JP5469952B2 - X-ray CT system - Google Patents

Description

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

  The X-ray CT apparatus irradiates X-rays from around the subject, collects data relating to the intensity of the X-rays transmitted through the subject with an X-ray detector, and based on the collected data, X-rays inside the subject are collected. This is an apparatus for imaging the distribution information of the linear absorption coefficient. In recent years, an automatic dose control function for calculating and controlling an optimal dose in order to reduce the X-ray dose while achieving the required image quality has been developed to reduce the exposure dose. In this automatic dose control function, a cross-sectional model at each position in the body axis direction of the subject is generated based on scanogram projection data obtained before the scan is executed, and the optimal dose (in the series of scans) based on these cross-sectional models ( Scan tube current etc.) is calculated. Therefore, unless the scanogram projection data is acquired accurately, it becomes impossible to calculate an optimal dose, and it becomes difficult to obtain a desired image quality.

  By the way, the intensity and quality (spectral distribution) of X-rays emitted from the X-ray generation source of the X-ray CT apparatus fluctuate with time. So-called reference correction is performed on the acquired projection data. In reference correction, generally, one to a plurality of channels at both ends of an X-ray detector are used as calibration detection elements (hereinafter referred to as reference channels), and X-rays that do not pass through the subject are directly detected by the reference channel. The image calculation device of the X-ray CT apparatus calibrates the X-ray intensity levels obtained in other channels with reference to the X-ray intensity detected in the reference channel, and uses the calibrated signals to obtain a scanogram image, a tomographic image, etc. Generation is in progress.

  However, for example, when the subject has a large physique or when the subject is shifted from the center of rotation of the scanner and placed on the bed, X-rays directed to the reference channel are blocked by the subject and a normal reference signal can be obtained. There may not be. In order to solve such problems, in Patent Document 1, signals obtained from the reference channels at both ends are compared with a predetermined threshold value, and it is determined whether the reference channel is blocked by the magnitude of the signal strength. When one is blocked, only the other reference channel signal is used as reference correction data, and artifacts are suppressed from appearing. Further, in Patent Document 2, the maximum signal value up to the previous view of the view to be calculated is held for each rotation of the scanner in each of the reference channels on both sides, and the reference signal on the side below a predetermined threshold is stored. On the other hand, a process of replacing with the maximum signal value is added. Thereby, even when the reference channels on both sides are blocked, the reference correction is moderately performed.

JP 2000-060840 A JP 2003-144427 A

  However, if the reference channels on both sides from the first view are covered by the subject, the reference correction cannot be performed properly even using the methods disclosed in Patent Document 1 and Patent Document 2 described above. Therefore, when the reference correction on the both sides is covered by the subject etc., if the reference correction is performed by the conventional method, the cross-sectional model obtained from the scanogram image may be calculated smaller than the actual subject size, The scan tube current calculated by the above-described dose automatic control function may be smaller than necessary. As a result, there is a problem that a desired image quality cannot be obtained.

  The present invention has been made in view of the above problems. Even when the reference channels on both sides of the X-ray detector are covered with the subject or the like, the X-ray intensity is appropriately compensated from the first view. An object of the present invention is to provide an X-ray CT apparatus capable of obtaining good image quality.

In order to achieve the above-described object, the present invention irradiates a subject with X-rays from each view direction around the subject, and detects transmitted X-rays transmitted through the subject with an X-ray detector. Projection data collection means for collecting data, and correction means for correcting the collected projection data based on reference data collected by reference channels for monitoring X-ray intensity provided at one or both ends of the X-ray detector And image calculation means for reconstructing an image based on the corrected projection data, and air calibration data of each view acquired by the reference channel in the absence of the subject in the X-ray CT apparatus Reference data holding means for holding the reference channel as reference data in advance and whether or not the reference channel is covered with the subject at the time of subject imaging. Determination means for determining each view based on data, scan tube current / tube voltage monitoring means for measuring a scan tube current and a scan tube voltage during scanning, and a view of each detection element of the X-ray detector during scanning Integration time monitoring means for measuring each integration time, and the correction means corresponds to the view for which the determination means determines that the reference channel is covered by the subject. The pseudo reference data includes the reference data, each measured value of the scan tube current and the scan tube voltage by the scan tube current / tube voltage monitoring unit, and the integration time measured by the integration time monitoring unit. calculated using the parameters is the X-ray CT apparatus characterized by the use as the reference data of the view

  According to the X-ray CT apparatus of the present invention, even when the reference channels on both sides of the X-ray detector are covered with the subject or the like, the X-ray intensity is appropriately compensated from the first view to obtain good image quality. It becomes possible.

