頭部CT撮影における頭部固定具が画質と線量に及ぼす影響

摘要

Purpose: For emergency or pediatric head CT scans, a simplified pillow made of hard sponge instead of a dedicated head holder may be used if it is difficult to immobilize the head. However, the radiation dose when using a simplified head holder may be increased due to radiation absorption by the patient couch if the automatic exposure control (AEC) system is used. In this phantom study, we compared the radiation dose delivered when using a dedicated and a simplified head holder. Materials and Methods: We used a dedicated-type and a pillow-type head holder made of hard sponge (simplified head holder). We placed a 20 cm-diameter cylindrical phantom made of water-equivalent material and an anthropomorphic head phantom in the head holders and then scanned them five times with a 64-detector CT scanner (VCT, GE Healthcare). We performed step-and-shoot and helical scanning with AEC; the noise index was set to 2.8. We measured the radiation dose using fluorescent glass dosimeters in the head phantom and the image noise at five sites in the cylindrical phantom. All values were averaged. Results: With step-and-shoot scans, the mean image noise with the dedicated and the simplified head holder was 3.30 ± 0.05 [SD] and 3.20 ± 0.05, respectively. With helical scans they were 3.00 ± 0.09 and 2.88 ± 0.03, respectively. There was no statistically significant difference (p = 0.02 and 0.04, Student's t-test). The radiation doses with the dedicated and the simplified head holder were 58.6 and 70.4 mGy, respectively, for step-and-shoot scanning and 41.8 and 49.0 mGy, respectively, for helical scanning. The doses were thus significantly higher with the simplified head holder for both step-and-shoot and helical scanning (p ＜ 0.01 and ＜ 0.01). Conclusion: We recommend the use of a dedicated head holder for head scanning with AEC since the radiation dose was lower than with the simplified head holder.%頭部computed tomography（CT）は，頭部の骨に囲まれている解剖学的な構造に加えて，発症から時間が経過したくも膜下出血や急性期の脳梗塞などコントラストが低い病変も対象となるため，画質を維持するためには体幹部と比較して線量を増加させる必要がある.一般的に，頭部CTにおいて線量を増加するためには，1回転あたりの時間（ローテーションタイム）を長く設定することや，ヘリカルスキャンにおけるピッチファクタを低く設定するなどの手法が用いられている.ただし，これらの変更はトータルの撮影時間の延長へつながり，結果的に患者による動きの影響を受けやすく画質の劣化が懸念される.そこで，頭部CTでは固定具などを使用して，撮影時に頭部の動きを抑制し撮影することが重要である.