Measuring arterial pulsatility with Dynamic Inflow MAgnitude Contrast (DIMAC)

Poor arterial health is increasingly recognised as an independent risk factor for cerebrovascular disease, however there remain relatively few reliable methods for assessing the function and health of cerebral arteries. In this study, we outline a new MRI approach to measuring pulsatile flow in cerebral arteries that is based on the inflow phenomenon associated with fast gradient-recalled-echo acquisitions. Unlike traditional phase-contrast techniques, this new method, which we dub Dynamic Inflow MAgnitude Contrast (DIMAC), does not require velocity-encoding gradients as sensitivity to flow velocity results purely from the inflow effect. We achieved this desired effect using a highly accelerated single slice EPI acquisition with very short TR (15 ms) and a 90 degree flip angle, thus maximising inflow contrast. Simulating the spoiled GRE signal in the presence of large arteries and perform a sensitivity analysis to demonstrate that in the regime of high inflow contrast it shows much greater sensitivity to flow velocity over blood volume changes. We support this theoretical prediction with in-vivo data collected in two separate experiments designed to demonstrate the utility of the DIMAC signal contrast. We perform a hypercapnia challenge experiment in order to experimentally modulate arterial tone within subjects, and thus modulate the arterial pulsatile flow waveform. We also perform a thigh-cuff release challenge, designed to induce a transient drop in blood pressure, and demonstrate that the continuous DIMAC signal captures the complex transient change in the pulsatile and non-pulsatile components of flow. In summary, this study proposes a new role for a well established source of MR image contrast, and demonstrate its potential for measuring both steady-state and dynamic changes in arterial tone.