Towards clinically useful coded apertures for planar nuclear medicine imaging.
Date
2010-03-26T06:16:51Z
Authors
Starfield, David Mark
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Abstract
Coded apertures have the potential to increase imaging e ciency in nuclear medicine
without degrading resolution, but near- eld artifacts are present even under idealised
aperture and imaging conditions. The purpose of this work is to reduce artifacts
prior to reconstruction, and to work towards coded apertures that are clinically
useful. A ray-tracing simulator was developed. Far- eld conditions produce nearperfect
images, but the simulation of distributed sources under idealised near- eld
conditions results in the presence of artifacts.
Three core concepts are introduced in this thesis: a novel rotatable array of identical
limited- eld-of-view coded apertures; the use of high-resolution aperture projections;
and the deliberate and counter-intuitive use of thin, highly transparent aperture
material.
An array of identical limited- eld-of-view coded apertures, which can be rotated so as
to implement an existing artifact-reduction technique, was simulated. The artifacts
that exist for a single coded aperture under idealised conditions are removed. This
novel technique remains e ective when realistic near- eld conditions are introduced
into the simulation. However, realistic apertures increase artifacts due to nite
pinhole widths and nite thickness of the aperture material. To address the pinhole
width problem, high-resolution patterns, in which the smallest hole corresponds to a
projection area of 1 1 detector pixels, o er the best trade-o between e ciency and
resolution despite the partial volume e ect. The nite aperture thickness problem
is addressed by another novel concept; viz. the deliberate reduction in material
thickness, which results in a highly transparent coded aperture. Simulation shows
that this counter-intuitive approach diminishes collimation e ects.
The implementation of any or all of these three core concepts, however, reduces count
statistics. An ultra-near- eld geometry, which would ordinarily result in severe
artifacts, can theoretically be used to maintain count statistics, without altering
either patient dose or image acquisition time. This was veri ed by simulation.
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ABSTRACT
A prior-state-of-the-art 1 mm thick tungsten coded aperture, and a deliberately
highly transparent 100 m tungsten foil coded aperture, were constructed for use
with a dual head gamma camera. Phantom studies of Technetium-99m point,
line, syringe and printed distributed sources were performed. The experimental
acquisitions veri ed the simulation results, for both the prior-state-of-the-art coded
aperture and the novel high-transparency coded aperture.
The results and arguments presented in this thesis point to the potential for these
three core developments to produce high-quality coded aperture images in a fraction
of the time that is taken for a collimator acquisition. The limiting factor appears to
be the poor count statistics that result from the low sensitivity of current gamma
cameras; a situation which looks set to change given current research trends.