Delayed convergence of iterative reconstruction in dopamine transporter SPECT with multiple-pinhole collimators

Multiple-pinhole (MPH) collimators for brain SPECT improve the resolution-sensitivity tradeoff compared with conventional parallel-hole and fan-beam collimators. The system count sensitivity profile with MPH collimators exhibits a peak at the center of the field-of-view and significant decline in count sensitivity toward the periphery. In MPH SPECT with [¹²³I]FP-CIT, this makes the difference in relative statistical noise between the striatum and extrastriatal (reference) regions even bigger. This could slow down the convergence of iterative reconstruction compared with [¹²³I]FP-CIT using conventional collimators. This study evaluated the impact of the effective number of reconstruction iterations in [¹²³I]FP-CIT SPECT with MPH collimators.

671 patients (43.8% females, 67.3±11.3y) were included retrospectively. Projection data were acquired with a triple-head camera (Mediso AnyScan Trio) equipped with 2^nd generation general-purpose brain MPH collimators. MPH projections from 12 min total scan duration were reconstructed with the iterative 3d Tera-Tomo Monte Carlo algorithm of the system software. The effective number of iterations was varied between 12 and 300. To evaluate the impact of the iteration number on diagnostic performance, a data-driven Gaussian mixture model approach was used to assess the power of the putaminal specific binding ratio (SBR) for differentiation between reduced and normal scans. In a subset of 72 patients, reconstructed images were visually evaluated twice by 7 independent readers with respect to parkinson-like reduction of striatal [¹²³I]FP-CIT uptake using a Likert 6-score (-3=clearly reduced, …, 3=clearly normal). The impact of the iteration number on the Likert 6-score was tested using repeated measure analysis of variance (ANOVA) with iteration number, reader and session as within-subject factors. The majority vote binary classification (reduced, normal) was taken into account as between-subject factor. Within- and between-readers agreement of visual scoring were characterized by Cohen's and Fleiss' kappa.

Iterative reconstruction converged much slower in the extrastriatal reference region than in the striatum and its subregions. Considerable improvement of spatial resolution with increasing iteration number came with an acceptable increase of statistical noise. The discriminative power of the putaminal SBR was highest with 300 effective iterations. There was a small but significant reduction of reader confidence and between-readers agreement with 300 compared to 24 iterations, particularly in normal cases (Likert 6-score 2.325 versus 2.468, p < 0.001).

300 iterations provide a good compromise between spatial resolution, robustness and diagnostic power of semi-quantitative SBR analyses on one side versus statistical noise and between-readers variability on the other side. In order to avoid relevant loss of between-readers stability, readers should be specifically trained for reading [¹²³I]FP-CIT MPH SPECT images reconstructed with a high number of iterations.