Performance evaluation of a PET insert for preclinical MRI in stand-alone PET and simultaneous PET-MRI modes

Gaelle Emvalomenos; Sofie Trajanovska; Binh T T Pham; Peter Doughty; Jerome Burnet; Isabelle Smith; Ruslan Garipov; Marie-Claude Gregoire; Nana Sunn; John McGrath; Steven R Meikle

doi:10.1186/s40658-021-00415-1

Performance evaluation of a PET insert for preclinical MRI in stand-alone PET and simultaneous PET-MRI modes

EJNMMI Phys. 2021 Oct 9;8(1):68. doi: 10.1186/s40658-021-00415-1.

Authors

Gaelle Emvalomenos^{1

2}, Sofie Trajanovska³, Binh T T Pham³, Peter Doughty⁴, Jerome Burnet⁴, Isabelle Smith⁵, Ruslan Garipov⁴, Marie-Claude Gregoire^{6

7}, Nana Sunn³, John McGrath⁴, Steven R Meikle^{8

6}

Affiliations

¹ Sydney School of Health Sciences, The University of Sydney, Camperdown, NSW, 2050, Australia. gaelle.emvalomenos@sydney.edu.au.
² Brain and Mind Centre, The University of Sydney, 100 Mallett Street, Camperdown, NSW, 2050, Australia. gaelle.emvalomenos@sydney.edu.au.
³ Sydney Imaging Core Research Facility, The University of Sydney, Camperdown, NSW, 2050, Australia.
⁴ MR Solutions, Guildford, UK.
⁵ School of Physics, The University of Sydney, Camperdown, NSW, 2050, Australia.
⁶ Brain and Mind Centre, The University of Sydney, 100 Mallett Street, Camperdown, NSW, 2050, Australia.
⁷ Australian Nuclear Science and Technology Organisation, Lucas Heights, NSW, 2234, Australia.
⁸ Sydney School of Health Sciences, The University of Sydney, Camperdown, NSW, 2050, Australia.

Abstract

Background: This study aimed to evaluate the performance of a preclinical PET insert in three configurations: as a stand-alone unit outside the MRI bore, inside the bore of a cryogen-free 3T MRI and, finally, while performing simultaneous PET/MRI studies.

Methods: The PET insert consists of two rings of six detectors, each detector comprising 8 × 12 SiPMs reading out dual offset layers of pixelated LYSO crystals with a 1.4-mm pitch. The inner diameter is 60 mm, transaxial field of view (FoV) 40 mm and axial FoV 98 mm. Evaluation was based on NEMA NU 4-2008 guidelines with appropriate modifications. Spatial resolution and sensitivity were measured inside and outside the MR bore. Image quality, count rate and quantitative performance were measured in all three configurations. The effect of temperature stability on PET sensitivity during fast spin echo sequences was also evaluated. B₀ field homogeneity and T1 and T2 relaxation times were measured using a water-filled phantom, with and without simultaneous PET operation. Finally, PET and MRI scans of a mouse injected with 10 MBq [¹⁸F]NaF and a mouse injected with 16 MBq [¹⁸F]FDG were performed in sequential and simultaneous modes.

Results: Peak absolute sensitivity was 10.15% with an energy window of 250-750 keV. Absolute sensitivity values outside and inside the MR bore with MR idle agreed to within 0.1%. Outside the MR bore, spatial resolution was 1.21/1.59 mm FWHM (radial/tangential) 5 mm from the centre of the FoV which compared well with 1.19/1.26 mm FWHM inside the MR bore. There were no substantial differences between all three scan configurations in terms of peak NEC rate (175 kcps at 17 MBq), scatter or random fractions. Uniformity and recovery coefficients were also consistent between scanning modes. B₀ field homogeneity and T1 and T2 relaxation times were unaltered by the presence of the PET insert. No significant differences were observed between sequential and simultaneous scans of the animals.

Conclusions: We conclude that the performance of the PET insert and MRI system is not significantly affected by the scanning mode.

Keywords: NEMA; PET insert; PET/MRI; Preclinical PET; Simultaneous PET/MRI.