🫁 CT Pulmonary Angiography Clot Visualization Simulator

Contrast Bolus Timing and Pulmonary Arterial Opacification

Iodinated contrast timed to peak pulmonary arterial opacification for clot detection.

• 60–100 mL: Contrast bolus volume (typical iodinated dose used)
• ~15–20 s: Scan delay (bolus-tracking triggered acquisition)
• ≤1 mm: Slice thickness (thin collimation for small vessels)
• >250 HU: Arterial HU target (threshold for good opacification)

Bolus tracking and scan geometry

Region of interest placed in main PA; scan triggers automatically.

Thin collimation resolves vessels down to subsegmental caliber.

Reading the arterial tree systematically

Radiologists trace central to peripheral branches in order.

Each branch checked for contrast cutoff or filling defect.

Large Central Filling Defects and Hemodynamic Compromise

Central clots straddle the trunk or main branches, often causing severe compromise.

• >50%: Typical clot burden (of trunk cross-section occluded)
• High: Right heart strain risk (RV/LV ratio frequently elevated)
• Bifurcation: Saddle embolus pattern (straddles main PA bifurcation)
• Very high: Reader agreement (large defects rarely missed)

Why central clots are dangerous

Large defects abruptly cut flow to both lungs downstream.

Right ventricle strain and shock can follow quickly.

Imaging appearance

Bright contrast surrounds a dark, low-attenuation filling void.

Distal branches often appear dim from reduced flow.

Central saddle emboli are a classic indication for aggressive, urgent intervention.

Moderate Filling Defects in Mid-Tree Branches

Lobar and segmental branches show smaller, more localized filling defects.

• 20–50%: Typical clot burden (of branch cross-section occluded)
• 1–2: Lobes affected (often confined to single lobe)
• Moderate: Clinical significance (usually treated, less urgent)
• High: Reader agreement (generally well visualized)

Anatomic level and flow impact

Lobar and segmental vessels feed a limited lung territory.

Occlusion here rarely destabilizes the whole right heart.

Treatment implications

Standard anticoagulation is typically sufficient at this level.

Size and symptom burden still guide final decisions.

Small Distal Defects and Diagnostic Uncertainty

Subsegmental clots are small, harder to see, and debated clinically.

• <20%: Typical clot burden (of small branch occluded)
• Lower: Inter-reader agreement (more variability among radiologists)
• ~5–10%: Isolated SSPE frequency (of positive CTPA studies)
• Ongoing: Management debate (treat vs. watchful monitoring)

Why subsegmental clots are tricky

Tiny vessel caliber and motion artifact obscure small defects.

False positives and missed calls both occur here.

Clinical decision-making

Isolated subsegmental clots may not always need treatment.

Risk factors and symptoms weigh heavily on the choice.

Isolated subsegmental PE remains one of the more debated calls in emergency imaging.

Clot Location, Severity, and Treatment Urgency

Location on the arterial tree correlates with severity and urgency of care.

• Highest: Central clot mortality (without prompt intervention)
• Lowest: Subsegmental mortality (often minimal added risk)
• Central-heavy: Aggressive therapy use (thrombolysis, embolectomy considered)
• Subsegmental: Watchful approach (select low-risk isolated cases)

Matching treatment to location

Central, high-burden clots push toward aggressive management fast.

Small distal clots often allow a measured approach.

Putting it together

Location plus burden plus patient status guides final urgency.

CTPA remains the reference standard for this decision.

Anatomic level of clot burden is a core driver of pulmonary embolism triage.