Comparison of gadolinium loaded and empty carbon nanotubes as high performance MRI contrast agent

Abstract

Introduction: Gadolinium loaded ultra-short single walled carbon nanotubes (US-tubes) has been proposed as an effective high r2 relaxivity contrast agent for use in MR molecular imaging. However, the true active ingredients for inheriting its high performance r2 characteristics, is remain unknown. In this study, we compare the R2 and R2* relaxation rate of the US-tubes with/without Gadolinium ions encapsulation. Methods: Two types of US-tubes MRI phantoms were prepared: (i) Gadonanotubes with 41 μM Gd and (ii) empty US-tubes. Each type of nanotubes was dispersed in pluronic solution with carbon concentration of 275 mg/L and transferred to a 1 cc syringe for MRI imaging. MRI imaging was performed in a 3T MRI scanner using a 35 mm volume coil with 2D spin echo (TR=1500 ms, TE=20–80 ms with 10 ms increment) and 2D gradient echo (alpha=90, TR=1500 ms, TE=20–80 ms with 5 ms increment) sequences to evaluate the R2 and R2* respectively. A 1 mm thick single slice was obtained at each TE cutting in the middle of the 3.5 cm long filled syringe placed parallel to the main magnetic field. T2 and T2* were determined by linear fitting of the measured image intensities at each TE at each phantom into log(S) = log(S0)-TE/T2 and log(S) = log(S0)-TE/T2*. Results and Discussions: Table 1 shows the estimated R2 and R2* relaxation rates of the phantoms. R2 represents the coefficient of determinations of the linear fitting. Both Gadonanotubes and empty US-tubes show high R2 and R2* relaxation rates with the R2* being 6–11% stronger than R2. The slight increase in R2* may attribute to bulk magnetic susceptibility effects due to the presence of metal ions inside the US-tubes. Nethertheless,the R2* observed is small when compared with that of other superparamagnetic contrast agents which usually show a few times increase in R2*. This may be due to the low concentration (in μM) and lose density packing of the metal ions inside the tubes. For empty US-tubes, it is observed that its relaxivities are as good as Gadolinium loaded US-tubes. Additional Gd3+ loading does not significantly increase the relaxivities of the US-tubes. Therefore, it is reasonable to consider using empty US-tubes as a high performance MRI contrast agent as well as drug/gene delivery carrier, with the delivery items being encapsulating inside its hollow interior space.