Using a different workstation viewer (ZioTerm 2009, Ziosoft, Minato-ku, Tokyo, Japan), a circular region of interest (ROI) with a radius of 10 mm was placed on the aortic arch by a single observer (TY).13 Image noise at each ROI was determined as the standard deviation of the CT values within the ROI. Because this analysis required measuring comparable regions on the aortic arch on different scan series, all six series with and without AIDR3D from a single patient were displayed simultaneously and set side-by-side on the screen. Thus, the observer was aware of the scanning protocol used for each scan series.