JPWO2009096361A1 - X-ray CT system - Google Patents

Abstract

To provide an X-ray CT system that can speed up daily air measurement and reduce the burden of air measurement. During the initial setting of the X-ray CT apparatus 1, the air measurement is performed for all the combinations of multiple filters that adjust the intensity of the X-rays incident on the subject 19, and initial air correction information is acquired. The relative information of the initial air correction information for the reference filter combination is calculated and stored in the storage device 9. Then, during normal times, air measurement is performed using a reference filter combination, reference normal air correction information is measured, and reference normal air correction information is developed for all filter combinations using relative information. Is calculated. The projection data of the subject 19 measured by the X-ray CT apparatus 1 is air-corrected using normal air correction information.

Description

  The present invention relates to an X-ray CT apparatus that images and displays a biological tissue of a subject. More specifically, the present invention relates to an X-ray CT apparatus that uses a combination of a plurality of compensation filters and soft line removal filters, performs image correction, and images a subject.

  An X-ray CT (Computed Tomography) apparatus is an apparatus that captures a tomographic image of a living tissue of a subject and displays it on a display device.

  The X-ray detector of the X-ray CT apparatus acquires the X-ray dose irradiated from the X-ray tube and transmitted through the subject as projection data. The image processing apparatus of the X-ray CT apparatus performs a predetermined correction process on the acquired projection data, performs image reconstruction, and displays a tomographic image on the display apparatus.

  The X-ray CT apparatus requires air correction in order to correct variations in sensitivity characteristics of X-ray detection elements constituting the X-ray detector. The operator performs air measurement to measure air in a state where the subject is not in the scanner of the X-ray CT apparatus before performing the main measurement for imaging the subject. Various corrections (offset correction, logarithmic conversion, etc.) are performed on the projection data at the time of air measurement and stored as air correction data.

  Since the sensitivity of the X-ray detector varies from element to element, air correction data is stored for each element. For example, in the case of a multi-slice X-ray detector having I channels in the channel direction and J columns in the slice direction, I × J air correction data is stored.

  In addition, since the air correction data varies depending on the tube voltage and collimation, the air correction data is measured under each condition.

The image processing device of the X-ray CT apparatus performs various corrections (offset correction, logarithmic conversion, etc.) on the projection data obtained by imaging the subject, and then performs air correction using the stored air correction data. Do. The image processing apparatus further performs a predetermined correction on the projection data after the air correction, and then performs an image reconstruction process.
In addition, air measurement is routinely performed after warming up the X-ray tube after starting the X-ray CT apparatus, and the air correction data is updated.

  In the X-ray CT apparatus, various filters are installed between the X-ray tube and the subject. The compensation filter adjusts the intensity distribution of the X-rays incident on the subject, and the soft line removal filter removes soft rays in the low energy region of the X-rays incident on the subject.

  When the cross section from the chest to the abdomen of the human body that is the subject is regarded as an ellipse, the transmission length of the X-ray at the center of the human body is longer than the transmission length of the peripheral portion. By inserting a compensation filter between the X-ray tube and the subject so that the transmission length is short at the center and the transmission length increases as the distance from the center increases, the intensity of the X-ray that passes through the subject is made uniform. There is a way to make adjustments.

  The compensation filter is made of a material such as aluminum or Teflon (registered trademark). The cross-section of the object to be imaged by the X-ray CT apparatus is not limited to an ellipse such as a chest to abdomen, but may be a nearly circular shape such as a head. In addition, there are X-ray CT apparatuses provided with several types of compensation filters having different shapes and materials because the size of the subject itself varies from patient to patient, even from the same ellipse or circle, from infants to adults. In addition, there has been proposed a compensation filter capable of changing the cross-sectional shape of the X-ray absorption portion into various shapes by using a movable means (see “Patent Document 1”).

  In addition, the low energy region of X-rays containing a wide range of energy is almost absorbed by the human body and thus hardly contributes to the projection data, resulting in an invalid exposure for the human body. Therefore, a soft wire removal filter having a uniform thickness in the X-ray incident direction may be used in combination with the compensation filter. The soft wire removal filter is made of a material such as copper. An invention has been proposed that includes several types of soft wire removal filters having different thicknesses and materials depending on the imaging region and size (see “Patent Document 2”).

JP 2005-34173 A Japanese Patent Laid-Open No. 2002-102217

  However, when several types of compensation filters with different shapes and materials are provided, or when using a compensation filter whose shape changes according to `` Patent Document 1 '', the work of performing air measurement and obtaining air correction data is performed on the subject. There is a problem that it is necessary to perform all the compensation filters used for photographing, and it takes time burden and work burden on the operator.

