Maximum efficiency radiofrequency shimming: Theory and initial application for hip imaging at 7 tesla

Cem Murat Deniz, Ryan Brown, Riccardo Lattanzi, Leeor Alon, Daniel K. Sodickson, Yudong Zhu

    Research output: Contribution to journalArticle

    Abstract

    Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.

    Original languageEnglish (US)
    Pages (from-to)1379-1388
    Number of pages10
    JournalMagnetic resonance in medicine
    Volume69
    Issue number5
    DOIs
    StatePublished - May 1 2013

    Keywords

    • B shimming
    • adiabatic excitation
    • hip
    • hip articular cartilage
    • radiofrequency power deposition
    • radiofrequency shimming
    • specific absorption rate
    • ultra-high-field MRI

    ASJC Scopus subject areas

    • Radiology Nuclear Medicine and imaging

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