The Effects of Surface Mapping Corrections with Synthetic-Aperture Focusing Techniques on Ultrasonic Imaging

Image processing to improve the resolution of ultrasonic imaging systems requires an accurate knowledge of the geometry and properties of the media through which the ultrasound travels. This is particularly true when a method such as the synthetic-aperture focusing technique (SAFT) is used. For SAFT to function properly, it is necessary to know accurately the path traveled by the ultrasound from transducer to target and back again. A form of SAFT imaging is described in which the imaging routine also constructs a map of the target surface. This map is then used to calculate accurately the propagation history of the ultrasound for the SAFT processing. The resolution and positional accuracy of unprocessed images, unmapped SAFT images, and surface mapped SAFT images are compared. All but the surface mapping images show significant errors in positional accuracy for relatively small surface deviations. The surface mapping correction, however, brings the imaging accuracy back to within the limits of the mechanical experimental error. A more severely distorted surface destroys the phase relationships required for processing unless the surface variations are accounted for. In addition, results achieved with a flat ultrasonic transducer suggest significant simplifications that may ease field implementation of SAFT systems.