Radiofrequency heating in porcine models with a "large" 32 cm internal diameter, 7 T (296 MHz) head coil

Abstract

Temperatures were measured in vivo in four pigs (mean animal weight = 110.75 kg and standard deviation = 6.13 kg) due to a continuous wave radiofrequency (RF) power irradiation with a 31.75 cm internal diameter and a 15.24 cm long, 7 T (296 MHz), eight channel, transverse electromagnetic head coil. The temperatures were measured in the subcutaneous layer of the scalp, 5, 10, 15, and 20 mm deep in the brain, and rectum using fluoroptic temperature probes. The RF power was delivered to the pig's head for ∼3 h (mean deposition time = 3.14 h and standard deviation = 0.06 h) at the whole head average specific absorption rate of ∼3 W kg(-1) (mean average specific absorption rate = 3.08 W kg(-1) and standard deviation = 0.09 W kg(-1)). Next, simple bioheat transfer models were used to simulate the RF power induced temperature changes. Results show that the RF power produced uniform temperature changes in the pigs' heads (mean temperature change = 1.68°C and standard deviation = 0.13°C) with no plateau achieved during the heating. No thermoregulatory alterations were detected due to the heating because the temperature responses of the pre-RF and post-RF epochs were not statistically significantly different. Simple, validated bioheat models may provide accurate temperature changes.

FIG. 1

A typical location of the fluoroptic probes in the porcine head. The probes were placed at the scalp and 5, 10, 15, and 20 mm in the porcine brain after the dura.

FIG. 2

Absolute temperatures in the scalp, brain, and rectum of an anesthetized, heated animal. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 3

RF power induced temperature changes in the scalp of swine. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 4

RF power induced temperature changes 5 mm deep in the brain of swine. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 5

RF power induced temperature changes 15 mm deep in the brain of swine. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 6

RF power induced temperature changes 10 cm deep in the rectum of swine. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 7

RF power induced temperature changes after 3 h in the pig-head cross-section where the scalp and brain temperature probes were placed.

FIG. 8

Effect of cooling on the RF power induced temperature changes in swine. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]

FIG. 9

Comparisons between the temperature changes produced by a CW and a pulse wave RF power deposition for the whole head average SAR of 3 W kg⁻¹ and forced cooling. [Color figure can be viewed in the online issue, which is available at wileyonlinelibrary.com.]