Optimizing the mapping of finger areas in primary somatosensory cortex using functional MRI

Functional magnetic resonance imaging mapping of the finger somatotopy in the primary somatosensory cortex requires a reproducible and precise stimulation. The highly detailed functional architecture in this region of the brain also requires careful consideration in choice of spatial resolution and postprocessing parameters. The purpose of this study is therefore to investigate the impact of spatial resolution and level of smoothing during tactile stimulation using a precise stimuli system. Twenty-one volunteers were scanned using 23 mm3 and 33 mm3 voxel volume and subsequently evaluated using three different smoothing kernel widths. The overall activation reproducibility was also evaluated. Using a high spatial resolution proved advantageous for all fingers. At 23 mm3 voxel volume, activation of the thumb, middle finger and little finger areas was seen in 89%, 67% and 50% of the volunteers, compared to 78%, 61% and 33% at 33 mm3, respectively. The sensitivity was comparable for nonsmoothed and slightly smoothed (4 mm kernel width) data; however, increasing the smoothing kernel width from 4 to 8 mm resulted in a critical decrease (∼50%) in sensitivity. In repeated measurements of the same subject at six different days, the localization reproducibility of all fingers was within 4 mm (1 S.D. of the mean). The precise computer-controlled stimulus, together with data acquisition at high spatial resolution and with only minor smoothing during evaluation, could be a very useful strategy in studies of brain plasticity and rehabilitation strategies in hand and finger disorders and injuries.