The present invention relates to compositions and methods for use in medical diagnosis and patient's monitoring. It more particularly relates to a biocompatible gel comprising nanoparticles and/or nanoparticle aggregates, wherein i) the nanoparticle and/or nanoparticles of the aggregate comprise an inorganic material comprising at least one metal element having an atomic number Z of at least 25, each of said nanoparticle and of said nanoparticle aggregate being covered with a biocompatible coating； ii) the nanoparticles and/or nanoparticle aggregate concentration is of about or less than 0.5% (w/w)； and iii) the apparent viscosity at 2 s&minus；1 of the gel comprising nanoparticles and/or nanoparticle aggregates, is between about 0.1 Pa·s and about 1000 Pa·s when measured between 20° C. and 37° C. The composition of the invention typically allows the delineation and visualization of at least 40% of the target biological tissue when said tissue is observed using X-ray imaging equipment.