Hardware block diagram of X-ray CT apparatus 1 (first embodiment) The figure explaining the positional relationship between each part of the scanner and the subject 6 The figure explaining the positional relationship between the subject 6 and the reference channels RefL and RefR Flowchart explaining photographing process including replacement of reference data The figure explaining the bed moving speed Hardware block diagram of X-ray CT apparatus 100 (second embodiment) Hardware block diagram of X-ray CT apparatus 150 (third embodiment)

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

(First embodiment)
First, the configuration of the X-ray CT apparatus 1 according to the first embodiment will be described with reference to FIGS.
In this embodiment, the case where there is one X-ray tube will be described. However, the present invention is also applicable to a multi-ray source type X-ray CT apparatus. In addition, the X-ray CT apparatus is a rotation-rotation method (Rotate-Rotate method) in which an X-ray tube and an X-ray detector rotate together while irradiating a wide fan beam covering the entire subject, There are an electron beam scanning system (Scanning Electron Beam system) in which the target electrode is applied while being deflected, and other systems, but the present invention is applicable to any system of X-ray CT apparatus.

  As shown in FIG. 1, the X-ray CT apparatus 1 includes a scanner 2, a bed 3, and a console 4. The X-ray CT apparatus 1 acquires the projection data of the subject 6 by loading the subject 6 placed on the top plate on the bed 3 into the opening 104 of the scanner 2 and scanning it.

  The scanner 2 includes an X-ray tube 201, an X-ray control device 202, a collimator 203, an X-ray detector 205, a data collection device 206, a rotating plate 207, and a gantry control device 208.

  The X-ray tube 201 is an X-ray source, and is controlled by the X-ray control device 202 to irradiate the subject 6 with X-rays continuously or intermittently. The X-ray control device 202 controls the X-ray tube voltage and the X-ray tube current applied or supplied to the X-ray tube 201 in accordance with the X-ray tube voltage and the X-ray tube current determined by the system control device 401 of the console 4. To do.

  The collimator 203 irradiates the subject 6 with X-rays radiated from the X-ray tube 201 as X-rays such as fan beams, for example, and is controlled by a collimator control device (not shown). X-rays transmitted through the subject 6 enter the X-ray detector 205.

  The X-ray detector 205 includes, for example, about 1000 X-ray detection element groups configured by, for example, a combination of a scintillator and a photodiode in the channel direction (circumferential direction), for example, about 1-320 in the column direction (body axis direction). They are arranged and arranged so as to face the X-ray tube 201 through the subject 6. The X-ray detector 205 detects X-rays emitted from the X-ray tube 201 and transmitted through the subject 6, and outputs the detected X-ray data to the data collection device 206.

  The X-ray detector 205 uses one or more channels at one or both ends as reference channels for X-ray intensity monitoring. Data collected in the reference channel is referred to as reference data.

  The data collection device 206 is connected to the X-ray detector 205, collects X-ray dose data detected by individual X-ray detection elements of the X-ray detector 205, converts it into digital data, and uses the console 4 as projection data. Output to the image calculation device 402.

  An X-ray tube 201, a collimator 203, an X-ray detector 205, and a data collection device 206 are mounted on the rotating plate 207. The rotating plate 207 is rotated by a driving force transmitted from a rotating plate driving device controlled by a gantry control device 208 through a drive transmission system. The gantry control device 208 controls the rotary plate driving device according to a control signal input from the system control device 401 of the console 4.

  The bed 3 includes a top plate, moving mechanisms for moving the top plate in the up-down direction, the body axis direction, and the left-right direction with respect to the body axis, and the bed control device 301. The bed control device 301 controls the movement mechanism of the bed 3 in each direction to make the bed 3 have an appropriate height, and moves the top plate in the body axis direction or in the left-right direction with respect to the body axis. . Thereby, the subject 6 is carried in and out to an appropriate position in the X-ray irradiation space of the scanner 2.

  The console 4 includes a display device 7, an operation device 8, a system control device 401, an image calculation device 402, and a storage device 404. The system control device 401 is connected to the X-ray control device 202, the bed control device 301, and the gantry control device 208 in the scanner 2, and controls the scanner 2 and the bed 3. The image calculation device 402 is connected to the data collection device 206 in the scanner 2 and acquires projection data collected by the data collection device 206.

