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.