Over the next few weeks we will look at some of the diagnostic tools commonly used in both medicine and veterinary practice, such as radiography (X-rays) and ultrasonography (ultrasounds). As I keep referring to their use in various articles, I thought it might be interesting to explain – in a simplified way, because otherwise I will get confused too – how they work but also what we are looking for when using them in diagnosis.
X-rays, more technically known as radiographs, are easier for most lay people to interpret than ultrasound, especially if it is a fractured bone you are looking at. Without getting too bogged down in the physics, X-rays are a type of radiation that travels in straight lines and is not reflected. They will turn photographic emulsion black which is why the more X-ray that gets through, the blacker the picture will be. Very crudely, the higher the density of tissue in the way of the beam, the whiter it will show up, so metal and thicker bits of bone will show up bright white, soft tissue like internal organs and fluid various will be shades of grey, and air will be black. Practically, what this means is that if we are looking for a fracture of say the middle of the femur (thigh bone), it should show up easily with the picture revealing a white bone with a black space where the break is. Interpretation of soft tissue problems can be trickier, and if there is fluid in the chest or the abdomen this will effectively add a grey screen over everything which is helpful to tell us there is free fluid, but will make it harder to see the organs beneath it.
The other important thing to realise is that X-rays provide us with a 2D image of a 3D animal. All you get is one plane at a time, which is why it is usual to take at least two X-rays of the same area. With small animals, it is most common to have the X-ray machine fixed to a mobile stand and point down over a table with a plate on it that contains the photographic film. The table should be lined with a lead cover to absorb any X-rays that are not taken into the film. Great care is also taken to make sure that the amount of X-rays fired by the machine are suitable for the size of patient (obviously you need less to get through a kitten’s paw than a great Dane’s chest) and that the beams are aimed to not go beyond the plate. I was taught to remember that it’s like toast: if you’ve overdone it (too much X-ray sent) then the whole picture will look too black. Any movement while taking the images will greatly affect the picture which is why, alongside the exact positioning often needed to show the right angle, in the vast majority of cases animals need to be sedated.
An example of how it works in practice would be when looking at the heart. Often an image is taken with the patient lying on its right-hand side with the X-ray beam passing from the machine through the left chest first, into the chest cavity, out through the right chest wall and into the plate containing the photographic film on the table. In addition to this, a second X-ray is taken with the patient lying with its tummy on the table. The beam then passes down through the spine to the chest cavity and out through the brisket into the plate. By combining the two images, you are able to construct a 3D picture of what the heart looks like. Depending on what structure you want to see will alter how the animal is positioned. While the heart is best seen in this manner, a clearer picture of the lungs is found by having the patient lie on its back.
Next week we look at special kinds of contrast techniques used when X-raying to highlight various structures before moving to ultrasounds.