Method for evaluating performance of radiation shield body used in CT imaging

Abstract

To provide a method for correctly evaluating a performance of a radiation shield body when the shield body is used in a CT imaging.SOLUTION: A method for evaluating a performance of a radiation shield body used in a CT (Computed Tomography) imaging which is a helical scanning in which an X-ray tube revolves along a spiral path, calculates a dose reduction factor (DRF) based on: a dose distribution Dw,i with the shield body measured by a helical scanning with the shield body; and a dose distribution Do,i without the shield body measured by the helical scanning without the shield body.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for evaluating the performance of a radiation shield used in computed tomography (CT).

Background Art

[0002] When performing CT imaging by X-ray irradiation or the like, a radiation shield is used to reduce exposure to radiation (X-rays) of various tissues of the subject, such as the thyroid gland, the lens, and the gonads. A general method for evaluating the shielding performance such as the dose reduction factor (DRF) of the radiation shield is to insert the radiation shield to be evaluated between an irradiation source such as an X-ray tube and an area dosimeter, and to obtain it from the difference in the dosimeter measurement values with and without the shield, and generally, that value is specified in the product.

[0003] However, in actual CT imaging, helical scanning in which the X-ray tube is rotated along a helical orbit while imaging has become the mainstream.

[0004] A comparison example between the above general method for evaluating a radiation shield and CT imaging is shown in FIG. 14 as a conceptual diagram. As shown in this figure, since the X-ray tube moves in a spiral shape, the dose reduction rate will be different from the value measured by the general method for evaluating a radiation shield.

[0005] The present inventors have proposed that a SD distribution can be created from the variation (SD) of the CT values of a plurality of ROIs (regions of interest) set outside the subject part of the CT image, and the angle θ formed by the SD centroid can be obtained to estimate the incident direction of X-rays (Patent Document 1). SD The present invention is obtained by applying the above technique to study a method for evaluating a shield used in CT imaging. The present invention is obtained by applying the above technique to study a method for evaluating a shield used in CT imaging.

Prior Art Documents

Patent Documents

[0006] Patent Document 1 Japanese Patent Application No. 2023-111491 Summary of the Invention Problems to be Solved by the Invention

[0007] An object of the present invention is to provide an evaluation method capable of correctly evaluating the performance of a radiation shield when the radiation shield is used in CT imaging. Means for Solving the Problems

[0008] The method for evaluating the performance of a radiation shield according to the present invention is a method for evaluating the performance of a radiation shield used in CT (Computed Tomography) imaging, wherein the CT imaging is helical scanning in which an X-ray tube rotates along a helical orbit, and the dose distribution D with the shield measured by performing helical scanning with the shield in place w,i and the dose distribution D without the shield measured by performing helical scanning without the shield o,i to calculate a dose reduction factor (DRF: Dose Reduction Factor).

[0009] The present invention is characterized in that the effective performance of the shield when used in CT imaging is evaluated by obtaining a dose reduction factor (DRF) using the dose distribution D with the shield w,i and the dose distribution D without the shield o,i . However, when such measurements are made, a problem is that the X-ray incident direction when measuring the dose distribution D with the shield w,i differs from the X-ray incident direction when measuring the dose distribution D without the shield o,i .

[0010] Therefore, it is preferable that the dose distribution D with the shield w,i and the dose distribution D without the shield o,i are those in which the radiation incident angle is calculated from a medical image and the incident angles are made to correspond. Also, as a simple method, the dose distribution D with a shield w,i and the dose distribution D without a shield o,i may be arranged in descending or ascending order of the measured dose values and made to correspond to each other.

Advantages of the Invention

[0011] When evaluating the dose reduction factor (DRF) of a radiation shield, the dose distribution D with a shield w,i is measured, and when the dose distribution D without a shield o,i is measured, the incident angles of the radiation often do not match, and correct evaluation cannot be performed. However, by applying a calculation method that takes into account this difference in incident angles, the performance of the shield in CT imaging can be correctly evaluated. It is expected that this will enable a scientific evaluation of the effectiveness of using a shield in CT examinations.

Brief Description of the Drawings

[0012]

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Mode for Carrying Out the Invention

[0013] The flow of calculating the dose reduction factor (DRF) of the shield according to the present invention is shown in FIG. 1. First, the experimental results will be described. As the CT apparatus, SOMATOM Force (manufactured by Siemens) was used, and as the dosimeter, a nanoDot OSL dosimeter and a microSTARii OSL reader (manufactured by Nagase Landauer Co., Ltd.) were used.

