A revised monitor source method for practical deadtime count loss compensation in clinical planar and SPECT studies

The aim of the study is to verify the fundamental assumption in the monitor source method, i.e. uniform fractional count loss across the field of view (FOV), and to introduce a revised monitor source method for SPECT deadtime correction that minimally interferes with the clinical protocol. SPECT images of non-uniform phantoms (4GBq (99m)Tc) with and without monitor sources (2  ×  20MBq (99m)Tc) attached to each detector were acquired nine times over 48 h in the photopeak energy window and the scatter energy window. Fractional count loss uniformity across the FOV was evaluated by correlating count rates in different regions of interest on projection images at different deadtime loss levels. The correction factors were calculated as the ratios of monitor source count rates with and without the phantom. Such factors were applied to the phantom images acquired without the monitor sources. The counting efficiency (count rate per unit activity) of the camera was calculated as a function of activity in the FOV both prior to and after the deadtime count-loss correction. The deadtime correction effectiveness was assessed by the independence of the efficiency on the activity in the FOV. Methods to interpolate the projection deadtime loss, based on limited projections, were also investigated. The fractional deadtime count loss was uniform across the FOV (r > 0.99). After the deadtime correction, the efficiency was largely independent of the activity in the FOV. The median and maximum absolute errors after the deadtime count loss correction were ≤1% and ~2%, respectively. Measured deadtime loss from five views per detector can be used to estimate deadtime count loss with errors ≤1% for all SPECT projections. The revised monitor source method can effectively correct planar and SPECT deadtime loss. Sparse sampling of the projection deadtime loss allows the acquisition of high monitor source counts with minimal time added while preserving the entire useful FOV.