Tissue ultrasound absorption measurement with MRI calorimetry

Tomographic reconstruction of tissue properties and temperature increase for high-intensity focused ultrasound applications

The acoustic and thermal properties as well as the temperature change within a tissue volume during high-intensity focused ultrasound ablation are critically important for treatment planning and monitoring. Described in this article is a tomographic reconstruction method used to determine the tissue properties and increase in temperature in a 3-D volume. On the basis of the iterative finite-element solution to the bioheat equation coupled with Tikhonov regularization techniques, our reconstruction algorithm solves the inverse problem of bioheat transfer and uses the time-dependent temperature measured on a tissue surface to obtain the acoustic absorption coefficient, thermal diffusivity and temperature increase within the subsurface volume. Numerical simulations were performed to validate the reconstruction algorithm. The method was initially conducted in ex vivo experiments in which time-dependent temperature on a tissue surface was measured using high-resolution, non-invasive infrared thermography.

The design and characterization of an ultrasound phased array suitable for deep tissue hyperthermia

In this paper we describe the design and evaluation of a planar phased-array ultrasound transducer suitable for producing localized hyperthermia in solid tumors deep within the body. Simulation using a customized version of Ultrasim has been used to determine the relationship between the size and position of the focus and parameters of the array. These parameters include the overall size of the array and the size, shape and distribution of the individual elements. A 15-element prototype array has been constructed using the results of the simulation. Beam profile measurements on this transducer made in an acoustic tank were compared with the beam profile predicted by simulation. The results showed good agreement in the shape of the focal region, but with the focus closer to the surface of the physical transducer when compared with the simulation and with small high-intensity areas between the surface of the transducer and the focus in the measured profile. A sensitivity analysis using a simulated factorial experiment indicated that the presence of a secondary vibrational mode within the elements of the array was the principal cause for both the shift in the position of the focus and for the unwanted maxima close to the surface of the array. The results also showed that the array was tolerant of a large variation in output intensity of the individual elements in the array in producing a focal region. Extrapolation of the results obtained in this study indicate that an array of 60 elements, based on the design described, driven by 550 V peak-to-peak pulses would be capable of producing a peak focal intensity of 50 Wcm(-2) at a depth of 60 mm in tissue, which would be appropriate for hyperthermia used as an adjunct to radiotherapy or chemotherapy.