Mitigating undersampling errors in MR fingerprinting by sequence optimization

PurposeTo develop a method for MR Fingerprinting (MRF) sequence optimization that takes both the applied undersampling pattern and a realistic reference map into account. MethodsA predictive model for the undersampling error leveraging on perturbation theory was exploited to optimize the MRF flip angle sequence for improved robustness against undersampling artifacts. In this framework parameter maps from a previously acquired MRF scan were used as reference. Sequences were optimized for different sequence lengths, smoothness constraints and undersampling factors. Numerical simulations and in vivo measurements in eight healthy subjects were performed to assess the effect of the performed optimization. The optimized MRF sequences were compared to a conventionally shaped flip angle pattern and an optimized pattern based on the Cramer-Rao lower bound (CRB). ResultsNumerical simulations and in vivo results demonstrate that the undersampling errors can be suppressed by flip angle optimization. Analysis of the in vivo results show that a sequence optimized for improved robustness against undersampling with a flip angle train of length 400 yielded significantly lower median absolute errors in T1: 5.6%+/- 2.9% and T2: 7.9%+/- 2.3% compared to the conventional (T1: 8.0%+/- 1.9%, T2: 14.5%+/- 2.6%) and CRB-based (T1: 21.6%+/- 4.1%, T2: 31.4%+/- 4.4%) sequences. ConclusionThe proposed method is able to optimize the MRF flip angle pattern such that significant mitigation of the artifacts from strong k-space undersampling in MRF is achieved.