JPH08243101A - Characteristic improvement for x-ray detector and x-ray ct system - Google Patents

Abstract

PURPOSE: To correct the sensitivity deterioration or recovery of a detecting element by correcting data on each channel collected by scanning by using the deterioration ratio of sensitivity in accordance with an exposure dose by the scanning and the recovery rate of sensitivity in accordance with lapse time from X-ray irradiation just before calculated by the inverse function of the recovery rate. CONSTITUTION: The sensitivity of an X-ray detecting element is measured by applying the X-ray irradiation successively in a state in which no deterioration of sensitivity occurs (after the lapse of a sufficient long time from the X-ray irradiation just before) (step 1). Thence, the deterioration ratio of sensitivity at every channel is measured and preserved (step 2). After that, the recovery rate of sensitivity at every channel is measured and preserved (step 3). The scanning for tomographic photographing is executed (step 4), and the data of X-ray transmission quantity obtained by the scanning is corrected (step 5). The reconfiguration of an image is executed by using the data of X-ray transmission quantity to which correction processing is applied, and the image is displayed on a display part (step 6).

Description

Detailed Description of the Invention

[0001]

The present invention relates to an X-ray CT (Computer Tom)
The present invention relates to a characteristic improving method relating to the sensitivity of an X-ray detector used in an imaging device, and an X-ray CT apparatus in consideration of improving the sensitivity characteristic 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 CT apparatus is shown in FIG. In FIG. 6, reference numeral 1 denotes an X-ray tube that emits X-rays for performing X-ray tomography, and the emitted X-rays are collimators 2
The image reconstruction area 3 is illuminated by being shaped into a fan shape by.

An X-ray detector 4 detects the X-rays emitted from the X-ray source 1 and generates an electric signal proportional to the X-ray energy, and is composed of a large number of detection elements. In the X-ray CT apparatus having the above configuration, X emitted from the X-ray tube 1
The rays are shaped into a fan shape by the collimator 2, irradiate the image reconstruction area 3, and the X-rays transmitted through the image reconstruction area 3 are X-rays.
It is converted into an electric signal in the line detector 4.

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.

By the way, as the above-mentioned solid-state detector, there is a detector whose sensitivity is deteriorated by receiving X-ray irradiation. Such deterioration in sensitivity occurs according to the amount of X-ray irradiation, and the sensitivity characteristic can be expressed by an exponential function or the like. The solid-state detector having such properties is an X-ray C
It is used as the detection means of the T device.

Further, this type of solid-state detector has a characteristic that the receiving sensitivity gradually deteriorates upon receiving X-ray irradiation, but the sensitivity returns as time passes after the end of X-ray irradiation. .

[0009]

Therefore, in the case of using this type of X-ray detector, it is possible to correct the deteriorated sensitivity after accurately grasping the degree of deterioration of sensitivity and the degree of recovery of sensitivity. desirable.

The following problems can be considered when performing such correction. If the characteristics of sensitivity deterioration of the solid-state detector are known in advance, X
It is possible to calculate the history of X-ray irradiation and to calculate the sensitivity deterioration at the time of performing X-ray irradiation and detection. That is, the sensitivity deterioration can be calculated by applying the history of X-ray irradiation to the characteristic of the sensitivity deterioration.
It is necessary to perform time measurement and always make calculations not only during the time when the apparatus is operating but also during the period when the operation of the X-ray CT apparatus is stopped. Therefore, there is a problem that the calculation becomes extremely complicated.

The present invention has been made in view of the above points, and an object thereof is to provide a method for improving the characteristics of an X-ray detector capable of correcting sensitivity deterioration or restoration of a detection element having sensitivity fluctuation. It is to be realized.

Another object of the present invention is to realize an X-ray CT apparatus capable of correcting sensitivity deterioration or sensitivity recovery of a detection element having sensitivity fluctuation. A further object of the present invention is to realize an X-ray CT apparatus capable of correcting the characteristic deterioration or the characteristic restoration of the detecting means having the characteristic variation other than the sensitivity.

[0013]

The inventor of the present application has conducted earnest research to improve the drawbacks of the conventional method for improving the characteristics of an X-ray detector, and as a result, has found that the optimum arrangement of the reference detector and The present invention has been completed by finding a correction method and finding a process that satisfies both the correction of the output fluctuation of the X-ray tube and the measurement of the sensitivity deterioration of the X-ray detector.

That is, the present invention, which is means for solving the problems, is as described in (1) to (4) below. (1) A first means for solving the problem is to provide a plurality of channels for image reconstruction of a first type of detection element whose sensitivity changes upon receiving X-ray irradiation, and the sensitivity does not change upon receiving X-ray irradiation. A method for improving the characteristics of an X-ray detector equipped with a second type detection element as a reference detection element, wherein the sensitivity of the detection element of each channel is detonated by referring to the sensitivity of the second type detection element. A function of the sensitivity deterioration rate that occurs according to the radiation dose,
The sensitivity of the detection element of each channel is determined with reference to the sensitivity of the detection element of the second type, and at least one of the function of the recovery rate of the sensitivity generated according to the time from the X-ray irradiation is obtained and scanning is performed. Data of each channel is collected according to the exposure dose due to the scan, and the elapsed time from the immediately preceding X-ray irradiation calculated from the inverse function of the recovery rate using the data at each scan. From at least one of the recovery rate of the sensitivity from the above function,
This is a method of improving the characteristics of an X-ray detector, characterized in that the data of each channel collected by scanning is corrected using one or both of the deterioration rate and the recovery rate thus obtained.