  The display device 7 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 system control device 401. The display device 7 displays a reconstructed image and a scanogram image output from the image arithmetic device 402 and various information handled by the system control device 401.

  The operation device 8 includes, for example, an input device such as a keyboard, a mouse, and a numeric keypad, and various switch buttons. The operation device 8 outputs various instructions and information input by the operator to the system control device 401. The operator interactively operates the X-ray CT apparatus 1 using the display device 7 and the operation device 8.

  The system control device 401 includes a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), and the like. The system control device 401 controls the X-ray control device 202, the data acquisition device 206, the X-ray detector 205, and the gantry control device 209 in the scanner 2, and controls the bed control device 301 of the bed 3, A scan of each position in the body axis direction of the subject 6 (hereinafter referred to as a Z position) is performed. At this time, the system control apparatus 401 irradiates X-rays from a plurality of angular directions (hereinafter referred to as views) around the subject 6 and acquires measurement data (projection data) in view units. At the time of scanogram imaging, the system control apparatus 401 moves the bed 3 in the body axis direction while projecting data while irradiating X-rays from one direction (for example, from the back of the subject to the front) without rotating the rotating plate 207. To get.

Furthermore, the X-ray CT apparatus 1 performs a scanning operation without a subject. This is called air calibration, and is performed at a predetermined timing such as when the X-ray CT apparatus 1 is activated. In the X-ray CT apparatus 1, the image arithmetic unit 402 performs necessary correction operations such as sensitivity correction of the X-ray detector 205 based on a detection signal (referred to as air calibration data) obtained by air calibration.
The X-ray CT apparatus 1 of the present invention stores the air calibration data (air cal reference value) of each view acquired by the reference channel at the time of air calibration in the storage device 404 as reference data. The reference data of each view is referred to when the reference channel is covered with a subject or the like and appropriate reference data cannot be obtained, and is used for calculating pseudo reference data described later.

  The image calculation device 402 acquires measurement data collected by the data collection device 206 in the scanner 2 under the control of the system control device 401, and performs various correction processes such as reference correction and correction of physical characteristics of the X-ray detector 205. The scanogram image and the tomographic image are reconstructed using the corrected data.

  The storage device 404 is configured by a hard disk or the like, and is connected to the system control device 401. The storage device 404 stores scanogram projection data and scanogram images collected by the data collection device 206, imaging conditions input from the operation device 8, target image quality, and the like. In addition to these various data, a tomographic image generated by the image reconstruction device 402, a program for realizing the functions of the X-ray CT apparatus 1, and the like are stored.

Next, reference channels RefL and RefR provided in the X-ray detector 205 will be described with reference to FIGS.
The reference channel is a calibration channel for monitoring the intensity of X-rays, and is composed of one to a plurality of channels of detection elements. The reference channel is provided at one end or both ends of the X-ray detector 205. Hereinafter, the reference channel provided on the left side in FIGS. 2 and 3 is referred to as RefL, and the reference channel provided on the right side is referred to as RefR. Normally, as shown in FIG. 2, the reference channels RefL and RefR are provided at positions where X-rays are not blocked by the subject 6.

  However, as shown in FIG. 3A, when the subject 6 is placed close to one side of the bed 3, one reference channel RefL (or RefR) is blocked by the subject 6. Further, as shown in FIG. 3B, when the subject 6 is large in size, both reference channels RefL and RefR may be blocked by the subject 6. Further, depending on the geometry of the bed 3 and the positioning in the gantry, the reference channel may be blocked by the bed 3 instead of the subject 6 itself. In the following description, the case where the reference channel is covered with the subject 6 will be described.

  In the present embodiment, the image calculation device 402 of the X-ray CT apparatus 1 determines whether or not the reference channels RefL and RefR are covered (covered) by the subject 6 with a predetermined value, When the subject 6 is covered, the reference data used for reference correction is replaced with pseudo reference data described later.

  It should be noted that processing such as reference channel cover determination and calculation and replacement of pseudo reference data may be performed by the image arithmetic device 402 of the console 4 or may be performed by the system control device 401.

