Optical Coherence Tomography
1. What is Optical Coherence Tomography (OCT)
Optical Coherence Tomography is an invasive intravascular imaging technique that uses near-infrared light to generate ultraโhigh-resolution cross-sectional images of the coronary artery lumen and vessel wall.
- Analogous to โoptical ultrasoundโ
- Resolution is ~10โ20 ฮผm, the highest among coronary imaging modalities
- Allows histology-like visualization in vivo
2. OCT Technique (Stepwise)
Principle
- Uses near-infrared light (โ1300 nm)
- Measures back-scattered light using interferometry
- Creates high-resolution tomographic images
Procedural Steps
- Coronary wiring of the target vessel
- OCT catheter (2.6โ2.7 Fr) advanced distal to lesion
- Blood clearance achieved by:
- Rapid contrast injection (preferred)
- Saline or dextran (rare)
- Automated pullback (typically 20โ40 mm/s)
- Image acquisition during brief contrast flush
Key Technical Features
| Parameter | OCT |
|---|---|
| Axial resolution | 10โ20 ฮผm |
| Penetration depth | 1โ2 mm |
| Blood clearance | Mandatory |
| Radiation | None |
| Contrast use | Required |
3. Indications of OCT
A. Pre-PCI Assessment
- Plaque characterization
- Thin-cap fibroatheroma (TCFA)
- Plaque rupture vs erosion
- Calcified nodules
- Lesion morphology
- Thrombus (red vs white)
- Dissection
- Ambiguous angiographic lesions
B. PCI Guidance
- Stent sizing
- Accurate lumen diameter and area
- Optimization
- Stent under-expansion
- Malapposition
- Edge dissection
- Tissue prolapse
C. Post-PCI Evaluation
- Mechanisms of stent failure
- Stent thrombosis
- In-stent restenosis
- Neoatherosclerosis detection
D. Acute Coronary Syndromes (High-Yield)
- Differentiates:
- Plaque rupture
- Plaque erosion
- Calcified nodules
- Guides erosion-based conservative strategies (Erosion without rupture)
E. Research / Advanced Indications
- Healing response to stents
- Bioabsorbable scaffold assessment
- Drug-eluting stent strut coverage
4. Superiority of OCT Over Other Imaging Modalities
OCT vs Coronary Angiography
| Aspect | Angiography | OCT |
|---|---|---|
| Lumen assessment | 2D silhouette | True cross-sectional |
| Plaque characterization | No | Yes |
| Stent apposition | No | Gold standard |
| Dissections | Often missed | Clearly visualized |
Key Point: Angiography shows lumen only; OCT shows lumen + pathology.
OCT vs IVUS (Most Exam-Relevant Comparison)
| Feature | OCT | IVUS |
|---|---|---|
| Resolution | 10โ20 ฮผm (Superior) | 100โ150 ฮผm |
| Plaque microstructure | Excellent | Limited |
| Fibrous cap thickness | Measured accurately | Not possible |
| Stent strut visualization | Best | Inferior |
| Penetration depth | Limited | Better |
| Blood clearance | Required | Not required |
| Large vessels / LMCA | Limited | Preferred |
Bottom line:
- OCT = micro-details
- IVUS = deep penetration & vessel sizing
OCT vs CCTA
| Aspect | OCT | CCTA |
|---|---|---|
| Invasiveness | Invasive | Non-invasive |
| Resolution | Microscopic | Millimetric |
| Plaque erosion | Yes | No |
| Stent optimization | Yes | No |
5. Unique Strengths of OCT (Exam Pearls)
- Only modality that can:
- Measure fibrous cap thickness
- Identify plaque erosion
- Visualize individual stent struts
- Best for:
- ACS mechanism identification
- Stent optimization
- Edge dissections and malapposition
6. Limitations of OCT
- Requires contrast injection (renal risk)
- Limited penetration:
- Not ideal for large vessels (LMCA)
- Cannot assess full plaque burden
- Less useful in:
- Severe ostial lesions
- Hemodynamic instability
7. One-Line Exam Summary
OCT is an intravascular imaging modality using near-infrared light that provides ultra-high-resolution, histology-like images of coronary arteries, superior to IVUS and angiography for plaque characterization and stent optimization, but limited by shallow penetration and need for blood clearance.
Q1. The single most important OCT feature defining a thin-cap fibroatheroma (TCFA) is:
A. Lipid arc >180ยฐ
B. Fibrous cap thickness <65 ฮผm
C. Presence of macrophages
D. Spotty calcification
OCT-defined TCFA requires **fibrous cap thickness <65 ฮผm** overlying a lipid-rich plaque. Lipid arc alone is insufficient.
