JP5798787B2 - Image photographing apparatus and image photographing method - Google Patents

Description

  The present invention relates to image correction in an image capturing apparatus and the like, and more particularly to correction of an output signal from a detector having a defective element.

  The X-ray CT apparatus calculates an X-ray absorption coefficient from an X-ray transmission image (hereinafter referred to as projection data) of a subject taken from a plurality of directions, and obtains a tomographic image (hereinafter referred to as a reconstructed image) of the subject. It is a device and is widely used in the fields of medicine and nondestructive testing. In recent years, in particular, in the medical field, the number of X-ray detectors has increased in the direction of the rotation axis, and this has enabled imaging over a wide range with one rotation, thereby reducing imaging time. On the other hand, since the number of detection elements arranged in the X-ray detector is increased, the failure of the elements is increasing. Therefore, conventionally, the detection element and the detector including the same have been replaced with new ones, or image correction has been performed to remove the influence of defective elements. However, when replacing with a new one, not only is it expensive to prepare a new detector, but also the replacement work takes time. It was also necessary to prepare a replacement detector before the defect occurred. In addition, since it takes time to respond, it may be inconvenient in clinical practice. On the other hand, when image correction is performed to remove the influence of defective elements, it can be dealt with promptly at low cost, which is effective in clinical practice. In particular, when a defect (defect) occurs in the clinical field, it is possible to cope with almost no dead time (unusable period) of the apparatus, which is effective. As an image correction method, for example, as described in Patent Document 1, a method is used in which the values of defective elements are interpolated using pixel values of peripheral normal elements with respect to an acquired image.

  By the way, there are various causes for the failure of the detector element. For example, as described in paragraph [0004] of the specification of Patent Document 2, a part of the signal is transferred from the defective element to the peripheral element. In some cases, the output of peripheral elements that are leaked out and have no defects also becomes abnormal. This occurs, for example, when holes and electrons generated in a photodiode or the like cannot be read due to an abnormality in the reading circuit of the detector. The charge generated in the photodiode is stored in the capacitance of the element and partly recombined. However, due to the abnormality of the readout circuit, the readout of the charge is not performed, and the generated charge continues to be accumulated and exceeds the accumulation capacity. It is considered that a part of the electric charge flows into the defective peripheral element.

JP 2000-79109 A JP-A-8-75544

  As described above, since the output of the defective peripheral element into which the signal from the defective element flows is deviated from the true value, there is a problem that an artifact is generated when an image is generated as it is. Also, if the output of the defective element is interpolated using the output of the nearby element as in the conventional image correction, the interpolation is performed using the output of the defective peripheral element that is deviated from the true value. There was a problem of artifacts. Also, if a defective peripheral element is handled in the same way as a defect, the correction value is obtained without using the detection signal of the defective peripheral element at all, and only a signal from a farther element can be used, resulting in a decrease in correction accuracy. there were.

  The present invention has been made in view of the above problems, and even when the influence of a defective element of a detector extends to surrounding elements, it is possible to remove and suppress artifacts by performing image correction with high accuracy. It aims at providing a simple image photographing device.

In order to achieve the above-described object, the first invention provides a detector in which a plurality of detection elements that detect light and convert it into electric charges and output an electric signal corresponding to the amount of electric charges, and each of the detectors. A signal collection device that collects an electrical signal detected by the detection element as an output signal; a correction unit that performs predetermined correction processing on the output signal collected by the signal collection device to generate image data; and the image An image generation device including an image generation unit configured to generate an image based on data, wherein an influence amount parameter indicating an influence of a defective peripheral element around the defective element included in the detector from the defective element Is stored in advance, and the correction process performed by the correction unit includes the defective element based on an output signal obtained from a normal detection element among the output signals collected by the signal collection device. Includes a defect element correction process for estimating an output signal and correcting the output signal obtained from the defect peripheral element based on the estimated output signal of the defect element and the influence amount parameter stored in the storage means The defect element correction process further includes a re-estimation process for estimating an output signal of the defective element based on an output signal of the defective peripheral element after correction .

According to a second aspect of the present invention, there is provided a detector in which a plurality of detection elements that detect and convert light into electric charges and output an electric signal corresponding to the amount of electric charges, and the electric current detected by each detection element of the detector. A signal collection device that collects signals as output signals, a correction unit that performs predetermined correction processing on the output signals collected by the signal collection device to generate image data, and an image based on the image data An image capturing method using an image capturing apparatus comprising: an image generating unit; and storing in advance an influence amount parameter indicating an influence of a defective peripheral element around the defective element included in the detector from the defective element. The correction processing performed by the correction means estimates the output signal of the defective element based on the output signal obtained from the normal detection element among the output signals collected by the signal collecting device. , Defective element correction process for correcting the output signal obtained from the defective peripheral device based on the stored amount of influence parameters on the output signal and said storage means estimated the defective element is included, the defective element correction The processing further includes a re-estimation process for estimating an output signal of the defective element based on the output signal of the defective peripheral element after correction .

  According to the present invention, it is possible to provide an image capturing apparatus or the like that can perform image correction with high accuracy and remove and suppress artifacts even when the influence of a defective element of a detector extends to peripheral elements.

Hardware block diagram of the entire image photographing apparatus (X-ray CT apparatus) 1 The block diagram which shows the internal structure of the central processing unit 20 A flowchart showing the flow of processing in the image capturing apparatus (X-ray CT apparatus) 1 Flow chart showing the flow of defective element correction processing The figure which shows an example of the defect map 206 The figure which shows the example (diagonal arrangement) of the element utilized for interpolation in a defective element correction process The figure which shows the example (same line arrangement) of the element utilized for interpolation in a defect element correction process The figure which shows the flow of the defect map creation processing performed before this processing The figure which shows an example of the defect map 206B of the stage which discriminate | determined the normality / abnormality of the output value of each element of the detector 5 The figure which shows the flow of influence amount parameter calculation processing performed in advance of this processing The figure explaining adjustment of the amount of X-rays incident on each detection element The figure which shows the output value distribution of the element of each channel of the same slice obtained at the time of incident amount adjustment A normalized output value profile 41 in which the output values of the elements shown in FIG. 12 are standardized with the output values assumed when all the elements are normal. Profile 43 showing the ratio 157 of signal outflow (signal outflow function f (x)) at each input level The figure explaining the X-ray filter 111 utilized for X-ray irradiation amount adjustment Flowchart explaining the re-estimation process of the output value of the defective element in the defective element correction process The flowchart explaining the flow of the defective element correction process (2) of the second embodiment.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.
In the following embodiments, the X-ray CT apparatus 1 will be described as an example of an image capturing apparatus according to the present invention, but the present invention is not limited to this. The present invention can be applied to various medical imaging apparatuses such as an X-ray diagnostic apparatus, a fluoroscopic imaging apparatus, an X-ray cone beam CT apparatus, an X-ray CT apparatus and an X-ray imaging apparatus for baggage inspection and non-destructive inspection. .