In this case, it is also possible to obtain and correct either one of the deterioration characteristics and the restoration characteristics in addition to correcting the data. For example, when scanning is performed in a short time, the deterioration rate increases but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. Further, in the case of scanning with a low radiation dose at long intervals, the recovery rate becomes large but the deterioration rate is small. Therefore, it is possible to obtain and correct only the deterioration rate.

(2) A second means for solving the problem is an X-ray CT apparatus provided with a plurality of channels of the first type of detecting element whose sensitivity changes upon receiving X-ray irradiation for image reconstruction. The sensitivity of the detection element of each channel is generated according to the exposure dose by referring to the sensitivity of the second type of detection element for which the sensitivity does not change upon receiving the line irradiation and the sensitivity of the second type of detection element. At least one of a function of a deterioration rate of sensitivity and a function of a recovery rate of sensitivity that occurs according to time from X-ray irradiation with respect to the sensitivity of the detection element of each channel with reference to the sensitivity of the second type detection element. Depending on the calibration means to obtain the function of, the sensitivity deterioration rate according to the exposure dose by the scan, and the elapsed time from the immediately preceding X-ray irradiation calculated from the inverse function of the return rate using the data at each scan. Recovery rate of sensitivity And a data correction means for correcting data of each channel collected by scanning using one or both of the deterioration rate and the recovery rate thus obtained. It is an X-ray CT apparatus that does.

In this case, it is possible to obtain and correct either one of the deterioration characteristics and the restoration characteristics instead of the correction of the data. For example, when scanning is performed in a short time, the deterioration rate increases but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. Further, in the case of scanning with a low radiation dose at long intervals, the recovery rate becomes large but the deterioration rate is small. Therefore, it is possible to obtain and correct only the deterioration rate.

(3) A third means for solving the problem is to use an X-ray CT apparatus provided with a plurality of channels of a first type of detection element whose sensitivity changes upon receiving X-ray irradiation for image reconstruction. A first type of reference detection element whose sensitivity changes upon receiving X-ray irradiation, a second type reference detection element whose sensitivity does not change upon receiving X-ray irradiation, and a first type before scanning The normalized reference sensitivity obtained by normalizing the sensitivity of the reference detection element of No. 2 by the sensitivity of the second type of reference detection element to obtain the second reference sensitivity
The sensitivity deterioration rate of the detection element of each channel generated as a function of the sensitivity of the detection element of each channel is obtained as a function, and the sensitivity of the second type of detection element is referred to. Regarding the sensitivity of the detection element of the channel, the recovery rate of the sensitivity generated according to the time from the X-ray irradiation is obtained as a function, and the sensitivity of the detection element of the first type is determined by the second
Calibration means for obtaining the normalized reference sensitivity normalized by the sensitivity of the detection element of the type, and the deterioration rate of the sensitivity according to the exposure dose due to the scan from the above function, the initial value of the normalized reference sensitivity and each The elapsed time from the immediately preceding X-ray irradiation is obtained from the inverse function of the return rate using the value of the normalized reference sensitivity at the time of scanning, and the return rate of sensitivity according to this elapsed time is obtained from the function and collected by scanning. An X-ray CT apparatus comprising: a data correction unit that corrects the data of each channel according to the deterioration rate and the recovery rate.

A method for improving the characteristics of the X-ray detector characterized by the operation of the X-ray CT apparatus shown in the third means for solving this problem is also one of the means for solving the problem. In this case, the data is corrected by obtaining both the deterioration characteristic and the restoration characteristic, but it is also possible to obtain and correct either one. For example, when scanning is performed in a short time, the deterioration rate increases but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. Also, when scanning low doses at long intervals,
Since the recovery rate becomes large but the deterioration rate is small, it is possible to obtain and correct only the deterioration rate.

(4) A fourth means for solving the problem is an X-ray CT apparatus provided with a plurality of channels of the first kind of detecting means for changing characteristics other than sensitivity upon receiving X-ray irradiation for image reconstruction. In reference to the characteristics of the second type of detecting means and the characteristics of the detecting means of the second type with reference to the characteristics of the second type of detecting means for reference, which does not change upon receipt of X-ray irradiation. With reference to the function of the deterioration rate of the characteristic that occurs according to the amount and the characteristic of the second type detecting means, the characteristic of the detecting means of each channel is generated according to the time from X-ray irradiation. Calibration means for at least one of the function of the recovery rate of the characteristics, the deterioration rate of the characteristics according to the exposure dose by scanning, and calculated from the inverse function of the recovery rate using the data at each scan Immediately before Obtained from the function for at least one of the recovery rate of the characteristics according to the elapsed time from the line irradiation, one or both of the degradation rate and the recovery rate thus obtained for each channel collected by scanning An X-ray CT apparatus comprising: a data correction unit that corrects data.

In the fourth means for solving the above problems, the characteristics of the detecting means may be leak count characteristics, offset drift characteristics, afterglow characteristics, etc., and any of them can be applied. Further, as the detecting means, a data collecting unit and the like are included in addition to the detecting element, and can be applied.

