Which imaging test is best for a suspected bone stress injury?

Magnetic resonance imaging is the preferred test for a suspected bone stress injury, because it shows bone marrow oedema, an early sign, before changes appear on a plain radiograph, and it involves no ionising radiation. A radiograph is often normal in the early weeks, so a normal X-ray does not rule out a bone stress injury, and MRI is used when clinical suspicion is high, interpreted alongside the history and examination.

When is ultrasound preferred over MRI?

Ultrasound is preferred when dynamic, real-time assessment is useful, for example watching a tendon move, and for guiding injections and aspirations in real time. It is quick, portable, inexpensive and free of ionising radiation, and images superficial tendons and muscles well. Its limitations are that it is operator-dependent and cannot assess deep structures or see through bone, so MRI is preferred for deeper or intra-articular structures and a global view of a joint.

What are the Ionising Radiation (Medical Exposure) Regulations?

These are the UK regulations governing the medical use of ionising radiation, such as X-rays and computed tomography. They require every exposure to be justified, so the clinical benefit outweighs the radiation risk, and optimised, so the dose is kept as low as reasonably practicable while still answering the question. They set out responsibilities for the referrer, the practitioner who authorises the exposure, and the operator who performs it, underpinning safe imaging.

Principles of Imaging

General MSK Principles

Overview

Imaging supports the diagnosis of sport and exercise medicine problems, but it never replaces a careful history and examination, and choosing the right test depends on the suspected pathology, its depth and site, and the specific clinical question. Different modalities suit different problems: radiographs are often first-line for a suspected fracture, malalignment or osteoarthritis, ultrasound and magnetic resonance imaging (MRI) show most soft-tissue problems well, computed tomography (CT) gives detailed bony anatomy, and only some of these use ionising radiation. For the sport and exercise medicine (SEM) clinician, understanding how each modality works, what it shows, its radiation and safety implications, and how to interpret it in context prevents both missed diagnoses and unnecessary scans. This page covers the main imaging modalities and their uses, radiation safety and test selection, a structured approach to interpretation, and the role of image-guided procedures.

How do the main imaging modalities work, and what is each best for?

Several modalities cover most sport and exercise medicine imaging, and the right one depends on the clinical question rather than a fixed rule. The radiograph, or plain X-ray, passes a small dose of ionising radiation through the body and is often the first-line test for a suspected fracture, for joint alignment and for osteoarthritis, though it does not show every bone disorder and is limited for soft tissue. Ultrasound uses high-frequency sound waves, involves no ionising radiation, and is well suited to superficial soft tissue, particularly tendons, muscles and fluid collections; its distinctive strength is dynamic, real-time assessment, and it is widely used to guide injections and aspirations. Its main limitations are that image quality depends heavily on the operator, and it cannot see through bone or assess deep structures well.

A comparison figure of five imaging modalities as bordered panels, each stating what it is suited to and whether it uses ionising radiation. Radiograph (plain X-ray): often first-line for suspected fracture, alignment and osteoarthritis, uses radiation. Ultrasound: superficial soft tissue such as tendons and muscles, dynamic real-time assessment, guiding injections, no radiation. MRI: multiplanar soft tissue such as ligaments and cartilage and bone marrow oedema for bone stress injuries, no radiation. CT: complex fractures, detailed bony anatomy and metal hardware, uses more radiation. DXA: bone mineral density and body composition, very low radiation. — Five modalities cover most SEM imaging; the right one depends on the clinical question. Radiographs and CT use ionising radiation and give bony detail, ultrasound and MRI use none and assess soft tissue, and DXA (very low dose) measures bone density.

Magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves, involves no ionising radiation, and provides excellent multiplanar assessment of soft tissue and bone marrow, showing ligaments, cartilage, muscle and bone marrow oedema, which makes it particularly useful for suspected bone stress injuries. Magnetic resonance arthrography, which adds intra-articular contrast, is helpful in selected labral or instability questions rather than being routinely superior to standard MRI. Computed tomography (CT) takes cross-sectional X-ray images and gives excellent bony detail for complex fractures and bony architecture, and for assessing metal hardware, at the cost of a higher radiation dose than a plain radiograph. Dual-energy computed tomography can help identify urate deposition in selected cases of suspected gout. Finally, dual-energy X-ray absorptiometry (DXA) uses a very low dose of radiation to measure bone mineral density and body composition. Because no single modality answers every question, the choice follows the suspected pathology, its depth and site, and what the result will change.

