R2 and R2* mapping for sensing cell-bound superparamagnetic nanoparticles: in vitro and murine in vivo testing

Abstract

Purpose: To prospectively determine the cellular iron uptake by using R2 and R2* mapping with multiecho readout gradient-echo and spin-echo sequences.

Materials and methods: All experiments were approved by the institutional animal care committee. Lung carcinoma cells were lipofected with superparamagnetic iron oxides (SPIOs). Agarose gel phantoms containing (a) 1 x 10(5) CCL-185 cells per milliliter of agarose gel with increasing SPIO load (0.01-5.00 mg of iron per milliliter in the medium), (b) different amounts (5.0 x 10(3) to 2.5 x 10(5) cells per milliliter of agarose gel) of identically loaded cells, and (c) free (non-cell-bound) SPIOs at the iron concentrations described for (b) were analyzed with 3.0-T R2 and R2* relaxometry. Iron uptake was analyzed with light microscopy, quantified with atomic emission spectroscopy (AES), and compared with MR data. For in vivo relaxometry, agarose gel pellets containing SPIO-labeled cells, free SPIO, unlabeled control cells, and pure agarose gel were injected into three nude mice each. Linear and nonlinear regression analyses were performed.

Results: Light microscopy and AES revealed efficient SPIO particle uptake (mean uptake: 0.22 pg of iron per cell +/- 0.1 [standard deviation] for unlabeled cells, 31.17 pg of iron per cell +/- 4.63 for cells incubated with 0.5 mg/mL iron). R2 and R2* values were linearly correlated with cellular iron load, number of iron-loaded cells, and content of freely dissolved iron (r(2) range, 0.92-0.99; P < .001). For cell-bound SPIO, R2* effects were significantly greater than R2 effects (P < .01); for free SPIO, R2 and R2* effects were similar. In vivo relaxometry enabled accurate prediction of the number of labeled cells. R2' (R2* - R2) mapping enabled differentiation between cell-bound and free iron in vitro and in vivo.

Conclusion: Quantitative R2 and R2* mapping enables noninvasive estimations of cellular iron load and number of iron-labeled cells. Cell-bound SPIOs can be differentiated from free SPIOs with R2' imaging.