Analysis system for radiation dose in X-ray CT examinations

Abstract

The aim is to provide a robust X-ray CT scan radiation dose analysis system that is less affected by the imaging area and imaging conditions of the patient. [Solution] In an X-ray CT image of a subject, multiple regions of interest (ROIs) are set in the air region surrounding the imaging area of ​​the subject, the standard deviation (SD) of the CT value in each region of interest is calculated, and the incident angle θ of the X-rays is obtained from the SD distribution. in The angle of incidence θ to find in Having an analysis means, the incident angle θ in The incident angle θ of the X-ray obtained by the analysis method in It is characterized by having a dose correction function that corrects the measured value of the exposure dose by a dosimeter based on the information.

Description

[Technical Field]

[0001] The present invention relates to a system for measuring and analyzing the radiation dose received by a patient during an X-ray CT (computed tomography) examination. [Background technology]

[0002] X-ray CT scans are highly valued in medical settings because they allow for three-dimensional diagnosis of the internal environment, and these scans are performed numerous times every day. In recent years, helical scanning with a high pitch factor has been adopted, which shortens inspection time by increasing the scanning speed. Furthermore, in X-ray CT scans, managing the patient's radiation dose is considered important due to concerns about protection from medical radiation exposure.

[0003] The inventors have previously proposed a highly accurate radiation dose analysis system using a small number of dosimeters by measuring CT images and analyzing the direction of X-ray incidence (Patent Document 1). This invention was obtained as a result of investigating a highly accurate analysis method using an even simpler algorithm. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2024-132783 [Overview of the project] [Problems that the invention aims to solve]

[0005] The present invention aims to provide an analysis system for radiation dose in X-ray CT examinations that is less affected by the imaging area and imaging conditions of the subject and exhibits excellent robustness. [Means for solving the problem]

[0006] The radiation dose analysis system according to the present invention sets multiple regions of interest (ROIs) in the air region surrounding the imaging area of ​​the subject in an X-ray CT image of the subject, calculates the standard deviation (SD) of the CT value in each of the ROIs, and uses the SD distribution to determine the incident angle θ of the X-rays. in The angle of incidence θ to find in Having an analysis means, the incident angle θ in The incident angle θ of the X-ray obtained by the analysis method in It is characterized by having a dose correction function that corrects the measured value of the exposure dose by a dosimeter based on the information.

[0007] In X-ray CT images, it has been found that there is a strong correlation between the angle formed by the centroids of the SD distributions created from the variability (SD) of CT values ​​in each ROI, when multiple regions of interest (ROIs) are set in the air region surrounding the imaging area, and the angle formed by the centroids of the dose distribution (Patent Document 1). However, in actual X-ray CT scans of patients, accurate measurement of radiation dose was difficult due to factors such as the inability to determine the starting position of X-ray tube rotation in helical scanning, the inability to set a region of interest (ROI) in certain areas due to the examination table, and the influence of imaging conditions, body size, and the area being examined. Therefore, the present invention provides a predetermined X-ray incidence angle θ in and its angle of incidence θ in A key feature of this study is that it analyzes the relationship of the SD distribution in the given context and corrects the measured radiation dose values ​​for actual subjects based on that analysis. Therefore, the dose correction function according to the present invention is for predicting the dose distribution based on a predetermined measurement location, such as the center of the chest, as the reference for dose measurement.

[0008] In this invention, the dose correction function is an arbitrary incident angle θ of X-rays. in This is based on an SD reference function, and it is preferable that the SD reference function is based on an SD distribution analyzed using a simulated phantom or a subject. [Effects of the Invention]

