Magnetic Particle Imaging
Magnetic particle imaging (MPI) is a relatively new technique that is just now entering commercialization. This in vivo imaging technology offers many unique features that distinguish it from other clinical and research imaging modes, such as MRI and CT.
Although there are some commonalities with MRI, MPI is a very distinct imaging mode from MRI. The primary difference is the origin of the signal. In MRI, the magnetic signal derives from the magnetic moment of individual nuclei throughout the sample or specimen. In MPI, the magnets that produce the signal stem entirely from a tracer introduced into the animal. The tracers are biocompatible magnetic nanoparticles, such as superparamagnetic iron oxide nanoparticles. For their sizes, these particles are highly paramagnetic, and respond to magnetic fields very differently from biological tissue, which is generally weakly diamagnetic. Thus, unlike many other imaging modes, MPI does not image the body tissue itself, but only the tracer particles—another imaging method must be used to image the animal’s anatomy and then the MPI signal can be superimposed.
The imaging itself is effected by placing the subject in a complex magnetic field with a gradient, including a well-defined location where the magnetic fields cancel out: the field-free point (FFP). The magnetic particles are then subjected to an excitation magnetic field. In most of the sample, the particles are fixed in place by the magnetic field, but at the FFP, the particles are free to interact with the excitation field, and this response is measured. The size of the response quantifies the concentration of magnetic particles at that location. By adjusting the magnetic field gradient, the FFP can be moved throughout the sample, building up a 3-D image of the spatial distribution of the magnetic particles.
MPI can produce images with a spatial resolution of about 1 mm, and is fast enough to provide real time imagery at subsecond intervals, allowing the study of blood flow as it carries the tracer through the circulatory system. This is one of the primary applications for the technology. Other applications include pharmacokinetic studies, cell tracking and perfusion studies. Some tracers have been approved for use in humans, and clinical MPI is also developing rapidly, with similar applications focusing on cardiovascular and neurovascular imaging.
The technique was developed by Philips about 10 years ago, but commercial systems are a relatively new development. The current installed base is primarily composed of “home-built” systems developed at major research institutions. A partnership between Philips and Bruker produced the first commercial system in 2013. The Bruker-branded MPI system is comarketed by both firms. Magnetic Insight is a startup company whose products are based on technology developed at UC Berkeley. Lodespin Labs is focused on developing the tracing agents, commercializing research out of the University of Washington.
Magnetic Particle Imaging at a Glance:
Leading Suppliers
- Bruker/Philips
- Magnetic Insight
- Lodespin Labs
Largest Markets
- Academia
- Government
- Hospital/Clinical
Instrument Cost
- NA