2293
Comparing RF Heating Based on SAR vs. B1+RMS MRI Conditional Labeling
Negin Behzadian1 and Shiloh Sison2
1Research and Development, Abbott, Sylmar, CA, United States, 2Research and Development, Abbott, Sunnyvale, CA, United States
1Research and Development, Abbott, Sylmar, CA, United States, 2Research and Development, Abbott, Sunnyvale, CA, United States
Synopsis
MRI RF heating can cause thermal damage to biological tissues, especially in patients with leaded implantable devices, by concentrating power deposition at the implant-tissue interface. With the emergence of B1+RMS as a more precise RF exposure metric than SAR, implantable device manufacturers have begun MR conditional labeling to fixed B1+RMS levels, in addition to SAR labeling. Our study characterizes the relationship between RF heating per SAR vs. B1+RMS-based scaling methods in the context of 3T Normal Operating and First Level Controlled Mode MRI exposure.
Introduction
MRI RF heating can cause thermal damage to biological tissues. Assessment of 3T MRI RF-induced heating for active implantable medical devices (AIMDs) requires electromagnetic simulations of clinical RF coils to generate probable RF E-fields that can induce currents along the length of implants, leading to concentrated power deposition and local temperature increment at the implant-tissue interface. The simulated coil excitations must be consistent with the MR conditional labeling, ranging across Normal Operating Mode, First-Level Controlled Operating Mode, fixed whole-body Specific Absorption Rate (wbSAR) or fixed B1+RMS levels. B1+RMS has been recently adopted by AIMD manufacturers as an additional RF exposure metric for MR conditional labeling, given that it is neither estimated nor patient-dependent like SAR. Little has been published however comparing expected RF heating due to SAR vs. B1+RMS levels, which has generated questions when both SAR and B1+RMS levels are specified in MR conditional labeling. Our investigation, therefore aims to characterize equivalence between wbSAR-scaled and B1+RMS-scaled RF heating predictions in the context of 3T Normal Operating Mode and First Level Controlled Mode MRI exposure respectively, with a cardiovascular implantable electronic device (CIED).Methods
We evaluated two different cardiac pacing leads for In Vivo RF MC-2 heating limited by both 3T Normal Operating Mode and First Level Controlled Mode when connected to a CIED. In-vitro temperature rise values were derived for RF exposures scaled to a range of wbSAR values across 0-4.0 W/kg with a maximum B1+RMS scaling of 7 uT, as well as for fixed B1+RMS values across 0.5-5.5 uT with a maximum wbSAR scaling of 2.0 W/kg vs. 4.0 W/kg for Normal and First Level Operating Mode respectively. We identified wbSAR values at which heating predictions were equivalent to specific B1+RMS level, namely 1.3 and 2.8 uT, values which AIMD manufacturers currently use in their MR conditional labeling, as well as B1+RMS values at which heating prediction were equivalent to specific wbSAR levels of 2.0 and 4.0W/kg, maximum limits imposed by Normal and First Level Controlled Operating Mode respectively1-3.Results
The normalized in-vitro temperature rise was quantified for multiple 3T Normal Operating Mode wbSAR-scaled (black) and B1+RMS-scaled (red) RF heating simulations for two different cardiac leads (Figure 1). The same is presented for the constraint of First-Level Controlled Mode in (Figure 2).The (wbSAR, B1+RMS) levels at which the temperature rises are equivalent under Normal Operating Mode, are plotted in Figure 3, approximately identical across the two leads: (0.3, 1.3), (1.5, 2.8), (2.0, 4.5).
The (wbSAR, B1+RMS) levels at which the temperature rises are equivalent under First Level Controlled Operating Mode, are plotted in Figure 4, approximately identical across the two leads: (0.3, 1.3), (1.7, 2.8), (2.0, 3.0), (4.0, 5.6).
Discussion
Our study shows that B1+RMS labeling currently used by AIMD manufacturers such as 2.8 uT 1,2, is equivalent to a wbSAR limit of 1.5W/kg under Normal Operating Mode, and 1.7W/kg under First-Level Controlled Mode. Additionally, a wbSAR limit of 2W/kg is equivalent to 4.5uT under Normal Operating Mode and 4W/kg is equivalent to 5.6uT under First Level Controlled Mode.Conclusion
Under Normal Operating Mode, B1+RMS MR conditional labeling at 3T adopted by manufacturers today is found to have an equivalent wbSAR, in terms of RF heating generated, of less than 2W/kg. Furthermore, using B1+RMS conditional labeling at levels up to 4.5 uT is equivalent to wbSAR up to 2W/kg.Under First Level Controlled Operating Mode, B1+RMS MR conditional labeling at 3T adopted by manufacturers today is found to have an equivalent wbSAR, in terms of RF heating generated, of less than 4W/kg. Furthermore, using B1+RMS conditional labeling at levels up to 5.6 uT is equivalent to wbSAR up to 4W/kg.
It is expected that because of the margin added to the displayed wbSAR in addition to the limits imposed by short-duration SAR, wbSAR is more restrictive than alternative B1+RMS. However if that wbSAR margin were to decrease then typical 3T B1+RMS labeling would restrict sequences above 1.5W/kg and 1.7W/kg for Normal and First Level Controlled Operating Mode.
Acknowledgements
No acknowledgement found.References
- Abbott. MRI Procedure Information for Abbott Medical MR Conditional Deep Brain Stimulation Systems: Clinician’s Manual, 2020. <https://manuals.sjm.com/~/media/manuals/product-manual-pdfs/6/3/631c18a6-6b9d-476f-b938-57c07d53ebe6.pdf>
- Medtronic. AttestaTM MRI SureScanTM /SpheraTM MRI SureScanTM Pacing Systems: MRI procedural information for SureScanTM pacemakers and SureScanTM leads, 2017. <http://manuals.medtronic.com/content/dam/emanuals/crdm/CONTRIB_256428.pdf>
- Boston Scientific. MRI Technical Guide: IMAGEREADYTM MR Conditional Defibrillation System, 2019. <http://irm-compatibilite.com/documents/boston/specific_conditions_boston_defibrillation.pdf>
Figures
Figure
1: Normalized 3T MRI RF-Induced In-Vitro Temperature Rise at Different Fixed
wbSAR and B1+RMS Levels under Normal Operating Mode for Two
Different Cardiac Pacing Leads
Figure
2: Normalized 3T MRI RF-Induced In-Vitro Temperature Rise at Different Fixed
wbSAR and B1+RMS Levels under First Level Controlled Operating Mode
for Two Different Cardiac Pacing Leads
Figure 3: Points of Equivalent 3T MRI RF-induced Heating for B1+RMS
vs. wbSAR Scaling under Normal Operating Mode for Two Different Cardiac Pacing
Leads
Figure
4: Points of Equivalent 3T MRI RF-induced Heating for B1+RMS vs.
wbSAR Scaling under First Level Controlled Operating Mode for Two Different
Cardiac Pacing Leads