[Basics of MRI technique and MRI image interpretation]

Today magnetic resonance imaging (MRI) is an important routine diagnostic tool in orthopaedics. It is based on the nuclear magnetic resonance phenomenon: "free" hydrogen atoms (mainly from water) become field-parallel in a strong outer magnetic field. They reach a higher energy level by application of an additional electromagnetic field. After shutdown of this outer field the atoms send out electromagnetic waves (radiowaves), representing the MR signal. Different amounts of "free" water result in various image characteristics. Typically, a higher tissue water content is represented by high MR signals, e.g. in blastomas, inflammations and degenerative changes. Waterless structures such as calcified or fibrous tissues, tendons and ligaments show low MR signals. Fatty structures have high signals; proteins dissolved in "free" water change the water signal dramatically. Besides these basic parameters, para-, ferro- and super-paramagnetic materials--ether naturally present in the human body, such as methemoglobin, or artificially introduced, such as MR contrast media--and flow are responsible for different grey shades in MRI. MRI is not associated with ionizing radiation and allows imaging in all planes without changing the patient's position. Disadvantages of MRI are high costs and low availability. Future technical developments will result in shorter imaging times and broadening of the application spectrum, leading towards "MR fluoroscopy" and MR interventions.