MRI
MRI
A modern MRI scanner
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.
whole spine MRI, side view
When it should be done
In any condition where pressure on a nerve or the spinal cord is suspected, the MRI is the best imaging technique (i.e. sciatica or spinal cord compression). It is also very useful when investigating infections and tumours of the spine. It is also used to investigate other medical conditions effecting the spinal cord or brain (i.e. multiple sclerosis).
When it should not be done
Patients with some metal implants (cardiac pacemaker) cannot have an MRI scan due to effects of the strong magnetic field and powerful radio frequency pulses. Other metal implants like hip or knee replacements and spinal implants are usually safe. However, spinal implants can cause artefacts which could make the interpretation of the pictures difficult or impossible in areas close to the implants. Modern implants made from Titanium often give little artefacts and changing the setting of the scanner usually allows fairly good pictures. Although there is no prove that MRI harms the unborn baby, it is best avoided during pregnancy.
Commonly asked questions
Which investigation is better: MRI or CT?
These are two very different investigations and often complement each other. However, investigation of choice for many spinal conditions is the MRI.
What can I do if I am claustrophobic?
Many patients find the combination of the loud noise and narrow tunnel rather unpleasant. Most patients, however, can manage quiet well (often better than they think before!). Sedation (taking a sleeping tablet) before helps the vast majority of patients with claustrophobia. It is important to inform the MRI department about this! Most MRI departments can give sedation but it needs to be arranged beforehand. It is usually best to inform the doctor or therapist who arranges the scan so it can be mentioned when the request is issued. In extreme cases, the MRI can be done under general anaesthesia. There are also some larger MRI scans around (so-called open MRI) which have wider tunnels or upright scanners. Some can also perform scans in flexion and extension.