Pressure gradient measurement in the coronary artery using 4D PC-MRI: towards noninvasive quantification of fractional flow reserve
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Fractional flow reserve (FFR) is an invasive procedure evaluating the functional significance of an intermediate stenosis by measurement of pressure drop across stenosis. Noninvasive pressure measurement technique i.e. Phase-contrast (PC)-MRI has been studied in the cardiac chamber, aorta, and renal arteries. The purpose of this study was to investigate the feasibility of pressure gradient quantification using 4DPC-MRI in the coronary arteries, which may allow for the derivation of FFR associated with stenosis.
A 4D PC-MRI sequence with an acquisition window at the mid-diastole and end-expiration phase using ECG-triggering and navigator-gating to minimize motion-induced errors was implemented on a 3T system (MAGNETOM Verio, Siemens). The sequence measures the 4D flow velocity field through a cross-sectional 3D acquisition, in conjunction with the Navier-Stokes equations to calculate the pressure gradient within the vessel segment of interest. A flow phantom study (gadolinium-doped water flow at a constant volume velocity of 250 mL/min in a silicone tubing of 4.8-mm ID) was first performed to determine the feasibility of the technique to detect changes in pressure difference (ΔP) at six different stenosis cases: 0, 22%, 34%, 44%, 60%, 64% with appropriate combinations of VENCs in z (45, 60, ..., 200 cm/s) and x, y (20, 30, ..., 80 cm/s) directions. The sequence was then tested in 3 healthy male volunteers using a VENC of 90z40x40y on the left main, LCX and 60z30x30y on the proximal LAD, respectively. Imaging parameters for human studies were: spatial resolution = 0.78 × 0.78 × 2.00 mm3, flip angle = 15°, cardiac phase = 2-3 (77 ms/phase) coinciding with the quiescent period, scan time = 11-18 mins.
The preliminary results have suggested that quantification of pressure gradient in the coronary artery is feasible. As expected, healthy volunteers showed a near zero pressure gradient across the coronary arteries. Animal and clinical validations on real coronary stenosis are currently underway. Further technical improvements such as temporal/spatial resolution are warranted.
NHLBI HL38698, NIBIB EB002623.
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