[First Embodiment]
First, a configuration of an X-ray CT apparatus 1 that is an embodiment of an X-ray imaging apparatus according to the present invention will be described with reference to FIG.

  As shown in FIG. 1, an X-ray CT apparatus 1 includes an X-ray tube (X-ray source) 2, a collimator 3, an X-ray detector 5, a rotating body 7, a control device 10, a signal collection device 12, and a central processing device 20. , A display device 21, an input device 22, and a bed 30. Reference numeral 8 denotes a rotation axis direction (slice direction, body axis direction), and reference numeral 9 denotes a rotation direction (channel direction).

  The X-ray tube 2 is an X-ray source, and is controlled by the control device 10 to irradiate the subject 33 with X-rays continuously or intermittently. The control device 10 controls the X-ray tube voltage and the X-ray tube current applied or supplied to the X-ray tube 2 according to the X-ray tube voltage and the X-ray tube current determined by the central processing unit 20.

  The collimator 3 irradiates the subject 33 with X-rays radiated from the X-ray tube 2 as X-rays such as cone beams (conical or pyramidal beams), for example, and the aperture width is controlled by the control device 10. Is done. X-rays transmitted through the subject 33 enter the X-ray detector 5.

  The X-ray detector 5 includes, for example, about 1000 X-ray detection element groups configured by a combination of a scintillator and a photodiode in the rotation direction (channel direction) 9, and the rotation axis direction (slice direction, body axis direction) 8. For example, about 1 to 320 pieces are arranged and arranged so as to face the X-ray tube 2 with the subject 33 interposed therebetween. The X-ray detector 5 detects X-rays emitted from the X-ray tube 2 and transmitted through the subject 33, and outputs the detected transmitted X-ray data to the signal acquisition device 12.

  The signal collection device 12 is connected to the X-ray detector 5, collects the electric charges obtained by the individual detection elements of the X-ray detector 5, converts them into digital signals, and creates raw data collectively in view units. Output to the central processing unit 20.

  The central processing unit 20 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), a storage device, and the like, and includes a control device 10, a signal collecting device 12, a bed 30, a display device 21, and an input. The device 22 is controlled.

  As illustrated in FIG. 2, the central processing unit 20 includes a calculation unit 201, a storage unit 202, and an image reconstruction unit 203.

  The calculation unit 201 controls each part of the X-ray CT apparatus 1 according to imaging conditions and instructions input from the input device 22 or a program stored in the ROM, and performs X-ray irradiation, rotation body driving, bed movement, signal collection. Etc. are performed. In addition, the calculation unit 201 displays shooting conditions, shooting data, and a created image on the display device 21. Further, the calculation unit 201 acquires raw data collected by the signal collecting device 12, performs image processing (including correction processing illustrated in FIG. 3) on the raw data, and generates projection data (image data). The image is output to the image reconstruction unit 203.

In the storage unit 202, in addition to raw data collected by the signal collection device 12, correction data (sensitivity / X-ray distribution data 207, offset data 208, etc.) used for correction processing, defective elements of the X-ray detector 5, etc. A defect map 206 indicating the arrangement, an influence amount parameter 205 described later, and the like are stored. In addition, an image reconstructed by the image reconstruction unit 203 of the central processing unit 20 is stored. In addition, a program, data, and the like for realizing the functions of the X-ray CT apparatus 1 are stored.
The image reconstruction unit 203 performs an image reconstruction process on the projection data acquired from the calculation unit 201, creates a reconstruction image indicating the X-ray absorption coefficient distribution of the subject 33, and outputs it to the display device 21.

Further, the arithmetic unit 201 of the central processing unit 20 performs a defective element correction process for estimating an output value of a defective element included in the X-ray detector 5 in the above correction process (see FIGS. 3 and 4).
In this defective element correction process, the calculation unit 201 estimates the output signal of the defective element based on the output signal obtained from the normal detection element among the output signals of the X-ray detector 5 collected by the signal collection device 12. Further, the output signal obtained from the defective peripheral element is corrected based on the estimated output signal of the defective element and the influence amount parameter 205 stored in the storage unit 202.
By performing such defective element correction processing, charge flows out to the peripheral elements, and the output value of the element with the outflow type defect that affects normal peripheral elements (defective peripheral elements) that have no faults or abnormalities is estimated. In addition, an error in the output signal of the defective peripheral element can be corrected. In addition, since the output signal of the defective element itself is estimated using the values of the surrounding normal elements excluding the defective peripheral elements, it can be corrected with high accuracy.

Here, the influence amount parameter 205 will be described. The influence amount parameter 205 represents the degree of influence that a defective peripheral element around the defective element receives from the defective element. In the present embodiment, as an example of the influence amount parameter 205, the signal outflow from the defective element A signal outflow function f (x) 146, which is a ratio, is obtained and used to estimate (correct) the output value of the defective peripheral element.
In this way, by correcting the output value of the defective peripheral element using the influence amount parameter 205, the correction can be performed while using the measured value of the defective peripheral element, and the correction accuracy can be improved.

  In addition to the defective element correction process, the arithmetic unit 201 of the central processing unit 20 corrects offset correction for correcting the zero level of the X-ray detector 5, X-ray irradiation distribution, and sensitivity distribution of the X-ray detector 5. Perform air correction. Details of the correction process and the defective element correction process will be described later.

  An X-ray tube 2, a collimator 3, an X-ray detector 5, and a signal collection device 12 are mounted on the rotating body 7. The rotating body 7 rotates by a driving force transmitted from the rotating body driving device controlled by the control device 10 through a drive transmission system.

