JP4731151B2 - X-ray tube current determination method and X-ray CT apparatus - Google Patents

Description

  The present invention relates to an X-ray tube current determination method and an X-ray CT apparatus, and in particular, irradiates a subject with X-rays of a fan beam (fan beam) or cone beam (conical or pyramid beam) and transmits the subject. The present invention relates to a single-slice or multi-slice X-ray CT apparatus that obtains a tomographic image of a subject by measuring X-rays with an X-ray detector and backprojecting measurement data from multiple directions.

  In the multi-slice X-ray CT apparatus, as shown in FIG. 3, the subject 17 is irradiated with a cone beam, that is, a pyramid-shaped X-ray beam, from the X-ray tube 8, and the detection element 18 is arranged in a two-dimensional direction (channel direction and column). The X-rays after passing through the subject are measured by the detectors 11 arranged in the direction) to obtain projection data of the subject 17.

  In the single-slice X-ray CT apparatus, a detector 11 in which detector elements are arranged in one row, that is, in a one-dimensional direction (channel direction) is used, and a fan beam, that is, a fan-shaped X-ray beam is transmitted from the X-ray tube 8 to the subject 17. Irradiation and X-rays after passing through the subject are measured to obtain projection data of the subject.

  In any case, the opposite X-ray tube 8 and detector 11 are rotated around the subject 17 to acquire projection data from multiple directions, and after performing reconstruction filter processing for blur correction, the reverse is performed. The tomographic image of the subject 17 is reconstructed by projecting.

  The projection data is acquired at discrete X-ray tube positions (hereinafter referred to as “views”), and the obtained projection data is referred to as “projection data in the corresponding view”. The number of views per revolution typically ranges from hundreds to thousands. The operation of acquiring projection data of the number of views necessary for reconstructing one CT image is called “scan”. Further, projection data for one view is composed of data corresponding to the number of channels of the detector 11 times the number of columns (a single slice X-ray CT apparatus can be considered as the number of columns = 1 as described above).

  In Patent Document 1, in order to perform scanning so as to satisfy an image SD (Standard Deviation) value required for a reconstructed image in the above-described X-ray CT apparatus, the X-ray CT apparatus is subjected to scanning from scanogram projection data obtained by one-way scanogram imaging. Disclosed is a CT apparatus capable of calculating an elliptical section model of a specimen, calculating an appropriate tube current value from the projected area of the elliptical section and the aspect ratio of the elliptical section, and executing a scan.

Patent Document 2 discloses a technique for improving a method and apparatus for performing automatic exposure control in CT scan. Specifically, during the first half of one rotation of the X-ray focus around the object, the exposure controller calculates the actual attenuation profile of the target slice from the electrical signal generated by the radiation detector in the half. Calculating an extrapolation attenuation profile of the slice for the second half of the swirl from the profile, and depending on the profile to change the radiation dose delivered by the x-ray source during the second half of the swirl, eg tube current etc. The operating parameters of the X-ray source are adjusted. The extrapolation attenuation profile is calculated to achieve a specified target pixel noise in the image of the currently scanned slice.
JP 2001-043993 A JP 2004-073865 A

  The X-ray CT apparatus of Patent Document 1 applies a tube current value corresponding to the model of the subject for each rotation during scanning, but uses a constant tube current value during each rotation. In addition, the image SD target value can be realized in the CT image of the subject.

  However, for example, by appropriately modulating the tube current value corresponding to the change in the subject transmission length during one rotation, the exposure dose in the direction where the transmission length is relatively short is appropriately reduced, and the target image SD It cannot be applied to the case where a value is realized.

  Furthermore, when view direction weights are applied to the view used for image reconstruction for the purpose of spiral correction and body motion correction, the influence of these weights on image noise is not considered for each view. The above conventional technique cannot be applied when the view direction weight is applied or when the target image SD value is realized.

  Further, the X-ray CT apparatus disclosed in Patent Document 2 cannot predict the exposure dose or the X-ray power at the time of scanning planning. Further, the influence of the view direction weight needs to be considered together with the tube current value of the corresponding view and the subject attenuation amount, but such consideration is not made.

  The object of the present invention is to balance the image quality of the reconstructed image and the exposure dose while properly considering the influence of the tube current modulation and the influence of the view direction weight in consideration of the change in the transmission length of the subject with respect to the X-ray tube circulation phase. To provide an X-ray tube current determination method and an X-ray CT apparatus for an X-ray CT apparatus capable of determining an imaging condition to be optimized at the time of scanning planning.