  Similarly, when using several types of soft wire removal filters with different thicknesses and materials according to the imaging region and size according to `` Patent Document 2, '' the work of acquiring air correction data is a time burden on the operator, There was a problem that it took a work burden.

  Furthermore, when imaging a subject using a plurality of compensation filters or a plurality of soft line removal filters, it is necessary to perform air measurement for all combinations of filters used for imaging to obtain air correction data, Furthermore, there is a problem that the operator is burdened with time and work.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an X-ray CT apparatus that can speed up daily air measurement and reduce air measurement burden. is there.

  The present invention for achieving the above-described object includes an X-ray tube that emits X-rays, an X-ray detector that is installed at a position facing the X-ray tube and detects X-rays, and the X-ray tube. A plurality of filters that are installed between the X-ray detector and adjust the intensity of X-rays incident on the subject, a data acquisition circuit that collects projection data from the X-ray detector, and a data acquisition circuit that collects the data In an X-ray CT apparatus including an image processing unit that performs data processing of the projection data that has been reconstructed and a storage device, the image processing unit performs air measurement for all combinations of the filters during initial setting. Using the initial air correction information measuring means for measuring the initial air correction information and the measured initial air correction information, the initial error for all the combinations of the filters with respect to the reference filter combination. Relative information calculation means for calculating relative information of correction information and storing it in the storage device, and the normal air correction information is measured by performing the air measurement on the reference filter combination at a predetermined timing in normal time. A reference normal air correction information measuring unit that calculates the normal air correction information by developing the measured reference normal air correction information for all combinations of the filters using the calculated relative information, and the storage device. Normal air correction information calculation means to be held, correction processing means for correcting the collected projection data of the subject using the normal air correction information corresponding to the combination of the filters used at the time of imaging, An X-ray CT apparatus characterized by comprising:

  The plurality of filters may include a plurality of compensation filters that adjust the intensity distribution of X-rays incident on the subject and a soft line removal filter that removes soft rays in the low-energy region of the X-rays incident on the subject. Each of the plurality of compensation filters and the soft wire removal filter is installed so that it can be used or not used.

The initial setting is assumed when the X-ray CT apparatus is shipped or maintained.
Air measurement is measurement in a state where the subject is not in the scanner of the X-ray CT apparatus, and air projection data is obtained by air measurement.
The predetermined timing at the normal time is a timing after warm-up of the X-ray tube after starting the X-ray CT apparatus.

  The X-ray CT apparatus of the present invention measures initial air correction information by performing air measurement on all combinations of filters that adjust the intensity of X-rays incident on a subject at the time of initial setting. The X-ray CT apparatus calculates the relative information of the initial air correction information with respect to the reference filter combination for all filter combinations, and stores the information in the storage device.

  The X-ray CT apparatus measures the reference normal air correction information by performing air measurement for a reference filter combination at a predetermined timing during normal operation. The X-ray CT apparatus develops the reference normal air correction information for all combinations of filters using the relative information, calculates the normal air correction information, and holds it in the storage device.

  The X-ray CT apparatus corrects the collected projection data of the subject using normal air correction information corresponding to the combination of filters used at the time of imaging.

  As a result, it is only necessary to perform air measurement only for the reference filter combination in the normal state, so that it is possible to reduce an operator's work burden related to air measurement.

  In addition, the X-ray CT apparatus measures measurement information of a specific channel where X-rays do not pass through the subject during measurement of the subject, and relative information of the measurement information with respect to the reference normal air correction information (first relative information) for the specific channel Is calculated. The X-ray CT device compares the relative information calculated for the specific channel (first relative information) with the relative information read from the storage device (second relative information), and warns or notifies based on the comparison result May be performed. The specific channel may be a channel located at the end of the X-ray detector.

  As a result, it is determined whether it is necessary to recalculate the calculated relative information by monitoring the deviation between the relative information already calculated for use in air correction and the actual measurement value. Can be determined appropriately.

  According to the present invention, it is possible to provide an X-ray CT apparatus that can speed up daily air measurement and reduce the air measurement burden.

