CT Imaging Principles
Computed Tomography for Orthopaedic Surgeons
Hounsfield Unit Scale
Air: -1000 HU (black)
Fat: -50 to -100 HU (dark grey)
Water: 0 HU (reference grey)
Soft tissue/muscle: +40 to +80 HU (grey)
Cancellous bone: +300 to +500 HU (light grey)
Cortical bone: +800 to +1200 HU (white)
Metal: +3000 HU (bright white with artefact)
Key: The Hounsfield Unit is a linear transformation of the attenuation coefficient normalised to water — this is the fundamental unit of CT imaging
Critical Must-Knows
- CT uses a rotating X-ray tube and detector array to acquire cross-sectional images, eliminating superimposition.
- Hounsfield Units (HU) quantify tissue density: water = 0, air = -1000, dense bone = +1000, metal = +3000.
- CT delivers 100-300 times more radiation than a plain radiograph of the same region.
- Multiplanar reconstruction (MPR) and 3D volume rendering are generated from axial data without additional radiation.
- Window and level settings determine which HU range is displayed — bone window and soft tissue window show different pathology.
Examiner's Pearls
- "CT is the gold standard for complex fracture characterisation, especially acetabular fractures, tibial plateau, and calcaneus.
- "Dual-energy CT can differentiate urate crystals (gout) from calcium — increasingly used in crystal arthropathy diagnosis.
- "Metal artefact reduction sequences (MARS) improve imaging around orthopaedic implants but do not eliminate artefact completely.
- "CT angiography is essential in knee dislocation to exclude popliteal artery injury before reduction.
- "Always justify CT with clear clinical indication — the ALARA principle applies with even greater urgency given high doses.
Exam Warning
CT principles are examined in both physics viva stations and clinical decision-making scenarios. You must understand: Hounsfield Units and their derivation, window/level settings, radiation dose compared to plain radiography, specific orthopaedic indications (acetabular fractures, tibial plateau fractures, spinal injuries), and the role of 3D reconstructions in preoperative planning. A common viva trap is failing to mention the significantly higher radiation dose compared to plain radiography when discussing CT indications.
FACTSCT Indications in Orthopaedics
Memory Hook:FACTS: CT gives you the facts about fracture geometry that plain films cannot reveal.
AFOOTHounsfield Unit Reference Points
Memory Hook:AFOOT through the Hounsfield scale: Air, Fat, 0-water, Organs, then hard Tissue.
SLIMCT Dose Reduction Strategies
Memory Hook:SLIM the dose: CT is the biggest radiation contributor in medical imaging, so every reduction matters.
Overview
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.
Strengths of CT
Superior spatial resolution for cortical bone (0.5-1mm). Cross-sectional imaging eliminates superimposition. Multiplanar reconstruction (MPR) and 3D volume rendering from a single acquisition. Fast acquisition (seconds). Excellent for fracture characterisation, preoperative planning, and postoperative implant assessment. Can identify subtle non-displaced fractures missed by plain radiography. CT angiography for vascular assessment in dislocations.
Limitations of CT
High radiation dose (2-20 mSv depending on region). Significant metal artefact from orthopaedic implants (beam hardening and photon starvation). Limited soft tissue contrast compared to MRI — cannot reliably evaluate ligaments, cartilage, or bone marrow oedema. Cost significantly higher than plain radiography. Not portable — patient must travel to the CT scanner. Iodinated contrast carries risks of allergy and nephrotoxicity.
Clinical Imaging
Imaging Gallery
Systematic Approach
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:
Systematic CT Interpretation Framework
| Step | What to Assess | Key Questions |
|---|---|---|
| 1. Scout and range | Confirm correct region scanned, check for incidental findings at scan margins | Is the entire region of interest captured? Are there additional pathologies at the scan boundaries? |
| 2. Bone windows — cortex | Scan systematically through cortical bone in all three planes (axial, coronal, sagittal) | Are all cortical surfaces intact? Any fracture lines, cortical breaks, or periosteal reaction? |
| 3. Bone windows — articular surface | Assess joint surface congruity in the plane perpendicular to the articular surface | Any step, gap, or depression? Quantify displacement in millimetres. Any intra-articular fragments? |
| 4. Bone windows — alignment | Axes, rotation, angulation, subluxation | Is the joint congruent? Any rotational malalignment? Measure specific angles as needed. |
| 5. Soft tissue windows | Haematomas, effusions, soft tissue swelling, vascular injury, gas | Any expanding haematoma? Joint effusion? Soft tissue gas (open fracture)? Vascular contrast extravasation? |
| 6. 3D reconstructions | Global fracture pattern, fragment relationships, preoperative planning | What is the overall fracture geometry? Which surgical approach best addresses all major fragments? |
Critical Exam Pearl
Always review CT on BOTH bone and soft tissue windows. A common examination pitfall is identifying the fracture on bone windows but missing an expanding haematoma, vascular injury, or compartmental swelling visible only on soft tissue windows. In spinal trauma, always assess the spinal canal on soft tissue windows for retropulsed fragments and epidural haematoma.