In order to achieve the above object, an X-ray tube current determining method for an X-ray CT apparatus according to the present invention is an X-ray tube current for an X-ray CT apparatus that determines an X-ray tube current flowing through the X-ray tube of the X-ray CT apparatus. A determination method, in which an object is scanned only in one direction without rotating an X-ray tube and an X-ray detector, and an object cross-sectional model is obtained from scanogram data obtained from the X-ray detector during the scanning. Determining the X-ray tube current standard modulation curve of the X-ray tube current to be changed for each view based on the subject cross-sectional model, and the X-ray tube current on the X-ray tube current standard modulation curve The step of estimating the image noise variance value of the reconstructed image based on the image noise variance contribution value of each view when using the image and the view direction weight applied to each view, and an operator set Target image noise determined from image noise target value Including a variance value, a step of modifying the X-ray tube current on the X-ray tube current standard modulation curve on the basis of the ratio of the image noise variance value the estimated, the. For each tube voltage that can be used and for each phantom that is the reference for CTDI, the reference CTDIvol value in (Expression 1) (Expression 1) tube current 1 mA, irradiation time 1 s, movement pitch (top plate z movement amount per scanner rotation) / X-ray beam thickness) = 1 is stored in advance, and a predicted value of the exposure dose is calculated based on the reference CTDIvol value and the X-ray tube current on the modified X-ray tube current standard modulation curve. And may further include a step of displaying.

  The “image noise value” here refers to the standard deviation of the CT value in the reconstructed image, and has the same meaning as the image SD value. Here, the image noise value is used as an index for the operator to specify the image quality of the CT image. This is a general case where the image noise value indicates the S / N ratio (signal / noise ratio) of the CT image. This is because it is used as a simple indicator.

  The “image noise variance value” is a value corresponding to the square value of the corresponding image noise value. For example, the “image noise variance target value” corresponds to the square value of the “image noise target value”. .

An X-ray CT apparatus according to the present invention includes an X-ray source that includes an X-ray tube that irradiates X-rays and an X-ray tube control device that controls the X-ray tube, and a subject. An X-ray detector facing the X-ray source, detecting X-rays and outputting imaging data, and a rotating plate mounted with the X-ray source and the X-ray detector and rotatable at predetermined angles An X-ray CT apparatus having an image processing apparatus that performs reconstruction calculation processing based on the imaging data, and scans the subject only in one direction without rotating the X-ray tube and the X-ray detector. , Means for obtaining an object cross-sectional model from scanogram data obtained from the X-ray detector during scanning, and an X-ray tube current of an X-ray tube current changed for each view based on the object cross-sectional model A means for determining a standard modulation curve and an X-ray tube current on the X-ray tube current standard modulation curve are used. Means for estimating the image noise variance contribution value for each view in the case of use, the image noise variance contribution value for each view that the estimated, the reconstructed image based on the view direction weight that is applied to each view The X-ray tube current standard modulation curve based on a ratio between a means for estimating an image noise variance value, a target image noise variance value determined from an image noise target value set by an operator, and the estimated image noise variance value Means for modifying the above x-ray tube current. Also, for each usable tube voltage and for each CTDI phantom, the reference CTDIvol value in (Equation 2) (Equation 2) tube current 1 mA, irradiation time 1 s, movement pitch (top plate z movement amount per scanner rotation) / X-ray beam thickness) = 1, a predicted value of the exposure dose is calculated based on the reference CTDIvol value and the X-ray tube current on the modified X-ray tube current standard modulation curve. And a means for displaying.

  According to the present invention, the balance between the image quality of the reconstructed image and the exposure dose can be obtained while appropriately taking into consideration the influence of the tube current modulation and the influence of the view direction weight in consideration of the change in the transmission length of the subject with respect to the X-ray tube circulation phase. It is possible to provide an X-ray tube current determination method and an X-ray CT apparatus for an X-ray CT apparatus that can determine imaging conditions to be optimized at the time of scanning planning.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In addition, this invention is not limited to embodiment shown below. FIG. 2 shows a general overview of an X-ray CT apparatus to which the present invention is applied, and FIG.

  As shown in FIG. 2, the X-ray CT apparatus includes a scanner 1, a patient table 2, an operation console 3, a top 4 of the patient table 2, a display device 5, and an operation device 6. As shown in FIG. 1, the scanner 1 includes an X-ray tube 8 in which X-rays are controlled by an X-ray tube controller 7. The X-rays radiated from the X-ray tube 8 are converted into, for example, a pyramid-shaped X-ray beam, that is, a cone beam X-ray, by the collimator 10 controlled by the collimator control device 9, and irradiated to the subject 17. X-rays transmitted through the subject 17 enter the X-ray detector 11.

  As shown in FIG. 3, the detector 11 includes a plurality of X-ray detection elements 18 that are two-dimensionally arranged in the channel direction and the column direction. The configuration of the X-ray detector 11 will be described later. A data collection device 12 is connected to the X-ray detector 11. The data collection device 12 collects detection data of individual X-ray detection elements 18 of the X-ray detector 11.

  The above components from the X-ray tube control device 7 to the data collection device 12 are mounted on the rotating plate 13 of the scanner 1. The rotating plate 13 is rotated by the driving force transmitted through the driving force transmission system 16 from the rotating plate driving device 15 controlled by the rotation control device 14.

