High Yield Overview
CT Imaging Principles
Computed Tomography for Orthopaedics
1-2mmTypical Slice Thickness
0 HUWater Density
+1000Bone HU
2-10mSvTypical Dose Range
Hounsfield Unit Reference
Air
Pattern-1000 HU
TreatmentBlack on all windows
Fat
Pattern-100 to -50 HU
TreatmentDark grey
Water
Pattern0 HU
TreatmentReference standard
Soft Tissue
Pattern+20 to +80 HU
TreatmentGrey
Bone (Trabecular)
Pattern+300 to +500 HU
TreatmentLight grey/white
Bone (Cortical)
Pattern+1000 HU
TreatmentBright white
Metal
Pattern+3000 HU
TreatmentBright white + artifact
Critical Must-Knows
- Hounsfield Units (HU): Water = 0, Air = -1000, Bone = +400 to +1000, Fat = -100.
- Window Width: Controls contrast. Narrow = high contrast. Wide = low contrast.
- Window Level: Centers the display range. Bone window level approximately +300-400 HU.
- Multiplanar Reformats (MPR): Sagittal, coronal, 3D from axial data. Essential for fracture assessment.
- CT dose is significant: 2-10 mSv for spine/pelvis (100-500x limb X-ray). ALARA principle applies.
Examiner's Pearls
- "Bone window: Wide width (2000-4000), high level (300-700). Soft tissue window: Narrow width (350-400), level 40-50.
- "Thin slices (less than 1mm) essential for quality 3D reconstruction.
- "CT arthrography superior to MRI for labral tears in post-surgical shoulder.
- "Metal artifact reduction protocols improve visualization around implants.
- "Dual-energy CT can differentiate urate crystals (gout) from calcium.
Clinical Imaging
Imaging Gallery
Click to expand
CT Radiation Dose - Higher Than Plain Radiography
CT delivers significantly higher radiation doses than plain radiographs. A single CT abdomen/pelvis is equivalent to approximately 500 chest X-rays or 4-5 years of background radiation. Always justify CT imaging (benefit must outweigh risk) and consider dose-reduction protocols, especially in young patients.
CT Physics Basics
Image Acquisition
- X-ray Tube Rotation: Tube and detectors rotate around patient
- Data Acquisition: Multiple projections collected per rotation
- Reconstruction: Mathematical algorithms create cross-sectional images
- Display: Images viewed in axial plane with MPR capability
Generations of CT Scanners
CT Scanner Evolution
| Generation | Configuration | Scan Time | Clinical Use |
|---|---|---|---|
| 1st/2nd | Translate-rotate | Minutes | Historical only |
| 3rd | Rotate-rotate | 1-3 seconds | Early clinical |
| 4th | Rotate-stationary | Less than 1 second | Standard clinical |
| Helical/Spiral | Continuous rotation | Subsecond | Modern standard |
| Multi-detector (MDCT) | Multiple detector rows | Subsecond volumes | Current standard |
Technical Parameters
kVp (Kilovoltage)
Controls X-ray penetration
- Standard: 120 kVp
- Lower kVp (80-100): Reduces dose, enhances contrast
- Higher kVp: Better penetration for large patients
mAs (Tube Current)
Controls radiation quantity
- Higher mAs = more photons = less noise
- Higher mAs = higher patient dose
- Automatic exposure control (AEC) adjusts per slice
Pitch
Table speed relative to collimation
- Pitch = Table travel per rotation / Beam width
- Pitch greater than 1: Faster scan, potential gaps
- Pitch less than 1: Overlapping data, better quality
Slice Thickness
Determines z-axis resolution
- Thin slices (0.5-1mm): Better detail, more noise
- Thick slices (3-5mm): Less noise, volume averaging
- Thin acquisition allows thick reconstruction
Image Display
Windowing
Window Width (WW) and Window Level (WL) control how HU values map to display grayscale.
