Human eyes can only see so much on a routine CT scan

A computerised tomography (CT) scan makes use of computer-processed combinations of multiple X-ray measurements taken from different angles to produce cross-sectional images (virtual “slices”) of specific areas of a scanned object. This allows visualisation inside the object without cutting it open. In the NHS alone, ~6 million CT scans were performed each year (~150 million scans globally/ year).

The variations in physical density of different objects translates to differences in their radio-densities on a CT scan.  Bone, soft tissue and air are readily discernible on a CT scan because their radio-densities are dramatically different. However, human eyes (even trained radiologists) cannot discern circulating blood from other types of soft tissue, as their respective intensity ranges overlap substantially.  For example, bony spine (orange arrow) is distinctly more radio-opaque compared to muscle (blue arrow) or the arteries (red arrows). However, muscle, bowel and arteries  appear similar on a routine CT scan.

CT images without RCM injection

Many CT scans are done with an injection of radiological contrast media

In order to visualise the artery and the contents within, patients are given an injection of radiological contrast media (RCM) which ‘lights up’ the relevant contents. This is called a CT angiogram. RCM is used in about 60% of all CT scans.

Injection of RCM in the same patient gives 'CT angiogram' and better visualisation inside the body.

* Problems related to the injection of radiological contrast media *

Related to the insertion of a needle (‘drip’):

Pain and discomfort

Bleeding, bruising, haematoma

Injury of the artery nearby

Related to radiological contrast  media:

Allergic reactions / anaphylaxis which can be life threatening

Toxicity to kidneys causing kidney to malfunction

Leakage of RCM from a poorly placed needle/drip causes skin damage

In some cases, patients can’t even have contrast CT scans because of a history of severe reaction to RCM, chronic kidney problems, or needle phobia! This poses particularly challenges to the management of their medical issues.