Radiofrequency heating at 9.4T: in vivo temperature measurement results in swine

Abstract

In vivo temperatures were correlated to the whole head average specific absorption rate (SAR(avg)) at 9.4T using 12 anesthetized swine (mean animal weight = 52 kg, standard deviation = 6.7 kg). Correlating the temperatures and SAR(avg) is necessary to ensure safe levels of human heating during ultra-high field MR exams. The temperatures were measured at three depths inside the brain, in the rectum, and at the head-skin of swine. A 400 MHz, continuous wave RF power was deposited to the head using a volume coil. The SAR(avg) values were varied between 2.7-5.8 W/kg. The RF power exposure durations were varied between 1.4-3.7 hr. To differentiate the temperature response caused by the RF from that of the anesthesia, the temperatures were recorded in four unheated swine. To study the effect of the spatial distribution of the RF and tissue properties, the temperature probes were placed at two brain locations (n = 4 swine for each location). Results showed that the in vivo brain temperatures correlated to the SAR(avg) in a geometry-dependent manner. Additionally, 1) the skin temperature change was not the maximum temperature change; 2) the RF heating caused an inhomogeneous brain temperature distribution; and 3) the maximum temperature occurred inside the brain.

FIG. 1

Normalized temperature change vs. normalized RF heating duration at the depth of 10 mm for Location 1 with corresponding average parametric curve and 95% CI curves.

FIG. 2

Normalized temperature change vs. normalized RF heating duration at the depth of 10 mm for Location 2 with corresponding average parametric curve and 95% CI curves.

FIG. 3

Normalized temperature change vs. normalized RF heating duration at the head-skin with corresponding average parametric curve and 95% CI curves.

FIG. 4

Normalized rectal temperature change vs. normalized RF heating duration with corresponding average parametric curve and 95% CI curves.

FIG. 5

Average normalized parametric curves for the brain, rectum, and head-skin.

FIG. 6

Temperatures in the brain and head-skin of an anesthetized, unheated animal.

FIG. 7

Temperatures in the brain, rectum, and head-skin of an anesthetized, heated animal.