Q2. OCT hallmark of plaque erosion in ACS is:
A. Cap rupture with cavity
B. Deep calcium protrusion
C. Intact fibrous cap with luminal thrombus
D. Neoatherosclerosis
Plaque erosion shows **intact fibrous cap** with adherent thrombus, commonly white thrombus.
Q3. Which thrombus type appears as a high-backscattering, signal-rich mass with shadowing on OCT?
A. White thrombus
B. Red thrombus
C. Calcified thrombus
D. Platelet aggregate without fibrin
Red thrombus is RBC-rich โ **high backscatter with shadowing**. White thrombus is homogeneous and signal-rich without shadowing.
Q4. Minimal stent area (MSA) assessment by OCT is superior to angiography because it:
A. Estimates plaque burden
B. Visualizes vessel remodeling
C. Measures EEM area
D. Directly measures true lumen geometry
OCT provides **true cross-sectional lumen area**, detecting under-expansion missed by angiography.
Q5. Which OCT finding most strongly predicts early stent thrombosis?
A. Tissue prolapse
B. Edge dissection <200 ฮผm
C. Severe stent under-expansion
D. Thin neointima
**Under-expansion** is the strongest OCT predictor of early stent thrombosis, outweighing malapposition.
Q6. OCT-defined malapposition is best described as:
A. Any visible gap
B. Strut-to-wall distance exceeding strut thickness + polymer
C. Absence of neointima
D. Late lumen loss
True malapposition requires **distance > strut thickness plus polymer**, not mere visual separation.
Q7. Calcified nodules on OCT are characterized by:
A. Smooth calcium plate
B. Lipid pool with shadowing
C. Healed rupture
D. Protruding, irregular calcium with disrupted surface
Calcified nodules show **protruding, jagged calcium** breaking into the lumen, a distinct ACS mechanism.
Q8. Which OCT feature differentiates neoatherosclerosis from ISR due to hyperplasia?
A. Uniform neointima
B. Lipid-laden neointima with macrophages
C. Minimal lumen area
D. Late malapposition
Neoatherosclerosis resembles native atherosclerosis with **lipid pools and macrophages** inside the stent.
Q9. Which lesion is least suitable for OCT imaging?
A. Mid-RCA plaque rupture
B. ISR evaluation
C. Ostial left main disease
D. Distal LAD erosion
Large vessels and ostial LMCA are limited by **poor penetration and blood clearance**.
Q10. OCT is superior to IVUS in PCI because it best detects:
A. EEM area
B. Plaque burden
C. Positive remodeling
D. Edge dissections <200 ฮผm
OCTโs ultra-high resolution allows detection of **tiny edge dissections** invisible on IVUS.
Q11. On OCT, which finding mandates post-dilatation even if angiography appears optimal?
A. Tissue prolapse without flow limitation
B. Edge dissection <150 ฮผm
C. Minimal stent area below reference lumen area
D. Mild late malapposition
OCT-guided PCI prioritizes **adequate stent expansion**. Suboptimal MSA is the strongest indication for post-dilatation.
Q12. OCT feature most specific for plaque rupture is:
A. Superficial macrophages
B. Fibrous cap discontinuity with cavity formation
C. Luminal thrombus alone
D. Spotty calcium
Plaque rupture requires **cap disruption with an underlying cavity**, not just thrombus.
Q13. Which OCT characteristic best predicts difficulty in stent expansion?
A. Lipid arc >180ยฐ
B. Thin fibrous cap
C. Macrophage infiltration
D. Thick circumferential calcium (>270ยฐ)
**Calcium arc >270ยฐ and thickness >0.5 mm** strongly predicts stent under-expansion.
Q14. OCT measurement uniquely guiding calcium modification strategy is:
A. Calcium length
B. Calcium volume
C. Calcium thickness
D. Remodeling index
OCT uniquely measures **calcium thickness**, guiding atherectomy vs intravascular lithotripsy.
Q15. White thrombus on OCT appears as:
A. Low backscatter with shadowing
B. Homogeneous signal-rich mass without shadowing
C. Calcified protrusion
D. Signal-poor cavity
White thrombus is platelet-rich โ **signal-rich, homogeneous, minimal shadowing**.
Q16. Which OCT finding favors conservative management without stenting in ACS?
A. TCFA with rupture
B. Large lipid pool
C. Plaque erosion with preserved lumen
D. Calcified nodule
OCT-identified **plaque erosion** may be treated medically without stenting (EROSION concept).
Q17. Which OCT feature defines late stent malapposition?
A. Incomplete strut coverage
B. Small neointimal thickness
C. Tissue prolapse
D. Acquired separation of struts from vessel wall over time
Late malapposition results from **positive remodeling or thrombus resolution**, not deployment error.
