JP2012148028A - X-ray ct apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately calculate an actual exposure dose in consideration of the size change in the body axial direction of a subject and a photographic form in main scanning.SOLUTION: An exposure dose calculation device 27 detects a subject area in respective images from N two-dimensional image groups of the minimum image slice thickness (step S31). Then, the area Sn of the subject area detected from each of the two-dimensional image groups is calculated (step S32). Then, the tomographic image of a subject 17 determined from Sn is approximated to a circular shape and a radius rn is obtained (step S33). Then, from a unit exposure dose table, a unit exposure dose equivalent to the radius rn obtained in the step S33 is selected (step S34). Then, an exposure dose CTDIn is added (step S35), the steps S31-S35 are repeated for all the images (step S36), and the total exposure dose is calculated (step S37).

Description

  The present invention relates to an X-ray CT apparatus that obtains a medical image by irradiating a subject with X-rays. In detail, it is related with the evaluation technique of the exposure dose of a subject.

  In recent years, interest in medical exposure has increased, and there has been an increasing demand for each medical institution or individual to manage the exposure history of examinations. A conventional X-ray CT apparatus is configured to display CTDI (CT DoSe IndeX) defined in IEC 60601-2-44 as an exposure dose at the time of scanning on a display device of a console. ing. However, the exposure dose displayed on the display device of the console in accordance with the IEC standard is one for each of the head and the abdomen, taking into consideration the scanning conditions for actually applying the exposure dose measured in advance in the reference phantom. However, it is different from the exposure dose considering the actual physique of the subject.

As a technique for improving such an error, Patent Document 1 calculates a distribution of X-ray absorption coefficients from a CT value distribution acquired by an X-ray CT apparatus, and calculates a dose actually received by a subject. It has been proposed to do.
Further, Patent Document 2 proposes to segment each tissue from a reconstructed image generated by an X-ray CT apparatus (divide the region) and calculate an exposure dose corresponding to the size of the target region. ing.

Further, Patent Document 3 discloses size information of a cross section perpendicular to the subject body axis based on projection data of each channel obtained by scout scanning (also referred to as “scanogram imaging”) of the subject from at least one direction. It has been proposed to calculate the exposure dose with reference to information defining the relationship between size and dose.
Specifically, first, based on the projection data of each channel obtained by scout scan to obtain a profile of the object cross-section represented by the sequence of height t _j. Next, the maximum height a = max (t _j ) of the subject cross section is obtained based on the profile of the subject cross section. Next, the maximum width b = (m−1) × ΔCP of the cross section of the subject is obtained based on the channel pitch ΔCP of the X-ray detector. Next, when a <b, the ellipticity γ of the cross section of the subject is obtained by γ = b / a, and when a> b, it is obtained by γ = a / b. The exposure dose is calculated based on the normalized dose nCTDIw ′ corrected based on the ellipticity γ of the subject cross section.

JP 2006-74000 A JP 2007-181623 A JP 2004-73397 A

  However, none of the methods proposed in Patent Documents 1 to 3 can accurately calculate the actual exposure dose.

  In the method of Patent Document 1, the distribution of the X-ray absorption coefficient is calculated from the CT value distribution acquired by the X-ray CT apparatus, and the X-ray absorption coefficient is an index indicating the direct X-ray attenuation rate. It is not an index for calculating scattered X-rays in the subject. Therefore, the method of Patent Document 1 does not consider the influence of scattered radiation.

  In contrast to the problem in the method of Patent Document 1, the method of Patent Document 2 considers the influence of scattered radiation, and obtains the exposure dose by regarding each tissue, each part, and body as a cylinder. However, since the actual physique is not cylindrical, it cannot be said that the exposure dose considering the size change of the subject in the body axis direction is calculated even by the method of Patent Document 2.

