Composite image formation in z-shimmed functional MR imaging

A challenge in functional magnetic resonance imaging (fMRI) is to develop i maging methods that are highly sensitive to microscopic field inhomogeneiti es [the blood oxygenation level-dependent (BOLD) effect] and minimally sens itivity to macroscopic fields. z-Shimming compensates for the through-plane dephasing that arises in gradient-echo images due to magnetic field inhomo geneities. To date, an analysis of the formation of composite images from m ultiple z-shim acquisitions has not been presented. This work compares thre e strategies for forming composite images, one of which is introduced for t he first time, against the nominal image acquisition. True-versus false-pos itive rates of activation detection are considered, in addition to the time efficiency of the methods. It is shown that z-shimming can provide uniform spatial sensitivity, resulting in increased activation detectability, in m any cases outperforming the nominal imaging approach. Time efficiency is sh own to be dependent on field uniformity. Theory, computer simulations, and results from fMRI studies are used to demonstrate the performance of these methods. Magn Reson Med 42:110-117, 1999. (C) 1999 Wiley-Liss, Inc.