Parallel-transmission-enabled magnetization-prepared rapid gradient-echo T1-weighted imaging of the human brain at 7...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radio Frequency (RF) Engineering
• Neuroimaging

Background:

• Ultra High Field (UHF) MRI (7 Tesla) promises higher spatial resolution but suffers from Radio Frequency (RF) transmit field non-uniformity.
• This non-uniformity degrades image quality in sequences like MP-RAGE, hindering post-processing like automated tissue segmentation.
• Flip angle (FA) homogeneity is critical for consistent signal and contrast in MRI sequences.

Purpose of the Study:

• To evaluate the potential of parallel-transmission (pTx) for high-quality MP-RAGE brain imaging at 7 Tesla.
• To assess the efficacy of k(T)-point trajectory-based RF pulses for improving FA homogeneity and inversion fidelity.
• To compare pTx performance against conventional methods and 3 Tesla benchmarks.

Main Methods:

• Development and application of tailored non-selective transmit-SENSE pulses using an 8-channel transmit array.
• Utilized k(T)-point trajectory for both low-FA excitation and 180° inversion pulses in the MP-RAGE sequence.
• Compared pTx results with conventional RF-shimmed excitations and 3 Tesla scans; performed automated tissue classification for quantitative analysis.

Main Results:

• pTx with RF-shim significantly improved 7 Tesla MP-RAGE image quality.
• The k(T)-point method achieved excellent inversion fidelity comparable to 3 Tesla, reducing cumulative energy deposition by over 40%.
• FA uniformity at 7 Tesla with k(T)-point pulses surpassed typical 3 Tesla performance, confirmed by improved automated tissue segmentation.

Conclusions:

• k(T)-point based parallel-transmission is a highly effective solution for UHF 3D MRI, yielding superior image quality and efficiency.
• This technique provides diagnostic image quality at 7 Tesla, comparable to clinical 3 Tesla systems, while enhancing patient safety by reducing specific absorption rates.
• Further research should address local susceptibility effects near cranial cavities for optimal segmentation in UHF imaging.