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Characterization of microparticles of iron oxide for magnetic resonance imaging
Jérémie P Fouquet1, Dina Sikpa1, Réjean Lebel1
1Centre d'Imagerie Moléculaire de Sherbrooke, Département de Médecine Nucléaire et Radiobiologie, Université de Sherbrooke, 3001 12e Avenue Nord, J1H 5N4 Sherbrooke, QC, Canada.
Abstract:
Microparticles of iron oxide (MPIOs) are increasingly used for contrast generation in magnetic resonance imaging (MRI). In particular, Dynabeads® MyOne™ Tosylactivated MPIOs have enabled sensitive and targeted molecular imaging, e.g., to detect vascular inflammation. For the first time we measured the relaxivities as well as the molar susceptibility χM of these MPIOs at 7 T in agarose gels. They are r1 = 0.69 ± 0.03 s-1/mM, r2 = 220 ± 6 s-1/mM, r2* = 679 ± 14 s-1/mM, and χM = 0.66 ± 0.05 ppm/mM, when expressed with respect to the iron concentration. These material parameters are essential to optimize MRI protocols and progress toward quantitative imaging. To address the heterogeneous nature of the MPIO distributions over the size of a typical MRI voxel, we coupled the MPIOs to a fluorophore to create a bimodal phantom that can be imaged by both Light Sheet microscopy and MRI. In this phantom, the MPIOs produced contrast similar to that found in vivo . The submicron resolution of Light Sheet microscopy images provided a precise measurement of the MPIO spatial distribution in phantoms also imaged by MRI. MPIO aggregates occupying less than one MRI voxel were responsible for alterations in R2* and magnetic susceptibility χ across several MRI voxels. In these cases, the sum of R2* or χ over the affected MRI volume correlated better with the microscopically determined number of MPIOs. These findings were confirmed with simulations performed in the static dephasing regime. The microscopically determined MPIO distribution was also entered directly into the simulation framework, indicating that the bimodal phantom is a useful tool to test theoretical models against experimental measurements.
Insights
We measured magnetic properties of iron oxide microparticles for MRI. A bimodal phantom revealed how particle distribution affects MRI signals, improving quantitative imaging accuracy.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Biomedical Engineering
- Materials Science
Background:
- Iron oxide microparticles (MPIOs) are crucial for MRI contrast enhancement.
- Dynabeads® MyOne™ Tosylactivated MPIOs enable sensitive molecular imaging, particularly for vascular inflammation.
- Accurate material parameters are vital for optimizing MRI protocols and quantitative imaging.
Purpose of the Study:
- To measure the relaxivities and molar susceptibility of MPIOs at 7 Tesla.
- To develop and utilize a bimodal phantom for studying MPIO distribution effects on MRI.
- To correlate microscopic MPIO distribution with MRI signal alterations.
Main Methods:
- Characterization of MPIO relaxivities (r1, r2, r2*) and molar susceptibility (χM) in agarose gels at 7T.
- Creation of a bimodal phantom by coupling MPIOs to a fluorophore for simultaneous MRI and Light Sheet microscopy.
- Imaging the phantom using MRI and Light Sheet microscopy to assess MPIO distribution and MRI contrast.
- Performing simulations in the static dephasing regime using experimentally determined MPIO distributions.
Main Results:
- MPIOs exhibited specific relaxivities and molar susceptibility values at 7T.
- The bimodal phantom demonstrated in vivo-like MPIO contrast in MRI.
- Light Sheet microscopy revealed that sub-voxel MPIO aggregates caused widespread R2* and susceptibility changes.
- Summed R2* and susceptibility across MRI voxels correlated better with MPIO counts than individual voxel values.
- Simulations validated the experimental findings and the utility of the bimodal phantom.
Conclusions:
- The measured MPIO properties are essential for optimizing MRI protocols.
- Bimodal phantoms are valuable tools for bridging microscopic MPIO distribution and macroscopic MRI signals.
- Understanding MPIO aggregation effects is key to improving quantitative MRI accuracy.
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