A particle has, in a pharmaceutically acceptable shell, a magnetic iron oxide core having a diameter of 20 nm to 1 mu m. An overall particle diameter/core diameter ratio is less than 6. Independent claims are also included for: (1) a method for producing the inventive particles, comprising (i) mixing an aqueous mixed iron salt solution, which contains an iron(II) salt and an iron(III) salt, with an aqueous alkali solution in the presence of a synthetic polymer or copolymer, a starch, or their derivative, a dextran or its derivative, a cyclodextran or its derivative, a fatty acid, a polysaccharide, a lecithin or a mono-, di- or triglyceride or their derivative, until a colloidal solution of the particles is formed； (ii) passing the particle-containing solution through a magnetic gradient field； (iii) removing the magnetic gradient field； and (iv) recovering the particles retained in the gradient field； (2) a composition, in which >；=2% of the particles are contained in the composition； (3) a fluid containing the composition； and (4) use of the composition or fluid to produce a diagnostic mechanism for use in magnetic particle imaging (MPI) to diagnose a disease consisting of proliferative diseases, inflammatory diseases, autoimmune diseases, diseases of the digestive tract, arteriosclerosis, apoplexy, infarction, pathological changes in the vascular system, the lymph system, the pancreas, the liver, the kidneys, the brain and the bladder, and also diseases affecting electrical stimulus transmission and neurodegenerative diseases.