CT Physics and Acquisition
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.
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.
Hounsfield Unit Values for Common Tissues
| Tissue | HU Range | Clinical Significance |
|---|---|---|
| Air | -1000 | Reference lower bound; appears black on all windows |
| Lung parenchyma | -700 to -500 | Low density due to air content; lung windows needed |
| Fat | -50 to -100 | Negative HU distinguishes fat from water — key for lipoma vs other tumours |
| Water/CSF | 0 | Calibration reference point of the Hounsfield scale |
| Muscle | +40 to +80 | Standard soft tissue density; visible on soft tissue windows |
| Acute blood (haematoma) | +50 to +80 | Fresh blood is slightly denser than muscle — useful for identifying acute haematoma |
| Cancellous bone | +300 to +500 | Trabecular bone; density reflects mineralisation status |
| Cortical bone | +800 to +1200 | Dense cortical bone; visible on bone windows |
| Metal implants | +3000 or more | Extremely dense; causes beam hardening and streak artefact |
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.
Standard CT Window Settings for Orthopaedic Imaging
| Window | Width (HU) | Level (HU) | What It Shows |
|---|---|---|---|
| Bone window | 2000-4000 | +300 to +500 | Cortical detail, fracture lines, implant position, calcification |
| Soft tissue window | 250-400 | +40 to +60 | Muscle, haematoma, soft tissue masses, fluid collections |
| Lung window | 1500-2000 | -600 | Air-containing structures, pneumothorax |
| Brain window | 80-100 | +35 to +40 | Intracranial pathology (relevant for polytrauma assessment) |
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.
Image Reconstruction
Reconstruction Algorithms
Raw CT data (sinogram) must be mathematically reconstructed into cross-sectional images. Two main approaches exist:
Filtered Back Projection (FBP) is the traditional reconstruction algorithm that has been used since the development of clinical CT. It works by projecting the measured attenuation data back through the image matrix along the original ray paths, with mathematical filtering to remove blurring.
FBP is computationally fast and produces consistent, well-understood image characteristics. However, it is dose-inefficient — reducing the radiation dose increases image noise proportionally, and there is a minimum dose below which diagnostic quality is lost.
Convolution kernels (reconstruction filters) applied during FBP include:
- Soft tissue (smooth) kernel: Reduces noise but lowers spatial resolution — used for soft tissue assessment
- Bone (sharp) kernel: Maximises spatial resolution at the cost of increased noise — essential for fracture detection
- Lung kernel: Optimised for high-contrast air-tissue interfaces
FBP remains the standard of comparison for image quality assessment.
Multiplanar Reconstruction and 3D Rendering
One of the most powerful features of CT for orthopaedic surgery is the ability to generate multiplanar reconstructions (MPR) and 3D volume-rendered images from the original axial dataset without any additional radiation to the patient.
Multiplanar Reconstruction (MPR)
Coronal, sagittal, and oblique reformats generated from isotropic axial data. Essential for tibial plateau fracture assessment (coronal view for split/depression), acetabular fracture classification (coronal and sagittal), and spinal alignment assessment. Quality depends on slice thickness — thinner axial slices (0.5-1mm) produce better reformats than thick slices (3-5mm).
3D Volume Rendering
Three-dimensional surface reconstructions that provide an intuitive global view of complex fracture patterns. Invaluable for preoperative planning of acetabular fractures, complex periarticular fractures, and deformity correction surgery. Can digitally subtract overlying structures (e.g., show only the posterior column of the acetabulum). The surgeon can rotate the 3D model to understand fracture geometry from any angle. Increasingly used for 3D printing of patient-specific fracture models for preoperative planning.
CT Artefacts in Orthopaedic Imaging
CT artefacts are particularly relevant in orthopaedic practice because metallic implants are extremely common in the patient population.
