JP2021058418A - Image processing apparatus, image processing method, radiographic apparatus, and program - Google Patents

Abstract

To provide a technology to enable more accurate afterimage correction by taking into account a change in an amount of radiation entering a radiographic apparatus.SOLUTION: An image processing apparatus for processing radiation image data radiographed at a predetermined frame rate includes: a calculation unit for calculating an amount of change and a time change indicating whether it is positive or negative on the radiation image data; a determination unit for determining afterimage characteristics in the radiation image data on the basis of the time change calculated by the calculation unit; an estimation unit for estimating an afterimage amount contained in the radiation image data on the basis of the afterimage characteristics determined by the determination unit; and a correction unit for subjecting the radiation image data to afterimage correction on the basis of the afterimage amount estimated by the estimation unit.SELECTED DRAWING: Figure 2

Description

The present invention relates to an image processing apparatus and an image processing method for processing a radiographic image, a radiographic imaging apparatus, and a program.

In recent years, as an image pickup device used for medical image diagnosis and non-destructive inspection by radiation, a radiation image pickup device using a plane detector configured by arranging solid-state image sensors made of amorphous silicon or single crystal silicon in a two-dimensional manner has become widespread. It has been put to practical use. In such a radiography apparatus, an afterimage may occur when radiation is incident on the device. When an afterimage occurs, the afterimage component is added to the image data captured by the radiography apparatus and output, so that information incorrect for the radiological imaging conditions is acquired.

In order to remove the generated afterimage, image correction processing using information based on the offset signal is performed. The offset data is image data output from the radiography apparatus when no radiation is incident, and is called a dark image. However, the afterimage has the property that the amount of afterimage artifacts decreases with the passage of time. Patent Documents 1 and 2 propose a technique for correcting an image by estimating the amount of afterimage artifacts that change with the passage of time. In Patent Document 1, it is assumed that the afterimage component is caused by an impulse response composed of a plurality of exponential functions, and the afterimage included in the radiation detection signal is removed by performing recursive arithmetic processing from the radiation detection signal. In Patent Document 2, attention is paid to a change in radiation imaging conditions, and afterimage correction is performed based on the pixel value of the image captured immediately before. In Patent Document 2, correction is performed particularly for a region having a large pixel value.

Japanese Unexamined Patent Publication No. 2006-006387 JP-A-2009-254660

However, when the shooting conditions change due to changes in shooting conditions, movement of the subject, and the like, and the amount of radiation incident on the radiography apparatus changes, the attenuation characteristics of the afterimage change. In Patent Document 1, when such a change occurs, it becomes difficult to accurately correct the afterimage. Further, the method of Patent Document 2 cannot cope with a sudden change in the amount of radiation incident on the radiographing apparatus.

Therefore, the present invention provides a technique that enables more accurate afterimage correction by considering a change in the amount of radiation incident on the radiography apparatus.

The radiography apparatus according to one aspect of the present invention has the following configurations. That is,
An image processing device that processes radiographic image data taken at a predetermined frame rate.
A calculation means for calculating the amount of change and the time change indicating the positive / negative of the radiation image data,
A determination means for determining the afterimage characteristic in the radiographic image data based on the time change calculated by the calculation means, and a determination means.
An estimation means for estimating the amount of afterimage contained in the radiographic image data based on the afterimage characteristics determined by the determination means, and an estimation means.
A correction means for correcting an afterimage on the radiographic image data based on the amount of afterimage estimated by the estimation means is provided.

According to the present invention, more accurate afterimage correction is possible by correcting the afterimage in consideration of the change in the amount of radiation incident on the radiography apparatus.

The block diagram which shows the structural example of the radiography apparatus which concerns on embodiment. The block diagram which shows the functional structure example of the data collection part and the afterimage correction part which concerns on embodiment. The figure which shows the afterimage with respect to the difference of the pixel value of the image data which concerns on embodiment. The flowchart which shows the processing of the radiography apparatus which concerns on embodiment.

Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. The following embodiments do not limit the invention according to the claims. Although a plurality of features are described in the embodiment, not all of the plurality of features are essential to the invention, and the plurality of features may be arbitrarily combined. Further, in the attached drawings, the same or similar configurations are designated by the same reference numbers, and duplicate explanations are omitted.

In the following embodiments, afterimages when the time change of the amount of radiation incident on the radiographing apparatus is positive (hereinafter referred to as rising afterimage) and afterimages when negative (hereinafter referred to as falling afterimage) are considered. Will be done. Further, in the embodiment, it is assumed that each of the rising afterimage and the falling afterimage is caused by an impulse response composed of a plurality of exponential functions. The radiation in the embodiment is, for example, X-ray.

FIG. 1 is a block diagram showing a configuration example of the radiography apparatus 100 according to the embodiment. The radiography apparatus 100 is used, for example, in a medical field. The irradiation unit 101 irradiates the subject (subject) with radiation. The radiation detection unit 102 detects radiation that has passed through the subject and is incident, and generates radiation image data. The tube control unit 103 controls the radiation dose emitted from the radiation irradiation unit 101. The shooting condition setting unit 104 sets shooting conditions such as the number of continuous irradiations of radiation radiated to the subject, the irradiation dose, and the frame rate according to the operation of the operator. The detector control unit 105 controls the radiation detection unit 102 and the tube control unit 103 based on the signal (imaging condition) output from the imaging condition setting unit 104.

The data collection unit 106 and the afterimage correction unit 107 are examples of image processing units that perform image processing for removing afterimages from radiographic image data. The data collection unit 106 holds the radiation image data acquired by the radiation detection unit 102. In addition, the data collection unit 106 calculates the time change of the radiation amount incident on the radiation detection unit 102. In the present embodiment, the difference between the radiation image data in the current frame and the radiation image data in the previous frame is calculated as a time change of the radiation amount. The difference includes the amount of change and information indicating whether the amount of change is increasing or decreasing (positive or negative). The afterimage correction unit 107 corrects the afterimage by removing the afterimage from the radiation image data held in the data collection unit 106. More specifically, the afterimage correction unit 107 estimates the afterimage based on the time change of the radiation amount and the afterimage characteristic, and corrects the radiation image data based on the estimated afterimage. Details of the data collection unit 106 and the afterimage correction unit 107 will be described later with reference to FIG. The image display unit 108 outputs the radiation image data corrected by the afterimage correction unit 107 to a monitor or the like.

Next, the data collection unit 106 and the afterimage correction unit 107 will be described in more detail. FIG. 2 is a block diagram showing a detailed configuration example of the data collection unit 106 and the afterimage correction unit 107.

In the data collection unit 106, the image holding unit 201 holds the radiation image data (radiation image taken at a predetermined frame rate) including the afterimage acquired by the radiation detection unit 102. The image holding unit 201 holds, in addition to the radiation image data of the current frame to be corrected for afterimages, the radiation image data of one or more frames before that. The change amount calculation unit 202 calculates the time change (change amount and its positive / negative) of the radiation amount incident on the radiation detection unit 102. More specifically, the change amount calculation unit 202 includes the radiation image data to be corrected for afterimages and the radiation image data in the frames before that among the radiation image data acquired by the radiation detection unit 102. Calculate the difference between pixel values (change amount and its positive / negative).

For example, the change amount calculation unit 202 calculates the pixel value (I (x, k)) of the radiation image data at the kth frame and the radiation image at the k-1th frame immediately before it in the calculation of the time change in the number of captured frames k. The difference ΔI (x, k) from the pixel value (I (x, k-1)) of the data is obtained. It should be noted that the radiation image data of the immediately preceding frame does not necessarily have to be used in the calculation of the difference. For example, the change amount calculation unit 202 sets the pixel value of the radiation image data at the kth frame and the pixel value of the radiation image data at the kk'frame (k'= 2, 3, 4 ...) Before that. Statistical processing may be performed by obtaining the difference. In the former method, the radiographic image data held by the image holding unit 201 only needs to be the immediately preceding one, and the calculation speed is increased. On the other hand, in the latter method, since the change in the pixel value with respect to a long time change can be seen, the influence of noise in the radiation image between frames can be reduced.

