7.3 IVUS & OCT
Key Takeaways
- IVUS uses intravascular ultrasound (typically 20–45 MHz) to measure lumen area, plaque burden, and vessel size; OCT uses near-infrared light (~1,300 nm) for 10–20× higher resolution of superficial plaque and stent struts
- Minimum lumen area (MLA) on IVUS below approximately 4.0 mm² in the LAD (and similar thresholds in other vessels) supports physiologic significance in many studies, but values must be interpreted with lesion length and clinical context
- IVUS excels at vessel sizing before stent selection, detecting edge dissection, confirming stent expansion and apposition, and visualizing calcium depth; OCT excels at strut coverage, thin-cap fibroatheroma identification, and precise stent malapposition
- Blood must be cleared from the field for OCT (contrast or saline flush); IVUS tolerates blood better but still requires steady pull-back and coaxial catheter position
- RCIS staff set up consoles, manage pull-back speed, document measurements, and recognize artifacts that mimic dissection or malapposition on both modalities
IVUS & OCT
Quick Answer: IVUS uses sound waves for deep vessel wall imaging and accurate lumen/vessel sizing; OCT uses light for ultra-high-resolution surface detail of plaque, stent struts, and dissection flaps. Both guide PCI optimization when angiography alone is ambiguous. RCIS staff operate consoles, manage flush protocols, and ensure quality pull-backs for interpretable images.
Angiography is a lumenogram—it shows contrast column silhouette, not plaque composition or true vessel wall dimensions. IVUS and OCT answer questions angiography cannot: How much plaque is hidden? Is the stent fully expanded? Is there edge dissection or malapposition? Is the lesion truly significant?
Fundamental Technology Comparison
| Property | IVUS (Intravascular Ultrasound) | OCT (Optical Coherence Tomography) |
|---|---|---|
| Energy source | Ultrasound (~20–45 MHz) | Near-infrared light (~1,300 nm) |
| Axial resolution | ~100–200 µm | ~10–20 µm |
| Penetration depth | ~4–10 mm (visualizes media/adventitia) | ~1–2 mm (superficial wall detail) |
| Blood interference | Moderate; imaging possible in blood | Must clear blood (contrast/saline flush) |
| Vessel sizing | Excellent (EEM and lumen borders) | Excellent for lumen; less deep wall detail |
| Calcium | Shadowing behind calcium | Can penetrate superficial calcium better |
| Stent strut assessment | Visible but less detail | Excellent malapposition/coverage detail |
| Typical catheter | ~2.6–3.5 Fr compatible | ~2.7 Fr rapid-exchange |
Rule of thumb: Use IVUS when you need vessel size, plaque burden, and calcium depth; use OCT when you need stent strut-level detail, thin-cap identification, or subtle dissection.
IVUS: Image Interpretation Essentials
Normal Layer Appearance
From lumen outward:
- Intima — thin bright line
- Media — darker sonolucent layer
- Adventitia — bright outer border
The external elastic membrane (EEM) defines the true vessel border for area measurements. The lumen border is the inner blood-intima interface.
Key Measurements
| Measurement | Definition | Clinical Use |
|---|---|---|
| Lumen area (LA) | Cross-sectional blood channel | Compare minimum vs reference segments |
| EEM area | True vessel area including wall | Plaque burden calculation |
| Plaque burden | (EEM − Lumen) / EEM × 100% | Risk stratification; >70% at MLA site is high |
| MLA (minimum lumen area) | Smallest lumen cross-section | Lesion significance (context-dependent thresholds) |
| Reference area | Normal segment proximal/distal | Stent diameter and length selection |
MLA thresholds (guideline-supported ranges, not absolute rules):
- LAD: MLA ≤ ~4.0 mm² often correlates with ischemia (FFR validation studies)
- LCx/RCA: Higher thresholds (~6.0 mm² LCx, ~5.5 mm² RCA) due to larger reference vessels
Always integrate MLA with symptoms, FFR/iFR, lesion length, and angiographic appearance.
