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High resolution PET insert for high field preclinical MRI: evaluation of single ring system using 7T field strength
Willy Gsell1, Uwe Himmelreich1, Cindy Casteels2, Christophe M. Deroose2, Antonio J. Gonzalez3, Albert Aguilar3, Carlos Correcher4, Emilio Gimenez4, Cesar Molinos4, Ramiro Polo4, Thorsten Greeb5, Ralph Wissmann5, Sven Junge5, and Jose M. Benlloch3

1Biomedical MRI, Department of Imaging and Pathology, KU Leuven, Leuven, Belgium, 2Nuclear Medicine, Department of Imaging and Pathology, KU Leuven, Leuven, Belgium, 3i3M, Valencia, Spain, 4Oncovision, Valencia, Spain, 5Bruker Biospin, Ettlingen, Germany

Synopsis

We designed a novel PET insert based on monolithic LYSO crystals. From our first evaluation, we can conclude that sub-millimeter detector spatial resolution, combined with accurate photon DOI determination, make it possible to acquire high resolution reconstructed images. This enables us now to combine simultaneously high resolution and sensitivity PET with high field preclinical MRI to extract simultaneously complex data from anatomical to molecular information and to dynamically follow non-invasively animal models of different pathologies with no compromise in performance of each imaging modality.

Introduction

Positron Emission Tomography (PET) is a very sensitive molecular imaging modality, but it suffers from a lack of anatomical information. The combination of PET with magnetic resonance imaging (MRI) provides the ideal solution by having soft tissue contrast, multi-functional read-outs (MRI-diffusion, perfusion, dynamic contrast enhancement) and does not add additional exposure to ionizing x-ray radiation1-5, as Computed Tomography scanner do. However the design of a high resolution PET insert for preclinical imaging has to overcome several challenges due to the high static magnetic field, the fast switching of the gradient coils and potential interaction of the radiofrequency field with PET electronics6. We designed a novel PET insert based on monolithic LYSO crystals and high density arrays of SiPMs. The aim of this work was thus to characterize the performance of such a system for simultaneous PET-MRI acquisitions.

Methods

The PET system formed by 8 detectors has been tested inside the Bruker BioSpec 70/30 USR with a magnetic field of 7 T (Figure 1). The MR system was equipped with a BGA 20S-HP gradient coil. The inner and outer diameters of the PET scanner (insert) are 114mm and 198mm, respectively, fitting inside the MR gradient. Quadrature birdcage coils were used for all experiments (86mm inner diameter for phantom and rat studies and 40mm for mouse imaging). For the RF shielding, we implemented Carbon Fiber structures with tubular shape, surrounding the PET electronics. Fast Spin echo and EPI sequences were tested simultaneously with the PET insert. We assessed potential eddy currents induced by fast switching of the gradient, field homogeneity through B0 maps and PET/MR image quality (resolution, SNR, image homogeneity etc.). For PET characterization, we followed the NEMA protocol for sensitivity, and use mini-Derenzo like phantom (filled with 150uCi of 18F-FDG) for estimation of resolution and image quality. For all tests, PET data were reconstructed using Maximum Likelihood Estimation Method (MLEM) with either voxel size of 0.5 or 0.29mm3 and at least 12 iterations. Several animal models (mouse glioma, mouse stroke, xenografts in mice, rat heart, see Figures) were evaluated to provide real in-vivo data for the quality assessment of the simultaneous PET/MRI acquisitions.

Results

We tested different RF pulses (20ms and 630W, with 51us and 1ms duration) and MRI sequences (RARE, EPI, etc…) without observing PET degradation of the PET image quality or eddy currents that could produce a sub-optimal MR performance. The FieldMap sequences showed in the present study no change in the B0 field with a 55mm spherical phantom when the PET insert was inside the MR scanner (SNR variation with/without PET <6%). Both RARE and EPI sequences showed ghost levels of about 2.4 to 3.6%. The PET geometry and performance were almost identical to the current in-line Albira Si system6. Sub-millimeter image resolution (between 0.9 and 0.7mm) and homogeneous-FOV spatial resolution were reached, as shown in Fig.2 and Fig.3. The sensitivity for the one-ring PET, following the NEMA protocol, was determined to be beyond 3.5%. In-vivo evaluation demonstrated the addedd value of using simultaneously high resolution PET and MRI (Fig. 4). For example, a glioma as small as 0.6mm3 were visualized on the MRI and small SUV differences with contralateral side were demonstrated by PET. Dynamic data were acquired using cardiac PET and MRI acquisition in the rat heart.

Discussion and Conclusion

The design of a first prototype of a small animal PET insert is finalized and was successfully tested within a 7T MRI (Bruker Biospec). Typical MR image sequences for anatomical and functional imaging did not affect the PET performance. In-vivo experiments demonstrated the benefit of such combination in small animal models. PET detector spatial resolution nearing 1mm, combined with accurate photon DOI determination, make it possible to return high resolution reconstructed images visualizing the 750um rods of the micro Derenzo-like phantom. This enables us to combine high resolution and sensitivity PET with high-field preclinical MRI to extract simultaneously complex data from anatomical to molecular information and to dynamically follow non-invasively animal models of different pathologies.

Acknowledgements

MRI and PET imaging systems were founded by the Hercules foundation (PI: U. Himmelreich) and Stichting tegen Kanker (PI: C. Deroose)

References

1Schug D, Lerche C, Weissler B, et al. Initial PET performance evaluation of a preclinical insert for PET/MRI with digital SiPM technology. Phys Med Biol. 2016 Apr 7;61(7):2851-78.

2Von Schulthess G K and Schlemmer H P W. A look ahead: PET/MR versus PET/CT. Eur. J. Nucl. Med. Mol. Imaging. 2009, 36 3-9.

3Drzezga A et al. First clinical experience with integrated whole-body PET/MR: comparison to PET/CT in patients with oncologic diagnoses J. Nucl. Med. 2012, 53 845–55.

4Jadvar H. and Colletti P. M. Competitive advantage of PET/MRI Eur. J. Radiol 2014. 83 84–94.

5Vandenberghe S. and Marsden P. K. PET-MRI: a review of challenges and solutions in the development of integrated multimodality imaging. Phys. Med. Biol. 2015, 60 R115.

6Gonzalez A.J., et al., Trans. Nucl. Science 63, pp. 2471, 2016.

Figures

Figure 1. PET/MRI assembly. A: MRI system with PET insert on its trolley where all PET electronics are installed. B: PET insert inside the gradient coil.

Figure 2. PET insert performance. A: Mini-derenzo phantom with rods size ranging from 0.7 to 2.4 mm and the corresponding profile for the 0.7 and 0.9 rods. B: PET field of view homogeneity performed inside the MRI scanner using homogenous phantom filled with 400 mCi of FDG

Figure 3. Homogeneous-FOV spatial resolution.

Fig4: In-vivo imaging with simultaneous PET and MRI acquisition. A: glioma model with injection of 50,000 CT-2A cells into the right striatum. Images were acquired at day 8 post inoculation (3D-T2 weighted MRI and PET FDG). B: Xenograft model showing heterogeneity from MRI images and FDG hotspot localized to region with short T1 values. C: Mouse whole body imaging using 2 bed position. MRI and bone scan (18F-Na) fused together.

Proc. Intl. Soc. Mag. Reson. Med. 25 (2017)
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