Magnetization transfer in magnetic resonance fingerprinting

Abstract

Purpose: To study the effects of magnetization transfer (MT, in which a semi-solid spin pool interacts with the free pool), in the context of magnetic resonance fingerprinting (MRF).

Methods: Simulations and phantom experiments were performed to study the impact of MT on the MRF signal and its potential influence on T₁ and T₂ estimation. Subsequently, an MRF sequence implementing off-resonance MT pulses and a dictionary with an MT dimension, generated by incorporating a two-pool model, were used to estimate the fractional pool size in addition to the $B_1^{+}$ , T₁ , and T₂ values. The proposed method was evaluated in the human brain.

Results: Simulations and phantom experiments showed that an MRF signal obtained from a cross-linked bovine serum sample is influenced by MT. Using a dictionary based on an MT model, a better match between simulations and acquired MR signals can be obtained (NRMSE 1.3% vs. 4.7%). Adding off-resonance MT pulses can improve the differentiation of MT from T₁ and T₂ . In vivo results showed that MT affects the MRF signals from white matter (fractional pool-size ~16%) and gray matter (fractional pool-size ~10%). Furthermore, longer T₁ (~1060 ms vs. ~860 ms) and T₂ values (~47 ms vs. ~35 ms) can be observed in white matter if MT is accounted for.

Conclusion: Our experiments demonstrated a potential influence of MT on the quantification of T₁ and T₂ with MRF. A model that encompasses MT effects can improve the accuracy of estimated relaxation parameters and allows quantification of the fractional pool size.

Keywords: fingerprinting; magnetization transfer; quantitative imaging; relaxation.