Learning Objectives
By the end of this chapter you should be able to:
- Explain how an X-ray image is produced and why tissues look different.
- Name the five radiographic densities and apply them to a film.
- Choose and describe standard projections.
- Quote typical effective doses and apply ALARA.
- Use validated decision rules to justify imaging.
How X-rays Are Produced (the "Pathophysiology" of the Image)
X-rays are high-energy ionising electromagnetic radiation. They are produced in an X-ray tube when electrons are accelerated from a heated cathode filament toward a tungsten anode across a potential difference of roughly 40–120 kVp for musculoskeletal work. Less than 1% of the energy becomes X-rays (bremsstrahlung and characteristic radiation). The rest becomes heat.
Two technical factors control the image:
- kVp (peak kilovoltage) sets beam energy and penetration. Higher kVp gives a lower-contrast, more penetrating image.
- mAs (tube current × time) sets the number of photons. Higher mAs gives more blackening and more dose.
The beam passes through the patient and strikes a digital detector (DR) or a computed radiography (CR) plate. The detector records how much radiation emerged, and the image is a map of that attenuation.
Tissue Interactions and the Five Densities
Attenuation depends on tissue atomic number, density and thickness. Two interactions dominate:
- Photoelectric absorption is the main source of bone contrast. It is more likely in high-atomic-number material such as calcium, and it rises sharply at lower kVp.
- Compton scatter dominates at higher energies. It degrades the image and is the main source of staff exposure.
Five basic densities appear on every radiograph, from black to white:
| Density | Appearance | Examples |
|---|---|---|
| Air/gas | Black | Lungs, bowel gas |
| Fat | Dark grey | Subcutaneous fat, fat pads |
| Soft tissue/fluid | Mid grey | Muscle, effusion, heart |
| Bone/calcium | Near white | Cortex, calcification |
| Metal | Brightest white | Implants, jewellery |
Terms: radiolucent (dark, allows passage) and radiopaque (white, blocks passage). The silhouette sign occurs when two structures of equal density touch and their borders vanish. An example is right middle lobe consolidation obscuring the right heart border.
Fat pads are useful in MSK work. An elevated anterior fat pad or a visible posterior fat pad (the "sail sign") at the elbow suggests an effusion, and so an occult intra-articular fracture such as a radial head or supracondylar fracture.
Radiographic Views and Projections
Projections are named by the direction the beam travels:
- AP (anteroposterior): beam enters the front and exits the back.
- PA (posteroanterior): the reverse. The standard chest film is PA, which reduces magnification of the heart.
- Lateral: named by the side nearest the detector.
- Oblique: for example, scaphoid series and foot obliques.
- Special views: weight-bearing, stress, flexion/extension, skyline (patellofemoral), Y-view (scapula), open-mouth odontoid, and Zanca view (AC joint).
Rule of two: two views at 90° to each other, two joints (above and below a long bone fracture), and, where relevant, two sides for comparison in paediatric patients.
Other principles:
- Magnification increases with object-to-detector distance.
- Weight-bearing films show true joint space narrowing, alignment and instability. These are essential in knee OA, spondylolisthesis and scoliosis.
- Dynamic flexion/extension lumbar films assess translation. More than 4–5 mm of translation or more than 10–15° of angular change is commonly cited as instability.
Presentation: When Should a Patient Be Imaged?
Most musculoskeletal presentations do not need an X-ray. Imaging should change management. Indications include:
- Trauma with focal bony tenderness, inability to weight-bear, or deformity.
- Suspected fracture red flags in the spine: age >50 with trauma, prolonged corticosteroid use, osteoporosis, and significant trauma at any age.
- Suspected malignancy: past history of cancer, unexplained weight loss, night pain, and age >50.
- Infection: fever, IV drug use, immunosuppression.
- Inflammatory disease: morning stiffness >30–45 minutes in a patient under 40, suggesting axial spondyloarthritis. The sacroiliac joints on an AP pelvis are the first-line film.
- Post-operative or arthroplasty follow-up.
Non-specific low back pain without red flags should not be routinely imaged. Degenerative findings are common in asymptomatic people. In Brinjikji et al. (2015), disc degeneration was present in 37% of asymptomatic 20-year-olds and 96% of 80-year-olds.
Assessment: Decision Rules and Systematic Reading
Validated rules
- Ottawa Ankle Rules: X-ray if there is pain in the malleolar zone and bone tenderness at the posterior edge or tip of either malleolus (distal 6 cm), or inability to take four steps. For the midfoot, the triggers are tenderness at the base of the 5th metatarsal or at the navicular. Sensitivity is approaching 100% for fractures.
- Ottawa Knee Rule: X-ray if age ≥55, isolated patellar tenderness, fibular head tenderness, inability to flex to 90°, or inability to bear weight for four steps.
- Canadian C-spine Rule: high-risk factors are age ≥65, dangerous mechanism, or paraesthesias. Low-risk factors permit safe assessment of active rotation, and ability to rotate 45° left and right argues against imaging.
Systematic reading (ABCDS)
- Alignment (cortical lines, joint congruity)
- Bone (cortex, trabeculae, lucencies, periosteal reaction)
- Cartilage/joint space
- Devices and soft tissue (effusion, swelling, gas, foreign bodies, calcification)
- Summary, with comparison to prior films
Always check patient identifiers, date, laterality marker and projection first.
Selected normative values
- Cardiothoracic ratio on PA chest: <0.50
- Cobb angle: ≥10° defines scoliosis
- Böhler angle (calcaneus): 20–40°. A reduced angle suggests a compressed calcaneal fracture.
