Noise properties, the signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and signal responses were compared during functional activation of the human brain at 1.5 and 3.0 T. At the higher field spiral gradient-echo (GRE) brain images revealed an average gain in SNR of 1.7 in fully relaxed and 2.2 in images with a repetition time (TR) of 1.5 sec. The tempered gain at longer TRs reflects the fact that the physiological noise depends on the signal strength and becomes a larger fraction of the total noise at 3.0 T. Activation of the primary motor and visual cortex resulted in a 36% and 44% increase of "activated pixels" at 3.0 T, which reflects a greater sensitivity for the detection of activated gray matter at the higher field. The gain in the CNR exhibited a dependency on the underlying tissue, i.e., an increase of 1.8x in regions of particular high activation-induced signal changes (presumably venous vessels) and of 2.2x in the average activated areas. These results demonstrate that 3.0 T provides a clear advantage over 1.5 T for neuroimaging of homogeneous brain tissue, although stronger physiological noise contributions, more complicated signal features in the proximity of strong susceptibility gradients, and changes in the intrinsic relaxation times may mediate the enhancement. Magn Reson Med 45:595-604, 2001.
Copyright 2001 Wiley-Liss, Inc.