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

In the X-ray CT apparatus 1 of the present invention, air calibration is performed in advance before scanogram imaging or scanning. It is assumed that the output data (air cal reference value) of the reference channel acquired by this air calibration is held in the storage device 404 as reference data.
It is assumed that the reference data is acquired for each view for each condition such as an X-ray beam width and a tube voltage corresponding to an assumed slice thickness and held in the storage device 404.

  The X-ray CT apparatus 1 performs imaging of the subject 6 according to the procedure of the flowchart shown in FIG. In other words, the system control device 401 reads a program and data related to shooting processing from the storage device 404 and executes shooting processing based on the program and data.

  In the imaging process of FIG. 4, first, the system control device 401 controls each part of the X-ray CT apparatus 1 to perform scanogram imaging of the subject 6, and obtains scanogram projection data that is measurement data at this time (step) S101). In scanogram imaging, the X-ray tube 201 is not rotated and X-rays are irradiated in one direction (for example, from the back to the front) with respect to the subject 6 while the bed 3 is moved at a constant speed in the direction of the subject body axis. Scanogram projection data is acquired by the line detector 205.

Here, the system control apparatus 401 determines, for each view, whether or not the reference channels RefL and RefR are covered with the subject 6 based on the acquired scanogram projection data (step S102). Determination (cover determination) as to whether or not the reference channels RefL and RefR are covered by the subject 6 is performed based on the following equations (1) to (3).
In addition, the measurement data before Log conversion shall be used for each data in the numerical formula in the following description.

AirCalref_vwave_sum: Value obtained by adding all views to the average value for each view in the reference channel output data (air cal reference value or reference data) of the air calibration data (hereinafter referred to as air cal data) acquired without the subject (hereinafter referred to as air cal data). The air cal reference total value). This is a value calculated by calculating the average of air cal reference values for each view and adding them for the number of air cal acquired views.
AirCal_mA: Scan tube current at the time of air cal data acquisition. It is assumed that it is constant during the scan.
AirCal_vw : The number of acquired views of air cal data. It is a pre-defined value in the X-ray CT apparatus, and is assumed to be constant during scanning.
AirCalref_vw_nrm: a value obtained by normalizing the total value of the air cal reference with the scan tube current at the time of acquiring the air cal and the number of views at the time of acquiring the air cal data (hereinafter referred to as a normalized air cal reference value). That is, it is a value equivalent to 1 view and 1 mA of the air cal reference value.

  The above equation (1) is acquired for each condition of the X-ray focal spot size, the X-ray beam width corresponding to the assumed slice thickness, and the tube voltage, and is held in the storage device 404, for example.

RefD_L: Left reference data. When there are a plurality of reference channels, the average value of data of all the left reference channels is used.
RefD_R: Right reference data. When there are a plurality of reference channels, the average value of the data of all reference channels on the right side is used.
Cnst: a constant (for example, a value such as 0.9).
Scano_vw_rate: Measurement view rate (number of views / second) at the time of scanogram acquisition. It is assumed that this is a predetermined value and is constant during scanning.
Scano_mA: Scan tube current at the time of scanogram acquisition. It is assumed that it is constant during scanning.

  The above equations (2) and (3) are obtained by correcting the air cal reference value with parameters such as the scan tube current and the measurement view rate so that the air cal reference value becomes a value corresponding to each view at the time of scanogram acquisition. Is used as a reference channel cover determination index.

  As described above, the air cal data is acquired in advance for each of various conditions and stored in the storage device 404. The image calculation device 402 reads out the standardized air cal reference value (reference data) of the X-ray focal spot size, the scan tube voltage, and the X-ray beam thickness corresponding to the scan conditions at the time of scanogram imaging. A normalized air cal reference value [AirCalref_vw_nrm] corresponding to a dose equivalent to 1 view and 1 mA is calculated.

If there is no air cal data that corresponds to the scan conditions at the time of scanogram imaging, read the air cal data with the most conditions that match the scan conditions, and use X-ray beam using interpolation, extrapolation, proportional calculation, Monte Carlo method, etc. You may make it estimate using a simulation technique.
Further, the constants (Cnst) in the above formulas (2) and (3) may be simple additions, and are not limited to the multiplication form.

  When both the above equations (2) and (3) are satisfied, the image calculation device 402 determines that the reference channels RefL and RefR on both sides are covered, as shown in FIG. The process proceeds to step S103.

