In Vivo 3D Liver Imaging at 7 T Using kT-Point pTx Pulses and a 32-Tx-Channel Whole-Body Radiofrequency Coil Array.

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Radiofrequency (RF) Coil Technology
• Medical Physics

Background:

• Inhomogeneous B1+ transmit profiles at 7 Tesla (7T) cause flip angle variations, challenging MRI quality.
• Existing static parallel transmit (pTx) with local arrays struggles with homogeneity for large organs like the liver.
• Dynamic pTx is often required but presents its own complexities.

Purpose of the Study:

• To evaluate the benefits of a 32-Tx-channel remote array for improving B1+ homogeneity in 7T liver MRI.
• To assess the trade-off between flip angle homogeneity and RF power using varying numbers of k-T points.
• To compare the shimming performance of different channel numbers for static and dynamic pTx.

Main Methods:

• Acquisition of relative channel-wise 3D B1+ maps using a radial phase encoding (RPE) scheme at 7T in five healthy volunteers.
• Analysis of the optimal number of k-T point pTx pulses by comparing L-curves for flip angle (FA) homogeneity versus RF power.
• Evaluation of shimming performance with 8, 16, or 20 B1+ maps derived from the 32-channel array.

Main Results:

• The 32-Tx-channel remote array eliminated FA dropouts and achieved coefficient of variation (CV) values under 17% with magnitude and phase shimming.
• Two to three k-T points offered a favorable balance between FA homogeneity and RF power.
• Higher channel counts demonstrated superior shimming performance; 32 channels improved homogeneity 1.3- to 1.6-fold compared to 8 channels at the same RF power.

Conclusions:

• A remote 32-Tx channel array significantly enhances B1+ homogeneity for in vivo liver imaging at 7T.
• This advanced coil technology overcomes limitations of local arrays for large organ imaging.
• The findings support the use of remote multi-channel arrays for improved 7T MRI quality and diagnostic accuracy.