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

Comments on the NEMA NU 4-2008 Standard on Performance Measurement of Small Animal Positron Emission Tomographs