Computed Tomography (CT) scan
CT is an imaging technique based on x-rays. There are many applications for it and it can be used on every part of the body. There are usually a very few and specific indications to investigate problems of the spine. CT can be combined with the intravenous injection of contrast to enhance certain abnormal tissues or show blood vessels. It can also be combined with the injection of contrast via a lumbar puncture (CT-myelogram). The latter has become very rare thanks to the development of MRI. CT was initially called EMI scan and the first scanner in England was installed at Atkinson Morley Hospital in Wimbledon, London where the first brain CT was taken in 1972. In 1979 Allan Cormack and Godfrey Hounsfiled received the Nobel Price in medicine for the development of the CT.The German physicist Wilhelm Konrad Roentgen had already won the first Nobel price for physics in 1901 for the discovery of X-rays in 1895.
How it works
X-rays are send through the body and recorded by detectors. Various tissues absorb the x-rays differently, which is then measured. The camera and detectors rotate around the body (hence the donought like shape). If tissue absorbs a lot of x-ray beams it appears white (for example bone), if it absorbs very little, it appears black (like water). Most tissues are somewhat in-between and appear in different shades of grey. These differences can be very accurately measured (Hounsfield units) and are often characteristic for certain tissues.
Sometimes, a contrast medium is given as an injection into the blood, which can enhance in certain tissues or some disease processes.
Modern CT scanners often take only 1-2 minutes to obtain the images and can transform pictures into a three dimensional image. The patient lies on a trolley which is moved into a large donut. This usually does not cause problems for patients with claustrophobia.
What it shows
The main role in the spine is detecting problems with the bony structure. It also shows some soft tissues like muscles and intervertebral discs. However, this is not as detailed as MRI. It is a useful investigation, in particular in combination with contrast given through a lumbar puncture (so-called CT-myelogram), if MRI is not possible or contra-indicated.
What it does not show
Although neural structures (nerves and spinal cord) or intervertebral discs can be seen the quality is not anywhere near as good as on MRI. Therefore they do not show a trapped nerve or slipped disc very well. This means that they have no role in investigating conditions like sciatica unless there are very specific indications or reasons not to do an MRI.
When it should be done
Most acute spinal fracture should be investigated with a CT to show the extent of the fracture and help to make a judgement about the stability of the spine. Sometimes it can be helpful when planning a spinal fusion. In patients who have a cardiac pacemaker and therefore cannot have an MRI scan, a good quality CT is often the next best investigation.
When it should not be done
CT delivers a significant does of radiation and therefore should be avoided in pregnancy unless essential. However, a lead gown can protect the baby. It should not just be done because waiting times for MRI scan are often longer.
Commonly asked questions
What investigation is better: MRI or CT?
This depends on the condition. In principle, one is not better than the other but show different structures and often complement each other.