  The display device 21 includes a display device such as a liquid crystal panel and a CRT monitor, and a logic circuit for executing display processing in cooperation with the display device, and is connected to the central processing device 20. The display device 21 displays an image reconstructed by the central processing unit 20, imaging conditions to be set, various processing results, and the like.

  The input device 22 includes, for example, a keyboard, a pointing device such as a mouse, a numeric keypad, various switch buttons, and the like, and outputs various instructions and information input by the operator to the central processing unit 20. The operator interactively operates the X-ray CT apparatus 1 using the display device 21 and the input device 22.

  The bed 30 includes a top plate on which the subject 33 is placed, a bed control device, a vertical movement device, and a top plate drive device. The bed 30 is controlled by the control of the bed control device (not shown) to control the vertical movement device. Make the height of the appropriate. Further, the top plate driving device is controlled to move the top plate back and forth in the body axis direction, or to move in the direction perpendicular to the body axis and parallel to the top plate (left and right direction). Thereby, the subject 33 is carried into and out of the X-ray irradiation space.

  Next, the operation of the X-ray CT apparatus 1 will be described with reference to FIGS.

  The central processing unit 20 of the X-ray CT apparatus 1 of the present embodiment executes the process shown in FIG. 3 may be stored in the storage unit 202 as a program, read from the storage unit 202 by the arithmetic unit 201 of the central processing unit 20, and executed, or implemented as a processing circuit. You may do it.

  First, when the start of actual imaging is input from the input device 22, the central processing unit 20 controls the control device 10 to emit X-rays from the X-ray source 2. The X-ray is irradiated to the subject 33 placed on the bed 30 with an irradiation field limited by the collimator 3, passes through the subject 33, and is detected by the X-ray detector 5. This imaging is repeated while changing the X-ray irradiation angle by rotating the rotating plate 7 around the body axis of the subject 33, and measurement data for 360 degrees is acquired. Photographing is performed, for example, every 0.4 degrees. Further, the position control of the X-ray focal point is performed during this time. The signal collection device 6 collects the electric charge obtained by the X-ray detector 5, converts it into a digital signal, creates raw data collectively for each view unit, and outputs it to the central processing unit 20.

  When the arithmetic unit 201 of the central processing unit 20 acquires raw data from the signal collecting device 12 (step S1), the arithmetic unit 201 performs correction processing on the raw data, creates projection data, and outputs the projection data to the image reconstruction unit 203. (Step S2). Next, the image reconstruction unit 203 performs an image reconstruction process using projection data of a plurality of views, creates a reconstructed image of the X-ray absorption coefficient distribution of the subject 33 (step S3), and creates a reconstructed image. It is displayed on the display device 21 (step S4).

The correction process in step S2 will be described.
In the correction processing, the calculation unit 201 corrects offset correction (step S21), LOG conversion (step S22), sensitivity distribution of the X-ray detector 5, and X-ray irradiation distribution that corrects the zero level of the X-ray detector 5. Air correction (step S23) to be performed and defective element correction (step S24) to estimate the output value of the defective element are performed. Here, the flow of the correction process in FIG. 3 is an example, and the present invention is not limited to this. For example, the order of each process of step S21 to step S24 is not limited to this, and may be a different order. Other corrections may be added. There may also be cases where there is no offset correction or air correction.

In the example of FIG. 3, the arithmetic unit 201 first performs offset correction on the raw data.
In the offset correction (S21), the calculation unit 201 acquires the offset data 208 created in advance of the main shooting and saved in the storage unit 202, and makes a difference from the raw data. The offset data 208 is zero-level data, and is created, for example, by performing imaging without irradiating X-rays to obtain raw data and performing an averaging process on the view.
Next, the arithmetic unit 201 performs LOG conversion. In the LOG conversion (S22), the conversion process shown in the following equation (1) is performed.

  Y = aLOG (X) + b (1)

  Here, X is a value before conversion, Y is a value after conversion, and a and b are constant coefficients.

  Next, the calculation unit 201 performs air correction. In the air correction (S23), for example, the sensitivity / X-ray distribution data 207 created in advance of main imaging and stored in the storage unit 202 is subtracted from the raw data after LOG conversion. Sensitivity / X-ray distribution data 207 includes, for example, subjecting the raw data acquired when X-rays are irradiated from the X-ray tube 2 without providing the subject 33 to offset correction, view averaging processing, and LOG conversion. Created by doing.

Next, the calculation unit 201 performs defect element correction.
In the defective element correction (S24), the calculation unit 201 refers to the defect map 206 created in advance before the actual photographing (pre-processing A in FIG. 8; defect map creation processing). The defect map 206 represents the arrangement of each element of the X-ray detector 5, whether or not it is a defect, and the type of defect if it is a defect. As shown in FIG. 5, in the arrangement corresponding to the arrangement of all elements of the X-ray detector 5, the types of elements are, for example, “0: normal output value”, “1: abnormal output value (defective element)”, “ 2 (abnormal output value; defective peripheral element) ”. The defect map 206 in FIG. 5 illustrates the defect map 206 for the X-ray detector 5 composed of 8 × 8 elements, but the number of elements is not limited to this.

  Further, it is necessary to obtain the influence amount parameter 205 and store it in the storage unit 202 in advance of the main photographing (pre-processing B in FIG. 10; influence amount parameter calculation processing). The influence amount parameter is a parameter indicating an influence amount that a defective element exerts on a defect peripheral element, and is, for example, a signal outflow function f (x) described later.

  It is assumed that the defect map 206 and the influence amount parameter 205 are already stored in the storage unit 202, and the defective element correction process in step S24 will be described with reference to FIG.

