Single and Multipinhole Collimator Design Evaluation Method for Small Animal SPECT

High resolution functional imaging of small animals
is often obtained by single pinhole SPECT with circular orbit
acquisition. Multipinhole SPECT adds information due to its
improved sampling, and can improve the trade-off between
resolution and sensitivity. To evaluate different pinhole collimator
designs an ef cient method is needed that quanti es the
reconstruction image quality. In this paper, we propose a fast,
approximate method that examines the quality of individual
voxels of a post-smoothed MLEM reconstruction by studying
their linearized local impulse response (LLIR) and (co)variance
for a prede ned target resolution. For validation, the contrastto-
noise ratios (CNRs) in some voxels of a homogeneous sphere
and of a realistic rat brain software phantom were calculated
for many single and multipinhole designs. A good agreement
was observed between the CNRs obtained with the approximate
method and those obtained with post-smoothed MLEM reconstructions
of simulated noisy projections. This good agreement
was quanti ed by a least squares t through these results, which
yielded a line with slope 1.02 (1.00 expected) and a y-intercept
close to zero (0 expected). 95.4% of the validation points lie within
three standard deviations from that line. Using the approximate
method, the in uence on the CNR of varying a parameter in
realistic single and multipinhole designs was examined. The
investigated parameters were the aperture diameter, the distance
between the apertures and the axis-of-rotation, the focal distance,
the acceptance angle, the position of the apertures, the focusing
distance and the number of pinholes. The results can generally
be explained by the change in sensitivity, the amount of postsmoothing
and the amount of overlap in the projections. The
method was applied to multipinhole designs with apertures
focusing at a single point, but is also applicable to other designs.