Clinical Implementation of Dynamic Parallel Transmission in 7T Brain MRI: Improved Homogeneity and Contrast Using SPACE Sequences.

Abstract

Background and purpose: The adoption of routine clinical 7T MRI has been constrained by several challenges, with heterogeneity of the transmit field (B1+) being among the most notable. Dynamic parallel transmission (pTx) presents a promising strategy to enhance B1+ transmit homogeneity; however, associated technical challenges have limited its routine use.We assess performance of a prototype dynamic pTx implementation for 3D sampling perfection with application-optimized contrasts using different flip angle evolution (SPACE) sequence and hypothesize that signal homogeneity and tissue contrast will improve versus single transmit mode (sTx).

Materials and methods: Data from consecutive clinical patients undergoing 7T brain MRI for various indications were utilized. Signal homogeneity was assessed using coefficient of variation (CoV). Tissue contrast was assessed using an image intensity profile along an 8mm line crossing the cortex into underlying white matter. Additionally, stability of signal homogeneity across a larger cohort of clinical patients was assessed with CoV. Predicted local specific absorption rate (SAR) for the head between pTx and sTx sequences was also compared.

Results: For each sequence, 11 patients had both sTx and pTx scans. The comparison clinical cohort had 40 patients with pTx for each sequence. Image signal and contrast were significantly improved with pTx versus sTx for both T2-SPACE and FLAIR-SPACE (p=0.001). Tissue contrast between white matter and cortex was also significantly improved in the temporal lobe with pTx (p=0.001). CoV did not reveal any outlier cases across a large clinical cohort, demonstrating consistency in signal homogeneity. Despite increased SAR, T2-SPACE pTx consistently operated in first-level controlled mode, while FLAIR-SPACE pTx scans generally operated in normal mode.

Conclusions: We demonstrate the feasibility of a time-efficient prototype dynamic pTx implementation in T2-SPACE and FLAIRSPACE sequences, which significantly enhances signal and contrast across the brain compared to sTx, while exhibiting consistent and robust performance in a large cohort of clinical patients.

Abbreviations: BRISQUE = Blind/Referenceless Image Spatial Quality Evaluator; CoV = coefficient of variation; FOCUS = fast online-customized pulses; GM = gray matter; NIQE = Natural Image Quality Evaluator; pTx = parallel transmit mode; RF = radiofrequency; SAR = specific absorption rate; SPACE = 3D sampling perfection with application-optimized contrasts using different flip angle evolution; sTx = single transmit mode; WM = white matter.