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

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 cellbound 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.