Introduction

RF-induced heating is a major safety consideration during MRI examinations. To mitigate temperature increase, RF-power is controlled by limiting the specific absorption rate (SAR, W/kg). However, the scanner’s reported SAR is a simplified model-based estimation. During MRI exams an active implanted medical device (AIMD) is prone to RF-induced heating, mostly due to antenna effects. As a result, RF-energy is transformed into a temperature increase that can result in tissue heating, especially at the lead-tip. Because the interactions are localized at the device-tissue interface, the scanner’s reported SAR is an inadequate method to estimate RF-induced heating. Therefore, in general, analysis of RF-induced AIMD lead-tip heating is performed following the F2182−11a ASTM method, wherein the temperature increase is measured using fiber optic temperature probes that are placed near the device under examination. RF-induced heating depends on the immersive media’s electrical properties: conductivity (σ) and the dielectric permittivity (ε). While σ represents the ease with which RF-current conducts. ε characterizes the propagation of the electric-field in the media relates to the media’s polarization and the dissipation of electric energy into heat or loss factor (ε”; proportional to σ/ω, where ω is the RF-frequency) [1]. It has been shown that σ increases with temperature (Figure 1). The latter suggests that the baseline temperature (T_B) of the immersive media will affect RF-induced device heating. B₁ mapping techniques have been proposed as a method to estimate the media’s electrical properties [2]. Therefore, B_1-maps can carry information related to the rate of temperature increase. The objective of this work was to analyze RF-induced heating for titanium ASTM-reference rod placed in the ASTM phantom. Temperature data along with B₁-maps were acquired for different T_Bs. The media’s electrical properties were also analyzed.

Methods

Experiments were performed on an ASTM-phantom filled with poly-acrylic acid (PAA) gel at 3T (Siemens, Skyra). A 13cm, 1/8inch titanium ASTM-reference rod was placed on the left side of the phantom and at 9cm from the anterior position (Figure 2-A). This corresponds to a common AIMD implant location. Thermal data was acquired using fluoroptic temperature probes (Lumasense Technologies). One probe was fixed in a tip-to-tip configuration at the end of the rod. RF-induced heating experiments were separated into two parts:
Part-1 analyzed the effect of T_B on RF-induced heating only (Figure 2-B). Temperature data was recorded at SAR~4 W/kg for four repeated runs of a turbo spin-echo sequence (5 minutes each) with intervals of one minute between each run. T_B was defined as the mean temperature 30 seconds before each TSE sequence started.
Part-2 analyzed the relationship between the electrical properties and T_B (Figure 2-C). Experiments were repeated on the same phantom for two T_Bs. The first T_B corresponded to the scanner’s room temperature (18.7C); whereas the second T_B corresponded to the phantom warmed outside the scanner room (31.3C). For both T_Bs, RF-induced heating experiments were performed (15 minutes of RF exposure ~4 W/kg), σ measurements and B₁-maps were acquired. σ was measured using a conductivity meter, whereas the B₁-maps were acquired using an EPI double angle technique [3]. From the B₁-maps, ε-maps for both T_Bs were estimated [4].

Results

The results for Part-1 are shown in Figure 3. The temperature increase (𝚫T) for the four consecutive runs are shown in Figure 3-A. After each run, T_B increased but 𝚫T decreased. Maximum 𝚫T during RF-exposure (𝚫T_max) is shown in Figure 3-B. When T_B increased, 𝚫T_max decreased. For the first T_B (30.4C) 𝚫T_max was 4.7C. For the last run (T_B: 34.5C) 𝚫T_max was 2.6C.
The results for Part-2 are shown in Figure 4. Figure 4-A shows the calculated B₁-maps for both T_Bs (18.7C and 31.3C). Results suggest that the magnitude of B₁ decreased with increasing temperature. The estimated ε (Figure 4-B) shows an increased value with increasing temperature. σ also increased when increasing Temperature (0.49 S/m vs. 0.58 S/m). 𝚫T curves for both T_Bs are shown in Figure 4-C. As observed in the experiments in Part-1, RF-induced heating decreased when T_B increased. For a T_B=18.7C, 𝚫T_max was 8.3C. On the other hand, For T_B=31.3C, 𝚫T_max was 5.3C.

Discussion and Conclusion

Results here show that RF-induced heating decreases when the media baseline temperature increases. The latter is attributed to the increased value of the media’s electrical properties with temperature. Therefore, temperature control and monitoring of the media is important for device testing during RF exposure. Similar results have been observed during RF heating at 27 MHz [5]. Where an inverse relationship between the loss factor (ε”) and RF-heating rate was observed. Consequently, for greater ε”, a smaller amount of RF-energy was absorbed by the analyzed sample. Lastly, these results suggest that the evaluation of RF-induced heating of medical devices can benefit from the analysis of the media’s electrical properties using MRI-based techniques. More specifically, B₁-maps are a promising approach to measure the change of electrical properties when increasing temperature. Further work needs to be performed to more precisely calculate electrical properties from B₁-maps.

Acknowledgements

This project was supported, in part, by NIH R01 HL131823 to DBE