In magnetic resonance imaging (MRI), the use of magnetic nanoparticles (MNPs) as contrast agent (CA) greatly enhances the possibility to identify several diseases hardly diagnosed by other means. The efficacy of a new CA is described by the longitudinal and transverse relaxivity. Nuclear Magnetic Relaxation Dispersion (NMRD) profiles represent the evolution of relaxivities with magnetic field. Many efforts have been taken to develop theoretical models to depict water proton relaxation in presence of magnetic compounds. The use of theoretical models in junction with NMRD profiles has become a powerful tool to characterize MNPs as CA. In this work, a heuristical theoretical model was implemented, verified and assessed with different magnetic materials. It has been demonstrated that the model works well when using iron cores but fails with other magnetic compounds. A weighting factor associated with Langevin function was introduced to the model. This extra calibration enables the model to be used with other magnetic compounds to characterize new CAs in MRI.

No abstract

This work presents a study of photo-induced effects in biological-tissue models made from agar gel and porcine cornea samples. We used a Nd:YAG (5 ns) and a Ti:sapphire (90fs) lasers to irradiate the samples. The main objective in this study is to understand some aspects of the interaction between pulsed lasers and biological tissue, of especially interest for us are vascular and corneal tissues. Our research includes laser heating of vascular-like tissue and laserinduced cavitation bubble formation in cornea. For the laser heating studies we used tissue models composed by a single or up to four stack layers made of agar gel (of between 200 and 500µm thickness per layer); for the laser-induced bubble formation we used corneal tissue which was obtained from regular porcine corneas. In order to emulate blood vessels, we designed an organic dyed agar gel layer made off an organic dye whose absorption coefficient is similar to that of hemoglobin. We will present results of laser heating of vascular-like tissue, and its dependence on laser fluence and pulse duration. Also, we will present results of cavitation bubble formation (100-600 µm deep) for agar gel and corneal tissues. Our results show that there exists a well determined threshold fluence for the onset of bubble formation; the laser-induced bubbles on agar gel and cornea can be permanent or transient depending on the laser irradiation parameters. The diameter of the cavitation bubbles is clearly dependent on the laser fluence. Some interesting dermatological and ophthalmic applications related to the above effects will be suggested and discussed throughout this work.