One of the most common questions we get as a diagnosing practitioner is “Do I need a MRI, I’ve been told I should get a scan?”. There is no doubt technological improvements in radiology equipment and increased access to these services has become a huge help in us confirming and eliminating particular diagnosis. I thought it helpful to explain what each type of scan might be used for, and how each scan achieves its picture. In each individual situation, we will make a judgment on what we think will help you achieve the best outcome. This may be that we refer you off to get a scan, but we may also discourage this idea and will explain why.
The main types of diagnostic scans to image musculoskeletal problems are:
Computed (or computerized) axial tomography (CT or CAT) scans
Diagnostic ultrasound scans
Magnetic resonance imaging (MRI) scans
Each of these scans use different methods to achieve a picture of structures beneath the skin. X-rays and CT scans both use electromagnetic radiation in the form of X-rays. These rays pass through soft tissue more easily than they can through bone tissue. The X-ray picture is made up from X-rays from a single source hitting a sensor once they have passed through the body segment. As bone absorbs more X-rays, a white area is left as a shadow where bone is present and the darker areas are where more X-rays have passed through. X-rays will usually show a shadow where soft tissues (muscle, tendon, fat etc.) are present, but don’t show them well.
CT scans also use X-ray’s fired at the body segment from many angles. Computer systems then analyse the X-rays that have passed through the body part enabling them to create a 2-dimension or 3-dimension picture. As X-rays are used as the medium for obtaining an image, CT scans are ideal at looking at bone very accurately and can also show soft tissue moderately well. One of the major negatives to CT scans is the amount of radiation needed to create this type of image. Our bodies absorb natural amounts of radiation from the atmosphere and items around us. A multiphase abdomen and pelvis CT requires one of the highest doses of radiation to produce its image – one scan produces the equivalent of 10 years of natural background radiation.
Diagnostic ultrasounds are the process of an ultrasound beam being applied to the tissue and the amount of reflected beam detected. Ultrasounds are quite good as showing soft tissue under the skin such as muscle and tendon. Where they differ from other scans is that they are dynamic – meaning that you will continue to provide images as a body segment is moved. This is excellent in the case of structures pinching, catching and changing positions with movement. The equipment they require is much cheaper than other scans and they are fairly easy to complete. However the downside to ultrasounds scans are the findings have been shown to be less accurate than other scans, they are very user dependent and may only show a part of the problem.
Magnetic Resonance Imaging (MRI) is a scan where large magnetic fields and radiowaves to excite hydrogen molecules in fluid within the body. The machine then measures the rate at which these excited molecules return to equilibrium. This provides a picture showing the difference between tissue types. MRI scans are typically best in showing tissues with high proportions of water: muscle, tendon, ligament, fat, swelling etc. The sensitivity of MRI also gives it a very good ability to show bone. MRI’s are also great in that they don’t require the use of radiation, however they are time consuming and expensive to perform.
Bone scans are used to help detect very specific problems and work by the patient being injected with a radioactive material into their bloodstream. During times of damage, the body has systems where it will increase the blood flow to an area to provide nutrients for this repair to occur – inflammation is part of this process. This radioactive material moves with normal blood and increases in areas of bone repair and remodeling. There is a normal amount of bone remodeling that will occur during life – its believed the body will turn over about 10% of its mature bone each year and form new bone. Hence bone damage can be identified by the amount of repair that the body is conducting, but may also be identified by a lack of normal bone synthesis.
Written by Mark Fotheringham
Mayo Clin Proc. 2010 December; 85(12): 1142–1146. doi: 10.4065/mcp.2010.0260