  In contrast to the problem in the method of Patent Document 2, the method of Patent Document 3 takes into account the size change in the body axis direction of the subject and calculates the cross section of the subject calculated based on the projection data obtained by the scout scan. The exposure dose is obtained using the ellipticity γ. However, the scout scan is performed for an imaging plan (for example, positioning) of the main scan, and the X-ray irradiation mode is different from that of the main scan (for example, the scout scan is acquired without rotating the gantry). However, this scan is generally taken with the gantry rotating.) Therefore, as long as the projection data obtained by the scout scan is used, it cannot be said that the photographing mode in the main scan is accurately considered.

  The present invention has been made in view of the above-described problems, and its purpose is to determine the actual exposure dose in consideration of the size change of the subject in the body axis direction and the imaging mode in the main scan. An X-ray CT apparatus capable of calculating with high accuracy is provided.

  In order to achieve the above-described object, the present invention includes an X-ray source that irradiates a subject with X-rays, an X-ray detector that is disposed opposite to the X-ray source and detects X-rays transmitted through the subject, An image on which the X-ray source and the X-ray detector are mounted, a scanner that rotates around the subject, measurement data detected by the X-ray detector is acquired, and a tomographic image of the subject is reconstructed An X-ray CT apparatus comprising: a reconstruction device; and an image display device that displays a tomogram reconstructed by the image reconstruction device, wherein the exposure dose of the phantom is determined for each phantom size information and image slice thickness information. Storage means for storing a unit exposure dose calculated based on a measurement result, and a two-dimensional image of a cross section perpendicular to the body axis of the subject at predetermined intervals over the entire imaging range based on the measurement data Create images to create A size information calculating means for detecting a subject area that is a set of pixels having a CT value of the subject for each two-dimensional image and calculating size information of the subject area; and for each two-dimensional image In addition, the unit dose is specified by searching the phantom size information corresponding to the size information of the subject region and the image slice thickness information corresponding to the predetermined interval. An X-ray CT apparatus comprising: an exposure dose calculating unit that calculates the total exposure dose of the subject by adding the unit exposure doses to an image.

  According to the present invention, it is possible to provide an X-ray CT apparatus capable of accurately calculating an actual exposure dose in consideration of a change in the size of the subject in the body axis direction and an imaging mode in the main scan.

External view of X-ray CT system Configuration diagram of X-ray CT system Schematic explaining the relationship between X-ray detector and X-ray irradiation Schematic view of the relationship between the scanner, patient table, and subject from the side Flow chart showing the overall processing flow of the X-ray CT apparatus Flow chart showing the flow of exposure dose calculation processing Diagram showing a two-dimensional image Example of unit dose table Example of judgment formula for determining the phantom diameter used Display example of radiation dose calculation results Schematic diagram explaining the relationship between image slice thickness and image reconstruction interval in helical scan Example of relational expression between phantom radius and CTDI

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
First, an X-ray CT apparatus according to an embodiment of the present invention will be described with reference to FIGS.

As shown in FIG. 1, the X-ray CT apparatus mainly includes a scanner 1, a patient (subject) table 2, an operation console 3, a top 4 provided on the patient table 2, a display device 5, an operation device 6, and the like. Consists of.
As shown in FIG. 2, the scanner 1 mainly includes an X-ray tube control device 7, an X-ray tube 8 (X-ray source), a collimator control device 9, a collimator 10, an X-ray detector 11, a data collection device 12, and a rotation. The plate 13, the rotation control device 14, the rotary plate driving device 15, the driving force transmission system 16 and the like are configured.

The X-ray tube 8 irradiates the subject 17 with X-rays, and the tube voltage and tube current supplied to the X-ray tube 8 for irradiating the X-rays are controlled by the X-ray tube controller 7. The
The collimator 10 adjusts the X-ray irradiation field in order to irradiate the subject 17 with the X-rays irradiated from the X-ray tube 8 as, for example, a pyramidal X-ray beam, that is, a cone beam X-ray. It is controlled by the control device 9.
As shown in FIG. 3, the X-ray detector 11 includes a plurality of X-ray detection elements 18 provided two-dimensionally in the channel direction and the column direction, and is disposed at a position facing the X-ray tube 8. . X-rays emitted from the X-ray tube 8 pass through the subject 17 and enter the X-ray detector 11. The configuration of the X-ray detector 11 will be described in detail later.
The data collection device 12 is connected to the X-ray detector 11 and collects detection data of individual X-ray detection elements 18 of the X-ray detector 11.

