector used in the Tomography apparatus, and an X-ray CT apparatus that takes into consideration the improvement of the sensitivity characteristics of the X-ray detector.

[0002]

2. Description of the Related Art A radiation CT apparatus is an apparatus which irradiates radiation around the entire circumference of a subject, detects it by a detection device, and reconstructs a tomographic image for diagnostic purposes. The X-ray CT apparatus for irradiating rays is the best known.

A schematic configuration of the X-ray tomography apparatus is shown in FIG. In FIG. 3, reference numeral 1 denotes an X-ray tube that emits X-rays for performing X-ray tomography. The emitted X-rays are shaped into a fan shape by a collimator 2A, and further, the bowtie filter 2 is used.
The X-ray dose is adjusted by B, and the image reconstruction area 3
Is irradiated.

An X-ray detector 4 detects the X-rays emitted from the X-ray tube 1 and generates an electric signal proportional to the X-ray energy, and is composed of a large number of detection elements. The bow tie filter 2B adjusts the passing X-ray dose to
The purpose of this is to make the X-ray intensity after passing through the subject spatially uniform and suppress the range of the measurement range of the X-ray detector 4 to improve the resolution.

In the X-ray tomography apparatus having the above structure,
The X-rays emitted from the X-ray tube 1 are shaped into a fan shape by the collimator 2A, irradiate the image reconstruction area 3, and the X-rays transmitted through the image reconstruction area 3 are converted into electric signals in the X-ray detector 4. To be done.

Further, as the X-ray detector 4 used in such an apparatus, a Xe gas detector has been mainly used conventionally. In such a gas detector, it is generally said that the energy conversion efficiency from incident X-rays to collected charges is difficult to exceed 50%.

On the other hand, the solid-state detector directly converts energy from photons of X-rays and γ-rays and is efficient.
For this reason, solid-state detectors have recently come to be used as detectors for X-ray CT.

Then, in order to correct the variation in the sensitivity of each channel of the X-ray detector 4, a predetermined phantom is irradiated with X-rays before the actual scan in which the subject is irradiated with X-rays. Correction data is generated from output data.

Since such correction data is reference data, it is desirable that the SN ratio is good. Then, in order to improve the SN ratio of the data, it is necessary to increase the X-ray count value and relatively reduce the influence of random noise.

That is, by multiplying the X-ray tube current or the scanning time by n times, the X-ray count value becomes n times, and the power of random noise at this time becomes √n times. As a result, the SN ratio is improved by √n times (= n / √n).

Since the upper limit of the allowable tube current of the X-ray tube 1 is actually set, it can be dealt with by increasing the scan time. For example, when generating correction data in a device that actually scans at 1 second / 1 rotation,
A calibration scan is performed every second / one rotation.

[0012]

By performing the calibration scan for a long time as described above, it is possible to obtain correction data with a good SN ratio. However, the rotation speed is different between the actual scan and the calibration scan. Due to the difference, the following problems may occur.

Since the X-ray tube 1 rotates around the image reconstruction area 3, a large centrifugal force works. At that time, since the magnitude of the force acting on each part is different between the actual scan (1 second / revolution) and the calibration scan (3 second / revolution), the focus for generating X-rays in the X-ray tube 1 is increased. It has been confirmed that the position is slightly deviated.

Therefore, as shown in FIG. 4, the position of the X-ray passing through the bowtie filter 2B is different between the actual scan and the calibration scan. Therefore, the X-ray transmission lengths of the bow-tie filter are different, and the X-ray intensity distribution does not match between the actual scan and the calibration scan. Also, a large centrifugal force may act on the X-ray detector 4 to cause a deviation.

As a result, the sensitivity correction becomes inconsistent. This causes artifacts in the reconstructed image. The actual scan may be executed in a shorter time in the future, but the calibration scan is executed for a long time in order to improve the S / N ratio, so that it is expected that the above problems will be more serious.

The present invention has been made in view of the above points, and an object thereof is to cause a contradiction in sensitivity correction when the sensitivity correction is performed on the actual scan using the correction data obtained by the calibration scan. It is to realize a method for improving the characteristics of a non-existing X-ray detector.

