SPECT deadtime count loss correction using monitor source method

Objectives Deadtime count loss (DTloss) correction using monitor source (MS) requires: 1) uniform relative DTloss across FOV, 2) images of MS w/ & w/o the object and 3) high counting statistics for the MS. The aims are to validate condition 1 and to develop a practical protocol that satisfies conditions 2&3 with minimal additional study duration.

Methods SPECT images of non-uniform phantoms (4GBq 99mTc) along with MS (20MBq 99mTc) attached to each detector were acquired in photopeak and scatter energy window (EW) using Siemens SymbiaS and GE D670 systems multiples time over 48 hours. Planar images of the MS only were also acquired. Photopeak counts for the MS ROIs were >100 kcts. Relative DTloss uniformity across the FOV was evaluated by correlating count rates in different ROI on projection images at different DTloss levels. The correction factor for each SPECT projection at every time point was calculated as the ratio of time-corrected MS count rates w/o & w/ the phantom. The DTloss corrected projections for each SPECT acquisition were decay corrected to a common time point and correction accuracy was assessed against DTloss estimated by paralyzable detector response. The accuracy of DTloss correction for SPECT through correction of projections was evaluated. A method to model DTloss for every projection based on measured DTloss for a subset of projections was investigated. The relation of DTloss between photopeak and scatter EW was explored.

Results The fractional DTloss was found to be uniform across the FOV (r>0.99), hence validating condition 1. The MS method was accurate to >99% for planar and SPECT. Measured DTloss from 3-to-5 projections/detector can be used to estimate DTloss with accuracy >98% for all SPECT projections by modeling DTloss with measured projection rate. The correction factor in photopeak and scatter EW are equivalent with >99% agreement.

Conclusions MS method can accurately correct planar and SPECT DTloss. Sparse sampling of the projection DTloss allows acquiring MS counts with high statistics with minimal increase to the overall SPECT study duration making it practical for clinical implementation.