Dose Distribution in Pediatric CT Head Examination: Phantom Study R. Gotanda1, 2, T. Katsuda3, T. Gotanda1, 3, A. Tabuchi4, T. Kuwano2, H. Yatake5,

K. Yabunaka6, T. Akagawa7, H. Sato1 and Y. Takeda2

1 Department of Radiological Sciences, Ibaraki Prefectural University of health Sciences, Ibaraki, Japan 2 Graduate School of Health Sciences, Okayama University, Okayama, Japan

3 School of Medical Engineering, Faculty of Health Care Sciences, Himeji Dokkyo University, Hyogo, Japan 4 Department of Radiology, Kawasaki Medical School Kawasaki Hospital, Okayama, Japan

5 Department of Radiology, Kaizuka City Hospital, Osaka, Japan 6 Department of Radiology, Katsuragi Hospital, Osaka, Japan

7 Department of Radiology, Tokushima Red Cross Hospital, Tokushima, Japan

Abstract— To keep radiation doses during computed to-mography (CT) examinations as low as reasonably achievable, performing a detailed dose measurement is important. A flexi-ble acrylic sheet roll CT dosimetry phantom (SRCT-P) with radiochromic film (RF) was developed to estimate in detail the dose distribution during pediatric CT examination. The SRCT-Ps were cylindrically-shaped by rolling up flexible acrylic sheets (1.1 g/cm3). The dose distributions in the SRCT-P (diameters of 6 cm [premature baby], 10 cm [neonates], and 14 cm [infants]) were evaluated. RFs were positioned every 5 mm along the radius at each SRCT-P, starting at center and ending on the surface. In this study, the absorbed dose and the dose distribution in the phantom were measured at 100 or 120 kV of the x-ray tube voltage. The other scanning parameters of the CT were 250 mA, 1.0 sec/rot, mm slice thickness, and a 1.0 beam pitch. The mean center doses at 100 or 120 kV in 6, 10 and 14 cm of the SRCT-P were 37.0 or 64.2 mGy; 36.5 or 52.2 mGy; and 19.5 or 40.2 mGy, respectively. The mean surface doses in 100 or 120 kV at 6, 10 and 14 cm of the SRCT-P were 33.9 or 57.9 mGy; 34.0 or 46.8 mGy; and 20.0 or 47.3 mGy, respectively. The absorbed dose was increased with a decrease in the phantom size. However, at 100 kV, the mean depth dose distributions at 6 or 10 cm of the SRCT-P were shown about the same value. The results indicated that the dose distribution in pediatric head CT becomes complex with patient head size and the tube voltage.

Keywords— radiochromic film, computed tomography, phan-tom, radiation dosimetry, pediatric

I. INTRODUCTION

The radiation risks from computed tomography (CT) have been recently reported in many studies [1]. Caution should be exercised in the pediatric setting because children have more rapidly dividing cells than adults and a longer life expectancy [1]. To keep radiation doses during CT examination as low as reasonably achievable [1], it is im-portant that the CT scanning parameters are adjusted appro-priately for each individual’s weight and size, and thus the

accurate and high-resolution dose measurement and identi-fication of the maximum dose position are required.

Radiochromic films (RFs) have been developed for measurement of the absorbed dose of low energy photons [2] and are used to measure CT dose profiles [2, 3].

RFs are self-developing and radiation-sensitive. The amount of darkening is proportional to the absorbed dose. The RFs are very easy to handle due to insensitivity to inte-rior room light and resistance to water. To measure the CT dose profiles, the RFs are placed on an empty paper box or placed in the prove holes in the CT dose phantom [2, 3].

The CT dose phantom has been designed to measure the CT dose index (CTDI) [4, 5], the CTDI variations (CTDIw and CTDIvol), and the dose-length product (DLP) [5] using a pencil-type ionization chamber. There are two types of CT dose phantoms (a body phantom [32 cm] and a head phan-tom [16 cm]) [4]. The phantom shapes are circular cylinders of polymethyl-methacrylate (1.19 ± 0.01 g/cm3) [4]. Each phantom has 5 probe holes (the inside diameter is 1.31 cm) to insert a pencil-type ionization chamber. The phantom diameter, shape, and dose measurement position in the phantom are limited. In addition, the evaluation of the ab-sorbed dose in a local area and the detailed dose distribution in a phantom are difficult.