Another object of the present invention is to realize an X-ray CT apparatus which does not cause a contradiction in sensitivity correction when sensitivity correction is performed on an actual scan using correction data obtained by a calibration scan. That is.

[0018]

The present invention, which is a means for solving the problems, is as described in the following (1) to (3).

(1) In the first invention, a calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector, and measurement data of the object is measured. A method for improving the characteristics of an X-ray detector that corrects the measurement data obtained by an actual scan for
A feature is that correction data is generated from data obtained by a calibration scan of a plurality of rotations at the same rotation speed as the actual scan, and the data collected in the actual scan is corrected using the correction data thus obtained. Do X
This is a method for improving the characteristics of the line detector.

In the method for improving the characteristics of the X-ray detector according to the first aspect of the present invention, since the calibration scan is performed at the same rotation speed as the actual scan, the focus of the X-ray irradiation, the vicinity of the bow-tie filter or the vicinity of the X-ray detector is measured. No deviation occurs. Therefore, the sensitivity correction does not cause a contradiction between the actual scan and the calibration scan.

Since the calibration scan having the same rotational speed as the actual scan is performed a plurality of times, the SN ratio of the sensitivity correction data can be improved in the state where the contradiction of the sensitivity correction does not occur.

Therefore, it is possible to perform sensitivity correction without causing a contradiction in a state where the SN ratio is good, and it is possible to reconstruct a good image with no artifacts.

(2) In the second invention, a calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector, and measurement data of the subject is measured. Is an X-ray CT apparatus for correcting the measurement data obtained by the actual scan, and the control means for performing the calibration scan of the plurality of rotations at the same rotation speed as the actual scan, X comprising: calibration means for generating correction data from the obtained data; and data correction means for correcting the data collected by the actual scan using the correction data obtained by the calibration means. It is a line CT device.

In the X-ray CT apparatus of the second invention, since the calibration scan is performed at the same rotation speed as the actual scan, a shift occurs in the focus of X-ray irradiation, near the bow-tie filter or near the X-ray detector. do not do.

Therefore, the sensitivity correction does not cause a contradiction between the actual scan and the calibration scan. Since the calibration scan having the same rotation speed as the actual scan is performed a plurality of times, the SN ratio of the sensitivity correction data can be improved in the state where the contradiction of the sensitivity correction does not occur.

Therefore, it is possible to perform the sensitivity correction without causing a contradiction in a state where the SN ratio is good, and it is possible to reconstruct a good image in which no artifact is generated.

(3) In the third aspect of the invention, a calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector to measure the object. Is an X-ray CT apparatus that corrects the measurement data obtained by the actual scan, and the time required for one rotation of the calibration scan is set equal to the time required for the actual scan, and the calibration scan of a plurality of n rotations is performed. Control means for performing the calibration, and calibration means for averaging n pieces of data obtained by the calibration scan of n rotations to generate correction data,
An X-ray CT apparatus comprising: a data correction unit that corrects data collected by an actual scan using the correction data obtained by the calibration unit.

In the third invention, since the calibration scan is performed at the same rotation speed as the actual scan, X
No deviation occurs in the focal point of the ray irradiation, near the Bowtie filter, or near the X-ray detector. Therefore, the sensitivity correction does not cause a contradiction between the actual scan and the calibration scan.

Since the calibration scan having the same rotational speed as the actual scan is performed a plurality of times, the influence of random noise can be reduced by the averaging process in the state where the contradiction of the sensitivity correction does not occur.
The SN ratio of the sensitivity correction data can be improved.

Therefore, it is possible to perform the sensitivity correction without causing a contradiction in a state where the SN ratio is good, and it is possible to reconstruct a good image in which no artifact is generated.

[0031]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a flow chart showing a processing procedure of a characteristic improving method for an X-ray detector according to an embodiment of the present invention. Further, FIG. 2 shows an apparatus (X-ray CT
FIG. 2 is a block diagram showing a configuration of an X-ray CT apparatus according to an embodiment of the present invention.

