JP2005319152A - X-ray ct apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an X-ray CT apparatus which can reduce or eliminate an artifact caused by an afterglow of a detector and can prevent a decline of time resolution with high speed and accuracy corresponding to a photographing condition. <P>SOLUTION: The X-ray CT apparatus includes an X-ray source to irradiate a subject with X-rays, an X-ray detector which is comprised of two or more X-ray detecting elements to convert the X-rays which penetrate the subject to an electric signal, a correction means (301) to apply a correction processing on an output of the above X-ray detector, and an arithmetic processing means (302) to apply a reconstructive arithmetic processing on an output of the correction processing, wherein the correction means (301) has an afterglow correcting means which applies the correction of the afterglow on the output of the X-ray detector on the basis of an afterglow responding characteristic which exhibits the characteristic of the afterglow of the X-ray detector and an afterglow responding characteristic altering means which alters the afterglow responding characteristic on the basis of the photographing condition. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an X-ray CT apparatus.

  Currently, solid-state detectors, which have high sensitivity and can be miniaturized, are becoming mainstream as detectors for X-ray CT apparatuses. However, the solid-state detector has a phenomenon (afterglow) in which the image taken at the previous time remains in the projected image taken at the next time due to afterglow when X-rays are converted into light by the scintillator. Degradation of image quality due to the resulting artifacts and a decrease in temporal resolution occur in the reconstructed image. Several methods have been proposed for correcting the afterglow.

  One method is to correct the afterglow component with the shortest time constant at any time (see, for example, Patent Document 1), or from the superposition integration of the response characteristics of the X-ray detector and the corrected output data. A method of correcting the obtained afterglow amount by difference (for example, see Patent Document 2), a method of expressing response characteristics by a plurality of components having different time constants, and performing correction corresponding to each time constant for each component (for example, Patent Document 3).

  In order to correct this afterglow, it is necessary to determine the afterglow response function of the X-ray detector. This afterglow has a plurality of components, and each component can be expressed by a time constant and a component ratio. For this reason, the response function is written as the sum of a plurality of time constants and component ratios. Several methods have been proposed for determining the time constant and component ratio of the afterglow response function.

  As one of the methods, a step response function obtained by superimposing a plurality of impulse response functions having a plurality of time constants is fitted to an actually measured step response of the detector, and the error is minimized (for example, Patent Document 4), a method of using a genetic algorithm to minimize the error (see, for example, Patent Document 5), a response function represented by the sum of exponential functions, and adding components with different time constants There is a method of sequential optimization (see, for example, Patent Document 6).

Japanese Patent Laid-Open No. 4-300527

JP-A-6-343629 JP-A-7-090024 JP 2003-061945 A JP 2003-061945 A JP 2001-309915 A

  The afterglow has a problem in that the reconstructed image causes degradation in image quality due to a decrease in position resolution and artifacts.

  Also, afterglow components that affect image quality may differ depending on the shooting conditions. For example, an afterglow component with a short time constant causes a decrease in position resolution when photographing fine objects with a large absorption coefficient, such as bone, but this component is not necessary when high position resolution such as the abdomen is required. There is a possibility that afterglow components need not be removed until the SNR is reduced by correction. As another case, for example, when there are a plurality of shooting modes in which the shooting time differs due to different projection image acquisition cycles, the afterglow components that affect the image quality are different in each mode. Similarly, the afterglow component to be corrected may change when the imaging conditions such as the X-ray irradiation time and the type of subject are different. Further, when correction is performed by determining the afterglow response function, there is a problem that if the number of components increases, a longer time is required for correction.

  In addition, since the afterglow is a linear sum with respect to the component ratio, the calculation can be performed in a relatively short time. On the other hand, since the time constant is a non-linear sum, the calculation generally takes time. The calculation time for this time constant is explosive if calculation is performed on all X-ray detection elements because the number of X-ray detection elements has increased explosively since the current detectors for X-ray CT have increased in number. There is a problem that it increases continuously.

