10 AI-generated high-yield questions by our AI engine
Overview — Imaging Modalities in Orthopaedics
Computed tomography (CT) and magnetic resonance imaging (MRI) are the two most powerful cross-sectional imaging modalities in orthopaedic practice. CT excels at defining bony architecture, fracture patterns, and implant positions with unparalleled spatial resolution. MRI provides unrivalled soft tissue contrast, allowing assessment of cartilage, ligaments, tendons, bone marrow, and neural structures without ionising radiation. Understanding the physical principles, clinical indications, imaging sequences, and interpretive approach for each modality is essential for every orthopaedic surgeon — the ability to request, interpret, and communicate imaging findings appropriately is a core clinical competency.
CT physics: CT uses X-rays (ionising radiation) rotated around the patient; detectors measure the attenuation of X-rays as they pass through tissues; differences in attenuation create the CT image; attenuation is measured in Hounsfield Units (HU) named after Sir Godfrey Hounsfield (Nobel Prize 1979); the HU scale: water = 0 HU; air = −1000 HU; fat = −100 to −50 HU; soft tissue = +20 to +80 HU; cortical bone = +400 to +1000 HU; blood/haematoma (acute) = +50 to +80 HU; calcium/bone = white (high attenuation); air = black (low attenuation); the window level and window width settings determine which HU range is displayed — bone window (wide window) for fractures; soft tissue window for muscles/organs; lung window for chest
MRI physics: MRI uses strong magnetic fields and radiofrequency (RF) pulses to measure the relaxation behaviour of hydrogen protons (in water and fat); different tissues have different T1 and T2 relaxation times — these differences create the image contrast; no ionising radiation; contraindications: ferromagnetic metallic implants (cochlear implants, certain pacemakers, intracranial aneurysm clips — check MRI safety database); claustrophobia; pregnancy (relatively safe after first trimester); metallic foreign bodies near vital structures
CT — Key Sequences & Orthopaedic Applications
CT Application
What It Assesses
Key Uses in Orthopaedics
CT plain (without contrast)
Bony architecture; fracture pattern; fragment position; intra-articular extension; canal diameter; alignment; bone density
Vascular structures (arteries and veins enhance with contrast); soft tissue lesions (tumours, abscesses enhance); assessment of arterial injury (CT angiography)
CT angiography for popliteal artery injury (knee dislocation); CT chest-abdomen-pelvis for staging of bone tumours (pulmonary metastases, visceral primary in suspected metastatic disease); pelvic CT angiography for arterial bleeding after pelvic fracture; deep infection/abscess localisation
CT 3D reconstruction
3-dimensional surface rendering of complex fracture patterns; volume-rendered images
Hounsfield Units to memorise: air (−1000), fat (−50 to −100), water (0), soft tissue (+20 to +80), acute blood/haematoma (+50 to +80), bone (+400 to +1000), cortical bone (up to +1000); these values allow rapid identification of tissue types on CT — a `bright` lesion in a bone = calcification or new bone formation; a `dark` lesion within a tumour = fat (lipoma, liposarcoma, fibrous dysplasia with fat content)
MRI — Key Sequences & Signal Characteristics
Sequence
Fat Signal
Water/Fluid Signal
Key Orthopaedic Uses
T1-weighted
Bright (high signal)
Dark (low signal)
Anatomy; bone marrow assessment (normal marrow = bright/fat signal on T1; marrow replacement = dark on T1 = tumour, infection, oedema); fat-containing lesions (lipoma = bright on T1); AVN staging (T1 dark line = subchondral fracture/necrosis); cortical bone = dark (no signal — no mobile protons); muscle = intermediate grey
T2-weighted
Bright (high signal)
Bright (very high signal) — `water is white on T2`
Joint effusions (bright fluid); ligament tears (disruption + high signal within the ligament); disc herniation (T2 high signal in disc = hydrated nucleus or tear); soft tissue oedema; meniscal tears; articular cartilage assessment; spinal cord compression (T2 high signal = myelomalacia in CSM)
STIR (Short TI Inversion Recovery)
Suppressed (dark — fat signal nulled)
Very bright (water/oedema highlighted)
The most sensitive sequence for bone marrow oedema and subtle pathology; stress fractures (bright STIR = marrow oedema around the fracture line); occult fractures (STIR positive before plain X-ray); early infection/osteomyelitis; early AVN; soft tissue oedema; metastases in bone marrow; the preferred sequence for `is something wrong here?`
Proton Density (PD) ± fat suppression
Variable
Intermediate
