R2 and R2* mapping for sensing cell-bound superparamagnetic nanoparticles: in vitro and murine in vivo testing.
OBJECTIVE
To prospectively determine the cellular iron uptake by using R2 and R2* mapping with multiecho readout gradient-echo and spin-echo sequences.


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.


CONCLUSIONS
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.
Kuhlpeter
Rebecca
R
Department of Clinical Radiology, University Hospital of Muenster, Albert-Schweitzer-Str 33, D-48129, Muenster, Germany.
Dahnke
Hannes
H
Matuszewski
Lars
L
Persigehl
Thorsten
T
von Wallbrunn
Angelika
A
Allkemper
Thomas
T
Heindel
Walter L
WL
Schaeffter
Tobias
T
Bremer
Christoph
C
D000818:Animals; D045744:Cell Line, Tumor; D003287:Contrast Media; D005260:Female; D052203:Ferrosoferric Oxide / Q000737:chemistry / Q000493:pharmacokinetics*; D006801:Humans; D007089:Image Enhancement / Q000379:methods; D008175:Lung Neoplasms / Q000378:metabolism* / Q000473:pathology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D051379:Mice; D008819:Mice, Nude; D053758:Nanoparticles; D010316:Particle Size; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013113:Spin Labels; D051598:Whole Body Imaging / Q000379:methods*
D003287:Contrast Media; D013113:Spin Labels; C427127:superparamagnetic blood pool agent; D052203:Ferrosoferric Oxide