  Accordingly, an object of the present invention is to provide an X-ray CT apparatus capable of performing high-speed and high-accuracy in accordance with imaging conditions to reduce and eliminate artifacts caused by the afterglow of the detector and to prevent a decrease in time resolution. There is to do.

  In order to achieve the above object, the present invention has the following characteristics.

  (1) An X-ray source that irradiates a subject with X-rays, an X-ray detector that includes a plurality of X-ray detection elements that convert the X-rays transmitted through the subject into electrical signals, and an output of the X-ray detector In an X-ray CT apparatus for photographing a cross-sectional image of the subject, the correction means includes a correction means that performs correction processing on the image and a calculation processing means that performs reconstruction calculation processing on the output of the correction processing. Is based on afterglow response characteristics representing afterglow characteristics of the X-ray detector, and afterglow correction means for correcting the afterglow with respect to the output of the X-ray detector, And an afterglow response characteristic changing means for changing the afterglow response characteristic.

  (2) In the X-ray CT apparatus according to (1), the correction unit divides the afterglow characteristics of the X-ray detector obtained from the measured data into a plurality of components having a time constant and a component ratio. A component determination function, and the afterglow response characteristic is determined by using a plurality of time constant sets consisting of all or part of the time constant determined by the component determination function, and a time constant of the time constant set. The afterglow response characteristic is changed by changing the time constant of the time constant set according to the photographing condition.

  (3) In the X-ray CT apparatus according to (1), the correction means includes the imaging conditions including imaging time, X-ray intensity, X-ray detector signal readout cycle, subject type, and subject imaging region. It has a plurality of shooting modes that differ according to at least one shooting condition, and has a function of changing afterglow response characteristics according to the shooting mode.

  (4) In the X-ray CT apparatus of (1), the correction unit outputs the output of the X-ray detector to the arithmetic processing unit without correcting the afterglow under at least one imaging condition. It is characterized by that.

  (5) In the X-ray CT apparatus according to (2), the X-ray detection elements are two-dimensionally arranged in a row direction and a column direction, and a part of the row direction or the column direction, or a part of the region. And means for determining a time constant of the set of time constants using the distribution of the time constants obtained from the response characteristic data of the X-ray detection element belonging to the above.

  According to the present invention, it is possible to realize an X-ray CT apparatus capable of performing high-speed and high-precision in accordance with imaging conditions in order to reduce and eliminate artifacts caused by the afterglow of the detector and to prevent a reduction in time resolution. Can do.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

Example 1
The first embodiment of the present invention will be described below with reference to FIGS.

  FIG. 1 is a diagram showing an embodiment of the X-ray CT apparatus of the present invention including afterglow correcting means. FIG. 2 is a diagram for explaining a configuration example of the X-ray detector in the X-ray CT apparatus of FIG. FIG. 3 is a diagram for explaining an example of the configuration of the central processing means in the X-ray CT apparatus of FIG. FIG. 4 is a diagram for explaining an example of a method of afterglow correction processing performed by the correction unit shown in FIG. FIG. 5 is a diagram for explaining an example of a method for determining the time constant and component ratio of the afterglow used in FIG. FIG. 6 is a diagram for explaining the step response characteristics for measurement and storage in FIG. 5 and the impulse response characteristics for estimation. FIG. 7 is a diagram for explaining an example of a method for determining a time constant of afterglow used for each photographing condition. FIG. 8 is a diagram for explaining an example of a phantom used in the phantom shooting of FIG.

  As shown in FIG. 1, the basic configuration of the X-ray CT apparatus includes an X-ray tube 100 that emits X-rays, an X-ray detector 104 that detects X-rays and converts them into electrical signals, and an X-ray detector 104 A signal collecting unit 118 that collects a projection image, a central processing unit 105 that stores a signal (projection image) from the signal collecting unit 118 and performs image processing, a display unit 106 that displays a result of the image processing, a start of photographing, and parameter setting , An input means 119 for inputting, and a control means 117 for controlling the X-ray tube 100 and the X-ray detector 104. In the figure, 107 indicates the body axis direction (slice direction), and 108 indicates the rotation direction (channel direction) of the rotating body 101.