Configuration diagram of X-ray CT system 1 Diagram showing around the collimator case 17 Diagram showing soft wire removal filter 41 and compensation filters 45-0 to 45-2 Diagram showing types of compensation filters 45-0 to 45-2 Flow chart showing relative information calculation processing The figure which shows the calculation result of relative information Flow chart showing air correction information calculation processing The figure which shows an air correction information calculation process Flow chart showing measurement processing of subject 19 Flow chart showing monitoring of air correction value Diagram showing reference channel 55 of X-ray detector 25

Explanation of symbols

  1 X-ray CT system, 3 scanner, 7 memory, 9 storage device, 11 input device, 13 display device, 15 X-ray tube, 17 collimator case, 19 subject, 21 bed, 23 irradiation X-ray, 25 X-ray detector 27 Data acquisition circuit 29 Scanner control device 31 Image processing device 32 Data acquisition means 33 Correction processing means 35 Image reconstruction processing means 36 Initial air correction information storage section 37 Relative information storage section 38 Normal air Correction information storage unit, 39 Measurement information storage unit, 41 Soft wire removal filter, 43 Soft wire removal filter moving means, 45, 45-0 to 45-2 Compensation filter, 47 Compensation filter moving means, 48 Collimator, 49 channels, 51 X-ray Detector output, 53 Relative data, 54 Incident X-ray intensity, 55-1, 52-2 Reference channel, 201-206 Channel distribution of initial air correction information, 301-306 Channel distribution of relative information, 401-406 Normal air correction Channel distribution of information

BEST MODE FOR CARRYING OUT THE INVENTION

<First Embodiment>
Hereinafter, an X-ray CT apparatus according to a first embodiment of the present invention will be described in detail with reference to the accompanying drawings.

(1. Configuration of X-ray CT system 1)
First, the configuration of the X-ray CT apparatus 1 will be described with reference to FIG.
FIG. 1 is a configuration diagram of the X-ray CT apparatus 1.

  The X-ray CT apparatus 1 is an apparatus that irradiates the subject 19 with X-rays, acquires the X-ray dose transmitted through the subject 19 as projection data, performs image processing, and displays a tomographic image on the display device 13. The X-ray CT apparatus 1 includes an X-ray tube 15, a collimator case 17, an X-ray detector 25, a scanner 3 equipped with a data acquisition circuit 27, a bed 21 on which an object 19 is placed, an input device 11, a display device 13, and scanner control The device 29 includes an image processing device 31, a memory 7, a storage device 9, a system control device 10, and the like.

  The X-ray tube 15 irradiates the subject 19 with X-rays by applying a high voltage. The collimator case 17 stores a filter for adjusting the X-ray intensity, a collimator for determining the X-ray beam width, and the like (details will be described later). The X-ray detector 25 detects X-rays transmitted through the subject 19 and converts them into electric signals. The data collection circuit 27 collects projection data detected by the X-ray detector 25. The scanner 3 has an X-ray tube 15 and an X-ray detector 25 arranged opposite to each other with the subject 19 interposed therebetween, and has a rotation mechanism that rotates around the subject 19 and the bed 21 on which the subject 19 is placed.

  The input device 11 is a device such as a keyboard, a mouse, a trackball, a touch panel, or a button arranged in the X-ray CT apparatus 1. The operator operates the input device 11 to perform an imaging instruction, measurement operation instruction, parameter setting, input, and the like by the X-ray CT apparatus 1.

  The display device 13 is a CRT, a liquid crystal display device, or the like, and displays the result of image processing by the image processing device 31.

  The scanner control device 29 controls the X-ray tube 15, the filter and collimator in the collimator case 17, and the X-ray detector 25. For example, the scanner control device 29 performs switching control between a plurality of compensation filters and soft line removal filters in the collimator case 17.

  The image processing device 31 performs image processing of projection data collected by the data collection circuit 27 of the scanner 3. The image processing device 31 includes a data acquisition unit 32 that acquires the projection data collected by the data collection circuit 27, a correction processing unit 33 that performs various correction processes of the projection data, and an image that reconstructs a cross-sectional image from the projection data after the correction process. Reconfiguration processing means 35 is provided. The correction processing includes air correction, offset correction, reference correction, beam hardening correction, non-linear correction, and the like.

The memory 7 is a semiconductor storage device such as a RAM and a ROM.
The storage device 9 is a storage device such as a hard disk. The storage device 9 includes storage units such as an initial air correction information storage unit 36, a relative information storage unit 37, a normal air correction information storage unit 38, and a measurement information storage unit 39. The initial air correction information storage unit 36 is a storage unit for air correction information that is measured for all combinations of compensation filters and soft wire removal filters when the X-ray CT apparatus 1 is shipped or maintained. The relative information storage unit 37 is a storage unit for the relative information of the air correction information of all the filter combinations calculated based on the air correction information based on the reference filter combination.

