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WIREs Nanomed Nanobiotechnol
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Paramagnetic inorganic nanoparticles as T1 MRI contrast agents

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Magnetic resonance imaging (MRI) is one of the most powerful molecular imaging techniques and can noninvasively visualize and quantify biological processes within the living organisms. The introduction of exogenous contrast agents has allowed specific visualization of biological targets as well as enhanced the sensitivity of MRI. Recently, paramagnetic inorganic nanoparticles showing positive T1 contrast effect have been investigated as T1 MRI contrast agents. Since the first trials of spherical nanoparticles of manganese oxide and gadolinium oxide, inorganic nanoparticles of various compositions and shapes have been used for in vivo and in vitro MRI because of their distinct signal enhancement in MR images. However, for clinical applications, important and complex issues such as safety and efficiency should be investigated by active research encompassing multiple disciplines, including chemistry, biology, biomedical engineering, and medicine. This article is categorized under: Diagnostic Tools > Diagnostic Nanodevices Diagnostic Tools > In Vivo Nanodiagnostics and Imaging

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(a) Schematic illustration of formation of HMONs. MnO phase is gray and Mn3O4 phase is black. TEM image of WMONs (b) and HMONs prepared after immersion in water for (c) 1 and (d) 5 days. Insets of (c) and (d) show high‐resolution TEM (HRTEM) images, respectively. In vivo T1‐weighted MR images (e) and T2‐weighted MR images (f) of a mouse brain. (Reprinted with permission from Ref . Copyright 2009 Wiley‐VCH)
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(a) Schematic illustration of synthesis of lamellar structured ultrathin MnO nanoplates. (b,c) Transmission electron microscopy (TEM) image and scanning transmission electron microscopy (STEM) image of lamellar structured ultrathin MnO nanoplates with a width of 20 nm. In vivo T1‐weighted MR images showing liver, gall bladder, intestinal tract (d), and kidney (e). (Reprinted with permission from Ref . Copyright 2011 American Chemical Society)
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(a) Schematic illustration and corresponding TEM images of step‐wised multi‐shell growth. NaYF4: Tm3+, Yb3+ shell growth (middle, 1st @ 2nd) on NaYF4: Er3+, Gd3+ UCNP (left, 1st) followed by second NaGdF4 shell(right, 1st @ 2nd @ 3rd), (b) Positive lattice shielding effect demonstration in UCNPs. Emission band fluorescent intensities of 1st (blue), 1st @ 2nd (brown), and 1st @ 2nd @ 3rd (green). The enhancement factors were listed right near the emission bands. (c,d,e) Negative lattice shielding effect; (c) NaYF4: Er3+, Gd3+ (r1 = 1.24 mM−1s−1 ) (d) NaYF4: Tm3+, Yb3+ shell on UCNP (r1 = 0.037 mM−1s−1) (e) NaGdF4 shell growth (r1 = 1.18 mM−1s−1). All UCNPs were transferred into water by a silica shell with nearly same thickness of ca 10 nm. (Reprinted with permission from Ref . Copyright 2011 Wiley‐VCH)
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(a) Schematic illustration of dual imaging and PDT using UCNP‐Ce6. (b) TEM image of UCNPs. (c) Emission spectra of UCNP and UCNP‐Ce6 under 980‐nm excitation. (d) Upconversion luminescence images of nude mice bearing tumor after intravenous injection of UCNP‐Ce6. (Top row: bright field images, middle row: true color images of green luminescence, bottom row: pseudo‐color images.) (e) In vivo T1‐weighted images and corresponding color mapping images of a tumor‐bearing mouse injected with UCNP‐Ce6. (Reprinted with permission from Ref . Copyright 2012 Wiley‐VCH)
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(a) TEM image of 4‐nm iron oxide nanoparticles, (b) their r1 and r2 relaxivity of different PEG molecular weight, and (c) their r2/r1 value correlates with the hydrodynamic diameter. (Reprinted with permission from Ref . Copyright 2009 American Chemical Society). (d) TEM image of 3‐nm iron oxide nanoparticles and (e) size‐dependant magnetic properties. (f) High‐resolution blood pool MR image enhanced by 3‐nm iron oxide nanoparticles. (Reprinted with permission from Ref . Copyright 2009 American Chemical Society)
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(a) Schematic illustration of the synthesis of HMnO@mSiO2 nanoparticles. TEM image (b) and HRTEM image (c) of the HMnO@mSiO2 nanoparticles. (d) r1 value of HMnO@mSiO2 nanoparticles was evaluated and r2/r1 value was estimated at 11.13 when measured in 11.7 T MRI. (e) In vivo MR images of transplanted MSCs. Unlabeled MSCs were not detected in T1‐weighted image. (f) HMnO@mSiO2 nanoparticles‐labeled MSCs were detected with hyperintense signals (green arrow) until 14 days (Reprinted with permission from Ref . Copyright 2011 American Chemical Society)
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