studies revealed that these Gd3+n@US-tube species are linear superparamagnetic molecular magnets with MRI efficacies 40–90 times larger than any current Gd3+-based CA in clinical use. As such, gadonanotubes, with their embedded 2–5 nm superparamagnetic Gd3+n-ion clusters, demonstrate potential as a radically new synthon for the development of high-performance MRI CAs. Nuclear magnetic relaxation dispersion (NMRD) profiles can be of great help in determining the parameters influencing relaxivity, and they have played an important role in the devel- opment of our understanding of proton relaxivity. NMRD profiles are proton-spin relaxivities measured as a function of magnetic field. The NMRD profile (B= 5 × 10–4–9.4 T) for an aqueous solution of a Gd3+n@US-tube sample in 1% SDBS (sodium dodecylbenzene sulfonate, a surfactant employed to suspend the highly lipophilic SWNTs in water) solution at 37°C, is presented in Figure 1. Also presented, for comparative purposes, are data for one of the commercially available MRI CAs, [Gd(DTPA)(H2O)]2– (MagnevistTM). For any magnetic field in Figure 1, the relaxivity for the Gd3+n@ US-tubes is remarkably larger than for the clinical CA. This is true at the standard MRI field strength (nearly 40 times larger) for clinical imaging of 20–60 MHz (170 mM–1 s–1 vs 4.0 mM–1 s–1), but is even more pronounced (nearly 90 times larger) at very low fields such as 0.01MHz (635 mM–1 s–1 vs 7.0 mM–1 s–1). In addition to the exceptionally large relaxivity values obtained for the gadonanotubes, the shape of the NMRD curve, as shown in Figure 1, is also considerably different from that reported so far for any other Gd3+-based system. In particular, the relaxivities are continuously decreasing with increasing magnetic field at proton Larmor frequencies below 1 MHz, in contrast to the usual Gd3+ CAs which present constant values at these low fields. Even more remarkable is the finding that at high magnetic fields (>60MHz), the relaxivities remain practically constant, whereas a strong decrease is observed for the usual Gd3+ CAs. The Solomon-Bloembergen-Morgan theory is unable to predict the observed shape of the NMRD profile and thus does not appear appropriate for gadonanotubes. Clearly, further investigations are needed in order to explain both the extremely large relaxivities and the magnetic-f ield dependency of the proton relaxivities for the gadonanotubes and possibly also other nanoscalar MRI CA materials. However, the data can be used to evaluate their potential efficacy as advanced MRI CAs. To this end, we also recently conducted T1-weighted MRI phantom studies of the gadonanotubes at a 0.04 mM international Journal of Nanomedicine 2006:1(3) Gadonanotunes as Mri contrast agents (Gd3+). Additional vials containing the commercial contrast agent MagnevistTM and deionized water solutions were also included in the imaging section as a reference for comparative purposes. Representative T1-weighted MRI images of the vials are shown in Figure 2. At 0.04 mM (Gd3+) the gadonanotubes showed extremely large signal enhancement compared to MagnevistTM (ca. 100 times!) at the same concentration (Figures 2 and 3). Figure 2 T1-weighted magnetic resonance imaging (Mri ) phantoms of the Gd3+n@US-tubes solution at a 0.04 mM (Gd3+). For comparative purposes, Mri phantoms of MagnevistTM at a 0.04 mM (Gd3+) and pure water are also shown. All data were collected using a 1.5T Philips Mri imager. Figure 3 Magnitude signal-intensity curves for vials of Gd3+n@US-tubes ( ) and MagnevistTM ( ) at the same Gd3+ ion concentration of 0.04 mM, along with that of pure water (X). Note the substantially faster recovery time for the gadonanotubes probes when compared with the conventional CA. Future applications Universal Mr probes The high relaxivity values for the gadonanotubes at all field strengths (low MR fields, current clinical MR fields and high MR fields) make these new gadonanotube materials suitable for use as universal MRI CAs. At the standard MRI field strength for clinical imaging in the 20–60 MHz range, their values are nearly 40 times larger than clinical CAs (170 mM–1 s–1 vs 4.0 mM–1 s–1). The remarkably-high values at very low fields such as 0.01MHz (635 mM–1 s–1 vs 7.0 mM–1 s–1) will especially benefit microtesla MRI imaging technologies (McDermott et al 2004). At high magnetic fields (>60MHz), the ability of the gadonanotubes to maintain a nearly constant 293