In the calculation of the difference between the pixel values of the radiation image data, the difference may be calculated for each pixel of the entire image, or the difference may be calculated for each pixel of one or more partial regions. .. Further, these differences may be retained for each frame and recursive processing may be performed according to the time change. For example, the recursive coefficient is α (value of 0 to 1), and the difference values in the k frame and k-1 frame are X _k and X _k-1 , respectively. _{In this case, the time change (Y k} ) in the current frame (k frame) is acquired by the calculation of Y _k = α × X _k + (1-α) × X _k-1. Due to the recursive processing, even when a sudden pixel value fluctuation occurs (for example, when the irradiation field / collimator aperture is changed suddenly), the pixel value difference changes smoothly over time, so afterimage correction is performed. It prevents sudden changes in brightness of the image.

The afterimage determination unit 203 determines the afterimage characteristic (the ratio t of the characteristic of the rising afterimage, which will be described later) in the radiographic image data based on the time change (ΔI (x, k)) calculated by the change amount calculation unit 202. .. This afterimage characteristic is used by the afterimage amount estimation unit 206 to estimate the afterimage amount. The afterimage determination unit 203 holds, for example, a change in the afterimage with respect to the difference in the pixel values of the radiation image data acquired by the radiation detection unit 102 as a data table, as shown in FIG. This data table measures the change in the afterimage with respect to the difference in the pixel values of the image data acquired by the radiation detection unit 102 in advance, and holds the correspondence between the difference in the pixel values and the afterimage as a table.

Further, in the present embodiment, the afterimage characteristic in the radiation image data of the current frame is determined by synthesizing and using the characteristic of the rising afterimage and the characteristic of the falling afterimage as described later. That is, the ratio of the composition of the characteristics of the rising afterimage and the characteristics of the falling afterimage is determined so that the relationship between the difference and the amount of afterimage is the relationship shown in FIG. In the present embodiment, the ratio t of the rising afterimage (hereinafter, simply referred to as the ratio t) is used as the ratio of the synthesis. In the data table, a ratio t in which the change in the afterimage with respect to the difference in pixel values provides the relationship as shown in FIG. 3 is held in association with the difference in pixel values. Since the difference values registered in the data table are actually discrete, the ratio t corresponding to the difference value closest to the difference value calculated by the change amount calculation unit 202 is acquired. Alternatively, the ratio t to the calculated difference value may be acquired by acquiring the two ratios t corresponding to the two difference values sandwiching the calculated difference value from the data table and interpolating them. The acquired ratio t is held in the memory unit 204 described later.

The afterimage determination unit 203 determines that if the difference obtained by the calculation of the change amount calculation unit 202 is negative, it is a falling afterimage, and if it is positive, it is a rising afterimage, and changes according to the absolute value of the difference. The ratio t is selected from the data table. When selecting the ratio t, t in the previous frame may be retained, and recursive processing may be performed according to the change between frames to determine t. For example, (a value of 0 to 1) recursive coefficients alpha, k frames, and temporal change of the radiation dose the rate t obtained from (difference value) _t k _{respectively,} and _{t k-1} in k-1 frame, the current The ratio t in the frame (k frame) may be acquired by _{t = α × t k} + (1-α) × t _k-1.

In the afterimage correction unit 107, the memory unit 204 describes the characteristics of the rising afterimage and the falling afterimage with respect to the radiation image data acquired by the radiation detection unit 102 (the following, an1, an2, bn1 (k), bn2 (k)). To hold. The data representing the afterimage characteristics in the radiographic image data corresponding to the current frame is a combination of the parameter an and the parameter bn (k) in the calculation for estimating the afterimage amount A (x, k) (hereinafter [Equation 1]). It is represented by. an is the time constant or attenuation factor of the afterimage. bn (k) is the ratio or intensity of the afterimage. Hereinafter, for the sake of brevity, the attenuation factor an and the intensity bn (k) are described. In [Equation 1], the parameters (an, bn (k)) representing the afterimage characteristic are different between the case of the rising afterimage and the case of the falling afterimage. In addition, the intensity bn (k) has a characteristic that changes with each radiography depending on k.