The normal air correction information storage unit 38 is a storage unit for air correction information calculated for all combinations of compensation filters and soft line removal filters. The normal air correction information storage unit 38 is calculated using air correction information and relative information based on a combination of filters that are measured on a daily basis.
The measurement information storage unit 39 is a storage unit for projection data of the subject 19 that is corrected using normal air correction information.
The system controller (CPU) 10 controls the entire system of the X-ray CT apparatus 1.

(2. Filter configuration)
Next, a plurality of filters for adjusting the intensity of the irradiated X-ray 23 of the X-ray CT apparatus 1 will be described with reference to FIGS.

(2-1. Configuration around the collimator case 17)
FIG. 2 is a view showing the vicinity of the collimator case 17. In the collimator case 17, a plurality of compensation filters 45 selected according to the imaging region of the subject 19 and the size of the subject 19, a soft line removal filter 41, and compensation filter moving means 47 for switching the compensation filter 45 A soft wire removal filter moving means 43 for switching the soft wire removal filter 41, a collimator 48, and the like are disposed.

  The irradiated X-rays 23 exposed from the X-ray tube 15 pass through the soft-wire removal filter 41 to remove soft rays in the low-energy region of the X-ray, and pass through the compensation filter 45 so that the X-ray intensity distribution is reduced. The X-ray width is adjusted by the collimator 48 and the subject 19 is irradiated.

  In FIG. 2, the X-rays emitted from the X-ray tube 15 are transmitted in the order of the soft line removal filter 41 and the compensation filter 45, but are not limited to this order, and are in the order of the compensation filter 45 and the soft line removal filter 41. May be installed.

(2-2. Configuration of multiple filters)
FIG. 3 is a diagram showing the soft line removal filter 41 and the compensation filters 45-0 to 45-2.
When the soft wire removal filter 41 is used, it is disposed on the X axis through which the irradiation X-rays 23 are transmitted. When the soft wire removal filter 41 is not used, the soft wire removal filter moving means 43 is used to move in the Z-axis direction off the axis.

  In the X-ray CT apparatus 1, soft rays in the low energy region of X-rays are almost absorbed by the subject 19, so the soft wire removal filter 41 is used to prevent invalid exposure of the subject 19.

  In areas such as the head with many bones with high X-ray absorption, it is desirable to remove the low energy area of X-rays incident on the head in consideration of the beam hardening effect. A soft wire removal filter 41 is used. The beam hardening effect is that low-energy X-rays are selectively absorbed and the detected X-rays have high energy.

  In addition, since X-rays including soft lines in the low energy region have good low contrast resolution, the soft line removal filter 41 may not be used when imaging a region near the abdominal liver that is difficult to contrast. .

  A plurality of compensation filters 45-0 to 45-2 are installed along the Z-axis direction that is the body axis direction of the subject 19. The operator selects the compensation filter 45 to be used from among the compensation filters 45-0 to 45-2 according to the imaging region and size of the subject 19, and the compensation filter 45 selected using the compensation filter moving means 47. Is moved in the Z-axis direction and installed on the X-axis through which the irradiated X-rays 23 are transmitted.

  FIG. 4 is a diagram showing the shape types of the compensation filters 45-0 to 45-2. The compensation filter 45-1 has the largest curvature of the curved surface, and the compensation filter 45-0 and the compensation filter 45-2 have a curvature of curvature smaller than that. The compensation filter 45-1 is used when imaging a relatively small subject 19, such as the head, heart, or child, and the compensation filter 45-0 is used for the subject 19 such as a chest or a standard size abdomen, The compensation filter 45-2 is used for imaging the abdomen of the subject 19 having a large size.

Hereinafter, the compensation filter 45-0 of size M, the compensation filter C ₀ as a reference. The compensation filter 45-1 S size and the compensation filter C _1, the compensation filter 45-2 L size and the compensation filter C _2.

  As described above, the subject 19 is imaged by the X-ray CT apparatus 1 in which the compensation filter 45 and the soft line removal filter 41 are combined, depending on the imaging region and size of the subject 19.

(3. Air correction processing)
Next, referring to FIGS. 5 to 9, as an example, the air correction processing of the X-ray CT apparatus 1 capable of switching between the three types of compensation filters 45 and switching between the presence / absence of the soft line removal filter 41 will be described. To do. In the following, (3-1) Relative information calculation processing at initial setting, (3-2) Normal air correction information calculation processing, (3-3) Subject 19 measurement and air correction processing, Separately described.