In the defective element correction process, first, the arithmetic unit 201 of the central processing unit 20 refers to the defect map 206 stored in the storage unit 202 (step S241).
In the present embodiment, the pixel (3, 3) in the defect map 206 shown in FIG. 5 corresponds to the outflow type defective element, and the adjacent (3, 2), (2, 3), (4, 3), An example in which the influence is exerted on the pixel (3, 4) will be described. The outflow type defect is that the readout circuit of the X-ray detector 5 has a defect, so that the photodiode is in an open state, and further, the charge generated in the photodiode is not read out to the adjacent element (peripheral element). This is a defect in which an output abnormality occurs also in adjacent pixels (peripheral pixels). That is, there is no failure in the elements in (3, 2), (2, 3), (4, 3), and (3, 4), and the output value is influenced by the influence from peripheral defective elements (outflow type defective elements). An abnormality has occurred. Further, since the generated charge cannot be read from the element corresponding to the defective (3, 3) pixel, an abnormality occurs in the output value.
Note that the defect map 206 in FIG. 5 shows an example in which the charge flows out only to the element adjacent to the slice direction and the channel direction of the outflow type defective element, but the charge outflow range is not limited to the adjacent element, Since it is possible to extend to a farther element, the “defect peripheral element” includes all elements affected by the outflow of charge from the outflow type defect element.

  In the defect map 206 of FIG. 5, “1” is given to the defective element (outflow type defective element). In addition, an element that is determined to be abnormal output due to a signal outflow from the outflow type defective element, such as the pixels of (3, 2), (2, 3), (4, 3), and (3, 4). And “2” is assigned to the defect map 206. In addition, an element with a normal output value is a normal element, and “0” is assigned to the defect map 206.

Referring to this defect map 206, operation unit 201 estimates the output value of the defective element (value “1” in FIG. 5) (step S242). The calculation unit 201 estimates the output value of the defective element by interpolation using the output value of the normal element.
The elements used for the interpolation are located diagonally with the defective element (3, 3) as the center (positioned in a direction that bisects the slice direction and the channel direction), as shown by the hatched portion in FIG. The output values of the elements (2, 2), (2, 4), (4, 2), (4, 4) that are not peripheral elements can be used. Any mathematical expression may be used for the interpolation calculation, for example, it may be obtained from an average value of output values of the elements (2, 2), (2, 4), (4, 2), (4, 4). it can.

  As described above, since the output value of the defective element is estimated using the output signal from the normal element that is located in the vicinity of the defective element and is not the defective peripheral element, highly correlated and highly reliable interpolation can be performed. Further, since the correction is performed without using the signal of the element (defect peripheral element “2”) in which a signal flows from the defective element, the correction can be performed with high accuracy.

  In the above description, an example (FIG. 6) in which the correction value of the outflow defect element is calculated using four normal elements diagonally centered on the outflow defect element (3, 3). The correction values are calculated using the outputs of some of the normal elements (2, 2), (2, 4), (4, 2), and (4, 4) indicated by the hatched portion in FIG. It may be. In particular, when the outflow type defective element is located at the end of the X-ray detector 5, there is no normal element on the end side, so some of the normal elements located obliquely from the outflow type defective element (3, 3). It is effective to interpolate using. For example, when there is an outflow type defective element in (2, 1), interpolation may be performed using the output of the elements (1, 2) and (3, 2). The average of the output values of the normal elements is used as the correction value. However, the present invention is not limited to this, and an addition average with different weights may be used, or estimation may be performed using a function.

  Alternatively, the output value of the outflow type defective element may be interpolated using the output value of the element that is in the same row (same channel or same slice) as the outflow type defective element and is not a defective peripheral element. For example, as indicated by the hatched portion in FIG. 7, the elements (1, 3) and (5, 3) (or the non-defective peripheral elements that are in one of the channel direction and the slice direction of the defective element (3, 3) (or The output value may be estimated by linear interpolation using the output values (3, 1) and (3, 5). As described above, if the outflow-type defect element and the defect peripheral element are sandwiched and the interpolation is performed using the output value of the normal element in the same row, the present invention can be applied to imaging of one slice or two slices. The present invention can also be applied to a one-dimensional detector in which the elements of the X-ray detector 5 are arranged in one row. Furthermore, the elements used for interpolation need not be limited to two points, and may be interpolated using data of a larger number of the same channel or the same slice. For example, the values of the outflow defect elements of (3, 3) may be obtained using the values of the elements of (3, 1), (3, 5), and (3, 6). In this case, weighted addition according to the distance from the defective element may be performed, or estimation may be performed using some function.

Returning to the description of FIG.
Next, the calculation unit 201 of the central processing unit 20 uses the defective element (3, 2), (2, 3), (4, 3), (3,4) as the defective element (step S242) obtained in step S242. 3 and 3) and the influence amount parameter 205 stored in the storage unit 202 are used for correction (step S243). As the influence amount parameter 205, a signal outflow function f (x) 146 is used. The signal outflow function f (x) 146 represents the ratio of the amount of charge outflow to the defective peripheral element with respect to the output value of the defective peripheral element when the defective element normally outputs.

  In the present embodiment, for simplicity of explanation, a common signal outflow function f (x) is applied to four peripheral peripheral elements. However, the outflow amount is small for each peripheral peripheral element. Since it may be different, an individual signal outflow function f (x) may be applied to each defective peripheral element. A method for determining the signal outflow function f (x) will be described in detail later (see FIG. 10).

  Since the estimated value z of the defective element (3, 3) calculated in step S242 is considered to be substantially equal to the output value x when the defective element normally outputs, it is assumed that the defective peripheral element (3 3, 2), (2, 3), (4, 3), and (3,4), a signal flows in by z · f (z). The corrected output value of each defective peripheral element is a value (measurement value−z · f (z)) obtained by subtracting this inflow from the measurement value (before correction) (step S243).

  In this way, by obtaining the correction value of the defect peripheral element using the output value of the defect peripheral element obtained by the measurement, the X-ray signal incident on the defect peripheral element can be used. More accurate correction can be realized than when estimation is performed by interpolation or extrapolation from an element.

Next, the defect map creation process will be described with reference to FIGS.
In the defect map creation process, the arithmetic unit 201 determines an abnormal output element using the sensitivity / X-ray distribution data 207 held as correction data. That is, the calculation unit 201 reads the sensitivity / X-ray distribution data 207 from the storage unit 202 (step S51), and determines an output value range for determining an abnormal element based on the sensitivity / X-ray distribution data 207 (step S52). The element showing the output value corresponding to the output value range is determined as an abnormal element (step S53).
For example, the average value and the standard deviation of the output values of all elements are calculated using the sensitivity / X-ray distribution data 207 at the time of X-ray irradiation of a predetermined intensity, and the output value range separated from the average value by, for example, 5 times the standard deviation Is the abnormal output range. Then, it is determined whether or not each element is in the abnormal output range, and “1” is given to the corresponding pixel position in the defect map 206 if it is the abnormal output range and “0” if it is not the abnormal output range. FIG. 9 shows a defect map 206B in a state indicating whether or not the output range is an abnormal output range. Pixels (3, 2), (2, 3), (3, 3), (4, 3), and (3, 4) are abnormal output values, and the other pixels are determined to have normal output values.

