The first MPI systems reported used permanent magnets to generate a static field gradient. However, these typically only had a field-of-view (FOV) big enough to image individual organs in mice, due to limits on magnetic field strength and the associated radio frequency absorption by the scan subject. Overcoming this, researchers from the University of Würzburg in Germany developed travelling wave MPI (TWMPI), which uses an array of coils to create a dynamic field gradient and enables an FOV large enough to scan an entire mouse. In new work, they report its first in vivo images, a dynamic scan of a beating mouse heart (Phys. Med. Biol. 61 6620).

In TWMPI's simplest mode, saddle coils steer the small low-field region from which the nanoparticles' RF signal is detected - the field-free point (FFP) - in a straight line parallel to the scanner axis. In this so-called line-scanning mode (LSM), 3D data are acquired by repeating the line scan at multiple locations in the plane perpendicular to the scan axis. However, the mode is slow and, at 7 mm, has poor radial resolution.

To improve upon this, the researchers modified their system to enable a slice-scanning mode (SSM) where the FFP is steered along a sinusoidal path acquiring a 2D slice in a single trajectory. Contiguous slices are acquired to image a 3D volume. "The advantage of this mode is that it is possible to scan an entire slice out of the FOV within the same time as one line using the LSM," said first author Patrick Vogel. "This makes this mode very fast, up to almost 2000 frames per second."

In vivo imaging
Exploiting the high temporal resolution of the technique, the researchers performed a dynamic scan of the heart of an anaesthetized mouse. Positioned prone on the scanning bed, the mouse was injected with a 50 µl bolus of iron oxide nanoparticle solution during data acquisition. The TWMPI system acquired data for a single 7-mm coronal slice over 10 seconds at 20 frames per second. An MRI scan acquired beforehand provided a reference anatomical map. Two small glass spheres filled with a nanoparticle solution with fixed locations were used to co-register the two data sets.

When the researchers applied a region-of-interest (ROI) around the heart, they demonstrated temporal fluctuations in MPI signal strength as the heart emptied and filled with nanoparticles with each beat. Using a Fourier transform of the ROI data, the researchers measured the mouse's heart rate and demonstrated a match with that measured by an independent monitor. "The results show that the TWMPI scanner is sufficiently fast and sensitive to determine the distribution of an iron oxide tracer," said Vogel.

"For us it is a very important step," said Volker Behr, MPI group leader at Würzburg. "We demonstrated that the TWMPI approach is an alternative MPI scanner design that is able to work at the same level as other MPI systems based on the original design by Gleich and Weizenecker."

While the new scanning mode significantly improved the system's spatial resolution in the sinusoid scan plane, perpendicular to the scanner axis, it remained poor perpendicular to the scan plane. However, in separate research, the authors have demonstrated improvements by rotating the scan plane about the system's axis.

As part of ongoing research, the group are optimizing individual hardware components in the TWMPI system and improving the system's image reconstruction algorithm. They also plan to develop multimodal scanners combining MPI with MRI or CT. By maximizing the efficiency of data collection, hybrid scanners are key to the routine application of MPI in pre-clinical and clinical imaging, Behr told medicalphysicsweb.

Related stories
'Immobilized' calibration improves MPI
Multi-colour MPI expands imaging options
Particle-loaded blood cells enhance MPI
Magnetic particle imaging: moving ahead