Plain radiography
X-ray production
Electrons are accelerated from cathode to anode in an X-ray tube. Their interaction with the target produces X-rays, which are attenuated differently by tissues.
Image formation
Greater attenuation produces a more radiopaque appearance.
Approximate order from most to least radiopaque:
- metal
- bone
- soft tissue/fluid
- fat
- air
Radiation protection
Use the ALARA principle: keep exposure as low as reasonably achievable.
Practical measures:
- minimise screening time
- maximise distance from source when possible
- use appropriate shielding
- collimate to the required field
- avoid unnecessary repeat imaging
- position staff away from the highest scatter region
Distance is especially important because radiation intensity falls approximately with the square of distance from a point source.
CT
CT provides cross-sectional X-ray attenuation data and is particularly useful for:
- complex fracture anatomy
- articular surface assessment
- rotational alignment
- bone loss
- preoperative planning
Hounsfield units
The CT scale expresses attenuation relative to water. Air is strongly negative, water is near zero and dense bone is positive.
CT provides excellent cortical and three-dimensional bony detail and is particularly valuable for:
- complex articular fractures
- pelvic and acetabular injury
- rotational assessment
- preoperative planning
- some implant and fusion assessments
Its disadvantages include ionising radiation and metal artefact, although modern reconstruction can reduce artefact.
MRI
MRI uses a magnetic field and radiofrequency pulses rather than ionising radiation.
T1-weighted images
Typically provide excellent anatomy and marrow-fat contrast. Fat is usually bright.
T2-weighted images
Fluid is generally bright and these sequences are useful for detecting oedema and many pathological processes.
Actual appearance depends on sequence design, fat suppression and other parameters.
MRI provides excellent soft-tissue, marrow and neural detail without ionising radiation.
Sequences are selected to emphasise different tissue properties. Fluid-sensitive sequences highlight oedema and many pathological processes; T1-weighted images are useful for marrow anatomy and fat.
Contraindications and safety concerns relate to ferromagnetic objects, certain implanted devices and patient factors. Implant compatibility should be verified rather than assumed.
MRI artefacts
Important examples:
- metal susceptibility artefact
- chemical shift
- motion
- partial-volume effects
Metal artefact reduction sequences can improve imaging around orthopaedic implants.
Nuclear medicine bone scan
Technetium-labelled diphosphonates localise in areas of active bone turnover.
A traditional three-phase study assesses:
- perfusion
- blood pool
- delayed skeletal uptake
Bone scintigraphy is sensitive to increased turnover but often lacks specificity.
Imaging principle
Choose the modality according to the clinical question.
- radiograph: alignment, fracture, joint space, implant position
- CT: cortical detail and 3D fracture anatomy
- MRI: marrow, cartilage, soft tissue, infection and tumour assessment
- nuclear medicine: physiological bone activity and selected whole-skeleton questions
Viva principle
Do not describe imaging as a list of technologies. Link each modality to what physical property it measures and why that answers the orthopaedic question.
Plain radiographs
Plain radiography remains the first-line investigation for many orthopaedic problems because it demonstrates:
- alignment
- cortical integrity
- joint-space relationships
- mineralisation
- implant position
- gross bone lesions
Two orthogonal views are a basic requirement for fracture assessment whenever feasible. Special views are selected according to anatomy and suspected pathology.
X-ray production and dose
X-rays are generated when high-energy electrons interact with a target. Image quality and patient dose are influenced by tube voltage, current, exposure time, collimation and detector technology.
The practical radiation-protection principles are:
- justify the exposure
- optimise technique
- minimise unnecessary field size
- maximise distance where possible
- use shielding and barriers appropriately
- limit exposure time
Fluoroscopy
Intraoperative fluoroscopy can expose both patient and staff. Scatter from the patient is a major source of staff exposure.
Dose-reduction strategies include:
- pulsed rather than continuous screening where possible
- tight collimation
- minimising magnification
- keeping the detector close to the patient
- optimising C-arm position
- standing on the lower-scatter side where practical
- using lead protection
Ultrasound
Ultrasound is dynamic, inexpensive and free of ionising radiation. It is useful for:
- superficial tendon pathology
- effusions
- infant hip assessment
- image-guided procedures
- some peripheral nerve problems
It is operator dependent and limited by bone and air.
Nuclear imaging
Bone scintigraphy and related nuclear techniques demonstrate physiological activity rather than fine anatomy. Increased uptake is sensitive but often non-specific. Hybrid imaging can improve localisation.
Image interpretation
Use a systematic sequence:
- confirm patient/date/view
- assess alignment
- inspect bone cortex and trabeculae
- review joint surfaces and soft tissues
- assess implants and interfaces
- compare previous imaging
FRCS synthesis
An imaging question is usually about choosing the modality that answers the clinical question with acceptable risk. Explain what information is needed and why a given modality provides it.