Radioprotection 2025, 60( 1), 37 – 43 © K. Mitsui et al., Published by EDP Sciences, 2025 https:// doi. org / 10.1051 / radiopro / 2024033 Available online at: www. radioprotection. org
ARTICLE
Relationship between radiation dose indices and patient size in adult brain computed tomography
K. Mitsui 1, Y. Inoue 1,*, H. Itoh 2, H. Hata 2, H. Miyatake 2 and Takuro Yamane 1, 3
1 Department of Diagnostic Radiology, Kitasato University School of Medicine, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa, Japan. 2 Department of Radiology, Kitasato University Hospital, 1-15-1 Kitasato, Minami-ku, Sagamihara, Kanagawa, Japan. 3 Department of Radiology, JCHO Sagamino Hospital, 1-2-30 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa, Japan.
Received: 12 May 2024 / Accepted: 5 August 2024
Abstract – The relationship between radiation dose indices and patient size was investigated in adult brain CT scans to assess the significance of considering the patient size for radiation dose management. Two hundred patients who underwent brain CT using automatic exposure control were enrolled. The radiation dose indices( volume CT dose index, CTDIvol; size-specific dose estimate, SSDE; dose length product, DLP) were compared with the head size indices( effective diameter, ED; water-equivalent diameter, WED) and body weight. The CTDIvol and SSDE correlated positively with the ED and WED. The correlations were higher for the WED than the ED and for the CTDIvol than the SSDE. Although the DLP also correlated positively with the ED and the WED, the correlations were lower than those for the CTDIvol and the SSDE. The CTDIvol and SSDE showed significant positive correlations with body weight, but the correlations were lower than those for the ED. In conclusion, the radiation dose indices in adult brain CT correlate with the head size indices. Consideration of the head size indices is recommended for radiation dose management in brain CT. The relationship between the radiation dose indices and body weight may be of some significance when the head size indices are not readily available.
Keywords: Radiation dose / computed tomography / brain / automatic exposure control
1 Introduction
Epidemiologic studies have demonstrated an increased incidence of brain tumours in children who underwent computed tomography( CT) of the brain( Pearce et al., 2012; Mathews et al., 2013; Meulepas et al., 2019). When using CT, the radiation dose to the patients should be reduced while maintaining clinical benefits for the patients. The volume CT dose index( CTDIvol) and dose length product( DLP) commonly serve as indices of CT radiation dose( ICRP, 2007, 2017). For brain CT, the CTDIvol is calculated from the imaging parameters and the absorbed dose measured using a 16-cm dosimetry phantom. The DLP is an integral of the CTDIvol over the longitudinal scan range and reflects total radiation exposure in an imaging series.
Even when CTDIvol values are identical in brain CT, the actual absorbed dose to the patient varies depending on the size of the patient’ s head. As the head size increases, X-rays become more severely attenuated in sections that are imaged and the proportion of X-rays that reach tissues distant from the incident
* Corresponding author: inoueys34 @ gmail. com side decreases, resulting in a reduced absorbed dose. The sizespecific dose estimate( SSDE) is a dose index that considers differences in attenuation between the patient and the dosimetry phantom( AAPM, 2011, 2019). The effective diameter( ED) and water equivalent diameter( WED) are often used as indices of the cross-sectional size. The ED is defined as the geometric mean of the anteroposterior and lateral diameters. Even at an identical ED, attenuation is stronger in sections containing more bony tissues. The WED is calculated considering differences in attenuation strengths among tissues. The SSDE for brain CT can be calculated from the CTDIvol and WED( AAPM, 2019).
In a larger patient, X-ray attenuation in the imaging section is stronger; therefore, more radiation exposure is required to preserve image quality. Automatic exposure control( AEC) evaluates the strength of attenuation of the imaging subject mainly based on the scout image and automatically modulates the tube current and thus the radiation dose( Kalra et al., 2004; Lee et al., 2008; Inoue, 2022a). AEC is an essential tool to optimize the radiation dose in CT.
In brain CT, it is appropriate that the dose index increases with increasing head size, from the aspect of optimization. In children, the head size shows a rapid initial increase during the early stage of birth, followed by a slow increase( Kleinman
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