High Yield Overview
Nuclear Medicine in Orthopaedics
Functional Molecular Imaging
Tc-99mBone Scan Tracer
3-PhasePerfusion/Pool/Delay
2-4hDelay Phase Timing
95%+Sensitivity for Mets
Common Radiotracers
Tc-99m MDP
PatternBone scintigraphy
Treatment6 hour half-life, binds hydroxyapatite
Tc-99m HMPAO WBC
PatternLabelled white cells
TreatmentInfection imaging
F-18 FDG
PatternPET glucose metabolism
TreatmentMalignancy, infection
F-18 NaF
PatternPET bone agent
TreatmentHigher resolution bone imaging
Ga-67 Citrate
PatternInflammation/infection
TreatmentLargely replaced by FDG
Critical Must-Knows
- Tc-99m MDP: Binds to hydroxyapatite at sites of active bone formation/turnover.
- Three-phase scan: Phase 1 (flow), Phase 2 (blood pool), Phase 3 (delayed/bone). All three positive = bone pathology.
- High sensitivity, low specificity: Hot spots indicate increased turnover but not cause.
- Cold lesions: AVN, bone infarct, aggressive lytic tumors (no reactive bone).
- PET-CT with FDG: Most sensitive for malignancy and infection, provides anatomical localization.
Examiner's Pearls
- "Superscan = diffuse metastases (no kidney visualization, uniform uptake).
- "Stress fracture positive on bone scan 2-3 weeks before X-ray changes.
- "Triple-phase positive (all three phases) suggests osteomyelitis over cellulitis.
- "Labelled WBC scan more specific for infection than bone scan.
- "Photopenic (cold) lesions can be aggressive tumors or AVN.
Clinical Imaging
Imaging Gallery
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Bone Scan Limitations
Bone scintigraphy is highly sensitive but poorly specific. A positive scan indicates increased bone turnover but does not distinguish between fracture, infection, tumor, or arthritis. Clinical correlation and often additional imaging (MRI, CT) are required for definitive diagnosis.
Bone Scintigraphy
Mechanism
Tc-99m MDP (Methylene Diphosphonate):
- Binds to hydroxyapatite crystite at sites of active bone formation
- Uptake reflects osteoblastic activity and blood flow
- Excreted by kidneys (normal kidneys visible on scan)
What Causes Increased Uptake
Increased Bone Turnover
- Fracture healing
- Infection (osteomyelitis)
- Tumour (primary or metastatic)
- Arthritis
- Paget disease
Increased Blood Flow
- Inflammation
- Hyperemia
- Soft tissue infection (early phases)
Three-Phase Bone Scan
Perfusion/Angiogram
Soft Tissue Phase
Bone Phase
Interpretation Patterns
Three-Phase Scan Interpretation
| Pattern | Phase 1 | Phase 2 | Phase 3 | Suggests |
|---|---|---|---|---|
| Osteomyelitis | + | + | + | All three positive = bone infection |
| Cellulitis | + | + | - | Soft tissue phases only = no bone involvement |
| Fracture | ± | ± | + | Delayed phase positive, earlier phases variable |
| Tumor (bone) | ± | + | + | Blood pool and delayed positive |
| AVN | +/- | +/- | -/Cold | May be cold (no osteoblast activity) |
Clinical Applications
Infection Imaging
Bone Scan Findings:
- All three phases positive (triple-phase positive)
- Focal increased uptake at infection site
- 90-95% sensitivity
- Poor specificity (also positive for fracture, tumor, etc.)
