¹H MRI phase thermometry <i>in</i><i>vivo</i> in canine brain, muscle, and tumor tissue

TLDR

The chemical shift of water changes by ~0.01 ppm/°C, enabling temperature monitoring in vivo or in vitro through phase variations in gradient‑echo MR images. At 1.5 T, the authors heated canine brain, muscle, and gel phantoms with a 130 MHz RF annular phased‑array hyperthermia antenna while simultaneously acquiring 64 MHz images, using band‑pass filters that isolated the RF fields by >100 dB to preserve image SNR. Phase sensitivity of 4.41 ± 0.02 °/°C in gel and 3.0–3.2 °/°C in brain, muscle, and tumor at TE = 20 ms allowed detection of 0.22 °C changes in 50 mm³ and 0.6 °C changes in 16 mm³ volumes within <10 s, demonstrating the method’s usefulness despite sensitivity to field inhomogeneity, stability, and motion.

Abstract

The temperature sensitivity of the chemical shift of water (approximately 0.01 ppm/°C) provides a potential method to monitor temperature changes in vivo or in vitro through the changes in phase of a gradient‐echo magnetic resonance (MR) image. This relation was studied at 1.5 T in gel materials and in vivo in canine brain and muscle tissue, heated with a radio frequency (rf) annular phased array hyperthermia antenna. The rf fields associated with heating (130 MHz) and imaging (64 MHz) were decoupled using bandpass filters providing isolation in excess of 100 dB, thus allowing simultaneous imaging and rf heating without deterioration of the MR image signal‐to‐noise ratio. In a gel, temperature sensitivity of the MR image phase was observed to be (4.41±0.02) phase degrees/°C for T e =20 ms, which allowed temperature changes of 0.22 °C to be resolved for a 50‐mm 3 region in less than 10 s of data acquisition. In vivo , for T e =20 ms, the temperature sensitivity was (3.2±0.1) phase degrees/°C for brain tissue, (3.1±0.1) phase degrees/°C for muscle, and (3.0±0.2) phase degrees/°C for a muscle tumor (sarcoma), allowing temperature changes of 0.6 °C to be resolved in a 16‐mm 3 volume in less than 10 s of data acquisition. We conclude that, while the technique is very sensitive to magnetic field inhomogeneity, stability, and subject motion, it appears to be useful for in vivo temperature change measurement.