About MRI (Magnet Resonance Imaging)
Aka MRT (Magnet Resonance Tomogram)
The technology behind MRI is complex. In short, certain molecules in the human body react to the strong magnetic field produced by the scanner. The scanner can detect this and produce a picture of normal and abnormal tissues and organs in the body. This is an investigation often used in spinal conditions. Unlike computed tomography (CT) MRI uses no ionizing radiation and is generally a very safe procedure. Patients lie in a narrow tunnel and some find this rather uncomfortable. The scanner produces quiet loud noises but many MRI departments offer earplugs. A single level scan (i.e. neck or lower back) takes around 20min.
How it works
In more details, the body is composed of water molecules. These contain two hydrogen nuclei (protons). When a person goes inside the powerful magnetic field of the scanner, these protons align with the direction of the field. A radio frequency electromagnetic field is then briefly turned on, causing the protons to absorb some of its energy. When this field is turned off the protons release this energy, which can be detected by the scanner. The position of protons in the body can be determined by applying additional magnetic fields during the scan, which allows an image of the body to be built up. This creates the knocking sounds heard during the MRI scan. The signal from diseased tissue, such as a tumour or infection can be detected because the protons in different tissues return to their equilibrium state at different rates. By changing the parameters on the scanner this effect is used to create contrast between different types of body tissue. Sometimes contrast is injected intravenously to enhance the appearance of certain tissues, blood vessels or diseased organs.
What it does show
In the spine, MRI shows in particular well the nerves and spinal cord, intervertebral discs, ligaments and muscles around the spine. It also shows abnormalities within the vertebral body or abnormal blood vessels within the spinal canal (vascular malformations). In some sequences (called STIR) bone swelling (oedema) can be seen which is useful to see inflammation, bony healing and some pathological processes such as tumours better.
What it does not show
Bone is only visible indirectly because the MRI shows the bone marrow. Calcium, which is an important component of bone, does not produce a signal. Plain x-rays or a CT scan are usually better.