Combining TMS and fMRI localisations of function

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The table above shows four visualisations of fMRI data including an electrode placed using TMS guidance. Each image is the result of combining structural and functional MRI scans. The fiducial electrodes placed using functional determination by TMS are also visible, and fMRI activity can be seen in the cortex immediately "below" the motor site electrode.

This study is being conducted by

Mark S. George, MD, Donna Roberts, MS, Diana Vincent, PhD, and Andrew Speer, MD

at the Functional Neuroimaging Division, Departments of Psychiatry, Radiology, and Neurology, Medical University of South Carolina, Charleston, USA.

Dr George writes:

The [upper left] image is a superimposition of a structural scan (FSE) taken through the same axial slices where we acquired echoplanar BOLD fMRI data. We first used single-pulse TMS to map the area on the skull which optimally stimulates the left hand abductor pollicis brevis. We then placed a copper sulfate fiducial on this spot and a control vitamin E capsule on the left prefrontal cortex. The subject then entered the Picker 1.5 Tesla scanner equipped with whole body high performance gradients, and had the structural scans taken. We then acquired BOLD T2* weighted images (TE 40, TR 3000, FOV 25.5 cm, slices 5mm thick, no interslice interval) over 6 minutes, with the subject alternating between rest and performing a complex finger opposition task with the left hand. BOLD images were then realigned for movement and compared with a paired t-test. Areas of significant activation were then superimposed on the structural scan, where the fiducials were readily apparent. On these axial images, the fiducial is not on the same plane as the area of BOLD activation, because the peak magnetic field from the TMS magnet travels down in the z direction through the slices.

[The other three images] are 3-dimensional volume rendered images from a recent study in a healthy volunteer. As before, we used a figure-eight coil to locate with TMS the optimum position for stimulating the left hand APB. We then put a copper-sulfate fiducial on this spot, and on a spot 5 cm anterior and in a parasagittal line. (This second fiducial represents the spot in this person that corresponds to the region we and Alvara Pascual-Leone have stimulated in studies of depressed or healthy controls that appears to be able to influence mood). We then moved the person to the Picker 1.5 tesla MRI scanner and acquired 15 coronal slices structurally, and 120 15-slice BOLD images over 6 minutes while resting or performing the finger opposition task with the left thumb. This image is a merged image of the significant areas of activation during thumb apposition (p < 0.01) onto the 3-D volume scan. You can see that the motor fiducual is directly over the area that fMRI also shows controls motor movement in the left thumb. There is also significant activation in the cerebellum on the same side as the movement, opposite from the site of stimulation.

We think we have demonstrated good concordance between these two methods of mapping brain-behavior relationships. Moving from TMS to conventional functional imaging and back likely represents a powerful new frontier in brain exploration.