Q18. OCT hallmark of neoatherosclerosis causing late stent thrombosis is:
A. Homogeneous neointima
B. Lipid-laden neointima with cap rupture
C. Chronic malapposition
D. Edge restenosis
Neoatherosclerosis behaves like native plaque and may **rupture inside the stent**.
Q19. Compared with IVUS, OCT is inferior primarily because it:
A. Cannot assess stent apposition
B. Has lower resolution
C. Has limited tissue penetration
D. Requires radiation
OCT penetration is **1โ2 mm**, limiting assessment of large vessels and total plaque burden.
Q20. OCT is contraindicated or least useful in:
A. Stent optimization
B. ACS mechanism analysis
C. ISR evaluation
D. Hemodynamically unstable patient with contrast intolerance
OCT requires **contrast injection**, limiting use in shock or severe renal dysfunction.
Q21. In bifurcation PCI, OCT most reliably identifies which predictor of side-branch compromise?
A. Medina classification
B. Plaque burden by area
C. Carina vs plaque shift mechanism
D. Vessel diameter mismatch
OCT distinguishes **carina shift from plaque shift**, directly influencing need for side-branch protection.
Q22. OCT criterion that best predicts need for intravascular lithotripsy (IVL) is:
A. Calcium length >10 mm
B. Calcium thickness โฅ0.5 mm
C. Calcium arc >90ยฐ
D. Spotty calcification
**Calcium thickness โฅ0.5 mm** is the strongest OCT predictor of balloon failure and IVL need.
Q23. Which OCT feature differentiates acute stent thrombosis from chronic ISR?
A. Neointimal hyperplasia
B. Neoatherosclerosis
C. Late malapposition
D. Intraluminal thrombus with uncovered struts
Acute stent thrombosis shows **fresh thrombus and uncovered/malapposed struts**, not mature neointima.
Q24. OCT finding that mandates extension of stent length is:
A. Distal reference lipid plaque
B. Minor tissue prolapse
C. Edge dissection involving media
D. Mild malapposition
**Medial or deep edge dissections** on OCT require stent extension regardless of angiographic appearance.
Q25. Which OCT parameter best correlates with stent restenosis risk?
A. Neointimal heterogeneity
B. Minimal stent area
C. Strut thickness
D. Polymer type
Across trials, **smaller minimal stent area** is the strongest predictor of ISR.
Q26. OCT feature most consistent with healed plaque rupture is:
A. Intact smooth cap
B. Calcified nodule
C. Layered plaque with heterogeneous signal
D. Homogeneous neointima
Healed rupture appears as **layered, heterogeneous plaque**, reflecting repeated thrombosis and healing.
Q27. OCT advantage over IVUS in LMCA PCI is limited mainly because:
A. Poor spatial resolution
B. Inability to see stent struts
C. Lack of lumen measurement
D. Limited penetration and blood clearance issues
In LMCA, **IVUS is preferred** due to deeper penetration and no need for contrast flush.
Q28. Which OCT finding best supports the diagnosis of stent fractureโrelated ISR?
A. Uniform neointimal thickening
B. Focal strut discontinuity with restenosis
C. Lipid-rich neointima
D. Late malapposition
OCT can directly visualize **strut discontinuity**, confirming fracture-related ISR.
Q29. OCT-based criterion for optimal stent expansion is:
A. Symmetric angiographic appearance
B. Absence of malapposition
C. MSA โฅ90% of distal reference lumen area
D. No edge dissection
Optimal expansion targets **MSA โฅ80โ90% of reference lumen area**, not angiographic symmetry.
Q30. OCT-detected feature most strongly linked to very late stent thrombosis is:
A. Small edge dissection
B. Neoatherosclerosis with cap rupture
C. Mild under-expansion
D. Tissue prolapse
**Neoatherosclerotic plaque rupture inside the stent** is the dominant OCT mechanism of very late stent thrombosis.
Q31. OCT feature that best predicts need for repeat revascularization is:
A. Malapposition
B. Tissue prolapse
C. Edge lipid plaque
D. Small minimal stent area
Consistently, **small MSA** drives restenosis and repeat PCI.
Q32. OCT is uniquely capable of identifying:
A. Plaque burden
B. External elastic membrane
C. Fibrous cap thickness
D. Vessel remodeling
Only OCT has sufficient resolution to **measure fibrous cap thickness**.
Q33. In DES follow-up, homogeneous neointima on OCT suggests:
A. Neoatherosclerosis
B. Stable healing response
C. Thrombotic risk
D. Impending rupture
Homogeneous neointima reflects **benign healing**, unlike heterogeneous lipid-rich tissue.