  The shooting procedure will be described with reference to FIG. When the start of imaging is input from the input unit 119, X-rays are emitted from the X-ray source 100 toward the subject 102 placed on the bed top plate 103. The X-ray passes through the subject 102 and is converted into an electric signal by the X-ray detector 104. This electric signal is AD converted by the signal acquisition circuit 118 to become a projected image. This imaging is repeatedly performed by rotating the rotating body 101 on which the X-ray tube 100 and the X-ray detector 104 are mounted to change the X-ray irradiation angle with respect to the subject to obtain a projection image of 360 degrees. . This projection image is taken every 0.4 degrees, for example. At this time, the control circuit 117 controls the rotation of the rotating body 101 and the reading of the X-ray detector 104. The projected image is subjected to image correction processing and reconstruction calculation by the central acquisition circuit 105. The result is displayed on the display means 106.

  A configuration example of the X-ray detector 104 in the X-ray CT apparatus of the present invention will be described with reference to FIG. A plurality of X-ray detectors 104 shown in FIG. 2 are arranged in an arc shape, and are arranged to face the X-ray tube 100 as shown in FIG. An X-ray detector 104 shown in FIG. 2 includes a scintillator element 112 that converts X-rays into light, a photodiode substrate (photoelectric conversion substrate) 111 on which a plurality of photodiodes that convert light into electrical signals are formed, It is composed of an electrode pad 120 for outputting a signal and a wiring board 113 having wiring for it. The scintillator element 112 and the photodiode substrate 111 are bonded by an optically transparent adhesive 310, and these are supported by the wiring substrate 113.

  A case where X-rays are incident on the X-ray detector 104 will be described with reference to FIG. The incident X-ray is converted into light by the scintillator element 112. The scintillator element 112 is divided by a separator 130. A photodiode that converts light into an electrical signal is provided on the photodiode substrate 111 corresponding to each scintillator element 112. The photodiode and the scintillator element 112 constitute an X-ray detection element, and an electric signal converted from light by the photodiode is output for each X-ray detection element. The electrode of the photodiode is electrically connected to the electrode pad 120. From this electrode pad 120, the electric signal generated by the X-ray is read out to the signal collecting means 118 shown in FIG. The electrical signals are collected for all the X-ray detection elements to form a projection image. This projection image is acquired at a certain time interval (ΔT), and a plurality of projection images are sequentially acquired. The Nth is called an Nth view projection image.

  Although a projected image is created in this way, not all signals generated by X-rays are read out as a projected image immediately after X-ray irradiation. That is, it has an afterglow phenomenon. For example, a part of the signal emitted immediately before the projection image of the Nth view is read after the projection image of the (N + 1) th view. The cause is a delay (afterglow) when the X-ray is converted into light by the scintillator and an unread reading in the circuit. Such afterglow causes afterimages in the projected images after the (N + 1) th view and causes artifacts in the reconstructed image.

A configuration example of the central processing unit 105 will be described with reference to FIG. The projected image input from the signal collecting unit 118 to the central processing unit 105 is first recorded in the data storage unit 300. Thereafter, the correction unit 301 performs image correction processing. The processing performed at this time is, for example, offset correction that excludes the output of the dark current of the detector, afterglow correction that corrects the influence of the afterglow of the detector, and correction of variations in X-ray distribution and detector sensitivity. Sensitivity correction, defect correction for correcting the output of defective pixels, and the like. Next, the reconstructing unit 302 reconstructs the cross-sectional image of the X-ray absorption coefficient distribution of the subject by applying convolution (convolution) and back projection (back projection) processing to the projection image. This cross-sectional image is displayed on the display means 106. Parameters used in the image correction processing and reconstruction processing, and correction data used in the image correction processing are stored in the parameter storage unit 303. These parameters can be changed by the parameter determination unit 330 with respect to the photographing condition input from the input unit 119.

  FIG. 4 is a diagram showing an example of a method of afterglow correction processing performed by the correction unit 301 shown in FIG. A case where afterglow correction processing is performed on the projection image of the Nth view will be described.

