Functional and metabolic MRI techniques have been rapidly evolving and have tremendous potential for clinical brain disorders research. Clinical activation fMRI studies are performed at 1.5 and at 3.0 Tesla using blood oxygenation level dependent (BOLD) contrast method and arterial spin tagging (AST) techniques. Gradient-echo echo-planar fMRI experiments were performed at 1.5 T and 3.0 T with varying acquisition echo time and bandwidth, and with a 4 mm isotropic voxel size. To analyze the BOLD sensitivity, the relative contributions of BOLD signal amplitude and thermal and physiologic noise At 1.5 T, the number of activated pixels and the average t-score showed appeared to be optimum over a TE range of 60-160 msec. At 3.0 T, the optimum range was between TEs of 30-130 msec. The average number of activated pixels and t-scores were 59% and 18% higher at 3.0 T than at 1.5 T, respectively, an improvement that was lower than the observed 100% to 110% increase in signal-to-noise ratio predicted at 3.0 T. The anticipated improvements in BOLD contrast was not observed as predicted presumably due to increased physiological noise with the experimental conditions. Improvements in fMRI bold imaging can be accomplished by reducing the image acquisition bandwidth.
