Reducing inhomogeneity artifacts in functional MRI of human brain activation - Thin sections vs gradient compensation

We evaluated two methods for correcting inhomogeneity-induced signal losses in magnetic resonance gradient-echo imaging that either use gradient compe nsation or simply acquire thin sections. The strategies were tested in the human brain in terms of achievable quality of T2*-weighted images at the le vel of the hippocampus and of functional activation maps of the visual cort ex. Experiments were performed at 2.0 T and based on single-shot echo-plana r imaging at 2.0 x 2.0 mm(2) resolution, 4 mm section thickness, and 2.0 s temporal resolution. Gradient compensation involved a sequential 16-step va riation of the refocusing lobe of the slice-selection gradient (TR/TE = 125 /53 ms, flip angle 15 degrees), whereas thin sections divided the 4-mm targ et plane into either four 1-mm or eight 0.5-mm interleaved multislice acqui sitions (TR/TE = 2000/54 ms, flip angle 70 degrees). Both approaches were c apable of alleviating the inhomogeneity problem for structures in the base of the brain. When compared to standard 4-mm EPI, functional mapping in the visual cortex was partially compromised because of a lower signal-to-noise ratio of inhomogeneity-corrected images by either method. Relative to each other, consistently better results were obtained with the use of contiguou s thin sections, in particular for a thickness of 1 mm. Multislice acquisit ions of thin sections require minimal technical adjustments. (C) 2000 Acade mic Press.