Magnetic Particle Imaging for Radiation-Free, Sensitive and High-Contrast Vascular Imaging and Cell Tracking

Abstract

Magnetic particle imaging (MPI) is an emerging ionizing radiation-free biomedical tracer imaging technique that directly images the intense magnetization of superparamagnetic iron oxide nanoparticles (SPIOs). MPI offers ideal image contrast because MPI shows zero signal from background tissues. Moreover, there is zero attenuation of the signal with depth in tissue, allowing for imaging deep inside the body quantitatively at any location. Recent work has demonstrated the potential of MPI for robust, sensitive vascular imaging and cell tracking with high contrast and dose-limited sensitivity comparable to nuclear medicine. To foster future applications in MPI, this new biomedical imaging field is welcoming researchers with expertise in imaging physics, magnetic nanoparticle synthesis and functionalization, nanoscale physics, and small animal imaging applications.

Introduction

Magnetic particle imaging (MPI) is a new tracer imaging technique first introduced by Philips, Hamburg¹. MPI directly images the location and concentration of superparamagnetic iron oxide nanoparticle (SPIO) tracers with time-varying magnetic fields and has remarkably high sensitivity and contrast. Several SPIOs are clinically approved and currently on the market, including Feraheme (ferumoxytol), which is FDA approved for treatment of anemia in chronic kidney disease (CKD) patients². Outside of the United States, SPIOs are available for patient imaging (Resovist), sentinel lymph node localization (Sienna) and hyperthermia (NanoTherm) applications^3–5. Safe, radiation-free scanning in MPI combined with high-contrast, high-sensitivity imaging gives MPI fundamental advantages in vascular imaging and cell-tracking, which we discuss in this review.

The physics of MPI relies on the electronic magnetization of SPIOs¹. When an excitation field is applied to SPIOs in the field-of-view (FOV), the magnetic dipoles reorient rapidly in response. Much like in MRI, the change in magnetization can be visualized via Faraday’s law with a receiver coil. Unlike MRI, the change is of electronic magnetization, rather than nuclear magnetization. This results in a higher sensitivity for MPI, as the electronic magnetization of iron is 22 million times stronger than that of the nuclear magnetization of water at 7 Tesla⁶. To localize this signal, a large gradient field is used. Outside of a small region with a close to zero field, termed the field free region (FFR), the gradient locks SPIOs in place even if the excitation field is applied. Inside the FFR, the SPIOs reorient in response to the excitation field. By rastering this FFR across each point in the FOV, an image is created.

MPI sensitivity can be as low as nanograms of iron (corresponding to as few as ~200 cells for cell tracking applications), and resolution can be as fine as ~1 mm^7,8. These specifications were obtained on academic prototype scanners. Commercial preclinical MPI scanners were only recently introduced by Bruker GmbH and Magnetic Insight Inc, and specifications are steadily improving. Theoretical work predicts that a human MPI scanner could have picogram sensitivity in a 1 second scan⁹ and technical improvements have been made approaching this goal¹⁰. MPI has no view limitations, and it works robustly even in the lungs and bones, where MRI and Ultrasound routinely fail. For instance, Fig. 3b clearly resolves SPIO-labeled stem cells in the lungs. Indeed, researchers have demonstrated proof-of-concept studies for MPI imaging of lung perfusion and ventilation^11–13. MPI also has no depth limitations, unlike optical imaging methods. Many molecular reporters employed in cell culture studies and small animals employ optical fluorescence or bioluminescence probes, which are fundamentally limited by optical scattering and attenuation to surface applications. Last, unlike radiotracers, the SPIOs reporter “half life” is essentially infinite, enabling researchers to track the location of labeled cells even three months after introduction to a murine model¹⁴. This may be enabling since Nuclear Medicine reporters last for only hours (FDG 2 hours half life, Tc-99m 6 hours, In-111 2.8 days) while many pathophysiologic processes require weeks to manifest. Finally, MPI obtains a dose-limited sensitivity that is already competitive with Nuclear Medicine method on prototype MPI scanners.

Figure 3. Selected MPI cell tracking applications.

