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.
ABBREVIATIONS: ASTM, American Society for Testing and Materials
CEM, cumulative effect minutes
CNS, central nervous system
DBS, deep brain stimulation
SAR, specific absorption rate
SCS, spinal cord stimulation
WBHT, whole-body human hyperthermia
*Research, Clinical, and Medical Safety, Medtronic Neuromodulation, Minneapolis, Minnesota;
‡Research, Medtronic Neuromodulation, Minneapolis, Minnesota
Correspondence: Robert J. Coffey, MD, 353 Dryden St, Thousand Oaks, CA 91360. E-mail: email@example.com
Received August 05, 2013
Accepted October 28, 2013