  Next, the calculation unit 201 classifies whether an element indicating an abnormal output is an outflow type defective element or a defective peripheral element (step S54). The classification method is determined, for example, from the arrangement of elements that show abnormal output. Specifically, for example, the elements in which the abnormal elements are adjacent to each other and are sandwiched between the plurality of abnormal elements are classified as outflow-type defect elements (value “1” in FIG. 5). Also, abnormal elements other than outflow type defective elements are separated from defective peripheral elements (value “2” in FIG. 5). The calculation unit 201 stores the defect map 206 created in the above-described procedure in the storage unit 202.

  The above-described defect map creation process is an example, and another method may be adopted. For example, a separate image may be acquired without using the sensitivity / X-ray distribution data 207 in order to determine an abnormal element in the abnormal element determination step of step S53. For example, an air image may be obtained using X-rays, or a plurality of types of images may be obtained using a phantom, and used for determination of abnormal elements. Furthermore, an abnormal element may be determined using an image acquired for other correction. Moreover, the function which irradiates light to the X-ray detector 5 may further be provided, and a defective element may be extracted from an image obtained when the light is irradiated. Also, instead of determining abnormal elements in advance of actual shooting, whether artifacts are extracted from the reconstructed image using the actual shooting projection data, and the output value is abnormal based on the position where the artifact is generated It may be determined whether or not. In the abnormal output range determination step in step S52, the abnormal output range is obtained using the average value and the standard deviation. However, the present invention is not limited to this, and it may be determined by a predetermined threshold value.

In the defective element determination step in step S54, both the outflow type defective element and the defective peripheral element are detected as abnormal elements, but only the outflow type defective element may be detected. In many cases, the outflow-type defect element has no change in output even when irradiated with X-rays (light), and can be detected relatively easily from an X-ray image or the like. In this case, an element adjacent to the outflow type defective element may be determined as a defective peripheral element. However, it is desirable to separate the outflow type defect element and the non-outflow type defect element in advance.
Furthermore, in the defective element classification step of step S54, an element in which an element showing an abnormal output is sandwiched in the channel direction and the slice direction is determined as an outflow type defective element. However, the present invention is not limited to such a classification method. Alternatively, an element sandwiched only in one direction of the channel direction or the slice direction may be determined as an outflow-type defective element, and an element adjacent to the element may be determined as a defective peripheral element. Further, the defective element and the defective peripheral element may be determined based on the output value. In this case, for example, since the photodiode is in an open state in the outflow type defective element and the defect peripheral element is increased by a part of the signal flowing in, the X-ray image such as sensitivity / X-ray distribution data is obtained. It is also possible to use a pixel having an output of a certain level or less as an outflow-type defective element and determine an element having an output of a certain level or more as a defective peripheral element.

Next, the influence amount parameter calculation processing will be described with reference to FIGS.
The influence amount parameter calculation process is performed as a preliminary process for the main photographing.
In the influence amount parameter calculation processing, an X-ray shield is provided on the front surface of the X-ray detector 5 and imaging is performed with the X-ray dose incident on the defect peripheral element adjusted (shielded) to obtain raw data (step S61). , S62). For example, a lead plate or a tungsten plate is used as the X-ray shield. The X-ray shield shields incident X-rays on the defect peripheral element (element 152 in FIG. 11), and is arranged so that X-rays are incident on the outflow-type defect element (element 151 in FIG. 11).
Thus, in a state where the X-ray dose incident on the defect peripheral element is adjusted (shielded), the calculation unit 201 acquires raw data from the signal collection device 6, and corrects the raw data with the correction shown in the flowchart of FIG. Among the processes, a defective element correction process and a correction process excluding LOG conversion are performed to create projection data (step S63). That is, offset correction (S631) and air correction (S633) are performed to create projection data.

Here, FIG. 12 shows an example of the output distribution in the channel direction of a certain slice among the obtained projection data.
In FIG. 12, a dotted line 155 represents the end position of the X-ray shield. The solid circle represents the output of the third slice in which the outflow defect element 151 is included. Also, the dotted circle represents the output of, for example, the fifth slice with no outflow type defective element and defective peripheral element. A dotted circle 153 or the like is on the curve 154.
Reference numerals 151, 152, and 153 shown in FIG. 12 represent output values of elements having the same reference numerals in FIG.

Using the output distribution shown in FIG. 12, the calculation unit 201 outputs the output of the third slice including the defective element (solid line circle) to the output of the slice not including the defective element (for example, the fifth slice) (dotted line circle). Standardize with. Then, the profile 41 shown in FIG. 13 is obtained. In normal elements other than the outflow-type defect element 151 and the defect peripheral element 152, the output values of the third slice and the fifth slice are substantially equal, and thus the normalized output value is on the straight line 156. That is, the normalized output value of the normal element is about “1”.
On the other hand, since the defect peripheral element 152 shows different output values in the third slice and the fifth slice, the output value after normalization is a deviation 157 from the straight line 156 (normal element output value “1” after normal element). Just shift. This shift amount 157 is the amount of charge flowing from the outflow defect element 151 into the defect peripheral element 152. If the defective peripheral element 152 is not shielded, the input amount of X-rays is considered to be approximately the same as that of the adjacent element 153. Therefore, if the defective peripheral element 152 is not shielded and is a normal element, it is approximately equal to the output value 158 shown in FIG. Expected to produce output. Therefore, the ratio of the signal outflow to the defective peripheral element 152 can be obtained from the above-described deviation amount (charge inflow amount) 157 and the output value 158 when the defective peripheral element 152 is normal.

  FIG. 14 is a profile 43 showing the rate of signal outflow at each input X-ray dose level. By fitting the result, a function representing signal outflow can be determined, and from this, a signal outflow function f (x) 146 representing the ratio of signal outflow after LOG conversion can be determined.