Here, k indicates that the radiation imaging is the kth time, that is, the number of imaging frames is the kth frame or the number of irradiations of the radiation is the kth radiation image data. x indicates a pixel, for example, a pixel number x. Further, A (x, k) is the amount of afterimage in the pixel x of the kth radiation image data.

The afterimage characteristic parameters an and bn (k) in [Equation 1] will be described. In the present embodiment, the characteristics of the rising afterimage are set to, for example, the rising step response function RSRF representing the signal output by the radiation detection unit 102 when the radiation is irradiated under certain imaging conditions from the state where the radiation is not irradiated. It is obtained by fitting [Equation 2] of. RSRF is an abbreviation for Rising Step-Response Function. From [Equation 2], an1 and bn1 (k), which are the characteristics of the rising afterimage, can be obtained.

In [Equation 2], u (k) is a discontinuous unit step function (discrete unit step function). Further, the afterimage characteristic of the falling afterimage is, for example, in the descending step response function FSRF representing the signal output by the radiation detection unit 102 when the radiation is not irradiated immediately after being irradiated under certain imaging conditions. It is obtained by fitting the number 3]. FSRF is an abbreviation for Falling Step-Response Function. From [Equation 3], an2 and bn2 (k), which are the characteristics of the falling afterimage, can be obtained. In [Equation 3], Nf is the number of times of irradiation.

Here, an1, b01 and bn1 (k) are data on the characteristics of the rising afterimage, and an2 and bn2 (k) are data on the characteristics of the falling afterimage. The data representing the afterimage characteristic in [Equation 1] (time constants an and bn (k) can be expressed as the following [Equation 4] by the ratio t determined by the afterimage determination unit 203.

The amount of afterimage in FIG. 3 converges to the amount of afterimage of the rising afterimage, which starts irradiation from a state where no radiation is irradiated when the difference between the pixel values of the image data is positively diverged, and is negative. When it diverges, it converges to the amount of afterimage of the falling afterimage, which stops the irradiation from the state where the radiation is irradiated under certain imaging conditions.

S (x, k) in [Equation 1] will be described. S (x, k) is image information derived from the previously acquired radiation image data, and is image information including the destination radiation image data after afterimage correction. S (x, k) is calculated by the following [Equation 5]. Hereinafter, S (x, k) is referred to as "S value" or "S". For example, the S value is a value in which the information of the previous frame is stored, and is updated for each shooting by [Equation 5]. The initial value S (x, 1) of the S value is set to 0 or a predetermined value.

Here, the IC (x, k-1) is the radiation image data after the afterimage correction at the pixel number x of the k-1th frame (kth radiation image data), and is represented by the following [Equation 6]. To. In [Equation 6], I (x, k-1) is the radiation image data before afterimage correction at the pixel number x of the k-1th frame (k-1th radiation image data).

The frame information acquisition unit 205 acquires and holds the radiation image data after afterimage correction in the previous frame and the S value in the current frame from the S value in the previous frame. The afterimage amount estimation unit 206 includes the ratio t including the rising afterimage determined by the afterimage determination unit 203, the afterimage characteristic data held in the memory unit 204, and the current frame held by the frame information acquisition unit 205. The afterimage amount A (x, k) is estimated using the S value. The afterimage removing unit 207 subtracts the afterimage estimated by the afterimage amount estimation unit 206 from the image data including the afterimage acquired by the radiation detection unit 102 held by the image holding unit 201.

Next, with reference to FIG. 4, a process from shooting the subject to displaying the radiographic image data will be described.