A method for improving the characteristic of the X-ray detecting means characterized by the operation of the X-ray CT apparatus shown in the fourth means for solving this problem is also one of the means for solving the problem. In this case, it is possible to obtain and correct either one of the deterioration characteristics and the restoration characteristics, instead of performing the correction of the data. For example, when scanning is performed in a short time, the deterioration rate increases but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. Further, in the case of scanning with a low radiation dose at long intervals, the recovery rate becomes large but the deterioration rate is small. Therefore, it is possible to obtain and correct only the deterioration rate.

[0023]

In the characteristic improving method of the X-ray detector which is the first means for solving the problem, the sensitivity deterioration rate and the recovery rate of the detection element of each channel are referred to by referring to the sensitivity of the second type detection element. Deterioration rate at each scan in consideration of at least one of the sensitivity deterioration due to X-ray irradiation of the detection element of each channel and the sensitivity recovery according to the elapsed time from the immediately previous scan. By correcting one or both of the recovery rate and the recovery rate, the X-ray can be accurately detected.

X-ray CT which is the second means for solving the problems
In the apparatus, the calibration processing means refers to the sensitivity of the second type detection element to obtain at least one function of the deterioration rate and the recovery rate of the detection element sensitivity of each channel, and the data correction means determines each channel. In consideration of at least one of sensitivity deterioration due to X-ray irradiation of the detection element and sensitivity recovery according to the elapsed time from the immediately previous scan, one or both of the deterioration rate and the recovery rate at each scan is obtained and correction is performed. This enables accurate detection of X-rays.

In the third means for solving the problem, the calibration processing means obtains the sensitivity deterioration due to the X-ray irradiation of the detection element of each channel and the sensitivity recovery according to the elapsed time from the immediately preceding scan, and obtains the data. Accurate detection of X-rays can be performed by obtaining and correcting the sensitivity deterioration rate at each scan by the correction unit.

Then, when the sensitivity recovery of the detection element is obtained, the recovery time is applied to the function of the sensitivity recovery to obtain it by calculation. When calculating this return time,
It is possible to obtain only from the measured value of the first type reference detection element that causes sensitivity deterioration, but it is necessary that the output of the X-ray tube is constant. Therefore, the recovery time is obtained by the normalized reference sensitivity obtained by normalizing the measurement value of the first type of detection element with the measurement value of the second type of detection element that does not cause sensitivity deterioration. By normalizing the measured value in this way, the state is not affected by the output fluctuation of the X-ray tube at each measurement timing. Therefore, even if an output fluctuation occurs in the X-ray tube at each timing for obtaining the recovery time, it is not affected by the fluctuation,
It becomes possible to accurately determine the recovery time. As a result, it is not necessary to continuously measure the time from the previous scan to the current scan, and the sensitivity recovery rate can be accurately obtained from the calculated recovery time Δt. Therefore, the sensitivity deterioration rate and the sensitivity recovery rate can be accurately obtained and corrected.

In the fourth means for solving the problem, the calibration processing means refers to the characteristics of the second type of detecting means to determine at least one of the deterioration rate and the recovery rate of the characteristics of the detecting element of each channel. The function is calculated, and the deterioration rate and the recovery rate at each scan are considered in consideration of at least one of the deterioration due to the X-ray irradiation of the detection means of each channel by the data correction means and the recovery according to the elapsed time from the immediately previous scan. Accurate detection of X-rays can be performed by obtaining one or both and performing correction.

In each of the means for solving the above problems, when scanning is performed in a short time, the deterioration rate becomes large but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. , X-rays can be accurately detected. Further, in the case of scanning with a low radiation dose at long intervals, the recovery rate becomes large but the deterioration rate is small, so it is possible to obtain and correct only the deterioration rate.
It enables accurate detection of lines.

[0029]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a flowchart showing a processing procedure of an X-ray detector characteristic improving method according to an embodiment of the present invention. Further, FIG. 2 is a block diagram showing a configuration of an apparatus (X-ray CT apparatus) used in a method for improving the characteristics of an X-ray detector according to an embodiment of the present invention and 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 one embodiment of the present invention, and the X-ray CT which is one embodiment of the present invention. The configuration of the device will be described with reference to FIG.

The X-ray tube 1 emits X-rays for performing X-ray tomography, and the emitted X-rays are shaped into a fan shape by the collimator 2 and irradiate the image reconstruction area 3. X-ray detector 5
Is a detector that converts X-rays that pass through the image reconstruction area 3 and enter into an electric signal proportional to the energy thereof, and is composed of detection elements for a plurality of channels. Also,
As shown in FIG. 3, the X-ray detector 5 converts the X-rays, which are transmitted through the image reconstruction area 3 and are incident, into electric signals by a solid-state detector whose sensitivity changes upon receiving X-ray irradiation. And a first type reference detection element ref_1 whose sensitivity changes upon receiving X-ray irradiation, and a second type reference detection element whose sensitivity does not change upon receiving X-ray irradiation.
It has ref_2 and.

The reference detection elements ref_1 and ref_2 are provided at the ends of the X-ray detector 5 in order to receive the X-rays outside the image reconstruction area 3 and measure the reference values. . The data collection unit 6 collects a signal proportional to the energy of the X-ray from the X-ray detection unit 5, sends the output of ref_1 and ref_2 and each detection element to the calibration processing unit 7 described later, and also serves as a reference. The output of each detection element other than the detection elements of No. 1 is sent to the data correction unit 8.