  The X-ray detector 11 described above has a configuration in which a plurality of X-ray detection elements 18 are two-dimensionally arranged in the channel direction and the column direction as shown in FIG. The X-ray detection element 18 as a whole forms a cylindrical surface or an X-ray incident surface curved in a polygonal line shape with respect to the channel direction. The channel number i is, for example, about 1-1000, and the column number j is, for example, about 1-1000. It is. Further, the X-ray detection element 18 is constituted by a combination of a scintillator and a photodiode, for example. The spread angle of the cone beam X-ray in the channel direction in the X-ray detector 11, that is, the fan angle is α, and the cone beam X-ray in the X-ray detector 11 coincides with the column arrangement direction. The column direction spread angle, that is, the cone angle is γ.

  As shown in FIG. 4, after the subject 17 placed on the top 4 of the patient table 2 is carried into the opening of the scanner 1, the cone beam X-ray whose cone angle γ is adjusted by the opening width of the collimator 10 is taken. When the specimen 17 is irradiated, the image of the subject 17 irradiated with the cone beam X-ray is projected onto the X-ray detector 11, and the X-ray transmitted through the subject 17 is detected by the X-ray detector 11.

  The patient table 2 shown in FIG. 1 controls the patient table vertical movement device 21 by the patient table control device 20 to an appropriate table height, and controls the top plate driving device 22 by the patient table control device 20 to adjust the table. The plate 17 is moved back and forth so that the subject 17 is carried into and out of the X-ray irradiation space of the scanner 1 as shown in FIG.

  The console 3 shown in FIG. 1 has a system control device 19. The scanner 1 and the patient table 2 are connected to the system control device 19.

  More specifically, the X-ray tube control device 7, the collimator control device 9, the data collection device 12, and the rotation control device 14 in the scanner 1 are controlled by the system control device 19. The patient table control device 20 in the patient table 2 is controlled by the system control device 19.

  Data collected by the data collection device 12 in the scanner 1 is input to the image reconstruction device 23 under the control of the system control device 19.

  The image reconstruction device 23 creates a scanogram image using scanogram projection data (subject fluoroscopy data) collected by the data collection device 12 at the time of scanogram imaging. Use to perform CT image reconstruction.

  A storage device connected to the system control device 19 includes a scanogram image generated by the image reconstruction device 23, a reconstructed CT image, various data, a program for realizing the functions of the X-ray CT apparatus, and the like. 24.

  The system controller 19 is also connected to the display device 5 and the operation device 6. The display device 5 displays the reconstructed image output from the image reconstruction device 23 and various information handled by the system control device 19. The operation device 6 is operated by an operator and inputs various instructions and information to the system control device 19. The operator interactively operates the X-ray CT apparatus according to the present embodiment using the display device 5 and the operation device 6.

  A scan planning device 25 is also connected to the system control device 19, and a scan condition pre-plan is created using an instruction input by the operator using the operation device 6 and a scanogram image read from the storage device 24. It can be carried out. That is, the scanogram image read from the storage device 24 is displayed on the display device 5, and the operator uses the operation device 6 on the displayed subject scanogram image to perform a CT image reconstruction position (hereinafter referred to as a slice position). By designating the coordinates, it is possible to plan the slice position. Further, the information on the slice position planned here is stored in the storage device 24, and is also used by the scan planning device 25 for making plans such as tube current control conditions.

  In the X-ray CT apparatus according to the present embodiment, various preparation operations are performed in order to set imaging conditions before a scan for acquiring a CT image of the subject 17. As this preparatory operation, scanning of a scanogram image for setting the imaging position of the subject, analysis of the scanogram data, determination of an optimal tube current change pattern as imaging conditions based on the scanning, etc. It is carried out under 19 controls. In particular, analysis of scanogram data and determination of an optimal tube current change pattern as an imaging condition based on the analysis are important functions of the scan planning device 25 connected to the system control device 19.

  As main components involved in these preparation operations, the system control device 19, the scan planning device 25, the operation device 6, the display device 5, the X-ray tube 8, and the detector 11 in FIG. Etc.

  In this preparatory operation, the operating device 6 first inputs X-ray conditions such as an X-ray tube voltage and an X-ray tube current set value to the system.

  While the X-ray tube 8 and the X-ray detector 11 are stationary (with the rotating plate 13 not rotated), the table 2 and the rotating plate 13 are moved relative to each other along the body axis of the subject 17 to obtain a scanogram. An image is taken and scanogram projection data and scanogram image data are stored in the storage device 24.

  The scan planning device 25 analyzes the scanogram projection data, and models the estimated cross section at an arbitrary position along the body axis of the subject 17 as an elliptical cross section having an X-ray absorption coefficient equivalent to water, for example. This model is a three-dimensional model in which the major axis length and minor axis length of the elliptical cross section change depending on the position along the body axis of the subject 17 (hereinafter referred to as the z position) (hereinafter referred to as the subject 3). Called a dimensional model). Data of the subject three-dimensional model is stored in the storage device 24.

  The scan planning device 25 includes an image noise target value, a tube voltage, a tube current setting value, an X-ray collimation condition, a time per one rotation of the scanner (hereinafter referred to as a scanning time), and a scan planning device 25. Based on the data of the created three-dimensional object model, values of the following item (1) and item (2) are calculated and displayed on the display device 5 via the system control device 19.

