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.