  Assume that afterglow consists of components with multiple time constants. The time constant of the i-th (i = 1,..., M) afterglow component is τi, and the component ratio is Ai. These determinations are made by the parameter determination means 330, and details thereof will be described with reference to FIG.

  As shown in FIG. 4, the correction data storage means 311 of the correction means 301 stores the projected image of the past view before the afterglow correction process and / or the corrected projection image and / or the afterglow amount data 320. . In addition to this, the attenuation rate 316 of the afterglow component i calculated from the time constant τi and the component ratio Ai stored in the parameter storage unit 303, the projection image 314 before the afterglow correction of the Nth view, and the like are used. The afterglow amount data 312 of the view eye is estimated. The N-th view afterglow amount data 312 is calculated into a projection image 314 before the N-th view afterglow correction to obtain a projection image 315 after the N-th view afterglow correction, and the reconstruction unit 302 receives the calculated image 315. Output.

  An example of a method for determining the afterglow time constant τi and the component ratio Ai (i = 1,..., M) will be described with reference to FIG. In order to determine these, first, a step response characteristic F (v) is measured 350 without providing a subject. The step response characteristic F (v) is a distribution of the relative intensity of the output of the detector obtained when the X-ray irradiation is stopped for a fixed time. v represents the vth view.

  FIG. 6A shows a step response characteristic F (v) measured when X-rays are irradiated from the 0 view to the N view. As shown in FIG. 6C, the step response characteristic F (v) is an impulse response characteristic f (v) obtained when only the n-th view as shown in FIG. 6B is irradiated with X-rays. The amount obtained by convolution integration is the amount generated from the 0 view to the N view.

  As shown in FIG. 5, F (v) obtained by the measurement 350 of the step response characteristic F (v) is stored in the data storage unit 300. Next, using this F (v), the parameter determination means 330 estimates the impulse response characteristic f (v) 352. In this estimation 352, for example, a fitting function F ′ obtained by convolving and integrating the impulse response function f ′ (v) indicated by the sum of the components of the afterglow as shown in (Equation 1) by N views from 0 view. By fitting (v) to the step function F (v) stored in the data storage means 300, the afterglow time constant τi and the component ratio Ai (i = 1,..., M) are estimated.

ここで、ΔＴは上記のようにビュー間隔の時間である。このフィッティングには、例えば最小２乗法を用いる。次に、決定したパラメータのうち時定数τiのみ、アフターグロウ時定数τiの記憶３５３にて記憶する。

Here, ΔT is the time of the view interval as described above. For this fitting, for example, the least square method is used. Next, of the determined parameters, only the time constant τ i is stored in the afterglow time constant τ i storage 353.

  Next, when the shooting condition determination 357 is performed from the input unit 119, a set of time constants used for afterglow correction for each shooting condition from the stored time constants τi (i = 1,..., M). Decision 354 is made. The correspondence between the shooting conditions and the time constant set is determined by prior evaluation. The time constant selected in the time constant set can be from one component to all M components.

  Next, the impulse response f (v) is re-estimated 355 using the time constant τi of the time constant set, and the component ratio Ai of the afterglow component i is determined. Here, for example, the component ratio Ai is determined by the method of least squares. At this time, since the fitting function F ′ (v) is linearly coupled to the component ratio Ai, the time constant τi to be nonlinearly coupled is determined. The estimation can be performed in a short time compared to. The time constant τ i and the component ratio A i selected in this way are stored in the parameter storage means 303 and used in the afterglow correction performed in the correction means 301 as shown in FIG. Such processing is performed for each X-ray detection element.

  By such a method, afterglow correction can be performed according to the photographing conditions. Further, by correcting only the necessary afterglow component, it is possible to correct at high speed and with high accuracy.

  FIG. 7 is an explanatory diagram illustrating an example of an embodiment of a method for determining a time constant of an afterglow used for each imaging condition. Here, the imaging conditions are a certain fixed imaging time, X-ray intensity, and signal readout cycle of the X-ray detector, and imaging is performed for the case where the imaging conditions differ depending on the type of subject and the imaging location of the subject. This is a case where a mode is provided and a set of time constants of components to be subjected to afterglow correction performed in each photographing mode is determined.

