Simultaneous Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) in small animals would enable entirely new approaches to drug development and yield to a better functional understanding of living organisms. However, preclinical systems that allow truly simultaneous PET and MRI are not yet commercially available. We present the integration of an optimized microPET camera with ultra-high magnetic field strength MRI.
Equipment: The prototype PET system (SynchroPET, Inc.; Stony Brook, NY) is based on a previously presented platform3, consisting of 12 scintillator crystals (LYSO,18.5x9.6x6mm3,4x8 pixels), APD and ASICS microchips (Fig.1). Experiments were performed at a preclinical 9.4T MRI (Bruker Biospin, Biospec 94/20 USR) equipped with a 440 mT/m imaging gradient insert with 114 mm inner diameter and a volume transceiver coil (outer/inner diameter=44/23mm).
Integration: We designed (FreeCAD v0.16) and 3D-printed (Stratasys Eden 260V;VeroWhite) an attachable integration module for the MRI’s motorized sample positioning system (AutoPAC,Bruker Biospin) that holds PET ring and RF coil (Fig.2).
Experiments: Despite physical independence of the imaging mechanisms, interactions between the two modalities include induced magneto-static inhomogeneities of the main magnetic field (B0), electromagnetic interactions of the PET electronics with RF pulses and rapidly switching imaging gradients. We studied effects of PET on MRI performance in three configurations: MRI-only and PET-MRI with PET powered off/on. For MRI we used a glass sphere phantom filled with CuSO4-doped water (9mm diameter) positioned in the center of the RF coil. The protocol included: PRESS (TR/TE=2500/20ms;averages = 1,no water suppression) after iterative linear shimming process to assess field inhomogeneity; 2D-T1-RARE (TR/TE=1500/8.5ms,factor=4,slice-thickness=2mm,voxel-size=100x100µm2,excitation/refocusing pulse-power=18/63W), a 2D-T2-SE (TR/TE=283.8/8.6ms,slice-thickness=1.5mm,voxel-size=105x105µm2,excitation/refocusing pulse-power=18/63W), a 2D-FLASH (TR/TE=350/5.4ms,flip=40º,slice-thickness=0.5mm,voxel-size=80x78µm2,pulse-power=20W), a 3D-multi-echo-GRE (TR/TE1=22.5/1.33ms,ΔTE=1.50ms,10 echoes,flip=7º,FOV=20x20x20mm3,200µm isotropic,bandwidth=357kHz,pulse-power=11W,78% duty-cycle) and a noise-dedicated 2D-FLASH without RF excitation pulse (TR/TE=350/5.4ms,FOV=55x55mm2,slice-thickness=0.5mm,voxel-size=100x100µm2, readout bandwidths: 100 kHz,200 kHz,300 kHz,400 kHz,500 kHz,550 kHz,570 kHz,and 1 MHz). To assess the high-resolution MRI performance we applied a 3D FLASH sequence (80µm isotropic, bandwidth=50 kHz,pulse-power=128 W) to a Gadolinium-perfused rat brain immersed in a perfluoropolyether (GaldenHT80). Effects of MRI on PET were investigated using a 50μCi Na-22 point source (0.23mm diameter;Eckert&Ziegler) in five configurations: PET outside magnet without/with coil, PET inside magnet without/with coil (no MR imaging), and PET inside magnet with coil (active MR imaging).
Analysis: We compared MRI images acquired in different settings side-by-side with respect to image artifacts and SNR. Static field inhomogeneities were assessed by calculating the water-line width. For PET experiments, we assessed the scintillation counts per second in each setting for 10 minutes.
1. Nagy, Kálmán, et al. "Performance evaluation of the small-animal nanoScan PET/MRI system." Journal of Nuclear Medicine 54.10 (2013): 1825-1832.
2. Vandenberghe, Stefaan, and Paul K. Marsden. "PET-MRI: a review of challenges and solutions in the development of integrated multimodality imaging." Physics in medicine and biology 60.4 (2015): R115.
3. S. H. Maramraju, S. D. Smith, S. S. Junnarkar, D. Schulz, S. Stoll, B. Ravindranath, M. L. Purschke, S. Rescia, S. Southekal, J.-F. Pratte, P. Vaska, C. L. Woody, and D. J. Schlyer, “Small animal simultaneous PET/MRI: initial experiences in a 9.4 T microMRI.” Physics in medicine and biology, vol. 56, no. 8, pp. 2459–80, 2011.