On the rotating plate 13, the above-described X-ray tube control device 7, X-ray tube 8, collimator control device 9, collimator 10, X-ray detector 11, and data collection device 12 are mounted. 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.
As shown in FIG. 2, the patient table 2 mainly includes a top plate 4, a patient table control device 19, a patient table vertical movement device 20, a top plate drive device 21, and a top plate position sensor 25.
The patient table control device 19 controls the patient table vertical movement device 20 based on information from the top plate position sensor 25 to control the top plate 4 to an appropriate table height. Further, the patient table control device 19 controls the top plate driving device 21 to move the top plate 4 back and forth. As a result, the subject 17 is carried into and out of the X-ray irradiation space of the scanner 1.

As shown in FIG. 2, the console 3 mainly includes a display device 5, an operation device 6, an image reconstruction device 22, a storage device 23, a scan planning device 24, a system control device 26, and an exposure dose calculation device 27. The
The display device 5 is connected to the system control device 26, and displays a reconstructed image output from the image reconstruction device 22 and various information handled by the system control device 26.
The operation device 6 is connected to the system control device 26, and inputs various instructions, information, and the like to the system control device 26 by an operator.
The operator can interactively operate the X-ray CT apparatus using the display device 5 and the operation device 6.

The image reconstruction device 22 receives data collected by the data collection device 12 in the scanner 1 under the control of the system control device 26, and scanogram projection data (subject fluoroscopy data) collected by the data collection device 12 during scanogram imaging. ) To create a scanogram image, and at the time of scanning, a CT image (tomographic image) is reconstructed using projection data of a plurality of views collected by the data collection device 12.
The storage device 23 is connected to the system control device 26, and a program for realizing the functions of a scanogram image, a reconstructed CT image, various data, and an X-ray CT apparatus created by the image reconstruction device 22 Etc. are stored.

  The scan planning device 24 is connected to the system control device 26, and an operator creates a scan advance plan using an instruction input from the operation device 6 and a scanogram image read from the storage device 23. It is. That is, the scanogram image read from the storage device 23 is displayed on the display device 5, and the operator uses the operation device 6 on the displayed subject scanogram image to reconstruct the CT image (hereinafter, “slice position”). ")"), The slice position can be planned. Furthermore, the information on the slice position planned here is stored in the storage device 23, and is also used for making a plan such as an X-ray control condition by the scan planning device 24.

  The system control device 26 is connected to the scanner 1 and the patient table 2, and controls the X-ray tube control device 7, the collimator control device 9, the data acquisition device 12, and the rotation control device 14 in the scanner 1, and The patient table control device 19 in the patient table 2 is controlled.

  The exposure dose calculation device 27 is connected to the system control device 26 and calculates the exposure dose of the subject 17 from the measurement data of the subject 17 detected by the X-ray detector 11. A method for calculating the exposure dose of the subject 17 will be described in detail later.

Here, the X-ray detector 11 will be described in detail. As shown in FIG. 3, the X-ray detector 11 has a plurality of X-ray detector elements 18 configured by combining a scintillator and a photodiode, for example, two-dimensionally in the channel direction and the column direction (slice direction). It has become the composition.
The X-ray detection element 18 as a whole forms a cylindrical surface or an X-ray incidence surface curved in a polygonal line with respect to the channel direction, and the channel number i is, for example, about 1 to 1000 (that is, the X-ray detection element 18 is a channel). The column number j is, for example, about 1 to 320 (that is, the X-ray detection elements 18 are arranged about 1 to 320 in the column direction).
As shown in FIGS. 3 and 4, the cone beam X-ray adjusted in the fan angle α and the cone angle γ by the opening width of the collimator 10 is placed on the top 4 of the patient table 2 and carried into the opening of the scanner 1. The irradiated subject 17 is irradiated. In contrast, the X-ray detector 11 detects X-rays transmitted through the subject 17.