  First, before actually photographing a subject for diagnosis, a phantom image 400 corresponding to a region to be imaged is performed. Here, the phantom corresponding to the imaging region is not limited to a sample of a human or animal region, but may be a simulated one. For example, in the abdominal imaging mode, a phantom in which water is placed in a cylindrical container having a diameter of 10 cm, 20 cm, 30 cm, 40 cm, or the like, each simulating the size of a subject. For example, assuming a bone part, a tomographic plane for creating a reconstructed image is a cylinder made of a material having a relatively high CT value from the center of a cylindrical container containing water, as shown in FIG. A phantom 455 in which samples are arranged may be used. Here, in the phantom 455 shown in FIG. 8, a sample 450, a sample 451, and a sample 452 having different diameters and a sample 453 having a CT value different from these are provided.

  As shown in FIG. 7, afterglow correction is performed on the projected image of the phantom by dividing it into two paths. One of the paths is a case where m time constants are selected 401 as components for performing afterglow correction. Here, for the m time constants, all the components of the afterglow obtained using FIG. 5 at first (M) are used. Next, the component ratio is determined 402, the afterglow correction 403 is performed on the previously captured phantom projection image, and a reconstructed image is created 404. The other path is to select 410 (m−1) time constants obtained by removing one component i from m components, and similarly determine the component ratio 411, perform afterglow correction 412 and create a reconstructed image. 413 is performed. This reconstructed image is created when the excluded component i is all components. Next, a difference image is created 420 from these reconstructed images, and a standard deviation SD ′ and a maximum value MAX are calculated 422. In these calculations, a value in a specific area is calculated. For example, in the phantom illustrated in FIG. 8, calculation is performed in a plurality of regions including each of the sample 450, the sample 451, the sample 452, and the sample 453. Further, the standard deviation SD is calculated 421 from the same region of the reconstructed image created in the reconstructed image creation 404.

  Next, the standard deviation SD, SD ′ and the maximum value MAX are processed under conditions such as the condition 423, for example. In this process, even if one component is YES, the time constant deletion 424 that satisfies the condition is performed, and the process is similarly performed from the beginning with the reduced number of components. Here, a and b are certain constants. If all of the conditions 423 are NO, a set of time constants 425 corresponding to the imaging region is determined, and these time steps are stored in the parameter storage unit 303. When the time constant is different for each region where the standard deviation SD is calculated, all the time constants included in one of them are stored as time constants corresponding to the imaging region.

  In this method, the photographing modes are provided as photographing conditions only by the type of subject and the photographing part of the subject, but the present invention is not limited to this. An imaging mode is provided under different conditions with respect to imaging time, X-ray intensity, and signal readout cycle of the X-ray detector, and a component for performing afterglow correction is determined by a method as shown in FIG. 7 for each condition. It doesn't matter. In addition, the photographer selects a set of time constants at the time of imaging in diagnosis from one or more sets of time constants determined by the method of FIG. The time constant may be determined by repeatedly performing afterglow correction on the subject and confirming the resulting image.

  Further, afterglow correction is not necessarily performed for all shooting modes. In a shooting mode that does not require afterglow correction, afterglow correction may be performed in at least one necessary shooting mode without performing afterglow correction. This can be considered that there is a shooting mode in which 0 time constants are determined in the time constant set.

(Example 2)
The second embodiment of the present invention differs from the first embodiment in the method of determining the afterglow time constant τi and the component ratio Ai (i = 1,..., M). FIG. 9 shows an explanatory diagram of the determination method in the X-ray CT apparatus in the second embodiment, and FIG. 10 shows an example of a specific method for determining the afterglow time constant from the estimation of the impulse f (v). Show.

