MR thermometry with high precision and temporal resolution by quadratic phase MR fingerprinting

To map temperature via the proton resonance frequency- (PRF-) shift with a high frame rate and high precision using quadratic RF excitation phase-magnetic resonance fingerprinting (qRF-MRF).

A continuous balanced qRF-MRF sequence was implemented using a constant low-flip-angle excitation with quadratic RF excitation phase increments, which impart sensitivity to resonance frequency changes from heating by repeatedly sweeping the sequence's resonance frequency between $-1$ /(2TR) and +1/(2TR) Hz, while minimizing sensitivity to $T_1$ and $T_2$ . Temperature maps were reconstructed from sliding windows using the conjugate gradient (CG) algorithm, dictionary matching, and conventional PRF temperature calculations using MRF-synthesized gradient-recalled echo (GRE) images. Monte Carlo simulations were performed to optimize the sequence. qRF-MRF temperature precision was compared to acquisition time-matched 2DFT GRE temperature maps at 3 Tesla in simulations, phantom imaging, and in vivo imaging. The ability to image dynamic temperature changes was validated in a phantom-focused ultrasound (FUS) heating experiment.

Compared to 2DFT GRE, the optimized qRF-MRF sequence achieved an 85% reduction in temperature standard deviation in phantom simulation (0.092 vs. 0.014 ${\hspace{0.17em}}^{\circ }C$ ), 71% reduction in phantom imaging (0.065 vs. 0.019 ${\hspace{0.17em}}^{\circ }C$ ), and 55% reduction in vivo (0.321 vs. 0.147 ${\hspace{0.17em}}^{\circ }C$ ). CG MRF reconstruction improved dictionary match inner products $\sim$ 2 $\times$ and reduced temperature standard deviation 30% compared to gridding. In FUS heating, qRF-MRF-reconstructed heating pattern and temperature curve closely matched the 2DFT results. qRF-MRF also enabled the reconstruction of linewidth maps.

Continuous low-flip-angle qRF-MRF is capable of temperature imaging using the PRF shift with similar frame rates but higher precision than conventional GRE thermometry.