  Next, a processing flow of the X-ray CT apparatus will be described with reference to FIG. The following processing is performed under the control of the system control device 26. Here, the description will be made assuming a non-helical scan, but the embodiment of the present invention is not limited to the non-helical scan, and can also be applied to a helical scan. A special process when applied to the helical scan will be described in detail later.

  First, the operator operates the operation device 6 to instruct the system control device 26 whether to perform scanogram imaging, and the following processing is determined based on the instruction (step S10). .

When scanning a scanogram (YES in step S10), the process proceeds to a process of setting scan conditions using scanogram data (steps S11 to S12). That is, scanogram imaging of the subject is performed under the control of the system control device 26 (step S11), and a scan plan is set by the scan planning device 24 using the captured scanogram image (step S12). .
The scan plan set here is, for example, the position of the head CT image / end CT image in the body axis direction, the CT image reconstruction interval in the body axis direction, the X-ray tube voltage, the X-ray tube current, the scan time (one rotation). Time), X-ray collimation conditions, types of reconstruction filter functions, field size, and the like.

When scanogram imaging is not performed (NO in step S10), a scan plan is set by the scan planning device 24 without a scanogram image (step S13).
The scan plan set here is the same as that described in step S12, but the position of the head CT image and the tail CT image in the body axis direction is determined by a positioning projector (not shown). Thus, it can be determined by the relative position to the reference position.

When the scan plan is set, scanning is performed (steps S14 to S23).
First, the patient table control device 19 moves the top 4 to a predetermined body axis direction initial position (step S14).
Next, a scan is performed at a predetermined position in the body axis direction under the control of the system control device 26 (step S15).

Next, various correction / conversion processes (steps S16 to S18) will be described.
For the measurement data of the subject 17 obtained by scanning, the image reconstruction device 22 outputs the output (offset output) from the data collection device 12 when no X-ray is irradiated from the data collection device 12 at the time of subject measurement. (Step S16), sensitivity variation correction processing for correcting the sensitivity variation of the X-ray detector element 18 is performed (step S17), and the measurement data after correction is X-rays in the X-ray transmission path. A Log conversion process is performed to convert the projection data in proportion to the absorption coefficient integral value (step S18).
The data created by various correction / conversion processes (steps S16 to S18) for the measurement data for creating the projection data of the subject 17 is used for the CT image reconstruction process (steps S19 to S20). Used. Thereafter, the reconstructed data is used for the dose calculation processing (step S21) by the dose calculation device 27.

Next, CT image reconstruction processing (steps S19 to S20) will be described.
The image reconstruction device 22 performs reconstruction filtering processing for blur correction on the projection data of the subject 17 created in step S18 (step S19). Then, the image reconstruction device 22 reconstructs the CT image at a predetermined position in the body axis direction of the subject 17 by back projecting the projection data after the reconstruction filtering process (step S20).

Next, the exposure dose calculation device 27 analyzes the data created by the image data reconstruction process (steps S19 to S20), and performs the exposure dose calculation process (step S21). The calculation process of the exposure dose will be described in detail later.
Next, the CT image reconstructed in step S20 is stored in the storage device 24 together with the incidental dose calculated in step S21 and additional information such as other scanning conditions and displayed on the display device 5 ( Step S22). A display example will be described in detail later.

Next, the system controller 26 determines whether or not all scans have been completed (step S23).
If all scans have been completed (YES in step S23), a series of processing is terminated.
If all scans have not been completed (NO in step S23), the patient table control device 19 moves the top 4 to the next top position in the body axis direction (step S24). Thereafter, the scan is executed again (step S15). In this way, scanning is repeated as many times as necessary, and a series of processing ends.

Hereinafter, the exposure dose calculation processing will be described in detail with reference to FIGS.
The exposure dose calculation device 27 previously determines the exposure dose to the phantom at least for each phantom radius rn (phantom size information) and collimation width (minimum image slice thickness M (mm) × number of columns R: image slice thickness information). Based on the measurement result, CTDI (unit dose) per image is calculated and stored in the storage device 23 as a unit dose table.