<Structure of X-ray CT Apparatus> First, the X-ray CT apparatus used in the method for improving the characteristics of the X-ray detector, which is an embodiment of the present invention, and the embodiment of the present invention will be described. The configuration of a certain X-ray CT apparatus will be described with reference to FIG.

The X-ray tube 11 is used for X-ray tomography.
X-rays are radiated, the radiated X-rays are shaped into a fan shape by the collimator 12, and the passing X-ray dose is adjusted by the bow-tie filter 13 to irradiate the image reconstruction area 14. The bow-tie filter 13 functions so that the X-ray intensity after passing through the subject becomes spatially uniform.

The X-ray detector 15 is a detector that converts the X-rays that have entered through the image reconstruction area 14 and is incident into an electric signal proportional to the energy thereof, and is composed of detection elements for a plurality of channels. .

The data collection unit (DAS) 16 collects a signal (raw data) proportional to the energy of the X-ray from the X-ray detector 15, and the output of the detection element of each channel will be described later. It is sent to the raw data storage unit 31.

The operator console 21 is an operating means for receiving various instructions from the operator, and the system controller 22 controls the entire apparatus in accordance with the instructions from the operator console 21. Further, the scan control unit 23 controls the entire scan operation such as rotation of the gantry 10 according to the control of the system control unit 22.

The raw data storage unit 31 is the data collection unit 16
It is a storage means for temporarily storing the raw data collected in. The pre-processing unit 32 executes various processes such as correction of the variation of the detection element based on the correction data, and constitutes a data correction means.

The image reconstructing unit 33 reconstructs an image by backprojecting the raw data that has been preprocessed.
The display device 34 is an image display unit for displaying the reconstructed image.

The calibration section 40 is a correction data generation means for generating correction data as a calibration scan for calibration, and is composed of an averaging processing section 41, a correction data calculation section 42 and a correction data storage section 43. . The calibration unit can be configured as an independent processing unit or can be realized inside the CPU such as the system control unit 22.

The averaging processing section 41 is a processing means for averaging the raw data obtained by a plurality of calibration scans. The correction data calculation unit 42 is a calculation unit that generates correction data for correcting variations in the sensitivity of the detection element of each channel from the averaged raw data of the calibration scan. Correction data storage unit 4
Reference numeral 3 is a storage means for temporarily storing the generated correction data.

In the above configuration, each unit is configured by hardware such as various processing processors, processing programs and the like, or firmware in which these are combined.

<Processing Procedure of X-ray Detector Characteristic Improvement Method>
The procedure of the method for improving the characteristics of the X-ray detector according to the embodiment of the present invention and the operation of the X-ray CT apparatus according to the embodiment of the present invention are roughly divided into the following,
,,, and each process. This step will be described step by step.

Data collection (calibration (1), steps 3 to 5 in FIG. 1): First, the system controller 22 determines whether the scan to be executed is a calibration scan or an actual scan according to an instruction input from the operator console 21. (FIG. 1S1).

When a calibration scan is to be executed, a positive integer C of 2 or more is set to the rotation speed n (S2 in FIG. 1). Further, regarding the rotation speed, the calibration scan and the actual scan are set to be the same.

Then, the phantom is placed on the image reconstruction area 14 and the phantom scan is executed (FIG. 1S).
3). The raw data obtained by this phantom scan is stored in the raw data storage unit 31.

Since the raw data obtained by the phantom scan in this way is performed at the same rotational speed as that of the actual scan, it is obtained in a state in which the displacement of each part due to the difference in centrifugal force does not occur. Has become.

Further, the system control unit 22 gives an instruction to the scan control unit 23 to repeat this phantom scan C times (S3 to S6 in FIG. 1). As a result, the rotating part of the gantry 10 performs continuous C scans and collects data.

The C pieces of data thus detected by the X-ray detector 15 and collected by the data collecting section 16 are stored in a predetermined area of the raw data storage section 31. Averaging process (calibration (2), step 7 in FIG. 1): Raw data by C calibration scans stored in the raw data storage unit 31 is read out, and C raw data is read by the averaging process unit 41. Are averaged (S1 in FIG. 1).