Meniscal and cartilage assessment (best sequence for meniscal tears); ligament evaluation; articular cartilage grading; the sequence of choice for knee MRI in most centres
Gadolinium-enhanced T1 (T1+Gd)
Bright
Enhancement in vascular/inflamed areas
Differentiation of viable tumour from necrotic tissue; assessment of tumour extent and vascularity; infection (abscess ring enhancement); post-operative assessment (scar vs recurrent tumour); MR arthrography (contrast injected intra-articular) for labral tears (hip/shoulder), loose bodies, and articular cartilage defects
MRI Signal Mnemonics & Key Patterns
The essential MRI memory rule: `Fat is bright on T1; Water is bright on T2; STIR suppresses fat and highlights water/oedema`; T1 = anatomy and fat; T2 = pathology and fluid; STIR = sensitive screening; this three-sentence summary allows correct interpretation of most orthopaedic MRI studies
Bone marrow signal: normal marrow = bright on T1 (fat-containing yellow marrow in adults); pathological marrow (tumour, infection, infarction, oedema) = dark on T1 + bright on STIR/T2; this `T1 dark, STIR bright` pattern in bone marrow = abnormal and must be explained
Ligament and tendon: normal ligament = low signal (dark) on all sequences (tightly packed collagen fibres); torn ligament = disruption of low-signal band + high signal (fluid/oedema) within and around the tear on T2/STIR; complete tear = complete discontinuity of the band; partial tear = focal high signal within an otherwise intact band
AVN (avascular necrosis) MRI pattern: T1 — dark curvilinear subchondral line (the `double line sign` on T2 — an outer dark line + inner bright line around the necrotic segment = pathognomonic of AVN; this double line represents the reactive interface between viable and necrotic bone); STIR — high signal marrow oedema; the `crescent sign` on plain X-ray (subchondral lucency) indicates subchondral collapse — a late finding; MRI detects AVN before X-ray changes appear
Osteomyelitis MRI: T1 dark + STIR/T2 bright replacement of normal bright marrow signal; cortical destruction (loss of normal dark cortical line); periosteal new bone formation; adjacent soft tissue oedema; subperiosteal abscess (fluid collection); gadolinium enhancement of the infected bone + ring-enhancing abscess
Radiation Doses & Safety
Investigation
Approximate Effective Dose
Background Radiation Equivalent
Chest X-ray
0.02 mSv
~3 days background radiation
Pelvis X-ray
0.7 mSv
~4 months
CT head
2 mSv
~1 year
CT chest
7 mSv
~3 years
CT abdomen/pelvis
10 mSv
~4–5 years
CT chest-abdomen-pelvis (trauma)
15–20 mSv
~7 years
MRI (any)
0 mSv (no ionising radiation)
None
Isotope bone scan (Tc-99m)
4–6 mSv
~2 years
Exam Pearls
HU values: air = −1000; fat = −50 to −100; water = 0; soft tissue = +20 to +80; bone = +400 to +1000; acute haematoma = +50 to +80 (hyperdense vs brain); calcium = white/bright; air = black/dark; these allow instant tissue identification on CT
MRI core rule: T1 = Fat bright, Water dark (anatomy); T2 = Fat bright, Water bright (pathology/fluid); STIR = Fat suppressed, Water bright (sensitive for oedema, occult fractures, marrow pathology)
Bone marrow: normal = T1 bright (yellow fat); abnormal (tumour, infection, oedema) = T1 dark + STIR bright; `T1 dark marrow` = always explain; the most important marrow assessment principle
AVN double line sign: T2 — outer dark + inner bright curvilinear line around the necrotic femoral head segment = pathognomonic of AVN; MRI detects AVN before plain X-ray; STIR shows marrow oedema
STIR: most sensitive sequence for occult fracture, stress fracture, early osteomyelitis, bone marrow oedema, and early metastases; when to order STIR — `X-ray negative but clinical suspicion high` → MRI with STIR
MRI contraindications: ferromagnetic implants (cochlear implants, certain pacemakers, intracranial clips — check MRI safety database); metallic foreign bodies near eyes/brain; MRI-conditional implants — check manufacturer`s documentation; claustrophobia (open MRI or sedation options); pregnancy (avoid first trimester where possible; gadolinium crosses placenta — avoid in pregnancy)
CT angiography: investigation of choice for popliteal artery injury (knee dislocation) when hard signs absent but ABI <0.9; sensitivity ~95–100% for significant arterial injury; provides road map for vascular surgical planning
Radiation dose principle (ALARA — As Low As Reasonably Achievable): CT carries significant radiation dose (pelvis CT ~10 mSv = 4–5 years background); MRI is dose-free; for imaging of young patients and repeated studies, choose MRI over CT where diagnostic quality is equivalent; justify each CT request with clinical indication
