Impact of image noise levels, scout scan dose and lens shield on image quality and radiation exposure in z-axis dose-modulated neck MSCT on 16- and 64-slice Toshiba Aquilion scanners

Abstract

Objective Assessing the impact of image noise (IN) levels, scout scan dose and lens shield use on image quality and radiation exposure in neck multislice CT (MSCT) when using z-axis dose modulation (DM). Methods Neck MSCT phantom studies with/without z-axis DM were performed by using different IN levels (S.D. 7.5–30 HU) and scout scan tube currents (7.5–50 mA) on Toshiba Aquilion scanners (16-/64-slice). Image quality indices were evaluated by two radiologists and radiation exposure parameters calculated. Cadaveric phantom measurements elucidated lens shield interactions with DM efficacy. The lowest dose scan protocol with diagnostic image quality was introduced into the clinical imaging routine and retrospectively evaluated in 20 age-matched patients undergoing neck MSCT with/without DM. Results The highest image noise level in DM neck studies with comparable image quality to standard neck CT amounted to 20 HU, resulting in a mean tube current of 50 mAs (CTDI w 6.3 mGy). DM reduced effective dose by 35% and organ dose figures (lens, thyroid) by 33%. Scout scan dose lowering to 20 mA resulted in an effective dose (ED) decrease of 0.06 mSv (5%). Avoiding lens shield placement during scout scan effected an organ dose decrease of 20%. Overall contour sharpness and image contrast did not differ significantly (DM/without DM) whereas image noise was rated higher in DM neck CT studies ( p < 0.05). Conclusions z-Axis dose modulation, as assessed on 16- and 64-slice Toshiba Aquilion scanners, is effective and mandatory in neck MSCT. DM efficacy can be enhanced by optimising scout scan doses and lens shield use.