To increase the survival rates of patients with breast cancer, an ultrasound imaging system must detect tumors when they are small, with a diameter of 5 mm or less. This requires an understanding of how propagation of ultrasound energy is affected by the complex structure of the breast. In this paper, a Finite-Difference Time-Domain (FDTD) method is developed to simulate ultrasound propagation in a two-dimensional model of the human breast. The FDTD simulations make it possible to better understand the behavior of an ultrasound signal in the breast. For example, here the simulations are used to investigate the effect of fat lobes adjacent to the skin layer in a simple breast model. Experimental work performed at the University of Pennsylvania has shown that strong refraction caused by the fat lobes results in nulls in the forward transmitted field. This result was duplicated with the FDTD simulations, and it was shown that the effect of refraction is clearly evident for energy exiting the breast. The existence of strong refraction has a significant impact on ultrasound imaging since it implies that an imaging method based on a weak scattering assumption is unlikely to work well.

In recent publications [Chew et al., IEEE Trans. Blomed. Eng. BME-9, 218-225 (1990); Borup et al., Ultrason. Imaging 14, 69-85 (1992)] the inverse imaging problem has been solved by means of a two-step iterative method. In this paper, a third step is introduced for ultrasound imaging of the breast. In this step, which is based on statistical pattern recognition, classification of tissue types and a priori knowledge of the anatomy of the breast are integrated into the iterative method. Use of this material classification technique results in more rapid convergence to the inverse solution--approximately 40% fewer iterations are required--as well as greater accuracy. In addition, tumors are detected early in the reconstruction process. Results for reconstructions of a simple two-dimensional model of the human breast are presented. These reconstructions are extremely accurate when system noise and variations in tissue parameters are not too great. However, for the algorithm used, degradation of the reconstructions and divergence from the correct solution occur when system noise and variations in parameters exceed threshold values. Even in this case, however, tumors are still identified within a few iterations.

In this paper a model based on transmission line theory is used to predict the behavior of an eccentrically coated asymmetric antenna applicator for use in intracavitary hyperthermia. Theoretical results for the heating rate (HR) of the applicators are compared to experimental results. The experimental results were obtained at City of Hope National Medical Center using four different 915-MHz applicators, each with a different antenna size and eccentricity of the coating. A parameter delta is defined where delta << 1.0 is a thin wire approximation; delta is primarily a function of the eccentricity of the coating, the antenna diameter, and the coating diameter. It is found that when delta approximately less than 0.5, the theoretical model works well. In particular, it predicts the directivity due to the eccentricity of the coating. However, as this eccentricity is increased or as the antenna diameter is increased (delta approximately greater than 0.6), the model no longer accurately predicts directivity. Thus, the model that can be used to predict the HR profiles for an eccentrically coated asymmetric antenna only when delta approximately less than 0.5.