Magnetic Particle Imaging (MPI) Technology
Magnetic Particle Imaging (MPI) uses two core technologies, a specialized imaging hardware and an imaging tracer, to produce 3d images of how a patient’s body is working. Considering these technologies in concert is essential. The imaging system’s resolution, sensitivity, and contrast are driven by the interaction between the imager’s main field gradient and the superparamagnetic iron oxide (SPIO) tracer’s magnetic properties. Additionally, the ability of a tracer to visualize an application, such as looking at the immune system, is driven by the nanoparticle’s behavior in tissue.
How does MPI work?
An MPI imager directly detects a tracer known as a Super-Paramagnetic Iron Oxide particle or SPIO. The technology detects only the SPIOs and does not see tissue. These tracers can be modified to suit the application desired. For example, suppose tracers selectively label immunotherapy cells such as CAR T-cells. In that case, we can use MPI to image the distribution of those cells after patient administration. After tracer administration, the patient is positioned in the imaging region of the MPI scanner, and we acquire a three-dimensional image of the tracer distribution.
Key to the MPI scanner operation is the strong magnetic field gradient produced inside the imaging volume by the main magnet. Inside this magnetic field gradient, a small region with low magnetic field strength exists, known as the field-free region or FFR. Outside of the FFR, the magnetic field is strong. You can see what this field looks like in the figure, where the magnets are shown in blue/green and the FFR in orange.
Rapidly moving the FFR over SPIOs causes the magnetization of any SPIOs passing through the FFR to “flip,” inducing a signal in the imager’s receive coil. Outside of the FFR, the strong magnetic field prevents magnetic nanoparticles from rotating, and particles that do not pass through the FFR do not induce a signal in the receiver coil.
During MPI image acquisition, the FFR is moved so that its trajectory samples the entirety of the imaging volume. Since the FFR location is known at all times, we can assign the received MPI signal to a specific location, producing a quantitative MPI image.
MPI or MRI?
Magnetic Particle Imaging (MPI) and Magnetic Resonance Imaging (MRI) are fundamentally different techniques despite apparent similarities in acronyms and the use of magnets. MRI detects the resonance of nuclear spins (typically the H nucleus) and is generally used for anatomical imaging. MPI detects magnetization in nanoparticles and is used mainly for cellular imaging. These two technologies also use different main magnet architectures.
MRI creates a uniform field to produce an image using weak gradients (mT/m) and strong field strengths (T).
Two strong magnets pointing at each other produce a magnetic field gradient with an FFR at the center. The FFR is then rapidly moved across the sample to produce an image using strong gradients (T/m) and weak field strengths (mT).