Noninvasive localized ultrasonic measurement of tissue properties

We present in vitro and in vivo results validating a new localized noninvasive ultrasonic measurement of both tissue absorption and perfusion. The method employs sub-second low-intensity focused ultrasonic beams to generate a brief temperature rise on the order of 1°C. The RF data from an imaging transducer is processed to produce a noninvasive temperature estimate in the localized heated volume using published speckle tracking or frequency-domain algorithms (Seip, R., 1996; Simon, C. et al., 1998). Absorption can be obtained from the initial heating rate, while perfusion can be estimated from the initial decay. Due to the small size of the heated spot, the noninvasive temperature estimation can produce results that are virtually free of thermal lensing effects. Furthermore, since the measurements are based on heating and decay rates, only the parameters of the transient bioheat equation (density and heat capacity) are needed for the estimate. Both ex vivo and in vivo results show two-fold to three-fold increases in tissue absorption. For the ex vivo results, the change in absorption was estimated using direct fine wire thermocouple measurements at the treatment site in addition to the noninvasive temperature estimation. The decay rate of the in vivo estimated temperature increased by two-fold, indicating increased perfusion in the tumor surrounding the small lesion. This is very likely in the single-shot lesion formation experiment, while the opposite effect can be expected in volumetric lesion formation. The in vivo results show very clearly the feasibility of perfusion estimation based on decay rate.