  As shown in the flowchart of FIG. 10, the arithmetic unit 201 of the central processing unit 20 obtains the amount of charge flowing from the defective element to the defective peripheral element using the projection data after the correction process of S63 (step S64 of FIG. 10). ) And the ratio of signal outflow is obtained (step S65). Further, the processing of steps S61 to S65 is repeated while changing the input X-ray dose, the ratio of the signal outflow at each X-ray dose level is obtained, and the signal outflow function f (x 146 is obtained and stored in the storage unit 202 as the influence amount parameter 205 (step S66).

  The input X-ray dose is adjusted in the channel direction by the X-ray shield, and the influence (signal outflow function f (x)) of the outflow defect element on the channel direction is obtained. This is an example, and the slice direction is In addition, the input X-ray dose can be adjusted to determine the influence (signal outflow function) on the slice direction by the outflow type defect element. For example, the input X-ray dose may be adjusted in the slice direction using an X-ray shield, and the signal outflow function f (x) 146 may be determined in the same manner as in the case of the element 152. Furthermore, the input X-ray dose may be adjusted simultaneously in both the channel direction and the slice direction, and the signal outflow function f (x) 146 may be determined.

Further, the X-ray filter 111 is used to provide an inclination in the X-ray irradiation distribution, and the signal outflow function f (x) 146 is determined using the air image at this time (image taken without the subject). Also good. FIG. 15 shows an example of the X-ray filter 111.
FIG. 15 is a schematic view of a cross section in the rotation axis (slice) direction of the X-ray CT apparatus 1 of FIG.

The X-ray CT apparatus 1 has a structure in which the X-ray emitted from the X-ray tube 2 is limited to the irradiation field by the X-ray collimator 3 and reaches the X-ray detector 5. Further, an X-ray filter 111 can be inserted. By adding a structure, the X-ray irradiation distribution is inclined.
As shown in FIG. 15, the X-ray filter 111 is inserted between the X-ray tube 2 and the collimator 3. The material is preferably a material that does not completely shield X-rays and is somewhat transparent. For example, a material such as aluminum is used. The X-ray filter 111 has a shape corresponding to a desired X-ray irradiation distribution. For example, when an inclination of the irradiation distribution is provided in the slice direction, the thickness changes in the slice direction, and the irradiation distribution in the channel direction. When the slope is provided, the thickness changes in the channel direction. FIG. 15 illustrates an X-ray filter 111 having a triangular prism shape having an inclination in the slice direction. The X-ray filter 111 is moved and inserted by the X-ray filter moving mechanism 112 in the direction of arrow 106 in FIG. The X-ray filter moving mechanism 112 includes a driving device, and the operation is controlled by the control device 10.

When the X-ray filter 111 whose thickness changes in the rotation axis direction (slice direction) is inserted between the X-ray tube 2 and the X-ray collimator 3, the X-ray irradiation distribution is inclined in the rotation axis direction (slice direction). The distribution is input to the X-ray detector 5. If this change in distribution is used, the signal outflow function f (x) 146 in the slice direction can be determined as in the case of using an X-ray shield as shown in FIG.
In other words, the X-ray filter 111 and the X-ray filter moving mechanism 112 are further provided, and the X-ray filter moving mechanism 112 moves the position of the X-ray filter 111 with various irradiation amounts (input amounts to the X-ray detector). Thus, the signal outflow function f (x) 146 can be calculated easily and in a short time. As a result, an outflow-type defective element occurs over time, and the signal outflow function f (x) 146 (influence amount parameter) is obtained even when the signal outflow function f (x) 146 (influence amount parameter) is not stored in advance. The correction process including the defect element correction process can be quickly performed.

  Note that the case where the X-ray filter 111 is provided with an inclination in the X-ray irradiation distribution in the slice direction has been described as an example, and an inclination may be provided in the channel direction or in both directions. In the above description, the case where the signal outflow function f (x) is common to all the defective peripheral elements is described, but this is an example, and different functions may be used in the channel direction and the slice direction. Further, for the defective peripheral element located at the end, a signal outflow function f (x) different from that of other elements may be used. The signal outflow function f (x) is not limited to a constant value, and may be expressed by various functions such as a proportional function, a linear function, and a polynomial function. For example, the signal outflow function f (x) may be a function that should have positive or negative power, a polynomial function that consists of the sum, an exponential function, a logarithmic function, or the like. Further, for example, the signal outflow function f (x) may be expressed by a step function corresponding to the input range or by switching the function in the input range, and further obtained by four arithmetic operations of the function. Various functions are possible. The signal outflow function f (x) may be determined by simulation.

As described above, according to the X-ray CT apparatus 1 of the first embodiment, the calculation unit 201 of the central processing unit 20 classifies detection elements that indicate abnormal output according to the type of defect, A defect map 206 is created and stored in the storage unit 202. For an outflow type defective element in which charge flows out to an adjacent element (or a peripheral element), a signal outflow function f (x) 146 representing a ratio of signal outflow to the defective peripheral element is used as the influence amount parameter 205. Stored in the storage unit 202. Then, as a correction process, in addition to the commonly performed offset correction, air correction, etc., the defect element correction process is performed, and the output signal of the outflow type defect element is obtained based on the output signal obtained from the normal detection element. In addition to the estimation, the output signal of the defective peripheral element is corrected based on the estimated output signal of the outflow type defective element and the above-described influence amount parameter.
Accordingly, since the output value of the defective element is estimated by interpolation using the output signal from the normal element, highly reliable correction using the signal of the correlated element can be performed. Further, since the signal of the element (defect peripheral element “2”) into which a signal flows from the defective element is not used, the interpolation accuracy can be improved. Further, for the correction of the output value of the defective peripheral element, the correction value is obtained by removing the influence of the defective element from the output value of the defective peripheral element obtained by measurement. The detection signal (measured value) can be used. Therefore, it is possible to realize correction with higher accuracy than simply performing estimation from adjacent elements by interpolation or extrapolation.

