Magnetic Particle Imaging
Magnetic Particle Imaging (MPI) is a new imaging modality that promises detection of nanomolar concentrations of super-paramagnetic iron oxide (SPIO) nanoparticles without depth limitations. The MPI method directly detects the bulk magnetization from an SPIO whose saturation magnetization approaches 0.6 T. The MPI method has extraordinary promise for sensitivity and contrast because this bulk SPIO magnetization is 10 million times more intense than the nuclear paramagnetism of water detected with a 7 Tesla MRI scanner.
SPIO tracers have shown exceptional utility in human MRI studies for liver and lymph node cancer detection and are routinely used in animal MRI experiments to tag and track cells. MRI detects the presence of SPIO nanoparticles indirectly, typically by detecting the dephasing of the signal from the nuclear paramagnetism of water surrounding the SPIO. Consequently, the detection limit is typically low, but there has been some progress in MRI detection of single SPIO tagged cells under ideal conditions. Numerous groups have developed positive and negative contrast methods, which aim to localize and quantify SPIO concentrations. MPI's goal in detecting nanomolar concentrations of SPIO particles would provide a 200-fold gain in sensitivity over MRI with positive contrast, and should enable reliable tracking of tens of tagged cells in vivo. Because there is no background signal from tissue, MPI should have excellent contrast as well.
Beyond cell tracking, MPI's excellent resolution and sensitivity would have medical applications including angiography using venous injections of SPIOs, examining allograft rejection, and tracking inflammation.
Abstracts
Direct Imaging of Ferumoxides using Magnetic Particle Imaging: Instrument Construction, Sensitivity, and 3d Imaging Society for Molecular Imaging 2008, Nice, France
Ultra-Sensitive Direct Detection of Ferumoxides for Small Animal Imaging Patrick Goodwill and Gary Lee and Greig Scott and Steve Conolly Society for Molecular Imaging 2007, Providence, Rhode Island