Magnetic resonance imaging (MRI) calorimetry for tissue ultrasound (US) absorption measurement

grantor: University of TorontoThe renewed interest in the use of high intensity focused ultrasound (US) for minimally invasive, MRI guided thermal therapy has stimulated a review of the interaction mechanisms of high intensity US with tissue. In particular, the variation of ultrasound absorption with frequency and temperature needs to be well understood to apply this technique optimally. While the study of US properties of tissues has been conducted extensively, agreement on the measured value of tissue US absorption are poor. Available data from various studies show as much as a four fold variation in the US absorption coefficient for the same frequency and tissue at room temperature. This is largely a result of experimental assumptions and the use of invasive thermocouples to measure tissue temperature elevation, which themselves, alter the heating and absorption properties of the tissue. A new approach to measure US tissue absorption has been studied here. It is based on a form of MRI calorimetry which allows non-invasive energy measurement. Validation data show excellent agreement of this method with the actual energy delivered. Calorimetry for US heating of freshly-excised bovine liver tissue sample has been conducted. This, together with hydrophone measurement of the incident US field, has led to a measured US absorption coefficient of 0.0581/cm or 0.504dB/cm (at 1 MHz and room temperature). As this approach can be applied over a range of frequencies, tissues and temperatures, it should provide a much improved means of measuring absolute tissue US absorption coefficients to improve US therapy planning, future transducer design and US dosimetry models.M.Sc