September 15, 2016 (Vol. 36, No. 16)
Magnetic Particle Imaging Facilitates Research and Clinical Cell Therapy Success
Over the past decade, cell therapy has emerged as one of the fastest growing therapeutic directions for treating devastating conditions such as cancer and organ failure. This emergence has been enabled by advances in cellular engineering, improvements in the safety of cell-based grafts, and optimizations in culture manufacturing.
Current technology struggles to monitor grafts that survive and repair the body after administration. A reliable imaging method capable of cell tracking and monitoring engraftment could substantially improve the success rate of cellular therapies.
Presently, clinicians and scientists rely on measurements that take months to years after engraftment to assess treatment efficacy. These measurements, such as symptomatic improvement, functional imaging, and histological measurements, provide little insight into the engraftment process. A true cell tracking modality for stem cell therapy would allow clinicians to monitor engraftment in real-time, and would ideally have the following properties.
In this tutorial, we explore the capabilities of magnetic particle imaging (MPI), which is a disruptive imaging approach that is near-ideal for cell tracking.
MPI Technology
MPI is an imaging modality capable of sensitive imaging of superparamagnetic nanoparticle (SPIO) tracers with positive contrast and no depth limitations. The technique has demonstrated applications in whole-body, sensitive cell tracking, at sub-millimeter resolution scales. The technique’s high sensitivity is due to the detection of the intense electronic magnetization (0.6 T) in the SPIOs. This is very different than magnetic resonance imaging (MRI), which detects the nuclear paramagnetism of water. MPI images a magnetization that is stronger by a factor of more than 10 million than what we image in MRI.
Safety
MPI is safe and well-suited for clinical cell tracking. SPIO contrast agents used in MPI are commonly used in MRI imaging and as therapeutics. MPI uses benign, low-frequency magnetic fields for imaging, and does not ionizing radiation such as what is used in X-ray and PET imaging. The precise measurements suffer no tissue attenuation, a physical challenge with ultrasound and optical imaging. SPIOs readily tag cells, and, when used properly, have no short- or long-term effects on cell viability, proliferation, and differentiation.
SPIO Cell Labeling and Administration
A wide array of cell types including stem cells can be loaded with SPIO nanoparticles using a standard cell-labeling protocol. Cells are first seeded and grown to confluence, and then labeled with a SPIO such as VivoTrax™ (Magnetic Insight) by simple incubation. The intracellular iron uptake can be quantified in vitro using either MPI or ICP.
No significant change in cell viability occurred at working SPIO concentrations, which was confirmed by MTT viability assay.
Cells can be administered locally as a bolus or systemically with or without a targeting moiety. Human mesenchymal stem cells were labeled with an SPIO tracer and injected intravenously into a rat (Figure 1). Animals were then scanned up to a period of 12 days. Cells become localized in the rat lungs as seen in Figure 1A. MPI images can also be co-registered to either CT (computed tomography) or MRI.
Figure 1. (A) SPIO-labeled hMSCs localize to the lungs 5 min after tail vein injection. FOV: 6 cm x 4 cm x 4 cm; scan time: 15 min. (B) No signal attenuation occurs in MPI regardless of depth of the SPIO in tissue; however, significant attenuation is observed in optical imaging. (C) MPI signal resulting from SPIO-labeled hMSCs is linearly quantitative anywhere in the body, and cell numbers can easily be determined.
Signal Attenuation
The MPI signal is not attenuated by tissue (Figure 1B). Optical techniques are the most frequently used cell-tracking technique, but the nonlinear attenuation with depth and photon diffusion make true cell quantification and localization problematic. Only magnetic and ionizing imaging techniques do not suffer from depth attenuation. Furthermore, tissue is invisible to MPI, and so MPI images have zero signal background.
A standard curve can be generated using a range of a known number of cells and known cell iron-loading levels. This curve can then be used in conjunction with the MPI signal to accurately determine the number of cells present within the body. The data demonstrates the utility of linearly quantitative MPI signal, a sensitive cell detection limit, and sub-millimeter resolution images (Figure 1C).
Longitudinal Cell Tracking
MPI is well suited to longitudinal cell tracking. Here we demonstrate long-term (48 day) tracking of implanted myoblast grafts in the hind leg of rats (Figure 2). For comparison, we compare the MPI image to an MRI image of the same animal taken at the last time point. The background signal from water can makes it difficult to unambiguously identify the transplanted cells in vivo, and indeed interpretation of T2/T2* weighted images can be challenging even for experienced radiologists.
Quantification of a tracer in MRI also presents a challenge, as the effect of SPIOs on MRI images is inherently nonquantitative. The lifetime of the tracer can be as long as on the order of months as the tracer slowly breaks down in the low pH of lysosomes. MPI studies tracking neural progenitor cells implanted into the forebrain were monitored for 87 days post administration.
Figure 2. Longitudinal tracking of mouse myoblasts implanted contralaterally in the hind legs on days 2, 23, and 49. The signal at day 49 is fourfold lower due to clearance of the cells from the animal by the immune system. The MRI image at day 49 is nonquantitative and can be difficult to analyze. White arrows show iron. MPI FOV: 5 cm x 4 cm x 6 cm (X,Y,Z), 5 minute scan. MRI FOV: 5.5 cm x 5.5 cm, 5 minute scan. Images are not co-registered.
Conclusion
MPI is an excellent tool for identifying the best cell types, timing of delivery, dose, and delivery route of a stem cell therapy. MPI technology is new, and no other single imaging modality possess the unique capabilities of the technique. This technology has only recently been released to the market in the form of Magnetic Insight’s Momentum™ MPI scanner (Figure 3) , which is tailor-made for ultra-sensitive stem-cell tracking.
Figure 3. The Momentum magnetic particle imaging system offers ease of operation, has a highly sensitive detection limit, and allows high-resolution imaging of small animal models up to small rats.
Patrick Goodwill, Ph.D. ([email protected]), is CTO at Magnetic Insight. Bo Zheng, Ph.D., is from UC Berkeley, and Ryan Spitler, Ph.D., is from Stanford University.
