8.0-Tesla human MR system: temperature changes associated with radiofrequency-induced heating of a head phantom

To investigate if the heat induced in biological tissues by typical radio frequency (RF) energy associated with an 8.0-Tesla magnetic resonance (MR) system causes excessive temperature changes.

Fluoroptic thermometry was used to measure temperatures in multiple positions in a head phantom made of ground turkey breast. A series of experiments were conducted with measurements obtained at RF power levels ranging from a specific absorption rate (SAR) of up to 4.0 W/kg for 10 minutes.

The highest temperature increases were up to 0.7 degrees C. An inhomogeneous heating pattern was observed. In general, the deep regions within the phantom registered higher temperature increases compared to the peripheral sites.

The expectation of an inhomogeneous RF distribution in ultra high field systems (> 4 T) was confirmed. At a frequency of 340 MHz and in-tissue RF wave length of about 10 cm, the RF inhomogeneity was measured to create higher temperatures in deeper regions of a human head phantom compared to peripheral tissues. Our results agree with the computational electromagnetic calculations for such frequencies. Importantly, these experiments indicated that there were no regions of heating that exceeded the current FDA guidelines.