X-Ray, Ultrasound, CT and MRI: What Each Shows and When Imaging Is Worth Having

Four Tools That See Different Things

Imaging is not a single test. Each method shows a different kind of tissue, so the right choice depends on the question being asked. X-ray shows bone well and is commonly used to look for fractures and to assess joint alignment and bone changes; it shows soft tissues poorly. CT uses X-rays to build detailed cross-sectional images of bone and is often used for complex fractures or detailed bony anatomy. X-ray and CT both involve ionising radiation.

Ultrasound and MRI for Soft Tissue

Ultrasound uses sound waves, involves no radiation, and can show superficial tendons, muscles and fluid collections, including during movement. The quality of the result depends considerably on the operator. MRI uses a magnetic field and shows soft tissues such as discs, cartilage, ligaments, tendons and muscle in more detail, without ionising radiation, though it is more expensive and takes longer. Neither is automatically better; the choice depends on the structure of interest and the clinical question.

Why a Finding Is Not Always a Cause

A systematic review of imaging of the spine in people without symptoms found that degenerative features such as disc degeneration, disc bulges and protrusions were common in asymptomatic individuals and became more frequent with age. The authors concluded that many of these features are probably part of normal ageing and unassociated with pain, and that imaging findings must be interpreted in the context of the person's clinical condition (Brinjikji et al., 2015). A report that describes degeneration therefore does not by itself explain someone's pain, and an apparently alarming wording on a report can add worry without adding useful information.

When Imaging Is Worth Having

A systematic review and meta-analysis of randomised trials found that routine, immediate lumbar imaging in people with low back pain and no indication of a serious underlying condition did not improve clinical outcomes compared with usual care, and concluded that routine immediate imaging should be avoided in acute or subacute low back pain without features suggesting serious pathology (Chou et al., 2009).

Imaging is more likely to be useful when a fracture or other serious cause is suspected, when there are concerning features, when symptoms are not progressing as expected, or when the result will change management, such as before considering surgery or injection. Whether to refer is a decision for a medical practitioner or an appropriately qualified clinician after an assessment.