Contrast Media for Magnetic Resonance Imaging and Ultrasound

The substances most suitable as CM for MRI are those that change the relaxation times of the anatomic structures to be depicted. Both paramagnetic and superparamagnetic substances are appropriate for this purpose. The magnetic moments of the individual atoms or molecules of paramagnetic substances point randomly outside a magnetic field. When an external magnetic field is applied, they align themselves in such a way that, as a whole, an “induced” magnetic field parallel to the external one arises that increases linearly in strength proportional to the strength of the external field. Superparamagnetic substances are microscopically small (Fe3O4 < 0.035 μm in diameter), solid particles, in which even outside of a magnetic field the magnetic moments of the individual atoms or molecules are parallel in alignment as a result of the forces between these particles. The resulting magnetic moments of the individual particles are independently aligned when outside of a magnetic field. Since these magnetic moments align in parallel in an external magnetic field, a strongly induced magnetic field arises when such a field is applied. Initially, it grows linearly in strength proportional to the strength of the external field. However, as the strength of the latter increases, the strength of the induced field begins to grow more slowly as equilibrium is approached.