In step S401, the imaging condition setting unit 104 sets the radiation imaging conditions (imaging conditions) of the subject, such as the imaging mode, the radiation irradiation dose, and the tube voltage of the radiation irradiation unit 101, according to the operation of the operator. .. The set shooting conditions are output to the detector control unit 105. In step S402, the variable k representing the number of continuous irradiations of radiation or the number of shooting frames is set to 0. In the following description, the number of shooting frames is referred to as k for the sake of brevity. In step S403, the detector control unit 105 outputs a dose control signal to the tube control unit 103 based on the imaging conditions set in step S401. The tube control unit 103 outputs a radiation signal to the radiation irradiation unit 101 based on the dose control signal. The irradiation unit 101 receives the irradiation signal and irradiates the subject with radiation.

The processes of steps S404 to S412 are processes that are repeated at the frame rate of imaging for each imaging of radiation. In step S404, 1 is added to the number of shooting frames k. In step S405, the detector control unit 105 outputs an image data acquisition signal to the radiation detection unit 102 based on the imaging conditions set in step S401. The radiation detection unit 102 acquires the radiation image data I (k) captured based on the imaging conditions set in step S401, and holds the radiation image data I (k) in the image holding unit 201.

In step S406, the change amount calculation unit 202 determines the pixel value of the pixel x of the radiation image data I (k) of the current frame and the radiation image data I (k-1) of the previous frame held in step S405. The difference (Δ (x, k)) of is calculated. For example, ΔI (x, k) is calculated by I (x, k) -I (x, k-1). Note that I (0) (or I (x, 0)) is set to 0 or a predetermined value. The calculated difference ΔI (k) is provided to the afterimage determination unit 203.

In step S407, the afterimage determination unit 203 determines that the difference ΔI (k) calculated in step S406 (time change of image data) is positive, it is a rising afterimage, and if it is negative, it is a falling afterimage. Then, the afterimage determination unit 203 acquires the ratio t of the rising afterimage from the data table according to the determination result of whether the rising afterimage or the falling afterimage and the absolute value of the difference. The afterimage determination unit 203 holds the acquired ratio t in the memory unit 204.

In step S408, the afterimage amount estimation unit 206 acquires the ratio t acquired in step S407 from the memory unit 204. Further, the afterimage amount estimation unit 206 is provided by the frame information acquisition unit 205 and the characteristics (an1, bn1 (k), an2, bn2 (k)) of the rising afterimage and the falling afterimage held in the memory unit 204. Get the S value. Then, the afterimage amount estimation unit 206 uses the ratio t, the afterimage characteristics an1, bn1 (k), an2, bn2 (k), and the S value, and uses the pixel I (x,) of the radiation image I (k) of the k frame. The afterimage amount A (x, k) at k) is estimated.

In step S409, the afterimage removing unit 207 corrects the afterimage by removing the afterimage from the radiation image using the afterimage amount estimated by the afterimage determination unit 203 in step S408. That is, the afterimage removing unit 207 performs IC (x, k) = I (x, k) -A (x, k) for all the pixels (x) to obtain the image IC (k) after afterimage correction. obtain. In step S410, the image display unit 108 displays the radiation image data IC (k) corrected for afterimages in step S410.

In step S411, the frame information acquisition unit 205 calculates the S value used in estimating the afterimage amount in the next frame using the afterimage-corrected image, and provides it to the afterimage amount estimation unit 206. In step S412, it is determined whether or not the radiation is continuously irradiated. When the radiation is continuously irradiated, the process returns to step S404. When the radiation exposure reaches the desired number of times, the imaging is completed and the flow is completed.

As described above, according to the present embodiment, the ratio of the rising afterimage to be included is determined based on the amount of change in the pixel value and the determination of whether the rising afterimage is the rising afterimage or the falling afterimage. Then, the afterimage characteristic is determined by synthesizing the characteristic of the rising afterimage and the characteristic of the falling afterimage according to the determined ratio, and the amount of the afterimage is estimated using the determined afterimage characteristic. Therefore, in the afterimage correction process of the present embodiment, the amount of afterimage generated at the time of falling and the amount of afterimage generated at the time of rising can be obtained more accurately, and more accurate afterimage correction can be realized.

