Fast field-cycling magnetic resonance detection of intracellular ultra-small iron oxide particles in vitro: Proof-of-concept

ABSTRACT
PURPOSE
Inflammation 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.
METHODS
Relaxation rates were measured in suspensions of J774 macrophage-like cells loaded with USPIO (0-200μg/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 90o flip angle and 20 different evolution fields selected logarithmically between 80 μT and 0.2T (3.4kHz 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.
RESULTS
The T1 dispersion curves derived from FFC-MR were excellent in detecting USPIO at all concentrations examined (0-200μg/ml Fe as ferumoxytol) vs. control cells, p ≤ 0.001. FFC-NMR was capable of reliably detecting cellular iron content as low as 1.12ng/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 ≥ 10µg/ml USPIO.
CONCLUSIONS
We 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.