  A NEMA performance evaluation was conducted on the new General Electric (GE)
digital Omni Legend PET-CT system with 32-cm extended field-of-view. This study
marks the introduction of the first-ever commercially available clinical
digital bismuth germanate technology. Testing was performed in accordance with
the NEMA NU2-2018 standard. A comparison was made with the performance of two
other commercial GE scanners with extended fields-of-view. A digital lutetium
yttrium orthosilicate system (Discovery MI - 6 ring) and a non-digital bismuth
germanate system (Discovery IQ). For the Omni assessment, the tangential,
radial, and axial spatial resolutions at 1 cm radial offset were measured as
3.76 mm, 3.73 mm, and 4.25 mm FWHM. The total system sensitivity to a line
source at the center was 44.36 cps/kBq. The peak NECR was 501 kcps at 17.8
kBq/mL. The scatter fraction at NECR peak was 35.48%, and the maximum
count-rate error at and below NEC peak was 5.5%. Sphere contrast recovery
coefficients were from 52% (10 mm) to 93% (37 mm). The system does not use time
of flight; thus, no assessment of timing resolution was made. The PET-CT
co-registration accuracy was 2.4 mm. The performance of the Omni Legend
surpassed that of the Discovery MI on all NEMA tests, except for assessments of
background variability (image noise). Time of flight is associated with
inherent improvements in signal-to-noise ratio. In lieu of time of flight
capabilities, the Omni provides software corrections in the form of a
pre-trained neural network (trained on non-ToF to ToF). With such corrections,
average performance is competitive when compared to ToF systems. Further
validation is required to optimize clinical imaging protocols and
hyperparameters associated with such software corrections and to examine the
effect of non-linear corrections with varying target size, particularly for
real world, clinical scans.