The calibration processing unit 7 uses the reference X
Receiving the signals of the line detection elements ref_1 and ref_2 and the respective detection elements, the deterioration rate and the restoration rate of the sensitivity of each detection element of the X-ray detector 5 are obtained. Here, the sensitivity deterioration rate refers to a rate at which the sensitivity is deteriorated in proportion to the amount of X-ray irradiation, and the sensitivity return rate is once determined according to the time elapsed from the X-ray irradiation. It is the rate at which the degraded sensitivity recovers. The deterioration rate Di of the sensitivity of each detection element
= Fi and recovery rate Di = gi (where i is the channel number of each detection element), the calibration processing unit 7 obtains and holds the same.

The data correction unit 8 corrects the X-ray intensity data transmitted through the image reconstruction area 3 and detected by referring to the deterioration rate Di = fi and the recovery rate Di = gi of the sensitivity of the X-ray detector 5. The processing (data correction processing) is executed.

The image processing section 9 performs image reconstruction using the data after the data correction processing to generate a desired tomographic image. The display unit 10 is a display unit that displays the reconstructed image.

The control section 11 controls each section of the X-ray CT apparatus. For example, in the example shown in FIG.
To control the irradiation amount, and to the data correction unit 8, data such as the X-ray irradiation amount and the time after completion of X-ray irradiation.

In the above configuration, the calibration processing unit 7, the data correction unit 8, the image processing unit 9, and the control unit 11 are implemented by hardware such as various processing processors, processing programs, etc., or firmware combining these. It is configured.

<Procedure of Method for Improving Characteristic of X-ray Detector> Procedure of method for improving characteristic of X-ray detector which is one embodiment of the present invention,
In addition, the operation of the X-ray CT apparatus according to the embodiment of the present invention is as follows.
It is roughly divided as shown below ...
It is composed of each step of. This step will be described step by step.

Initial measurement of sensitivity (calibration (1), step in FIG. 1): First, X-rays are continuously measured in a state where there is no deterioration in sensitivity (a state in which a sufficiently long time has passed since the last X-ray irradiation). Irradiation is performed to measure the sensitivities of the X-ray detection elements ref_1 and ref_2.

In this case, each data is converted into DATA.
(Ref_1) and DATA (ref_2), ε ₀
= (DATA (ref_1)) / (DATA (ref_2))
As a result, the normalized reference sensitivity ε ₀ is stored. In this way, by normalizing the data of the detection element ref_1 that causes sensitivity deterioration with the detection element ref_2 that does not cause sensitivity deterioration, X
The initial setting value (normalized reference sensitivity) of the sensitivity data can be saved without being affected by the value of the output of the wire tube.

Here, the reason for obtaining the normalized reference sensitivity will be described. In order to obtain the sensitivity deterioration that occurs depending on the X-ray irradiation dose of the solid-state detector as described above, a solid-state detector of the same type (reference solid-state detector) that does not emit X-rays is prepared, and this X-ray is used. It is conceivable to measure the output of a solid-state detector that does not irradiate a ray and use it as a reference for sensitivity before sensitivity deterioration occurs.

However, in the measurement of such sensitivity deterioration,
It is not possible to eliminate the influence of the output fluctuation during X-ray irradiation when measuring the sensitivity. That is, if the reference solid state detector and the solid state detector in which sensitivity and sensitivity have deteriorated have the same X-ray irradiation dose, only the sensitivity deterioration is obtained as the difference between the outputs of the respective solid state detectors.

However, if there is a difference in the amount of X-ray irradiation applied to the reference solid-state detector and the solid-state detector in which sensitivity / sensitivity deterioration has occurred, data containing an error corresponding to the variation of the X-ray irradiation amount. Therefore, it is not possible to measure only the deterioration of sensitivity. That is, it is difficult to accurately measure the deterioration of sensitivity due to the output fluctuation of the X-ray tube.

For this reason, the detection element that causes sensitivity deterioration is also used as the reference detection element ref_1. In an ideal state where the output of the X-ray tube does not change, only the second reference detection element ref_2 may be used.

Sensitivity deterioration rate measurement (calibration (2), step in FIG. 1): Next, the sensitivity deterioration rate Di is measured for each channel and stored. Deterioration rate D of this sensitivity
i is the amount of X-ray sensitivity deterioration depending on the integrated value of the count number of X-rays (data I obtained in each channel) (in the present specification, this is referred to as exposure dose and is represented as Σcount). The rate of increase is such that the sensitivity deteriorates at a constant rate as shown in FIG.

In the characteristic diagram shown in FIG. 4, the horizontal axis represents the exposure dose Σcount and the vertical axis represents the deterioration rate Di [%] of the sensitivity. As is clear from FIG. 4, the exposure dose Σ
As the count increases, the sensitivity deterioration rate Di [%]
It can be seen that also increases exponentially.

Here, the X-ray irradiation is continuously performed, and the integral value (Σcount) of the count value in the data collecting unit 6 for each channel and each i-channel during X-ray irradiation are set at a constant timing. Sensitivity ratio ε between the detector element and ref_2
(Ε = (DATA (i)) / (DATA (ref_2
))) Is measured, and the deterioration rate D (= 1−ε / ε ₀ ) as shown in FIG.

