Imaging the invisible in preclinical models using magnetic particle imaging (MPI Magnetic Particle Imaging)

The Magnetic Partcile Imaging (MPI) workflow combines some of the best features of existing medical imaging methods: (1) easy as optical imaging, (2) specific as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) imaging, (3) fast as computed tomography (CT) imaging, and (4) safe as magnetic resonance imaging (MRI). This will be explored in detail below. MPI has the sensitivity and ease of use of optical imaging. With a nanomolar detection limit, MPI sensitivity can be compared with in vivo fluorescence imaging, but without the confounding tissue absorption and signal scattering. MPI excels at tracer experiments such as visualizing functionalized nanoparticles and tracking the migration of cells deep within an animal. MPI has the translatability and depth penetration of nuclear imaging technique, such as PET and SPECT, without the complex workflow, safety, and half-life limitations of a radioactive tracer. The MPI signal and image resolution do not change with tracer depth since tissue does not attenuate magnetic fields. Neither the physics nor engineering are barriers to scaling MPI to human scales. MPI’s tracers are safe, long-lasting, and linearly quantifiable. Researchers have long worked with nanoparticle contrast agents in MRI as well as CT, and many groups have demonstrated nanoparticles targeting inflammation, atherosclerotic plaques, and cancers, among others. MPI can target the same applications, but with nanoparticles operating as the sole signal-producing tracer – providing improved contrast, sensitivity, and quantitation. Importantly, MPI’s imaging physics enables absolute quantitation of tracer in a region of interest, such as the number of cells at an implant site, or the amount of tracer deposited in the liver. MPI tracers are also safe and translatable; FDA- and EU-approved iron oxide tracers are currently available. MPI tracers do not photo-bleach, do not require a substrate injection to produce a signal, and unlike nuclear medicine, are stable in cells for weeks to months. Finally, while MPI’s unique capabilities are complementary to current imaging technologies, we expect MPI inflammation images will be taken in addition to standard MRI, CT, and PET scans. For example, MPI can be acquired in combination with CT, to provide a co-registered anatomic reference scan, in order to produce MPI/CT images similar in concept to PET/CT hybrid imaging.