Generalized double-acquisition imaging for radiofrequency inhomogeneity mitigation in high-field MRI: Experimental proof and performance analysis

Transmit arrays have been developed to compensate for radiofrequency inhomogeneities in high-field MRI using different excitation schemes. They can be classified into static or dynamic shimmings depending on the target: homogenizing the radiofrequency field directly or homogenizing the flip angle distribution using the Bloch equation. We have developed an intermediate solution to compare shimming performances between different transmit arrays. This solution, called generalized double-acquisition imaging, is easier to implement than most dynamic shimming methods and offers more degrees of freedom than static shimmings. It uses two acquisitions so that the second acquisition complements the excitation of the first one to obtain by superposition an image that minimizes radiofrequency artefacts. For validation, the method is demonstrated experimentally for a gradient echo sequence on a spherical homogeneous phantom and by simulation on a human head model. Magn Reson Med, 2011. (C) 2011 Wiley-Liss, Inc.