In the defect element correction process, the outflow type defect element value may be re-estimated using the corrected defect peripheral element value.
That is, as shown in FIG. 16, the calculation unit 201 calculates correction values for defective elements and defective peripheral elements in steps S241 to S243, and further corrects outflow-type defective elements using the correction values for defective peripheral elements. The value is recalculated (step S244).
In the re-estimation process (S244), when estimating the value of the outflow-type defect element, the value of the adjacent defect peripheral element is used. That is, the interpolation is performed using the correction value of the defective peripheral element that is closer than the normal element value used in step S242. For the interpolation, for example, an average value of correction values of four defective peripheral elements may be calculated. Further, at the time of re-estimation of the outflow type defective element, interpolation may be performed by combining the value of the defect peripheral element after correction and the value of the surrounding normal element. Further, for example, it may be estimated using the values of normal elements adjacent to defective peripheral elements of the same channel. In this case, for example, the value of the defective element of (3, 3) in FIG. 6 is changed to the output value of (1, 3), the correction value of (2, 3), the correction value of (4, 3), (5 , 3) is used for calculation. Similarly, re-estimation may be performed with the same slice, or the weight may be calculated according to the distance.
By performing the re-estimation process, it is possible to estimate the output value of the defective element by using the value of a nearby element, so that highly accurate correction can be realized.

[Second Embodiment]
Next, an image capturing apparatus (X-ray CT apparatus) 1 according to a second embodiment of the present invention will be described. In the imaging apparatus (X-ray CT apparatus) 1 according to the second embodiment, the same components as those in the X-ray CT apparatus 1 according to the first embodiment are denoted by the same reference numerals, and redundant description is given. Omitted.

  In the first embodiment, the correction of the signal value in the case where there is an outflow type defective element in which a generated charge is not read and a part of the signal flows into the peripheral element has been described. In the embodiment, correction when a defect due to another cause is included will be described.

  For example, when a failure occurs in a circuit (analog-digital converter or the like) after a signal is read out from the X-ray detector 5, or in an analog reading circuit from a photodiode, In some cases, the photodiodes are short-circuited. An element in which charges generated in a photodiode are read out but an output value is abnormal is referred to as a non-outflow type defective element.

In the second embodiment, when the correction process is performed, the non-outflow type defect element and the outflow type defect element are classified, and the correction process according to the classification result is performed.
FIG. 17 is a flowchart showing the flow of the defective element correction process (2) in the second embodiment.
In the defective element correction process (2) of FIG. 17, the calculation unit 201 first acquires raw data from the signal collection device 6 (step S71), and determines whether the output value is normal or abnormal for each element. (Step S72). If the output value is normal (step S72; Yes), the process is terminated without performing the correction process, and it is determined whether or not the output value is normal for the next element. When the output value is not normal (step S72; No), the calculation unit 201 measures the signal inflow amount to the adjacent element (step S73). The measurement of the signal inflow amount can be performed in the same manner as in step S64 in FIG. If the inflow amount is smaller than the predetermined threshold (step S74; Yes), it is determined as a non-outflow defect element, and the process proceeds to the correction process in step S75.
On the other hand, when the inflow amount is equal to or greater than the above-described threshold value (step S74; No), it is determined as an outflow type defective element, and correction processing similar to that in the first embodiment (defective element correction processing (1); FIG. 4). (Step S76).

In step S75, for an element determined as a non-outflow type defective element, if the adjacent element is a normal element, an interpolation operation is performed using the output value of the adjacent element to correct the output value. For example, when the element (3, 3) is determined to be a non-outflow type defect element, (3, 2), (2, 3), (2, 4), (3, Interpolate using the output value of 4). In the non-outflow type defective element, since the electric charge generated in the photodiode is read out, there is no outflow of a signal to the peripheral elements, and therefore it can be used for interpolation.
In this way, if different correction processes are performed depending on the cause of the defect, since there is no outflow of charge to the surroundings for non-outflow type defective elements, interpolation using the output value of the most adjacent element is possible, and highly reliable correction Can be done.

As described above, the image capturing apparatus 1 of the present invention stores in the storage unit 202 in advance the influence amount parameter 205 that indicates the influence of the defective peripheral element around the defective element included in the detector 5 from the defective element. As the correction process, the output signal of the defective element is estimated based on the output signal obtained from the normal detection element among the output signals collected by the signal collecting device 6, and the estimated output signal of the defective element is Based on the influence amount parameter 205 stored in the storage unit 201, the signal value including the defective element correction process for correcting the output signal obtained from the defective peripheral element is corrected.
That is, in the defective element correction process, the output value of the defective element is obtained by interpolation using an output signal from a normal element located near the defective element, so that a correlated signal can be easily obtained and highly reliable correction can be performed. . Further, since the signal of an element (defect peripheral element) into which a signal flows from the defective element is not used, the interpolation accuracy can be improved. Further, for the correction of the output value of the defective peripheral element, the correction value is obtained by removing the influence of the defective element from the output value of the defective peripheral element obtained by measurement. Since the detection signal (measured value) of the above can be used, it is possible to realize correction with higher precision than simply performing estimation by interpolation or extrapolation from adjacent elements. As a result, the generation of artifacts due to the outflow type defect element and the defect peripheral element can be removed and suppressed.

The defective element correction process may further include a re-estimation process for estimating the output signal of the defective element based on the corrected output signal of the defective peripheral element.
By the re-estimation process, data of defective peripheral elements closer to each other and having a high correlation can be used for the estimation calculation, so that correction can be performed with high accuracy.

Also, based on the output signal collected by the signal collecting device, whether the defective element is an outflow type defective element that affects peripheral elements or a non-outflow type defective element that does not affect peripheral elements. It is desirable to make a determination and perform a correction process according to the determination result. In particular, when the defective element is an outflow type defective element that affects peripheral elements, the above-described defective element correction process is performed.
In this way, more accurate correction can be performed by performing correction processing according to the type of defect.

In addition, the amount of light incident between the defective element and the defective peripheral element is adjusted by a shielding plate, an X-ray filter, etc. (adjustment means), and the output signal of each element is acquired and acquired with the light incident amount adjusted. It is desirable to calculate a signal outflow ratio from the defective element to the defect peripheral element based on the output signal, and to calculate a correction value for the defect peripheral element using the signal outflow ratio as an influence amount parameter.
Thus, the ratio of the signal outflow from the defective element to the defective peripheral element is obtained and used for correction, whereby the output signal of the defective peripheral element can be corrected with high accuracy.

