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ECR 2019 / C-1825
Certificate of Merit
Where do the scattering X-rays that are exposed to the eye lens come from during neonatal cardiac CT examination?
Congress: ECR 2019
Poster No.: C-1825
Type: Educational Exhibit
Keywords: Education and training, Physics, Dosimetry, CT, Radiation physics, Paediatric, Eyes
Authors: Y. Mihara1, H. Hayashi2, T. Uehara2, E. Tomita2, S. Goto2, K. Tada3, Y. Kanazawa1, T. Okazaki 4, T. Hashizume4; 1Tokushima/JP, 2Kanazawa/JP, 3Izumo/JP, 4Tsukuba/JP
DOI:10.26044/ecr2019/C-1825

Findings and procedure details

For the phantom study, we used a neonate phantom (Kyoto Kagaku Co., Ltd.) and 320-row CT equipment (Aquillion ONE, Canon Medical System Corporation, Japan).  The irradiation conditions were as follows: 80 kV, CTDI=10 mGy, and rotational speed of 0.275 s/rotation.  The scanning range was determined so as to the upper range limit is the submandibular, and the scan length is 16 cm, from where heart is set at almost center.  The lower range is the upper edge of the iliac crest.

 

First, in order to evaluate dose contributions of each angle, a square type shielding box was made from lead (thickness of 2 mm).  As shown in Fig. 7 the box can be opened at each side.  This allows the angular contribution from each side to be experimentally derived.  Each surface of the shielding box was numbered as shown in the inset of Fig. 7.  The dosimeters were placed on the surface of the phantom corresponding to the position of the left eye lens; in the area of the left eye lens a normal nanoDot OSL dosimeter is placed, and for the right eye lens position the other dosimeter is placed in a shielding box as described below. 

Fig. 7: Measurements of dose contribution from each direction. As shown in the right photograph, we made lead box which can measure individual contributions. The directions are defined by the schematic drawing in the figure. The photograph shows an example for measuring direction 1.
References: Tokushima University - Tokushima/JP

 

Next, we made prototype lead shields (one is a neck brace, and the other is an eye mask) to carry out the preliminary experiment for obtaining basic data.  The X-ray shields were made of a lead plate having a thickness of 1 mm.  The outer size of the eye mask type X-ray shield is 6 cm in length and 16 cm in width.  The outer size of neck brace type is 3 cm in length and 28.5 cm in width.  These sizes were optimized to use on the neonate phantom.  Both eye lens can be covered with the eye mask, and the neck brace can surround the neck from submandibular to the apex of the lung.  In order to test the shields performance, we performed four different experimental conditions as shown in Fig. 8; in condition “i” both neck brace and eye mask were used, in condition “ii” only the neck brace was used, in condition “iii” only the eye mask was used, and in condition “iv” the experiment was performed without any X-ray shields.  As presented in the Fig. 8 photograph, nanoDot OSL dosimeters were placed at the eye lens positions.

Fig. 8: Phantom study to measure exposure dose to the eye lens. We performed four experimental settings: in the four settings i, ii, iii and iv, the phantom was scanned with and without the neck brace and eye mask.
References: Tokushima University - Tokushima/JP

 

 

Figure 9 shows the dose contribution from each angle.  The highest dose contribution was from the side where the dosimeter was in contact with the body surface (direction 6 in the figure) and it was about 40% of the total dose.  Subsequently, a large dose contribution was from the side of dosimeter facing the foot (direction 5 in the figure) which was approximately 30% of the total dose.  In contrast, the dose contribution from the open side, which is the surface of the dosimeter (direction 1 in the figure), was approximately 14%.  These results indicate that dose contributions to the eye lens are mainly caused by scattering X-rays coming from the patient body and there are small amounts of scattering X-rays from the direction of body surface.  

 

Fig. 9: Measured results for dose contribution from each direction. We found that directions 5 and 6 are efficient; the exposure dose from the direction of 5 was caused by scattering radiation from the scanning area, and that of 6 was caused by scattering X-rays from the patient’s body.
References: Tokushima University - Tokushima/JP

 

Figure 10 shows the measurement results when we used the X-ray shielding items.  The table on the upper left of Fig. 10 summarizes the experimental results for each condition.  In the upper right graph, the horizontal axis represents each condition and the vertical axis plots the relative dose reduction ratio.  The value obtained by dividing the dose under the condition using the shield (condition “i”, “ii”, or “iii”) by the dose under the condition without using the shield (condition “iv”) was defined as the relative dose reduction ratio.  When the relative dose reduction ratio in the case of not using a lead shield was defined as 100%, the relative dose reduction ratio can be reduced to 23% when only the neck brace type shield is worn and by about 43% when only the eye mask type.  It was found that 70% of the relative dose reduction ratio can be reduced by using both eye mask type and neck brace type.  From this fact, we assumed the passing of the scattering X-rays is depicted in the illustration, as presented in lower right area of Fig. 10.  X-rays are irradiated directly to the patient, and scattered X-rays are generated from inside the patient; because current CT equipment has good collimation technique, therefore there is no significant leakage radiation.  In some cases, scattered X-rays may reach the eye lens as represented by the blue line and have an influence, and in another case, scattered X-rays may not reach the eye lens as shown by the red line, the neck brace.  From these data, we expect that the radiation dose can be reduced. 

Fig. 10: Estimation of reduction of exposure doses using shielding items. Using the both neck brace and eye mask, exposure doses can be reduced to below 30%. It means reduction rate of 70% compared without any shielding items.
References: Tokushima University - Tokushima/JP
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