  When it is determined that the reference channels RefL and RefR on both sides are covered, the system control device 401 calculates pseudo reference data [Ref_alt] corresponding to the case where the reference channels RefL and RefR are not covered. The pseudo reference data [Ref_alt] is calculated by the following equation (4) (step S103).

  Next, the image calculation device 402 replaces the pseudo reference data calculated by Expression (4) as reference data of the view (Step S104).

  The image arithmetic device 402 performs reference correction on the acquired measurement data of each view (step S105), and generates scanogram projection data. At this time, if it is determined that both reference channels RefL and RefR are covered, reference correction is performed using the pseudo reference data generated and replaced in steps S103 to S104. When only one reference channel is covered, or when the reference channel is not covered, reference correction is performed by a conventional method.

  In the above description, in the reference channel cover determination processing in step S102, when both the above-described equations (2) and (3) are satisfied and both reference channels RefL and RefR are covered, The process proceeds to step S103 and the pseudo reference data is generated / replaced. However, one of the above-described equations (2) and (3) is satisfied, and only one reference channel is covered. In the case where there is, pseudo reference data shown in the following formula (5) may be generated and replaced.

here,
Ref_nocov: Reference data on the uncovered side.

  The above-mentioned formula (5) is the average value of the pseudo reference data calculated based on the air cal reference value (see the above formula (4)) and the reference data on the uncovered side. Means that.

  In the method of Patent Document 1 (Japanese Patent Laid-Open No. 2000-060840), reference correction is performed from reference data on only one side, whereas in the above equation (5), in addition to reference data on one side that is not covered Since the reference correction is performed using both of the reference data calculated based on the reference data, the reliability of the reference data is improved.

In addition, although said Formula (1)-Formula (5) shall perform pseudo | simulation reference data production | generation and substitution using the measurement data before Log conversion, this invention is not limited to this, After Log conversion The same processing may be performed using data.
In the above description, scanogram projection data is generated while performing processing such as cover determination of the reference channels RefL and RefR, generation / replacement of pseudo reference data, and reference correction for each acquired view. Without being limited thereto, after acquiring scanograms at all positions, reference channel cover determination may be performed for each view, and processing such as generation / replacement of pseudo reference data and reference correction may be performed. However, when processing is performed in the flow of steps S101 to S105, a scanogram image can be reconstructed in real time from the scanogram projection data after the reference correction and displayed on the display device 7.

Step S106 and subsequent steps are the same as the conventional general scan operation.
That is, in step S105, the system control device 401 analyzes the reference-corrected scanogram projection data, and a cross section at each position (hereinafter referred to as a Z position) along the body axis of the subject is an X-ray equivalent to, for example, water. Modeled as an elliptical cross section having an absorption coefficient (step S106). 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 Z position of the subject. The calculated subject cross-sectional model data is stored in the storage device 404. The object cross-section model data calculated here is scanogram projection data that has been subjected to appropriate reference correction by the processing in steps S102 to S104, even when both sides of the reference channels RefL and RefR are covered. Therefore, a highly reliable cross-sectional model that matches the actual physique of the subject is obtained.

  Next, using the scanogram image displayed on the display device 7, the scan area is set from the operation device 8 (step S107), and the moving pitch of the bed 3, the scan tube voltage, the scan tube current, and the scan. When various scanning conditions such as time, X-ray collimation condition, reconstruction filter function type, field size, etc. are set (step S108), the system control device 401 performs the processing based on the set scanning area and scanning condition. A scan of the specimen 6 is executed (step S111).

  When the automatic dose control function is used in the scan condition setting process in step S108, the target image quality (step S109) such as an image SD (Standard Deviation) input by the operator, and the step S106 are further entered. Based on the calculated cross-sectional model, the system control device 401 calculates an optimum scan tube current value at each Z position (step S110). The calculation of the optimum scan tube current using the three-dimensional cross-sectional model is described in, for example, Japanese Patent Application Laid-Open No. 2003-033346, and detailed description thereof is omitted.

  The system control device 401 controls each unit of the scanner 2 to scan the subject based on the scan region and scan conditions set in step S107 and step S108 and the optimum scan tube current calculated in step S110. Get scan data.

