Proceedings Article | 19 March 2007
Proc. SPIE. 6510, Medical Imaging 2007: Physics of Medical Imaging
KEYWORDS: Sensors, Signal attenuation, Skin, Photons, Lung, Bone, Monte Carlo methods, Radioisotopes, Single photon emission computed tomography, Systems modeling
Small animal SPECT using low energy photons of I-125 and approaching resolutions of microscopic
levels, imaging parameters such as pinhole edge penetration, detector blur, geometric response, detector
and pinhole misalignment, and gamma photon attenuation and scatter can have increasingly noticeable
and/or adverse effects on reconstructed image quality. Iterative reconstruction algorithms, the widelyaccepted
standard for emission tomography, allow modeling of such parameters through a system
matrix. For this Monte Carlo simulation study, non-uniform attenuation correction was added to the
existing system model. The model was constructed using ray-tracing and further included corrections
for edge penetration, detector blur, and geometric aperture response. For each ray passing through
different aperture locations, this method attenuates a voxel's contribution to a detector element along
the photon path, which is then weighted according to a pinhole penetration model. To lower the
computational and memory expenses, symmetry along the detector axes and an incremental storage
scheme for the system model were used. For evaluating the nonuniform attenuation correction method,
3 phantoms were designed of which projection images were simulated using Monte Carlo methods. The
first phantom was used to examined skin artifacts, the second to simulate attenuation by bone, and the
third to generate artifacts of an air-filled space surrounded by soft tissue. In reconstructions without
attenuation correction, artifacts were observed with up to a 40% difference in activity. These could
be corrected using the implemented method, although in one case overcorrection occurred. Overall,
attenuation correction improved reconstruction accuracy of the radioisotope distribution in the presence
of structural differences.
