The Gibraltar Sign: An Anatomic Landmark for Predicting Transverse Sinus Dominance Laterality on Conventional MRI

ABSTRACT BACKGROUND AND PURPOSE Physiologic asymmetry of transverse sinus (TS) caliber is common and can lead to misinterpretation of magnetic resonance venography (MRV) studies of the brain. This retrospective study assesses the ability of multiple anatomic features on axial noncontrast T1‐weighted images (T1WIs) of the brain to predict congenital TS dominance. METHODS One hundred consecutively acquired combined MRI‐MRV studies of the brain were reviewed. On noncontrast axial T1WI, each reviewer measured (1) inclination of the groove for the superior sagittal sinus (SSS), (2) angulation of the posterior falx, (3) laterality of the SSS flow void, and (4) laterality of the inferior‐most occipital lobe. TS cross‐sectional area was measured on noncontrast sagittal 2‐dimensional time‐of‐flight MRV images and served as the reference standard. RESULTS The bony groove for the SSS sloped down to the right in 65 of 100 patients and correctly predicted right TS dominance in 63 of 65 cases (97% positive predictive value [PPV]). The groove sloped down to the left in 24 of 100 patients and correctly predicted left TS dominance in 22 of 24 cases (92% PPV). Laterality of the SSS flow void, laterality of the inferior‐most occipital lobe, and angulation of the posterior falx had 91% PPV, 82% PPV, and 74% PPV for predicting TS dominance, respectively. CONCLUSIONS Multiple anatomic features seen on conventional axial T1WI can be used to predict the laterality of congenital TS dominance and can help inform the interpretation of MRV studies of the brain.