(3-1. Relative information calculation processing)
FIG. 5 is a flowchart showing a relative information calculation process at the time of initial setting.
The initial setting is before shipment of the X-ray CT apparatus 1 or during regular maintenance.

In the following description, the combination of the compensation filter 45 and the soft line removal filter 41 is represented as (C _m , F _n ). The subscript “m” of “C” indicates the shape of the compensation filter 45 to be used. When the compensation filter 45-0 (C ₀ ) is used, “m = 0”, and when the compensation filter 45-1 (C ₁ ) is used, “m = 1”, so that the compensation filter 45- When 2 (C ₂ ) is used, “m = 2”. The subscript “n” of “F” indicates whether or not the soft line removal filter 41 is used. When the soft wire removal filter 41 is used, “n = 0”, and when the soft wire removal filter 41 is not used, “n = 1”. Further, all combinations (C _m , F _n ) indicate all possible combinations of the compensation filter 45 and the soft line removal filter 41. For example, when “m = 0, 1, 2” and “n = 0, 1”, there are 3 × 2 = 6 combinations as all combinations (C _m , F _n ).

An operator (physician or engineer) inputs a command for air measurement at the time of initial setting to the input device 11, and performs air measurement for all combinations (C _m , F _n ) of the compensation filter 45 and the soft wire removal filter 41. The air correction data is acquired by the data acquisition means 32 (step 1001). The air measurement is performed in a state where the subject 19 is not in the scanner of the X-ray CT apparatus 1.

  When the measurement of the air correction data is completed, the system control device 10 performs offset correction and reference correction on the air correction data acquired by the correction processing means 33 (step 1002). The offset correction is correction for canceling the dark current components of the X-ray detector 25 and the data acquisition circuit 27. Reference correction is correction that cancels fluctuations in X-rays during and after scanning.

The air correction data after the offset correction and the reference correction is stored as initial air correction data AIr0 [(C _m , F _n )] in the initial air correction information storage unit 36 of the storage device 9 (step 1003).
The air correction data is actually data acquired for each channel and each projection angle of the X-ray detector 25.

Next, the system controller 10 sets the relative data Trans [(C for the reference initial data AIr0 [(C ₀ , F ₀ )] as the reference for all the initial air correction data AIr0 [(C _m , F _n )]. _m , F _n )] is calculated (step 1004).

The relative data Trans [(C _m , F _n )] is calculated by the equation (1).
_{Trans [(C m, F n} )] = AIr0 [(C m, F n)] - AIr0 [(C 0, F 0)] ··· (1)

Note that the initial air correction data AIr0 [(C ₀ , F ₀ )] is the reference initial data AIr0 [(C ₀ , F ₀ )] as a reference, and therefore the relative data Trans [(C ₀ , F ₀ )] is “0”.

In the equation (1), the relative data Trans [(C _m , F _n )] is the initial air correction data AIr0 [(C _m , F _n )] and the reference initial data AIr0 [(C ₀ , F ₀ ), respectively. However, the calculation formula is not limited to the calculation of the difference. Division or other relative data calculation formulas may be used.

The calculated relative data Trans [(C _m , F _n )] is stored in the relative information storage unit 37 of the storage device 9 (step 1005).

FIG. 6 is a diagram illustrating a calculation result of relative information.
FIG. 6A to FIG. 6C are graphs showing the initial air correction information acquired in the processing of step 1003. The horizontal axis of this graph is the channel 49 of the X-ray detector 25, and the vertical axis is the X-ray detector output 51. The graph 201, the graph 203, and the graph 205 indicate the initial air correction information acquired when the soft line removal filter 41 is used (F ₀ ). The graph 202, the graph 204, and the graph 206 show the initial air correction information acquired when the soft line removal filter 41 is not used (F ₁ ).

  6 (d) to 6 (f) are graphs showing the relative information calculated in the process of step 1004. The horizontal axis of this graph is the channel 49 of the X-ray detector 25, and the vertical axis is the relative data 53. The relative information graphs 301 to 306 are respectively calculated based on the initial air correction information graphs 201 to 206 in the processing of step 1004.

(3-2. Air correction information calculation process)
FIG. 7 is a flowchart showing a normal air correction information calculation process.
The normal time is, for example, the timing after warm-up of the X-ray tube after the daily X-ray CT apparatus 1 is started.