[0014] The arrangement of the dosimeters is shown in FIG. 2. For the purpose of shielding the thyroid gland, as Experiment A, a bib type covering the entire neck was carried out, and as Experiment B, a type arranged on the front of the neck was carried out.

[0015] Experiment A will be described. Twelve OSL dosimeters were evenly arranged around the neck at intervals of 30 degrees. FIG. 3 shows the positioning image corresponding to the polar coordinate system of the X-ray source and the slice number when helical scanning is executed. FIG. 4 schematically shows the method of calculating the X-ray incident direction at each slice cross section. Here, the ROI (Region of Interest) used for image analysis is set in plural so as to surround the subject in the CT image, and the angle θ formed by the center of gravity of the dose distribution indicating the X-ray incident angle is measured by the OSL dosimeter.Dose and the angle θ formed by the centroid of the standard deviation (SD) distribution of the CT value variations among the plurality of ROIs SD has a negative correlation, specifically a 180-degree shift. Therefore, the X-ray incident angle for each slice number shown in FIG. 3 can be estimated (see Patent Document 1).

[0016] In the experiment, 10 scans were independently performed, and the measurement data for up to 3 of these scans are shown in FIG. 5. In Scan NO.1~3, the direction indicated by the arrow is the X-ray incident direction. It can be seen that the X-ray incident direction is different between without the shielding body and with the shielding body. FIG. 6 shows the average values of the 10 scans, and FIG. 7 shows the results of calculating the DRF for each arrangement angle of the dosimeter using these 10 average values. From these results, it can be seen that the shielding rate also varies depending on the position where the dosimeter is arranged.

[0017] Therefore, as shown in FIG. 8, multiple measurements were performed without the shielding body, and the X-ray incident angle (θ on ) was calculated for each data. Further, multiple measurements were performed with the shielding body, and the X-ray incident angle (θ wn ) was calculated for each data, and the data set was sorted so that they generally match. The results are shown in FIG. 9. FIG. 9 shows the results of Experiment A and Experiment B. (a-1) The raw roll-up type shielding body plotted the measured values without the shielding body (without) and with the shielding body (with) in the scan order (the order of experiment implementation) as they are, and calculated the DRF value from the difference between them. (a-2) The sorted roll-up type shielding body was sorted and plotted so that the X-ray incident angles with and without the shielding body generally corresponded in the same direction, and the DRF was calculated from the difference between them. The DRF (%) calculated based on the original scan order varies greatly in the range from about 25% to about 60%, and it cannot be said that the DRF is accurately obtained. On the other hand, the DRF calculated based on the data sorted by adjusting the X-ray incident angle varies in the range of about 45% to about 55%, and the DRF is accurately obtained. (b-1) The raw shielding body of the placing type is the one that plots the measured values as they are in the scanning order, and (b-2) the sorted shielding body of the placing type is sorted and plotted so that the calculated X-ray incident angles correspond to generally the same direction with and without the shielding body, and the DRF is calculated from the difference between them. Also in this case, it can be seen that by sorting so that the incident angles match, a more accurate DRF with less variation can be obtained.

[0018] In the above experiment, since the importance of considering the influence of the X-ray incident angle when calculating the DRF was shown, scanning was sequentially performed with and without the shielding body, and by sorting the measured dose data in descending order of dose (Dose) as shown in the lower part of Fig. 10, an analysis considering the influence of the X-ray incident angle was simply performed. The upper part of Fig. 10 shows a conceptual diagram when four experiments are performed under the conditions of without and with the shielding body, and the lower part is a conceptual diagram of the analysis of pairing by sorting the data measured without the shielding body in descending order of the measured dose and similarly sorting the data measured with the shielding body in descending order of the measured dose. For example, in the experiment without the shielding body, the 1st, 2nd, 3rd, and 4th data were obtained in this order, but it shows that after sorting in descending order of dose, the data order is 2, 4, 1, 3. Fig. 11 shows a block diagram of this procedure. In Fig. 12, the original data is the one that plots the dose and DRF for the data set obtained in the scanning order, and the sorted data shows the result of sorting the measured data in descending order of dose to pair a new data set and plotting the dose and DRF. These results were obtained by analyzing the experiment of Experiment A. (a-1) and (a-2) are the measurement results at the 0-degree position (directly above the thyroid gland), and (b-1) and (b-2) are the calculation results at the 90-degree position (side of the neck). In both cases, it can be seen that the accuracy of the DRF is improved by sorting. Specifically, the DRF calculated based on the data of (a-1) plotted in the scan order is distributed in the range of about 45% to about 65% and has a very large variation. However, for the data of (a-2) sorted in descending order of dose, the DRF is within the range of about 50% to about 55%, with less variation and a more accurate DRF obtained. Similarly, while the variation of the DRF calculated based on the data of (b-1) plotted in the scan order is as large as about 25% to about 60%, for the data of (b-2) sorted in descending order of dose, the value of the DRF is within the range of about 40% to about 50%, with less variation and a more accurate DRF obtained.

