A two-dimensional temperature estimation method was developed based on the detection of shifts in echo location of backscattered ultrasound from a region of tissue undergoing thermal therapy. The echo shifts are due to the combination of the local temperature dependence of speed of sound and thermal expansion in the heated region. A linear relationship between these shifts and the underlying tissue temperature rise is derived from first principles and experimentally validated. The echo shifts are estimated from the correlation of successive backscattered ultrasound frames, and the axial derivative of the accumulated echo shifts is shown to be proportional to the temperature rise. Sharp lateral gradients in the temperature distribution introduce ripple on the estimates of the echo shifts due to a thermo-acoustic lens effect. This ripple can be effectively reduced by filtering the echo shifts along the axial and lateral directions upon differentiation. However, this is achieved at the expense of spatial resolution. Experimental evaluation of the accuracy (0.5 degrees C) and spatial resolution (2 mm) of the algorithm in tissue mimicking phantoms was conducted using a diagnostic ultrasound imaging scanner and a therapeutic ultrasound unit. The estimated temperature maps were overlaid on the gray-scale ultrasound images to illustrate the applicability of this technique for image guidance of focused ultrasound thermal therapy.

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A prototype 16-element needle hydrophone array has been designed, fabricated and characterized. The primary use of this array is to provide acoustic feedback during ultrasound hyperthermia treatments. This feedback can be used to compensate for patient motion and tissue inhomogeneities by controlling the phased array driving patterns. It can also be used in adaptive dynamic focusing, a procedure which enables the phased array to focus at points away from specified control points. The hydrophone array consists of a PVDF sheet, which covers a silicon substrate carrier that contains the signal electrodes of the individual acoustic sensors. A complete description of the hydrophone array and its characteristics is given in this paper. The aberration correction and motion compensation algorithms are also described, and some experimental results are shown. Finally, a Taylor series based adaptive dynamic focusing method for phased arrays based on a set of discrete hydrophone array measurements is described. This algorithm does not require any prior knowledge of the applicator geometry and all the parameters needed for correction can be measured directly at the hydrophone array sensor locations.

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A computationally efficient hybrid ray-physical optics (HRPO) model is presented for the analysis and synthesis of multiple-focus ultrasound heating patterns through the human rib cage. In particular, a ray method is used to propagate the ultrasound fields from the source to the frontal plane of the rib cage. The physical-optics integration method is then employed to obtain the intensity pattern inside the rib cage. The solution of the matrix system is carried out by using the pseudo inverse technique to synthesize the desired heating pattern. The proposed technique guides the fields through the intercostal spacings between the solid ribs and, thus, minimal intensity levels are observed over the solid ribs. This simulation model allows for the design and optimization of large-aperture phasedarray applicator systems for noninvasive ablative thermal surgery in the heart and liver through the rib cage.