  9 differs from the method of the first embodiment shown in FIG. 5 in the method of estimating 352 of the impulse response characteristic f (v). In the method shown in FIG. 5, the estimation 352 is performed for each X-ray detection element. However, in the method shown in FIG. 9, a specific X-ray detection element, in particular, a specific slice (row) (L The estimation is performed using only the data of the X-ray detection element (slice). The time constant obtained in this way is used as the time constant of all X-ray detection elements. This is because the afterglow component is caused by factors such as the afterglow in the scintillator and the unreadness in the circuit as described above, and it is considered that there are many X-ray detection elements whose time constants are approximately the same. Because it is.

  Such a process is particularly advantageous when the number of X-ray detection elements is large, such as a multi-slice two-dimensional X-ray detector. The estimation 352 has a particularly large processing amount due to nonlinear parameter fitting of a function as shown in (Equation 1). For this reason, it is effective to determine the time constant from a specific X-ray detection element, which greatly reduces the processing amount.

  FIG. 10 is a diagram for explaining an example of a specific method for determining the afterglow time constant 353 from the estimation 352 of the impulse response characteristic f (v) shown in FIG. First, as described above, the estimation 352 is performed only on the response characteristic data of the X-ray detection element of the Lth slice. Next, the time constants obtained in this way are totaled, and a histogram 520 of the afterglow time constant is created. In this histogram, the horizontal axis represents a time constant, and the vertical axis represents the number of X-ray detection elements having the time constant. A time constant 521 common to the number of X-ray detection elements is estimated from this histogram, and this is stored as a time constant common to all X-ray detection elements.

  In this method, afterglow possessed only by a specific X-ray detection element is not estimated. For this reason, there may be a case where the effect of the afterglow is not removed by the afterglow correction, and an artifact occurs in the reconstructed image. In such a case, only a specific X-ray detection element is provided with a time constant other than a common time constant and stored, or estimation 352 is performed separately from other pixels to estimate the afterglow time constant. And apply the method of memorizing.

  By such a method, the afterglow time constant can be determined efficiently by reducing the amount of processing.

  In this embodiment, the histogram creation 520 is performed using the response characteristics of the X-ray detection element of a specific slice, but the present invention is not limited to this. There may be a case where the response characteristic of the X-ray detection element common to a specific channel (column) direction orthogonal to the slice is used, or a case where the response characteristic of the X-ray detection element in a specific region is used.

  The present invention is not limited to the above-described embodiments, and is effective for an X-ray CT apparatus equipped with an X-ray detector having a multi-component afterglow. For example, the present invention can be applied to an X-ray CT apparatus equipped with an X-ray detector that directly converts X-rays into electrical signals.

  Further, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention at the stage of implementation. Further, the above-described embodiments include various stages, and various embodiments can be extracted by appropriately combining a plurality of disclosed components. For example, a configuration in which some components are deleted from all the components shown in the above-described embodiment may be adopted.

  As described above, according to the present invention, X-ray CT that can reduce and remove artifacts caused by the afterglow of the detector and prevent the deterioration of temporal resolution at high speed and with high accuracy according to the imaging conditions. A device can be realized. In addition, an X-ray CT apparatus having a function capable of preventing the components used for the afterglow correction from increasing more than necessary and selecting and changing the components that need to be corrected for each imaging can be realized.