Also in this case, the data of the detection element of each channel which causes the sensitivity deterioration is detected by the detection element re which does not cause the sensitivity deterioration.
By normalizing with f_2, it is possible to measure the state of sensitivity deterioration without being influenced by the fluctuation of the output of the X-ray tube 1 at each timing when the deterioration rate D is obtained.

Then, the deterioration rate Di for each channel measured in this way is fitted by an exponential function fi using an index such as e ^X, and the integrated value (Σcount) of the count values in the data collection unit 6 Is used as a parameter, and is represented by an exponential function fi for each channel as fi (Σcount) (1). Then, the calibration processing unit 7 stores the exponential function fi for each channel. The exponential function fi is called a sensitivity deterioration characteristic function.

In the above D = 1-ε / ε _O , ε
Normalization is performed by using the detection element ref_2 for each of ε and ε _O. For this reason, the denominator and the numerator have the same proportion of X.
Since the output fluctuation of the X-ray tube 1 acts, the fluctuation of the output of the X-ray tube 1 at the time of measuring the sensitivity at each timing can be ignored.

Further, this sensitivity deterioration characteristic function fi can be expressed as fi (0, Σcount, A) (1) '.

Where i is the channel number of each detector,
0 is the initial value of sensitivity deterioration (indicating that sensitivity deterioration is 0 in the initial state), and A is the current value [mA] supplied to the X-ray tube 1 per unit time.

That is, as the property of the detecting element, the deterioration rate may be changed by changing the X-ray irradiation amount per unit time. Therefore, in addition to the integral value Σcount of the X-ray count value, the tube current of the X-ray tube 1 may be changed. Also can be a parameter. When such a sensitivity deterioration characteristic function fi can be expressed, the sensitivity deterioration characteristic function fi including the tube current A of the X-ray tube 1 is obtained and stored for each detection element.

As described above, the sensitivity deterioration characteristic function is obtained for all the elements and stored in the calibration processing section 7 as a correction vector (correction amount) for each channel.

In the above explanation, the deterioration of sensitivity is caused by the integrated value Σ.
It was about relying on count. In addition, if the sensitivity deterioration depends on the dose rate (count / t), the integrated value (Σ (count) per unit time) of the dose rate (count value per unit time) is used instead of the above integrated value (Σcount). /
t)) can also be used. When using such an integrated value of the dose rate, t is an arbitrary unit time related to X-ray irradiation such as a view time or a scan time.

Sensitivity recovery rate measurement (calibration (3), step in FIG. 1): Next, the sensitivity recovery rate is measured for each channel and stored. The recovery rate of this sensitivity is X
This is the rate at which the deterioration of X-ray sensitivity is restored according to the elapsed time t from the end of the radiation. As shown in FIG. 5, the characteristic is such that the sensitivity deterioration Di is restored at a constant rate.

Incidentally, the recovery rate means that the deterioration rate is recovered. In the characteristic diagram shown in FIG. 5, the horizontal axis represents the elapsed time t from the end of X-ray irradiation, and the vertical axis represents the sensitivity. Deterioration rate D
i [%] is shown.

As is clear from FIG. 5, the deterioration rate Di of the sensitivity di
It can be seen that [%] is gradually returning exponentially. Here, from the state in which the sensitivity has been deteriorated to some extent by X-ray irradiation, scanning is performed at fixed time intervals Δt, and the data of the detection element of each channel i during X-ray irradiation and the re
Sensitivity ratio to f_2 ε (ε = (DATA (i)) / (DA
TA (ref_2))) and plotting ε for each Δt from these, the deterioration rate (recovery rate) as shown in FIG.
D (= 1-ε / ε ₀ ) is calculated.

Since the scan performed at this time is for obtaining the return amount, it is necessary to perform the scan so as not to cause the above-mentioned deterioration of sensitivity. For example, the scan with a reduced X-ray irradiation amount is performed, or the scan is performed with a scanning technique that does not deteriorate the sensitivity. As such a scanning technique, for example, a short X-ray irradiation may be considered by setting the scanning time to several milliseconds.

Also in this case, the data of the detection element of each channel which causes sensitivity deterioration is detected by the detection element re which does not cause sensitivity deterioration.
By normalizing with f_2, it is possible to measure the state of sensitivity deterioration in a state where it is not affected by fluctuations in the output of the X-ray tube 1 at each timing when the recovery rate D is obtained.

Then, the deterioration rate Di for each channel measured in this way is fitted with an exponential function gi using an index such as e ^X, and the elapsed time t from the end of the previous scan is used as a parameter to obtain gi (T) (2) is represented by the exponential function gi for each channel. Then, the calibration processing unit 7 stores the exponential function gi for each channel. The exponential function gi will be referred to as a sensitivity recovery characteristic function.

In the above D = 1-ε / ε ₀ , ε
Normalization is performed using the detection element ref_2 at each of ε ₀ and ε ₀ . For this reason, the denominator and the numerator have the same proportion of X.
The output variation of the X-ray tube 1 acts, and the variation of the output of the X-ray tube 1 at the time of measuring the sensitivity at each timing for each Δt can be completely ignored.