Q34. OCT limitation that directly affects renal outcomes is:
A. Radiation exposure
B. Catheter size
C. Pullback speed
D. Need for contrast flush
OCT requires **contrast injection**, increasing AKI risk in vulnerable patients.
Q35. OCT-guided PCI trials have primarily demonstrated:
A. Reduced mortality
B. Reduced stroke
C. Improved procedural optimization
D. Less contrast use
Evidence supports **better stent expansion and optimization**, not hard mortality endpoints.
Q36. OCT is superior to angiography in detecting:
A. TIMI flow
B. Small edge dissections
C. Vessel tortuosity
D. Calcification severity
Angiography misses **micro-dissections** easily seen on OCT.
Q37. OCT finding that does NOT usually require correction is:
A. Severe under-expansion
B. Medial edge dissection
C. Large malapposition
D. Minor tissue prolapse
Small tissue prolapse without flow compromise is usually **clinically benign**.
Q38. OCT is preferred over IVUS for evaluating:
A. Vessel remodeling
B. Plaque burden
C. Stent strut coverage
D. EEM sizing
OCT is the **gold standard for strut-level analysis**.
Q39. OCT appearance of lipid-rich plaque is:
A. High backscatter with sharp borders
B. Signal-poor region with diffuse borders
C. Homogeneous bright signal
D. Acoustic shadowing
Lipid appears **signal-poor with poorly defined borders**.
Q40. The single greatest strength of OCT in ACS management is:
A. Measuring plaque burden
B. Assessing vessel remodeling
C. Estimating ischemia
D. Identifying ACS mechanism (rupture vs erosion)
OCT uniquely defines **ACS pathophysiology**, enabling mechanism-based therapy.
Q41. OCTโFFR discordance where OCT shows severe MLA but FFR is >0.80 is best explained by:
A. Pressure wire drift
B. Microvascular dysfunction
C. Short lesion length with preserved hyperemic flow
D. Calcium shadowing
FFR reflects **physiology**, not anatomy. Short focal lesions may have low resistance despite small MLA.
Q42. OCT shows acceptable stent expansion but post-PCI FFR is 0.76. Most likely cause is:
A. Stent malapposition
B. Neoatherosclerosis
C. Edge dissection
D. Residual diffuse disease outside stented segment
OCT optimizes focal segments; **FFR may remain low due to diffuse atherosclerosis**.
Q43. In LM disease, OCT-derived MLA thresholds are unreliable mainly because:
A. Low resolution
B. Limited penetration and incomplete wall visualization
C. Poor catheter deliverability
D. Blood artifact only
LMCA assessment requires **full vessel wall visualization**, favoring IVUS over OCT.
Q44. In distal LM bifurcation, OCT most directly guides which critical step?
A. SYNTAX scoring
B. Medina classification
C. Stent sizing based on distal reference lumen
D. CABG vs PCI decision
OCT excels in **precise distal reference sizing**, critical for LM bifurcation PCI.
Q45. OCT finding that best predicts side-branch ostial compromise after main-vessel stenting is:
A. Carina angle
B. Medina 1,1,1 pattern
C. Side-branch diameter
D. Eccentric plaque opposite side-branch ostium
**Plaque shift**, not carina shift, is the dominant mechanism of SB compromise.
Q46. In cardiogenic shock, OCT is least appropriate primarily due to:
A. Catheter size
B. Imaging time
C. Need for contrast flush and flow clearance
D. Radiation exposure
Shock patients cannot tolerate **contrast-dependent blood clearance**.
Q47. OCT shows plaque erosion in STEMI with TIMI 3 flow. Best management strategy is:
A. Immediate DES implantation
B. Intensive antithrombotic therapy without stenting
C. CABG
D. Atherectomy
OCT-guided **erosion-based therapy** allows stent avoidance in selected cases.
Q48. OCT feature that correlates best with poor post-PCI FFR improvement is:
A. Edge dissection
B. Malapposition
C. Residual diffuse disease outside stent
D. Tissue prolapse
FFR integrates **entire vessel physiology**, not just treated segment.
Q49. OCT-defined mechanism of very early DES thrombosis (<24 h) is most often:
A. Neoatherosclerosis
B. Hypersensitivity
C. Late malapposition
D. Severe under-expansion with thrombus
**Mechanical factors**, especially under-expansion, dominate early thrombosis.
Q50. OCT is superior to IVUS for guiding atherectomy because it:
A. Measures calcium volume
B. Measures calcium thickness
C. Assesses plaque burden
D. Shows EEM
Calcium **thickness** determines balloon and atherectomy failure.