The operator (doctor or engineer) inputs a command to the input device 11 at a predetermined timing before the start of the CT examination, and measures the air for the combination of the compensation filter 45 (C ₀ ) and the soft wire removal filter 41 (F ₀ ). To obtain air correction data (step 2001).
When the air correction data of (C ₀ , F ₀ ) is measured, the correction processing means 33 performs offset correction and reference correction on the acquired air correction data (step 2002).

The air correction data after the offset correction and the reference correction is stored in the storage device 9 as reference normal data AIr1 [(C ₀ , F ₀ )] (step 2003).

Next, the system control device 10 compares the measured reference normal data AIr1 [(C ₀ , F ₀ )] and the relative information (relative data Trans [(C _m , F _n )]), the normal air correction data AIr1 [(C _m , F _n )] is calculated for all combinations (C _m , F _n ) of the compensation filter 45 and the soft line removal filter 41. (Step 2004).

The normal air correction data AIr1 [(C _m , F _n )] is calculated by equation (2).
AIr1 [(C _m , F _n )] = Trans [(C _m , F _n )] + AIr1 [(C ₀ , F ₀ )] (2)

In the equation (2), the normal air correction data AIr1 [(C _m , F _n )] is the relative data Trans [(C _m , F _n )] and the reference normal data AIr1 [(C ₀ , F ₀ ), respectively. However, the calculation formula is not limited to addition. A calculation formula corresponding to the calculation formula of the relative data used in the processing of step 1004 may be used.

The system control device 10 performs logarithmic conversion on the calculated normal air correction data AIr1 [(C _m , F _n )] (step 2005) and stores it in the normal air correction information storage unit 38 of the storage device 9 (step 2006). ).

FIG. 8 is a diagram showing the air correction information calculation process (FIG. 7: Step 2004) and the result.
The relative information graphs 301 to 306 in FIG. 8 are the same as the relative information graphs 301 to 306 in FIG. 6, and are calculated by the processing in step 1004. The horizontal axis of the normal air correction information graph is the channel 49 of the X-ray detector 25, and the vertical axis is the incident X-ray intensity 54. Since the incident X-ray intensity 54 in FIG. 8 is a calculated value, it is distinguished from the X-ray detector output 51 in FIG. 6 which is an actual measurement value. The reference normal data graph 401 is obtained by the processing in step 2003.
The normal air correction information graph 401 to graph 406 are calculated from the relative information graph 301 to graph 306 and the reference normal data graph 401 in the processing of step 2004, respectively.

(3-3. Measurement of subject 19 and air correction)
FIG. 9 is a flowchart showing measurement of the subject 19 and air correction.
The operator selects a compensation filter 45C _p (p is 0, 1, or 2) and a soft line removal filter 41F _q (q is either 0 or 1) corresponding to the imaging region of the subject 19 and the size of the subject 19. ) Is selected. The selected compensation filter 45 and soft line removal filter 41 are set on the X axis in FIG. In this state, a CT scan of the subject 19 is performed, and the measurement data Data0 [(C _p , F _q )] is acquired by the data acquisition means 32 (step 3001).

The correction processing means 33 performs offset correction and reference correction on the acquired measurement data Data0 [(C _p , F _q )] (step 3002).
From the normal air correction information stored in the normal air correction information storage unit 38 of the storage device 9, the correction processing means 33 calculates normal air corresponding to the combination (C _p , F _q ) of the compensation filter 45 and the soft wire removal filter 41. The correction data AIr1 [(C _p , F _q )] is read and the air correction of the measurement data Data 0 [(C _p , F _q )] is performed to obtain the measurement data Data 1 [(C _p , F _q )] (step 3003 ).

The correction processing means 33 performs various corrections such as beam hardening correction and nonlinear correction on the calculated measurement data Data1 [(C _p , F _q )] (step 3004), and the corrected measurement data Data1 [(C _p , F _q )] is stored in the measurement data storage unit 39 of the storage device 9 (step 3005).
The image reconstruction processing means 35 of the X-ray CT apparatus 1 performs image reconstruction processing based on the corrected measurement data Data1 [(C _p , F _q )] (step 3006).

