Clinical radiography is an exceptionally vital tool in the diagnostic process for our patients. It is a fast efficient way to rule out very simple paths of diagnosis.
Radiography is wonderful in evaluating anatomic regions of stark contrast in density, but reveals its limitations when evaluating areas of minute tissue density differences. For example, when doing radiographs of the chest, X-ray provides a good representation of the anatomy because the majority of tissue is either air (no density) or bone (high density). There is little in the way of tissue between those two extremes, so abnormalities in the lung field are very obvious. The same goes with imaging of bones. In evaluating for fractures, the radiograph shows the presence or absence of bone, and reveals a fracture as a dark line in the cortical structure.
Beyond these situations, radiography is unfit to give an honest representation of the anatomical structures involved. Though radiography is established on a grey scale of contrast and density, the structures overlay each other and result in an image that roughly only contains four separate densities.
I know that the radiography purists out there might be upset by that comment, but please hear me out. If we were able to remove all of the contents of your body, and lay them out sided-by-side and take a radiograph, then we would see that each structure does have a unique tissue density to an extent. Unfortunately though, when all of those structures are mixed together in your body and we take a two-dimensional radiograph, all of those densities get averaged together and produce an image with a significantly lower dynamic image contrast.
We are able to improve this somewhat with Computed Tomography, but CT still falls short in the area of soft tissue evaluation. Even though CT gives a wider contrast range, the tissue structures are still so close that our brain averages them together and they look more similar than they actually are. MRI, however, does not rely solely on the physical density of tissue to create an image. It uses characteristic information from the tissue based on chemical make-up, position, resonant frequency, and magnetic susceptibility (to name a few) in order to give a more comprehensive breakdown of the structures involved.
Here is an example. In images 1 (left) and 2 (right), there is good contrast between the air in the lungs and the dense bone of the ribs and the spine. However, the spinal cord is superimposed by the vertebra and shown as a streak of grey instead of a separate structure.
MRI images 3 and 4 show same patient as in the X-rays, taken on the same day. Notice how the MRI reveals a large obvious anomaly within the spinal cord on both images, at the level of T12-T13. This animal had presented with an acute onset of spastic paraplegia and the MRI report revealed this abnormality to be a large, left-sided disk herniation causing marked compression of the spinal cord and syringohydromyelia. This diagnosis is good news for the patient and its owners because, with surgery, the chance of a full recovery is very high.
Now, the prognosis isn’t always great, but the information is never less vital to have. In this second case, radiographs show rather normal tissue in the vertebra (image 5, left). However, the contrast-enhanced MRI tells a much different story. This patient came in with overall weakness, pain and difficulty with standing or walking. Notice on the MRI (image 6, right) that the vertebra is black with large white blobs in it. On this particular image, the bone should be completely black because we’ve nulled all of the signal from the structures containing fat. The white blobs in the bone are contrast-enhancing metastatic lesions that extend throughout the spine, pelvis, and beyond.
Though diagnostic radiography is a vital part of the diagnostic process, rarely should we depend on it to give us all of the answers we desire. In this day and age with the resources we have available, we owe it to ourselves and our clients to utilize every opportunity to further diagnosis and promote health and longevity.