Since each detection element of the X-ray detector 5 has individual differences, the above-mentioned sensitivity recovery characteristic function gi is measured for all the detection elements, and correction is performed for each channel in the same manner as fi described above. It is stored in the calibration processing unit 7 as a vector (correction amount).

Scan (step in FIG. 1): A scan for tomography is executed by a normal procedure. It should be noted that the control unit 11 prepares a scan plan and the control unit 1
The X-ray dose of the X-ray tube 1, the X-ray irradiation time, the X-ray irradiation time, the rest period, etc. are controlled according to the command from 1. Therefore,
Data such as the X-ray dose in the X-ray tube 1, the irradiation time, the time when the X-ray irradiation is performed, and the like are held in the storage unit for the scan plan of the control unit 11.

Further, each time the scan is executed, the calibration processing section 7 finds and stores the normalized reference sensitivity. For example, the normalized reference ε _K is measured from the sensitivities of the X-ray detection elements ref_1 and ref_2 when the kth scan is executed. In this case, each data is DA
When TA (ref_1) and DATA (ref_2) are set,
ε _K = (DATA (ref_1)) / (DATA (ref_2
)) Save as.

Also in this case, the detection element which causes the sensitivity deterioration
The data of ref_1 is detected by the detector ref_2 that does not cause sensitivity deterioration.
By normalizing with, the data of the normalized reference sensitivity is stored without being affected by the fluctuation of the output of the X-ray tube 1 when obtaining this data (k-th scan).

Data correction processing (step in FIG. 1): The data correction unit 8 corrects the data of the X-ray transmission amount obtained by the scan. Here, the sensitivity correction characteristic function fi and the sensitivity recovery characteristic function gi as the correction vector for each channel are received from the calibration processing unit 7, and the data correction is performed by receiving the X-ray irradiation time and the elapsed time from the control unit 11. Execute the process.

That is, for each view (or each scan), the correction by the deterioration of the sensitivity is executed by the integrated value (Σcount) of count, and the correction by the restoration of the sensitivity is executed by the elapsed time from the previous scan.

Therefore, the sensitivity-corrected data Ii 'of the data Ii obtained in the i-channel of the detection element can be expressed as Ii' = Ii * 1 / (1-Di) (3).

Here, the correction by the equation (3) will be described in detail. First, the inverse function g of the already obtained function g
^{Using -1} , the above-mentioned ε _K and ε ₀ previously obtained are used to obtain Δt as follows. It should be noted that this Δt is the time (return time) elapsed between the current k-th scan and the previous k−1-th scan. Δt = g ⁻¹ ((1−ε _k / ε ₀ ) − (1−ε _k−1 / ε ₀ )) (4) Then, the sensitivity deterioration occurs for each view and each channel in this k-th scan. And deterioration rate d in consideration of sensitivity recovery
Find i, j, k. Here, k is the scan number, j is the number of views (1 to n), and i is the channel number.

First, the sensitivity recovery rate gi corresponding to the deterioration rate di, j, k-1 and the recovery time .DELTA.t in the previous (k-1) th scan, which is already stored in the calibration processing unit 7.
The initial value d0i, k of the deterioration rate in the kth scan is calculated in consideration of (Δt). d0i, k = di, j, k-1 + gi (Δt) (5) Then, with reference to the initial value d0i, k of the deterioration rate in the kth scan obtained by the equation (5), Deterioration rate caused by scanning is the sensitivity deterioration characteristic function f
In consideration of the above, the deterioration rate di, j, k of each view in the kth scan is obtained for each channel. di, j, k = d0i, k + fi (d0i, k, ΣIi, m, k, A) (6) In this equation (6), m is 1 to j as the number of views.
Is calculated by.

The value thus obtained by the equation (6) is the deterioration rate of the i channel in the j view in the kth scan. Each value ε _k , di, j, k obtained by this data correction process is used for the data correction process of the next k + 1-th scan, and is therefore stored in the storage unit of the calibration processing unit 7.

Then, by using the value obtained by the above equation (6), the measurement value of the detection element of each channel is corrected in consideration of sensitivity deterioration and sensitivity recovery. That is, if the above equation (3) is rewritten, Ii, j, k '= Ii, j, k x1 / (1-di, j, k) (7) This correction process is executed for each view. I showed an example
Depending on the accuracy of the correction, the timing for calculating the correction parameter may be roughened and the parameter may be calculated for each scan to perform the correction.

In this correction process, the data is corrected by obtaining both the deterioration characteristic and the restoration characteristic, but it is also possible to obtain and correct either one.
For example, when scanning is performed in a short time, the deterioration rate increases but the recovery rate is small. Therefore, it is possible to obtain and correct only the deterioration rate. Further, in the case of scanning with a low radiation dose at long intervals, the recovery rate becomes large but the deterioration rate is small. Therefore, it is possible to obtain and correct only the deterioration rate.

Image reconstruction (step in FIG. 1): The image processing unit 9 executes image reconstruction by using the X-ray transmission amount data corrected as described above, and displays the image on the display unit 10. .

<Effects Obtained by Method and Apparatus for Improving Characteristics of X-ray Detector: Comparison with Conventional Example> As described above, X-rays for correction using two types of reference detecting elements for sensitivity changes According to the detector characteristic improving method and the X-ray CT apparatus, the following effects can be obtained.

