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B1+interferometry for the calibration of RF transmitter arrays


Brunner, D O; Pruessmann, K P (2009). B1+interferometry for the calibration of RF transmitter arrays. Magnetic Resonance in Medicine, 61(6):1480-1488.

Abstract

Multiple-channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B(1) (+), of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B(1) (+) and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B(1) (+) fields of individual transmitter elements are measured indirectly by observing their interference with a SAR-efficient baseline RF field. In this fashion even small RF fields can be observed in the B(1) (+) -sensitive large-flip-angle regime. Based on a set of such experiments B(1) (+) maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off-resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8-channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T(1) or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B(1) (+) maps accurately reflect the linearity of RF superposition.

Multiple-channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B(1) (+), of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B(1) (+) and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B(1) (+) fields of individual transmitter elements are measured indirectly by observing their interference with a SAR-efficient baseline RF field. In this fashion even small RF fields can be observed in the B(1) (+) -sensitive large-flip-angle regime. Based on a set of such experiments B(1) (+) maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off-resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8-channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T(1) or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B(1) (+) maps accurately reflect the linearity of RF superposition.

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30 citations in Web of Science®
32 citations in Scopus®
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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:04 Faculty of Medicine > Institute of Biomedical Engineering
Dewey Decimal Classification:170 Ethics
610 Medicine & health
Language:English
Date:2009
Deposited On:18 Nov 2009 12:40
Last Modified:05 Apr 2016 13:33
Publisher:Wiley-Blackwell
ISSN:0740-3194
Publisher DOI:10.1002/mrm.21893
PubMed ID:19353666
Permanent URL: http://doi.org/10.5167/uzh-24019

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