In vitro study of MRI - related heating of deep brain stimulation electrodes at 1.5 - tesla

Deep brain stimulation (DBS) is used increasingly in the field of movement disorders. The implanted electrodes create not only a priori risk to patient safety during MR imaging but also a unique opportunity in the collection of functional MRI (fMRI) data conditioned by direct neural stimulation. We performed safety tests of commercially available bilateral neurostimulation systems (Soletra® 7482 Medtronic) extensions and leads in a clinical 1.5 T MR system (Siemens Germany) across a range of MR acquisitions of typical and worst case scenarios. The primary safety risk arises from RF induction of current through the electrode leads. It depends on the RF energy deposited during specific imaging sequences on physical properties of the electrodes and on the low impedance and/or resonant paths within the lead geometry. In vitro testing was performed using a phantom filled with a semisolid gel simulating the thermal convection and dielectric properties of human tissues. Each DBS electrode was positioned with a single extension loop around each pulse generator and a single loop at the "head" end of the phantom. The clinical MRI sequences had calculated RF power depositions ranging up to 1.2 W/kg or 3.4 W/kg for a worst case and was unaffected by turning the stimulators on. Temperature increases at the lead tip measured using a fluoroptic thermometry system (Luxtron CA) were less than 1° C for all clinical imaging sequences and were 2.1° C for the "artificial" worst-case conditions. Temperature rise was unaffected by turning the stimulators on. These results show that while some heating will occur temperature elevations in clinical and fMRI sequences can be within an acceptable safety range.