Radiofrequency energy deposition and radiofrequency power requirements in parallel transmission with increasing distance from the coil to the sample

Cem M. Deniz, Manushka V. Vaidya, Daniel K. Sodickson, Riccardo Lattanzi

    Research output: Contribution to journalArticle

    Abstract

    Purpose We investigated global specific absorption rate (SAR) and radiofrequency (RF) power requirements in parallel transmission as the distance between the transmit coils and the sample was increased. Methods We calculated ultimate intrinsic SAR (UISAR), which depends on object geometry and electrical properties but not on coil design, and we used it as the reference to compare the performance of various transmit arrays. We investigated the case of fixing coil size and increasing the number of coils while moving the array away from the sample, as well as the case of fixing coil number and scaling coil dimensions. We also investigated RF power requirements as a function of lift-off, and tracked local SAR distributions associated with global SAR optima. Results In all cases, the target excitation profile was achieved and global SAR (as well as associated maximum local SAR) decreased with lift-off, approaching UISAR, which was constant for all lift-offs. We observed a lift-off value that optimizes the balance between global SAR and power losses in coil conductors. We showed that, using parallel transmission, global SAR can decrease at ultra high fields for finite arrays with a sufficient number of transmit elements. Conclusion For parallel transmission, the distance between coils and object can be optimized to reduce SAR and minimize RF power requirements associated with homogeneous excitation.

    Original languageEnglish (US)
    Pages (from-to)423-432
    Number of pages10
    JournalMagnetic resonance in medicine
    Volume75
    Issue number1
    DOIs
    StatePublished - Jan 1 2016

    Keywords

    • parallel transmission
    • specific absorption rate
    • transmit coils
    • ultimate intrinsic SAR

    ASJC Scopus subject areas

    • Radiology Nuclear Medicine and imaging

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