The effects of resolution recovery on estimation of binding potential from brain SPECT images

We investigated the effects of resolution recovery and SPECT system sensitivity on performance in estimating binding potential (BP) from dynamic brain SPECT data obtained using I-123-altropane, a dopamine transporter imaging agent. BP is estimated by Fischman´s approach, whereby a gamma variate function is fitted to the difference between striatal and occipital time-activity curves (TAC). The TAC were obtained using an approach published by Huesman (1984), who estimated the activity concentration in a ROI directly from the projection dataset without reconstructing the image. We modified this method by incorporating resolution recovery, using a Metz filter. We simulated dynamic projection datasets of a simple striatal phantom and determined the accuracy and precision of estimation of striatal activity concentration and binding potential, both for current system sensitivity and for the higher sensitivity of a new collimator, presently being manufactured. This collimator is expected to increase sensitivity at the center of the brain by a factor of 3, without degrading resolution. The parameter, P, of the Metz filter, which controls the extent of resolution recovery, was varied from 1 to 10⁵. For estimation of striatal activity concentration, increasing the value of P over this range reduced bias and gradually increased variance. For estimation of BP, however, increasing the value of P beyond 2 (for current sensitivity) and beyond 10 (for increased sensitivity) dramatically increased variance. For estimation of striatal activity, there was a broad minimum in RMSE of ∼12% for P between 7 and 100 at current sensitivity, and ∼10% for P between 10 and 300 for improved sensitivity. For estimation of binding potential, the minimum RMSE was 32% (P=2) for current sensitivity, and 17% (p=7) for improved sensitivity. The differences in the effects of resolution recovery on estimation of binding potential and striatal activity concentration are due to the nonlinear nature of the former task.