CT Imaging Principles

Computed tomography (CT) revolutionised orthopaedic imaging by eliminating the superimposition problem inherent to plain radiography. By acquiring cross-sectional images, CT allows direct visualisation of fracture geometry, articular surface congruity, fragment displacement, and the relationship of bone to surrounding soft tissues in all three planes.

In orthopaedic practice, CT is primarily used for: complex fracture characterisation (especially intra-articular fractures), preoperative planning, postoperative assessment of reduction and implant position, assessment of union and nonunion, tumour staging, and evaluation of spinal pathology. Its superior spatial resolution for cortical bone (0.5-1mm) far exceeds MRI, making it the modality of choice when precise bony anatomy is needed.

The major disadvantage of CT is radiation dose — a single CT scan of the pelvis delivers 100-300 times more radiation than an AP pelvic radiograph. This makes clinical justification and dose optimisation essential for every CT requested.

Systematic Approach to CT Interpretation in Orthopaedics

A structured approach to CT interpretation prevents missed findings and ensures comprehensive assessment. For every orthopaedic CT, apply the following systematic review:

Data Acquisition

A CT scanner consists of an X-ray tube mounted on a rotating gantry opposite a detector array. The tube rotates continuously around the patient (360 degrees per rotation in approximately 0.3-0.5 seconds on modern scanners), emitting a fan-shaped or cone-shaped X-ray beam. As the beam passes through the patient, different tissues attenuate (absorb and scatter) the beam to varying degrees. The detectors on the opposite side measure the intensity of the transmitted beam at thousands of angles around the patient.

Modern multi-detector CT (MDCT) scanners have 64 to 320 detector rows, allowing simultaneous acquisition of multiple slices per rotation. This dramatically reduces scan time and enables isotropic voxel resolution (equal resolution in all three planes), which is essential for high-quality multiplanar reconstructions.

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Helical Acquisition and Pitch

Modern CT is acquired helically (spiral): the table moves continuously while the tube rotates, so the X-ray focus traces a helix relative to the patient and a whole volume is acquired in one breath-hold or pass. The single most examined acquisition parameter is pitch:

Pitch = table travel per gantry rotation / total nominal beam width (collimation)

• Pitch = 1: the table advances exactly one beam-width per rotation (contiguous acquisition).
• Pitch over 1: the table moves faster than the beam width — faster coverage and (all else equal) lower dose, but interpolation gaps can reduce z-axis resolution and increase helical (windmill) artefact.
• Pitch under 1: overlapping acquisition — higher z-axis resolution and better for fine detail, but at the cost of higher dose.

In practice, high pitch is chosen to freeze motion (polytrauma, paediatric or uncooperative patients) and low pitch for high-resolution work. The viva point is that pitch is the trade-off between speed/coverage, dose, and z-axis resolution; many scanners automatically adjust tube current with pitch to hold image quality constant, so always interpret pitch alongside the displayed CTDIvol rather than in isolation.

Hounsfield Units

The fundamental unit of CT imaging is the Hounsfield Unit (HU), named after Sir Godfrey Hounsfield who developed clinical CT. The HU is a linear transformation of the X-ray attenuation coefficient, normalised to water:

HU = 1000 × (μ tissue - μ water) / μ water

Where μ is the linear attenuation coefficient. This normalisation gives water a value of exactly 0 HU and air approximately -1000 HU, providing a standardised scale across all CT scanners.

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Window and Level Settings

A CT image dataset contains a far wider range of HU values than a monitor can display (typically 256 grey levels). Window width determines the range of HU values displayed, and window level (centre) determines the midpoint of this range.

The critical concept for the examination is that changing window settings does NOT change the data — it simply changes which portion of the Hounsfield scale is displayed on screen. The same dataset can be viewed on bone windows (to assess fractures) and soft tissue windows (to assess haematomas) without rescanning the patient.

Choosing CT vs Other Modalities

A frequent viva theme is justifying CT over an alternative. The clinical question — bone geometry, soft tissue, marrow, or vascular — should drive modality choice, not habit.

CT delivers substantially higher radiation doses than plain radiography and is the single largest contributor to medical radiation exposure in developed countries. Understanding CT dose metrics and optimisation strategies is essential for fellowship examinations.

CT Dose Metrics

CT Dose Metrics Defined

A frequent physics-viva trap is to quote effective dose (mSv) without knowing the quantities the scanner actually reports. Distinguish three things:

The key concept: CTDIvol and DLP are physical dose quantities measured on a standard phantom, whereas effective dose is a calculated, risk-weighted estimate (not a measured patient dose). The k-factor is larger for the trunk (radiosensitive viscera) and small for the extremities — which is why a limb CT carries a far lower effective dose than its CTDIvol alone might suggest. Diagnostic reference levels (DRLs) are benchmarked in CTDIvol and DLP, not mSv.

Dose Optimisation

Dual-energy CT (DECT) acquires data at two different kVp settings simultaneously, allowing material decomposition based on the energy-dependent attenuation properties of different tissues. This has several orthopaedic applications:

CT is the single largest contributor to medical radiation exposure worldwide. Across high-income health systems, CT accounts for only a minority of imaging studies (typically under 10% of examinations) yet contributes the majority (around 60-70%) of the collective dose from diagnostic radiology — the central justification for diagnostic reference levels (DRLs) and the ALARA principle. CT utilisation continues to rise globally, fastest in middle-income settings as scanner access expands, which makes appropriate-use criteria as relevant in resource-limited settings as in tertiary centres.

Global Epidemiology of CT Use

• CT volumes per capita are highest in the United States, Japan, and parts of Europe and lowest in low-resource settings where access — not over-use — is the dominant problem.
• Orthopaedic CT demand is driven by the ageing population, rising fragility-fracture burden, and the growing prevalence of in-situ metalwork (revision arthroplasty volumes are increasing across all major arthroplasty registries, including the NJR, AJRR, AOANJRR, and Scandinavian registries), which increases the need for metal-artefact-reduction CT.

Side-by-Side Society Guidance

High- vs Limited-Resource Practice Variation

• Well-resourced settings: routine thin-slice MDCT with iterative (or deep-learning) reconstruction, automatic tube-current modulation, dual-energy capability, and vendor MAR; CT is standard for preoperative planning of complex fractures and for 3D-printed patient-specific models.
• Limited-resource settings: older single- or few-slice scanners, limited dual-energy and MAR availability, and constrained access mean plain radiography (and, where available, MRI) carries more of the diagnostic load. Justification matters even more where each scan competes for scarce capacity, and protocol optimisation and shared reference levels are the highest-yield safety interventions.

Diagnostic reference levels, not billing pathways, are the appropriate global benchmark for governing CT use — dose audit against DRLs is recommended by ICRP, the IAEA, and national regulators regardless of health-system funding model.