Therefore, since C pieces of raw data by phantom scan executed at the same rotation speed as the actual scan are averaged, the signal component becomes C times and the noise component becomes √C times. As a result, the SN ratio is √C times (= C /
√C).

For example, if the actual scan for 1 second and the calibration scan for 3 seconds have been performed, the calibration scan for 1 second is executed three times. As a result, the S / N ratio becomes √3 times without any shift of the focus position.

Correction data calculation (calibration (3), step 8 in FIG. 1): The correction data calculation unit 42 is used by using raw data whose SN ratio is improved by the averaging process.
Generates correction data (data for correcting variations in the sensitivity of the detection element of each channel) (S8 in FIG. 1). Then, the generated correction data is stored in the correction data storage unit 43.

Actual scan (steps 9 and 10 in FIG. 1):
A scan for tomography is executed according to a normal procedure (S9 in FIG. 1). That is, the system control unit 22 gives an instruction to the scan control unit 23 to execute a predetermined actual scan. As a result, the rotating part of the gantry 10 performs a predetermined scan to collect data (S10 in FIG. 1). The raw data thus obtained by the actual scan is stored in the raw data storage unit 31.

Data correction processing (preprocessing, step 11 in FIG. 1): The data of the X-ray transmission amount obtained by the actual scan is read from the raw data storage section 31 and the preprocessing section 32 is read.
In step S11 of FIG.

That is, the correction data for correcting the variation in the sensitivity of each channel of the X-ray detector 4 is read from the correction data storage section 43, and the pre-processing section 32 reads the raw scan row from the raw data storage section 31. Correct the data. In this way, the variation of the sensitivity of each channel is corrected by the integrated value (Σcount) of count for each view (or each scan).

The correction data used here has the improved SN ratio as already described, and the focus position when the correction data is generated is generated so as to be the same condition as the actual scan. Conflict does not occur due to sensitivity correction.

Image reconstruction and display (step S1 in FIG. 1)
2, 13): The image reconstruction unit 33 executes image reconstruction using the X-ray transmission amount data corrected as described above (FIG. 1S12), and the reconstructed image is displayed on the display device 3
An image is displayed on screen 4 (S13 in FIG. 1). Further, the reconstructed image data is stored in an image data storage unit (not shown) as needed.

In this case, since the sensitivity correction is performed by the correction data generated under the condition that the focal position and the like are the same in the actual scan and the calibration scan, the contradiction of the sensitivity correction does not occur and the reconstructed image is not generated. No artifacts will occur.

<Effects Obtained by Embodiments: Comparison with Conventional Example> As described above, the time required for one rotation of the calibration scan is set equal to the time required for the actual scan, and a plurality of calibration scans are performed. The correction data is generated by averaging a plurality of data obtained by executing the correction, and the sensitivity correction is performed using the correction data. X
According to the method for improving the characteristics of the line detector and the X-ray CT apparatus, the effects described below can be obtained.

Since the calibration scan is performed at the same rotation speed as the actual scan, no shift occurs in the focus of X-ray irradiation, near the bow-tie filter, or near the X-ray detector. Therefore, the sensitivity correction does not cause a contradiction between the actual scan and the calibration scan.

Since the calibration scan having the same rotation speed as the actual scan is performed a plurality of times,
The averaging process can reduce the influence of random noise and improve the SN ratio of the sensitivity correction data in the state where the contradiction of the sensitivity correction does not occur.

As a result of the above, it is possible to perform sensitivity correction without causing a contradiction in a state where the SN ratio is good, and it is possible to reconstruct a good image in which no artifacts occur.

<Other Preferred Examples> In addition to the above description of the embodiments, the following modified examples are also conceivable.

Instead of averaging the raw data obtained by a plurality of calibration scans and then calculating the correction data, the plurality of correction data may be generated and then averaged. In order to do so, the averaging processing unit 41 may be provided in the subsequent stage of the correction data calculating unit 42, and the averaging processing unit 41 may perform the averaging process of the correction data.

