Thalamic stimulation and functional magnetic resonance imaging: localization of cortical and subcortical activation with implanted electrodes - Technical note

The utility of functional magnetic resonance (fMR) imaging in patients with implanted thalamic electrodes has not yet been determined. The aim of this study was to establish the safety of performing fMR imaging in patients wi th thalamic deep brain stimulators and to determine the value of fMR imagin g in detecting cortical and subcortical activity during stimulation. Functional MR imaging was performed in three patients suffering from chroni c pain and two patients with essential tremor. Two of the three patients wi th pain had undergone electrode implantation in the thalamic sensory ventra lis caudalis (Vc) nucleus and the other had undergone electrode implantatio n in both the Vc and the periventricular gray (PVG) matter. Patients with t remor underwent electrode implantation in the ventralis intermedius (Vim) n ucleus. Functional MR imaging was performed during stimulation by using a p ulse generator connected to a transcutaneous extension lead. Clinically, Vc stimulation evoked paresthesias in the contralateral body, PVG stimulation evoked a sensation of diffuse internal body warmth, and Vim stimulation ca used tremor arrest. Functional images were acquired using a 1.5-tesla MR imaging system. The Vc stimulation at intensities provoking paresthesias resulted in activation o f the primary somatosensory cortex (SI). Stimulation at subthreshold intens ities failed to activate the SI. Additional stimulation-coupled activation was observed in the thalamus, the secondary somatosensory cortex (SII), and the insula. In contrast, stimulation of the PVG electrode did not evoke pa resthesias or activate the SI, but resulted in medial thalamic and cingulat e cortex activation. Stimulation in the Vim resulted in thalamic, basal gan glia, and SI activation. An evaluation of the safety of the procedure indicated that significant cur rent could be induced within the electrode if a faulty connecting cable (de fective insulation) came in contact with the patient. Simple precautions, s uch as inspection of wires for fraying and prevention of their contact with the patient, enabled the procedure to be conducted safely. Clinical safely was further corroborated by performing 86 MR studies in patients in whom e lectrodes had been implanted with no adverse clinical effects. This is the first report of the use of fMR imaging during stimulation with implanted thalamic electrodes. The authors' findings demonstrate that fMR i maging can safely detect the activation of cortical and subcortical neurona l pathways during stimulation and that, stimulation does not interfere with imaging. This approach offers great potential for understanding the mechan isms of action of deep brain stimulation and those underlying pain and trem or generation.