After execution of the scan, the exposure dose calculation device 27 creates a two-dimensional image of a cross section perpendicular to the body axis of the subject 17 at predetermined intervals over the entire imaging range based on the measurement data. For example, the exposure dose calculation device 27 creates a two-dimensional image group having N minimum image slice thicknesses.
Next, the exposure dose calculation device 27 detects a subject region that is a set of pixels having CT values of the subject 17 for each two-dimensional image, calculates the area of the subject region, and rounds the subject region. The approximate radius (object area size information) is obtained.
Next, the exposure dose calculation device 27 specifies the unit exposure dose by searching the phantom size information corresponding to the size information of the subject region and the image slice thickness information corresponding to the predetermined interval for each two-dimensional image. Further, the total exposure dose of the subject 17 is calculated by adding the unit exposure doses for all the two-dimensional images.
In the case of spiral scanning, a two-dimensional image group created at the same interval as the image slice thickness set as the imaging condition for obtaining measurement data is used.

The process shown in FIG. 6 is performed by the exposure dose calculation device 27.
First, the exposure dose calculating device 27 detects a subject area in each image from the two-dimensional image group having the minimum image slice thickness created in step S20 (step S31). The subject region is detected by setting a CT value threshold.

  Here, the threshold value will be described. The X-ray irradiation path roughly includes air and the subject 17. X-rays are absorbed when passing through the subject 17 but are not absorbed by air. In the image data created in step S20, the difference between air and the subject 17 is clearly expressed as a CT value. Therefore, the threshold value A is set from the CT value of air included in the two-dimensional image (around −1000 HU). For example, the threshold A is set to about −800 HU. A region having a CT value higher than the threshold value A is determined to perform X-ray absorption, and “1” is determined. A region having a CT value lower than the threshold value A is determined to have no X-ray absorption, and is set to “0”. By subjecting to binarization, the subject region is detected.

Moreover, the top plate 4 also exists in the X-ray irradiation path. Therefore, a threshold value may be provided based on the CT value of the top plate 4 in order to eliminate the influence of the top plate 4. Since the top plate 4 is for supporting a person, a material close to the CT value of air (small X-ray absorption amount) is used. On the other hand, the threshold value for removing the top 4 needs to be a CT value smaller than the CT value of fat having the smallest CT value in the subject 17 (near -100HU). Therefore, the threshold value for removing the top 4 may be in the range of −1000 <top CT value <−100.
In addition, methods for detecting a subject region have been reported in various documents (for example, Japanese Patent Application Laid-Open No. 2007-181623), and not only this method but also a known method may be used.

Next, the exposure dose calculation device 27 calculates the area Sn of the subject region detected from each of the two-dimensional image groups (step S32).
The area calculation process of the subject region in the nth image in the two-dimensional image group will be described with reference to FIG. In FIG. 7, the n-th two-dimensional image 30 is simply illustrated for each pixel. The two-dimensional image 30 displays, for example, 512 × 512 pixels in each of the X direction and the Y direction, and has a pixel value of “0” in the air region 32 and “1” in the subject region 31.
For example, the exposure dose calculation device 27 calculates the area per pixel from the FOV size D [mm] of the two-dimensional image 30 and multiplies the total number of pixels Tn having a pixel value of “1” (the following equation) Reference), the area of the subject region 31 is obtained.

In addition, although it demonstrated as a 512x512 pixel display here, it is not limited to this.
Next, the exposure dose calculation device 27 approximates the tomographic image of the subject 17 from the obtained Sn to a circle, and obtains the radius rn by the following equation (step S33).

  A method for obtaining the area of the subject region 31 in the two-dimensional image 30 has been reported in various literatures (for example, 2007-181623), and a known method may be used without being limited to this method. .

  Next, the dose calculation apparatus 27 selects a unit dose corresponding to the radius rn obtained in step S33 from the unit dose table (step S34).