  Further, in the defective element correction processing, the output signal of the defective element is estimated using the output signal of the normal element in the same row as the defective element, and the estimated output signal of the defective element and the influence amount stored in the storage unit 202 It is desirable to correct the output signal of the defective peripheral element based on the parameter (signal outflow function). As a result, the defective element correction processing of the present invention can be applied to a detector having a one-dimensional array or imaging of only one slice or two slices, which is practical.

  In the defective element correction process, the output signal of the defective element is estimated using the output signal of the normal element located diagonally with the defective element as the center, and the estimated output signal of the defective element is stored in the storage unit 202. The output signal of the defective peripheral element may be corrected based on the effect amount parameter (signal outflow function). As a result, correction can be performed using the output signals of normal elements that are located closer to each other and have a high correlation, so that highly reliable correction can be performed. This is effective when performing multi-slice using a two-dimensional array detector.

  In the above-described embodiment, the X-ray detector 5 has an example in which the elements are two-dimensionally arranged. However, the present invention can also be applied to a one-dimensionally arranged detector. Moreover, although the indirect type detector which consists of a scintillator and a photodiode was demonstrated as the X-ray detector 5, this is an example and another kind of detector may be sufficient. For example, it may be a direct conversion type detector that converts X-rays directly into an electrical signal. FIG. 1 illustrates the X-ray CT apparatus 1 in which the X-ray detector 5 and the central processing unit 20 are integrated. However, the present invention is not limited to this. You may provide as an apparatus. That is, image data obtained by an external X-ray CT apparatus may be acquired, and the correction processing (including defective element correction processing) described in this embodiment may be performed based on the acquired image data. Furthermore, the present invention is not limited to an X-ray detector, and can be applied to, for example, a detector that detects light of various wavelengths such as visible light, infrared light, and ultraviolet light, and a radiation detector that detects gamma rays, for example. . In addition, it is obvious that those skilled in the art can come up with various changes and modifications within the scope of the technical idea disclosed in the present application, and these naturally belong to the technical scope of the present invention. It is understood.

1. X-ray CT system (imaging system)
2 ... X-ray tube 3 ... Collimator 5 ... X-ray detector 7 ... Rotating body 8 ... Rotating axis direction (slice direction, body axis direction) )
9 ... Direction of rotation (channel direction)
DESCRIPTION OF SYMBOLS 10 ... Control apparatus 12 ... Signal collection device 20 ... Central processing unit 201 ... Calculation part 202 ... Storage part 203 ... Image reconstruction part 21 ... display device 22 ... input device 30 ... bed 33 ... subject 202 ... storage unit 205 ... influence parameter 146 ... ..Signal outflow function f (x)
206 ... Defect map

Claims (7)

A detector in which a plurality of detection elements that detect light, convert it into electric charges, and output an electric signal corresponding to the amount of electric charges are arranged, and electric signals detected by the respective detection elements of the detector are collected as output signals A signal collecting device that performs a predetermined correction process on the output signal collected by the signal collecting device to generate image data, and an image generating unit that generates an image based on the image data. An image photographing apparatus provided,
A storage means for storing in advance an influence amount parameter indicating an influence of a defective peripheral element around the defective element included in the detector;
The correction process performed by the correction means includes
Estimating an output signal of the defective element based on an output signal obtained from a normal detection element among output signals collected by the signal collecting device, and storing the estimated output signal of the defective element and the storage means A defect element correction process for correcting the output signal obtained from the defect peripheral element based on the influence amount parameter that has been performed ,
The defective element correction process further includes:
An image photographing apparatus comprising: a re-estimation process for estimating an output signal of the defective element based on an output signal of the defective peripheral element after correction . The correction means includes
Based on the output signal collected by the signal collection device, whether the defective element is an outflow type defective element that affects peripheral elements or a non-outflow type defective element that does not affect peripheral elements. It further comprises a defective element discriminating means for discriminating,
Imaging device according to claim 1, when it is determined that the outflow-type defective elements by said defective elements determining means, and performing the defect element correction process.   For each element of the detector, a defect map that classifies whether it is a defective element, a defective peripheral element affected by the defective element, or a normal element and generates a defect map that represents the classification result together with element arrangement data The image photographing apparatus according to claim 1, further comprising a generation unit. Adjusting means for adjusting the amount of incident light between the defective element and the defective peripheral element;
The output signal of each element is acquired in a state where the amount of incident light is adjusted by the adjusting means, and the ratio of signal outflow from the defective element to the defective peripheral element is calculated based on the acquired output signal, and the influence An influence amount parameter calculation means used as an amount parameter;
The image photographing device according to claim 1, further comprising:   In the defective element correction process, the output signal of the defective element is estimated using the output signal of the normal element in the same row as the defective element, and the estimated output signal of the defective element is stored in the storage means The image capturing apparatus according to claim 1, wherein an output signal of the defective peripheral element is corrected based on an influence amount parameter.   The detection elements are two-dimensionally arranged, and in the defect element correction process, an output signal of the defect element is estimated and estimated using an output signal of a normal element located diagonally with the defect element as a center. 2. The image photographing apparatus according to claim 1, wherein the output signal of the defective peripheral element is corrected based on the output signal of the defective element and the influence amount parameter stored in the storage means. A detector in which a plurality of detection elements that detect light, convert it into electric charges, and output an electric signal corresponding to the amount of electric charges are arranged, and electric signals detected by the respective detection elements of the detector are collected as output signals A signal collecting device that performs a predetermined correction process on the output signal collected by the signal collecting device to generate image data, and an image generating unit that generates an image based on the image data. An image photographing method using the provided image photographing device,
Preliminarily storing an influence amount parameter indicating an influence of a defective peripheral element around the defective element included in the detector, which is affected by the defective element;
The correction process performed by the correction means includes
Estimating an output signal of the defective element based on an output signal obtained from a normal detection element among output signals collected by the signal collecting device, and storing the estimated output signal of the defective element and the storage means A defect element correction process for correcting the output signal obtained from the defect peripheral element based on the influence amount parameter that has been performed ,
The defective element correction process further includes:
An image photographing method comprising: a re-estimation process for estimating an output signal of the defective element based on an output signal of the defective peripheral element after correction .

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