Magnetic Particle Imaging: A New Imaging Modality

“MPI is the most promising emerging imaging technology in the last 20 years and is expected to change the landscape of modern medical imaging and in vivo translational research.”
– IWMPI 2014

Magnetic Particle Imaging (MPI) is a new imaging modality that directly detects iron oxide nanoparticle tracers using time varying magnetic fields. Because the tracer is not normally found in the body, MPI images have exceptional contrast and high sensitivity. The MPI technique is straight forward and can be described classically with three key concepts:

1. Magnetic Nanoparticles Align with an Applied Magnetic Field

Magnetic nanoparticles act like nanoscale bar magnets that align to an applied magnetic field. When the nanoparticles are aligned, we say that their magnetization is “saturated,” and their magnetization is at a maximum. When there is no applied field, the nanoparticles randomly orient and are “unsaturated.”

Magnetic Neoparticles

Figure 1: Magnetic nanoparticles saturate with an applied magnetic field. (A, C) With a strong field, magnetic nanoparticles magnetically saturate and align. (B) With no applied field, magnetic nanoparticles randomly orient and produce no magnetization.

2. A Strong Magnetic Field Gradient Produces a Field Free Point (FFP)

A strong magnetic field gradient produces a special magnetic field where magnetic nanoparticle tracers are unsaturated at only one position, the FFP (Figure 2). Only nanoparticles passing through the FFP can produce a signal.

Magnetic Field Gradients

Figure 2: Two strong magnets pointing at each other (shown here as loops of wire) produce a magnetic field gradient with an FFP at the center. The FFP is then rapidly moved across the sample to produce an image.

3. Rapidly Moving the FFP Selectively “Flips” SPIO’s, Inducing a Signal in a Receive Coil

Rapidly moving the FFP causes the nanoparticles passing through the FFP to flip from a saturated negative value (Figure 1A) to a saturated positive value (Figure 1C). This change in magnetization induces a signal in a receive coil. Since we know where the FFP is at all times, we can assign the signal to the known position of the FFP to produce an MPI image.

Video 1: Induced flipping of SPIOs and signal development.