Abstract
PurposeInflammation is central in disease pathophysiology and accurate methods for its detection and quantification are increasingly required to guide diagnosis and therapy. Here we explored the ability of Fast Field-Cycling Magnetic Resonance (FFC-MR) in quantifying the signal of ultra-small superparamagnetic iron oxide particles (USPIO) phagocytosed by J774 macrophage-like cells as a proof-of-principle. MethodsRelaxation rates were measured in suspensions of J774 macrophage-like cells loaded with USPIO (0–200 μg/ml Fe as ferumoxytol), using a 0.25 T FFC benchtop relaxometer and a human whole-body, in-house built 0.2 T FFC-MR prototype system with a custom test tube coil. Identical non-imaging, saturation recovery pulse sequence with 90° flip angle and 20 different evolution fields selected logarithmically between 80 μT and 0.2 T (3.4 kHz and 8.51 MHz proton Larmor frequency [PLF] respectively). Results were compared with imaging flow cytometry quantification of side scatter intensity and USPIO-occupied cell area. A reference colorimetric iron assay was used. ResultsThe T1 dispersion curves derived from FFC-MR were excellent in detecting USPIO at all concentrations examined (0–200 μg/ml Fe as ferumoxytol) vs. control cells, p ≤ 0.001. FFC-NMR was capable of reliably detecting cellular iron content as low as 1.12 ng/µg cell protein, validated using a colorimetric assay. FFC-MR was comparable to imaging flow cytometry quantification of side scatter intensity but superior to USPIO-occupied cell area, the latter being only sensitive at exposures ≥ 10 µg/ml USPIO. ConclusionsWe demonstrated for the first time that FFC-MR is capable of quantitative assessment of intra-cellular iron which will have important implications for the use of USPIO in a variety of biological applications, including the study of inflammation.
Highlights
Tissue inflammation is a recognised disease process in many cardiovascular conditions, whether it involves the myocardiumAbbreviations: FC-MRI, Fast Field-Cycling Magnetic Resonance Imaging; FFCNMR, Fast Field-Cycling Nuclear Magnetic Resonance; MRI, Magnetic Resonance Imaging; SSC, Side Scatter; ultrasmall superparamagnetic iron oxide (USPIO), Ultra-Small Superparamagnetic Iron Oxide; SPIO, Superparamagnetic Iron Oxide.⇑ Corresponding author at: Aberdeen Cardiovascular and Diabetes Centre, uk
The detection range (0.115 ± 0.118 ng/mg to 12.398 ± 0.233 ng/mg protein) of the colorimetric iron quantification assay of non-fixed cells is shown for reference (p < 0.001 vs. control at all concentrations) as the ‘absolute’ intracellular iron amount against which all comparisons were made
The uptake of USPIO by J774 cells is modelled in a curvilinear fashion, as detected by this technique (Fig. 2)
Summary
Tissue inflammation is a recognised disease process in many cardiovascular conditions, whether it involves the myocardium⇑ Corresponding author at: Aberdeen Cardiovascular and Diabetes Centre, uk Inflammatory cell tracking with imaging in vivo is likely to become part of an individualised diagnostic pathway, assessing the severity of disease and monitoring the response to anti-inflammatory therapies. Inflammatory macrophages are well suited for MR imaging due to their strong phagocytic ability to uptake contrast agents such as ultrasmall superparamagnetic iron oxide (USPIO) particles [2]. Phagocytosed USPIOs are retained in inflammatory cells after the intravascular pool is cleared. Due to their superparamagnetic properties, USPIOs internalised by macrophages can be tracked with Magnetic Resonance Imaging (MRI) techniques in vivo [3].
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