Limitations:
- Cannot distinguish osteomyelitis from neuropathic joints
- Difficult to interpret adjacent to hardware or recent surgery
Tumour Imaging
Bone Metastases
Bone scan is excellent screening tool:
- Sensitivity greater than 95% for osteoblastic metastases
- Whole body imaging in single study
- Detects metastases before X-ray changes
Common primaries: Prostate, breast, lung, kidney, thyroid
Limitations
May miss:
- Purely lytic metastases (no osteoblastic response)
- Multiple myeloma (rarely positive)
- Very aggressive tumors
Superscan: Diffuse uptake throughout skeleton, loss of kidney visualization - suggests widespread metastatic disease
Cold (Photopenic) Lesions
Causes of Decreased Uptake
- Aggressive lytic tumors: No reactive bone formation
- Multiple myeloma: Minimal osteoblast stimulation
- Avascular necrosis: No blood flow to deliver tracer
- Bone infarct: Dead bone with no activity
- Prior radiation: Reduced bone metabolism
- Metal artifact: Attenuates photons
Stress Fractures
Bone scan advantages:
- Positive 2-3 weeks before radiographic changes
- Sensitivity approaches 100%
- Can detect multiple sites in single study
Limitations:
- Poor anatomical detail
- Non-specific (must exclude tumor, infection)
- Being replaced by MRI in many centers
Avascular Necrosis
Bone scan findings:
- Early: Cold (photopenic) area due to loss of blood supply
- Later: Hot rim (reactive bone around necrotic segment)
- Late: Diffusely increased uptake (repair/collapse)
MRI is now preferred for AVN diagnosis
SPECT Imaging
Single Photon Emission Computed Tomography
SPECT adds to planar bone scan:
- 3D tomographic images (like CT reconstruction)
- Improved anatomical localization
- Better contrast and sensitivity
- Can be fused with CT (SPECT-CT)
Orthopaedic Applications
SPECT Applications in Orthopaedics
| Application | Advantage of SPECT |
|---|---|
| Spine (pars defects) | Localizes to specific vertebral level and structure |
| Facet arthropathy | Identifies active facet disease for injection |
| Occult fractures | Better detection than planar imaging |
| Hardware complications | Localizes uptake to specific component |
| Osteoid osteoma | Precise localization for surgery/ablation |
SPECT-CT
Hybrid imaging combining:
- Functional information from SPECT
- Anatomical detail from CT
- Precise localization of abnormal uptake
- Superior to either modality alone
Particularly useful for:
- Spine pathology (pars, facets)
- Foot and ankle (stress fractures, tarsal coalition)
- Knee (osteochondral lesions)
PET Imaging
Positron Emission Tomography
F-18 FDG (Fluorodeoxyglucose):
- Glucose analog taken up by metabolically active cells
- Malignant cells have increased glucose metabolism
- Most sensitive modality for detecting malignancy
Orthopaedic Applications
Oncology
- Staging bone and soft tissue sarcomas
- Detection of metastatic disease
- Treatment response monitoring
- Detecting recurrence
- Higher sensitivity and specificity than bone scan
Infection
- Chronic osteomyelitis
- Periprosthetic joint infection
- Spondylodiscitis
- FDG uptake in infection and inflammation
F-18 Sodium Fluoride (NaF) PET
PET bone agent:
- Similar mechanism to Tc-99m MDP (binds to hydroxyapatite)
- Higher resolution than conventional bone scan
- Faster imaging (1 hour vs 3-4 hours)
- Increasingly available
Applications:
- Bone metastases (superior to Tc-99m bone scan)
- Metabolic bone disease
- Stress fractures
Specific Conditions
Paget Disease
Bone scan findings:
- Intense uptake in affected bones
- Characteristic patterns (skull, spine, pelvis, femur, tibia)
- Useful for determining extent/polyostotic involvement
- Can monitor treatment response (uptake decreases)
Metabolic Bone Disease
Hyperparathyroidism:
- Diffuse increased skeletal uptake
- Brown tumors may appear as focal hot spots
- Salt-and-pepper skull pattern
Osteomalacia:
- Diffuse increased uptake
- Looser zones may show as hot spots
- Rib and pelvic involvement common
Complex Regional Pain Syndrome
Three-phase bone scan classic pattern:
- Phase 1 and 2: Increased periarticular flow
- Phase 3: Diffuse periarticular uptake in affected limb
- Sensitivity approximately 50%, specificity approximately 90%
Limitations: Can be normal, especially in later stages
Radiation & Safety
Radiation Dose
Nuclear Medicine Radiation Doses
| Examination | Effective Dose (mSv) | Comparison |
|---|---|---|
| Tc-99m Bone Scan | 4-5 | 200 chest X-rays |
| SPECT-CT | 5-8 | Adds CT dose |
| FDG PET-CT | 8-15 | Includes diagnostic CT |
| WBC Scan | 3-5 | Depends on agent |
Contraindications
Nuclear Medicine Precautions
- Pregnancy: Contraindicated (radiotracer crosses placenta)
- Breastfeeding: Interrupt for 24-48 hours depending on agent
- Renal impairment: Not a contraindication (most agents not nephrotoxic)
- Recent nuclear medicine: Allow adequate decay before repeat studies
- Patient isolation: Minimal precautions needed for Tc-99m (low energy, short half-life)
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
Bone Scan Interpretation
EXAMINER
"How do you interpret a three-phase bone scan and what patterns suggest osteomyelitis versus cellulitis?"
EXCEPTIONAL ANSWER
A three-phase bone scan has three components: Phase 1 (flow) is a dynamic acquisition showing arterial perfusion, Phase 2 (blood pool) at 5-10 minutes shows soft tissue perfusion, and Phase 3 (delayed) at 2-4 hours shows osteoblastic bone activity. In osteomyelitis, all three phases are positive - there is hyperemia on flow, increased soft tissue activity on blood pool, and focal increased uptake on delayed images at the site of bone infection. In cellulitis alone (without bone involvement), the flow and blood pool phases show increased activity due to soft tissue hyperemia, but the delayed bone phase is negative or shows only diffusely increased uptake from hyperemia rather than focal bone involvement. This triple-phase pattern helps differentiate bone from soft tissue infection, though MRI has largely superseded bone scan for this indication due to superior anatomical detail.