[0064]

According to the method of improving the characteristics of the X-ray detector and the X-ray CT apparatus described in detail above, the following (1) to
The effect as shown in (3) is obtained.

(1) In the characteristic improving method of the X-ray detector according to the first aspect of the invention, the correction data is generated from the data obtained by the calibration scan of a plurality of rotations at the same rotation speed as the actual scan. By correcting the data collected by the actual scan using the correction data obtained as described above, it is possible to generate the sensitivity correction data with a good SN ratio without causing a contradiction in the sensitivity correction, so that no artifact is generated. A good image can be reconstructed.

(2) In the X-ray CT apparatus of the second invention, the correction data is generated from the data obtained by the calibration scan of a plurality of rotations at the same rotation speed as the actual scan, and the correction thus obtained is performed. By correcting the data collected by the actual scan using the data, it is possible to generate the sensitivity correction data in which the contradiction of the sensitivity correction does not occur in a state with a good SN ratio, so that a good image without artifacts can be obtained. Can be reconfigured.

(3) In the X-ray CT apparatus of the third invention, since the calibration scan is performed at the same rotation speed as the actual scan, the deviation of the focal point of X-ray irradiation, the vicinity of the bow-tie filter or the vicinity of the X-ray detector. Since it does not occur, the sensitivity correction does not cause a contradiction between the actual scan and the calibration scan.

Since the calibration scan having the same rotation speed as the actual scan is performed a plurality of times, the influence of random noise can be reduced by the averaging process in the state where the contradiction of the sensitivity correction does not occur.
The SN ratio of the sensitivity correction data can be improved.

Therefore, it is possible to perform the sensitivity correction without causing a contradiction in a state where the SN ratio is good, and it is possible to reconstruct a good image in which no artifact is generated.

[Brief description of drawings]

FIG. 1 is a flowchart showing a procedure of a method of improving a characteristic of an X-ray detector according to an embodiment of the present invention.

FIG. 2 is a configuration diagram showing a configuration example of an X-ray CT apparatus according to an embodiment of the present invention.

FIG. 3 is a configuration diagram showing a configuration example of a main part of an X-ray CT apparatus.

FIG. 4 is an explanatory diagram showing a state of deviation of X-ray irradiation between an actual scan and a calibration scan.

[Explanation of symbols]

 11 X-ray tube 12 Collimator 13 Bowtie filter 14 Image reconstruction area 15 X-ray detector 16 Data acquisition section 21 Operator console 22 System control section 23 Scan control section 31 Raw data storage section 32 Pre-processing section 33 Image reconstruction section 34 Display Device 40 Calibration unit 41 Averaging processing unit 42 Correction data calculation unit 43 Correction data storage unit

Claims (3)

[Claims] 1. A calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector, and the correction data is obtained by an actual scan for measurement of an object. A method for improving the characteristics of an X-ray detector that corrects the measured data, in which the correction data is generated from the data obtained by the calibration scan of a plurality of rotations at the same rotation speed as the actual scan, and is obtained in this way. A method for improving the characteristics of an X-ray detector, characterized in that the correction data is used to correct the data collected in the actual scan. 2. A calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector, and the correction data is obtained by an actual scan for measuring an object. An X-ray CT apparatus for correcting the measured data, the control means for performing a calibration scan of a plurality of rotations at the same rotation speed as the actual scan, and the correction data from the data obtained by the calibration scan of the plurality of rotations. An X-ray CT apparatus, comprising: a calibration unit that generates the data; and a data correction unit that corrects data collected by an actual scan using the correction data obtained by the calibration unit. 3. A calibration scan for initial setting of sensitivity is performed to generate correction data for correcting sensitivity unevenness of each channel of the X-ray detector, and the correction data is obtained by an actual scan for measuring an object. An X-ray CT apparatus for correcting the measured data, wherein the time required for one rotation of the calibration scan is set equal to the time required for the actual scan, and a control means for performing the calibration scan of a plurality of n rotations; N obtained by rotation calibration scan
A calibration means for averaging individual data to generate correction data, and a data correction means for correcting the data collected by an actual scan using the correction data obtained by the calibration means are provided. X-ray CT device.