Standard CT Windows
| Window | Width (HU) | Level (HU) | Application |
|---|---|---|---|
| Bone | 2000-4000 | 300-700 | Fractures, cortical detail |
| Soft Tissue | 350-400 | 40-50 | Muscles, organs, masses |
| Lung | 1500-2000 | -600 | Pulmonary parenchyma |
| Brain | 80-100 | 35-40 | Intracranial structures |
Window Adjustment Principles
- Narrow window width: High contrast, fewer grey shades (good for subtle differences)
- Wide window width: Low contrast, many grey shades (good for bone detail)
- Window level: Centers the display range on tissue of interest
Reconstruction Algorithms
Bone/Sharp Kernel
- High spatial resolution
- Edge enhancement
- More image noise
- Use for: Fractures, cortical bone, hardware
Soft Tissue/Smooth Kernel
- Lower spatial resolution
- Noise reduction
- Better low-contrast resolution
- Use for: Soft tissue masses, joints
Multiplanar Imaging
Multiplanar Reformats (MPR)
Click to expand
Modern MDCT acquires isotropic (equal resolution in all directions) data, enabling high-quality reformats:
- Axial: Standard acquisition plane
- Coronal: Front-to-back view
- Sagittal: Side-to-side view
- Oblique: Along any arbitrary plane
- Curved planar: Along curved structures (vessels, nerves)
3D Reconstruction Techniques
3D Rendering Methods
| Technique | Description | Orthopaedic Use |
|---|---|---|
| Surface Rendering | Creates surface shell from thresholded data | Fracture visualization, patient education |
| Volume Rendering | Displays all voxel data with transparency | Complex anatomy, surgical planning |
| Maximum Intensity Projection (MIP) | Shows brightest voxel along ray | CTA for vessels |
| Minimum Intensity Projection (MinIP) | Shows darkest voxel along ray | Airways, subtle lucencies |
Contrast Enhancement
Intravenous Contrast
Iodinated Contrast Agents
Mechanism: Iodine absorbs X-rays (high atomic number) → increased HU
Enhancement patterns:
- Arterial phase (15-25s): Vessels, hypervascular tumors
- Portal venous phase (60-70s): Organs, most masses
- Delayed phase (3-5min): Urinary system, some tumors
Orthopaedic uses:
- Vascular injury assessment
- Tumor characterization (enhancement suggests vascularity)
- Infection (rim enhancement in abscess)
Contrast Complications
Contrast-Induced Nephropathy (CIN)
- Risk increases with pre-existing renal impairment
- Check eGFR before IV contrast in at-risk patients
- Hydration is primary prevention strategy
- Consider alternative imaging if eGFR less than 30
- Metformin: Hold for 48 hours post-contrast if eGFR less than 45
Allergic-like Reactions
- Mild (1-3%): Urticaria, nausea, warmth
- Moderate (0.04%): Bronchospasm, facial edema
- Severe (0.004%): Anaphylaxis, cardiac arrest
- Premedication for previous reactors
Risk Factors
- Previous contrast reaction (5x risk)
- Asthma (2x risk)
- Multiple allergies
- Renal impairment
- Cardiac disease
Orthopaedic Applications
Fracture Assessment
CT excels for:
- Complex fractures: Tibial plateau, acetabulum, calcaneus, pilon
- Articular involvement: Quantifying depression, fragment count
- Preoperative planning: Entry points, screw trajectories
- Occult fractures: Hip (elderly), scaphoid (if X-ray/MRI equivocal)
Mnemonic
PACTCT Indications for Fractures
P
Preoperative Planning
Complex fractures needing surgical planning
A
Articular Involvement
Quantify joint surface disruption
C
Characterization
Better define fracture pattern
T
Three-Dimensional
Spatial relationships for complex anatomy
Memory Hook:Make a PACT to CT complex fractures
Specific Applications
Indications:
- Trauma: Vertebral fracture characterization, posterior element involvement
- Preoperative: Pedicle dimensions, anatomy for instrumentation
- Postoperative: Hardware position, fusion assessment
Protocol: Thin slices (less than 1mm), bone and soft tissue reconstructions, sagittal/coronal MPR
CT Arthrography
Technique: Intra-articular iodinated contrast + CT scan
Advantages over MRI:
- Superior spatial resolution
- Better in post-surgical patients (less artifact)
- Faster acquisition
Applications:
- Labral tears (shoulder, hip) post-surgery
- Cartilage defects
- Loose bodies
- Rotator cuff tears (when MRI contraindicated)
Radiation Dose
Typical Effective Doses
CT Dose Comparison
| Examination | Effective Dose (mSv) | Equivalent X-rays | Background Equivalent |
|---|---|---|---|
| Limb X-ray | 0.01 | 1 | 1 day |
| CT Head | 2 | 200 | 8 months |
| CT Spine | 6 | 600 | 2 years |
| CT Abdomen/Pelvis | 10 | 1000 | 4 years |
| CT Angiogram | 5-15 | 500-1500 | 2-6 years |
Dose Reduction Strategies
Technical Optimization
- Automatic exposure control (AEC)
- Iterative reconstruction algorithms
- Lower kVp for smaller patients
- Limit scan length to area of interest
Clinical Optimization
- Justify every scan (risk vs benefit)
- Avoid repeat scanning
- Consider alternative modalities (MRI, US)
- Single-phase protocols when possible
Artifacts
Common CT Artifacts
CT Artifacts in Orthopaedics
| Artifact | Cause | Solution |
|---|---|---|
| Metal/Streak | High-density implants | Metal artifact reduction (MAR), dual-energy CT |
| Beam Hardening | Preferential absorption of low-energy photons | Calibration, filtration, iterative reconstruction |
| Motion | Patient movement during scan | Faster acquisition, immobilization, sedation |
| Partial Volume | Tissue averaging in voxel | Thinner slices |
| Ring | Faulty detector element | Detector calibration |
Metal Artifact Reduction
Strategies:
- Increased kVp: More penetrating beam
- MAR algorithms: Iterative reconstruction to reduce streaks
- Dual-energy CT: Material decomposition to remove metal
- Positioning: Orient long axis of hardware parallel to scan plane
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
CT Physics Viva
EXAMINER
"Explain the concept of Hounsfield units and how you adjust windowing for different tissues."
