Investigating and reducing the effects of confounding factors for robust T1 and T2 mapping with cardiac MR fingerprinting

Abstract

This study aims to improve the accuracy and consistency of T and T measurements using cardiac MR Fingerprinting (cMRF) by investigating and accounting for the effects of confounding factors including slice profile, inversion and T preparation pulse efficiency, and B. The goal is to understand how measurements with different pulse sequences are affected by these factors. This can be used to determine which factors must be taken into account for accurate measurements, and which may be mitigated by the selection of an appropriate pulse sequence. Simulations were performed using a numerical cardiac phantom to assess the accuracy of over 600 cMRF sequences with different flip angles, TRs, and preparation pulses. A subset of sequences, including one with the lowest errors in T and T maps, was used in subsequent analyses. Errors due to non-ideal slice profile, preparation pulse efficiency, and B were quantified in Bloch simulations. Corrections for these effects were included in the dictionary generation and demonstrated in phantom and in vivo cardiac imaging at 3 T. Neglecting to model slice profile and preparation pulse efficiency led to underestimated T and overestimated T for most cMRF sequences. Sequences with smaller maximum flip angles were less affected by slice profile and B. Simulating all corrections in the dictionary improved the accuracy of T and T phantom measurements, regardless of acquisition pattern. More consistent myocardial T and T values were measured using different sequences after corrections. Based on these results, a pulse sequence which is minimally affected by confounding factors can be selected, and the appropriate residual corrections included for robust T and T mapping.

Water–fat Dixon cardiac magnetic resonance fingerprinting

“…Accurate water T 1 and T 2 values were obtained with the proposed approach in comparison to spin echo‐based reference values (R 2 > 0.99, NRMSE T1 = 1.4% and NRMSE T2 = 5.4%). A slice profile correction was included in the dictionary to obtain accurate measurements (for T 2 especially) when using short pulses . A Bloch simulation of the inversion efficiency was also included, impacting mainly the T 1 measurements.…”

“…Inversion pulses are applied every 5 heartbeats with delays of [10, 300, 10] ms, whereas T 2 preparations with TEs of [No T 2 prep, No T 2 prep, 40, 80, 160] ms are repeated 3 times over the 15 heartbeats. Additional contrast encoding is provided with the flip angle pattern composed of a linear ramp‐up of 20 RF pulses from 5° to 30° followed by a fixed 30° flip angle . Low flip angles, fixed TR and gradient spoiling were used to reduce the signal’s sensitivity to B 0 and B 1 inhomogeneities .…”

“…27,39 A Bloch simulation of the inversion efficiency was also included, 27 impacting mainly the T 1 measurements. To limit the impact of B 1 inhomogeneities, adiabatic inversions 47 and refocusing pulses in the T 2 preparation prepulses 48 were used, with the maximum excitation flip angle 27 limited to 30°. To extend the pattern from single echo to 3-echo Dixon encoding, the TEs were chosen based on a previously reported Cramer-Rao lower bound (CRLB) optimization.…”

Simultaneous multislice cardiac magnetic resonance fingerprinting using low rank reconstruction

“…At 3 T, myocardial T 1 values between 1477 and 1569 ms have been reported with SASHA . In this study, the cMRF dictionary included corrections for some confounding factors that could introduce quantitative errors if not taken into account . Specifically, the dictionary simulation modeled both the non‐ideal slice profile, and T 1 and T 2 relaxation during the inversion and T 2 preparation pulses.…”

“…As previously mentioned, magnetization transfer effects are not taken into account. No B 1 + corrections were performed, although previous work has shown that this particular cMRF sequence is relatively robust to B 1 + variations . The slice profile correction that was included in the dictionary assumes there is only one tissue type across the slice direction of each voxel, and it does not consider partial volume or multi‐compartment effects.…”