I am often asked by my patients whether or not they should have an x-ray or other diagnostic test to rule out certain injuries such as shin splints, stress fractures, etc. Patients are often curious as to which test is best suited for their particular injury, so I thought Id take the time to outline the different imaging techniques available to health professionals and talk a little about how they work.
Plain film radiographs, or x-rays, are the simplest and most common diagnostic tool used by doctors when evaluating skeletal problems. It is typically used first before any other sophisticated tests are done due to its ease of access and cost effectiveness. X-rays pass radiation through the body and onto a film. Bones absorb the radiation while soft tissues like muscle and fat allow the radiation to pass right through. This contrast in absorption is what gives the white and black differences on the film. X-rays do have their limitations, though. As I mentioned above, because the radiation passes through the soft tissues you can only get a good picture of the bone. As well, x-rays arent extremely sensitive. This means that if you have a tumor or other lesion in the bone, you will need to have 30-50% loss of bone density before it will show up on the film. However, conditions such as fractures show up readily on x-ray so we generally use these films when we are looking for obvious fractures, dislocations, and postural abnormalities. As an aside, when having spinal x-rays done it is best to take them standing so that any postural abnormalities that might be hidden while lying down (such as a short leg) are evident on the final film.
Imaging such as myelography, angiography, etc. use an opaque contrast medium which is injected into the soft tissues in the area to be studied, and then an x-ray is taken. For example, myelography involves injecting a contrast medium into the spinal canal and then x-rays are taken to see if there is pressure anywhere on the spinal cord.
Computerized Tomography is an advanced form of x-ray which uses the addition of a computer to generate better films. The premise is that the internal structure of an object can be reconstructed from multiple projections of that object. The patient lies on a table and an x-ray unit rotates around the patient, taking multiple little slices of x-ray. Because of the computers ability to manipulate digital data, it can create images that show cross sections of the area being studied. So, in contrast to plain film x-rays which only show front or side views, CT scans can actually give you a cross section through the middle of the bone. Very useful for imaging conditions such as disc herniations and tumors in bone. While CT scans generally use more radiation and take more pictures than conventional x-rays, because each slice is very thin, actual exposure incurred by the patient is comparable to that of a plain film series of x-rays. The downside is that CT scans are more expensive than plain films and less readily available.
Magnetic Resonance Imaging is by far the most exciting of the imaging techniques. It doesnt use any radiation but it produces some of the most amazing pictures of the human body. It is based on the principle that all matter is made up of atoms that have magnetic properties due to the electrical charge of the protons and electrons in each atom. These charged particles are randomly aligned in the body. Now, the body is made up of mostly water, and different tissues have different concentrations of water. For example, fat has more water than muscle which has more water than bone. With an MRI, the patient is placed in a tube which is actually a large magnet. This magnet creates a field which aligns the protons in the patients body with the magnet field. Then a radio frequency is passed through the patient and this changes the orientation of the protons, thereby randomizing them again. When the radio frequency is turned off, the protons re-align with the magnetic field and the computer picks up on this change and creates images based on the movement of the protons. The data can be manipulated in various planes to produce side views, cross sections, whatever angle you want. MRIs display organs and bone equally well. Because of the high cost of these units and their lack of availability, they are only used when other diagnostic procedures are not enough. Hence, there is often a long waiting list - unless you are a professional football or baseball player.
Bone scans fall under the heading of Radionuclide Imaging and they are based on the principle that certain compounds are selectively absorbed by certain organs. In bone imaging, the most common isotope used is Technium-99m phosphate, and less frequently, Gallium-67. These isotopes are injected into the body and then after a waiting period an imaging device called a gamma camera will record the bone-emitted gamma rays as the isotope degenerates. It takes a full body picture which isnt terribly crisp but if there is an area where the bone is damaged it will show up as a "hot spot" because the isotope is being absorbed rapidly in that area. Areas that typically show up as "hot" include stress fractures, infections, tumors, as well as the elbow where the injection was given and the bladder where the isotope concentrates before excretion. While bone scans do not produce very clear images, they are extremely sensitive which means they only require 3-5% bone destruction to be visible which is up to 10 times more sensitive than plain film radiographs. But these images are of activity only - they cant tell you if you are dealing with an infection or a tumor so further testing would be required.
Considerations for the Athlete
So how does this relate to the injured athlete? Lets use an example like a stress fracture in the shin. Because stress fractures are tiny cracks in the bone, they rarely show up on plain film x-rays unless they are old enough in which case we sometimes see evidence of the healing process. So, because they are hard to detect a bone scan would be the test of choice due to its ability to sense minute changes in bone.
In closing, the majority of overuse injuries that the multi-sport athlete will encounter should not require advanced imaging techniques to be diagnosed properly. A good doctor with experience treating athletes should be able to diagnose your injury through history and examination alone. Good luck with your training and racing this year!
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