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 T₁ 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 R₁ 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.