Mitigation of Radio-frequency Induced Active Implantable Medical Device Heating During MRI Exams
MRI examinations for patients with active implantable medical devices (AIMDs) pose significant safety concerns. Among them, induced currents by the radiofrequency (RF) electromagnetic field can result in tissue damage. While the magnetic component of the RF field (B1) is necessary for spin excitation, the electric component does not have a significant role during an MRI exam but it is mostly responsible of the induction of currents that get concentrated by the AIMD. The induced current propagates in the AIMD lead and when it reaches the end of the lead, a temperature increase is observed at the lead,
commonly known as lead-tio heating (LTH), which can damage the surrounding tissue. A large number of factors impact the magnitude of the induced currents. Therefore, the analysis can be extremely complex, which contributes to difficulty associated with understanding how to safely examine patients with an AIMD using MRI. Consequently, and unfortunately, MRI examinations for patients with AIMDs are often denied. To reduce safety risks, device manufacturers have created MR-conditional AIMDs which under specific conditions safety risks are reduced. The conditions often include the specific absorption rate (SAR). SAR is a measure to estimate RF exposure measured by the MRI scanner which is often limited to a certain value to mitigate temperature increase. However, meeting SAR conditions can be challenging. In this dissertation, a thorough examination of the parameters that affect RF-induced AIMD heating is performed. Novel techniques for mitigating device heating are addressed including a workflow that provides guidance for modifying protocols to reduce the applied RF power with limited impact on diagnostic image quality. Additional work provides an analysis of the magnitude of the RF electric fields with the aim of reducing the induced device heating.
The motivation behind this thesis is provided in Chapter 2. A review of the risks associated with MRI exams for patients with MR-conditional and non-conditional cardiac AIMDs. The current clinical practice for performing MRI examinations is provided in Chapter 3. Chapter 4 is a brief introduction to the physics behind the static magnetic and time varying electromagnetic fields used in MRI. A brief introduction to nuclear magnetic resonance (NMR) is also provided.
Chapter 5 main goal is to find a method to allow MRI clinics to modify standard protocols while limiting the effect on image quality and scan time. To do so, a workflow
that modifies SAR dependent sequence parameters in a specific order was suggested and tested for head, C-spine, L-spine and T-spine vendor provided protocols. The protocols were modified to meet SAR levels as low as 0.1 W/kg which is ~95% less than SAR for conventional examinations. We believe that the proposed workflow will solve the potential examination denials of MR-conditional devices due to the lack of guidance to meet really low SAR conditions.
Chapter 6 presents a simple method for reducing RF induced lead-tip heating (LTH) for patients with MRI unsafe cardiac AIMDs. LTH is known to depend on the lead-path and distance to isocenter. Herein we suggest that by simply changing patient orientation from a head-first to a feet-first orientation LTH can be mitigated. The principle behind this is steeped in the fact that the RF electric field is spatially asymmetric and temperature increase is proportional to the magnitude of the electric field. Thus, by moving to a lower E-field magnitude, LTH can be reduced. Hence, for MR-unsafe cardiac AIMDs implanted on a left side switching from a head-first to a feet-first orientation was then suggested. To do so temperature data was acquired for MR-unsafe cardiac devices following different lead paths and at nine positions within the human body. The results show that changing orientation can reduce LTH. There was no scenario in which LTH was significantly worse in a feet-first orientation.
In Chapter 7 a comparison of LTH at 1.5T and 3T is provided. With 1.5T being the standard field strength used for scanning patients with cardiac AIMDs, examinations at 3T are generally contraindicated. The principal motivation is that SAR increases with the square of field strength, thus the rationale suggests that for patients with cardiac AIMDs, RF-induced device heating will increase as well. Notably, however, the interactions
between the cardiac AIMD and the RF-electromagnetic field are subject to resonant length effects, hence LTH may not be worse. In addition to resonant effects, LTH depends