CT Artefacts Relevant to Orthopaedic Practice
| Artefact | Cause | Appearance | Reduction Strategies |
|---|---|---|---|
| Beam hardening | Preferential absorption of low-energy photons by dense material (bone or metal) | Dark bands between dense structures (Hounsfield bar between petrous bones); cupping artefact | Beam hardening correction algorithms, increased kVp, hardware filtering |
| Metal streak artefact | Photon starvation and beam hardening from orthopaedic implants | Bright and dark streaks radiating from metallic hardware, obscuring adjacent anatomy | Metal artefact reduction sequences (MARS/O-MAR/SEMAR), dual-energy CT, increased kVp, increased mAs |
| Partial volume averaging | Voxel containing multiple tissue types is assigned their average HU value | Blurring of interfaces; small fracture lines may be missed if slice is too thick | Thinner slice thickness (0.5-1mm); volumetric acquisition |
| Motion artefact | Patient movement during acquisition | Blurring, double contours, streak patterns | Faster scan time (wider detectors), immobilisation, breath-hold for torso scans |
| Ring artefact | Miscalibrated detector element | Concentric ring pattern centred on the rotation axis | Detector calibration, quality assurance programme |
Metal Artefact Reduction
Metal artefact reduction (MAR) is increasingly important as the orthopaedic population with existing implants grows. Vendor-specific algorithms (O-MAR by Philips, SEMAR by Canon, iMAR by Siemens) use iterative techniques to reduce streak artefact around metal hardware. Dual-energy CT approaches can also help by generating virtual monoenergetic images at higher keV, which are less affected by beam hardening. However, no current technique completely eliminates metal artefact — MRI with metal artefact reduction sequences (MAVRIC/SEMAC) may be preferable for soft tissue assessment around implants.
Orthopaedic Applications
CT in Fracture Assessment
CT is most valuable when plain radiographs suggest a complex fracture that requires detailed characterisation for surgical planning. The key orthopaedic trauma indications include:
Acetabular Fractures: CT is considered mandatory for all acetabular fractures. It changes the Letournel-Judet classification in up to 40% of cases compared to plain films alone. CT reveals: column involvement, wall fragments, marginal impaction (the 'gull sign'), intra-articular fragments, femoral head injury, and dome arc measurements.
Tibial Plateau Fractures: CT quantifies articular depression depth (greater than 2-3mm is a common surgical threshold), identifies split fragments, reveals posterior column involvement (often missed on plain films), and helps plan surgical approach.
Calcaneal Fractures: CT with coronal reformats demonstrates the posterior facet depression, calcaneocuboid joint involvement, sustentaculum tali fragment position, and allows measurement of the Bohler angle in the sagittal plane.
Pilon Fractures: CT defines the articular injury pattern, identifies the number and position of articular fragments, and guides the surgical approach.
Spinal Injuries: CT is the primary investigation for thoracolumbar burst fractures, assessing canal compromise, posterior element fractures, and vertebral body comminution.
CT is indispensable for preoperative planning of these injuries.
Radiation Dose and Safety
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 Radiation Dose Estimates by Region
| Examination | Effective Dose (mSv) | Equivalent Plain Radiographs | Equivalent Background Radiation |
|---|---|---|---|
| CT extremity (wrist, ankle) | 0.1-0.5 | 10-50x limb X-ray | 1-8 weeks |
| CT cervical spine | 2-4 | 100-200x C-spine X-ray | 8-16 months |
| CT lumbar spine | 5-10 | 3-7x lumbar X-ray series | 2-4 years |
| CT pelvis | 6-10 | 8-14x AP pelvis X-ray | 2-4 years |
| CT abdomen/pelvis | 10-20 | 500-1000x chest X-ray | 4-7 years |
| CT chest/abdomen/pelvis (polytrauma) | 20-30 | 1000-1500x chest X-ray | 7-12 years |
A single polytrauma CT (head, cervical spine, chest, abdomen, pelvis) delivers approximately 20-30 mSv — equivalent to 7-12 years of natural background radiation. While this is justified in the acute trauma setting, the cumulative dose from serial CT imaging (follow-up, surveillance) must be considered, particularly in young patients. Always ask: 'Can this clinical question be answered by plain radiography, ultrasound, or MRI instead?'
Dose Optimisation
Technical Optimisation
Automatic tube current modulation (adjusts mAs to patient size and body region). Iterative reconstruction (40-60% dose reduction). Low-kVp protocols for extremities. Scan length limitation (only scan the region of interest). Appropriate clinical indication and justification for every scan.
Clinical Optimisation
Use plain radiography as first-line investigation. Reserve CT for when plain films are insufficient for clinical decision-making. Avoid repeat CT when previous imaging can answer the question. Use MRI for soft tissue questions (ligaments, bone marrow oedema) rather than CT. Consider low-dose CT protocols for follow-up imaging.
Dual-Energy CT
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:
Dual-Energy CT Applications in Orthopaedics
| Application | Mechanism | Clinical Utility |
|---|---|---|
| Gout crystal detection | Urate crystals have a unique dual-energy signature different from calcium | Non-invasive diagnosis of gout with reported sensitivity of 78-100% and specificity of 89-100%; can detect asymptomatic tophi |
| Virtual non-calcium imaging | Subtraction of calcium signal reveals underlying bone marrow oedema | Detection of occult fractures (bone bruises) without MRI — useful in acute trauma when MRI is unavailable or contraindicated |
| Metal artefact reduction | Virtual monoenergetic images generated at higher keV reduce beam hardening | Improved visualisation around orthopaedic implants compared to conventional CT; 130-190 keV virtual monoenergetic images optimal |
| Tendon and ligament assessment | Collagen-rich structures have different dual-energy signatures | Emerging application for Achilles tendon assessment and ligament integrity, though MRI remains superior |
Exam Pearl: DECT for Gout
Dual-energy CT can identify monosodium urate crystal deposits as small as 2mm, even in the absence of tophi visible on physical examination. The crystals are colour-coded green on commercial DECT software (calcium is coded purple/blue). False positives can occur with nail bed artefact, skin calluses, and motion artefact. This is increasingly asked about in fellowship examinations as it represents a genuine paradigm shift in gout diagnosis.