  Item (1) is a series of tube current values, that is, tube current change patterns that change with time in accordance with changes in the transmitted X-ray dose of the imaging region of the subject 17 during the scan. The tube current change pattern is determined mainly based on the input image noise target value and the subject three-dimensional model. The determined tube current change pattern is stored in the storage device 24 and is sequentially called according to the imaging region of the subject 17 at the time of scanning to change the tube current of the X-ray tube 8.

  Item (2) is an estimated exposure dose. First, a projected area (described later) of a CTDI measurement phantom defined in IEC 60601 and an actual measurement value or a theoretical calculation value of the exposure dose in the phantom are stored in the storage device 24 as a database, Estimate by comparing the dimensional model and recommended X-ray conditions. The estimated exposure dose is evaluated by the operator by being displayed on the display device 5, and the operator can change the X-ray condition by operating the operation device 6 again as necessary. Since the influence of scattered radiation is not negligible with respect to the problem of exposure dose, when the exposure dose in the CTDI measurement phantom is obtained by theoretical calculation, calculation in consideration of the influence of both direct rays and scattered rays is necessary.

  FIG. 5 shows a flow chart of a series of preparatory operations prior to scanning by the X-ray CT apparatus according to the embodiment of the present invention. Hereinafter, each step in FIG. 5 will be described in the order of steps.

  In the scanogram imaging step of step S100, a scanogram image of the subject 17 is captured. The procedure for capturing a scanogram image of the subject 17 and the procedure for capturing a CT image in a scan are basically the same. In step S <b> 100, scanogram projection data is obtained by irradiating the subject 17 with X-rays from a certain direction, for example, the back direction, without rotating the rotating plate 13 and capturing projection data by the detector 11. This scanogram projection data is sent from the X-ray detector 11 to the image reconstruction device 23 via the system control device 19, and a scanogram image is created in the image reconstruction device 23.

  The scanogram image obtained at this time is an image of X-rays transmitted from a front direction from a certain direction, for example, from the back to the front. This scanogram image is used for setting the slice position (CT image reconstruction position) of the subject 17 at the time of scanning. Further, the scanogram projection data is not only used for creating a scanogram image, but also used in the present invention for determining a tube current change pattern for tube current control particularly in scanning.

  In steps S110 to S130, the operator inputs a top plate movement pitch, a scan start position, and a scan end position as imaging conditions from the operation device 6 with reference to the scanogram image. By using these input data, the scan planning device 25 determines the CT image capturing range of the subject 17, the slice position z, and the phase angle (phase angle of the rotating plate 13) β of the X-ray tube 8. Here, the scan start position and the scan end position mean the z position of the first CT image and the z position of the last CT image obtained by a series of scans, respectively.

  In step S140, the operator inputs a tube voltage setting value, an X-ray collimation condition, a type of reconstruction filter function, and a visual field size as imaging conditions from the operation device 6.

  In step S150, the operator inputs an image noise target value as an image quality target from the operation device 6.

  Next, in the scanogram projection data analysis step in step S160 and the subject three-dimensional model generation step in step S170, the scanogram projection data is analyzed by the scan planning device 25, and a subject three-dimensional model of the subject 17 is generated. . This subject three-dimensional model is obtained by approximating each cross section of the subject 17 corresponding to the z position as an elliptical cross section having an X-ray absorption coefficient equivalent to water.

  In step S180, an X-ray attenuation index T for each position z in the body axis direction and each phase angle β of the X-ray tube 8 is calculated. Here, the X-ray attenuation index is an integral value of the X-ray absorption coefficient distribution along the X-ray transmission path passing through the center of the elliptical section in (z, β) of the subject three-dimensional model. Since this data can be obtained from the previously generated three-dimensional model of the subject, the scan planning device 25 calls and calculates the three-dimensional model of the subject from the storage device 24. The X-ray attenuation index calculation result is expressed as T = T (z, β).

  Next, in the step S190, a scan time as an imaging condition is input from the operation device 6. When the scan start position, the scan end position, the top plate movement pitch, and the scan time are determined, the position (z, β) of the X-ray tube 8 during the scan can be expressed as a function of the elapsed time t after the scan starts. Therefore, the X-ray attenuation index T of the subject 17 at each scan position can also be expressed as a function T = T (t) of time t. For this reason, in the process of step S200, the function of the X-ray attenuation index T is converted from T = T (z, β) to T = T (t).

