56. Dose Evaluation and Effective Dose Estimation for Multi-Detector CT

Kanae Nishizawa, Masaki Matsumoto, Kazuo Iwai¹, Ayako Tonari ², Takashi Yoshida², Makoto Takayama² (¹ Nihon Univ., ² Kyorin Univ.)

Keywords: Patient dose, multi-detector CT, effective dose, medical exposure

Computed tomography (CT) has devolved remarkably though device improvement and advancement of peripherals, including the computer. In 1999, multi detector-row CT (MDCT) appeared and made high-speed scanning possible. However, utility of clinical MDCT applications has not been gauged. Since CT examinations need a comparatively high dose, it is necessary to evaluate patient exposure prior to the introduction of the MDCT.

The CT scanners used for the measurements were Somatom Plus 4 (Siemens Medical System, Germany), Aquilion/M (Toshiba Medical, Japan) and QX/i (GE, USA). The dose measurements were carried out under routine operating conditions. An anthropomorphic phantom (Rando; Alderson Research Lab. USA, 163cm stature and 53kg weight) was used as the model for a Japanese adult. Two types of glass encapsulated thermo luminescence radiation dosimeters (TLDs) were used for the measurements of the organ or tissue doses in/on the phantom. They were UD-170A (BeO) and UD-110S (CaSO₄: Tm) (Panasonic, Japan). The UD-170A type TLDs were calibrated within the direct beam field while the UD-110S tips were calibrated outside the field with the tissue equivalent phantom. The exposure at each position in the phantom where the TLD was placed was determined with an ionization chamber traceable to the Japanese National Standard of the Electrotechnical Laboratory in Tsukuba, Japan. Dose measurements were performed in the organs or tissues in a phantom to which tissue weighting factors (W_T) were assigned by the 1990 Recommendation of the International Commission on Radiological Protection for whole pulmonary (about 37cm axial scan) as the chest scan and the abdomen-pelvis scan (about 38cm axial scan) with normal conditions, as used in hospitals for routine examinations.

The effective doses were in the range of 9.4-28mSv for the chest examination and 13-34 mSv for the abdomen-pelvis (Table11). The average surface doses varied from 14 to 25mGy for the chest examination and 20-37mGy for the abdomen-pelvis. In the chest examination, the organ or tissue doses and the effective dose showed large differences depending on CT equipment. Plausible reasons were significant differences in table speed and beam width and existence of beam overlapping. In the abdomen-pelvis examination, overlapping of beams was also found in the one apparatus.

The advantages of MDCT in clinical uses are that multiple slice data can be obtained by one scanning with multiple detectors and a thin slice image can be taken rapidly during one breath stop. As for CT, it has been conventionally thought that higher radiation exposure is delivered to the patient, compared with other X-ray diagnosis techniques. Since MDCT uses multi-detector rows at the same time, it was expected that exposure dose could be smaller in contrast to the conventional system and the reconstruction of the scan could be done with a low radiation dosage with keeping the same image quality of the conventional CT. However, the exposed doses differed very much according to scanning method and technical conditions and it seems that dose reduction by MDCT has not been realized yet.

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