[0019] Based on 35 scan data evaluating the performance of the collar-type shield of Experiment A, Fig. 13 shows a graph comparing the DRF obtained from the scan order (original data) and the sorted data in descending order of dose. [1] For the original data, the DRF is calculated from the measured values plotted in the scan order. Even when the number of experimental trials is increased to calculate the average value, the variation (SD: standard deviation) of the DRF does not decrease. In contrast, [2] for the sorted data, the variation of the DRF is small, and by analyzing using data with an experimental number of about 10 times, a sufficiently accurate DRF can be calculated.

[0020] Based on the experimental data so far, the block diagram shown in Fig. 1 is the summary of the DRF calculation procedure. There is no particular limitation on the dosimeter used for measurement. For example, using an OSL dosimeter and an OSL reader, the dose D o,i is measured in the state without a shield, and the dose D w,i is measured in the state with a shield. In helical scanning, as shown in the polar coordinates shown in FIG. 3, the X-ray tube, which is a radiation source, rotates along a helical orbit. Therefore, for each slice, by the method shown in FIG. 4, in the CT image, the standard deviation (SD) of the variation in CT values in a plurality of regions of interest (ROIs) is measured, and the angle θ formed by the SD centroid in the polar coordinate system SD is used to analyze the X-ray incident angle. This analysis regarding the incident angle is performed for a plurality of slice images with n > 1, plotted on the polar coordinate plane, and a helical function is fitted to estimate the helical orbit. Next, as shown in FIG. 3, at the installation location of the shielding body arranged outside the imaging range, the position of the X-ray tube (X-ray incident angle) is analyzed. The X-ray shielding body assumed in this embodiment is a thyroid shielding body, but the same analysis can also be applied to performance tests of shielding bodies for gonad protection and the like. In this way, based on the data measured through multiple experiments, the dose distribution D without a shielding body o,i and the dose distribution D with a shielding body w,i are obtained together with information on the X-ray incident angle. In the chart of FIG. 1, the experiment numbers are expressed as i = 1, 2, 3, ···, n. Here, instead of directly calculating the DRF using the data in the order of the experiment numbers, in the present invention, the measured values of the doses without and with a shielding body (D o,i and D w,i ) are rearranged using the information on the X-ray incident angle so that the X-ray incident angles are approximately the same, and then the DRF is calculated. Also, in the present invention, it is clarified that as a simple method instead of rearranging by the X-ray incident angle, the DRF may be calculated based on the data without and with a shielding body rearranged in descending or ascending order of dose.

Claims

1. A method for evaluating the performance of a radiation shield used in CT (Computed Tomography) imaging, wherein the CT imaging is helical scanning in which an X-ray tube rotates along a helical orbit, and a dose distribution D with the shield when helical scanning is performed with the shield is measured ｗ，ｉ and a dose distribution D without the shield when helical scanning is performed without the shield is measured ｏ，ｉ A method for evaluating the performance of a radiation shield, characterized by calculating a dose reduction factor (DRF: Dose Reduction Factor) from these

2. The dose distribution D with the shielding body ｗ，ｉ and the dose distribution D without the shielding body ｏ，ｉ are obtained by calculating the incident angle of the radiation from a medical image and corresponding to the incident angle, and the method for evaluating the performance of the radiation shielding body according to claim 1 is characterized in that.

3. Dose distribution D with the shielding body ｗ，ｉ and dose distribution D without the shielding body ｏ，ｉ The method for evaluating the performance of a radiation shielding body according to claim 1, characterized in that they are arranged in descending or ascending order of the measured dose values and corresponding to each other.

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