Referenceless phase velocity mapping using balanced SSFP

Abstract

Phase contrast MRI (PC-MRI) is an established technique for measuring blood flow velocities in vivo. Although spoiled gradient recalled echo (GRE) PC-MRI is the most widely used pulse sequence today, balanced steady state free precession (SSFP) PC-MRI has been shown to produce accurate velocity estimates with superior SNR efficiency. We propose a referenceless approach to flow imaging that exploits the intrinsic refocusing property of balanced SSFP, and achieves up to a 50% reduction in total scan time. With the echo time set to exactly one half of the sequence repetition time (TE = TR/2), we show that non-flow-related image phase tends to vary smoothly across the field-of-view, and can be estimated from static tissue regions to produce a phase reference for nearby voxels containing flowing blood. This approach produces accurate in vivo one-dimensional velocity estimates in half the scan time compared with conventional balanced SSFP phase-contrast methods. We also demonstrate the feasibility of referenceless time-resolved 3D flow imaging (called "7D" flow) in the carotid bifurcation from just three acquisitions.