Think about when a horse comes into the clinic for a lameness exam. The horse is examined visually, with hoof testers, and with a doctor's hands. The horse is trotted to determine the degree of lameness and ascertain which limb(s) are in pain. The lameness is localized by local anesthetic nerve blocks and that area is scrutinized with a radiograph series. What is the magic behind the x-ray machine and the views it produces? What type of things can we see or not see? The answers to these questions and others will be described in further detail.
X-rays are electromagnetic radiation, just as television, radio waves, microwaves and visible light. X-rays have the shortest wavelength of the above-mentioned waves and therefore have the highest energy level. The science of radiology is based on the high level of energy the x-ray beams have. These x-ray beams are able to penetrate various densities of matter and by capturing the differing amounts of x-rays that pass through a subject on film gives us a picture of that object internally. X-ray waves pass more readily through less dense objects and are absorbed by more physically dense objects. This gradation of x-ray particles going through the object exposes different parts of the film in different intensities. When we look at a radiograph there are five different levels of intensity seen, depending on the density or radiopacity of the object. The least dense or radiolucent to radiodopaque is air, fat, then water which includes muscle, bone and metal. This differing radiolucency is transformed onto the film by air being revealed as black to metal being revealed as white and shades of gray for the other densities in between.
Once the films have been exposed to the x-rays and developed we then interpret what has been captured. First we must consider several questions: Is this film diagnostic? Does it need to be retaken for better positioning, better technique or because the horse moved? Are there enough views to make a diagnosis? We are looking at a two-dimensional radiograph and attempting to match it to a three-dimensional object (the horses' foot, leg, etc.) so every radiograph needs at least two views if not more to confirm the diagnosis. Once the radiographs are found to be diagnostic, then interpretation can proceed. This takes the knowledge of understanding what normal anatomy looks like both, grossly and with radiographs. It also takes the knowledge to discern the minor changes in what is considered normal becoming abnormal.
Here are a few abnormalities seen on radiographs. Fractures are probably the thing we think about when are getting an x-ray exam done when we are injured. Broken bones have black lines through them were the bone should be all white to gray in color. Some places were there is superimposition of soft tissue over air can mislead one into seeing a fracture line. Ringbone is seen quite well on radiographs. This is a condition where usually, the short or long pastern bone has had some trauma to it. The bone's response to the injury is to produce more bone around the injury. This produces a very roughened appearance to what should be a smooth outline of the bone. Navicular bone degeneration or cysts can be seen radiographically. These look like dark spots or have what is called a "lollipop" shape in what should be the white or gray bone. Channels into the bone made from blood vessel show up darker compared to the bone because blood is primarily water and is more radiolucent. Degeneration of the deep flexor tendon can also be seen when there is calcification in that area of the navicular bone. The position and condition of the coffin bone within the hoof is very important to know when dealing with laminitis and pedal osteitis cases. The views from the side of the coffin bone gives both the veterinarian and farrier a guide to needed therapy in laminitis cases. A view looking down at the hoof at about 60 degrees from the front of the horse can give the veterinarian an idea of the degree of inflammation a coffin bone might have. Foot imbalance can also be detected by taking a view from directly in front of the horse, parallel with the ground, which can contribute to improper loading of the joints in the foot and leg higher up. One other abnormality seen on radiographs is osteoarthritis of the joints. This can be seen as small abnormal points coming off a normally smooth margin of a bone adjacent to the joint in question.
There are some drawbacks to radiographs. Larger areas that are thicker in mass need more energy to penetrate. This takes more powerful machines than what can be transported around in a veterinary truck. Not only does it take more power to penetrate large body parts, it takes more time in the amount of radiation exposure. This requires the subject to be absolutely still to have diagnostic radiographs taken. This requirement necessitates the horse not just sedated but under general anesthesia. Another drawback is although many abnormalities can be seen by a radiographic exam; abnormalities that are of soft tissue of origin cannot be distinguished with radiographs. Soft tissue abnormalities are better seen with ultrasound exams and magnetic resonance imagery (MRI).
Radiographs are an excellent way to document abnormalities and confirm a diagnosis involving the musculoskeletal system.