[0009] In the present invention, in an X-ray CT image, an X-ray incident angle θ is obtained from an SD distribution obtained from a plurality of ROIs set around a subject in , and the measured dose value can be corrected by using the dose correction function for this incident angle θ in , whereby the influence of the incident angle can be reflected in the exposure dose measurement, thus improving the measurement accuracy of the exposure dose. Furthermore, by using a single SD reference function and a single dose correction function, the system is less susceptible to the influence of the bed and imaging conditions in X-ray CT examination, and has excellent robustness. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] [Figure 1] Fig. 1 is an explanatory diagram showing the characteristics of the exposure dose analysis system according to the present invention. [Figure 2] Fig. 2 is an explanatory diagram showing a method for correcting measured exposure dose values. [Figure 3] Fig. 3 shows an algorithm of the analysis system according to the present invention. [Figure 4] Fig. 4 is a conceptual diagram showing an ideal system for X-ray incident direction analysis using X-ray CT images and an actual X-ray CT examination. [Figure 5] Fig. 5 shows the relationship between the X-ray incident angle θin and the SD in an ROI set at an arbitrary position. [Figure 6] Fig. 6 shows an SD reference function at an arbitrary X-ray incident angle θin. [Figure 7] Fig. 7 is an explanatory diagram showing a phantom used in a verification experiment. [Figure 8] Fig. 8 shows analysis results of X-ray incident angles. [Figure 9] Fig. 9 shows measurement results of exposure dose. [Figure 10] Fig. 10 shows measurement results of exposure dose. [Figure 11] Fig. 11 is an explanatory diagram showing dose modulation in the body axis direction in helical scanning and in the rotation axis direction at an arbitrary scanning position. [Figure 12] Fig. 12 is an explanatory diagram showing modulation of surface dose in helical scanning. [Figure 13] Fig. 13 is an explanatory diagram showing a conventional method for obtaining an average dose value. [Modes for carrying out the invention]

[0011] Before describing the analysis system according to the present invention, we will first explain the technical challenges in X-ray CT examinations using Figures 11 to 13. When performing a helical scan using an X-ray tube, a dosimeter such as a small OSL dosimeter is placed on the subject to measure the radiation dose. As shown in Figure 11(a), the X-ray dose varies depending on the area being examined, and as shown in Figure 11(b), it also changes depending on the axis of rotation of the X-ray tube. In particular, in helical scanning, as shown in Figure 12, imaging is performed by the X-ray tube tracing a helical trajectory, and since both dose modulation in the axial direction and dose modulation in the rotational direction have an effect, the surface dose shows very large fluctuations. As a result, as shown in Figure 13, even at the same slice position, the values ​​vary greatly from one measurement run to the next (Run No.).

[0012] Next, we will explain the challenges of setting a region of interest (ROI) in the air region surrounding the subject in CT images. Figure 4(a) shows an explanatory diagram illustrating the variability (SD) of CT values ​​in the surrounding ROI and the positional relationship of the X-ray source in an ideal system using a cylindrical phantom, while Figure 4(b) shows an explanatory diagram in a real-world X-ray examination. The standard deviation (SD) of an ROI near the X-ray tube is relatively small, while the SD of an ROI farther from the X-ray tube is relatively large. Therefore, the direction of the centroid vector of the SD value and the incident angle θ of the X-ray are related. in There is a strong correlation between them. Figure 4(a) shows an explanatory diagram of the case where 12 ROIs are set at equal intervals in the air region surrounding the subject. In contrast, as shown in Figure 4(b), in real-world X-ray examinations, there is a patient table, and some areas cannot be analyzed as ROIs. Therefore, the present invention creates a reference function to estimate the incident direction of X-rays from the SD distribution obtained in an actual X-ray inspection as shown in Figure 4(b), and performs fitting.

[0013] Figure 1 shows a conceptual diagram of the measurement system according to the present invention based on this. To measure the radiation dose, a dosimeter is attached to the subject, and a CT scan is performed using a helical scan. Let this number be N (a natural number). Based on this SD distribution, the incident angle θ of the X-rays in Analyze it. When the SD distribution is determined in this way, as shown in Figure 4(b), there are gaps in the measured values, resulting in a discrepancy from the original SD distribution. Therefore, by fitting a pre-created SD reference function to the SD distribution obtained from actual measurements, the incident angle θ of the X-rays is determined. in Analyze it. Also, the dose correction function (dose correction function) f Dose (θ in θ) is the incident angle of the X-rays θ in It is created based on the dose values ​​at each angle of incidence. The average value Σf based on the aforementioned dose correction function. Dose / N is calculated, and the correction value is calculated from the dose correction function value, average value, and ratio at each incidence angle. This corrects the measured dose value, allowing for correction of the X-ray incidence angle θ. in It is possible to obtain dose values ​​that have been corrected for the effects of [unspecified factors]. Here, the dose correction function is the incident angle θ of the X-rays. in This data is dependent on the measurement location, and if the measurement location changes, the dose distribution will also change. Therefore, in Figure 1, for example, a small dosimeter was placed in the center of the chest, and this measurement position was created with θ=0° as the reference.

