An MRI calorimetry technique to measure tissue ultrasound absorption

High-intensity focused ultrasound (US) surgery guided by magnetic resonance imaging (MRI) is a very promising form of minimally invasive thermal therapy. To apply this technique optimally, the interaction mechanisms of high-intensity US with tissue need to be better understood, in particular, the variation of ultrasound absorption with frequency and temperature. However, agreement on the value of measured tissue US absorption is poor, largely because of intrinsic experimental complications of prior investigations. A new approach toward measuring tissue US absorption, based on a form of MRI calorimetry, is proposed here, which allows non-invasive energy measurement through spatial temperature mapping with MRI. A modified two-dimensional spoiled gradient-echo sequence has been implemented to map temperature based on proton resonance frequency (PRF) shift. Validation experiments show excellent agreement of MRI measured energy with that delivered by a calibrated source. MRI calorimetry of US heating of tissue-mimicking polyethylene glycerol material has been performed. Using a hydrophone measurement of the incident US field, its US absorption coefficient was measured as 0.032 cm-1. 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.