The aim of the study was to test the hypothesis that fast spin echo T 1 -weighted images can be used to quantify the temperature in fat during thermal therapy in vivo. An MR compatible positioning device was used to manipulate focused ultrasound transducers in an MRI scanner. This system was used to sonicate fat tissue around the kidneys of 12 rabbits at various power levels for 10 to 20 sec. The scan parameters of T 1 -weighted fast spin echo (FSE) sequence were varied to optimize signal intensity characteristics while maintaining short scan times. An invasive optical probe was used to calibrate the temperature related signal intensity changes. For the T 1 -weighted FSE sequence, the signal intensity decreased with the temperature elevation at the rate of 0.97 ุ 0.02%/ o C. The single focused transducer produced a contrast-to-noise ratio more than 10 at power levels below the tissue damage threshold. The signal intensity was linearly dependent on the power, despite the measured temperatures being well above the coagulation threshold. This study demonstrates that T 1 -weighted FSE MRI sequences can be used to quantify the temperature elevation in fat in vivo during short focused ultrasound exposures. This can be very important for breast tumor surgery, fat ablation, and for treating deep seated tumors through superficial fat layers. Magn Reson Med 43:901-904, 2000.