Background
One of the key factors in successful hyperthermia treatment is the measurement and control of temperature in the tumor and also in surrounding normal tissue. While invasive thermometry provides accurate and precise measurements, complete temperature mapping of a region using imaging is expected to afford improvements in the control of the temperature therapy distribution. Previous work with magnetic resonance imaging (MRI) has shown the value of using the temperature sensitivity of the tissue water proton resonant frequency shift temperature change [Carter DL etal, Int J Rad Onc Biol Phys 40, 815-822, 1998]. However, tissues containing a mix of water and lipids, e.g. breast, confound most standard frequency shift approaches since lipids have no chemical shift dependencies with temperature change. Separation of the two components should provide a temperature sensitive and an insensitive reference value.

Methods
We used the IDEAL [Reeder S, etal, Magn Res Med 51, 35-45, 2004] model for water and fat separation using three gradient-echo MRI measurements at different echo times. A room temperature and a chilled sample of heavy cream (about 50% fat, 50% water) were scanned at 3Tesla (Siemens Trio) using a CP head coil. 18 time points over 85 minutes were taken while the chilled cream warmed to near room temp. Images were acquired at each time point with TR=100ms, FOV=180mm, 128x128 pts, one coronal slice 5mm thick, 310 Hz/pt and TE=[3.38, 4.17, 4.97ms]. Luxtron fluorescent probes measured actual temps after each acquisition.

Results
The fat-water phase difference change in the chilled cream from first to last time point was 21.6±0.95 degrees. This equates to an 11.6°C temperature change (assumes 124 MHz, 4.17ms TE, 0.01 ppm/°C). The Luxtron probes showed a temperature change of 11.1°C (±0.5°C) in chilled cream and 0.5°C in the room temp cream sample.

Conclusion
IDEAL water-fat separation methods that measure temperature dependent phase shifts using fat as an internal phase reference show great promise as a new approach for MR thermometry in fatty tissues such as the breast.