(a) Stem cell implant imaging in mice. MPI (A), MRI (B) and corresponding overlay MPI/MRI (C) of a mouse brain transplanted with 1 × 10⁵ (left hemisphere) or 5 × 10⁴ (right hemisphere) SPIO-labeled mesenchymal stem cells. Reproduced from²⁸ under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). b) Stem cell injection imaging in rats. 3D MPI-CT imaging of intravenously injected human mesenchymal stem cells (hMSCs) and SPIO control. (A) MPI imaging of hMSC tail vein injections <1 hr post-injection shows substantial hMSC localization to lung. (B) At 12 days, hMSC tail vein injections show significant total clearance and liver migration. (C) MPI imaging of SPIO-only tail vein injections less than one hour post-injection shows immediate uptake in liver and spleen. (D) Control injections of isotonic saline show no detectable MPI signal. Reproduced with permission from^27• under the Creative Commons Attribution (CC BY-NC) License (https://creativecommons.org/licenses/by-nc/4.0/). (c) Long-term stem cell implant imaging in rats. (LEFT) Longitudinal MPI imaging of 5 × 10⁵ SPIO-labeled human NPCs implanted in the forebrain cortex over 87 days. Scale bar 1 cm. Color intensity in ng/mm². (RIGHT) Postmortem Prussian blue (PB) staining confirms presence of iron-labeled cells at administration site. Adapted with permission from^27• under the Creative Commons CC BY License (https://creativecommons.org/licenses/by/4.0/).

In this review, we discuss recent applications in MPI, broadly divided into vascular imaging and cell tracking, as described in Fig. 1. Additionally, we discuss current needs in tracer development to enable future applications in MPI molecular imaging.

Figure 1. MPI Imaging Applications today.

Broadly, MPI researchers have pursued vascular imaging and cell tracking. In vascular imaging, researchers have used both tracers that passively highlight the physiology of interest, or are specifically targeted via an antibody or other moiety. In cell tracking, researchers have imaged several types of stem cells, and more recently interest has grown in imaging immune cells for infection imaging, immunotherapy tracking and early-stage cancer detection. Scanner schematic adapted with permission from²⁴. Copyright 2017 American Chemical Society. Vascular imaging phantom image courtesy of Justin Konkle. Stem cell tracking image courtesy of Bo Zheng. Image fusion of MPI (color) and CT (grey).

Vascular Imaging

The earliest applications of MPI focused on vascular imaging with untargeted SPIOs, such as 3D imaging of a beating mouse heart using Resovist¹⁵. This early work emphasized some of the inherent advantages of MPI – three-dimensional, high-contrast and fast imaging. Both imaging speed and circulation time of the MPI tracers is crucial for vascular applications.

SPIOs used in MPI typically have a hydrodynamic diameter between 50–100 nm, and they remain in the bloodstream until cleared by the reticuloendothelial system¹⁶. Circulation time varies from minutes to hours depending on the nanoparticle coating. Early MPI researchers typically relied on ferucarbutran (also known as Resovist or VivoTrax), which is a cyclodextrin coated SPIO originally designed as a MRI liver imaging agent to highlight cancerous lesions¹⁵. It targets the liver within minutes³. The Krishnan group at University of Washington developed SPIOs with 2-fold better spatial resolution compared to Ferucarbutran and extended the circulation time of MPI SPIOs to 2+ hours in mice and 4+ hours in rats using polyethylene glycol coatings^17,18. Alternative approaches to increasing MPI tracer circulation time include loading SPIOs into red blood cells^7,19,20. These long-circulating tracers are crucial for applications like cancer imaging via the enhanced permeability and retention (EPR) effect^21••, brain imaging for traumatic brain injury (TBI) and stroke via visualizing cerebral blood volume and cerebral blood flow^22••,23, and gastrointestinal (GI) bleed imaging²⁴. Each of these applications shown in Fig. 2 requires imaging over hours or days, and hence long circulating SPIO tracers offer MPI an advantage over nuclear medicine techniques, in which radionuclide decay limits the imaging time course. As an example, longer circulation time enables blood volume and blood perfusion studies in the brain, which could aid in stroke diagnosis and treatment planning. We can also appreciate the information available from the high resolution of MPI optimized tracers. For instance, in Fig. 2a at 10 minutes the initial wash-in of nanoparticles into the tumor margins is visible. This subtle rim enhancement is discernible hours before the SPIOs have highlighted the entire tumor at the 6 hour time point.

Figure 2. Selected MPI vascular imaging applications.

