  The National Electrical Manufacturers Association's (NEMA) NU 4-2008 standard
specifies methodology for evaluating the performance of small-animal PET
scanners. The standard's goal is to enable comparison of different PET scanners
over a wide range of technologies and geometries used. In this work, we discuss
if the NEMA standard meets these goals and we point out potential flaws and
improvements to the standard.
  For the evaluation of spatial resolution, the NEMA standard mandates the use
of filtered backprojection reconstruction. This reconstruction method can
introduce star-like artifacts for detectors with an anisotropic spatial
resolution, usually caused by parallax error. These artifacts can then cause a
strong dependence of the resulting spatial resolution on the size of the
projection window in image space, whose size is not fully specified in the NEMA
standard.
  We show that the standard's equations for the estimation of the random rate
for PET systems with intrinsic radioactivity are circular and not satisfiable.
We compare random rates determined by a method based on the standard to to
random rates obtained using a delayed coincidence window and two methods based
on the singles rates.
  We discuss to what extend the NEMA standard's protocol for the evaluation of
the sensitivity is unclear and ambiguous, which can lead to unclear
interpretations of publicizeded results.
  The standard's definition of the recovery coefficients in the image quality
phantom includes the maximum activity in a region of interest, which causes a
positive correlation of noise and recovery coefficients. This leads to an
unintended trade-off between desired uniformity, which is negatively correlated
with variance (i.e. noise), and recovery.
  With this work, we want to start a discussion on possible improvements in a
next version of the NEMA NU-4 standard.
