Impact of neonate haematocrit variability on the longitudinal relaxation time of blood: Implications for arterial spin labelling MRI

BACKGROUND AND PURPOSE: The longitudinal relaxation time of blood (T(1b)) is influenced by haematocrit (Hct) which is known to vary in neonates. The purpose of this study was threefold: to obtain T(1b) values in neonates, to investigate how the T(1b) influences quantitative arterial spin labelling (ASL), and to evaluate if known relationships between T(1b) and haematocrit (Hct) hold true when Hct is measured by means of a point-of-care device. MATERIALS AND METHODS: One hundred and four neonates with 120 MR scan sessions (3 T) were included. The T(1b) was obtained from a T(1) inversion recovery sequence. T(1b)-induced changes in ASL cerebral blood flow estimates were evaluated. The Hct was obtained by means of a point-of-care device. Linear regression analysis was used to investigate the relation between Hct and MRI-derived R(1) of blood (the inverse of the T(1b)). RESULTS: Mean T(1b) was 1.85 s (sd 0.2 s). The mean T(1b) in preterm neonates was 1.77 s, 1.89 s in preterm neonates scanned at term-equivalent age (TEA) and 1.81 s in diseased neonates. The T(1b) in the TEA was significantly different from the T(1b) in the preterm (p < 0.05). The change in perfusion induced by the T(1b) was −11% (sd 9.1%, p < 0.001). The relation between arterial-drawn Hct and R(1b) was R(1b) = 0.80 × Hct + 0.22, which falls within the confidence interval of the previously established relationships, whereas capillary-drawn Hct did not correlate with R(1b). CONCLUSION: We demonstrated a wide variability of the T(1b) in neonates and the implications it could have in methods relying on the actual T(1b) as for instance ASL. It was concluded that arterial-drawn Hct values obtained from a point-of-care device can be used to infer the T(1b) whereas our data did not support the use of capillary-drawn Hct for T(1b) correction.