(Other Examples)
The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It is also possible to realize the processing. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

The invention is not limited to the above embodiments, and various modifications and modifications can be made without departing from the spirit and scope of the invention. Therefore, a claim is attached to make the scope of the invention public.

100: Radiation imaging device, 101: Radiation irradiation unit, 102: Radiation detection unit, 103: Tube control unit, 104: Imaging condition setting unit, 105: Detector control unit, 106: Data collection unit, 107: Afterimage correction unit , 108: Image display unit, 201: Image holding unit, 202: Change amount calculation unit, 203: Afterimage determination unit, 204: Memory unit, 205: Frame information acquisition unit, 206: Afterimage amount estimation unit, 207: Afterimage removal unit

Claims (13)

An image processing device that processes radiographic image data taken at a predetermined frame rate.
A calculation means for calculating the amount of change and the time change indicating the positive / negative of the radiation image data,
A determination means for determining the afterimage characteristic in the radiographic image data based on the time change calculated by the calculation means, and a determination means.
An estimation means for estimating the amount of afterimage contained in the radiographic image data based on the afterimage characteristics determined by the determination means, and an estimation means.
An image processing apparatus comprising: a correction means for correcting an afterimage on the radiographic image data based on an afterimage amount estimated by the estimation means. The image processing apparatus according to claim 1, wherein the calculation means calculates the time change by subtracting the radiation image data corresponding to the immediately preceding frame from the radiation image data corresponding to the current frame. The image processing apparatus according to claim 2, wherein the subtraction is performed for each pixel of the entire image. The image processing apparatus according to claim 2, wherein the subtraction is performed for each pixel of one or more partial regions. The calculation means according to any one of claims 1 to 4, wherein the calculation means holds the time change from the immediately preceding frame for each frame and calculates the time change in the radiographic image data by performing recursive processing. The image processing apparatus described. Any of claims 1 to 5, wherein the determining means determines the afterimage characteristic by determining the ratio of combining the rising afterimage characteristic and the falling afterimage characteristic based on the time change. The image processing apparatus according to item 1. The characteristic of the rising afterimage is an ascending step response function corresponding to a signal output by a detector that detects the radiation image data when the radiation is irradiated under a predetermined imaging condition from a state in which the irradiation of the radiation is stopped. Yes,
The characteristic of the falling afterimage is a descending step response function corresponding to a signal output by the detector when the irradiation of radiation is stopped from the state of irradiating radiation under a predetermined imaging condition. The image processing apparatus according to claim 6. The image processing according to claim 6 or 7, wherein the determination means includes a table that holds the ratio of combining the characteristics of the rising afterimage and the characteristics of the falling afterimage with respect to a time change. apparatus. The determination means is characterized in that the ratio obtained from the table is retained for each frame, and the afterimage characteristic is determined using the ratio obtained by recursive processing of the retained ratio. 8. The image processing apparatus according to 8. The image processing apparatus according to any one of claims 1 to 9, wherein the afterimage characteristic includes a parameter indicating the time constant or attenuation rate of the afterimage and a parameter indicating the ratio or intensity of the afterimage. The image processing apparatus according to any one of claims 1 to 10.
Irradiation means to irradiate radiation and
A radiographic imaging apparatus comprising: a radiation detection unit that detects the radiation, generates radiographic image data, and supplies the radiographic image data to the image processing apparatus. An image processing method for processing radiographic image data taken at a predetermined frame rate.
A calculation process for calculating the amount of change in the radiographic image data and the time change indicating the positive or negative of the change,
A determination step of determining the afterimage characteristics in the radiographic image data based on the time change calculated by the calculation step, and a determination step.
An estimation step of estimating the amount of afterimage contained in the radiographic image data based on the afterimage characteristics determined by the determination step, and an estimation step.
An image processing method comprising: a correction step of correcting an afterimage on the radiographic image data based on an afterimage amount estimated by the estimation step. A program for operating a computer as each means of the image processing apparatus according to any one of claims 1 to 10.

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