Year: 2008
Type: dissertation
Abstract: Phase-based micro-computed tomography (microCT) techniques extend the possibilities of standard, bsorption based microCT by offering increased sensitivity. In the hard X-ray region, the increase in sensitivity can be several orders of magnitude. Phase measurements require that the X-ray beam has a defined phase, that is it has a certain degree of coherence. While several phase contrast mechanisms exist, a particularly simple one is to let the beam propagate in free space after interaction with an object. Such an image is called a Fresnel diffraction pattern. The phase in the image plane is lost, but the phase information of the object contributes to the image intensity. Both the object absorption and phase information are entangled in the recorded image. Such patterns can be used to calculate the phase shift through a process known as phase retrieval. It is a non-linear problem, but the most efficient phase retrieval algorithms so far rely on a linearized contrast model. These are the transport of intensity equation (TIE) and the contrast transfer function (CTF). The TIE is based on linearization in the propagation distance and is valid for short propagation distance only The CTF is based on linearization of the object function, and is valid for weak absorption and slowly varying phase
Author: Max Langer
Language: English
Subfield: Radiation
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W638070123