Evaluation of a separate-receive coil by magnetic particle imaging of a solid phantom

To aid the optimization and development of Magnetic particle imaging (MPI), an accurate and translatable apparatus and a methodology for the characterisation of imaging quality are required. This requires magnetic nanoparticles (MNP) as imaging tracer for MPI measurements in a long-term stable, defined state, since MNP in liquid suspension might tend to change their magnetic properties over time, e.g. as a result of particle aggregation, sedimentation, or disintegration. To this end, we have developed solid phantoms containing freeze-dried perimag in mannitol for the comparison of resolution capabilities between different MPI scanners. Freeze drying of MNP was determined to be the optimal immobilisation method for maintaining the dynamic magnetic properties of MNP as compared with polymer or starch techniques. Freeze-drying preserves the particle distribution, preventing the particles from aggregation or precipitation. Two different types of phantoms were developed, one (cross shaped) to seek for artefacts within reconstructed images, and one (coil shaped) to test the resolution limit. The resolution limit is determined from the coil phantom by measuring the smallest resolvable gap between the two adjacent tracer regions. Our resolution phantom contained a 2 mm wide channel filled with tracer. By using this phantom, the imaging performance of the preclinical MPI system and a prototype gradiometric separate receive coil installed within the same MPI scanner was determined. We found 2.1–2.2 mm resolution limit using the conventional set-up and 0.5–0.6 mm for the separate receive coil resulting in an improvement by factor four. Additionally, the resolution dependency on the channel wide was tested by a second resolution phantom contained a 1 mm wide channel. The long-term stability of the phantoms will be continuously monitored and improved.