What is a CT scan?
CT is an imaging technique used to examine the organs, limbs or body cavities. A CT machine contains a rotating head which circles the patient whilst emitting x-rays. This creates a 3D image throughout the body by taking a continuous picture in tiny slices (often only a few millimetres thick). This gives significantly more information than a 2D image, such as from an x-ray, as each organ can be individually identified. It is also easier to assess where structures are sitting in relation to each other, and how they are attached.
What are CT scans used for?
CT scans are good at examining most tissues, including:
- The nasal cavity
- The inner ear
- The lungs
- The abdominal organs
- The limbs
CT are not as good at examining the brain, spinal cord and nerves - MRI (magnetic resonance imaging) is better for these tissues.
Due to the thin slices x-rays it takes, CT scans are much more sensitive for detecting very small changes. This is most relevant in cancer diagnosis, where we often wish to screen for spread (known as metastasis) in the lungs or other organs. CT will detect much smaller abnormalities than is possible on an x-ray.
CT machines are very large and expensive to install and run, so are not available at every practice. You may need to travel to a specialist centre for a scan.
Most commonly, you will be asked to leave your pet at the practice for the morning. This is because CT scans are often part of a larger procedure list, and we do not know an exact time the exam will take place. Occasionally, you may be given a specific time slot for the scan.
To obtain a clear image, your pet must be very still during the scan. To ensure this is the case, animals will always need sedation or anaesthetic for a CT scan. The degree of anaesthesia required will depend on what is being scanned. Typically, for a scan of the lungs, the animal will need to be fully anaesthetised so that we have control of their airways. You will be asked to withhold food the night before the scan, so the stomach is empty. The scan itself is very quick, and not painful.
Depending on the areas being scanned, your vet may wish to give an injection of contrast agent into the bloodstream. This is distributed throughout the tissues, highlighting areas with increased blood flow. Contrast can help us interpret the images to make a diagnosis. If your pet requires contrast, they will usually have a blood test before the scan to ensure their kidneys are able to breakdown the contrast material. They may also be given intravenous fluids (a drip) afterwards to help flush the bloodstream. There are minimal risks with contrast, however a severe allergic reaction can occur extremely rarely – as with any medication.
CT images are created after the examination, as the computer processes the data to produce a final image. This is usually very quick, taking only a few minutes. Sometimes, serious problems can be spotted straight away.
CT images are often complex to interpret and so it can take a long time to study every tiny slice thoroughly. Most commonly the images are sent to a specialist vet who is highly trained in CT interpretation. They will study the images in detail and write a full report of their findings. This often takes a few days; however, more urgent reporting is sometimes available. Once your vet has the report back, they will be able to discuss the findings with you and any next steps.
As vets, we never recommend any test unless we think there is a good chance that it will give us a diagnosis. Unfortunately, not every problem can be identified on a CT scan, and sometimes images are more useful for ruling things out than providing a positive answer. Sometimes CT scans show us that there is something abnormal, but we cannot be certain exactly what is causing the changes. For example, an organ may look larger than normal. Images tell us that there is a structural change, but do not tell us whether the organ is functioning normally, or whether the enlargement is due to swelling, infection, or a mass.
Depending on what is seen on the CT scan this may be enough to decide a treatment plan. If it demonstrates changes but is inconclusive, we will need to gather more information. This may include additional imaging, taking a sample of cells or a surgical biopsy, taking organ-specific blood tests, or even exploratory surgery.