Sci-Fri AM: Imaging - 08: Comparison of single and dual layer detector blocks for pre-clinical MRI-PET

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

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.

Development of a PET scanner for simultaneously imaging small animals with MRI and PET

Recently, positron emission tomography (PET) is playing an increasingly important role in the diagnosis and staging of cancer. Combined PET and X-ray computed tomography (PET-CT) scanners are now the modality of choice in cancer treatment planning. More recently, the combination of PET and magnetic resonance imaging (MRI) is being explored in many sites. Combining PET and MRI has presented many challenges since the photo-multiplier tubes (PMT) in PET do not function in high magnetic fields, and conventional PET detectors distort MRI images. Solid state light sensors like avalanche photo-diodes (APDs) and more recently silicon photo-multipliers (SiPMs) are much less sensitive to magnetic fields thus easing the compatibility issues. This paper presents the results of a group of Canadian scientists who are developing a PET detector ring which fits inside a high field small animal MRI scanner with the goal of providing simultaneous PET and MRI images of small rodents used in pre-clinical medical research. We discuss the evolution of both the crystal blocks (which detect annihilation photons from positron decay) and the SiPM array performance in the last four years which together combine to deliver significant system performance in terms of speed, energy and timing resolution.