What is internal septation in a medical context?

Internal Septation in Medical Imaging

Internal septation refers to thin, wall-like structures or partitions that divide the interior of a cystic or fluid-filled lesion into multiple compartments. These septations are visible on various imaging modalities including ultrasound, CT, and MRI, and their characteristics help distinguish benign from malignant conditions.

Imaging Characteristics of Internal Septations

Ultrasound Findings

• Septations appear as thin linear structures within cystic lesions that divide the fluid-filled space into separate compartments 1
• On ultrasound, septations can be characterized as thin (typically <3 mm in height) or thick (>3 mm), with thick septations raising concern for malignancy 1
• Mobile thin septations within a cyst may indicate hemorrhagic content, particularly when associated with heterogeneous hyperechoic material corresponding to blood clots 1
• Complex internal septations in pleural effusions are highly suggestive of tuberculous etiology when combined with lymphocyte-rich fluid 1

CT and MRI Characteristics

• Septations may show varying signal intensities on MRI depending on their composition, with hemorrhagic septations appearing strongly hyperintense on T1-weighted images 1
• The number, thickness, and enhancement pattern of septations are critical features for risk stratification 1
• Contrast-enhanced imaging helps distinguish vascular septations from avascular structures like blood clots, with lack of enhancement suggesting clotting rather than viable tissue 1

Clinical Significance by Organ System

Hepatic Cysts

• Simple hepatic cysts may develop thin mobile septations after hemorrhage, which do not indicate malignancy 1
• In mucinous cystic neoplasms (MCNs) of the liver, thick septations and nodularity are worrisome features that suggest potential malignancy, particularly when combined with other findings 1
• Septations arising from the cyst wall without external indentation have very high association with MCNs rather than simple cysts 1

Pleural Effusions

• Highly complex internal septations in pleural effusions favor tuberculous etiology over malignancy, especially in lymphocyte-rich effusions 1
• Lentiform configuration with septations suggests infectious rather than malignant pleural disease 1

Ovarian/Adnexal Lesions

• Multilocular cysts with complete septations but no solid components >3 mm in height are classified as lower risk (O-RADS 3) 1
• Irregular septations or septal irregularity increases malignancy risk and warrants higher risk classification 1
• The presence of papillary projections arising from septations (≥3 mm height) significantly elevates concern for malignancy 1

Common Diagnostic Pitfalls

• Do not confuse post-hemorrhagic septations in simple cysts with the thick septations of neoplastic lesions—hemorrhagic septations typically show high T1 signal and lack enhancement on contrast studies 1
• Incomplete septations or wall irregularities should not be dismissed as they may represent early solid components requiring closer surveillance 1
• The number of septations alone is less important than their thickness, enhancement pattern, and associated features 1
• In peritoneal inclusion cysts, septations follow the contour of adjacent organs without mass effect, distinguishing them from ovarian pathology 1