MR Safety and In Vivo Thermal Characterization of an RF Coil at 9.4T

Correlating In vivo temperatures to the radio-frequency (RF) coil induced total RF power is necessary to ensure human safety in an ultra high field magnetic resonance (MR) application. Thus to ensure human safety in an ultra high field MR head imaging experiment, temperatures were measured as a function of time in the brain and surrounding cutaneous layer of twelve human sized, anesthetized swine (mean animal weight = 52kg, SD = ±6.7kg). In vivo temperatures were correlated to the RF power by developing coil and geometry specific normalized temperatures such that the RF coil induced cranial temperature change could be obtained during an MR exam by measuring only the whole head average specific absorption rate (ASAR) and the duration of the RF deposition. Thus, the feasibility of the thermal characterization of an RF volume head coil was shown. More specifically, a continuous wave (CW) RF was deposited in porcine cranium using a four loop RF head coil at 400 MHz (proton larmor frequency at 9.4T). Temperatures were recorded continuously using an inline probe placed at a predetermined location of 15mm inside the brain and a separate probe in the cutaneous layer. To differentiate the temperature response caused by the RF from that of anesthesia, the temperatures were recorded in four unheated, anesthetized swine for the complete duration of experiments (∼8hours). To study the effect of the spatial distribution of the RF as well as the tissue thermal/electrical properties and blood perfusion, the inline temperature probe was placed at two locations (N = 4 for each location). Results showed that the thermal characterization of an RF coil was possible such that the normalized temperature maps when multiplied by the ASAR and the RF heating time would predict In vivo temperature change during heating. Further, it was shown that at 9.4 T 1) the RF heating caused an inhomogeneous normalized temperature distribution in the brain; and 2) the skin temperature change was an unreliable parameter to assess In vivo temperature change.