Effects of radiofrequency channel numbers on B₁⁺ mapping using the Bloch-Siegert shift method

This paper aims to investigate the impact of the channel numbers on the performance of B₁⁺ mapping, by using the Bloch-Siegert shift (BSS) method. B₁⁺ mapping plays a crucial role in various brain imaging protocols.

We simulated the radiofrequency field of the human head model in six groups of multi-channel receive coil with a range of different channel numbers. MR signals were synthesized according to the standard BSS sequence, with quantified Gaussian added. Next, we combined the signals of each channel to reconstruct the B₁⁺ map by weighted averaging and maximum likelihood estimation strategies and evaluate the bias by relative standard deviation of each coil.

The simulation results revealed that the accuracy of B₁⁺ maps improved with the increasing of channel numbers, meanwhile the per channel efficiency of B₁⁺maps accuracy gradually decrease. Both trends slowed down when the channel numbers reached 12 or above.

Our finding suggests that increasing the channel numbers can improve the accuracy of B₁⁺map. However, a diminishing efficiency of per channel accuracy improvement was overserved, indicating that the relationship between quality of B₁⁺ map and the channel numbers is nonlinear. Based on these findings, our study provides a reference for determining channel numbers to achieve a balance of coil selection and manufacturing cost. It also provides a theoretical basis for evaluating other B₁⁺ mapping techniques.