MRI enables the safe visualisation of soft tissue at any point in the body. Its application in both routine diagnosis and in research is extensive, but principal uses include: 

There are around a trillion, trillion, trillion water molecules in the human body, where one trillion equal a million million. Each molecule of water (H2O) includes two hydrogen atoms, the nucleus of each of which is a single spinning proton that behaves like a tiny magnet that aligns with an applied magnetic field, just as a compass points in the direction of the Earth’s magnetic field. The alignment is not complete, however, and the axis of proton or nuclear spin wobbles or precesses about the magnetic field direction, rather like a spinning top or gyroscope does in the Earth’s gravitational field. The precession rate is dependent on the magnetic field strength; for every one Tesla, the protons in the hydrogen atoms of the water molecules in our living tissue precess at precisely 42.6 million times per second – i.e., at 42.6MHz.

When radio waves in the form of low power Radio Frequency (RF) pulses of electromagnetic energy are shone from a transmission coil close to the body, and at exactly the same frequency as the precession rate of those protons, some of the RF energy is absorbed by the protons, altering their precession dynamics. This is known as resonance. When the RF pulses switch off, the protons recover to their initial state, re-emitting RF energy at that same resonant frequency, which is picked up by a radio reception antenna and computer analysed. The duration of the recovery period differs between normal and abnormal tissue, permitting the detection of abnormal tissue such as tumours. The RF energy emitted by each resonant nucleus is miniscule, but there are so many nuclei involved that a viable signal is produced.

By superimposing a second precisely controlled but much weaker variable magnetic field – the gradient field - on to the strong field generated by the MRI magnet, the region of the body which is at the correct magnetic field for resonance can be focused into a layer or slice as thin as a few millimetres. As the gradient field slides through the body, so a stack of these slices can be imaged to complete the scan along one of the main x, y, z axes. A full three axes scan of the head, for example, can be completed in a few minutes with a high field MRI scanner. The detail is provided by the variation in density of the water molecules in the different tissues, and by the variation in recovery period for different tissues, and can be clearer than a slice of tissue prepared as a microscope slide, but without the surgical procedure.