  As described above, according to the first embodiment, the X-ray CT apparatus 1 is connected to both ends of the X-ray detector 205 at the time of air calibration data acquisition performed without a subject before an actual scan. Outputs of the provided reference channels RefL and RefR are held in the storage device 404 as reference data. Then, when imaging the subject 6 (during scanogram imaging in the above flowchart), the image calculation device 402 or the system control device 401 determines whether the reference channels RefL and RefR are covered by the subject 6 for each view. Judgment. When the subject 6 is covered, the air cal reference values (reference data) acquired by the reference channels RefL and RefR in the state where the subject 6 is not present are used as the view (view to be calculated). The pseudo reference data corrected so as to correspond is calculated. Then, the image calculation device 402 performs reference correction on the acquired projection data for the view using the calculated pseudo reference data.

Therefore, the pseudo reference data is calculated based on the air cal reference value acquired in advance without the subject and used for reference correction. Therefore, even if both reference channels RefL and RefR are covered, the first view Therefore, it is possible to appropriately perform reference correction.
As a result, even for scanogram projection data acquired before scanning, appropriate reference correction is performed from the first view, so that the object cross-section model used for the automatic dose control function, etc. can be accurately calculated, and the desired An image with high image quality can be obtained.

  Air calibration is acquired at predetermined time intervals adapted to changes in the time of each part of the X-ray CT apparatus 1 and changes in the external environment so that reliable data can always be obtained, and stored in the storage device 3 or the like. It is assumed that In addition, in order to obtain reliable data, it is confirmed that the predetermined time has elapsed since the last air calibration, or that the timing for performing air calibration has arrived after a predetermined number of inspections. A function for notifying the operator may be further provided.

  In the present embodiment, reference correction at the time of scanogram imaging has been described as an example. However, the present invention is not limited to this example, and the above-described simulation is performed during scanning to obtain a tomographic image of a subject. It also includes performing reference correction including replacement with reference data.

(Second Embodiment)
Next, an X-ray CT apparatus 100 according to the second embodiment will be described with reference to FIGS.
In the following description, the same parts as those in the configuration of the X-ray CT apparatus 1 of the first embodiment will be described with the same reference numerals.

  In the first embodiment described above, the scan at the time of acquiring the air cal data is added to the mathematical formulas (the above formulas (1) to (5)) used when determining the reference channel cover and calculating the pseudo reference data. Parameters such as a tube current [AirCal_mA], a measurement view rate [AirCal_vw_rate], a scan tube current [Scano_mA] at the time of scanogram acquisition (at the time of measurement subject to image calculation), a measurement view rate [Scano_vw_rate], and the like are included. These parameters use values set in advance, and are based on the assumption that the bed moving speed and the X-ray output are performed as theoretically.

  However, the actual operation of the X-ray CT apparatus may cause fluctuations in the X-ray output. Further, the bed moving speed is not always constant during scanning, but moves while accelerating until reaching a constant speed.

  As shown in FIG. 5, when the start position of scanogram imaging is O and the end position is b, the bed moving speed generally changes at a constant acceleration from the position of O to the position of a, and is constant speed after the position of a. Exercise. That is, for the section δ from the photographing start position O to the position a, the bed speed does not reach a constant speed. Therefore, strictly speaking, the view rate at the time of scanogram acquisition is not a constant value like [Scano_vw_rate] used in the above equation (4).

  Therefore, as shown in FIG. 5, in a situation where both reference channels RefL and RefR are covered over the entire range of the scanogram acquisition range, the simulation shown in Equation (4) of the first embodiment is performed. Even if the measurement data is subjected to reference correction using the reference data, artifacts may be mixed in the scanogram image of the section δ from the position O to the position a, which may cause a decrease in visibility and misperception.

  Therefore, in the X-ray CT apparatus 100 according to the second embodiment, the actual X-ray output and the bed speed are monitored at the time of air cal data acquisition or scanogram acquisition, and the actually measured values are represented by the above formula (the above formula). Applied to (1) to (5)), the reference channel cover is determined or pseudo reference data is calculated.

  As shown in FIG. 6, the X-ray CT apparatus 100 according to the second embodiment monitors the actual X-ray output and the bed speed, in addition to the configuration of the X-ray CT apparatus 1 according to the first embodiment. The scan tube current / scan tube voltage monitoring device 101 is connected to the X-ray control device 202, and the bed speed monitoring device 102 is connected to the bed control device 102.

  The scan tube current / scan tube voltage monitoring device 101 is connected to the X-ray control device 202, and at the time of acquiring air cal data or scanogram data / scan data, the actual scan tube current and scan tube voltage for each view. Is output to the image calculation device 402 of the console 4.

