Magnetic particle imaging
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Magnetic particle imaging (MPI) is an emerging cross-sectional imaging technique that in the future may be a new clinical imaging modality offering high resolution, dynamic functional imaging without utilizing ionizing radiation.
Magnetic particle imaging is a tracer imaging technique, which relies on superparamagnetic iron oxide nanoparticles (SPION) to generate signal. It uses a homogeneous, static magnetic field (termed a selection field) which saturates all SPIONs except in a select field-free region (field free line (FFL). The static field typically has a strength of 4 T/m in current experimental systems. Another important, oscillating field (termed the drive field) is also needed for an MPI scan, the latter typically having a field strength of 0.1-20 mT. The oscillating magnetic field can only exert an effect on SPIONs in the field-free region, which ensures spatial coding of the obtained information 3.
During an MPI scan the FFL rapidly sweeps over the imaging region, during which only SPIONs align their magnetic moment along the external magnetic field, thereby creating a net magnetization vector. The relaxation of SPIONs (either via Brownian or Néel relaxation) in turn creates a voltage in the receiver coils of the system, from which the amount of SPIONs in the FFL can be calculated 1,3.
Current use, potential future applications and benefits
Currently magnetic particle imaging scanners are only utilized in preclinical small animal imaging, however there is no known theoretical obstacle that would limit the construction of clinical systems. If the techniques tested in preclinical settings can be scaled up for clinical use (the main difficulty being the required extremely homogeneous drive field), magnetic particle imaging as a tracer-based imaging modality could be utilized for a diverse array of functional imaging applications similarly to PET/SPECT without ionizing radiation. As biologic tissues (including those with endogenous iron) do not generate any signal in an MPI system, the modality offers a very high intrinsic contrast, and low detection limits. To this date MPI has been tested in preclinical models for blood perfusion imaging and angiography using free SPIONs. Construction of targeted SPIONs for a variety of future applications (oncology, inflammation, stem cell tracking) is an area of active research 1,2,3.