(3-4. Effects of the first embodiment)
Through the above process, at the time of initial setting, the X-ray CT apparatus 1 performs air measurement for all combinations (C _m , F _n ) of the compensation filter 45 and the soft wire removal filter 41 to obtain initial air correction information, Relative information for the reference combination (C ₀ , F ₀ ) is calculated and held. X-ray CT apparatus 1 is in the normal, at a predetermined timing, the only combination that serves as a reference of the compensation filter 45 and the soft wire removal filter 41 (C _0, F ₀₎ performs air measurement, the combination of (C _0, F ₀₎ Based on the air measurement data and the relative information, normal air correction information is calculated for all combinations (C _m , F _n ) of the compensation filter 45 and the soft wire removal filter 41. When measuring the subject, the X-ray CT apparatus 1 reads normal air correction information corresponding to the combination (C _p , F _q ) of the used compensation filter 45 and soft line removal filter 41, and measures the X-ray projection of the measured subject. Perform air correction on the data.

Thus, even if the X-ray CT apparatus 1 includes a plurality of filters, it is only necessary to perform air measurement for the reference combination (C ₀ , F ₀ ) at normal times. Work burden on the person can be reduced. The X-ray CT apparatus 1 holds normal air correction information for all combinations (C _m , F _n ), and performs air correction according to the combination of filters used when measuring the subject. It is not necessary to acquire the air correction data every time the combination is changed.
Further, the exposure of the subject can be reduced by selecting a plurality of compensation filters and soft line removal filters suitable for the imaging region of the subject and the size of the subject.

<Second Embodiment>
(4. Monitoring air correction value)
Next, a second embodiment will be described with reference to FIG. 10 and FIG.
FIG. 10 is a flowchart showing an air correction value monitoring process.

The system control device 10 performs the measurement process of the subject 19 (FIG. 9) (step 4001), refers to the measurement information stored in the measurement information storage unit 39 of the storage device 9 at a predetermined timing, and performs a reference The measurement data Refch {Data 0 [(C _p , F _q )]} of channel 55 is acquired (step 4002).

FIG. 11 is a diagram showing the reference channel 55 of the X-ray detector 25. The reference channels 55-1 and 55-2 shown in FIG. 11 are arranged at positions exposed to X-rays (direct X-rays) that do not pass through the subject 19 when the subject 19 is measured. Accordingly, it can be considered that the air measurement is performed in the reference channels 55-1 and 55-2 simultaneously with the normal measurement of the subject 19. Therefore, the measurement data _{Refch {Data0 [(C p,} F q)]} can be considered normal air correction data _{Refch {AIr1 [(C p,} F q)]} and.

The reference channel 55 of the X-ray detector 25 shown in FIG. 11 is basically arranged at a position where data transmitted through the subject 19 does not enter, but the size of the subject 19 is very large, and the reference channel 55 When data transmitted through the subject 19 enters the channel 55, the measurement data Refch {Data0 [(C _p , F _q )]} is not used for monitoring the air correction value.

  For example, in order to determine whether or not the data transmitted through the subject 19 has entered the reference channel 55, it is determined using the detection value of the reference channel 55 of the scanogram image taken before the scan for the scan plan. Can do.

Returning to FIG. 10, the system control device 10 refers to the normal air correction information stored in the normal air correction information storage unit 38 of the storage device 9, and from the reference normal data AIr1 [(C ₀ , F ₀ )], Reference normal data Refch {AIr1 [(C ₀ , F ₀ )]} of the reference channel 55 is acquired (step 4003).

Next, the system control apparatus 10 acquires the measurement data Refch {Data0 [(C _p , F _q )]} (= normal air correction data Refch {AIr1 [(C _p , F _q ) acquired in step 4002 of the reference channel 55. )]}) And the reference normal data Refch {AIr1 [(C ₀ , F ₀ )]} of the reference channel 55 measured on a daily basis is calculated (step 4004). The difference value X1 is calculated by equation (3).

X1 = Refch {Data0 [(C p, F q)]} - Refch {AIr1 [(C 0, F 0)]}
= Refch {AIr1 [(C _p , F _q )]}-Refch {AIr1 [(C ₀ , F ₀ )]} (3)

Next, the system control device 10 uses the relative information stored in the relative information storage unit 37 of the storage device 9 to the combination (C _p , F _q ) of the compensation filter 45 and the soft wire removal filter 41 used during the CT scan. Corresponding relative data Refch {Trans [(C _p , F _q )]} in the reference channel 55 at the time of initial setting is read out to obtain the following X2 (step 4005).

The difference value X2 is expressed by equation (4). The difference value X2 is a difference value between the initial air correction data AIr0 [(C _p , F _q )] and the reference initial data AIr0 [(C ₀ , F ₀ )] in the reference channel 55 at the time of initial setting.

