Inter-scanner variability in Hounsfield unit measured by CT of the brain and effect on gray-to-white matter ratio

The density ratio of gray matter (GM) to white matter (WM) on brain computed tomography (CT) (gray-to-white matter ratio, GWR) helps predict the prognosis of comatose patients after cardiac arrest. However, Hounsfield units (HU) are not an absolute value and can change based on imaging parameters and CT scanners. We compared the density of brain GM and WM and the GWR by using images scanned with different types of CT machines.

102 patients with normal readings who were scanned using three types of CT scanners were included in the study. HU were measured at the basal ganglia level by two observers with circular regions of interest.

The difference in GM was 0.98-10.30 HU and WM was 1.05-7.55 HU. The mean value of measured HU and GWR were different for each CT group. The ANOVA test showed significant difference all variables. The post hoc test for GWR, which was used to compare the differences between each scanner, was statistically significant. Interclass correlation coefficients of measured GM and WM between the two observers were very high (Cronbach's α=0.995 and 0.990, respectively) and GWR was showed good confidence level (0.798).

In this study, the HU values of GM and WM in the normal adult brain differed up to 23% among scanners. Unfortunately, the GWR may not compensate for the HU difference between GM and WM occurring between scanners. Therefore, rather than applying consistent GWR cut-offs, the protocol or manufacturer differences between imaging scanners should be considered.