The present invention relates to compositions and methods for use in medical diagnosis and patient monitoring. It more particularly relates to a biocompatible gel comprising nanoparticles and/or nanoparticle aggregates, wherein: i) the nanoparticles and/or nanoparticles of said aggregate comprise an inorganic material comprising at least one metal element having an atomic number Z of at least 25, each of said nanoparticles and nanoparticle aggregates being covered with a biocompatible coating; ii) the nanoparticles' and/or nanoparticle aggregates' concentration is of about or less than 0.5% (w/w); and iii) the apparent viscosity at 2 s−1 of the gel comprising the 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 an X-ray imaging equipment.