Combined Study of Spin-Locked Balanced Steady-State Free-Precession and in MRI

Abstract

A spin-locked balanced steady-state free-precession (slSSFP) pulse sequence is defined as which combines a balanced gradient-echo acquisition with an off-resonance spin-lock pulse for fast MRI. The transient and steady-state magnetization trajectory was solved numerically using the Bloch equations and was shown to be similar to balanced steady-state free-precession (bSSFP) for a range of T2/T1 and flip angles, although the slSSFP steady-state could be maintained with considerably lower radio frequency (RF) power also the purpose is to develop a novel pulse sequence called spin-locked echo planar imaging (EPI), or (SLEPI), to perform rapid T1ρ-weighted MRI. In this SLEPI images were used to calculate T1ρ maps in two healthy volunteers imaged on a 1.5-T Sonata Siemens MRI scanner. The extremity coils and head were used for imaging the brain and blood in the popliteal artery, respectively. In both of the simulations and brain scans performed at 7T, slSSFP was shown to exhibit similar contrast and signal-to-noise ratio (SNR) efficiency to bSSFP, but with significantly lower power. At the end of this paper the sequence produced images of higher quality than a gradient-echo EPI sequence for the same contrast evolution times. We also discuss applications and limitations of the pulse sequence.