By taking into account the sensitivity deterioration of the detection element of each channel due to the X-ray irradiation and the sensitivity recovery according to the elapsed time from the immediately preceding scan, the sensitivity deterioration rate at each scan is obtained and corrected. , X-rays can be accurately detected. When the sensitivity recovery of the above-mentioned measuring detection element is calculated, the recovery time Δt is applied to the sensitivity recovery characteristic and the sensitivity recovery characteristic is calculated. When obtaining this recovery time Δt,
Measurement value DATA of the reference detection element that causes sensitivity deterioration
It can be obtained from (ref_1), but the output of the X-ray tube 1 needs to be constant. Therefore, the measured value D simultaneously measured in the detection element ref_2 that does not cause sensitivity deterioration
The measured value DATA (ref_1) is normalized by ATA (ref_2) to obtain Δt. By setting the measured value to a normalized value (normalized reference sensitivity) in this way, the output fluctuation of the X-ray tube 1 at the timing of each measurement is not affected. Therefore, even if an output variation occurs in the X-ray tube 1 at each timing for obtaining the restoration time Δt, it is not affected by the variation and the restoration time Δt can be accurately obtained. As a result, it is not necessary to continuously measure the time from the previous scan to the current scan, and the sensitivity recovery rate can be accurately obtained from the calculated recovery time Δt.

Therefore, the sensitivity deterioration rate and the sensitivity recovery rate can be accurately obtained and corrected. <Other Preferred Examples> In addition to the above description of each embodiment, the following modified examples are also possible.

In addition to the sensitivity characteristics of the detector described above, leak count, offset drift, and afterglow having a long time constant can be corrected by the same configuration as described above.

Here, the leak count is the count due to the detection element side in the count measured without X-ray irradiation. Offset drift is a phenomenon in which the count due to noise, which is always included in a device that converts data into digital data, fluctuates (drifts) due to external temperature or the like. The afterglow is a phenomenon in which the count does not become 0 when the X-ray irradiation is turned off, but gradually decreases due to the time constant when the detection element is turned on / off.

These cases are realized by using two types of reference detecting means having different characteristics to be corrected and executing the calibration process and the correction process. For example, in the case where the above-mentioned leak count characteristic deteriorates in accordance with the X-ray irradiation amount or returns in accordance with the return time, two types of characteristics are obtained in the same manner as the above embodiment. Different reference detecting means are provided.

In this case as well, the characteristic of deterioration is obtained,
In consideration of the deterioration of the detection means of each channel and the restoration according to the elapsed time from the immediately preceding scan, the correction is performed with reference to the detection means having a better deterioration rate characteristic at each scan as a reference. Can be accurately detected.

Further, when the return time Δt is obtained, the measurement values of the two kinds of detecting means are normalized to obtain no influence of the output fluctuation of the X-ray tube 1 at each measurement timing.

[0084]

According to the method of improving the characteristics of the X-ray detector and the X-ray CT apparatus described in detail above, the sensitivity deterioration characteristics of the detection element of each channel due to the X-ray irradiation or the elapsed time from the immediately preceding scan is determined. In consideration of at least one of the sensitivity recovery characteristics, the X-ray irradiation amount at each scan or the recovery time from the immediately preceding scan is used as a parameter to obtain one or both of the sensitivity deterioration rate and the sensitivity recovery rate and perform the correction. Therefore, it is possible to perform accurate and easy measurement with correction for sensitivity deterioration or sensitivity recovery.

Further, according to the method for improving the characteristics of the X-ray detector and the X-ray CT apparatus which use the first and second types of reference detection elements having different characteristics described above in detail, the detection element of each channel is provided. In consideration of at least one of the sensitivity deterioration characteristic due to the X-ray irradiation and the sensitivity restoration characteristic according to the elapsed time from the immediately previous scan, X at each scan is considered.
In the X-ray detector characteristic improving method and the X-ray CT apparatus which perform correction by obtaining the sensitivity deterioration rate using the radiation dose or the recovery time from the immediately preceding scan as a parameter, the sensitivity deterioration characteristic has X
Sensitivity deterioration is obtained by applying the parameters related to the line irradiation, and the value obtained by normalizing the measurement value of the reference detection element with sensitivity deterioration with the measurement value of the reference detection element without sensitivity deterioration is applied to the sensitivity recovery characteristic. Then, the recovery time is calculated, and the sensitivity recovery for each channel is calculated from this recovery time. By doing so, accurate and easy measurement in which sensitivity deterioration or sensitivity recovery is corrected can be performed without being affected by the output fluctuation of the X-ray tube during each measurement.

Further, according to the method for improving the characteristics of the X-ray detector and the X-ray CT apparatus described in detail above, the deterioration characteristics other than the sensitivity due to the X-ray irradiation of the detection element of each channel or the elapsed time from the immediately preceding scan. In consideration of at least one of the recovery characteristics other than the sensitivity, the deterioration rate or the recovery rate or both are determined using the X-ray irradiation amount at each scan or the recovery time from the immediately preceding scan as a parameter for correction. By performing the measurement, accurate and easy measurement in which deterioration or restoration of characteristics is corrected becomes possible.

[Brief description of drawings]

FIG. 1 is a flowchart showing a procedure of a method for 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 according to an embodiment of the present invention.

