On the effect of reference and sample volume on magnetic particle imaging quantification.

Magnetic particle imaging (MPI) is an emerging biomedical imaging modality that enables non-invasive quantification of superparamagnetic iron oxide nanoparticle (SPION) tracers by comparing signal in a region of interest (ROI) to that of reference samples of known tracer mass. However, despite growing interest and use of MPI, recent studies have raised concern over quantification accuracy as a function of the volume over which the tracer is distributed. Here, a constant mass dilution series of the tracer Ferucarbotran contained in well-defined sample geometries was used to evaluate the effect of relative reference and object sample volumes on MPI quantification accuracy. MPI images of this series were acquired under various scan modes, segmented using two commonly used methods, and iron mass quantified relative to two reference sample sets with different volumes. The maximum signal and percent error relative to ground truth were compared amongst these sample volumes, scan modes, analysis methods, and reference volumes. Tracer concentration influenced signal distribution and, in some cases, influenced iron estimation accuracy. Reference samples that were similar in volume to the volume of interest were found to provide the most accurate results across scan modes. The observed behavior is attributed to the partial volume effect, which arises due to the finite resolution in MPI. These results are discussed within the current scope of the field and suggest that care must be taken when selecting reference sample volume relative to test sample volume for quantitative MPI studies.