Room: Exhibit Hall | Forum 8
Purpose: MR Conditional implantable pulse generators (IPGs, e.g., neurostimulators, pacemakers, and implantable cardiac defibrillators) are typically designed and tested for diagnostic field strengths (1.5 and 3 T). MRI-guided radiation therapy (MR-IGRT) is routinely performed at low-field to minimize electron return effects. One of the promising applications of MR-IGRT is stereotactic cardiac radiosurgery in ventricular tachycardia patients that often have cardiac implantable electronic devices (CIEDs). However, treatment of patients with IPGs that are not cleared for low-field MRI constitutes off-label use and requires a benefit/risk assessment and its acceptance by the physician and patient. In this study, we examined the relative risks of RF heating of IPGs with leads at low field.
Methods: The risk of RF heating was considered based on antenna theory. RF wavelengths in tissue decreases with increasing Larmor frequency. Lead resonance is most likely to occur when the RF wavelength in tissue is equivalent to multiples of Î»/2. The RF wavelength (Î») was compared for various tissues at 0.35, 1.5, and 3 T. SAR was estimated for the 2D and 3D true fast imaging with steady state precession (TrueFISP) sequences used in low-field MR-IGRT.
Results: Assuming IPG lead lengths of 20-40 cm, resonance was most likely at 1.5 T and least likely at 0.35 T. SAR in normal tissues from the TrueFISP sequence was <0.5 W/kg.
Conclusion: The risks of treating patients with IPGs at low field are unknown unless the device is properly modeled and tested at the given field strength based on the actual geometry in the body and relative to the scanner. We need IPG manufacturers to test and verify device safety at low field to ensure patients with IPGs can be safely treated. Nevertheless, the risk of lead resonance should decline with low-field due to the longer RF wavelength and lower SAR.
Funding Support, Disclosures, and Conflict of Interest: Washington University in St. Louis receives research funding, and consulting and speaking fees from ViewRay although such research funding was not used in this study. Dr. Gach owns common stock shares in ViewRay.