Rapid 3D absolute B<sub>1</sub><sup>+</sup> mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart
James L. Kent, Iulius Dragonu, Ladislav Valkovič, Aaron T. Hess
Abstract
Purpose To shorten the acquisition time of magnetization‐prepared absolute transmit field (B 1 + ) mapping known as presaturation TurboFLASH, or satTFL, to enable single breath‐hold whole‐heart 3D B 1 + mapping. Methods SatTFL is modified to remove the delay between the reference and prepared images (typically 5 T 1 ), with matching transmit configurations for excitation and preparation RF pulses. The new method, called Sandwich, is evaluated as a 3D sequence, measuring whole‐brain and gated whole‐heart B 1 + maps in a single breath‐hold. We evaluate the sensitivity to B 1 + and T 1 using numerical Bloch, extended phase graph, and Monte Carlo simulations. Phantom and in vivo images were acquired in both the brain and heart using an 8‐channel transmit 7 Tesla MRI system to support the simulations. A segmented satTFL with a short readout train was used as a reference. Results The method significantly reduces acquisition times of 3D measurements from 360 s to 20 s, in the brain, while simultaneously reducing bias in the measured B 1 + due to T 1 and magnetization history. The mean coefficient of variation was reduced by 81% for T 1 s of 0.5–3 s compared to conventional satTFL. In vivo, the reproducibility coefficient for flip angles in the range 0–130° was 4.5° for satTFL and 4.7° for our scheme, significantly smaller than for a short TR satTFL sequence, which was 12°. The 3D sequence measured B 1 + maps of the whole thorax in 26 heartbeats. Conclusion Our adaptations enable faster B 1 + mapping, with minimal T 1 sensitivity and lower sensitivity to magnetization history, enabling single breath‐hold whole‐heart absolute B 1 + mapping.