Encapsulated gadolinium and dysprosium ions within ultra-short carbon nanotubes for MR microscopy at 11.75 and 21.1 T.

Single-walled carbon nanotubes (SWNTs) have gained interest for their biocompatibility and multifunctional properties. Ultra-short SWNTs (US-tubes) have demonstrated high proton relaxivity when encapsulating gadolinium ions (Gd(3+)) at clinical field strengths. At higher field strengths, however, Gd(3+) ions demonstrate decreased proton relaxation properties while chemically similar dysprosium ions (Dy(3+)) improve relaxation properties. This report investigates the first use of Gd(3+) and Dy(3+) ions within US-tubes (GNTs and DNTs, respectively) at ultra-high magnetic field (21.1 T). Both agents are compared in solution and as an intracellular contrast agent labeling a murine microglia cell line (Bv2) immobilized in a tissue-mimicking agarose phantom using two high magnetic fields: 21.1 and 11.75 T. In solution at 21.1 T, results show excellent transverse relaxation; DNTs outperformed GNTs as a T(2) agent with measured r(2)/r(1) ratios of 247 and 47, respectively. Additionally, intracellular DNTs were shown to be a better T(2) agent than GNTs with higher contrast percentages and contrast-to-noise ratios. As such, this study demonstrates the potential of DNTs at high magnetic fields for cellular labeling and future in vivo, MRI-based cell tracking.

Magnetic resonance contrast and biological effects of intracellular superparamagnetic iron oxides on human mesenchymal stem cells with long-term culture and hypoxic exposure.

BACKGROUND AIMS: Human mesenchymal stem cells (hMSCs) have gained interest for treatment of stroke injury. Using in vitro culture, the purpose of this study was to investigate the long-term detectability of hMSCs by magnetic resonance imaging (MRI) after transfection with a superparamagnetic iron oxide (SPIO) and evaluate the effects of SPIO on cellular activity, particularly under an ischemic environment. METHODS: hMSCs were exposed to low doses of SPIOs. After a short incubation period, cells were cultured for additional 1, 7 and 14 d to evaluate proliferation, colony formation and multilinear potential. Labeled cells were imaged and evaluated in agarose to quantify R2 and R2∗ contrast at each time point. Cells were placed in a low-oxygen, low-serum environment and tested for cytotoxicity. In addition, labeled cells were transplanted into an ischemic stroke model and evaluated with ex vivo MRI and histology. RESULTS: Cellular events such as proliferation and differentiation were not affected at any of the exposures tested when cultured for 14 d. The low iron exposure and short incubation time are sufficient for detectability with MRI. However, the higher iron dosage results in higher calcification and cytotoxicity under in vitro ischemic conditions. Transplantation of the hMSCs labeled with an initial exposure of 22.4 µg of Fe showed excellent retention of contrast in stroke-induced rats. CONCLUSIONS: Although SPIO labeling is stable for long-term MRI detection and has limited effects on the multilineage potential of hMSCs, high-dose SPIO labeling may affect hMSC survival under serum and oxygen withdrawal.

Intracellular SPIO labeling of microglia: high field considerations and limitations for MR microscopy.

The purpose of this study is to investigate MRI contrast as a function of magnetic field strength for microglia labeled with superparamagnetic iron oxide (SPIO) nanoparticles. A rat microglia cell line, Bv2, was incubated with SPIOs for 6 h. In two separate experiments conducted at 11.75 and 21.1 T, the impact of SPIO loading and cell count on T(1) , T(2) and T(2) * contrast were evaluated: (a) cells were incubated with 1, 2 or 5 µl of Feridex; and (b) cells incubated with 5 µl of Feridex were used to form layers of 25 000, 50 000, 100 000 or 200 000 cells. Intracellular iron was analyzed with ICP-MS and histological staining while cell viability was evaluated by Trypan blue dye exclusion. Bv2 cells displayed increases in intracellular iron concentration with SPIO exposure, with the highest labeling yielding 0.83 pg of Fe per cell. Although no differences were identified for T(1) mechanisms, both fields displayed trends toward increasing T(2) and T(2) * contrast with increasing SPIO loading or cell count, with few differences evident between fields. Bv2 cells can be labeled readily with commercially available SPIOs, with the potential of increasing the intracellular iron content over short incubation times without impacting viability. This phagocytotic cell line not only provides direct SPIO uptake but also plays a critical role in inflammation after brain injury, providing a possible neurodegeneration biomarker. With few differences between field strengths and limited ability to quantify intracellular iron content and cell count, this study demonstrates only a slight benefit of SPIO-based contrast agent at high fields based on susceptibility-based contrast and detection, necessitating unique agents for such applications.