- Acromiohumeral interval: about 7–14 mm. <7 mm suggests rotator cuff pathology.
- Lateral centre-edge angle (hip): 25–40°. <20° indicates dysplasia and >40° suggests pincer morphology.
- Kellgren–Lawrence grading for OA: 0 (none) to 4 (severe: large osteophytes, marked narrowing, sclerosis, deformity). Grade ≥2 is the usual threshold for radiographic OA.
- Prevertebral soft tissue (lateral C-spine): <7 mm at C2 and <22 mm at C6 in adults.
Radiation Dose and Safety Principles
Radiation effects are stochastic (probability rises with dose, no threshold assumed, e.g. cancer) or deterministic (tissue reactions above a threshold, e.g. skin erythema at around 2 Gy). Diagnostic plain film doses are far below deterministic thresholds.
Typical effective doses (Mettler et al., 2008):
| Examination | Effective dose (mSv) | Approx. equivalent background |
|---|---|---|
| Extremity | <0.001 | hours |
| Chest PA | 0.02 | ~2.5 days |
| Cervical spine | 0.2 | ~1 month |
| Pelvis | 0.6 | ~2 months |
| Lumbar spine | 1.5 | ~6 months |
| CT abdomen/pelvis | ~10 | ~3 years |
Average natural background is about 2.4–3 mSv/year.
Core principles:
- Justification (benefit exceeds risk) and optimisation (ALARA), as set out in ICRP 103 and IR(ME)R 2017 in the UK.
- Time, distance, shielding: intensity follows the inverse square law, so doubling distance cuts exposure to one quarter. Staff should stand at least 2 m from the tube or behind a protective screen.
- Collimation is the most effective dose-limiting technique in practice.
- Pregnancy: ask every woman of reproductive age. Fetal doses from extremity or chest films are negligible. The deterministic threshold for the fetus is about 50–100 mGy, well above plain film doses. Avoid direct abdominal and pelvic exposure unless it is clinically essential, and escalate to the radiographer and referrer.
- Shielding: routine gonadal shielding is no longer recommended in many jurisdictions (AAPM 2019). Follow local policy.
Management: A Staged Imaging Pathway for Physiotherapists
- Stage 1, clinical reasoning: take the history, screen for red flags and examine. Decide whether the result would change management.
- Stage 2, apply a rule: use the Ottawa or Canadian rules or local guidelines. If the rule is negative and no red flags are present, proceed with treatment and review at 1–2 weeks.
- Stage 3, refer or request: where the scope of practice and IR(ME)R training allow, request the appropriate projections. Otherwise, refer to the GP, ED or sports physician. Record the clinical question and pregnancy status.
- Stage 4, interpret: read the report alongside the film. Correlate the findings with symptoms and do not treat the image in isolation.
- Stage 5, act and communicate: for an unstable fracture, unexplained lytic lesion or suspected infection, stop loading and refer urgently. For stable findings, explain the result using plain language (avoid "wear and tear" labels that increase fear) and progress the rehabilitation plan.
Contraindications and cautions: known or possible pregnancy (relative, for abdominal or pelvic films), uncooperative or unstable patients (resuscitate first), and repeated imaging without a new clinical question.
Clinical Pearls
- Treat the patient, not the film. Imaging findings often do not correlate with pain.
- A normal film does not exclude fracture. Occult scaphoid and femoral neck fractures can be missed initially. If suspicion persists, immobilise and request MRI or repeat imaging at 7–10 days.
- Paediatric growth plates mimic fractures. Use comparison views and the Salter–Harris classification (I–V) for physeal injuries.
- A Looser zone or periosteal reaction needs medical investigation.
- Look at the edges of the film, since the lung apex and the other end of the clavicle are commonly missed.
- Lateral films are essential in the spine, where AP-only films miss spondylolisthesis and vertebral compression fractures.
Summary
X-rays are produced by accelerating electrons onto a tungsten target, and image contrast arises from differential attenuation. Five densities (air, fat, soft tissue, bone, metal) underlie every interpretation. Use at least two views and read systematically. Doses from plain films are small, but all exposure must be justified and optimised. Validated decision rules, such as the Ottawa and Canadian C-spine rules, help prevent unnecessary imaging while avoiding missed fractures.
References
- Lampignano JP, Kendrick LE. Bontrager's Textbook of Radiographic Positioning and Related Anatomy. 10th ed. St Louis: Elsevier; 2021.
- International Commission on Radiological Protection. The 2007 Recommendations of the ICRP. ICRP Publication 103. Ann ICRP. 2007;37(2–4).
- Mettler FA Jr, Huda W, Yoshizumi TT, Mahesh M. Effective doses in radiology and diagnostic nuclear medicine: a catalog. Radiology. 2008;248(1):254–263.
- Stiell IG, Greenberg GH, McKnight RD, et al. Decision rules for the use of radiography in acute ankle injuries: refinement and prospective validation. JAMA. 1993;269(9):1127–1132.
- Stiell IG, Wells GA, Hoag RH, et al. Implementation of the Ottawa Knee Rule for the use of radiography in acute knee injuries. JAMA. 1997;278(23):2075–2079.
- Stiell IG, Wells GA, Vandemheen KL, et al. The Canadian C-spine rule for radiography in alert and stable trauma patients. JAMA. 2001;286(15):1841–1848.
- Brinjikji W, Luetmer PH, Comstock B, et al. Systematic literature review of imaging features of spinal degeneration in asymptomatic populations. AJNR Am J Neuroradiol. 2015;36(4):811–816.
- The Ionising Radiation (Medical Exposure) Regulations 2017. UK Statutory Instrument 2017 No. 1322.