  The bed speed monitoring device 102 is connected to the bed control device 301, measures the actual moving speed of the bed 3 for each view, and operates the measured value when acquiring air cal data or scanogram data / scan data. The image is output to the image calculation device 402 of the table 4.

Next, the operation of the X-ray CT apparatus 100 according to the second embodiment will be described.
The operation flow of the X-ray CT apparatus 100 of the second embodiment is the same as the flowchart shown in FIG. 4, but the equations (1) and (4) used here are the following equations (6) ), (7).

  That is, the above equation (1) used for reference channel cover determination is replaced with the following equation (6). Further, the pseudo reference data calculated when both reference channels are covered uses the following formula (7) instead of the above formula (4).

here,
AirCal_mA_real: The average value of all the scan tube currents measured for each view at the time of air cal acquisition.
ε: a value calculated by referring to information for each view at the time of scanogram acquisition obtained by various monitoring devices, and is given by the following equation (8).

kV_set: Scan tube voltage at the time of setting scanogram acquisition.
kV_real: Scan tube voltage for each view at the time of scanogram acquisition.
f (kV_real, kV_set): a function of the X-ray intensity calculated from kV_real, kV_set. Reference is made to a value experimentally calculated in advance. In general, the fluctuation of the tube voltage for each view is smaller than the fluctuation of the tube current for each view, which will be described later. Therefore, this term can be regarded as 1.
Scano_mA_real: Scan tube current for each view at the time of scanogram acquisition.
Tsp_set: The bed moving speed when the set scanogram is acquired.
Tsp_real: The bed moving speed for each view at the time of scanogram acquisition.

  As described above, according to the second embodiment, in addition to the configuration of the X-ray CT apparatus 1 of the first embodiment, the scan tube current / scan tube voltage monitoring device 101 and the bed moving speed monitoring device 102 are provided. Is provided, and when the air cal data is acquired, when the scanogram data is acquired, and when the scan data is acquired, the actual measurement values of the X-ray output and the bed speed for each view are monitored. Based on the above, reference channel cover determination is performed for each view, or pseudo reference data is calculated.

  Therefore, in addition to the effects of the first embodiment, it is possible to perform reference correction with higher accuracy in accordance with fluctuations in X-ray irradiation and actual bed speed, and image quality is further improved.

(Third embodiment)
Next, an X-ray CT apparatus 150 according to the third embodiment will be described with reference to FIG.
In the following description, the same parts as those in the configuration of the X-ray CT apparatus 1 of the first embodiment will be described with the same reference numerals.

  The X-ray CT apparatus 150 according to the third embodiment monitors the actual X-ray output and the integration time of the X-ray detection signal at the time of air cal data acquisition or scanogram data / scan data acquisition. The value is applied to the above-described mathematical expressions (the above-described expressions (1) to (5)) to perform reference channel cover determination or calculate pseudo reference data.

  Here, the integration time of the X-ray detection signal is the time until the data acquisition device 206 acquires the X-ray detection signal from the X-ray detector 205 and outputs it as measurement data (projection data) before Log conversion. . That is, the data acquisition device 206 has a number of integrators equal to the number of channels of the X-ray detector 205, and integrates the X-ray detection signals of the respective channels by these integrators. A trigger is given to all channels of the X-ray detector 205 for each view, and the X-ray detection signal is integrated by the integrator starting from the trigger. Thereafter, the integrated value is A / D converted, and the converted digital data becomes data in the X-ray detection element. Processing such as gain multiplication is performed on this data to obtain data before Log conversion.

  By the way, since the equation (4) for calculating the pseudo reference data [Ref_alt] uses time-dependent parameters such as the measurement view rate [Scano_vw_rate] and [AirCal_vw_rate] in the view, the X-ray detection signal Integration time is a major influencing factor.

  As shown in FIG. 7, the X-ray CT apparatus 150 of the third embodiment monitors the actual X-ray output and the integration time of the X-ray detection signal, so that the X-ray CT apparatus of the first embodiment is used. In addition to the configuration of 1, the scan tube current / scan tube voltage monitoring device 101 is connected to the X-ray control device 202, and the integration time monitoring device 103 is connected to the data acquisition device 206.

