Quantitation of iodine-123-beta-CIT dopamine receptor uptake in a phantom model

The purpose of this study was to determine the effects of technical factors such as collimation and filtration on the measurement of 123I-beta-CIT uptake in the striatum.

All SPECT studies were performed using a brain phantom containing striata within a bone- and tissue-equivalent skull. The effects of collimator resolution and septal penetration were assessed from 99mTc and 123I studies containing variable activities in the striata and background regions. Optimum attenuation coefficients (mu) were determined from studies containing uniform activity in the brain.

For 99mTc, mu was 0.095 cm-1 and 0.07 cm-1 for parallel-hole and fanbeam collimators, respectively. For 123I, these values dropped to 0.09 cm-1 and 0.00 cm-1 (zero) for medium-energy and fanbeam collimators, respectively. Striatal uptake was significantly underestimated, particularly for medium-energy and general-purpose collimators. With 99mTc, fanbeam collimation gave a 50% increase in the measured striatal uptake, compared to medium-energy collimation. However, with 123I, this gain was eliminated by septal penetration and scatter. Increasing transaxial slice thickness, ROI size and decreasing filter cutoff frequency all degraded apparent striatal uptake.

Partial volume effects, combined with the averaging effects of increasing slice thickness and ROI size, are the most significant factors affecting measurement of striatal uptake of 123I-beta-CIT. The increased resolution of low-energy high-resolution collimators, compared to a medium-energy collimator, is offset by the increased septal penetration and scatter.