Optimized interferometric encoding of presaturated TurboFLASH B₁ mapping for parallel transmission MRI at 7 T:...

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 B₁ 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 T₁ mapping with pTx-MP2RAGE.

Global optimization of interferometric encoding was implemented by simulating the ability of the satTFL to reconstruct B₁ 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 optimization was compared to actual flip angle imaging. Optimized and non-optimized B₁ maps were then used to calculate pTx pulses for MP2RAGE T₁ mapping.

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. T₁ 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.

Optimization of the satTFL interferometric encoding improves B₁ maps in the spinal cord, in particular in low SNR regions. A linear correction of the satTFL was additionally shown to be required. The method was successfully used for quantitative phantom and in vivo T₁ mapping, showing improved results compared to non-optimized satTFL thanks to improved pTx-pulse generation.