B1(+) interferometry for the calibration of RF transmitter arrays.
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.
Institute for Biomedical Engineering, University and ETH Zurich, Zurich Gloriastrasse 35, Zurich, Switzerland. brunner@biomed.ee.ethz.ch
David O
DO
Brunner
Klaas P
KP
Pruessmann
D000465:Algorithms; D002138:Calibration; D007089:Image Enhancement / Q000592:standards*; D007090:Image Interpretation, Computer-Assisted / Q000592:standards*; D007368:Interferometry / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods* / Q000592:standards*; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013557:Switzerland; D014159:Transducers