Thermal Risks Induced by Active Implantable Medical Devices in MRI Exams with Fast Gradient Field Switching
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PO-GePV-I-43 (Sunday, 7/10/2022) [Eastern Time (GMT-4)]
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Purpose: It is well known that thermal risk in patients with MRI exams is primarily caused by RF transmission, and thermal risk from gradient field switching is insignificant due to its low frequency. However, for patients with active implantable medical devices (AIMD), thermal risk induced from the medical devices by fast switching gradient field may be significant. This study aims to compute the induced thermal risk in patients with AIMD from fast switching gradient field.
Methods: Sim4life v6.2 (ZMT Zurich MedTech AG, Switzerland) with a low frequency quasi static solver was used to simulate electromagnetic fields and thermal risk induced by fast switching gradient fields of gradient coils. An ASTM human body phantom with an embedded cardiac pacemaker including a generator and a lead was used for the simulation. Frequencies and gradients of switched gradient fields were set to be in the range used in typical MRI EPI pulse sequences. Specific absorption rate (SAR) over tissues around the generator and lead in the phantom were evaluated at different scanning conditions, including various generator orientations to the gradient field.
Results: SARs induced from the generator were at least 100 times higher than that from the lead tip. When the gradient switching frequency is less than 0.25 kHz, SARs from the generator were less than 1 W/kg. However, when the gradient switching frequency was increased to 0.5 and 1 kHz, SARs induced from the generator were significantly increased, especially when orientations of the generator was varied. In the worst scenario with 20 mT gradient field, 1 kHz gradient switching, and the generator is perpendicular to the gradient direction, the induced SAR from the generator was the highest at 13.22 W/kg.
Conclusion: Fast switching gradient fields of some MRI pulse sequences can impose non-negligible thermal risks in patients with AIMD during MRI exams.
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