博士学位論文

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博士学位論文

タングステン機能紙および臓器別管電流変調法を用いた頭部 CT 検査での水晶体被ばく低減

近畿大学大学院医学研究科医学系専攻小坂浩之

タングステン機能紙および臓器別管電流変調法を用いた頭部ＣＴ検査での水晶体被ばく低減

小坂浩之

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2 Doctoral Dissertation

Radiation dose reduction to the eye lens in head CT using tungsten functional paper and organ-based tube current modulation

October 2020

Department of Medical Physics,Major in Medical Sciences Kindai University Graduate School of Medical Sciences

Hiroyuki Kosaka

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Contents lists available atScienceDirect

European Journal of Radiology

journal homepage:www.elsevier.com/locate/ejrad

Radiation dose reduction to the eye lens in head CT using tungsten functional paper and organ-based tube current modulation

Hiroyuki Kosaka

^a

, Hajime Monzen

^a,

*, Morikazu Amano

^b

, Mikoto Tamura

^a

, Shota Hattori

^a

, Yuki Kono

^c

, Yasumasa Nishimura

^d

aDepartment of Medical Physics, Graduate School of Medical Sciences, Kindai University, 377-2 Ohno-higashi, Osaka-Sayama, Osaka, 589-8511, Japan

bDepartment of Radiology, Fujieda Municipal General Hospital, 4-1-11 Surugadai, Fujieda, Shizuoka, 426-8677, Japan

cDepartment of Central Radiology, Kindai University Hospital, 377-2 Ohno-higashi, Osaka-Sayama, Osaka, 589-8511, Japan

dFaculty of Medicine, Department of Radiation Oncology, Kindai University, 377-2, Ohno-Higashi, Osaka-Sayama, Osaka, 589-8511, Japan

A R T I C L E I N F O Keywords:

Tungsten functional paper Dose reduction

Organ-based tube current modulation Computed tomography

Eye lens

A B S T R A C T

Purpose:We investigated whether a tungsten functional paper (TFP) shield and/or organ-based tube current modulation (TCM) can reduce the dose to the eye lens.

Materials and methods:All scans were performed using our routine head examination protocol (spiral acquisition, 120 kVp, noise Index 3.5) with an anthropomorphic head phantom. The dose reduction rate was measured by the following methods with a scintillation fiber optic dosimeter: (a) without any dose reduction techniques (Original scan), (b) TFP shield, (c) TCM, and (d) TFP shield plus TCM. Image noise and CT number were obtained and compared between the three groups. In addition, image noise in method (d) was measured with varying distances between the TFP shield and eye lens.

Results:The reduction rates using TFP shield, TCM, and TFP shield plus TCM compared with those for the Original scan were 17.8 %, 13.6 %, and 27.7 %, respectively. Image noise (mean ± standard deviation) in the anterior region for the Original scan, TFP shield, TCM, and TFP shield plus TCM were 4.1 ± 0.2, 4.6 ± 0.2, 4.4

± 0.3, and 5.0 ± 0.2, while the CT numbers were 19.3 ± 0.8, 23.8 ± 0.8, 19.6 ± 0.8, and 24.1 ± 0.8, respectively. Increasing the distance between the TFP and the eye significantly decreased the CT number when using TFP shield plus TCM (p< .05).

Conclusion:TFP shield plus TCM reduced the dose to the eye lens in head CT while maintaining image quality with an air gap between the TFP and skin surface.

1. Introduction

Japan currently faces the problem of high medical radiation exposure, as limitations of tolerable dosages have been decreased, while the number of procedures resulting in medical exposure of patients has increased [1]. González et al. [1] reported that Japan had the highest annual frequency of diagnostic X-rays among the 15 countries studied and also had the highest attributable risks, with 3.2 % of the cumulative risk of cancer attributable to diagnostic X-rays. The Japan Network for Research and Information on Medical Exposures (J-RIME) summarized diagnostic reference levels (DRLs) in 2015 to optimize CT exposure doses according to volume CT dose index (CTDIvol) and dose length product (DLP) [2]. The International Commission on Radiological Protection (ICRP) [3] has classified radiation cataract as a tissue

reaction for which a threshold dose exists and recommended an equivalent dose limit of 20 mSv/year to the lens of the eye to prevent vision-impairing cataracts.

Lead is widely used in radiation protection devices in CT because of its excellent shielding ability against X-rays. However, the dis- advantages of protection devices made of lead include inflexibility and toxicity to the human body [4,5]. To overcome these problems, some researchers have explored ways to provide effective X-ray protection using alternative materials [6–9]. Bismuth shields have been used in CT to protect superficial organs such as the eye [10,11]. Wang J et al. [11]

reported that the dose to the eye lens was reduced by 26.4 % with a 1 mm thick bismuth shield in a phantom study.

