Two-dimensional temperature estimation using diagnostic ultrasound.

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.

A hybrid computational model for ultrasound phased-array heating in presence of strongly scattering obstacles.

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 phased-array applicator systems for noninvasive ablative thermal surgery in the heart and liver through the rib cage.

Multipoint temperature control during hyperthermia treatments: theory and simulation.

A real-time multipoint feedback temperature control system has been designed and implemented with an ultrasound phased-array applicator for hyperthermia. The control parameters are the total power available from the supply and the dwell times at a sequence of preselected heating patterns. Thermocouple measurements are assumed for temperature feedback. The spatial operator linking available heating patterns to temperature measurements is measured at the outset of the treatment and can be remeasured on line an adaptive implementation. A significant advantage of this approach is that the controller does not require a priori knowledge of either the placement of the thermocouples or the power distribution of the ultrasound heating patterns. Furthermore, the control loop uses a proportional integral (PI) gain in conjunction with a singular value decomposition (SVD) of the spatial transfer operator. This approach is advantageous for robust implementation and is shown to properly balance the power applied to the individual patterns. The controller also deals with saturation in the inputs without integrator windup and, therefore, without temperature overshoot. In this paper, we present the theoretical formulation and representative simulation results of the proposed controller. The control algorithm has been verified experimentally, both in vitro and in vivo. A subsequent paper describing these results and the practical implementation of the controller will follow.

Dynamic focusing in ultrasound hyperthermia treatments using implantable hydrophone arrays.

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.