Rapid 3D absolute B1+ 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
Purpose
To shorten the acquisition time of magnetization-prepared absolute transmit field (B1+) mapping known as presaturation TurboFLASH, or satTFL, to enable single breath-hold whole-heart 3D B1+ mapping.
Methods
SatTFL is modified to remove the delay between the reference and prepared images (typically 5 T1), 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 B1+ maps in a single breath-hold. We evaluate the sensitivity to B1+ and T1 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 B1+ due to T1 and magnetization history. The mean coefficient of variation was reduced by 81% for T1s 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 B1+ maps of the whole thorax in 26 heartbeats.
Conclusion
Our adaptations enable faster B1+ mapping, with minimal T1 sensitivity and lower sensitivity to magnetization history, enabling single breath-hold whole-heart absolute B1+ mapping.