The tungsten belongs to the group of refractory metals with a mo- lecular weight of 183.85 and the highest melting point (3410℃) of all

https://doi.org/10.1016/j.ejrad.2020.108814

Received 26 September 2019; Received in revised form 18 November 2019; Accepted 28 December 2019

⁎Corresponding author.

E-mail addresses:[email protected](H. Kosaka),[email protected](H. Monzen),[email protected](M. Amano),

[email protected](M. Tamura),[email protected](S. Hattori),[email protected](Y. Kono),[email protected](Y. Nishimura).

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elements except carbon, clinical follow-up in patients after tungsten coil implantation did not demonstrate any evidence for toxicity [12,13]. In previous studies, we reported on the electron beam,γ-ray, and X-ray shielding abilities of Tungsten [14–23].

Tungsten functional paper (TFP) is a novel paper with radiation shielding ability that is flexible and easy to process by methods such as cutting, folding, and sticking onto other materials [16]. Published TFP studies have shown that it provides adequate protection and usability in various situation [13,15–18]. In addition, an organ-based tube current modulation (TCM) technique has been implemented in CT scanners to reduce the dose to radiosensitive organs such as the breast, thyroid, and eye lens [11,24,25]. Dixon MT et al. [26] reported that TCM could reduce the CTDIvolvalue by approximately 20 % in a phantom study.

The purpose of this study was to investigate exposure to the eye lens using three kinds of exposure reduction methods, TFP shield, TCM, and TFP shield plus TCM, and to clarify the effectiveness of these methods by comparing their respective exposure and image noise levels in head CT examination.

2. Materials and methods 2.1. Characteristics of the TFP

The TFP sheet (Toppan Printing Co., Ltd, Tokyo, Japan) was 0.3 mm thick and contained tungsten powder at about 80 % of the mass. The elemental ratios of the TFP (mol %) were H: 24.2 %, C: 40.4 %, O: 20.2

%, W: 15.2 % [16].

2.2. CT scanning and exposure reduction methods

A tissue-equivalent anthropomorphic phantom was placed in a su- pine position (Fig. 1A). All scans were performed using the same CT scanner (Discovery 750HD, GE Healthcare, Little Chalfont, Buck- inghamshire, England) for investigations of dose exposure and image quality using our clinical routine head examination protocol (spiral acquisition, 120 kVp, Noise Index 3.5, rotation time 1 s, pitch 0.531, 5- mm-thick sections, 300-mm reconstruction field of view) from vertex to top of C1 lamina. To create the TFP shield, TFP (4×15 cm²) with a thickness of 0.3 mm was sandwiched between two eye masks. The lo- calizer was scanned, and then TFP was placed on both eyes of the phantom (Fig. 1C). The TCM technique employed a 90° arc, as per the head protocol, to modulate the tube current and reduce the radiation dose to radiosensitive organs such as the eye lens [26]. We applied the TCM protocol that was supplied by the manufacturer. In our study, the anterior tube current was reduced by 31.2 % compared with the posterior tube current.

2.3. CT dosimetry

CT dosimetry was performed to determine the eye surface dose of the tissue-equivalent anthropomorphic phantom using a scintillation fiber optic dosimeter (MIDSOF, Acrobio Co., Ltd, Tokyo, Japan) [27]

(Fig. 1B). The dosimeter was calibrated with an ionization chamber (T34069-154, PTW, Freiburg, Germany) according to Al filter thickness of the RQR5 (70 kV # 10 mA) radiation quality [28] on April 17, 2019.

The measurements for each scan were performed five times with both eyes individually. The dose to the exposed eye surface was measured by the following four methods: (a) without any shielding (Original scan), (b) TFP shield, (c) TCM, and (d) TFP shield plus TCM, and the dose reduction rate for each dose reduction technique was obtained using the following equation:

Dose reduction rate (%)=((D0- D) / D0)×100,

whereD0is the average absorbed dose of the Original scan andDis the average absorbed dose for the dose reduction techniques.

2.4. Image noise and CT number accuracy

Image noise levels were evaluated using the standard deviations (SDs) of CT numbers in regions of interest (ROIs) of approximately 100 mm²in the intracranial region. CT numbers within each ROI and their SDs were averaged over five adjacent images along the z axis. For methods involving a TFP shield, the shield was present on all five images included in the analysis. A total of nine ROIs were separated into three groups on the basis of their distance from the anterior mid- line (Fig. 2), as follows: (a) anterior area, three ROIs 9 cm below the surface; (b) central area, three ROIs 13 cm below the surface; and (c) posterior area, three ROIs 17 cm below the surface [11]. The average noise level within each group was compared among the four scanning techniques. The average CT numbers for each ROI were averaged within each group and compared among the different scanning techniques using ImageJ software (National Institutes of Health, Bethesda, Maryland, USA). Statistical analysis was performed with a paired two- tailed t-test (Excel; Microsoft, Redmond, Wash) by comparing the means and SDs of CT numbers from the same ROI group between the Original scan and dose reduction scanning techniques.P< .05 was considered to indicate a statistically significant difference.

