High Yield Overview
SPECT-CT: Orthopaedic Applications
—Sensitivity Improvement
20-50%over planar
—Specificity Improvement
—Significant (anatomic localisation)
—CT Component
—Low-dose or diagnostic
—Additional Time
15-20minutes
SPECT-CT Components
SPECT: 3D functional imaging (bone metabolism)
CT: Anatomic detail and attenuation correction
Fusion: Co-registered SPECT on CT anatomy
Key: The hybrid approach provides what, where, and why in a single examination
Critical Must-Knows
- SPECT = Single Photon Emission Computed Tomography
- Hybrid SPECT-CT combines functional and anatomic data
- Improves lesion localisation and diagnostic confidence
- Particularly useful for spine, foot, and complex anatomy
- Low-dose CT for attenuation correction and localisation
Examiner's Pearls
- "Spine: Differentiates facet, disc, pars pathology
- "Foot: Localises uptake to specific tarsal bones
- "Spondylolysis: Active vs inactive pars defect
- "CT can be diagnostic or low-dose (protocols vary)
- "Reduces need for additional imaging
Clinical Imaging
Imaging Gallery
Click to expand
Click to expand
Click to expand
Click to expand
Exam Warning
SPECT-CT significantly improves the diagnostic performance of bone scintigraphy. Know the key indications: spine (pars vs facet), foot (tarsal localisation), and equivocal planar findings. The CT component provides precise anatomic correlation that planar imaging cannot achieve.
SPECT-CT Principles
Planar vs SPECT vs SPECT-CT
| Feature | Planar | SPECT | SPECT-CT |
|---|---|---|---|
| Acquisition | 2D projection | 3D tomographic | 3D + anatomic CT |
| Depth information | None | Yes (tomographic) | Yes + anatomy |
| Lesion localisation | Limited | Moderate | Precise |
| Sensitivity | Baseline | Improved 20-50% | Similar to SPECT |
| Specificity | Limited | Improved | Significantly improved |
| Anatomic detail | None | None | Excellent (CT) |
| Scan time | Shortest | Longer | Longest |
How SPECT-CT Works
SPECT acquires gamma camera data from multiple angles around the patient, reconstructed into 3D tomographic images showing radiotracer distribution. CT (low-dose or diagnostic) provides anatomic reference and attenuation correction. Software fuses the two datasets, allowing precise localisation of metabolic activity to specific anatomic structures.
Clinical Applications
SPECT-CT for Spinal Pathology
| Indication | Planar Limitation | SPECT-CT Advantage |
|---|---|---|
| Spondylolysis | Cannot localise to pars | Confirms pars vs facet vs vertebral body |
| Facet arthropathy | Overlapping structures | Identifies specific facet joint |
| Discogenic pain | Limited visualisation | May show endplate uptake pattern |
| Post-fusion | Non-specific uptake | Localises to pseudarthrosis vs adjacent segment |
| Pedicle screw loosening | Artefact, non-specific | Better localisation around hardware |
Active vs Inactive Spondylolysis
SPECT-CT can differentiate active (metabolically active, potentially healing) pars defects from inactive (chronic, non-healing) defects. Active defects show increased uptake on SPECT with CT showing the bony defect. Inactive defects show the CT defect without significant SPECT uptake. This distinction guides management: active defects may respond to bracing, inactive defects unlikely to heal.
Interpretation Approach
Mnemonic
WHERE and WHATSPECT-CT Interpretation
W
W = What does the SPECT show? (uptake pattern, intensity)
H
H = How intense is the uptake? (qualitative/quantitative)
E
E = Exactly where is it? (use CT for precise localisation)
R
R = Relevant anatomy? (CT shows structural cause)
E
E = Explanation? (correlate function with structure)
Memory Hook:Start with the SPECT to identify abnormal uptake, then use CT to precisely localise and explain the cause
Mnemonic
FUSION Benefits
F
Functional data from SPECT (metabolic activity)
Functional data from SPECT (metabolic activity)
U
Unmatched accuracy with precise localisation
Unmatched accuracy with precise localisation
S
Superior to either modality alone
Superior to either modality alone
I
Improved diagnostic confidence and reduced uncertainty
Improved diagnostic confidence and reduced uncertainty
O
Optimal for complex anatomical regions (spine, pelvis)
Optimal for complex anatomical regions (spine, pelvis)
N
Non-invasive with same-day results
Non-invasive with same-day results
Memory Hook:FUSION imaging provides the best of both worlds
Reporting SPECT-CT Findings
| Element | Description |
|---|---|
| SPECT findings | Presence, pattern, and intensity of uptake |
| CT findings | Structural abnormality at uptake site |
| Correlation | Does CT explain the SPECT finding? |
| Clinical significance | Is the lesion likely symptomatic? |
| Differential | Alternative explanations if correlation unclear |
Technical Considerations
SPECT-CT Protocol Options
| CT Type | Purpose | Radiation Dose | When to Use |
|---|---|---|---|
| Low-dose CT | Attenuation correction, localisation | Low (0.5-2 mSv) | Most routine indications |
| Diagnostic CT | Full anatomic detail | Higher (5-15 mSv) | When CT alone would be indicated |
| No CT | SPECT only | SPECT dose only | If CT not needed or contraindicated |
When Diagnostic CT Is Warranted
Use diagnostic-quality CT when structural detail is essential: suspected fracture, bone tumour characterisation, complex post-surgical anatomy, or when CT would have been ordered separately. This avoids additional imaging and radiation. Communicate with the nuclear medicine physician about clinical question.