Plaque Morphology on IVUS
| Appearance | Likely Composition | PCI Implication |
|---|---|---|
| Soft/low echogenicity | Lipid-rich plaque | Higher embolization risk; consider distal protection |
| Bright with shadowing | Calcium | May require rotational/orbital atherectomy before stenting |
| Homogeneous moderate echo | Fibrous plaque | Standard technique often sufficient |
| Concentric vs eccentric | Remodeling pattern | Eccentric plaque may hide severity on angiogram |
OCT: Image Interpretation Essentials
OCT produces cross-sectional images that look like histology slices. Because light does not penetrate blood, automated or manual flush during pull-back is mandatory.
Stent Optimization Criteria (Common Research/GUIDELINE Targets)
| Parameter | Acceptable Target | Problem If Exceeded |
|---|---|---|
| Minimum stent area (MSA) | >90% of reference lumen area (protocol-specific) | Under-expansion → restenosis, thrombosis |
| Malapposed struts | Distance from strut to wall; acute malapposition >200–400 µm noted | Pooling, delayed endothelialization |
| Edge dissection | Flap depth/arc; significant if >200 µm depth or >60° arc | Acute closure risk; may need additional stent |
| Tissue prolapse | Protrusion of plaque through struts | Large prolapse may need further intervention |
OCT is the preferred modality for stent failure analysis (in-stent restenosis, stent thrombosis) because strut coverage and neoatherosclerosis are visible.
Pull-Back Technique and RCIS Workflow
IVUS pull-back:
- Advance IVUS catheter distal to the lesion (ideally in a stable branch or segment).
- Confirm coaxial position—off-axis catheters distort lumen measurements.
- Perform automated or manual pull-back at steady speed (typically 0.5–1.0 mm/s per system).
- Record cine for offline measurement; note markers relative to side branches.
OCT pull-back:
- Position catheter distal to target.
- Initiate contrast or saline flush via guide catheter or power injector.
- Trigger automated pull-back during blood clearance window (seconds matter).
- Verify frame quality before stent deployment decisions.
| Task | RCIS Responsibility |
|---|---|
| Console setup | Select vessel, calibrate, load pull-back protocol |
| Flush coordination | Syringe or injector timing with OCT |
| Documentation | Capture MLA, reference dimensions, stent MSA |
| Communication | Report measurement values verbally to physician |
| Artifact recognition | Bubble, non-coaxial, incomplete flush |
Artifacts and Pitfalls
| Artifact | Modality | Appearance | Fix |
|---|---|---|---|
| Non-coaxial catheter | IVUS/OCT | Eccentric lumen; distorted areas | Reposition; use guide support |
| Incomplete blood clearance | OCT | Streaks, signal dropout | Increase flush rate; check guide engagement |
| Acoustic shadowing | IVUS | Dark zone behind calcium | Not a dissection—rotate catheter |
| Bubbles | Both | Mobile bright spots | Flush; de-air system |
| Near-field saturation | IVUS | Ring-down near catheter | Adjust depth/gain |
Misreading artifact as dissection leads to unnecessary stenting; missing true edge dissection leads to acute closure. When OCT shows a flap, measure depth, length, and circumferential extent before deciding.
Clinical Scenarios on the RCIS Exam
- Ambiguous angiographic lesion — IVUS MLA and plaque burden determine whether to intervene.
- Left main or bifurcation — IVUS sizing selects stent diameter; OCT confirms strut apposition at bifurcation.
- Stent under-expansion — Post-dilatation guided by IVUS/OCT MSA targets.
- Spontaneous coronary artery dissection (SCAD) — OCT may visualize intimal flap; gentle technique required.
- Chronic total occlusion (CTO) — IVUS identifies true lumen and vessel course when angiography fails.
IVUS and OCT are complementary. Mastery of measurements, pull-back technique, and artifact recognition makes you an indispensable partner in precision PCI.
A lesion appears moderate on angiography. IVUS shows a minimum lumen area of 3.6 mm² in the mid-LAD with a plaque burden of 72% at that site. The most appropriate interpretation is:
Why must blood be cleared from the vessel during OCT imaging but is less critical for IVUS?
After stent deployment, OCT shows several struts separated from the vessel wall by more than 300 µm with no edge dissection. This finding best indicates: