  Accurate MR-based attenuation correction (MRAC) is essential for quantitative
PET/MR imaging of the brain. In this study, we analyze the regional bias caused
by MRAC based on Zero-Echo-Time MR images (ZTEAC) compared to CT-based AC
(CTAC) in static and dynamic PET imaging. In addition the results are compared
to the performance of the current default Atlas-based AC (AtlasAC) implemented
in the GE SIGNA PET/MR.
  Methods: Thirty static [18F]FDG and 11 dynamic [18}F]PE2I acquisitions from a
GE SIGNA PET/MR were reconstructed using ZTEAC (using a research tool, GE
Healthcare), single-subject AtlasAC (the current default AC in GE's SIGNA
PET/MR) and CTAC (from a PET/CT acquisition of the same day). In the 30 static
[18F]FDG reconstructions, the bias caused by ZTEAC and AtlasAC in the mean
uptake of 85 anatomical volumes of interest (VOIs) of the Hammers' atlas was
analyzed in PMOD. For the 11 dynamic [18}F]PE2I reconstructions, the bias
caused by ZTEAC and AtlasAC in the non displaceable binding potential BPnd in
the striatum was calculated with cerebellum as the reference region and a
simplified reference tissue model.
  Results: The regional bias caused by ZTEAC in the static [18F]FDG
reconstructions ranged from -8.0% to +7.7% (mean 0.1%, SD 2.0%). For AtlasAC
this bias ranged from -31.6% to +16.6% (mean -0.4%, SD 4.3%). The bias caused
by AtlasAC showed a clear gradient in the cranio-caudal direction (-4.2% in the
cerebellum, +6.6% in the left superior frontal gyrus). The bias in the striatal
BPnd for the [18F]PE2I reconstructions ranged from -0.8% to +4.8% (mean 1.5%,
SD 1.4%) using ZTEAC and from -0.6% to +9.4% using AtlasAC (mean 4.2%, SD
2.6%).
  Conclusion: ZTEAC provides excellent quantitative accuracy for static and
dynamic brain PET/MR, comparable to CTAC, and is clearly superior to the
default AtlasAC currently implemented in the GE SIGNA PET/MR.
