Computed Tomography (CT)

The field of diagnostic imaging is the most rapidly growing field in both human and veterinary medicine over the last 40 years. Although x-rays have been around for over 100 years, ultrasound, computed tomography (CT) and magnetic resonance imaging (MRI) have only been developed over last 40 years or so.

X-rays are present throughout the universe passing through space but are only present in very low quantities surrounding us as we go about our daily duties on the surface of planet Earth. However, we knew nothing about them until Wilhelm Roentgen discovered their existence in a laboratory in Germany in 1895.  When he discovered them however, he very quickly recognised their huge potential in the field of medicine. X-rays would for the first time in the history of the human race, allow mankind to look inside the human body and indeed animal bodies without having to cut them open! It was this discovery that resulted in Roentgen being awarded the first Nobel Prize for Physics in 1901.

Computed tomography or CT uses the same x-rays that Roentgen discovered in the 1890s but in a different way to produce a different type of image. The ability to use x-rays to produce a CT image was only made possible by the advent of another tremendous invention - the computer. Many people nowadays are familiar with CT having seen it on television or indeed having had a CT scan themselves. The patient lies flat on a table in the centre of a tunnel. So what happens now?

Imagine yourself for a moment at the centre of a large Ferris wheel like the London Eye for example. Except this Ferris wheel has 360 gondolas... You are lying still on your back looking up at the sky and all around you - above, below and to either side - are all these gondolas, each packed with sharphooters armed with magic x-ray guns. Once the operator presses the button to start the wheel, all the sharpshooters start firing at you at once and continue doing so as the wheel rotates around you. Luckily, you don't feel anything because the x-rays pass through you magically. Phew! OK, what happens next?

Because x-rays are being fired from 360° around the patient instead of a single static point, things are far more complicated than with a regular x-ray examination. The computer 'sees' the x-rays passing through the patient from all these different directions and is then able to construct a picture of a slice of the patient just as you would slice through an orange. This is very different to a regular x-ray which is more like a photograph of the orange for example.  This provides a major advantage over regular x-rays as the image avoids the  superimposition of tissues which is inevitable on a regular x-ray. Because the CT scan produces an image of a slice or cross-section through the patient, it is often referred to as cross-sectional imaging. Magnetic resonance imaging or MRI is also a form of cross-sectional imaging because it to produces images of slices through the body.

 A CT image differs from a regular x-ray image in another fundamental way. A regular x-ray shows fluid and all types of soft tissue (blood, urine, muscle, liver, spleen) as the same colour or opacity on the x-ray film. A CT image however is able to differentiate, not only between fluid and soft tissue, but different types of soft tissue. Thus, blood in the heart can be differentiated from the heart wall,   tumour is from the surrounding liver, bleeding or blood clots in the brain and so on.

In veterinary medicine, CT scans are becoming more available to patients and vets and are being used more commonly as the primary method to diagnose orthopoedic diseases such as poorly-mineralised or fragmented medial coronoid process, distal humeral OCD, elbow incongruency and incomplete ossification of the humeral condyles (IOHC) or in the search for spread of cancer to the lungs. Complex anatomic areas such as the skull or spine are frequently evaluated in more detail with CT as it avoids the problem of superimposition seen with regular x-rays. The chest and abdomen are also evaluated more frequently when more detail is required subsequent to an abnormality being seen initially on a regular x-ray or ultrasound examination.