To answer these questions, Canadian and Belgian researchers have conducted a series of investigations. The researchers: Marc-André Fortin and Luc Faucher from Université Laval, Québec, Canada, reported earlier this year on the use of US-Gd2O3 particles for "positively contrasted" cellular imaging, and were joined by Yves Gossuin from Université de Mons, Belgium.

Gadolinium (Gd) is a paramagnetic element widely used in MRI. However, most gadolinium complexes used nowadays are not suitable for molecular and cellular imaging because of the relatively weak unitary impact of each one of the gadolinium atoms on the relaxation time of hydrogen protons (providing the "signal" detected in MRI). To enhance the detectability of Gd-based contrast agents, one strategy is to bring hundreds of Gd atoms into so-called "ultra-small nanoparticles". Adequately coated, such particles are very efficiently ingested by cells and are considered as promising "probes" for cell tracking in MRI.

Field strength assessment

The team recently measured the efficiency of Gd2O3 suspensions for MRI applications (longitudinal and transverse relaxivities). Data acquired at magnetic field strengths typical of most hospital scanners (0.5–3 T), were compared with measurements at higher magnetic field strengths (7–11.7 T) as found in MRI systems mainly used in pre-clinical research.

At every magnetic field, the longitudinal relaxivity (r1) decreased upon agglomeration, while remaining high enough to provide positive contrast. The transverse relaxivity (r2) decreased slightly at 0.47 and 1.41 T, which is good for clinical MRI; however, it was enhanced at higher fields (7 and 11.7 T) upon agglomeration, thereby affecting the positive contrast enhancement effect. This is due to the total magnetic moment developed during agglomeration, since paramagnetic aggregates are considered as large magnetized spheres.

Given that agglomeration is susceptible to occur once particles are ingested in the cells, this is a significant finding to clearly establish the magnetic field strengths at which US-Gd2O3 particles might be preferentially used.

Hydrated coatings

The researchers suggest the development of highly hydrated nanoparticle coatings to maintain the interaction of surface gadolinium with 1H protons in spite of aggregation. Modulating the thickness of the nanoparticle polymer coating would also help to prevent the decrease of transversal relaxation time at high magnetic fields, which is caused by the close proximity of Gd2O3 nanocrystals upon agglomeration.

The study clearly establishes the potential of ultra-small Gd2O3 nanoparticles as a "positive" contrast agent for MRI, which is particularly efficient and potentially very useful at clinical magnetic field strengths (1–3 T).

More information can be found in the journal Nanotechnology.