Next, an example of the tube current change pattern setting method in step S210 will be described. Here, the number of views used to reconstruct the CT image Img (z) at the slice position z is M, and the convenient view number m is m = 0 to M−1. When the number of views per rotation is N, the number of used views M is not necessarily equal to the number of views N per rotation. Here, the aforementioned X-ray attenuation index T can also be expressed as a function T (m) of the view number to be used. Assuming that the maximum value of the X-ray attenuation index T in view numbers m = 0 to M−1 is Tmax (0: M−1), and the reference tube current value i_ref is assumed to correspond at this time, the tube current for the view number m The value i _v (m) is as follows:

[Equation 1]
i _v (m) = i_ref * exp (T (m)-Tmax (0: M-1))
On the other hand, assuming that the time trot for one rotation of the scanner is equal to the reference time trot_ref, and the X-ray attenuation index T is a constant value during that time, the tube voltage is xv, and the tube current value i is the reference tube current value i_ref. The image noise variance value V when the image thickness thk is reconstructed as the reference image thickness thk_ref using the reconstruction filter function g with equal weighting on the rotating view number N_ref is a function of the X-ray attenuation index T As the following equation.

A (xv) and b (xv, g) are stored in the storage device 24 in advance. The predicted image noise variance V ^* when using the tube current value i _v (m) expressed by the above-described equation 1 is expressed as the following equation.

  Here, w (m) in Equation 3 is a view direction weight applied to each view. The view direction weight is used when the number of views M used for reconstruction is different from the number of views N per rotation or when correcting artifacts due to the movement of the subject.

If the number of used views M is equal to the number of views N per rotation,
w (m) = 1 (m = 0 to N-1)
By so doing, so-called full scan reconstruction can be performed.

Here, from the image noise variance target value Vtgt (square value of SDtgt) determined from the image noise target value SDtgt input by the operator and the image noise variance predicted value V ^{* of the} equation (3), the tube current value i to be actually applied. _a (m) is determined as follows.

  As described above, the tube current modulation pattern for realizing the image noise target value input by the operator in the CT image at each slice position can be determined. If this tube current modulation pattern is I, I can be expressed as a function I (t) of the elapsed time t after the start of scanning. The tube current modulation pattern I = I (t) determined in this way is stored in the storage device 24, and sequentially called by the system control device 19 according to the imaging region of the subject 17 at the time of scanning. 7 to control the tube current during scanning.

Next, calculation and display of the predicted exposure dose value from step S220 to step S230 will be described. In the present embodiment, the exposure dose is predicted by applying CTDIvol defined in IEC 60601. The storage device 24 includes a tube current of 1 mA, an irradiation time of 1 s, a movement pitch (amount of movement of the top plate per scanner rotation / The reference CTDIvol value at X-ray beam thickness) = 1 is stored in advance. here,
CTDIvol_ref (xv, 160): Reference CTDIvol value for PMMA phantom with tube voltage xv and diameter 160mm
CTDIvol_ref (xv, 320): A reference CTDIvol value for a PMMA phantom having a tube voltage xv and a diameter of 320 mm. The storage device 24 also stores a projection area threshold value Pthr (xv) at the tube voltage xv in advance. Here, the projection area is a value obtained by adding projection data over all channels in the same view, and is proportional to the product of the cross-sectional area of the subject and the average value of X-ray absorption coefficients within the cross-section. The projection area threshold value Pthr (xv) is for distinguishing the size of the cross section of the subject three-dimensional model from the head equivalent and the abdomen equivalent. As the projection area threshold value Pthr (xv), the projection area of a PMMA phantom having a tube voltage xv and a diameter d [mm] is designated as SPMMA (xv, d).
Pthr (xv) = SPMMA (xv, 190)
Is used.

It is assumed that a series of scans to be planned are the scanner rotation speed R, the movement pitch p, the time trot per rotation, and the tube voltage xv in the X-ray irradiation state. Here, the scanner rotation number r is set to 0 to R-1 for convenience, and the average projected area Spat (xv, r) of the subject 17 during one rotation at an arbitrary rotation number r is predicted from the subject three-dimensional model. The predicted value is set to Spat_pred (xv, r). Further, the average tube current value i_mean (r) during one rotation at an arbitrary rotation number r can also be predicted using the above-described tube current modulation pattern determination result. Therefore, CTDIvol_pr which is a CTDIvol predicted value for each rotation is predicted as follows.

さらに、回転番号０〜Ｒ−１までの平均ＣＴＤＩｖｏｌ予測値である
ＣＴＤＩｖｏｌ＿ｐｒｅｄを次式のように予測する。

Furthermore, CTDIvol_pred which is an average CTDIvol predicted value from rotation numbers 0 to R-1 is predicted as the following equation.

  CTDIvol_pred obtained by the equation (6) is displayed as a predicted exposure dose on the display device 5 from the scan planning device 25 via the system control device 19.

  Next, in the image noise / exposure dose determination process in step S240, the operator looks at the predicted exposure dose value of the subject 17 displayed in step S230 and the target image noise value input in step S150. If it is determined that the balance is appropriate, the process proceeds to the scan execution process in step S250 to start the scan. If it is determined that the balance is not appropriate, step S150 is performed. Then, the image noise target value is input again.

  As described above, by inputting an image noise target value before an X-ray CT scan and determining a tube current modulation pattern suitable for it, a scan that optimizes the balance between image noise and exposure dose can be performed. Further, by predicting the exposure dose of the subject 17 and displaying the prediction result, the operator can know in advance the balance between the image noise of the subject 17 and the exposure dose according to the imaging conditions in advance. If the balance is unsatisfactory, the shooting conditions can be further changed.

