High-Throughput Total Body Dynamic Imaging in a Preclinical PET/MRI 4.7T System

1472 Introduction: PET tracer development is a key application of in vivo preclinical PET imaging in small rodents. Commonly, tracer candidates are costly, only available in small batches and, depending on the nuclide, they can be very short lived. This poses a strong motivation for simultaneous multi-animal imaging to enable sufficient statistical samples. MRI combined with PET has the potential for multiparametric imaging and hybrid tracer research. A PET/MRI system capable of high acquisition rates and a sufficiently large FOV for imaging the total body of up to 4 animals is presented here. Methods: The MRI subsystem features a 4.7T and 40 cm bore magnet. The PET subsystem is an insert of 20 cm in diameter with a FOV of 80 x 150 mm. An 86 mm quadrature volume coil for MRI signal excitation and detection was used. A purpose designed animal bed was employed allowing to image 4 mice. Animal warming was provided with a warm air supply to the system bore, temperature and respiration were monitored in a single animal, but the potential exists to acquire all 4 mice physiological signals. Data obtained using an F-18 labelled amino acid tracer is presented here but the same setup was used with a number of candidate PET tracers. An intravenous injection of 200 uCi in each animal was performed during the PET/MRI acquisition allowing for the dynamic biodistribution imaging of the PET tracer and the MRI contrast. The PET acquisition was 1.5 h and the data was reconstructed using MLEM, 12 iterations and 0.75 mm isotropic voxels. The following time bin schedule was used: 10 x 60 s; 10 x 180 s; 5 x 600 s. Besides anatomical MRI imaging, DCE and a Dixon based sequences for fat/water imaging were employed. The Turbo RARE sequence parameters employed for anatomical imaging were TE 45 ms, TR 800 ms FA 90, Matrix 260 x 340 x 50 and a voxel spacing of 0.25 x 0.25 and 1 mm. Results: Simultaneous total body PET/MR imaging in four mice was successfully achieved. Here we present the MRI morphological images co-registered with the PET during the first 5 min following the injection as well as the Time Activity Curves for the entire duration of the experiment. The full PET image is also presented separately. The results show the bolus injection through the inferior vena cava and the biodistribution allowing the study of the tracer kinetics in the target tissue, possible undesired organ accumulation as well as excretion rate. Conclusion: This work shows the possibility of simultaneous dynamic PET/MRI in 4 mice within an 86 mm RF coil. This methodology required a purpose designed animal bed combined with a PET/MRI system featuring a sufficiently large FOV capable of accurately quantifying relatively high rates (nearly 1 mCi). The methodology is being applied in a broad number of tracer development experiments. Acknowledgements: This work was possible thanks to the NIH shared instrumentation grant number S10 OD023503 01.