A new image based dead time correction method for planar dosimetry

Objectives Continuity Dead Time Correction (CDTC) method applies to whole-body images acquired with the step & shoot modality.

Methods A program compensates for the dead time induced image discontinuities between consecutive FOVs. A phantom sets filled with 5.5–6.8 GBq of ^99mTc, and ten scans taken before 48 hours of six patients administered with 4-10 GBq of ¹³¹I labelled agents were used for validation. Root mean square error (rms) and maximum deviation of monitor point-source counts in different FOVs served as figure of merit. Absorbed dose calculation (Area Under Curve, AUC) of 6 lesions was also estimated.

Results Phantom tests gave a 6% maximum deviation with respect to the true (decay corrected) counts. CDTC reduced the rms error from 36% to 2% and the maximum deviation from 50% to 2% on phantom, while rms error from 51% to 32% and maximum error reduction from 72% to 21% on patients. CDTC gave a correction of +56% of AUC, while +78% was obtained with standard source method.

Conclusions With respect to the standard source method, CDTC does not implies: a) the systematic over-correction; b) preparing a point-source; d) overlap between the patient’s arm and a point-source.With respect to methods based on the count rate characteristic curve, CDTC does not need: a) a phantom shape dependent calibration curve; b) handling high ¹³¹I activities for the calibration; c) the observed count rate; d) the demanding acquisition list mode.