  Here, the unit dose table will be described. The unit dose table is a table in which CTDI values measured using acrylic phantoms having several different radii are converted into CTDI values in image units. It is assumed that the storage device 23 of the X-ray CT apparatus of the present invention stores in advance CTDI values using a plurality of acrylic phantoms having different radii.

Hereinafter, in order to simplify the description, a case where eight acrylic phantoms having radii r1 mm, r2 mm,..., R8 mm are used will be described. Of course, the size and number of phantoms can be changed as appropriate.
The CTDI value registered in the unit dose table uses a value obtained by dividing the CTDI value measured for each collimation width by the maximum number of columns R in the collimation width so as to correspond to the minimum image slice thickness M [mm]. ing.
Here, the minimum image slice thickness M is the minimum image slice thickness that can be imaged by the X-ray CT apparatus, that is, the minimum value that can be set as the image slice thickness of the imaging conditions. The collimation width is the opening width of the collimator 10 in the column direction (see FIG. 3). The maximum number of columns R in the collimation width is the maximum number of X-ray detection elements 18 in the column direction that detect cone beam X-rays with the cone angle γ (see FIG. 3) adjusted.
For example, when the X-ray CT apparatus is a 64 column apparatus (an apparatus in which 64 X-ray detection elements 18 are arranged in the column direction), the collimation width is the minimum image slice thickness M [mm] × 64 columns. , CTDI value per image = calculated CTDI value / 64

In the unit dose table 40 shown in FIG. 8, CTDI values per image are registered for each tube voltage, collimation width (minimum image slice thickness M × number of columns R), and phantom radius.
For example, in the first table from the left, the phantom radius 44 is “depending on the imaging conditions where the tube voltage 41a is“ XXV ”and the collimation width 42a is“ minimum image slice thickness M (mm) × R1 ”. A value obtained by dividing _eight CTDI values “CTDI-a _{1 to} CTDI-a ₈ ” measured by using “r1 to r8” (phantom name 43 is “circular phantom 1 to circular phantom 8”) by the number of columns R1, Each of them is registered as CTDI 45 “CTDI-a ₁ / R1 to CTDI-a ₈ / R1” per image in association with the radius 44 of the phantom.
Further, for example, in the second table from the left, the phantom radius 44 is determined depending on the imaging conditions in which the tube voltage 41a is “XXV” and the collimation width 42b is “minimum image slice thickness M (mm) × R2”. The values obtained by dividing the eight CTDI values “CTDI-a _{9 to} CTDI-a ₁₆ ” measured by using “r1 to r8” by the number of columns R2 are associated with the phantom radius 44, respectively. It has been registered as per one CTDI45 _{"CTDI-a 9 / R2~CTDI-a} 16 / R2 ".
Further, for example, in the third table from the left, the phantom radius 44 depends on the imaging conditions in which the tube voltage 41b is “ΔΔKV” and the collimation width 42a is “minimum image slice thickness M (mm) × R1”. The values obtained by dividing the _eight CTDI values “CTDI-b _{1 to} CTDI-b ₈ ” measured by using “r1 to r8” by the number of columns R1 are associated with the phantom radius 44, respectively. It has been registered as per one CTDI45 _{"CTDI-b 1 / R1~CTDI-b} 8 / R1 ".

  The unit dose table 40 includes a CTDI per image at least for each phantom radius (phantom size information) and collimation width (minimum image slice thickness M (mm) × number of columns R: image slice thickness information). As long as is registered. Furthermore, as shown in FIG. 8, the CTDI per image is registered for each parameter that changes the tube voltage and other radiation qualities, so that the actual exposure dose of the subject can be calculated with high accuracy. Can do.

The correspondence between the radius rn obtained in step S33 and the phantom radius 44 registered in the unit dose table 40 is determined by the determination formula 52 in FIG.
For example, when rn ≦ (r1 + r2) / 2, the used phantom diameter 51 is r1. For example, when (r1 + r2) / 2 <rn ≦ (r2 + r3) / 2, the phantom diameter 51 used is r2. The same applies hereinafter.
The exposure dose calculation device 27 determines the use phantom diameter 51 in this way, specifies the CTDI 45 per image from the unit dose table 40, and sets it as the exposure dose CTDIn of the nth image.