The scan tube current / scan tube voltage monitoring apparatus 101 is the same as that described in the second embodiment.
The integration time monitoring device 103 is connected to each X-ray detection element of the data acquisition device 206 and measures the integration time of the X-ray detection signal for each view when acquiring air cal data or scanogram data / scan data. The actual measurement value is output to the image calculation device 402 of the console 4.

Next, the operation of the X-ray CT apparatus 150 according to the third embodiment will be described.
The operation flow of the X-ray CT apparatus 150 of the third embodiment is the same as the flowchart shown in FIG. 4, but the mathematical formulas used here are respectively replaced by the following mathematical formulas.
That is, the following equation (9) is applied instead of the above equation (1) used for the reference channel cover determination, and the following equation is substituted for the above equation (4) for calculating the pseudo reference data: Apply (10).

AirCal_ingrl_t: All view average value of integration time measured for each view at the time of air cal acquisition.
AirCal_vw: The number of acquired views of air cal data.
Scano_ingrl_t: integration time measured for each view at the time of scanogram acquisition.

  As described above, according to the X-ray CT apparatus 150 of the third embodiment, the pseudo reference data and the reference are obtained using the measured integration time and the measured scan tube current / scan tube voltage in each view. Since channel cover determination is performed, it is possible to perform reference correction with higher accuracy in consideration of minute influence factors such as data collection time. As a result, even when the reference channels RefL and RefR on both sides of the X-ray detector 205 are covered with the subject or the like, the X-ray intensity is appropriately compensated from the first view, and an image with better image quality can be obtained. Is possible.

  The preferred embodiment of the X-ray CT apparatus according to the present invention has been described above, but the present invention is not limited to the above-described embodiment. For example, in the first to third embodiments described above, reference correction at the time of scanogram acquisition has been described as an example. However, a scan for obtaining a tomographic image (data measured while an X-ray tube rotates around a subject) The same processing can be applied to (collection). In the above-described embodiment, the gantry type X-ray CT apparatus has been described, but a C-arm type X-ray CT apparatus may be used. In addition, it is obvious for those skilled in the art that various changes or modifications can be conceived 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 apparatus (first embodiment)
2... Scanner 3... Bed 4 .. Operation console 6 .. Subject 7... Display device 8. X-ray tube (X-ray source)
205 ... X-ray detector 401 ... System control device 402 ... Image arithmetic device 404 ... Storage device RefL, RefR ... Reference channel 100 ... X-ray CT device (second Embodiment)
101... Scanning tube current / scanning tube voltage monitoring device 102... Bed movement speed monitoring device 150... X-ray CT device (third embodiment)
103 ... Integration time monitoring device

Claims (2)

A projection data collection means for collecting projection data by irradiating the subject with X-rays from each view direction around the subject and detecting transmitted X-rays transmitted through the subject with an X-ray detector;
Correction means for correcting the collected projection data based on reference data collected in a reference channel for X-ray intensity monitoring provided at one or both ends of the X-ray detector;
In an X-ray CT apparatus provided with image calculation means for reconstructing an image based on the corrected projection data,
Reference data holding means for holding air calibration data of each view acquired in the reference channel in the absence of a subject in advance as reference data;
A determination unit that determines, for each view, whether or not the reference channel is covered with the subject at the time of subject imaging;
Scan tube current / tube voltage monitoring means for measuring scan tube current and scan tube voltage during scanning,
Integration time monitoring means for measuring the integration time for each view of each detection element of the X-ray detector during scanning, and
For the view in which the reference channel is determined to be covered by the subject by the determination unit, the correction unit converts the pseudo reference data corresponding to the view, the reference data, and the scan tube current / tube. It is calculated using parameters including each measurement value of the scan tube current and the scan tube voltage by the voltage monitoring unit and the integration time measured by the integration time monitoring unit, and is used as reference data of the view X-ray CT apparatus that is characterized. Air calibration tube current / tube voltage monitoring means for measuring tube current and tube voltage at the time of air calibration data acquisition;
Integration time monitoring means for measuring the integration time for each view of each detection element of the X-ray detector at the time of air calibration data acquisition,
The determination means uses the reference data as the measured values of tube current and tube voltage by the air calibration tube current / tube voltage monitoring means at the time of air calibration data acquisition, and the air measured by the integration time monitoring means. And determining whether or not the reference channel is covered with a subject using a value corrected to a value corresponding to the view by a parameter including an integration time at the time of acquisition of calibration data. The X-ray CT apparatus according to 1.

Images