What is the appearance of a chronic thrombus on computed tomography (CT) scan?

Appearance of Chronic Thrombus on CT Scan

A chronic thrombus on CT scan appears as a rigid, nondeformable intraluminal material with an irregular surface, possible calcifications, retraction with band or synechia formation, and a normal or decreased vessel size. 1

Key Characteristics of Chronic Thrombus on CT

Morphological Features

• Irregular surface compared to the smooth surface of acute thrombi 1
• Retraction of thrombotic material producing thin webs (synechiae) or thicker flat bands 1
• Incorporation into the vessel wall leading to regular or irregular wall thickening 1
• Normal or decreased vessel size in contrast to the distended vessels seen with acute thrombi 1

Density and Enhancement Characteristics

• Baseline attenuation of approximately 33.8 Hounsfield Units (HU) before contrast, which is lower than acute thrombi (54.9 HU) 2
• Contrast enhancement may be present in chronic thrombi (seen in approximately 54% of cases), whereas acute thrombi do not enhance 2
• Calcifications may be present within chronic thrombi, which is a distinguishing feature not seen in acute thrombi 1, 3

Structural Changes

• Rigid and nondeformable with probe pressure (on ultrasound) or similar appearance on CT 1
• Fibrosis producing scarring and wall thickening that may persist for years 1
• Development of collateral circulation is significantly associated with chronic thrombi (seen in 57% of cases) 2

Differentiating from Acute Thrombus

Acute thrombus on CT typically shows:

• Higher baseline attenuation (54.9 HU) 2
• No contrast enhancement 2
• Smooth surface with distended vessels 1
• Soft and deformable appearance 1

Clinical Implications

The ability to differentiate between acute and chronic thrombi is crucial for:

1. Treatment decisions: Chronic thrombi may require different management approaches than acute thrombi
2. Terminology clarification: The Society of Radiologists in Ultrasound recommends using "chronic postthrombotic change" rather than "chronic thrombus" or "residual thrombus" to avoid misinterpretation that might lead to inappropriate anticoagulation 1
3. Mortality risk assessment: Pulmonary embolism with CT-defined chronic thrombus is associated with higher mortality compared to acute thrombus alone 4

Common Pitfalls

• Misinterpreting chronic postthrombotic changes as acute thrombus, potentially leading to unnecessary anticoagulation 1
• Failing to recognize calcifications within thrombi, which strongly suggest chronicity 3
• Using the term "subacute thrombus," which should be avoided due to lack of unique imaging characteristics 1

By carefully evaluating these characteristics on CT, radiologists can accurately differentiate between acute and chronic thrombi, providing valuable information for clinical management.