A systematic increase in computed tomography (CT) number of approximately 0.13 Hounsfield unit per scan (HU/scan) was observed when serial DeltaScan 2020 CT scans of a uniform water phantom were equally spaced at 0.5, 1.0, or 2.0 min and a shaped aluminum beam-hardening filter was employed. Much smaller drifts (less than 0.06 HU/scan) were observed with flat aluminum or shaped beryllium oxide filters. This machine drift, which was not associated with a rise in water phantom temperature and did not consistently correlate with estimated x-ray tube heat, could result in a significant overestimation of regional cerebral blood flow (rCBF) for a xenon/CT rCBF protocol involving 5-7 sequential scans obtained at 1-min interscan intervals.
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Kearfott et al. (1984) studied this question.