2.5. Image noise and CT numbers with changed distance between TFP and skin surface

In the TFP shield plus TCM method, the distance between the TFP and the skin surface was categorized as (a) no gap, (b) 2 cm, (c) 3 cm, and (d) 4 cm gap and the ROI of the same position of a tissue-equivalent anthropomorphic phantom (Fig. 2). We inserted a 1 cm thick poly- styrene foam between the eyes and mask to make an air gap. The average image noise and CT number were measured and compared among these four distance levels [11].

3. Results 3.1. CT dosimetry

Table 1summarizes the dose and reduction rate to the eye (mean ± SD) in the Original scan, TFP shield, TCM, and TFP shield plus TCM conditions. The reduction rates for TFP shield, TCM, and TFP shield plus TCM were 17.8 %, 13.6 %, and 27.7 %, respectively. The dose reduction to the eye achieved by combining TFP shield and TCM was greater (approximately 10–14 %) than the other exposure reduction methods.

3.2. Image noise and CT number accuracy

Streak artifacts were observed in the orbits with TFP shield and TFP shield plus TCM (Fig. 3B, D) but were not presented with TCM (Fig. 3C).

However, no scanning technique produced noticeable artifacts in the brain region.

Table 2 shows the image noise and CT numbers in the anterior, central, and posterior regions. Significant increases in image noise and CT number were observed with the TFP shield and TFP shield plus TCM techniques (P< .05) compared with the Original scan, but there was no significant difference in the TCM only condition.

3.3. Image noise and CT number with changed distance between TFP and skin surface

Fig. 4shows the dose to the eye lens, image noise, and CT number of different distances between the TFP and skin surface in the TFP shield plus TCM method. The doses of the eye with distances between the TFP and skin surface of no gap, 2, 3, and 4 cm were 34.52 ± 0.14, 37.39 ± 0.37, 39.81 ± 0.29, and 42.80 ± 0.76 mGy, respectively.

The image noise levels of the anterior region according to the distance between the TFP and the skin surface of no-gap, 2, 3 and 4 cm, H. Kosaka, et al.

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were 5.0 ± 0.2, 4.7 ± 0.2, 4.9 ± 0.2 and 4.8 ± 0.4, while CT numbers were 24.1 ± 0.8, 22.7 ± 0.8, 22.1 ± 0.7, and 21.6 ± 0.7, respectively (Fig. 4). Increasing the distance between the TFP and the eye significantly decreased the CT number when using TFP shield plus TCM (p

< .05), while the image noise was not changed.

4. Discussion

In the present study, we investigated reduction of radiation exposure during head CT examination with TFP and/or TCM. Considering the gap between the TFP and skin surface, TFP had adequate shielding Fig. 1.Dose measurement geometries. (A) Photograph of the tissue-equivalent anthropomorphic phantom for CT scanning. (B) A scintillation fiber optic dosimeter.

(C) A Tungsten functional paper (TFP) eye shield was placed over the eyes of an anthropomorphic head phantom.

Fig. 2.The nine ROIs used to measure image noise and CT number. The three rows of ROIs were grouped according to distance from the anterior phantom’s surface [11].

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ability for reduction of exposure to the eye lens in head CT examination without deterioration of image quality. This study is the first report of the radioprotective properties of tungsten in CT examination.

The dose reduction to the eye achieved by combining a TFP shield with TCM was less than that achieved with other research. Wang et al.

[11] reported that the reduction rates to the eye provided by a bismuth shield and TCM were 26.5 % and 30.4 %, respectively. In contrast, our results indicated TFP shield and TCM provided smaller dose reduction

to the eye lens than indicated by their results. The reason is that they used optically stimulated luminescence (OSL) dosimeters, which can have large variation [29]. The energy dependence of OSL is almost 10

% from about 20 keV to 1 MeV. In addition, the orientation dependence of OSL is up to almost ±20 % in the range of 60° in both the horizontal and vertical directions in 83 keV X-rays [29], while that of a scintillation fiber optic dosimeter is within ±3 % in the range of 360° [27]. This 83 keV X-ray strength matches well with the 120 kVp X-rays widely Table 1

Dose and reduction rate to the eye surface with different scanning techniques.

