Magnetic particle imaging (MPI) represents a tomographic imaging method that is used to determine the spatial distribution of magnetic material injected into the bloodstream. Iron-oxide based superparamagnetic nanoparticles are used as a suitable material for the purpose of this quite novel imaging technique.
The concept of MPI was conceived in 2001 by Dr. Bernhard Gleich at the Philip Research Laboratories in Germany. This method takes the advantage of superparamagnetic iron oxide nanoparticles response to an oscillating magnetic field, in order to determine their local concentration and spatial distribution.
MPI represents the first medical application where nanoparticles are not merely supportive contrast vehicles (as in magnetic resonance imaging), but the single source of signal and, therefore, the only visualized constituent. Therefore iron-oxide nanoparticles are usually referred to as tracers, and not contrast agents.
Capabilities such as very high spatial and temporal resolution, no need for ionizing radiation and creation of three-dimensional images with a great contrast sets MPI apart from other imaging modalities that are already established in medicine. This predisposes MPI for a myriad of different medical applications, such as cardiovascular interventional and diagnostic procedures, as well as cell tracking and labelling.
Vascular imaging is currently performed by X-ray and digital subtraction angiography, or computed tomography and magnet resonance angiography. The latter two are still the gold standard for diagnostic purposes, whereas digital subtraction angiography is considered the method of choice for interventional purposes.
Nevertheless, all the aforementioned methods burden physicians and patients alike with a substantial amount of ionizing radiation. That is where MPI comes into play as an interesting and viable option for diagnostic purposes; additionally, fast 3D imaging properties combined with specially coated instruments can be employed for image guided interventions.
Instrument maker company Bruker offers high-quality preclinical MPI scanners, which give 3D images at millisecond intervals. It can perform imaging of up to 46 volumes per second, which enables real-time imaging of biological activities at an equal (or higher) spatial resolution in comparison to positron emission tomography.
Cellular and targeted imaging
The affinity of iron-oxide based superparamagnetic nanoparticles towards the cells of the reticuloendothelial system has been employed for inflammation imaging (for example, in arthritis or for atherosclerotic plaques). The application of MPI has been extended to tumor imaging as well, where enhanced permeability and detainment effect of tumor vessels were exploited.
Iron-oxide nanoparticles can also be modified by various coatings, especially by adding ligands (i.e. peptides, antibodies or polysaccharides) for active targeting. One other possible approach is to label certain cells with these nanoparticles ex vivo and then monitor their migration in vivo.
Most of these scenarios still need to be evaluated in human organism, but a big step in the right direction has been made by the development of commercial MPI system for small animals by Bruker. 3D distribution of administered iron oxide nanoparticles using signal of 25 kHz can be recorded smoothly from any depth within the animal, which enables high flexibility in choosing areas of interest.
- Weizenecker J, Gleich B, Rahmer J, Borgert J. Particle Dynamics of Mono-domain Particles in Magnetic Particle Imaging. In: Buzug T, Borgert J, Knopp T, Biederer S, Sattel TE, Erbe M, Lüdtke-Buzug K, editors. Magnetic Nanoparticles: Particle Science, Imaging Technology, and Clinical Applications: Proceedings of the First International Workshop on Magnetic Particle Imaging. World Scientific Publishing Co. Pte. Ltd., 2010; pp. 3-16.