(a) Cancer imaging of rats. MPI/CT of a human breast tumor xenograft shows enhanced image contrast 6 h after SPIO injection. Arrows indicate tumor volume. Adapted with permission from^21••. Copyright 2017 American Chemical Society. (b) Traumatic brain injury (TBI) imaging of rats. Blue dotted circle indicates impact site. Green circles indicate lymph nodes. The TBI rat has significant signal from the hemorrhage, as well as signal inside the lymph nodes, unlike the control²³. Copyright Institute of Physics and Engineering in Medicine. Adapted with permission of IOP Publishing. All rights reserved. (c) Stroke imaging of mice. MRI and MPI signals were plotted over time for certain selected regions of interest: filled black circles, MRI signal ischemic hemisphere; filled black squares, MRI signal healthy hemisphere; red dotted line, MPI signal ischemic hemisphere; red crosses, MPI signal healthy hemisphere). The concentration–time curves of the MPI and MRI showed similar progression and reduced wash-out of the contrast agents into the ischemic hemisphere. Reprinted with permission from^22••. Copyright 2017 American Chemical Society. (d) GI bleed imaging of mice. Dynamic projection MPI and subtraction MPI images, both co-registered to X-ray anatomical reference, allow detection and quantification of GI bleed in a ApcMin/+ mouse model predisposed to GI polyp development. Reprinted with permission from²⁴. Copyright 2017 American Chemical Society.

Recently, groups have employed both biochemical and biomechanical targeting moieties on SPIO surfaces to highlight pathophysiologies. Biochemical active targeting to cancers includes using lactoferrin to highlight brain glioma^25,26, while biomechanical targeting includes labeling macroaggregated albumin (MAA) with SPIOs to evaluate lung perfusion or directly administering nebulized SPIOs to the lung airways^11–13. Unlike radiotracers, SPIO signal is stable over time, obviating the need for preparation immediately before patient use. The convenience and image quality of 3D MPI with targeted SPIOs will enable MPI scans as a compelling alternative to FDG-PET or Tc99m-V/Q scintigraphy for diagnosing cancer and pulmonary embolism, respectively. Future work in vascular imaging will continue in this vein to highlight specific pathophysiologies and harness the increasing resolution of MPI technology to highlight previously hidden perfusion behavior.

Cell Tracking

In recent years, stem cell tracking has gained research traction as a theranostic technique. Successful stem cell therapy requires verification that stem cells reach their intended destination, remain there over time and maintain viability to eventually create functional tissues or organs. MPI tracking of stem cells, shown in Fig. 3, has been demonstrated to quantitatively image the biodistribution and fate of intravenously administered stem cells in rats, neural implants of stem cells in rats, and neural implants of stem cells in mice^27•,14,28. Of note is that the study tracking the biodistribution of intravenously administered stem cells was able to robustly image the presence of stem cells in the lungs, an area which is traditionally difficult for MRI and Ultrasound due to artifacts caused by air-tissue interfaces. Moreover, the neural implant work imaged stem cells over 87 days, a study which would not be possible with the clinical tracer modalities today due to the need for radioactively decaying tracers. These early studies on stem cell lines have sparked further interest in the field of MPI stem cell tracking, leading to work demonstrating that MPI can be used to image pancreatic islet transplants in mice and work developing new nanoparticle agents for multimodal imaging of stem cells, including via MPI^29,30. These studies demonstrated up to 200-cell sensitivity, which is far more sensitive than other medical imaging modalities³¹. Last, recent progress has been made in correlating MPI relaxation times to stem cell viability in vivo³². This technique relies on analyzing the harmonic spectra of the MPI signal (Magnetic Particle Spectroscopy - MPS) to determine if the SPIOs are still within the cell or have been released to the outside after cell death. With new particle and arbitrary excitation design³³, it may be possible to further distinguish stem-like states versus apoptotic or non-stem states.

Researchers have recently explored MPI for tracking prelabeled white blood cells (WBCs), which could emerge as a radiation-free alternative to infection imaging via traditional WBC-In111 scintigraphy (Gaudet et al., Zhang et al., Chandrasekharan et al., World Molecular Imaging Conference, Philadelphia, September 2017). Preliminary work has shown that MPI can track cultured macrophages in both healthy and middle cerebral artery occlusion (MCAO) stroke mice, and tumor-associated macrophages in a 4T1 mouse model of metastatic cancer. Future work in MPI cell tracking is expected to extend these imaging studies to theranostic applications, such as tracking stem cell migration for disease treatment, or immune cell migration for immunotherapy treatment of cancers.

We note that the theranostic potential of MPI extends far beyond guiding cell-based treatments. Although this review focuses on MPI imaging applications, extensive work has also been performed demonstrating the utility of MPI for ionizing radiation-free surgical guidance^34–37 and therapeutic guidance, such as with magnetic hyperthermia^38–42.