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
VIVA SCENARIOStandard
EXAMINER
"A 16-year-old fast bowler presents with lower back pain. Plain X-rays are normal. The sports physician requests a bone scan to assess for spondylolysis."
EXCEPTIONAL ANSWER
SPECT-CT significantly improves spondylolysis assessment compared to planar bone scan. Planar imaging cannot reliably localise uptake to the pars interarticularis versus facet joint versus vertebral body due to overlapping structures. SPECT-CT provides: (1) Precise localisation - can confirm uptake is specifically in the pars defect, (2) CT correlation - shows the structural defect (pars fracture line or sclerosis), (3) Activity assessment - helps differentiate active (healing potential) from inactive (chronic) defects. An active spondylolysis shows increased SPECT uptake at the pars with CT demonstrating the bony defect or stress reaction. This guides management: active lesions may respond to bracing and activity modification; inactive chronic defects are unlikely to heal and may require different management.
KEY POINTS TO SCORE
Planar cannot localise pars vs facet
SPECT-CT precisely localises to pars
CT shows structural defect
Active (uptake+) vs inactive (uptake-) guides treatment
Active defects may heal with bracing
COMMON TRAPS
✗Accepting planar as sufficient for pars assessment
✗Not understanding active vs inactive distinction
✗Not correlating SPECT uptake with CT structure
VIVA SCENARIOStandard
EXAMINER
"A patient with chronic midfoot pain has a positive bone scan showing uptake in the midfoot region. The planar images cannot determine which specific joints are affected."
EXCEPTIONAL ANSWER
The midfoot contains multiple small joints (tarsometatarsal, naviculocuneiform, intercuneiform, calcaneocuboid) that overlap on planar imaging, making localisation impossible. SPECT-CT would precisely identify which specific joints show metabolic activity. This matters because: (1) Treatment targeting - if selective fusion is considered, knowing which joints are symptomatic guides surgical planning. (2) Injection therapy - diagnostic or therapeutic injections can be directed to the specific affected joint. (3) Differential diagnosis - stress fracture vs arthritis vs coalition can be differentiated by the CT appearance at the uptake site. For example, uptake at the naviculocuneiform joint with CT showing joint space narrowing and osteophytes confirms symptomatic arthritis at that level.
KEY POINTS TO SCORE
Midfoot has multiple overlapping joints
Planar cannot localise to specific joint
SPECT-CT identifies which joint is symptomatic
Guides surgical planning (selective fusion)
Directs injection therapy
COMMON TRAPS
✗Accepting non-specific planar findings
✗Treating without knowing specific joint affected
✗Not using CT to characterise the pathology
VIVA SCENARIOStandard
EXAMINER
"A patient 18 months after lumbar fusion has ongoing pain. Plain X-rays are inconclusive for fusion status. A bone scan is considered."
EXCEPTIONAL ANSWER
SPECT-CT is valuable for post-fusion spine assessment. Interpretation: (1) Solid fusion typically shows no significant uptake at the fusion site by 12-18 months, with CT showing continuous bone across the fusion mass. (2) Pseudarthrosis shows persistent increased uptake at the fusion site on SPECT, with CT showing motion, lucency, or lack of bridging bone. (3) Adjacent segment degeneration - uptake at levels above or below fusion with CT showing disc degeneration or facet arthropathy. SPECT-CT can also identify hardware-related issues: loosening (uptake around screws with CT showing lucency) or stress reaction at screw tips. The combination of metabolic information (is it active?) and anatomic information (what's the structural problem?) guides whether revision surgery or conservative management is appropriate.
KEY POINTS TO SCORE
Solid fusion: No uptake by 12-18 months
Pseudarthrosis: Persistent uptake at fusion site
CT shows structural fusion or failure
Adjacent segment: Uptake at adjacent levels
Can assess hardware loosening
COMMON TRAPS
✗Not knowing timeline for normal post-fusion uptake
✗Using bone scan alone (poor specificity)
✗Missing adjacent segment disease
Evidence Base
Diagnostic Accuracy
1
2
Clinical Applications
3
4
Key Evidence Points
- SPECT alone: Poor spatial resolution and anatomical localisation
- CT alone: Functional information absent
- SPECT-CT fusion: Combines functional and anatomical data
- Clinical impact: Changes management in 30-40% of cases
SPECT-CT Quick Reference
High-Yield Exam Summary
SPECT-CT Advantages
- •3D functional + anatomic imaging
- •Precise lesion localisation
- •Improved specificity over planar
- •Answers 'what' and 'where'
Key Indications
- •Spine: Pars vs facet vs disc
- •Foot: Tarsal localisation
- •Spondylolysis: Active vs inactive
- •Post-fusion: Pseudarthrosis assessment
Spondylolysis Assessment
- •Active: SPECT uptake + CT defect
- •Inactive: CT defect, no SPECT uptake
- •Active may respond to bracing
- •Inactive unlikely to heal
CT Options
- •Low-dose: Localisation, attenuation
- •Diagnostic: Full anatomic detail
- •Choose based on clinical question