Initial Performance Assessment of a Combined microPET® Focus-F120 and MR Split Magnet System

This work is an initial evaluation of the performance of a microPET Focus-Fl20 system modified to fit in the split in a 1 Tesla MR imaging system to create a rodent scanner capable of simultaneous PET and MR acquisition. The MR imager is a novel design with a split in the magnet former, cryostat and gradient coils and with sufficient self-shielding to radically reduce the stray field at the cryostat edge. This permits the Focus-F120 detector geometry to be maintained with the addition of longer fibre bundles coupling the crystals to the PMT’s, which consequently are situated in a low magnetic field region outside the cryostat. Baseline PET performance was measured with gradient and shim coils in place but without RF coils in the PET FOV. Misalignment of the crystal blocks has decreased the PET spatial resolution, from 1.7 mm to 2.2 mm at 5 mm radial distance, compared to the Focus-F120. The peak absolute sensitivity is reduced to 5.3% compared to 6.8% for a 250-750 keV energy window and 6 ns coincidence window. The noise equivalent count rate (NECR-1R) peaks at about 215 kcps at 112 MBq for a NEMA rat phantom compared to 300 kcps for the Focus-F120 at 140 MBq. Magnetic field homogeneity was assessed as +/− 6 hertz over a 10 cm long and 8 cm diameter cylindrical space and a pulsed gradient strength of 200 mT/m, with a rise time of less than 100 microseconds, demonstrated on all three axes. Separate PET and MR imaging capability has been demonstrated as well as continuous, simultaneous acquisition with no detectable interference between the two imaging systems.