Tracer Development

In the early days of MPI, researchers had to rely on SPIOs designed for T2* or T1-weighted MRI contrast agents⁴³. Since MRI and MPI “see” SPIOs with completely different physics, the MPI performance of these SPIOs was not ideal. Dramatic improvements in sensitivity and image resolution have been enabled through tailoring of SPIOs to MPI’s unique physics^44–46. Both theory and experiment have shown that 5–20 nm core sizes show poor resolution^47–49. Larger core sizes (>27 nm) do not perform as well as the optimal range of 20–26 nm, due to relaxation blurring⁵⁰. While 1–2 mm resolution is comparable to nuclear medicine’s resolution, a dramatic improvement in spatial resolution (100 microns) would obviate perhaps the last technical weakness of MPI. Since image resolution is improved both by either better SPIOs or higher scanner gradients, these MPI-tailored SPIOs could dramatically reduce scanner cost. A ten-fold improvement in SPIO magnetic resolution could reduce the cost of a clinical MPI scanner by nearly one-hundred-fold. Hence, high-resolution SPIOs remain an exciting and crucial area in MPI research to enable safe and cost-effective human MPI.

Beyond improving MPI signal and resolution, harnessing MPI physics to provide molecular contrast is an exciting research area. Color MPI extracts information about SPIO relaxation to distinguish between nanoparticles with different relaxation dynamics and opens the door to multiplexing in MPI, similar to fluorescence multiplexing⁵¹. This work has recently been extended to in vivo imaging, as shown in Fig. 4 (Hensley et al., World Molecular Imaging Conference, Philadelphia, September 2017). Relaxation mechanisms include Néel and Brownian, which are strongly influenced by the viscosity of the medium. In general, smaller particles exhibit mainly Néel behavior and are unaffected by the media viscosity or by binding. Larger particles exhibit mainly Brownian behavior and are very sensitive to media viscosity and to binding. Small relaxation changes can be detected via color MPI, but even standard MPI can detect large relaxation changes^52•. MPI has seen only limited research into targeted nanoparticle coatings that target specific pathophysiologies^53•,54. It will be exciting to see the vast area of nanomedicine research applied to MPI-targeted SPIOs for high specificity and sensitivity diagnosis of disease with zero ionizing radiation.

Figure 4. Recent progress in tracer technologies.

(a) In vitro tri-color MPI. Three different MPI tracers are indistinguishable in a standard MPI reconstruction algorithm, but can be distinguished after applying a multi-color reconstruction algorithm. Adapted with permission from⁵¹ under the Creative Commons Attribution 3.0 license (http://creativecommons.org/licenses/by/3.0). (b) In vivo dual-color MPI. Rat lung and liver are targeted with two nanoparticles with different relaxation behavior. In standard MPI, the organs are indistinguishable, but after the colorizing algorithm the organs can be distinguished based on the relaxation behavior of the SPIOs within. Image courtesy of Daniel Hensley. (c) Multi-modal Janus iron oxide MPI tracers. SPIO tracers can be designed for multi-modality imaging. Mice were subcutaneously implanted with nanoparticle-labeled cells and imaged under MPI, fluorescence and T2-weighted MRI. Adapted with permission from^53•. Copyright 2017 American Chemical Society. (d) Lung perfusion imaging with MAA-SPIO. Large macroaggregated albumin conjugated to SPIOs are biomechanically trapped in the rat lung, allowing imaging of blood perfusion through the lungs¹¹. Institute of Physics and Engineering in Medicine. Adapted with permission of IOP Publishing. All rights reserved.

Conclusion

Magnetic particle imaging is an emerging SPIO tracer imaging modality with superb image sensitivity and zero tissue background signal, enabling unprecedented contrast-to-noise ratio (CNR) for a zero-radiation imaging modality. MPI uses no ionizing radiation and uses SPIO tracers, some of which have been used safely in patients. The MPI signal is the same anywhere and at any depth in the body, allowing for quantitative imaging, including in the lungs or bone marrow, which are challenging regions for MRI, Ultrasound and X-ray. Here, we focused on recent vascular perfusion imaging and cell tracking applications of MPI. Emerging applications include real-time guidance of surgery and magnetic hyperthermia. Color MPI could soon enable molecular contrast in MPI, distinguishing bound from unbound antibodies in vivo. Industry researchers are rapidly developing preclinical and, soon, clinical MPI scanners. MPI scanners may be integrated with MRI⁵⁵ or CT scanners, akin to hybrid PET-CT scanners. The high-contrast, ionizing radiation-free and superb sensitivity of MPI is expected to usher in new applications for preclinical and clinical researchers.