Design of a near-field coded aperture cameras for high-resolution medical and industrial gamma-ray imaging

Abstract

Coded Aperture Imaging is a technique originally developed for X-ray astronomy, where typical imaging problems are characterized by far-field geometry and an object made of point sources distributed over a mainly dark background. These conditions provide, respectively, the basis of artifact-free and high Signal-to-Noise Ratio (SNR) imaging. When the coded apertures successful in far-field problems are used in near-field geometry, images are affected by extensive artifacts. The classic remedy is to move away from the object until a far-field geometry is restored, but this is at the expense of counting efficiency and, thus, of the SNR of the images. It is shown in this thesis that the application to near-field of a technique originally developed to mitigate the effects of non-uniform background in far-field applications results in a considerable reduction of near-field artifacts. This result opens the way to the exploitation in near-field problems of the favorable SNR characteristics of coded apertures: images comparable to those provided by state-of-the-art imagers can be obtained in a shorter time or while administering a lower dose to patients. Further developments follow when the SNR increase is traded for better resolution at constant time and dose.

(cont.) The main focus of this work is on a coded aperture camera specifically designed for high-resolution single-photon planar imaging with a pre-existing gamma (Anger) camera. Original theoretical findings and the results of computer simulations led to an optimal coded aperture that was tested experimentally in phantom as well as in-vivo studies. Results include, but are not limited to, 1.66-mm-resolution images of 99mTc-labeled blood and bone agents in a mouse. The theoretical bases for extension to sub-millimeter resolution and higher-energy isotopes are also laid and a candidate aperture capable of 0.96-mm resolution proposed. Potential applications are in small-animal imaging, pediatric nuclear medicine and breast imaging, where increased resolution can result in earlier diagnosis of disease. The last Chapter of the thesis extends the ideas developed to the design of a coded aperture suitable for CAFNA (Coded Aperture Fast Neutron Analysis), a contraband detection technique that has been under development at MIT for a number of years.