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WIREs Nanomed Nanobiotechnol
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Quantitative magnetic resonance fluorine imaging: today and tomorrow

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Abstract Fluorine (19F) is a promising moiety for quantitative magnetic resonance imaging (MRI). It possesses comparable magnetic resonance (MR) sensitivity to proton (1H) but exhibits no tissue background signal, allowing specific and selective assessment of the administrated 19F‐containing compounds in vivo. Additionally, the MR spectra of 19F‐containing compounds exhibited a wide range of chemical shifts (>200 ppm). Therefore, both MR parameters (e.g., spin–lattice relaxation rate R1) and the absolute quantity of molecule can be determined with 19F MRI for unbiased assessment of tissue physiology and pathology. This article reviews quantitative 19F MRI applications for mapping tumor oxygenation, assessing molecular expression in vascular diseases, and tracking labeled stem cells. WIREs Nanomed Nanobiotechnol 2010 2 431–440 This article is categorized under: Diagnostic Tools > Biosensing Therapeutic Approaches and Drug Discovery > Emerging Technologies

(a) Representative 19F spectrum of perfluoropolyether (PFPE) and perfluorooctylbromide (PFOB) nanoparticles shows the chemical shift of 19F signatures. The single PFPE peak and five discernible PFOB peaks are easily detected and individually resolved. (b) Chemical structure of PFPE shows its 20 19F atoms. (c) Schematic of a PFPE nanoparticle functionalized with homing ligands in the outer phospholipid monolayer (shown in green). The PFPE nanoparticle provides 1H magnetic resonance (MR) contrast by its surface payload of ∼90, 000 Gd3+ (shown in gold) and 19F MR contrast by ∼100 M 19F in its core. (Reprinted with permission from Ref 10. Copyright 2004 John Wiley & Sons, Inc. and Ref 11. Copyright 2009 Elsevier).

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Localization of perfluorocarbon (PFC) nanoparticles labeled cells in mice using 19F magnetic resonance imaging (MRI). (a) 19F MRI trafficking of stem/progenitor cells labeled with either perfluorooctylbromide (PFOB) (green) or perfluoropolyether (PFPE) (red) nanoparticles. Labeled cells were locally injected into the skeletal muscle of mouse thigh before MRI. (b)–(d) At 11.7‐T field strength, 19F spectral discrimination permits respective imaging of ∼1 × 106 PFOB‐loaded cells (b) and PFPE‐loaded cells (c). The composite 19F (displayed in color) and 1H (displayed in grayscale) images (d) reveal the location of PFOB labeled cells in the left leg and PFPE labeled cells in the right leg (dashed line indicates 3 × 3 cm2 field of view for 19F images). (e) Similarly, a 19F image acquired at 1.5‐T field strength shows 19F signal from ∼4 × 106 PFPE nanoparticles labeled cells. (f) The composite 19F and 1H images show the location of PFPE nanoparticles labeled cells in a mouse thigh. Overall, the absence of background signal in 19F images (b), (c), and (e) enables unambiguous localization of PFC‐containing cells at both 1.5 and 11.7 T field strength. (Reprinted with permission from Ref 67. Copyright 2007 FASEB).

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Diffusion weighted 19F signal in the ear of K14‐HPV16 mice (open symbols), an animal model of squamous cell cancer with dysplastic lesions developed in the ear epidermis, and in the ear of control C57BL/6 mice (filled symbols). All animals were intravenously injected with αvβ3‐integrin targeted perfluorocarbon nanoparticles before magnetic resonance imaging. (a) Results show complete decay of 19F signal in control mouse ears when b value (i.e., an index of diffusion weighting) is > 1500 s/mm2. In contrast, a large fraction of 19F signal persisted in the ears of K14‐HPV16 mice at all b values, reflecting the specific binding of targeted nanoparticles to the ear neovasculature of K14‐HPV16 mice. (b) Diffusion weighted 19F signal in the ears of K14‐HPV16 mice persisted even when b values are > 10,000 s/mm2. (Reprinted with permission from Ref 55. Copyright 2008 John Wiley & Sons, Inc.).

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(a) A representative 19F spectrum of perfluoropolyether (PFPE) nanoparticle emulsion (−90 ppm) and trichlorofluoromethane reference standard (0 ppm) acquired at 4.7 T. (b) The calibration curve for PFPE nanoparticle emulsion shows a linear relationship between the quantity of perfluorocarbon nanoparticles and 19F signal intensity. (c) Left: an optical image of a human carotid endarterectomy sample shows moderate luminal narrowing and several atherosclerotic lesions. Middle: a 19F projection image acquired through the entire thickness of carotid artery sample shows high 19F signal along the lumen because of the binding of nanoparticles to fibrin. Right: The calculated concentration map of bound nanoparticles in the carotid sample based on 19F signal intensity in each voxel and the calibrated standard curve in (b). (Reprinted with permission from Ref 10. Copyright 2004 John Wiley & Sons, Inc.).

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Fluorocarbon relaxometry using echo planar imaging for dynamic oxygen mapping determined pO2 maps of two representative AT1 tumors in rats. The pO2 value was calculated pixel‐by‐pixel based on the quantified 19F R1 and a priori calibrated 19F R1–pO2 curve of hexafluorobenzene (HFB). (a) and (d) Composite 19F (displayed in color) and 1H (displayed in grayscale) magnetic resonance images show HFB distribution in a large tumor (a, 3.6 cm3) and a small tumor (d, 1 cm3). (b) and (e) Baseline pO2 maps show higher pO2 in the small tumor when both animals were breathing air. Mean pO2 of large and small tumors was 0.1 ± 1.8 and 25.4 ± 1.1 torr, respectively. (c) and (f) Tumor pO2 maps of the same animals obtained at 24 min after oxygen breathing; mean pO2 of large and small tumors was 8.1 ± 4.5 and 90.6 ± 3.9 torr, respectively. Both values were significantly higher than that of baseline (p < 0.01). (Reprinted with permission from Ref 52. Copyright 2007 Elsevier).

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