FDTD simulations for ultrasound propagation in a 2-D breast model.

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.