Rapid 3D absolute B₁⁺ mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart

Abstract

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.

Optimized interferometric encoding of presaturated TurboFLASH B1 mapping for parallel transmission MRI at 7 T: Preliminary application for quantitative T1 mapping in the spinal cord

“…The presented method only requires B 1 and B 0 maps, as well as accurate SAR 10g prediction (with VOPs for instance) to ensure the feasibility of the optimized encoding. It could therefore be combined with other variations of satTFL B 1 mapping, such as 3D acquisitions 7,10 or interleaved RF cycling and complementary reference modes (B 1 TIAMO), 14 aiming to provide robust and accurate inputs for better pTx techniques. The method and code will be shared on demand by directly contacting the authors.…”

“…Several B 1 mapping sequences have been proposed in the literature, each offering different characteristics, 6–8 with the presaturated TurboFLASH (satTFL) sequence 9 offering a great trade‐off between accuracy and speed. This sequence, which uses the ratio of two images with and without preconditioning pulse to calculate the saturation flip angle (FA), is particularly SAR‐efficient 8 and has shown robustness to flow and motion 10 . Although it has been developed for 2D B 1 mapping, 9 recent works have implemented it in 3D 7,10 .…”

“…This sequence, which uses the ratio of two images with and without preconditioning pulse to calculate the saturation flip angle (FA), is particularly SAR-efficient 8 and has shown robustness to flow and motion. 10 Although it has been developed for 2D B 1 mapping, 9 recent works have implemented it in 3D. 7,10 It is possible to measure individual B 1 maps with the satTFL by transmitting with only one channel at a time.…”

“…10 Although it has been developed for 2D B 1 mapping, 9 recent works have implemented it in 3D. 7,10 It is possible to measure individual B 1 maps with the satTFL by transmitting with only one channel at a time. However, this was shown to be particularly inefficient because low SNR leads to poor accuracy with increasing distance from the driven channel.…”

Rapid estimation of 2D relative B1+‐maps from localizers in the human heart at 7T using deep learning

“…However, when FA optimization over a 3D volume is desired, multi‐slice 2D or 3D $$$ {\mathrm{B}}_1^{+} $$$‐maps with scan times of more than 3 minutes 19 need to be acquired, requiring multiple breath‐hold scans or free respiratory navigation. Recent preliminary work showed fast $$$ {\mathrm{B}}_1^{+} $$$‐mapping based on pre‐saturated TurboFLASH sequences 20–22 in under a minute. However, reconstruction times of at least 8 minutes were needed.…”