MPI Imager Technology
Two decades of development have been invested by groups across the world in order to develop reliable Magnetic Particle Imaging (MPI) instrumentation. MPI imaging requires a complex interaction of hardware and software to produce an image. Briefly, the imager performs the following tasks:
- Uses a Selection Field Gradient to create a field-free region (FFR).
- Slowly moves the FFR using Slow Shift Fields & Mechanical Shifting to cover the Field of View (FOV).
- Rapidly moves the FFR and detects the MPI signal using a Transmit and Receive System.
- Image Reconstruction of the MPI signal.
The selection field magnets in the Momentum Imager are water-cooled electromagnets and mounted on a rotating gantry.
Selection Field Gradient
The static magnetic field gradient, known as a “selection” field, balances the competing requirements of image resolution, signal strength, and scanning speed. The Momentum imaging system, for example, uses a pair of water-cooled electromagnets and a laminated iron field return to generate a 6 tesla/meter selection field gradient. This selection field strength produces a small FFR for millimeter-scale image resolution.
Slow Shift Field & Mechanical Shifting
The mean position of the FFR must be shifted slowly across the FOV to cover a large area. For example, to move the FFR, the Momentum uses a pair of electromagnets to generate a shift field of +/-0.180 Tesla to magnetically shift the FFR by up to +/- 3 cm, and can mechanically shift the sample by up to 12 cm. Depending on the design of the system, the magnets may also be placed on a rotating gantry to produce 3D images.
Animation showing how x-space gridding of the received signal generates an image of SPIO location.
Transmit and Receive System
The FFR is moved back and forth by a high fidelity, low frequency sinusoidal radiofrequency (RF) “drive” field. Before transmission, the RF waveform is filtered to remove unwanted noise and distortion. The radio waves are transmitted to the sample using a transmit coil. The effect of the driving field is to induce magnetic field flips in the tracer, which are then detected by the receiver system as high-frequency signals. These high frequencies are not present in the driving field and indicate the presence of tracer. After filtering and amplification, the signal is digitized.
Once the time-domain signal is acquired, it is processed based on the “x-space” MPI theory. X-space MPI is a “sensitive point” based method, where the image is reconstructed by gridding the received signal over time to pixels.