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Purpose To develop a B1+ mapping during the transient phase of balanced steady state free precession (bSSFP) imaging which can be used for subsequent B1+ inhomogeneity compensation. Methods Two images with different flip angles (FA) are acquired using single-shot spiral technique during the transient phase of bSSFP with three consecutive RF pulses and balanced gradients. Under the assumptions that the transmit (B1+) field varies slowly in spatial domain and T1 and T2 relaxation effects are negligible during 2·TR, B1+ was estimated using the two magnitude images and bSSFP data was sequentially acquired. B1+ estimation error due to the assumptions and other factors such as FA and off-resonance were assessed using Bloch simulation. Phantom and in vivo experiments were performed with α-2α-3α scheme. Results The simulation results indicated that the proposed method was less sensitive to T1 relaxation and B1+ mapping FA (α) of approximately 60° produced minimum estimation error. The B1+-induced intensity variation was reduced with the proposed method in the phantom experiment. For both the phantom and in vivo experiments, the estimated B1+ map showed comparable to the conventional B1+ map using spin-echo DAM. Conclusion B1+ map was estimated during the transient phase of bSSFP and subsequently compensated bSSFP images. There was no scan time increment and hence the technique can be used in a prescan manner for B1+ mapping or shimming.

To develop a BTwo images with different flip angles (FA) are acquired using single-shot spiral technique during the transient phase of bSSFP with three consecutive RF pulses and balanced gradients. Under the assumptions that the transmit (BThe simulation results indicated that the proposed method was less sensitive to TB

To simultaneously acquire the B 1 + magnitude and B 1 + phase, a modified multi-echo actual flip-angle imaging (AFI) sequence is proposed. A multi-echo gradient echo sequence was integrated into every even TR of AFI to measure both magnitude and phase of B 1 + . In addition, to increase the signal-to-noise ratio of the B 1 + phase, a double-angle multi-echo AFI sequence, in which the flip-angle of the RF pulses is α at the odd TR and 2α at the even TR is proposed. Images were simulated to evaluate the performance of this method under various imaging and physical parameters. The performance was compared to the spin echo based B 1 + mapping method in phantom and in vivo studies. In the simulation, the estimation error decreased as TR 1 /T 1 decreased and TR 2 /TR 1 increased. For double-angle AFI, flip-angle ranges that could estimate B 1 + magnitude and phase better than the original AFI were identified. Using the proposed method, B 1 + phase estimation was similar to spin echo phase. In the phantom study, correlation coefficient between the estimated B 1 + phases using the spin echo and the proposed method was 0.9998. The results show that the B 1 + magnitude and B 1 + phase can be simultaneously acquired and accurately estimated using the proposed double-angle AFI method.

The radio frequency (RF) slice profile effects on T1 and T2 estimation in magnetic resonance fingerprinting (MRF) are investigated with respect to time-bandwidth product (TBW), flip angle (FA) level and field inhomogeneities. Signal evolutions are generated incorporating the non-ideal slice selective excitation process using Bloch simulation and matched to the original dictionary with and without the non-ideal slice profile taken into account. For validation, phantom and in vivo experiments are performed at 3T. Both simulations and experiments results show that T1 and T2 error from non-ideal slice profile increases with increasing FA level, off-resonance, and low TBW values. Therefore, RF slice profile effects should be compensated for accurate determination of the MR parameters.

A subject-specific local in vivo passive shimming method, focusing on the prefrontal and temporal regions, is proposed. The aim of the investigation is to show that subject variability exists in optimal passive shimming and that the proposed method can be effectively used to overcome these differences. A shimming structure capable of adjusting the position of the passive shims to within a millimeter resolution is built. The optimal shim positions for each individual subject are computed from obtained field map using a convex optimization algorithm. Passive shim experiments at predetermined fixed shim positions vs. individually adjusted shim positions were performed and compared. The results show that intersubject variability exists in the optimal shim positions and that the location-sensitive method proposed can be useful for improving main field homogeneity in vivo.