Speckle processing for ultrasonic imaging
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
A new approach to ultrasonic imaging which is designed to form faithful images in spite of severe propagation distortions is introduced. The difficulties associated with the formation of faithful images of soft mammalian tissue by conventional imaging methods are reviewed. The degrading effects of transmission speckle are demonstrated experimentally. The new approach is essentially statistical and involves the formation and subsequent combination of an ensemble of images with effectively independent distortions. Experiments show that the distortions of an ensemble of 128 images formed over contiguous narrow frequency bands spanning one octave (from 1.7 to 3.4 MHz) are usefully independent and isoplanatic when the propagation medium is either water on its own or water with animal liver immersed in it. It is also demonstrated that, in certain circumstances, a suitable ensemble may be generated entirely within a computer from a single image. Experimental results for both one-dimensional and two-dimensional imaging situations show that coherent combination of such ensembles of images by either speckle interferometry or the recently developed shift-and-add technique gives worthwhile improvements. Processing of the images by (modified) Wiener filtering is also beneficial. An important property of the processing, namely its ability to reduce the effects of aberrations of the imaging instrument on the processed images, is demonstrated. The images presented here are only of isolated objects. The latter were chosen mainly to allow repeatability of, and unambiguous interpretation of the results of, the experiments, rather than for their realism in important practical applications. However, the success of these preliminary trials suggests that it would be worthwhile to conduct further experiments using more realistic objects.