In Vivo T1 of Blood Measurements in Children with Sickle Cell Disease Improve Cerebral Blood Flow Quantification from Arterial Spin-Labeling MRI

Abstract

BACKGROUND AND PURPOSE:Children with sickle cell disease have low hematocrit and elevated CBF, the latter of which can be assessed with arterial spin-labeling MR imaging. Quantitative CBF values are obtained by using an estimation of the longitudinal relaxation time of blood (T1 blood ). Because T1 blood depends on hematocrit in healthy individuals, we investigated the importance of measuring T1 blood in vivo with MR imaging versus calculating it from hematocrit or assuming an adult fixed value recommended by the literature, hypothesizing that measured T1 blood would be the most suited for CBF quantification in children with sickle cell disease.

Pseudo continuous arterial spin labeling quantification in anemic subjects with hyperemic cerebral blood flow

“…By assuming equitable relaxation times, the consensus document ASL model neglects the interaction between transit time and tissue T 1 values. While this assumption is acceptable for gray matter measurements in healthy subjects (T 1b =1600 ms and T 1t =1330 ms)(34) this assumption is less appropriate for white matter ASL quantification (T 1t =850), and ASL in anemic subjects (T 1b >1800 ms)(9,11). Shortening of δ in anemic subjects(8) compounds quantitation errors that arise when identical intravascular and extravascular longitudinal relaxation is assumed.…”

“…A 3D T 1 weighted sequence (1mm 3 isotropic resolution) was collected for calculation of total brain volume (Brainsuite.org, v.15a)(26). A brain density of 1.05g/ml was assumed for conversion of brain volume to brain mass(11,27). For calculation of CBF from phase contrast, total flow through the carotid and vertebral arteries was normalized to the combined mass of the cerebrum, cerebellum, midbrain, pons and brain stem, excluding cerebrospinal fluid.…”

“…When this is unavailable, a population specific T 1b of 1818ms should be used instead. For this reason, we used a T 1b of 1818ms in all subjects with SCD(11). …”

“…Anemic subjects have elevated CBF(6), high blood velocities(7), shorter vascular transit times(8) and longer blood relaxation times(9,10). In sickle cell disease (SCD), quantification is complicated further by abnormal blood properties (shorter T1 for a given hematocrit(11)) and cerebrovascular disease (stroke(12,13), vessel tortuosity(14)). As a result, historically there are widely variable reports of ASL CBF values in SCD(8,15–20).…”

Assessment of cerebral blood flow with magnetic resonance imaging in children with sickle cell disease: A quantitative comparison with transcranial Doppler ultrasonography

“…A CBF-weighted map was calculated by subtraction of tag and control images (ΔM), and converted into absolute CBF by fitting to a single-compartment kinetic model on a voxel-wise basis (Wong, Buxton, & Frank, 1998a): where λ is the blood/tissue water partition coefficient (assumed 0.98 ml/g) and α is the inversion efficiency (assumed 95%). Assumed values of T 1a were taken as 1818 ms for SCD patients (Václavů et al, 2016) and 1650 ms for healthy controls (Alsop et al, 2015).…”