A method for safety testing of radiofrequency/microwave‐emitting devices using MRI

Abstract

Purpose Strict regulations are imposed on the amount of radiofrequency (RF) energy that devices can emit to prevent excessive deposition of RF energy into the body. In this study, we investigated the application of MR temperature mapping and 10-g average specific absorption rate (SAR) computation for safety evaluation of RF-emitting devices. Methods Quantification of the RF power deposition was shown for an MRI-compatible dipole antenna and a non–MRI-compatible mobile phone via phantom temperature change measurements. Validation of the MR temperature mapping method was demonstrated by comparison with physical temperature measurements and electromagnetic field simulations. MR temperature measurements alongside physical property measurements were used to reconstruct 10-g average SAR. Results The maximum temperature change for a dipole antenna and the maximum 10-g average SAR were 1.83° C and 12.4 W/kg, respectively, for simulations and 1.73° C and 11.9 W/kg, respectively, for experiments. The difference between MR and probe thermometry was <0.15° C. The maximum temperature change and the maximum 10-g average SAR for a cell phone radiating at maximum output for 15 min was 1.7° C and 0.54 W/kg, respectively. Conclusion Information acquired using MR temperature mapping and thermal property measurements can assess RF/microwave safety with high resolution and fidelity.

Heat equation inversion framework for average SAR calculation from magnetic resonance thermal imaging

“…Gaussian noise with 0 mean and standard deviation of 0.035 °C was injected into temperature difference maps. Noise was chosen based on empirical noise figures in MR thermal mapping phantom data in MRI literature [Alon et al, 2015]. HEI reconstruction framework used simulated temperature maps to reconstruct SAR using different lambda values between 0 and 80 in a logarithmic scale.…”

“…Gultekin and Moeller [2013] have shown that magnetic resonance (MR) methods can be used to measure temperature increase due to RF energy absorption, where the magnitude of temperature increase was a function of RF power and exposure time in brain equivalent gels and brain tissue [Gultekin and Moeller, 2013]. Recently, high-resolution MR thermal mapping has been utilized for assessment of RF energy absorption from handheld wireless devices [Alon et al, 2015] that output less than 1W of power.…”

Prospects for Millimeter-Wave Compliance Measurement Technologies [Measurements Corner]

“…E-field probe-based systems [7], electro-optical (EO) probe-based systems [8], and optical fiber thermal sensor-based systems [9] are classified as invasive measurement systems. Infrared (IR) measurement systems [10], thermal scanner systems [11], power density (PD) measurements [12], and optical SAR systems [13], [14] can be classified as noninvasive measurement systems with minimal to no interactions between the device under test (DUT) and the measurement system.…”

“…Recently, the technique has been expanded to accommodate high-frequency wireless device sources that have traditionally been considered to be incompatible with MRI [11]. The method is sensitive to small temperature changes (<0.1°C) and evaluates SAR with millimeter resolution [11]. Recent studies have shown that from the 3-D temperature change, SAR can be computed via direct inversion of the heat equation [26].…”

Estimation of Specific Absorption Rate Using Infrared Thermography for the Biocompatibility of Wearable Wireless Devices

“…MRI temperature mapping and understanding of RF heating while using MRI scanners for MRI compatible devices was given by [13,14]. MRI temperature mapping to characterize heating from wireless devices that are not necessarily MRI compatible and calculation of 10 g average SAR using the heat mappings has been suggested by [15]. Accurate calculation of SAR using magnetic resonance temperature images and measured thermal properties of the phantom have been done by [16].…”