Magnetic resonance imaging conditionally safe neurostimulation leads: investigation of the maximum safe lead tip temperature.

BACKGROUND: Magnetic resonance imaging (MRI) is preferred for imaging the central nervous system (CNS). An important hazard for neurostimulation patients is heating at the electrode interface induced, for example, by 64-MHz radiofrequency (RF) magnetic fields of a 1.5T scanner.
OBJECTIVE: We performed studies to define the thermal dose (time and temperature) that would not cause symptomatic neurological injury.
METHODS: Approaches included animal studies where leads with temperature probes were implanted in the brain or spine of sheep and exposed to RF-induced temperatures of 37 °C to 49 °C for 30 minutes. Histopathological examinations were performed 7 days after recovery. We also reviewed the threshold for RF lesions in the CNS, and for CNS injury from cancer hyperthermia. Cumulative equivalent minutes at 43 °C was used to normalize the data to exposure times and temperatures expected during MRI.
RESULTS: Deep brain and spinal RF heating up to 43 °C for 30 minutes produced indistinguishable effects compared with 37 °C controls. Exposures greater than 43 °C for 30 minutes produced temperature-dependent, localized thermal damage. These results are consistent with limits on hyperthermia exposure to 41.8 °C for 60 minutes in patients who have cancer and with the reversibility of low-temperature and short-duration trial heating during RF lesion procedures.
CONCLUSION: A safe temperature for induced lead heating is 43 °C for 30 minutes. MRI-related RF heating above 43 °C or longer than 30 minutes may be associated with increased risk of clinically evident thermal damage to neural structures immediately surrounding implanted leads. The establishment of a thermal dose limit is a first step toward making specific neurostimulation systems conditionally safe during MRI procedures.