Longitudinal monitoring of reconstructed activity concentration on a clinical time-of-flight PET/CT scanner

Positron emission tomography (PET) images are potential quantitative biomarkers. Understanding long-term (months/years) biomarker variability is important for establishing confidence intervals on studies using such biomarkers over these time frames. PET biomarkers are derived from activity concentration (ρ) extracted from PET images. Over 30 months, we measured the stability of decay-normalized counts (Nn) and ρ by scanning the same 4.5-cm-diameter Ge-68 cylinder weekly, the same Na-22 point source daily, and a refilled 20-cm F-18 cylinder phantom monthly on a clinical TOF-PET/CT scanner. Longitudinal and adjacent-measurement variability was characterized. We found no drift in ρ or Nn for properly calibrated images over 24 months. During this time, ρmean ranged ±5% to 6% for count-matched Ge-68 and F-18 images, with coefficient of variation (COV) across time of 2.3% (Ge-68, 81 scans) and 3.2% (F-18, 24 scans). At typical patient image count levels the Ge-68 ρmean (ρmax) COV across time was 6.9% (9.6%). Changes in ρmean between adjacent F-18 scans (Δdays=34) ranged between ±10%, with corresponding date-matched changes in Ge-68 ρmean ranging ±2%. We recommend (1) tracking trends in Nn with image ρ as a check of quantitative data corrections/calibrations and (2) tracking both mean and COV of ρ (single time point measures) to hundredths precision using standardized uptake values. © 2016 Society of Photo-Optical Instrumentation Engineers (SPIE).