A ‘gold standard’ refers to the best possible, the standard by which others are compared. With imaging (x-ray, MRI, CT, ultrasound), each method has it’s pros and cons and some imaging techniques are much better at visualising certain things.
X-Ray for example is very good at imaging bone – if you have a suspected fracture, X-Ray is your man! X-Ray can often be the ‘first port of call’ for imaging as it is simple and cost effective. It is good for visualising bony abnormalities and joint spaces (although not in detail).
MRI is very good at visualising soft tissues – ligaments, intervertebral discs, meniscus. If you have a suspected ACL tear or herniated disc, MRI is the scan for you! It is an expensive imaging route to take, and as such if the results are not needed to guide treatment (as is the case with many lower back issues), it may not be deemed necessary.
CT is great at modelling complex fractures, and assessing for various health conditions (different cancers etc). It uses multiple X-Rays and different tissue densities to create an image of an area.
Ultrasound is used to visualise superficial muscle/tendon/ligament issues. It is very dependent on the user knowing what they are doing / what to look for and thus is the least reliable form of imaging. It is however quick and easy, and for simple needs (eg a torn ATFL in the ankle) it can be sufficient and cost effective.
Hopefully this provides some insight into the different types of scans and why they might be used (or not).
Chris Mooney, Physiotherapist