KEY POINTS TO SCORE
Three phases: flow (0-60s), blood pool (5-10min), delayed (2-4h)
All three positive = bone pathology (osteomyelitis)
Only flow/pool positive = soft tissue (cellulitis)
Bone scan sensitive but not specific
COMMON TRAPS
✗Forgetting the timing of each phase
✗Not mentioning that diabetic foot is difficult to interpret
✗Not acknowledging MRI is now often preferred
LIKELY FOLLOW-UPS
"When would you add a labelled WBC scan?"
"How do you assess for prosthetic joint infection?"
"What is a superscan?"
VIVA SCENARIOStandard
Metastatic Bone Disease
EXAMINER
"A 65-year-old man with prostate cancer has back pain. His bone scan shows multiple focal hot spots throughout the spine and pelvis. What is your interpretation?"
EXCEPTIONAL ANSWER
This bone scan pattern with multiple focal areas of increased uptake throughout the axial skeleton in a patient with known prostate cancer is highly consistent with widespread skeletal metastases. Prostate cancer typically produces osteoblastic metastases, which stimulate reactive bone formation and therefore show intense uptake on bone scintigraphy. The distribution - spine, pelvis, ribs, proximal femora - follows the pattern of red marrow distribution. I would recommend correlating with PSA levels and plain radiographs. If there's a specific symptomatic area, MRI would assess for epidural extension or pathological fracture risk. If the uptake were so diffuse that individual lesions couldn't be distinguished and the kidneys weren't visible, this would be a 'superscan' indicating very extensive metastatic disease. Bone scan has greater than 95% sensitivity for osteoblastic metastases, making it an excellent screening tool.
KEY POINTS TO SCORE
Multiple focal hot spots = likely metastases
Prostate cancer produces osteoblastic mets (bone scan positive)
Axial distribution follows red marrow
Superscan = diffuse uptake, no kidneys visible
COMMON TRAPS
✗Not mentioning other causes of multiple hot spots (Paget, polyostotic fibrous dysplasia)
✗Forgetting that lytic mets may be cold
✗Not recommending further imaging for symptomatic areas
LIKELY FOLLOW-UPS
"What cancers may have a false-negative bone scan?"
"How does PET compare to bone scan for metastases?"
"How would you manage a solitary bone lesion?"
VIVA SCENARIOStandard
Stress Fracture Imaging
EXAMINER
"An athlete has shin pain with normal X-rays. What is the role of bone scan versus MRI for suspected stress fracture?"
EXCEPTIONAL ANSWER
Both bone scan and MRI can detect stress fractures before radiographic changes appear. Bone scan is highly sensitive (approaching 100%) and becomes positive 2-3 weeks before X-ray changes due to the osteoblastic healing response. It can screen the entire skeleton in one study, useful for athletes with multiple potential injury sites. However, bone scan lacks specificity and anatomical detail - it cannot distinguish stress fracture from infection, tumor, or other causes of increased bone turnover. MRI has become the preferred modality for several reasons: it shows both bone marrow edema and the fracture line, provides excellent anatomical detail for grading severity, has no radiation exposure, and can visualize associated soft tissue injuries. MRI can also grade stress injuries along a continuum from stress reaction to complete fracture, which helps guide return-to-play decisions. In current practice, MRI is generally first-line for localized symptoms, while bone scan may still be useful for screening when symptoms are diffuse or bilateral.
KEY POINTS TO SCORE
Both positive before X-ray changes
Bone scan: very sensitive, whole body, lacks specificity
MRI: shows bone marrow edema + fracture line, grades severity
MRI preferred for most stress fracture diagnosis now
COMMON TRAPS
✗Saying bone scan is more sensitive than MRI (both very high)
✗Not mentioning radiation considerations
✗Forgetting MRI shows soft tissue injuries too
LIKELY FOLLOW-UPS
"How would you grade a stress fracture on MRI?"
"What is the typical return-to-play for a tibial stress fracture?"
"When might bone scan still be preferred over MRI?"
Nuclear Medicine Exam Day Cheat Sheet
High-Yield Exam Summary
Three-Phase Bone Scan
- •Phase 1 (Flow): 0-60 seconds - arterial perfusion
- •Phase 2 (Blood Pool): 5-10 min - soft tissue
- •Phase 3 (Delayed): 2-4 hours - bone uptake
- •All 3 positive = osteomyelitis; 1+2 only = cellulitis
Hot Lesions (Increased Uptake)
- •Fracture (stress, traumatic)
- •Infection (osteomyelitis)
- •Tumor (metastases, primary)
- •Arthritis, Paget disease
Cold Lesions (Decreased Uptake)
- •AVN / bone infarct
- •Aggressive lytic tumors
- •Multiple myeloma
- •Prior radiation
Key Patterns
- •Superscan: Diffuse mets, no kidney visualization
- •Stress fracture: Hot 2-3 weeks before X-ray
- •PET-CT: Most sensitive for malignancy
- •WBC scan: More specific for infection