B interferometry for the calibration of RF transmitter arrays

✍ Scribed by David O. Brunner; Klaas P. Pruessmann

• Publisher: John Wiley and Sons
• Year: 2009
• Tongue: English
• Weight: 703 KB
• Volume: 61
• Category: Article
• ISSN: 0740-3194
• DOI: 10.1002/mrm.21893

✦ Synopsis

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, of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B 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 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 ‐sensitive large‐flip‐angle regime. Based on a set of such experiments B 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 maps accurately reflect the linearity of RF superposition. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.