[0014] Next, the specific analysis procedure will be explained based on Figures 2 and 3. First, as shown in Figures 2(b) and 3(b), a cylindrical phantom is used as a simulated phantom to acquire CT images. In the obtained CT images, multiple regions of interest (ROIs), for example, at 10° intervals, are set around the cylindrical phantom, and SD measurements are performed at the angle θ where each ROI is located. The incident angle of X-rays (θ) obtained from this measurement data and log data in Based on this, we perform a functionalization of the SD value distribution and create the SD criterion function f SD (θ,θ in Create ).

[0015] On the other hand, as shown in Figures 2(a) and 3(a), a small dosimeter is placed in the center of the subject's chest, and a CT scan using helical scanning is performed to measure the radiation dose, as well as the standard deviation (SD) at the angle θ where each ROI is located. In actual CT scans, there are ROIs (Regions of Interest) at positions (θ) where SD measurement is not possible. Therefore, the SD reference function f SD (θ,θ in By fitting the X-ray incident angle θ, in Analyze it. The measured dose is the incident angle θ of the X-ray. in Since it depends on the measurement position in the center of the subject's chest, in this embodiment, the dose correction function f is used with θ=0° as the reference. Dose (θ in I created ). Based on this, Figure 2 shows the incident angle θ of the X-rays. in The measured value at =x° was corrected as a corrected dose value based on the predicted dose distribution.

[0016] Next, the SD reference function f SD (θ,θ in This section explains an example of how to create a file. A helical scan was performed using a 25cm diameter cylindrical water phantom under the following conditions. ·Clinical CT equipment • Detector row count / width: 96 rows / 0.6 mm • X-ray tube voltage: 120kV Pitch Factor: 1.2 <Image reconstruction conditions> • Slice thickness: 1mm • Slice spacing: 1mm • FOV: 500mm Figure 5(a) shows the CT images of the water column phantom and the analyzed standard deviation (SD) distribution. The incident angle θ of the X-rays in the aforementioned SD distribution in The temperature was 26°. The position (angle) of each ROI at this time is θ in The relationship between and SD is shown in Figure 5(b). Based on this, the SD reference function f was created. SD (θ,θ in An example of this is shown in Figure 6.

[0017] Next, we will describe the demonstration experiments conducted using various simulated phantoms. In the demonstration experiment, the cross-sectional phantom shown in Figure 7(a) was used to set the pitch factor (PF) in helical scanning to 1.2, 0.9, and 1.5 under three conditions, while the whole-body phantom shown in Figure 7(b) was used to set the pitch factor (PF) in helical scanning to 1.2. • Imaging range: Lung field ±2cm • Dosimeter placement: Above the sternum, on the lower edge of the sternum The shooting conditions were the same as those used when creating the SD reference function.

[0018] The results of the analysis of the X-ray incidence angle are shown in Figure 8. The upper graph shows the relationship between the true X-ray incidence direction in CT scans of cylindrical water phantoms (PF: 1.2), cross-sectional phantoms (PF: 1.2, 0.9, 1.5), and whole-body phantoms (PF: 1.2), and the X-ray incidence direction obtained in this experiment. The lower graph shows the deviation from the straight line y=x. We were able to determine the incident X-ray direction with an accuracy of 1σ = 13° for various phantoms and PFs.

[0019] Figures 9 and 10 show the results of the dose distribution analysis for each simulated phantom. The graph plots the measured dose value and the corrected dose value. In all cases, the calculated standard deviation (SD) is smaller, indicating an improvement in the accuracy of dose analysis.

Claims

1. In the X-ray CT images taken of the subject, Multiple regions of interest (ROIs) are set in the air region surrounding the imaging unit of the subject, the standard deviation (SD) of the CT values ​​in each region of interest is determined, and the incident angle θ of the X-rays is obtained from the SD distribution. ｉｎ The angle of incidence θ to find ｉｎ Having analytical means, The incident angle θ ｉｎ The incident angle θ of the X-ray obtained by the analysis means ｉｎ An exposure dose analysis system characterized by having a dose correction function that corrects the measured exposure dose value by a dosimeter based on information.

2. The dose correction function is an arbitrary incident angle θ of X-rays. ｉｎ The exposure dose analysis system according to claim 1, characterized in that it is based on an SD reference function in the context of the following.

3. The exposure dose analysis system according to claim 2, characterized in that the SD reference function is based on an SD distribution analyzed using a simulated phantom or a subject.

4. The exposure dose analysis system according to any one of claims 1 to 3, characterized in that the dose correction function is for predicting the dose distribution based on dose measurement at the center of the chest.

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