The exposure dose calculation device 27 adds the exposure dose CTDIn in step S35, and repeats steps S31 to S35 for all images (step S36).
That is, the exposure dose calculation device 27 calculates the total exposure dose as follows (step S37).

  FIG. 10 shows a display example of the exposure dose. 10A is a top view of the subject 17, FIG. 10B is a side view of the subject 17, FIG. 10C is an exposure dose (CTDI value) for each region, and FIG. 10D. These show the change of the use phantom diameter (mm) accompanying the shape change of a body-axis direction.

  Since the exposure dose calculation device 27 calculates the exposure dose for each two-dimensional image, as shown in FIG. 10C, the exposure dose is calculated and displayed for each arbitrary region in the body axis direction. Can do.

In the graph of FIG. 10D, the horizontal axis indicates the position of the subject 17 in the body axis direction, and the phantom diameter (phantom size information) specified by the exposure dose calculation device 27 is the vertical axis.
Up to now, the reference phantom defined by IEC 60601-2-44 has used only two types of head region (radius 80 mm) and trunk region (radius 160 mm) as shown in FIG. The shape change in the body axis direction could not be considered. On the other hand, in the present invention, as shown in FIG. 10 (d), when determining the exposure dose, the area of the subject is converted into an acrylic phantom radius equivalent for each image in the body axis direction, and the shape of the subject in the body axis direction is calculated. It is possible to express the exposure dose considering changes.

  Conventionally, it has been very difficult to determine photographing conditions for photographing an image suitable for interpretation in obese patients. On the other hand, the graph with the phantom diameter used as the vertical axis shown in FIG. 10D is information that is helpful when determining the shooting conditions in the next shooting. Because the measurement results by the phantom are known in advance as to the imaging conditions and the quality of the image quality, if the body shape of the subject can be approximated to a phantom, the imaging conditions for capturing an image suitable for interpretation are also available. This is because it can be easily judged.

  In the graph of FIG. 10D, the phantom diameter is the vertical axis, but the vertical axis may be CTDI (unit dose) per image specified by the dose calculation device 27. By setting the vertical axis as the unit exposure dose, the detailed exposure situation of the image unit can be understood.

  According to the embodiment of the present invention, since it is possible to obtain the exposure dose in consideration of the shape change of the subject in the body axis direction, the exposure dose can be calculated with high accuracy. Further, since the size information of the subject region is calculated based on the measurement data of the main scan, not the measurement data of the scanogram imaging, and the exposure dose is obtained based on the size information of the subject region, The photographing mode is accurately taken into consideration.

In the above description, in order to calculate the dose calculation in the body axis direction with higher accuracy, the unit dose based on the minimum image slice thickness is registered in the unit dose table. The unit exposure dose based on a thick image slice thickness obtained by adding several thicknesses may be registered.
In the above description, the tomographic image of the subject 17 is approximated to be circular and the radius rn is obtained on the premise of the circular phantom. However, on the premise of the elliptical phantom, the tomographic image of the subject 17 is elliptical. May be approximated.

<Modification 1>
A special process when applied to the helical scan will be described.
In the helical scan, image reconstruction can be performed from anywhere in a long spiral locus, and many images can be reconstructed. However, in this case, the slice thickness of each sheet is due to collimation and helical pitch, and overlapping images can be obtained by reducing the image reconstruction interval.

As shown in FIG. 11, if a two-dimensional image (helical scan image) based on measurement data acquired by a helical scan is an image created at the same image reconstruction interval as the image slice thickness, the regions overlap each other. There is no.
On the other hand, by making the image reconstruction interval finer than the image slice thickness, the spiral scan images become images that overlap each other.
Therefore, when a two-dimensional image is created with the image reconstruction interval being smaller than the image slice thickness, the total exposure dose calculated by the exposure dose calculation device 27 is calculated to be larger than the actual exposure dose. Become.
Therefore, in the case of a spiral scan, the dose calculation device 27 calculates a dose using a two-dimensional image group created at the same interval as the image slice thickness set as the imaging condition for obtaining measurement data. To do.

