Abstract
PurposeTo present a modeling workflow for the evaluation of a lead electromagnetic model (LEM) consisting of a transfer function (TF) and a calibration factor. The LEM represents an analytical relationship between the RF response of a lead and the incident electromagnetic field. The study also highlights the importance of including key geometric details of the lead and the electrode when modeling multi-electrode leads.MethodsThe electrical and thermal responses of multi-electrode leads with helical and straight wires were investigated using 3D electromagnetic (EM) and thermal co-simulations. The net dissipated power (P) around each lead electrode and the net temperature increase at the electrodes (ΔT) were obtained for a set of incident EM fields with different spatial distributions. A reciprocity approach was used to determine a TF for each electrode based on the results of the computational model. The evaluation of the calibration factors and the TF validation were performed using the linear regression of P versus the LEM predictions.ResultsP and ΔT were investigated for four multi-electrode leads and four single-electrode leads containing either helical or straight wires. All electrodes of the multi-electrode lead were found to be points of high power deposition and temperature rise. The LEMs for the individual electrodes varied substantially. A significant dependence of the calibration factors on the surrounding tissue medium was also found. Finally, the model showed that the TF, the calibration factor, P and ΔT for multi-electrode leads differ significantly from those for single-electrode leads.ConclusionThese results highlight the need to evaluate a LEM for each electrode of a multi-electrode lead as well as for each possible surrounding medium. It is also shown that the results derived from simulations based on simplified single-electrode leads can significantly mislead multi-electrode lead analyses.
Highlights
Radiofrequency (RF)-induced heating of tissues near an electrode of an active implantable medical device (AIMD) is a potential problem for patients undergoing magnetic resonance imaging because tissue damage may occur for sustained exposure above critical temperatures (MRI) [1,2,3,4,5]
A mesh adaption procedure in HFSS increased the number of mesh elements until the variation of P or ‖S‖max between two consecutive meshes was less than 3%
Left edge center right edge multi-electrode lead with straight wire, 1st electrode multi-electrode lead with straight wire, 8th electrode multi-electrode lead with helical wire, 1st electrode multi-electrode lead with helical wire, 8th electrode single electrode lead with helical wire of ∅ = 1.1 mm single electrode lead with helical wire of ∅ = 0.9 mm single electrode lead with straight wire of ∅ = 0.73 mm single electrode lead with straight wire of ∅ = 1.0 mm
Summary
Radiofrequency (RF)-induced heating of tissues near an electrode of an active implantable medical device (AIMD) is a potential problem for patients undergoing magnetic resonance imaging because tissue damage may occur for sustained exposure above critical temperatures (MRI) [1,2,3,4,5]. Where σ is the electrical conductivity of the surrounding medium, Etotal(v) is the electrical field with the lead in place, Ebackgnd(v) is the electrical field without the lead in place, HSIV is the hot spot integration volume, ΔTtotal is the temperature increase at the electrode with the lead in place, and ΔTbackgnd is the temperature increase at the electrode without the lead in place. Recent publications have analyzed leads containing only one electrode [10,11,12], in spite of the fact that multi-electrode leads are more common in AIMDs. Multi-electrode leads have been analyzed [13,14,15,16,17,18], substantial simplifications of the lead wire structure were made, e.g., substitution of a multi-wire design with a single wire. These geometric simplifications are a concern because they can have a significant impact on safety assessments
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