X2 = Refch {Trans [(C _p , F _q )]}
_{= Refch {AIr0 [(C p} , F q)]} - Refch {AIr0 [(C 0, F 0)]} ··· (4)
The system control device 10 compares the difference value X1 with the difference value X2, and determines whether or not the comparison result exceeds the threshold value T (step 4006). As an example of the comparison method, the ratio X1 / X2 between the difference value X1 and the difference value X2 is compared with the threshold value T.
If the ratio X1 / X2 does not exceed the threshold T (NO in step 4006), the process returns to the measurement process of the subject 19 and the air data comparison process (step 4001).

  If the threshold value T <X1 / X2 (YES in step 4006), the system control device 10 re-reads “Please perform all air measurements” on the display device 13 when the scan of the subject 19 is completed. A warning display for prompting measurement is performed (step 4007). As the difference between the difference value X1 and the difference value X2 increases, the relative information stored in the relative information storage unit 37 differs from the actual value, and the accuracy of air correction deteriorates.

  The operator who has confirmed the warning display performs air measurement in all combinations of the compensation filter 45 and the soft-wire removal filter 41 provided in the X-ray CT apparatus 1 and updates the relative information.

  Further, the remote maintenance function of the X-ray CT apparatus 1 may be used to notify the management facility (such as a service center) of the X-ray CT apparatus 1 that X1 / X2 exceeds the threshold value T (step 4008).

  Through the above process, the X-ray CT apparatus 1 uses the air data of the reference channel 55 simultaneously with the measurement of the subject 19, and monitors the deviation of the relative information used for air correction from the actual measurement data.

As described above, in the second embodiment, as in the first embodiment, it is only necessary to perform air measurement for the reference combination (C ₀ , F ₀ ) at the normal time. Therefore, the operator's work load related to air measurement is reduced. It can be reduced. In the second embodiment, it is determined whether it is necessary to recalculate the calculated relative information by monitoring the deviation between the relative information already calculated for use in air correction and the actual measurement value. Therefore, it is possible to appropriately determine the maintenance timing.

(5. Others)
The technical scope of the present invention is not limited to the embodiment described above. It will be apparent to 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 are naturally within the technical scope of the present invention. Understood.

Claims (4)

An X-ray tube that emits X-rays, an X-ray detector that is installed at a position facing the X-ray tube and detects X-rays, and is placed between the X-ray tube and the X-ray detector. A plurality of filters that adjust the intensity of X-rays incident on the specimen, a data acquisition circuit that collects projection data from the X-ray detector, and data processing of the projection data collected by the data acquisition circuit In an X-ray CT apparatus provided with an image processing unit for reconfiguring and a storage device,
The image processing unit
Initial air correction information measuring means for measuring initial air correction information by performing air measurement for all the combinations of the filters at the time of initial setting;
Using the measured initial air correction information, for all the filter combinations, relative information of the initial air correction information with respect to the reference filter combination is calculated and stored in the storage device. Means,
Reference normal air correction information measuring means for measuring the reference normal air correction information by performing the air measurement on the combination of the filters to be the reference at a predetermined timing during normal time,
Normal air correction information calculation means for expanding the measured reference normal air correction information for all combinations of the filters using the calculated relative information, calculating normal air correction information, and storing the normal air correction information in the storage device; ,
Correction processing means for correcting the collected projection data of the subject using the normal air correction information corresponding to the combination of the filters used at the time of imaging;
An X-ray CT apparatus comprising:   The plurality of filters include a plurality of compensation filters for adjusting an intensity distribution of X-rays incident on the subject, and a soft line removal filter for removing soft rays in a low energy region of the X-rays incident on the subject. 2. The X-ray CT apparatus according to claim 1, wherein each of the compensation filter and the soft-wire removal filter is installed so as to be switched between non-use and use. The image processing unit further includes:
A specific channel measurement means for measuring measurement information of a specific channel in which the X-ray does not pass through the subject during measurement of the subject;
First relative information calculating means for calculating first relative information of the measurement information with respect to the reference normal air correction information for the specific channel;
Second relative information reading means for reading out the relative information as second relative information from the storage device for the specific channel;
Comparison means for comparing the first relative information calculated by the first relative information calculation means and the second relative information read by the second relative information reading means;
Warning notification means for performing warning or notification based on the comparison result by the comparison means;
3. The X-ray CT apparatus according to claim 1, further comprising:   4. The X-ray CT apparatus according to claim 3, wherein the specific channel is a channel located at an end of the X-ray detector.

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