The rapid and successful advancement in ultra-high field (UHF) magnetic resonance (MR) scanners (7 tesla (T) or higher) has led to improvement in the spatial and temporal resolution and the signal-to-noise ratio (SNR) per unit time of MR images. However, imaging of human-sized objects at high frequency (Larmor frequency of 1H ≥ 300 MHz) presents a key limitation quantified by an excessive radiofrequency (RF) power deposition and localized tissue heating due to the shortened in-tissue wavelength. The safety index provided by current MRI scanners is the global specific aborption rate (SAR), which measures the power delivered per mass of tissue, whereas local SAR is generally not given and hard to predict.
To date, existing MR safety-assessment methods based on temperature or electric field measurement are unable to assess the local SAR accurately and non-invasively in vivo.
In 2017, Winkler et al. proposed a direct SAR monitoring tool based on the thermoacoustic effect. During the RF transmission phase, the RF energy absorbed by the biological tissue increases the local temperature causing local thermal expansion, resulting in a pressure distribution that propagates through the human body as an acoustic wave. The lab’s research aims to benefit from the thermoacoustic effect to develop a setup consisting of an array of transducers to predict spatially varying local SAR patterns in vivo at 7T.