Geometric and intensity distortion in echography.

Anatomic structures possessing varying sonic propagation velocities refract ultrasonic beams and create distortions in the sonographic image. The distortions consist of inaccurate positioning of echogenic locations (geometric distortions) and of inaccurate display of ultrasonic intensities (intensity distortions). Artifacts of both types occur in the region distal to a structure of circular cross section with an internal sonic propagation velocity lower than that of its surroundings. In an attempt to better understand these distortions, a model is developed from first principles of the production of sonograms of such a region. Assuming a uniform ultrasonic beam and uniform echogenicity of the surrounding tissue, a mathematical expression has been derived for the intensity of the sound arriving at each point and returning to the transducer. Computer simulations of the resulting sonographic image are provided for visualization. In spite of many simplifying assumptions, this model is shown to be consistent with several known artifacts, and provides insight into the mechanisms of their production.