Non-invasive in vivo temperature mapping of ultrasound heating using magnetic resonance techniques

A major problem with conventional methods of measuring heating in vivo is that they are invasive and therefore interfere with heat propagation. A sensitive non-invasive method for temperature measurement using in vivo magnetic resonance spectroscopy (MRS) of the temperature dependent chemical shift of the cobalt (III) nucleus has been developed. Initial experiments demonstrate that this technique can be used to measure ultrasound induced temperature changes in the liver. Tris(ethylenediamine) cobalt (III) trichloride was encapsulated in liposomes and injected into seven rats. Heating was performed using a calibrated unfocused transducer operating at 3.41 MHz. After 5 minutes of CW ultrasound exposure, the chemical shift of the cobalt complex indicated that the temperature rise within the liver was 2.0±1.2°C. This was seen to return to normal upon cessation of heating. The acoustic power was determined in a water bath using a calibrated hydrophone. Theoretical calculations based on the transducer calibration characteristics using the monopole-source solution for estimating tissue temperature increase yielded 2.0°C based on steady state conditions. These results indicate that experimental values agree with the heating theory