Slice‐selective adiabatic magnetization T₂‐preparation (SAMPA) for efficient T₂‐weighted imaging at ultrahigh field strengths

Abstract

Purpose At high field, T2-weighted (T2w) imaging is limited by transmit field inhomogeneity and increased radiofrequency power deposition. In this work we introduce SAMPA (Slice-selective Adiabatic Magnetization T2 PrepAration) and demonstrate its use for efficient brain T2w imaging at 7T. Methods SAMPA was designed by subsampling an optimized B1 insensitive rotation (BIR4) waveform with small tip angle linear subpulses. In order to perform T2w imaging, SAMPA was inserted before a fast gradient echo acquisition. The off-resonance behavior, B1 robustness and slice selectivity of the novel T2 preparation module were analyzed using Bloch simulations. The performance of SAMPA for T2w imaging was demonstrated in phantom experiments as well as in the brains of healthy volunteers at 7T. Results Based on simulations, the proposed design operates at peak B1 of 15 μT and higher, within a 400 Hz bandwidth. T2 values were in strong agreement with spin echo-based T2 mapping in phantom experiments. Whole brain, interleaved multislab 3D imaging could be acquired with 0.8 mm3 isotropic resolution in 5:36 min per T2 weighting. Conclusion Compared with previous adiabatic T2 preparation techniques, SAMPA allows for slice-selectivity, which can lead to efficient and robust acquisitions for T2w imaging at high field.

Cerebrospinal fluid–suppressed T2‐weighted MR imaging at 7 T for human brain

“…In practice, our experience with the transceiver with the adiabatic refocusing pulses has not been shown to be a major problem for this over the entire head. Nonetheless, this sensitivity can be improved with incorporation of an adiabatic excitation spin‐echo module, such as that used by Dyvorne and Balchandani, although at added SAR cost.…”

“…In practice, our experience with the transceiver with the adiabatic refocusing pulses has not been shown to be a major problem for this over the entire head. Nonetheless, this sensitivity can be improved with incorporation of an adiabatic excitation spin-echo module, such as that used by Dyvorne and Balchandani, 16 although at added SAR cost. With the CSF suppression generated from the normalization strategy performed between the multiple acquisition blocks, acquisition of differing resolution images but at the same delay times can change the resulting contrast.…”

“…8,12 Alternatively, to altogether avoid multiple refocusing pulses, a driven equilibrium spinecho strategy to longitudinally encode T 2 contrast (i.e., T 2 preparation) has been useful. [13][14][15] To reduce the sensitivity of the acquisition to B 1 + inhomogeneity at 7 T, Dyvorne and Balchandani 16 implemented the T 2 preparation with an adiabatic spin-echo module in a multislab approach to generate excellent T 2 -weighted whole-brain coverage at 0.8-mm 3 isotropic resolution (approximate 5.5-minute acquisition). The FLAIR imaging at 7 T, however, remains challenging.…”

A simplified framework to optimize MRI contrast preparation

“…This allows the combination of excellent T 2 ‐weighted contrast and fast acquisition sequences such as ultra‐short (UTE) or even zero echo time (ZTE). Various declinations of this approach have been proposed to increase and/or B 1 robustness, in the context of coronary imaging and black blood imaging . The second T 2 ‐weighted preparation strategy aims at enhancing short or ultra‐short T 2 (<1 ms) tissues.…”