Radiation, safety and choosing the right test

A key distinction is that radiographs and CT use ionising radiation, whereas ultrasound and MRI do not. Ionising radiation carries a small, dose-dependent risk, so its medical use in the UK is governed by the Ionising Radiation (Medical Exposure) Regulations, which require every exposure to be justified (the benefit must outweigh the risk) and optimised (the dose kept as low as reasonably practicable). A CT scan delivers many times the radiation of a single chest radiograph, which is one reason CT is not used lightly.

A concept figure split into two halves. Left half titled 'Ionising radiation': radiograph and CT, with a relative-dose bar showing a plain radiograph as a small mark and a CT scan many times larger. Right half titled 'No ionising radiation': ultrasound and MRI. Beneath, two guiding principles from the UK Ionising Radiation (Medical Exposure) Regulations, 'Justify: benefit must outweigh risk' and 'Optimise: dose as low as reasonably practicable', and two questions, 'Will the result change management?' and 'Could a test without radiation answer it?'. — Radiographs and CT use ionising radiation (CT far more than a plain film); ultrasound and MRI use none. UK regulations require every exposure to be justified and optimised, and radiation-free tests are preferred where they can adequately answer the question, including in pregnancy where necessary imaging is still justified.

Pregnancy does not mean that necessary imaging is simply withheld. A radiograph or CT that is clinically necessary is still justified and optimised, and ultrasound or MRI are preferred where they can answer the clinical question adequately, so that radiation is avoided when a radiation-free test would do. MRI is free of ionising radiation but has its own safety considerations, and patients are formally screened before scanning: many modern cardiac and other implants are conditional for MRI under defined conditions rather than being absolute contraindications, so screening determines what is safe. Over-imaging also has real downsides, since incidental findings such as symptomless degenerative changes are common and can lead to anxiety and further unnecessary tests, so imaging is best requested to answer a defined clinical question rather than to reassure. Referral guidance such as the Royal College of Radiologists iRefer resource supports this, and the guiding questions remain whether the result will change management and whether a test without radiation could answer it.

Interpreting images and image-guided procedures

Interpreting imaging benefits from a systematic approach rather than jumping to the obvious. For a radiograph, a simple structure runs through adequacy (are the required views present for this examination), alignment, bone (cortex and texture), the joint spaces and articular surfaces, and finally the surrounding soft tissues, so that subtle findings are not missed; cartilage itself is not directly seen on a radiograph, so it is the joint space and articular surfaces that are assessed. MRI is read sequence by sequence, using the different weightings, such as T1, T2 and fat-suppressed or short tau inversion recovery images, to characterise tissue and highlight oedema. In every case the images are interpreted alongside the clinical picture, because a finding only matters if it fits the patient in front of you, and incidental abnormalities are common.

Imaging is increasingly not just diagnostic but interventional. Ultrasound in particular allows real-time guidance of procedures, which generally improves the accuracy of needle placement for joint and soft-tissue injections and aspirations compared with landmark-only techniques; better placement does not automatically guarantee better pain, function or safety outcomes for every injection, and the value depends on the procedure and the clinical situation. For the sport and exercise medicine clinician, this blurring of the line between imaging and treatment, especially with point-of-care ultrasound, is a useful development in day-to-day practice, provided it is used within the limits of training and always tied back to the clinical assessment.

Exam Tips

•Choose the imaging test by the suspected pathology, depth, site and clinical question; radiographs are often first-line for suspected fracture, alignment or osteoarthritis.
•MRI gives excellent multiplanar soft-tissue and marrow assessment and shows bone marrow oedema, useful for suspected bone stress injuries; ultrasound is ideal for tendons and dynamic assessment.
•Ionising radiation exposures must be justified and optimised under the Ionising Radiation (Medical Exposure) Regulations; necessary imaging in pregnancy is justified and optimised, not simply withheld.
•MRI is free of ionising radiation but needs formal safety screening; many modern implants are conditional for MRI rather than absolute contraindications.
•Use a structured radiograph review (adequacy, alignment, bone, joint spaces and articular surfaces, soft tissues) and always correlate with the clinical picture.
•Ultrasound guidance generally improves needle-placement accuracy, though better placement does not guarantee better outcomes for every injection.

Useful Links

British Society of Skeletal Radiologists: musculoskeletal ultrasound imaging standards in the UK

bssr.org.uk

Guidance to the Ionising Radiation (Medical Exposure) Regulations 2017 (justification and optimisation)

gov.uk

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