  FIG. 6 is a flowchart for explaining the operation in the scan preparation operation executed according to the second embodiment of the present invention. The same steps as those in FIG. 5 are indicated by the same step numbers.

Here, the tube current modulation pattern determination method in step S260 will be described with reference to FIG. The number of views used to reconstruct the CT image Img (z) at the slice position z is M, and the convenient view number m is m = 0 to M-1. When the number of views per rotation is N, the number of used views M is not necessarily equal to the number of views N per rotation. Further, the phase angle of the X-ray tube 8 at the view number m = 0 is β ₀ . However, the case where the X-ray tube 8 is located directly above is defined as a phase angle of 0 radians, and the circulation direction of the X-ray tube 8 is defined as a positive direction. Further, the horizontal axis length of the elliptical cross section of the subject three-dimensional model is A, and the vertical axis length is B. Here, when X-ray irradiation is performed in the major axis direction of the elliptical section of the subject three-dimensional model, the reference tube current value i_ref is made to correspond, and X-ray irradiation is performed in the minor axis direction of the elliptical section of the subject three-dimensional model Is made to correspond to the tube current value η * i_ref using an appropriate tube current modulation coefficient η (0 <η ≦ 1), and in other views, i_ref and η * i_ref are set according to the phase angle of the X-ray tube 8. Assuming that the result interpolated with the trigonometric function is applied, the tube current value i _v (m) for the view number m is expressed by the following equation.

The tube current value i _a (m) to be actually applied is determined using the equations (7) and (2) to (4). According to the present embodiment, even when the X-ray tube control device 7 in which the change rate of the tube current cannot be increased so much is used, the X-ray tube control device is appropriately determined by setting the tube current modulation coefficient η. The tube current modulation can be performed within the capacity range of 7, and the image quality according to the image noise target value can be realized. Even in the case where the tube current modulation coefficient η = 1, the predicted image noise variance V ^* according to Equation 3 also changes in accordance with the change of the major axis length and minor axis length of the elliptical cross section according to the slice position z. Therefore, the applied tube current ia (m) calculated using Equation 4 also changes, and it is possible to realize image quality along the image noise target value. Although this modification uses trigonometric function interpolation, the object of the present invention can be similarly achieved by using simpler linear function interpolation.

  FIG. 8 is a flowchart for explaining the operation in the scan preparation operation according to the third embodiment of the present invention. The same steps as those in FIG. 5 are indicated by the same step numbers.

Here, the tube current modulation pattern determination method in step S270 will be described. As the tube current value i _v (m) assumed for the view number m in order to predict the image noise, any one of the above formulas 1 and 7 may be used depending on the system.

Here, it is known that the moire-like artifacts of the image can be reduced by using a method called “opposite data addition” instead of simply multiplying the projection data by the view direction weight for the view number m. When using opposite data addition, how to use view direction weights and projection data is as follows.
(a) If there is a view number m with view number <N / 2 and weight value> 1, the following processing (a.1) to (a.2) is performed.
(a.1) For view number m corresponding to (a) above, the projection data of view number m is multiplied by 1 (not weight w (m)) and used for image reconstruction.
(a.2) In the opposite view number m + N / 2 of the view number m corresponding to (a) above, the weight w (m + N / 2), the channel number i, and the view in the view number m + N / 2 Projection data Proj (i, m + N / 2) of number m + N / 2, weight w (m) at view number m, channel number Ki (opposite side channel) at view number m, and projection data of its neighboring channels Proj (K-i + p, m) (where p =-(λ / 2) +1, ..., 0, ..., λ / 2 where λ is the number of channels used for channel direction interpolation), Equation 8 is used for image reconstruction as the channel direction interpolation coefficient f _p .

Here, the channel interpolation data is used for the opposite side, that is, the side having a view number different by N / 2 when the arrangement of the X-ray detector 11 is a so-called 1/4 channel offset as shown in FIG. This is because the counter data is shifted by 1/2 channel.
(b) When there is a view number m with a view number ≧ N / 2 and a weight value> 1, the following processes (b.1) to (b.2) are performed.
(b.1) For view number m corresponding to (b) above, the projection data of view number m is multiplied by 1 (not weight w (m)) and used for image reconstruction.
(b.2) For the opposite view number mN / 2 of the view number m corresponding to (b) above, the projection of the weight w (mN / 2), channel number i, and view number mN / 2 at the view number mN / 2 Data Proj (i, mN / 2), weight w (m) at view number m, channel number Ki (opposite side channel) at view number m and projection data Proj (K-i + p, m) of its neighboring channels (where, p = - (λ / 2 ) +1, ..., 0, ..., number of channels lambda with lambda / 2 used in the channel direction interpolation), number as the coefficient f _p in the channel direction interpolation 9 Equations are used for image reconstruction.

When the counter data addition is used as described above, the predicted image noise variance V ^* when the tube current value iv (m) is used is expressed by the following equation.