EXCEPTIONAL ANSWER
Hounsfield units (HU) are a standardized scale for CT density based on X-ray attenuation relative to water. Water is defined as 0 HU, air as -1000 HU, and dense bone approximately +1000 HU. The display window is controlled by two parameters: window level (WL) sets the center of the displayed range, and window width (WW) sets how many HU are displayed. For bone, I use a wide window (2000-4000 HU) centered high (300-700 HU) to see cortical detail. For soft tissue, I use a narrow window (350-400 HU) centered around soft tissue density (40-50 HU) to maximize contrast between similar tissues.
KEY POINTS TO SCORE
HU scale: Air -1000, Water 0, Bone +1000
Window level = center of display range
Window width = range of HU displayed
Wide window for bone, narrow for soft tissue
COMMON TRAPS
✗Confusing window level and width
✗Not knowing specific window values
✗Forgetting that fat is negative HU
LIKELY FOLLOW-UPS
"What is the dose from a CT scan compared to X-ray?"
"How do you reduce metal artifact from implants?"
"When would you choose CT over MRI for a fracture?"
VIVA SCENARIOStandard
CT vs MRI Selection
EXAMINER
"A 45-year-old has a tibial plateau fracture on X-ray. When would you order CT versus MRI?"
EXCEPTIONAL ANSWER
For tibial plateau fractures, CT and MRI provide complementary information. CT is my first choice for surgical planning as it provides superior bone detail - I can accurately assess depression depth, fragment number and size, and articular surface involvement using axial images and coronal/sagittal reformats. 3D reconstructions help visualize the fracture pattern. However, if I suspect significant soft tissue injury - MCL, ACL, or meniscal damage - I would also order MRI. The Schatzker classification can be made on CT, but associated ligament injuries (present in 20-30% of plateau fractures) require MRI. In practice, most operatively managed plateau fractures get both modalities.
KEY POINTS TO SCORE
CT for bone detail and surgical planning
MRI for soft tissue (ligaments, menisci)
20-30% have associated ligament injuries
Often need both modalities for complete assessment
COMMON TRAPS
✗Only ordering one modality
✗Not considering soft tissue injuries
✗Forgetting about radiation dose considerations
LIKELY FOLLOW-UPS
"What Schatzker type would you expect in a young vs elderly patient?"
"How does CT help with surgical approach planning?"
"What is the role of 3D printing from CT?"
VIVA SCENARIOStandard
CT Contrast Safety
EXAMINER
"A patient needs a CT angiogram after a pelvic fracture. Their creatinine is elevated. How do you proceed?"
EXCEPTIONAL ANSWER
I would first calculate the eGFR to quantify renal function. For contrast-enhanced CT, the main concern is contrast-induced nephropathy (CIN). If eGFR is greater than 45, I can proceed with standard precautions and hydration. If eGFR is 30-45, I would ensure adequate IV hydration before and after, use the minimum contrast volume necessary, and avoid nephrotoxic medications. If the patient is on metformin, I would hold it for 48 hours post-contrast and recheck renal function before resuming. If eGFR is less than 30, I would discuss with radiology and consider alternatives - however, in the setting of suspected vascular injury with a pelvic fracture, the clinical benefit likely outweighs the risk. I would involve nephrology, ensure optimal hydration, and use iso-osmolar contrast if proceeding.
KEY POINTS TO SCORE
Calculate eGFR, not just creatinine
Hydration is primary prevention for CIN
Risk-benefit analysis - vascular injury is life-threatening
Hold metformin 48 hours post-contrast if eGFR less than 45
COMMON TRAPS
✗Blanket refusal of contrast in renal impairment
✗Forgetting to check metformin status
✗Not considering clinical urgency
LIKELY FOLLOW-UPS
"What is the mechanism of contrast-induced nephropathy?"
"What premedication would you use for contrast allergy?"
"What alternative imaging could assess for vascular injury?"
CT Imaging Exam Day Cheat Sheet
High-Yield Exam Summary
Hounsfield Units
- •Air = -1000 HU (black)
- •Fat = -100 HU (dark grey)
- •Water = 0 HU (reference)
- •Soft tissue = +20-80 HU
- •Bone = +400-1000 HU (white)
Window Settings
- •Bone: WW 2000-4000, WL 300-700
- •Soft Tissue: WW 350-400, WL 40-50
- •Wide window = low contrast (bone)
- •Narrow window = high contrast (soft tissue)
Orthopaedic Indications
- •Complex fractures: plateau, acetabulum, calcaneus
- •Preoperative planning and 3D reconstruction
- •Articular surface assessment
- •CT arthrography post-surgery
Safety
- •Dose: CT spine 6mSv, pelvis 10mSv
- •CIN risk: Check eGFR before contrast
- •Metformin: Hold 48h if eGFR less than 45
- •Metal artifact reduction for implants