  The presence of tumor hypoxia is known to correlate with poor patient
prognosis. Measurement of tissue oxygen concentration can be challenging, but
recent advancements using positron annihilation lifetime spectroscopy (PALS) in
three-dimensional positron emission tomography (PET) scans have shown promise
for hypoxia detection. In this work, a novel method for estimating the
orthopositronium lifetime in PALS is presented. This method is analytical and
uses moments of the time-difference histogram from photon arrival times. For
sufficient statistical power, the method produces monotonic, stable estimates.
For cases with a lower number of photon counts, the method was characterized
and solutions are presented to correct for bias and estimation variability.