Where u _p (m) (p = (-λ / 2) +1, ..., 0, ..., λ / 2) is the number of channels used for channel direction interpolation, λ (even), any The channel range used for interpolation corresponding to the channel number i is expressed by the following equation as i-λ / 2 + 1 to i + λ / 2 and the channel direction interpolation coefficient f _p .

Sumvalue is as follows.

Therefore, the tube current value i _a (m) to be actually applied is determined by using Formula 1 or Formula 7, Formula 2, Formula 10 to Formula 12, and Formula 4. According to the present embodiment, it is possible to realize image quality according to the image noise target value while reducing moire artifacts by adding the opposing data. If f ₀ = 1 and p ≠ 0 and f _p = 0 in equation (11), the calculation amount increases from equation (3), but the calculation result of equation (10) is the same as equation (3), and the opposite data Even when the addition is not used, the equation (8) can be applied.

  Although the present invention has been described based on the above embodiment, it goes without saying that the present invention is not limited to the above-described embodiment.

As described above, in the X-ray CT apparatus of the present invention, the scan planning device 25 (1) scanogram analysis / object 3D model generation function for generating a 3D model of the object 17 from the scanogram projection data of the object 17 Or
(2) The image noise corresponding to the imaging region of the subject 17 is predicted from the three-dimensional model of the subject 17, and the modulation pattern of the tube current is automatically calculated by comparing the predicted image noise value with the target image noise value input by the operator. Tube current setting function to set automatically,
(3) A function for predicting the exposure dose of the subject 17 based on the tube current value,
And the display device 5 for displaying the processing results of the storage device 24 and the scan planning device 25 for storing the database file storing the data used for the prediction of the image noise and the exposure dose, and the imaging conditions of the X-ray CT apparatus. Since the setting device 6 is provided, the tube current change pattern during scanning can be automatically set by inputting the image SD target value as an imaging condition, and X to the subject 17 can be set. The radiation exposure can be assessed in advance.

  Furthermore, when the operator determines that the balance between the image SD target value and the predicted exposure dose value is not appropriate, the image SD target value can be reset. This makes it possible to easily perform an X-ray CT examination with an appropriate balance between exposure dose and image quality.

Overall configuration diagram of an X-ray CT apparatus to which the present invention is applied Overview of X-ray CT apparatus to which the present invention is applied Schematic diagram illustrating the configuration of a detector of an X-ray CT apparatus to which the present invention is applied and the relationship with X-ray irradiation The figure which shows the relationship between the scanner of X-ray CT apparatus to which this invention is applied, a patient table, and a subject from a side surface direction. Operation flow diagram of preparatory operations prior to scanning of an X-ray CT apparatus to which an embodiment of the present invention is applied Operation flow diagram of scan preparation operation according to second embodiment of the present invention The figure which shows the positional relationship of the vertical axis | shaft of the cross section of a to-be-examined object three-dimensional model, a horizontal axis, and a X-ray tube phase angle. Operation flow diagram of scan preparation operation according to the third embodiment of the present invention Explanatory diagram of channel position relationship of opposite data

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Scanner, 2 ... Patient table, 3 ... Console, 4 ... Top plate, 5 ... Display apparatus, 6 ... Operation apparatus, 7 ... X-ray tube control apparatus, 8 ... X-ray tube, 9 ... Collimator control apparatus, 10 DESCRIPTION OF SYMBOLS ... Collimator, 11 ... X-ray detector, 12 ... Data acquisition device, 13 ... Rotating plate, 14 ... Rotation control device, 15 ... Rotating plate drive device, 16 ... Driving force transmission system, 17 ... Subject, 18 ... X-ray Detection element, 19 ... system control device, 20 ... patient table control device, 21 ... patient table vertical movement device, 22 ... top plate drive device, 23 ... image reconstruction device, 24 ... storage device, 25 ... scan planning device

Claims (10)

An X-ray tube current determining method for an X-ray CT apparatus for determining an X-ray tube current to be passed through an X-ray tube of an X-ray CT apparatus,
Scanning the subject only in one direction without rotating the X-ray tube and the X-ray detector, and obtaining a subject cross-sectional model from the scanogram data obtained from the X-ray detector at the time of scanning;
Determining an X-ray tube current standard modulation curve of an X-ray tube current to be changed for each view based on the subject cross-sectional model;
The image noise variance contribution value of each view in the reconstructed image when the X-ray tube current in each view on the X-ray tube current standard modulation curve is used, and the view direction weight applied to each view On the basis of estimating an image noise variance value of a reconstructed image when using an X-ray tube current on the X-ray tube current standard modulation curve,
Based on a ratio between a target image noise variance value determined from an image noise target value set by an operator and the estimated image noise variance value, the X-ray tube current is set so as to achieve the target image noise variance value. Correcting the x-ray tube current on the standard modulation curve;
A method for determining an X-ray tube current of an X-ray CT apparatus. In the X-ray tube current standard modulation curve of the X-ray tube current changed for each view, the tube current value i _v (m) with respect to the view number m,
Let the image noise variance value of the estimated reconstructed image be the image noise variance value V ^* ,
Correction of the X-ray tube current on the X-ray tube current standard modulation curve is performed by the following (Equation 1).