The figure explaining the structure of the X-ray CT apparatus of 1st Example of this invention. The figure explaining the example of 1 structure of the X-ray detector in the X-ray CT apparatus of FIG. The figure explaining the example of 1 structure of the central processing means in the X-ray CT apparatus of FIG. The figure explaining an example of the method of the afterglow correction | amendment process performed by the correction | amendment means shown in FIG. The figure explaining an example of the determination method of the time constant and component ratio of the afterglow used in FIG. The figure explaining the step response characteristic which measures and memorize | stores in FIG. 5, and the impulse response characteristic which estimates. Explanatory drawing for demonstrating an example of the determination method of the time constant of the afterglow used for every imaging | photography condition. The figure explaining an example of the phantom used by the phantom imaging | photography of FIG. The figure explaining an example of the determination method of the time constant and component ratio of the afterglow in a 2nd Example. The figure explaining an example of the concrete method of determining an afterglow time constant from estimation of the impulse f (v) in a 2nd Example.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 100 ... X-ray source, 101 ... Rotating body, 102 ... Subject, 103 ... Bed top plate, 104 ... X-ray detector, 105 ... Central processing unit, 106 ... Display device, 107 ... Rotation axis direction, slice direction, 108 ... Rotation direction, channel direction, 111 ... photoelectric conversion substrate, 112 ... scintillator element, 113 ... wiring substrate, 117 ... control circuit, 118 ... signal collecting means, 119 ... input means, 120 ... electrode pad, 130 ... separator, 300 ... data Storage means 301 ... Correction means 302 ... Reconstruction means 303 ... Parameter storage means 310 ... Adhesive 311 ... Correction data storage means 312 ... N-view eye afterglow amount data 314 ... N-view eye afterglow correction Previous data, 315... N-th view afterglow corrected data, 316... Afterglow component i attenuation rate, 320 ... Previous view after-grow corrected / after-corrected projection image / after-grow amount data, 330 ... parameter determining means, 350 ... measurement of step response F (v), 351 ... storage of step response characteristic F (v), 352 ... impulse response characteristic f (v) estimation, 353 ... afterglow time constant τi (i = 1, ..., M), 354 ... determination of time constant τi used for afterglow correction, 355 ... time constant τi Re-estimation of impulse response characteristics f (v), 356... Time constant τi, storage of component ratio Ai (i = 1,..., M), 400 ... phantom imaging according to imaging region, 401. Selection (m), 402, 411 ... Determination of component ratio Ai, 403, 412 ... Afterglow correction, 404, 413 ... Reconstructed image creation, 410 ... Selection of time constant (excluding τi component (m- 1) pieces), 420 ... difference Creation of image, 421 ... calculation of standard deviation SD, 422 ... calculation of standard deviation SD ', calculation of maximum value MAX, 423 ... condition, 424 ... removal of time constant that also satisfies the condition, 425 ... time according to imaging region Determination of constant set, 450, 451, 452, 453 ... sample, 455 ... phantom, 520 ... afterglow time constant histogram creation, 521 ... estimation of common time constant from histogram.

Claims (5)

  An X-ray source that irradiates the subject with X-rays, an X-ray detector comprising a plurality of X-ray detection elements that convert the X-rays transmitted through the subject into electrical signals, and an output of the X-ray detector In an X-ray CT apparatus that includes a correction unit that performs correction processing and a calculation processing unit that performs reconstruction calculation processing on the output of the correction processing, the correction unit includes: Based on afterglow response characteristics representing the afterglow characteristics of the X-ray detector, afterglow correction means for correcting the afterglow with respect to the output of the X-ray detector, and the afterglow response depending on imaging conditions An X-ray CT apparatus comprising afterglow response characteristic changing means for changing characteristics.   2. The X-ray CT apparatus according to claim 1, wherein the correction unit determines a component for dividing the afterglow characteristic of the X-ray detector obtained from actually measured data into a plurality of components having a time constant and a component ratio. And the afterglow response characteristic is determined using a plurality of time constant sets consisting of all or part of the time constants determined by the component determination function and the time constants of the time constant sets. And an afterglow response characteristic is changed by changing a time constant of the time constant set according to the imaging condition.   2. The X-ray CT apparatus according to claim 1, wherein the correction unit includes at least one of the imaging conditions of imaging time, X-ray intensity, X-ray detector signal readout cycle, subject type, and subject imaging region. An X-ray CT apparatus having a plurality of imaging modes depending on one imaging condition and having a function of changing afterglow response characteristics depending on the imaging mode.   2. The X-ray CT apparatus according to claim 1, wherein the correction unit outputs the output of the X-ray detector to the arithmetic processing unit without correcting the afterglow under at least one imaging condition. X-ray CT apparatus that is characterized.   The X-ray CT apparatus according to claim 2, wherein the X-ray detection elements are two-dimensionally arranged in a row direction and a column direction, and belong to a part of the row direction or the column direction or a part of a region. An X-ray CT apparatus comprising means for determining a time constant of the set of time constants using a distribution of the time constant obtained from response characteristic data of the X-ray detection element.

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