To solve these problems, the sheet roll CT dosimetry phantom (SRCT-P) with RF was developed [6]. The SRCT-P was made by rolling up the sheet. The SRCT-P shape can assume any form because the SRCT-P shape is determined by the central core. Also, the diameter of the SRCT-P is variable due to control of the sheet length. The length of the SRCT-P is arbitrarily decided by the width of the sheet. Therefore, the SRCT-P can reproduce a wide range of body sections. Furthermore, the dose measurement positions were determined at any place in the SRCT-P with 360o and at the least 0.5 mm depth intervals by inserting the RF between the sheets. Thus, SRCT-P is flexible in terms of the deter-mination of the phantom size and shape, and the depth of the measurement position in the phantom is arbitrarily de-

Á. Jobbágy (Ed.): 5th European IFMBE Conference, IFMBE Proceedings 37, pp. 547–550, 2011. www.springerlink.com

cided by inserting RFs between the sheets. Additionally, the identification of the maximum dose position in the SRCT-P, evaluations of the absorbed dose in a local area, and the detailed dose distribution in the phantom are facilitated [6, 7]. However, the RFs used in these studies were designed for the radiotherapy, in particular intensity modulated radia-tion therapy (IMRT).

In this study, the CT dose was measured by the RF which designed for the low energy photons (20 kVp to 200 kVp). The SRCT Ps were cylindrically-shaped for accurate measurement of the adsorbed dose at 100 or 120 kV tube voltage. The SRCT-Ps were cylindrically-shaped by rolling up flexible acrylic sheets (1.1 g/cm3). The dose distributions in the SRCT-P (diameters of 6 cm [premature baby], 10 cm [neonates], and 14 cm [infants]) were evaluated.

II. MATERIALS AND METHODS

A. The flexible acrylic sheet roll CT dosimetry phantom (SRCT-P)

The CT dosimetry phantom was cylindrically-shaped by rolling up the flexible acrylic sheet (specific gravity, ap-proximately 1.1 g/cm3; SANVIC, Inc., Tokyo, Japan). The thicknesses of the sheets were 0.5 mm, and the width of the sheets was 17 cm each. The phantom center core is shown in Fig. 1.

The phantom sizes were arbitrarily set by controlling the length of the sheet. The measurement position of the ab-sorbed dose was also decided arbitrarily. The depth of the measurement position was made at 0.5 mm intervals by inserting RFs between the sheets.

Fig. 1. The central core of the SRCT-P.

GAFCHROMIC XR-QA2 film (XR-QA2 film, Lot No.

A12090904A; International Specialty Products [ISP], Wayne, NJ, USA) was used as a dosimeter. The measuring

dose range of the XR-QA2 film was designed to be 0.1-20 cGy. XR-QA2 film is a reflective-type film, and an eco-nomical flatbed scanner can be used for the measurement of film density [4].

B. CT dose measurement

A schematic arrangement of the exposure method for CT dose dosimetry is shown in Fig. 2. The SRCT-P was held on the CT bed for measurement of the CT dose. The center of each SRCT-P was always located at the isocenter of the gantry. The SRCT-P sizes were set at 6 [premature baby], 10 cm [neonates], and 14 cm [infants] to measure the rela-tionships among the tube voltage, the phantom sizes, the absorbed doses, and the depth of dose distributions. The XR-QA2 films were cut to a width of 20 mm and a length of 170 mm. The XR-QA2 films were positioned every 5 mm along the radius from center-to-surface at each SRCT-P. The arrangement of films is shown in Fig. 2. The scanning parameters of the CT (ECLOS; Hitachi Medical Corpora-tion, Tokyo, Japan) were 100 or 120 kV, 250 mA, 1.0 sec/rot, 1.25 x 8 mm slice, a beam pitch of 1.0, and a 95 mm exposure range.

