Subsegmental Pulmonary Embolism (SSPE)

Definition and Anatomy

• Main pulmonary artery → Lobar arteries → Segmental arteries → Subsegmental arteries → sub-subsegmental arteries

CT Imaging: The Key Modality

Imaging Appearances on CTPA

Direct Signs (Filling Defects)

• Central low-attenuation filling defect (thrombus)
• Surrounded by a peripheral rim of enhancing contrast
• Gives a "polo mint" or ring appearance

• Central low-density filling defect flanked by two parallel lines of contrast
• Indicates non-occlusive thrombus

• Complete absence of contrast within an expanded subsegmental vessel
• The affected vessel may appear slightly dilated
• Arrowhead-shaped defect at the branch point

• Low-density thrombus abutting one side of the vessel wall
• Forms an acute angle with the vessel wall (distinguishes acute from chronic PE, which forms obtuse angles)

Textbook Images

Image 1: Solitary Subsegmental PE on CTPA with V/Q Correlation

Image 2: Radiopaedia - PE at the Subsegmental Level (Axial CTPA)

Technical Requirements for SSPE Detection

Pitfalls and Artifacts (Important for SSPE Specifically)

Incidence and Epidemiology

Clinical Significance and Prognosis

• Lower rates of hemodynamic compromise
• Lower short-term mortality
• Lower rate of right ventricular dysfunction
• Many patients have underlying cardiopulmonary reserve sufficient to compensate

• Multiple subsegmental vessels involved bilaterally
• Coexisting DVT is present
• Patient has poor cardiopulmonary reserve (cancer, COPD, heart failure)
• Patient is pregnant or anticoagulation was recently stopped

Management: Anticoagulation vs. Surveillance

2026 Meta-Analysis (Nicoletto et al., Thrombosis Research, PMID: 41962426)

• 8 observational studies, 674 patients
• VTE recurrence: No statistically significant difference between anticoagulated and non-anticoagulated groups (RR 0.61, 95% CI 0.28-1.35)
• Bleeding: Significantly higher in anticoagulated patients (RR 3.10, 95% CI 1.39-6.88)
• All-cause mortality: No significant difference
• Conclusion: Available evidence does not support routine anticoagulation for isolated SSPE

2024 Systematic Review (Chin et al., Am Surg, PMID: 38058129)

• Supports clinical surveillance as a valid strategy in selected low-risk patients

Guideline Recommendations (ESC/ACCP)

• Anticoagulate if: concurrent proximal DVT, poor cardiopulmonary reserve, high recurrence risk, cancer
• Surveillance (no anticoagulation) may be considered if: isolated SSPE, no proximal DVT on lower extremity ultrasound, low recurrence risk, high bleeding risk
• Must perform bilateral leg ultrasound before withholding anticoagulation

Diagnostic Algorithm for Isolated SSPE

CTPA shows isolated SSPE
         ↓
Bilateral lower extremity venous ultrasound
         ↓
DVT present? → YES → Anticoagulate
         ↓ NO
Assess risk factors for VTE recurrence
         ↓
High risk (cancer, thrombophilia, recent surgery)?
  → YES → Anticoagulate
  → NO  → Consider clinical surveillance
         ↓
Consider V/Q scintigraphy to confirm/refute CTPA finding
(especially if isolated single-vessel SSPE, uncertain quality)

Key Teaching Points

1. SSPE = thrombus in 4th-order (subsegmental) pulmonary arteries, smallest branches visible on CTPA
2. Filling defect is the hallmark - "polo mint" sign on axial view, "railway sign" on longitudinal view
3. False positives are common - always review in all planes; inter-observer agreement is lower for SSPE than proximal PE
4. Isolated SSPE without DVT may not require anticoagulation in low-risk patients - a critical management distinction from proximal PE
5. V/Q scintigraphy can confirm questionable CTPA findings and guide management decisions
6. The incidence of CT-detected SSPE has risen with improved scanner resolution, raising questions about overdiagnosis

Segmental Pulmonary Embolism

Definition and Anatomy

• Diameter: approximately 3-6 mm on CTPA
• Clearly visible on standard-quality CTPA without special technical requirements
• The target level for "adequately opacified vessels" on diagnostic quality assessment

• Main PA → Right/Left PA → Lobar arteries → Segmental arteries (3rd order) → Subsegmental arteries (4th order)

CT Imaging (CTPA)

Image 1: Axial CTPA - Segmental + Lobar PE (Tintinalli's Emergency Medicine, Figure 56-3)

Image 2: Coronal CTPA - Normal vs. PE with Filling Defects (Labelled)

Direct CT Signs of Segmental PE

1. "Polo Mint" Sign (= "Doughnut Sign")

• Seen when imaging orthogonal (perpendicular) to the vessel long axis
• Central low-attenuation filling defect (thrombus) surrounded by a peripheral ring of bright contrast
• The affected vessel is typically expanded/dilated
• The filling defect edge forms an acute angle with the vessel wall (differentiates acute from chronic PE)
• Classic at the segmental level where vessel diameter is ideal for this appearance

2. "Railway Sign" (= "Tram-Track Sign")

• Seen when imaging parallel to the vessel long axis
• Central linear filling defect flanked by two parallel stripes of contrast
• Indicates non-occlusive thrombus
• Best demonstrated on coronal or sagittal reformats along the segmental artery axis
• Example: right lower lobe superior segmental artery running in the plane of imaging

3. Occlusive / "Cut-Off" Sign

• Complete absence of contrast within the segmental vessel
• The vessel itself may appear dilated (expanded) compared to the contralateral equivalent
• Abrupt termination of the contrast column

4. Eccentric Mural Defect

• Thrombus adherent to one wall
• Low-density material forming acute angle with vessel wall
• Partial opacification of remaining lumen

5. "Vessel Expansion" Sign

• Affected segmental artery is visibly wider than the paired/contralateral vessel
• Caused by distension from thrombus

Anatomy: Which Segmental Arteries Are Involved?

• Greater blood flow volume
• Gravitational preferential flow
• Larger caliber

Secondary (Indirect) Signs on CTPA at Segmental Level

Segmental vs. Subsegmental PE: Key Differences

Management: Segmental PE Always Requires Anticoagulation

• Carries significant clot burden
• Has well-established VTE recurrence risk
• Falls clearly above the threshold for anticoagulation in all major guidelines (ESC, ACCP/CHEST, AHA)

• DOAC (rivaroxaban, apixaban - preferred first-line in most patients)
• LMWH bridge to warfarin
• LMWH alone (cancer-associated)

Pitfalls Specific to Segmental Arteries

Key Teaching Summary

1. Segmental PE = 3rd-order pulmonary arteries, diameter 3-6 mm - reliably seen on CTPA
2. Direct signs: polo mint sign (axial), railway sign (longitudinal), cut-off, eccentric mural defect, vessel expansion
3. Lower lobes dominate (80-90%), especially posterior basal segments bilaterally
4. Always anticoagulate segmental PE - no debate, unlike subsegmental
5. Secondary signs (Hampton's hump, pleural effusion, oligaemia) support the diagnosis and indicate infarction
6. Inter-observer agreement is high at segmental level - confident, actionable diagnosis