Abstract
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Objectives Clinical use of I-123 radiopharmaceuticals is expanding rapidly. Experiments were performed to evaluate collimation (MEGP vs LEHR) and reconstruction methods for I-123 SPECT-CT of the brain. I-123 has a primary emission at 159keV (83%) and several low incidence, higher energy emissions (< 3% total, 248-784keV). Downscatter from the higher energy photons can be significant with low energy collimation, leading some to use medium energy collimation.
Methods A Striatal Head Phantom (RSD, Long Beach, CA) was set up with a caudate-putamen to brain I-123 activity ratio of 8.0. SPECT-CT used MEGP and LEHR collimators on a Symbia T6 (Siemens, Hoffman Estates, IL) and an Infinia-Hawkeye (GE, Waukesha, WI). Scatter energy windows were acquired (2 for Siemens, 1 for GE). Images were reconstructed with no attenuation correction (AC) / scatter correction (SC), AC, and AC/SC using each manufacturer’s OSEM based methods (Siemens also modeled collimator response). Volumes of interest were drawn about each caudate-putamen pair (target) and in the brain chamber. Corresponding ROIs were drawn on lateral planar projections.
Results Quantitative analysis is given in the table below. Qualitatively, LEHR images had better resolution and superior delineation between the caudate and putamen chambers. The count contribution from down-scattered high energy photons was greater with LEHR than MEGP in planar images and to a lesser extent in SPECT.
Conclusions Use of commercially available AC/SC improves quantitative accuracy of I-123 SPECT-CT. Medium energy collimation may improve quantitation compared to low energy collimation by reducing down-scattered photons but at a loss of resolution that may be clinically unacceptable.
Research Support The phantom and isotope used in this work were provided by GE Healthcare