@misc{9248575,
  abstract     = {{Background and Aim: A new PET/CT scanner using BGO crystals was recently introduced
in the clinic. Unlike scanners with LSO/LYSO crystals, this scanner does not include the time-
of-flight (TOF) property, due to low timing resolution of BGO crystals. To compensate for the
lack of TOF, the reconstructed images can be filtered with an AI model, called Precision DL
(PDL), which has been trained on images with and without TOF. It still remains unknown
how well the AI model can enhance small lesions in a patient compared to a system with TOF,
especially in more challenging conditions, such as larger patients. The aim of this project is to
compare detectability of small lesions between a PET/CT system with true TOF (GE Health-
Care’s Discovery MI) and a PET/CT system with AI based TOF (GE HealthCare’s OMNI
Legend).

Materials and Methods: To compare detectability of small lesions between the two sys-
tems, a Jaszczak phantom and a 3D printed XL shell to Jaszczak were used. The XL phantom
was divided into two cases, the shell filled with and without background activity. Six hot spheres
in different sizes were placed in Jaszczak, in a sphere-to-background ratio (SBR) of 10:1, where
the spheres had an activity concentration of 12 kBq/ml at start of scan. The PET radiophar-
maceutical used was 18F-FDG and the phantoms were scanned sequentially on the two PET
systems (DMI and OMNI). Images were acquired in listmode and rebinned into ten 90-s frames
per phantom and PET-system, with one bed position and matrix sizes of 256×256 and 384×384.
Phantom images were reconstructed using OSEM + PSF together with a post filter of 3 mm
and BSREM with β value of 500. The reconstructions were made with and without TOF for the
DMI scanner, whereas the BSREM reconstructions for the OMNI scanner were combined with
the three PDL levels. Recovery coefficients (RC) were calculated from peak values in the spheres
and the calculated activity concentrations in the spheres at the start of scan, and signal-to-noise
ratio (SNR) was determined in the background for each phantom. A peak-to-peak (P2P) value
was calculated as the ratio of the peak in a sphere to the peak in the background and was
evaluated as a detectability value. Non-blinded readers did a visual evaluation of a sample of
the images to visually estimate the lesion detectability.

Results: TheresultsshowedthatbothreconstructionmethodswithTOF(OSEMandBSREM)
with the DMI scanner gave higher RC than all reconstructions from the OMNI scanner across
all three phantom cases. The TOF effect was clearly shown for the DMI scanner, while the
PDL filters seemed to give little to none effect on the recovery. A decrease in the background
SNR was seen in reconstructions with TOF compared to those without TOF, while the PDL
models had an increasing effect on the SNR. The P2P values for the DMI tend to be higher
for reconstructions with TOF compared to those without. For the Jaszczak phantom, the DMI
scanner reaches higher P2P values than the OMNI scanner, while the opposite trend is seen for
the XL phantom. In general, higher P2P values were reached for Jaszczak compared to the XL
phantom, which means that more spheres were thought to be seen in the images for the smaller
phantom. This result was confirmed through the visual evaluation, which also resulted in higher
means of detected spheres for the BGO system. The results of the P2P values for both systems
also showed a large spread of the data points within the ten time frames of the XL phantom.

Conclusion: The P2P values together with the visual evaluation conclude that a smaller phan-
tom has greater detectability than a larger one. The P2P values show that the benefit of true
TOF is greater for the Jaszczak phantom than the XL phantom, which contradicts the theory.
The benefits of the BGO system were also shown to be larger in the phantom case with no
activity in the XL shell, which is the most patient like phantom case. Further studies are needed
to confirm the effect of true TOF versus the sensitivity of the BGO crystals for larger patients.}},
  author       = {{Sohlman, Marielle}},
  language     = {{eng}},
  note         = {{Student Paper}},
  title        = {{Comparison of Detectability Between Digital LYSO and BGO-Based PET/CT Scanners: A Phantom Study}},
  year         = {{2026}},
}