Optimized interferometric encoding of presaturated TurboFLASH B₁ mapping for parallel transmission MRI at 7 T: Preliminary application for quantitative T₁ mapping in the spinal cord

Purpose: The acquisition of accurate B1 maps is critical for parallel transmit techniques (pTx). The presaturated turboFLASH (satTFL) method has been widely used in combination with interferometric encoding to provide robust and fast B-1 maps. However, typical encodings, mostly evaluated on brain, do not necessarily fit all coils and organs. In this work, we evaluated and improved the accuracy of the satTFL for cervical spine at 7 T, proposing a novel interferometric encoding optimization. The benefits of such improvements were investigated in an exploratory study of quantitative T-1 mapping with pTx-MP2RAGE. Methods: Global optimization of interferometric encoding was implemented by simulating the ability of the satTFL to reconstruct B-1 maps, with varying encoding and inclusion of complex noise, inside a region of interest covering the cervical spine. The performance of satTFL before and after optimizationwas compared to actual flip angle imaging. Optimized and non-optimized B-1 maps were then used to calculate pTx pulses for MP2RAGE T-1 mapping. Results: Interferometric encoding optimization resulted in satTFL closer to actual flip angle imaging, with substantial gain of signal in regions where non-optimized satTFL could fail. T-1 maps measured with non-adiabatic pTx pulses were closer to standard non-pTx results (which used adiabatic pulses) when using optimized-satTFL, with substantially lower specific absorption rate. Conclusion: Optimization of the satTFL interferometric encoding improves B-1 maps in the spinal cord, in particular in low SNR regions. A linear correction of the satTFLwas additionally shown to be required. The methodwas successfully used for quantitative phantom and in vivo T-1 mapping, showing improved results compared to non-optimized satTFL thanks to improved pTx-pulse generation.