However,
2. The X-ray CT apparatus according to claim 1, wherein: i _a (m): tube current to be actually applied v _tgt : target image noise variance value determined from an image noise target value set by the operator X-ray tube current determination method. In the step of estimating the image noise variance value V ^* of the reconstructed image, to estimate the image noise variance value V ^* by the following (Equation 2),

However,
N: 1 bi-menu number per revolution
M: number of views used to reconstruct a CT image at a desired slice position
m: View number (m = 0 to M-1)
w (m): View direction weight applied to view number m (w (m) = 1 (m = 0 to N−1) in full scan reconstruction)
T (m): X-ray attenuation index at view number m
trot: Time for one scanner rotation
thk: image thickness V (T (m), i _v (m), trot, thk): image noise variance value when reconstructed as T (m), i _v (m), trot, thk An X-ray tube current determination method for an X-ray CT apparatus according to claim 2 . In the step of estimating the image noise variance value V ^* of the reconstructed image, to estimate the image noise variance value V ^* by the following (Equation 3),

The method of determining an X-ray tube current of an X-ray CT apparatus according to claim 2 . For each tube voltage that can be used and for each phantom that is the standard of CTDI, the standard CTDIvol value in (Equation 5) below,

Tube current 1 mA, irradiation time 1 s, movement pitch (top plate z movement amount per scanner rotation / X-ray beam thickness) = 1 (Equation 5)

Is stored in advance,
Calculating and displaying a predicted value of the exposure dose based on the reference CTDIvol value and the X-ray tube current on the modified X-ray tube current standard modulation curve;
The X-ray tube current determination method of the X-ray CT apparatus according to claim 1, wherein the X-ray tube current is determined. An X-ray source comprising: an X-ray tube that emits X-rays; and an X-ray tube control device that controls the X-ray tube;
An X-ray detector facing the X-ray source across the subject, detecting X-rays and outputting imaging data;
A rotating plate mounted with the X-ray source and the X-ray detector and capable of rotating at predetermined angles;
An image processing device that performs reconstruction calculation processing based on the imaging data;
An X-ray CT apparatus comprising:
Means for scanning the subject only in one direction without rotating the X-ray tube and the X-ray detector, and obtaining a subject cross-sectional model from the scanogram data obtained from the X-ray detector at the time of scanning;
Means for determining an X-ray tube current standard modulation curve of an X-ray tube current to be changed for each view based on the subject cross-sectional model;
Means for estimating an image noise variance contribution value of each view in a reconstructed image when using an X-ray tube current in each view on the X-ray tube current standard modulation curve;
Reconstruction using the X-ray tube current on the X-ray tube current standard modulation curve based on the estimated image noise variance contribution value in each view and the view direction weight applied to each view Means for estimating an image noise variance value of the image;
Based on a ratio between a target image noise variance value determined from an image noise target value set by an operator and the estimated image noise variance value, the X-ray tube current is set so as to achieve the target image noise variance value. Means for correcting the x-ray tube current on the standard modulation curve;
An X-ray CT apparatus comprising: Means for correcting the x-ray tube current on the x-ray tube current standard modulation curve;
In the X-ray tube current standard modulation curve of the X-ray tube current changed for each view, the tube current value i _v (m) with respect to the view number m,
When the estimated image noise variance value of the reconstructed image is an image noise variance value V ^* , correction of the X-ray tube current on the X-ray tube current standard modulation curve is performed by the following (Equation 6).

However,
The X-ray CT apparatus according to claim 6, wherein: i _a (m): tube current to be actually applied v _tgt : target image noise variance value determined from an image noise target value set by the operator. The means for estimating the image noise variance value of the reconstructed image estimates the image noise variance value V ^{* according} to (Equation 7) below.

However,
N: 1 bi-menu number per revolution
M: number of views used to reconstruct a CT image at a desired slice position
m: View number (m = 0 to M-1)
w (m): View direction weight applied to view number m (w (m) = 1 (m = 0 to N−1) in full scan reconstruction)
T (m): X-ray source weak index at view number m
trot: Time for one scanner rotation
thk: image thickness V (T (m), i _v (m), trot, thk): image noise variance value when reconstructed as T (m), i _v (m), trot, thk The X-ray CT apparatus according to claim 7 . The means for estimating the image noise variance value of the reconstructed image estimates the image noise variance value V ^{* according} to (Equation 8) below.

The X-ray CT apparatus according to claim 7 . For each usable tube voltage and for each CTDI standard phantom, the standard CTDIvol value in (Equation 10) below,

Tube current 1 mA, irradiation time 1 s, movement pitch (top plate z movement amount per scanner rotation / X-ray beam thickness) = 1 (Equation 10)

Means for storing
Means for calculating and displaying a predicted value of the exposure dose based on the reference CTDIvol value and the X-ray tube current on the modified X-ray tube current standard modulation curve;
The X-ray CT apparatus according to claim 6, wherein

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