<Modification 2>
In the above description, the unit exposure dose is specified using the determination formula 52 of FIG. 9, but the embodiment of the present invention is not limited to this example.
In the second modification, a correlation formula between the acrylic phantom radius and CTDI is calculated from a plurality of acrylic phantoms having different radii in advance, and the total exposure dose of the subject is calculated from the correlation formula.

  FIG. 11 shows a relational expression 60 between the phantom radius and CTDI. As shown in FIG. 11, it can be seen that the exposure dose decreases as the phantom radius increases. Therefore, by creating a correlation formula between the phantom radius and the exposure dose at this time, the exposure dose can be obtained without being caught by the measured phantom radius. The correlation equation at this time can be expressed by the following equation.

However, Z: coefficient, r: acrylic radius, k: number of measurement phantoms (k ≧ 2).
At this time, the larger the number of measurement phantoms, the better. The correlation equation also requires measurement of at least two or more phantoms.
The unit dose calculation device 27 calculates the used phantom diameter and the CTDI (unit dose) per image based on the correlation equation obtained here.
In this example, since the CTDI value per image having the minimum image slice thickness can be calculated without being limited to the number of acrylic phantoms, the total exposure dose can be determined more accurately than in the above example.

  The preferred embodiments of the X-ray CT apparatus and the like according to the present invention have been described above with reference to the accompanying drawings, but the present invention is not limited to such examples. 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 naturally belong to the technical scope of the present invention. Understood.

DESCRIPTION OF SYMBOLS 1 ......... Scanner 2 ......... Patient table 3 ......... Console table 4 ......... Top panel 6 ......... Display device 6 ......... Operation device 7 ......... X-ray tube control device 8 ......... X-ray Tube 9... Collimator control device 10... Collimator 11... X-ray detector 12... Data collection device 13. ......... Driving force transmission system 17 ......... Subject 18 ......... X-ray detection element 19 ......... Patient table control device 20 ......... Patient table vertical movement device 21 ......... Top plate driving device 22 ......... Image reconstruction device 23... Storage device 24... Scan planning device 26... Top plate position sensor 26... System controller 27.

Claims (3)

Equipped with an X-ray source for irradiating a subject with X-rays, an X-ray detector arranged to face the X-ray source and detecting X-rays transmitted through the subject, and the X-ray source and the X-ray detector An image reconstructing device that acquires measurement data detected by the scanner that rotates around the subject and the X-ray detector and reconstructs a tomographic image of the subject, and is reconstructed by the image reconstructing device. An X-ray CT apparatus comprising an image display device for displaying a configured tomogram,
Storage means for storing a unit dose calculated based on a measurement result of the dose to the phantom for each phantom size information and image slice thickness information;
An image creating means for creating a two-dimensional image of a cross section perpendicular to the body axis of the subject at predetermined intervals over the entire imaging range based on the measurement data;
Size information calculation means for detecting a subject region that is a set of pixels having a CT value of the subject for each of the two-dimensional images and calculating size information of the subject region;
For each of the two-dimensional images, the unit dose is specified by searching the phantom size information corresponding to the size information of the subject region and the image slice thickness information corresponding to the predetermined interval, A dose calculation means for calculating a total dose of the subject by adding up the unit doses for all the two-dimensional images;
An X-ray CT apparatus comprising:   2. The X-ray CT according to claim 1, wherein, in the case of a helical scan, the image creating unit makes the predetermined interval the same as an image slice thickness set as an imaging condition for obtaining the measurement data. apparatus. A graph with the horizontal axis representing the position of the subject in the body axis direction and the vertical axis representing the unit dose and / or phantom size information identified by the dose calculation means is displayed on the image display device. Display means,
The X-ray CT apparatus according to claim 1, further comprising:

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