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Colloidal suspensions of iron oxide and metal iron nanoparticles prepared by laser pyrolysis have been obtained by coating the particles with dextran in an aqueous media giving rise to biocompatible ferrofluids. The structural characteristics of the powders and the size of the particles and the aggregates in the colloidal suspensions have been analysed and correlated with the magnetic properties of both solids and fluids. For the first time, to our knowledge, a stable ferrofluid based on metal particles (<10 nm) has been obtained with aggregate sizes of  nm. In comparison to iron oxide based products, this material exhibits higher saturation magnetization (45 emu g−1) and susceptibilities (4000 emu/g T). In addition, the nuclear magnetic resonance response of the ferrofluids has been measured in order to gain information about the influence of the crystallochemical and magnetic properties on their relaxation behaviour. The main parameter affected by the presence of the magnetic nanoparticles is the transversal relaxation time T2 and the corresponding relaxivity R2 value that is of the order of 400 (mmol/l)−1 s−1. It has been shown that R2 value increases not only by using iron metal instead of iron oxide but also by increasing the crystal size of the particles. From this study an evaluation of the possibilities of these materials as contrast agents for magnetic resonance imaging has been made.

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Diffusion weighted 19 F images of rat lung in vivo using SF 6 are presented. Projection-reconstruction images were acquired by filling the rat lung with a mixture of SF 6 and air, during 64 successive apneas. Each apnea lasted for 6 s, the time required to perform 100 accumulations of each k-space radial phase step for the five values of the diffusion gradient (TR ‫؍‬ 10 ms). After diffusion images were acquired, an apparent diffusion coefficient (ADC) map was generated, yielding an average value for the ADC of 2.22 ؋ 10 -6 m 2 /s and SD for ADC values of 1.27 ؋ 10 -6 m 2 /s. To the best of our knowledge, this is the first in vivo diffusion weighting imaging application and the first ADC map obtained using 19

Biocompatible magnetic dispersions have been prepared from γ-Fe2O3 nanoparticles (5 nm) synthesized by continuous laser pyrolysis of Fe(CO)5 vapours. The feasibility of using these dispersions as magnetic resonance imaging (MRI) contrast agents has been analysed in terms of chemical structure, magnetic properties, 1H NMR relaxation times and biokinetics. The magnetic nanoparticles were dispersed in a strong alkaline solution in the presence of dextran, yielding stable colloids in a single step. The dispersions consist of particle-aggregates 25 nm in diameter measured using transmission electron microscope and a hydrodynamic diameter of 42 nm measured using photon correlation spectroscopy. The magnetic and relaxometric properties of the dispersions were of the same order of magnitude as those of commercial contrast agents produced using coprecipitation. However, these dispersions, when injected intravenously in rats at standard doses showed a mono-exponential blood clearance instead of a biexponential one, with a blood half-life of 7 ± 1 min. Furthermore, an important enhancement of the image contrast was observed after the injection, mainly located at the liver and the spleen of the rat. In conclusion, the laser pyrolysis technique seems to be a good alternative to the coprecipitation method for producing MRI contrast agents, with the advantage of being a continuous synthesis method that leads to very uniform particles capable of being dispersed and therefore transformed in a biocompatible magnetic liquid.