Temperature and SAR measurement errors in the evaluation of metallic linear structures heating during MRI using fluoroptic® probes

Abstract

The purpose of this work is to evaluate the error associated with temperature and SAR measurements using fluoroptic temperature probes on pacemaker (PM) leads during magnetic resonance imaging (MRI). We performed temperature measurements on pacemaker leads, excited with a 25, 64, and 128 MHz current. The PM lead tip heating was measured with a fluoroptic thermometer (Luxtron, Model 3100, USA). Different contact configurations between the pigmented portion of the temperature probe and the PM lead tip were investigated to find the contact position minimizing the temperature and SAR underestimation. A computer model was used to estimate the error made by fluoroptic probes in temperature and SAR measurement. The transversal contact of the pigmented portion of the temperature probe and the PM lead tip minimizes the underestimation for temperature and SAR. This contact position also has the lowest temperature and SAR error. For other contact positions, the maximum temperature error can be as high as -45%, whereas the maximum SAR error can be as high as -54%. MRI heating evaluations with temperature probes should use a contact position minimizing the maximum error, need to be accompanied by a thorough uncertainty budget and the temperature and SAR errors should be specified.

Feasibility of simultaneous intracranial EEG-fMRI in humans: A safety study

“…Using a similar setup as the one used in this work, a 500% uncertainty in temperature values was reported, linked to non-optimal positioning of the temperature-sensing optical-fibre tips relative to the electrode contacts [39]. In our study the temperature probes were therefore positioned transversely relative to the electrode contacts enabling reproducible measurements [17,39].…”

“…In our study the temperature probes were therefore positioned transversely relative to the electrode contacts enabling reproducible measurements [17,39]. Uncertainty in the physical properties of the phantom gel may have added to uncertainty in temperature measurements, although this effect is likely small considering the consistency of results obtained in this and our previous study [17].…”

“…In light of these results the temperature sensors were sited at the following positions for experiment set A temperature-sensor fibre tips were of comparable scale to the electrodes [17], ensuring they were sensitive to localised temperature changes adjacent to the electrode. The sensors were placed such that they lay in a transverse position relative to the electrode contacts to minimise measurement error [17,39].…”

Role of the lead structure in MRI‐induced heating: In vitro measurements on 30 commercial pacemaker/defibrillator leads

“…The background noise was in the range of Luxtron resolution. For passive-fixation leads, the terminal portion of the temperature probes were placed in transversal contact (i.e., the probe is perpendicular to the body of the lead-wire axis) with the lead tip: this contact configuration was demonstrated to minimize the measurement errors associated to the physical dimensions of the probes (27). For active-fixation leads, the temperature sensor was placed inside the helix tip of the lead (Fig.…”

“…In addition, also the temperature probe positioning differs for the two types of lead: for active implants, the temperature sensor (pigmented terminal portion of the optic fibre) is inserted into the screw and it is in contact with the metal electrode for almost its entire length (Fig. 2); for passive-fixation leads, the temperature probe is placed in transversal contact with the lead tip, causing a measurement error that has to be taken into account (27). In any case, the two different ways used to measure the temperature at the tip of active and passive implants are the ones that most closely reproduce the realistic contact situations between the pacing electrode and the heart tissue for the two systems.…”