Evidence Base
CT Scanning for Acetabular Fractures
- CT identified marginal impaction (dome impaction) in 32% of acetabular fractures that was not apparent on plain radiographs.
- CT changed the fracture classification in 15-40% of cases compared to plain films alone.
- Identification of marginal impaction changed the surgical plan in a significant proportion of patients.
CT Classification of Tibial Plateau Fractures
- CT identified fracture patterns not visible on plain radiography in 26% of tibial plateau fractures.
- CT demonstrated posterior column involvement (increasingly recognised as important for surgical planning) that was missed on standard AP and lateral radiographs.
- 3D CT reconstructions improved the accuracy of fracture classification by orthopaedic surgeons.
CT evidence consistently supports its use for complex periarticular fracture characterisation.
Australian Context
In Australia, CT scanning is widely available across metropolitan and regional centres, with an increasing rate of utilisation that has raised concerns about population radiation dose. ARPANSA data shows that CT accounts for approximately 67% of the total medical radiation dose in Australia despite representing only approximately 5% of all medical imaging examinations — highlighting the importance of judicious CT requesting.
The Royal Australian and New Zealand College of Radiologists (RANZCR) publishes imaging pathway guidelines that recommend CT as a second-line investigation after plain radiography for most orthopaedic conditions, with specific indications including complex periarticular fractures, spinal injuries, and tumour staging. Australian diagnostic reference levels for CT are published by ARPANSA and serve as benchmarks for dose optimisation.
Dual-energy CT for gout diagnosis is increasingly available at Australian tertiary centres and may reduce the need for invasive joint aspiration in selected cases. Australian data from the AOANJRR demonstrates the growing volume of revision arthroplasty, which increases the demand for CT with metal artefact reduction around existing implants. Australian Medicare funding for CT imaging is subject to specific clinical indications and referral pathways, reinforcing the principle that CT should be requested only when clinically justified.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
"A 45-year-old female has sustained an acetabular fracture in a road traffic collision. Plain radiographs show a both-column fracture pattern. You request a CT scan."
"An examiner shows you a CT scan of a knee with significant metal streak artefact from a previous tibial plateau plate. They ask you to explain the artefact and describe strategies to improve image quality."
"A concerned parent asks about the radiation risk of a CT scan ordered for their 12-year-old child who has a complex tibial plateau fracture."
CT Imaging Principles — Exam Day Reference
High-Yield Exam Summary
Hounsfield Units
- •Water = 0 HU (reference), Air = -1000 HU, Fat = -50 to -100 HU
- •Muscle = +40-80 HU, Cancellous bone = +300-500 HU, Cortical bone = +800-1200 HU
- •HU = 1000 x (mu tissue - mu water) / mu water
- •Changing window/level does NOT change data — only changes display
Key Orthopaedic Indications
- •Acetabular fractures: MANDATORY — changes classification in up to 40%
- •Tibial plateau: quantifies depression, reveals posterior column involvement
- •Calcaneus: posterior facet assessment on coronal reformats
- •Spinal trauma: canal compromise, posterior element fractures
- •CT angiography in knee dislocation for popliteal artery assessment
Radiation Dose
- •CT delivers 100-300x more radiation than plain radiography of same region
- •CT accounts for 67% of medical radiation dose but only 5% of imaging volume
- •Extremity CT: 0.1-0.5 mSv; Pelvis CT: 6-10 mSv; Polytrauma CT: 20-30 mSv
- •Iterative reconstruction reduces dose by 40-60% vs filtered back projection
Metal Artefact
- •Beam hardening: preferential low-energy photon absorption by metal
- •Photon starvation: complete absorption at some projection angles
- •Reduction: higher kVp, MAR software (O-MAR/iMAR/SEMAR), dual-energy CT at 130-190 keV
- •MRI with MAVRIC/SEMAC may be superior for soft tissue around implants
Dual-Energy CT
- •Gout: urate crystal detection (87% sensitivity, 84% specificity)
- •Virtual non-calcium: detects bone marrow oedema without MRI
- •Virtual monoenergetic images: reduce metal artefact
- •False positives: nail bed, skin calluses, motion artefact