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Fig 2. A,B, Attenuation characteristics of standard-dose and low-dose head CT studies in a 7-year-old girl with a shunt for hydrocephalus. (A) Transverse standard-dose supraventricular CT image with sample ROIs (white circles) showed GM and WM attenuations of 31.6 HU and 25.6 HU, respectively, with noise (SD, HU within ROI) levels of 1.6 HU and 1.8 HU. Low-dose image (B) showed no significant change in attenuation (30.6 HU and 26.8 HU) despite increased noise levels (4.3 HU and 2.8 HU). CNR of standard-dose CT (1.88) was significantly higher (P < .01) than that of low-dose CT (0.47). However, both readers ranked standard-dose (grade 4) and low-dose (grade 3) studies as diagnostically acceptable.
