Abstract
Simple SummaryMagnetic Particle Imaging (MPI) is an emerging imaging technique that provides quantitative direct imaging of superparamagnetic iron oxide nanoparticles. In the last decade, MPI has shown great prospects as one of the magnetic methods other than Magnetic Resonance Imaging with applications covering cancer diagnosis, targeting enhancement, actuating cancer therapy, and post-therapy monitoring. Working on different physical principles from Magnetic Resonance Imaging, MPI benefits from ideal image contrast with zero background tissue signal, enabling hotspot-type images similar to Nuclear Medicine scans but using magnetic agents rather than radiotracers. In this review, we discussed the relevance of MPI to cancer diagnostics and image-guided therapy as well as recent progress to clinical translation.Background: Magnetic Particle Imaging (MPI) is an emerging imaging modality for quantitative direct imaging of superparamagnetic iron oxide nanoparticles (SPION or SPIO). With different physics from MRI, MPI benefits from ideal image contrast with zero background tissue signal. This enables clear visualization of cancer with image characteristics similar to PET or SPECT, but using radiation-free magnetic nanoparticles instead, with infinite-duration reporter persistence in vivo. MPI for cancer imaging: demonstrated months of quantitative imaging of the cancer-related immune response with in situ SPION-labelling of immune cells (e.g., neutrophils, CAR T-cells). Because MPI suffers absolutely no susceptibility artifacts in the lung, immuno-MPI could soon provide completely noninvasive early-stage diagnosis and treatment monitoring of lung cancers. MPI for magnetic steering: MPI gradients are ~150 × stronger than MRI, enabling remote magnetic steering of magneto-aerosol, nanoparticles, and catheter tips, enhancing therapeutic delivery by magnetic means. MPI for precision therapy: gradients enable focusing of magnetic hyperthermia and magnetic-actuated drug release with up to 2 mm precision. The extent of drug release from the magnetic nanocarrier can be quantitatively monitored by MPI of SPION’s MPS spectral changes within the nanocarrier. Conclusion: MPI is a promising new magnetic modality spanning cancer imaging to guided-therapy.
Highlights
Magnetic Particle Imaging (MPI) is an emerging magnetics-based imaging technique first introduced by Philips, Hamburg in 2005 [1]
Unlike Magnetic Resonance Imaging (MRI), where the signal comes from the precession of nuclear spin magnetic moments of the target nuclei (e.g., 1H, 2H, 13C, 17O, 19F, 23Na, 31P), the MPI signal is obtained from the ensemble magnetization of superparamagnetic iron oxide nanoparticles (SPION) as described by the Langevin model [2]
Because there are no SPIONs found in native biological tissue unlike the 1H in water and biological tissue sensed by MRI, MPI benefits from zero tissue background signal and achieves excellent image contrast comparable to tracer images typical of nuclear medicine scans such as positron emission tomography (PET) or single-photon emission computerized tomography (SPECT), which are the gold standard for diagnostic cancer imaging [3,4,5]
Summary
Magnetic Particle Imaging (MPI) is an emerging magnetics-based imaging technique first introduced by Philips, Hamburg in 2005 [1]. The same SPIONs in an MRI scan are typically semi-quantitative as they produce contrast changes via susceptibility differences (Figure 1a), yielding a non-linear indirect effect on the 1H signal [2]. The 2 ng value was calculated from the 370 MBq FDA-mandated dose limit for 18-FDG divided by the specific (radio)activity of 1ng of 18-fluorine (in MBq, averaged specific activity value) This implies that MPI can increase the systemic administered dosage to compensate and ensure tumor detection at 260 nM Fe sensitivity, even though this is poorer sensitivity than the 2 pM of PET [20]. If SPIONs can achieve similar targeting efficiencies to tumors as 18-FDG, MPI can be expected to be competitive with PET on a dose-limited comparison, and help avoid radiation dose (especially important for pediatrics). It is important to note that these qualities are not mutually exclusive and an SPION can be designed with multiple of these qualities such as a high heating performance magnetic core with stealth PEG-coating
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