Imaging in a Distorting Medium
Thesis DisciplineElectrical Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Images of an object embedded in an inhomogeneous medium are shown to contain speckle artefacts. The efficacy of a speckle processing technique called 'shift-and-add' for removing these artefacts is investigated. Shift-and-add is known to be applicable to any object which is dominated by a bright isolated point scatterer. This scatterer is called the 'reference'. Evidence that certain naturally occurring biological structures are capable of acting as a reference is presented. It is shown that an idealised version of shift-and-add, in conjunction with Fourier imaging, provides an approximate method for imaging an arbitrary object. Experimental results are reported which show that under most circumstances shift-and-add is able to reduce artefacts caused by multiple scattering. When the object is not dominated by point scatterers the shift-and-add image contains a significant 'ghosting' artefact. A theoretical model which accounts for this artefact is proposed and verified by simulation. Several extensions of shift-and-add are reported which allow the artefacts to be reduced and so allow an appreciably larger class of objects to be imaged. In particular, most objects dominated by several bright point scatterers can be reconstructed. This is verified experimentally for a simple object embedded in animal tissue. Additionally, a wide class of piecewise-constant extended objects can be reconstructed. One dimensional simulations are presented to support this conclusion. A generalised shift-and-add principle, which in theory allows arbitrary objects to be reconstructed, is proposed. It is shown that, in practice, the class of objects which can be reconstructed by the extensions to shift-and-add introduced in this thesis is considerably wider than that which can be successfully imaged by previously reported versions of shift-and-add.