Fig. 2. Schematic arrangement of the CT dose dosimetry.

C. Analysis of the EBT-film

For image data acquisition, XR-QA2 films were scanned before and after exposure using a flat bed scanner (Epson ES-2200; Seiko Epson Co., Nagano, Japan) in RGB (48 bit) mode, 150 dpi, with the protection film of a liquid crystal (LCD-150; Sanwa Supply Inc., Okayama, Japan). The XR-QA2 films were placed on the center of scanner bed. To remove any increase in density error due to time differences, films were scanned at a constant time (12 hours) from post-exposure [4].

Image data of the XR-QA2 films were changed into the red mode and 16-bit gray scale with Adobe Photoshop 6.0 (Adobe Systems Incorporated, San Jose, CA, USA), and

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were analyzed with Image J 1.36 (National Institutes of Health [NIH], Bethesda, MD, USA).

In the image date before and after exposure, a region of interest (ROI) for analysis was determined at the same posi-tion on each film, and the pixel values of the ROI were measured.

To adjust the color shading in the clear polyester layer of the XR-QA2 film and to evaluate the practical density in-crease of each film, the pixel values of the ROIs on the pre-exposure films were subtracted from the pixel values of the ROIs on the 12 hour post-exposure films. The pixel values after this subtraction were defined as the net pixel value (NPV).

D. CT dose evaluation

For evaluation of the dose distribution along the z-axis direction (axis of the body), the size of the ROI (50 pixels x 1000 pixels) was set at the center of each film. The NPV of each film was analyzed and the dose distributions at each phantom size were calculated using the calibration curve(Fig. 3). The absorbed dose and the mean depth dose distri-bution among different phantom sizes and tube voltages were analyzed using spreadsheet software (Excel 2010; Microsoft Co., Redmond, WA, USA).

Fig. 3. Calibration curve for EBT-film.

III. RESULTS

The dose distributions along the z-axis direction at 6 cm SRCT-P are shown in Figs. 4 or 5, respectively. The maxi-mum dose area was the surface at each SRCT-P, except for the 10 cm SRCT-P in 120 kV. At 10 cm of the SRCT-P at 120kv, the maximum dose area was 6 cm depth. The maxi-mum dose value at 6, 10, and 14 cmof the SRCT-P at 100 or 120 kV were 44mGy, 44 mGy, and 28 mGy; or 78 mGy, 61 mGy, and 60 mGy, respectively. The minimum dose area

was the surface at each SRCT-P, except for the 10 cm SRCT-P at 120kV. At 10 cm of the SRCT-P at120 kV, the minimum dose area was 5 cm depth.

Fig. 4. The dose distribution along the z-axis direction in 120kV.

Fig. 5. The dose distribution along the z-axis direction in 100 kV.

Fig. 6. The mean depth dose distribution.

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The minimum dose value at 6, 10, and 14 cmof the SRCT-P at 100 or 120 kV were 42mGy, 38 mGy, and 34 mGy; or 27 mGy, 30 mGy, and 16 mGy, respectively.

The mean depth dose distribution at each SRCT-P is shown in Figs. 6. At 120 kV, the absorbed dose was in-creased with a decrease in the phantom size. However, at 100 kV, the mean absorbed doses at 6 and 10 cm of the SRCT-P were shown about the same value. At each x-ray tube voltage, the mean depth dose distributions at same phantom size were shown the same tendency. The maxi-mum mean dose area at 6or 14 cm of the SRCT-P were center or surface, respectively. At 10 cm of the SRCT-P, the mean depth dose distribution was almost homogeneous.

