The authors present a formulation for calculating the field distributions for an ultrasonic imaging system when focusing in biological media. A biological medium is characterized in terms of its complex and frequency-dependent compressibility that, in turn, depends on the medium relaxation time and specific heat ratio. The analysis is based on a spatial/temporal transfer function formalism and can predict the effects of dispersion, frequency-dependent attenuation, aperture apodization and pulse shape. These effects can be observed on a ;snapshot' of the field in time and space. A number of simulations for a 3-MHz broadband system are presented. The results indicate that, aside from geometrical considerations (F/number), the pulse-width and the dispersive processes can play a major role in imaging performance.