Rapid B₁⁺ mapping using a preconditioning RF pulse with turboFLASH readout

In MRI, the transmit radiofrequency field (B₁⁺) inhomogeneity can lead to signal intensity variations and quantitative measurement errors. By independently mapping the local B₁⁺ variation, the radiofrequency-related signal variations can be corrected for. In this study, we present a new fast B₁⁺ mapping method using a slice-selective preconditioning radiofrequency pulse. Immediately after applying a slice-selective preconditioning pulse, a turbo fast low-angle-shot imaging sequence with centric k-space reordering is performed to capture the residual longitudinal magnetization left behind by the slice-selective preconditioning pulse due to B₁⁺ variation. Compared to the reference double-angle method, this method is considerably faster. Specifically, the total scan time for the double-angle method is equal to the product of 2 (number of images), the number of phase-encoding lines, and approximately 5T₁, whereas the slice-selective preconditioning method takes approximately 5T₁. This method was validated in vitro and in vivo with a 3-T whole-body MRI system. The combined brain and pelvis B ₁⁺ measurements showed excellent agreement and strong correlation with those by the double-angle method (mean difference = 0.025; upper and lower 95% limits of agreement were -0.07 and 0.12; R = 0.93; P < 0.001). This fast B₁⁺ mapping method can be used for a variety of applications, including body imaging where fast imaging is desirable.