IV. DISCUSSION

In this study, the SRCT-P sizes were set at 6 cm for premature baby, 10 cm for neonates and 14 cm for infants. In premature baby, the maximum or minimum dose areas at each x-ray tube voltage were at the surface; however, the mean center dose was higher than the mean surface dose. In neonates, the maximum dose areas at 100 kV or 120 kV were at the surface or 6 cm in depth. The minimum dose area at each x-ray tube voltage was at the surface. However, the mean depth dose distribution was almost homogeneous at each x-ray tube voltage. In infants, the maximum dose area at each x-ray tube voltage was at the surface. The min-imum dose areas at 100 kV or 120 kV were at the surface or 5 cm in depth. However, the mean surface dose was higher than the mean center dose at each x-ray tube voltage.

At 100 kV, the mean depth dose distributions at 6 or 10 cm of the SRCT-P were shown about the same value. This result indicated that the absorbed dose was constant with phantom size below 10 cm of diameter. It considered that the decreased incident x-ray was substituted with the in-creased transmission x-ray.

This study indicated that the dose distribution became complex with patient size and x-ray tube voltage. It is im-portant that the CT scanning parameters are adjusted appro-priately for each individual’s weight and size and for the region being scanned [1]. Therefore, CT dose measurement with SRCT-P was important to obtain an accurate dose evaluation in the pediatric head CT. On any parameter, the accurate and high-resolution dose measurement and identi-fication of the high dose areas are possible using SRCT-Ps with RFs. In particular, the SRCT-P is flexible in terms of the phantom shape, size, and dose measurement position. Therefore, the SRCT-P combined with RF dosimetry is suitable for the evaluation of the absorbed dose from CT examination and the optimization of the CT parameters.

V. CONCLUSIONS

In the current study, the absorbed doses in premature ba-by and neonates were shown about same value at 100 kV. The results indicated that the dose distribution in pediatric head CT becomes complex with patient size and x-ray tube voltage. Accurate dose measurement and maximum dose areas were identified using SRCT-P with RF dosimetry. To keep radiation doses during CT examination as low as rea-sonably achievable, it is important that the CT scanning parameters be adjusted appropriately for each individual’s size and weight under an identification of the high dose area. The dose measurement by SRCT-P with RF will become one of the evaluation methods for adjusting parameters.

ACKNOWLEDGMENT

We would like to sincerely thank Kawagoe Tohru of INABATA & CO., LTD. (Tokyo, Japan) and Motohisa Masuda of SANVIC, Inc. (Tokyo, Japan) for providing flexible acrylic sheets.

REFERENCES

1. FDA Public Health Notification. Reducing Radiation Risk from Computed Tomography for Pediatric and Small Adult Patients. Available at: http://www.fda.gov/cdrh/safety/110201-ct.html.

2. International Specialty Products web site. Available at: http://www.ispcorp.com/products/dosimetry/content/gafchromic/index.html.

3. International Specialty Products web site. GAFCHROMIC_XR-CT dosimetry film User protocol: Rev 0.2. Available at:http://www.ispcorp.com/products/dosimetry/content/gafchromic/content/products/xrct/pdf/XRCTProtocol.pdf.

4. Title 21. Food and Drugs Subchapter J. Radiological Health Part 1020; Performance Standards for Ionizing Radiation Emitting Prod-ucts, Sec. 1020.33. Computed tomography [CT] equipment. Available at: http://www.accessdata.fda.gov/scripts/cdrh/cfdocs/search/search.cfm?db=CFR&ID=1020.33

5. McNitt-Gray M.F(2002) AAPM/RSNA Physics tutorial for residents: Topics in CT. Radio.Graphics 22:1541-1553

6. Gotanda R, Katsuda T, Gotanda T et al. (2007) Computed tomogra-phy phantom for radiochromic film dosimetry. Australas Phys Eng Sci Med 30:194-199

7. Gotanda R, Katsuda T, Gotanda T et al. (2008) Dose distribution in pediatric CT head examination using a new phantom with radio-chromic film. Australas Phys Eng Sci Med 31:339-344 Author: Rumi Gotanda Institute: Ibaraki Prefectural University Street: 4669-2 Ami, Ami-machi City: Inashiki-gun Country: Japan Email: gotanda@ipu.ac.jp

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