Quantification of neurovascular compliance with retrospectively gated phase-contrast MRI

Neurovascular compliance (NVC) is the change in the brain’s arterial tree blood volume, ΔV, divided by the change in intra-vascular blood pressure, ΔP, during the cardiac cycle. The primary aim of this work was to evaluate the performance of MRI measurement of NVC obtained from time-resolved measurements of internal carotid artery (ICA) and vertebral artery (VA) flow rates. A secondary aim was to explore whether NVC could be estimated from common carotid (CCA) flow in conjunction with prior knowledge of mean ICA and VA fractional flow rates, given the small cross-section of ICA and VA in some populations, in particular small children.

ΔV was quantified from the blood flow rate measured at the ICA and VA for actual NVC derivation. It was further estimated from individually measured CCA flow rate and mean flow fractions ICA/CCA and VA/CCA (which could alternatively be obtained from literature data), to yield estimated NVC. Time-resolved blood flow rate in CCA, ICA and VA was obtained via retrospectively-gated 2D PC-MRI at 1.5 T in healthy subjects (N = 16, 8 women, mean age 36 ± 13 years). ΔP was determined via a brachial pressure measurement.

Actual and estimated mean NVC were 27 ± 15 and 38 ± 15 μL/mmHg, respectively, and the two measurements were strongly correlated (r = 0.80; p = 0.0002) with test–retest intra-class correlation coefficients of 0.964 and 0.899.

Both methods yielded excellent retest precision. In spite of a large bias, actual and estimated NVC were strongly correlated.