FIG. 4 is a characteristic diagram showing an example of characteristics of an X-ray detector used in an embodiment of the present invention.

FIG. 5 is a characteristic diagram showing an example of characteristics of an X-ray detector used in one embodiment of the present invention.

FIG. 6 is a configuration diagram showing a configuration example of a main part of a conventional X-ray CT apparatus.

[Explanation of symbols]

 1 X-ray tube 2 Collimator 3 Image reconstruction area 5 X-ray detector 6 Data acquisition unit 7 Calibration processing unit 8 Data correction unit 9 Image processing unit 10 Display unit 11 Control unit

Front Page Continuation (72) Inventor Masatake Nakai 127 GE Yokogawa Medical Systems Co., Ltd. 4-7 Asahigaoka, Hino City, Tokyo

Claims (4)

[Claims] 1. A first type of detection element, the sensitivity of which changes when exposed to X-rays, is provided in a plurality of channels for image reconstruction, and
A method for improving the characteristics of an X-ray detector equipped with a detection element of a second type whose sensitivity does not change when exposed to radiation, as a reference detection element, and the sensitivity of the detection element of the second type is referred to. For the sensitivity of the detection element of each channel, refer to the function of the deterioration rate of the sensitivity that occurs according to the exposure dose, and the sensitivity of the detection element of the second type, and the sensitivity of the detection element of each channel from the X-ray irradiation. At least one of the function of the recovery rate of sensitivity generated according to time is obtained, and the data of each channel is collected by scanning, and the deterioration rate of sensitivity according to the exposure dose by scanning and Of at least one of the sensitivity recovery rate according to the elapsed time immediately before the X-ray irradiation calculated from the inverse function of the recovery rate using the above data, and the deterioration rate and the recovery rate thus obtained When On the other hand, or characteristic improving method of the X-ray detector and corrects the data of each channel collected in scan using both. 2. An X having a plurality of channels of a first type detecting element whose sensitivity is changed by X-ray irradiation for image reconstruction.
In a line CT apparatus, referring to the sensitivity of the second type of reference detection element and the sensitivity of the second type of detection element whose sensitivity does not change upon receiving X-ray irradiation, the sensitivity of the detection element of each channel With reference to the function of the sensitivity deterioration rate that occurs according to the amount and the sensitivity of the second type of detection element, the sensitivity recovery rate that occurs according to the time from X-ray irradiation for the sensitivity of the detection element of each channel Of at least one of the above functions, the deterioration rate of sensitivity according to the exposure dose due to the scan, and the X-ray irradiation immediately before calculated from the inverse function of the recovery rate using the data at each scan. From at least one of the recovery rate of the sensitivity according to the elapsed time from the from the function, each of the char collected by scanning using one or both of the deterioration rate and the recovery rate thus obtained X-ray CT apparatus characterized by comprising a data correcting means for correcting the Le data. 3. An X provided with a plurality of channels of a first type detecting element whose sensitivity is changed by receiving X-ray irradiation for image reconstruction.
In the X-ray CT apparatus, a first type reference detection element whose sensitivity changes upon receiving X-ray irradiation, a second type reference detection element whose sensitivity does not change upon receiving X-ray irradiation, and a scan The normalized reference sensitivity obtained by normalizing the sensitivity of the detection element for the first type of the reference with the sensitivity of the detection element for the second type of the reference is obtained, and the sensitivity of the detection element of the second type is referred to. For the sensitivity of the detection element of each channel, the deterioration rate of the sensitivity generated according to the exposure dose is obtained as a function, and the sensitivity of the detection element of each channel is referred to by X-ray irradiation with reference to the sensitivity of the second type of detection element. The recovery rate of the sensitivity generated according to the time from is obtained as a function, and the normalized reference sensitivity obtained by normalizing the sensitivity of the first type detection element with the sensitivity of the second type detection element during scanning is obtained. The required calibration method and scanning The deterioration rate of sensitivity according to the exposure dose is calculated from the above function, and the elapsed time from immediately before the X-ray irradiation is calculated from the inverse function of the recovery rate by using the initial value of the normalized reference sensitivity and the value at each scan. The X-ray is provided with data correction means for calculating the recovery rate of sensitivity according to the elapsed time from the function and correcting the data of each channel collected by scanning with the deterioration rate and the recovery rate. CT device. 4. An X-ray CT apparatus provided with a plurality of channels for image reconstruction of a first type of detecting means whose characteristics other than sensitivity change upon receiving X-ray irradiation. With reference to the characteristics of the second type of detection means that does not change and the characteristics of the second type of detection means, the deterioration rate of the characteristics of the characteristics of the detection means of each channel that occurs according to the exposure dose And a function of the recovery rate of the characteristic generated in response to the time from X-ray irradiation for the characteristic of the detecting means of each channel with reference to the characteristic of the second type detecting means. Calibration means for obtaining the function, the deterioration rate of the characteristics according to the exposure dose by scanning,
From the function, at least one of the recovery rate of the characteristic according to the elapsed time from the immediately preceding X-ray irradiation calculated from the inverse function of the recovery rate using the data at each scan,
X is provided with data correction means for correcting the data of each channel collected by scanning using one or both of the deterioration rate and the recovery rate thus obtained.
X-ray CT equipment.