Scanning Technique CTDIvol(mGy)^a Dose to the right eye (mGy)^b Dose to the left eye (mGy)^b Reduction rate of right eye (%) Reduction rate of left eye (%)

Original scan 52.25 47.98 ± 0.14 47.54 ± 0.15 – –

TFP shield 52.25 39.35 ± 0.58 39.16 ± 1.51 18.0 17.6

TCM 47.01 41.98 ± 0.04 40.54 ± 0.08 12.5 14.7

TFP shield plus TCM 47.01 35.88 ± 0.03 33.15 ± 0.03 25.2 30.3

aCTDIvol= volume CT dose index.

b Data are averages ± standard deviations.

Fig. 3.CT images of a tissue-equivalent anthropomorphic phantom acquired by different scanning techniques. (A) Original scan, (B) TFP shield, (C) TCM, (D) TFP shield plus TCM.

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used in CT exams. On the other hands, even though we set up the symmetry condition present in the phantom geometry, there was a difference in the dose reduction rate between left and right eyes using TCM technique. Ishihara [30] reported that TCM has a slightly different dose reduction between right side and left side for clinical protocol with clockwise rotation. The report matched our result, the dose reduction rate of left eye was higher than right eye.

Image noise and CT number in the brain parenchyma of a tissue- equivalent anthropomorphic phantom were evaluated as image quality (Table 2). The image noise level increased with TFP shield and the combination of TFP shield and TCM (p< .05) because of a reduction in the number of photons available to generate the CT image. Geleijns et al. [31] reported a modest increase in image noise of about 1–2 HU with the use of an eye shield. These findings align with our results using TFP (Table 2), which indicates that the image noise produced by the TFP is acceptable for clinical use. The CT number was increased with TFP shield and the combination of TFP shield and TCM (p< .05) because of a beam hardening effect. In addition, image noise and CT number were evaluated with different distances between the TFP and skin surface (Fig. 4). Increasing the distance between the TFP shield and the eye lens can reduce CT number and artifacts near the orbit and nasal cavity. This CT number increase was seen as acceptable for head examination. The image noise level was also acceptable for head CT

examination because the variation was within 2 HU, similar to results with bismuth described by Wang et al. [11]. Therefore, considering the gap between the TFP and skin surface, the combination of TFP shield and TCM is useful because it has the largest shielding ability and acceptable image quality for routine head CT examination, although the TCM technique may also reduce the patient’s eye lens exposure while maintaining image quality.

TFP has potential for application as a radiation shielding material and several benefits for head CT examination. It can be used in appli- cations where skin contact with the shielding material may occur because it is lead free [16]. Additionally, TFP can be used in all patients and situations because its paper property makes it more flexible, and it is therefore easy to process and fit to the patient’s body contours compared with other shielding materials.

In conclusion, TFP shield plus TCM can reduce the adequate eye lens exposure and maintain the image quality in the brain for head CT examination by considering the distance between the TFP and skin surface.

Declaration of Competing Interest

Hajime Monzen has a consultancy agreement with, and a financial interest in, Toppan Printing Co., Ltd., Tokyo, Japan.

Table 2

Image noise and CT number in brain regions with different scanning techniques.

Anterior Region Central Region Posterior Region

Scanning Technique Noise CT Number Noise CT Number Noise CT Number

Original scan 4.1 ± 0.2 19.3 ± 0.8 4.2 ± 0.2 19.6 ± 0.5 4.2 ± 0.2 20.3 ± 0.4

TFP shield 4.6 ± 0.2* 23.8 ± 0.8* 4.4 ± 0.2* 22.5 ± 0.6* 4.4 ± 0.2* 22.1 ± 0.4*

TCM 4.4 ± 0.3 19.6 ± 0.8 4.3 ± 0.2 19.8 ± 0.5 4.3 ± 0.2 20.4 ± 0.3

TFP shield plus TCM 5.0 ± 0.2* 24.1 ± 0.8* 4.8 ± 0.2* 22.5 ± 0.5* 4.6 ± 0.2* 22.1 ± 0.4*

Note. – data are averages ± standard deviations (in Hounsfield units).pvalues were calculated to test whether there was a significant difference between the noise or CT number value with reference scanning and other scanning techniques. Expect where noted, allpvalues were less than .05.

*p< .05.

Fig. 4.Radiation dose to the eye lens with TFP shield plus TCM at different distances from the eye surface (left). Error bars represent measurement variation between two eyes. Image noise (middle) and CT number (right) in brain regions for original scan and scans with TFP shield placed at varying distances from the phantom surface. Error bars represent measurement variation in five adjacent images along the z axis.

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Acknowledgements

This work was supported partly by JSPS KAKENHI grant number 19K08211. We thank Richard Lipkin, PhD, from Edanz Group (www.

edanzediting.com/ac) for editing a draft of this manuscript.

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