Skull-Base Foramina of the Middle Cranial Fossa: Assessment of Normal Variation with High-Resolution CT

ABSTRACT

PURPOSE: To recognize foraminal variants of the foraminae of the skull base in the middle cranial fossa, and to thus understand and distinguish normal and potentially abnormal structures. MATERIALS AND METHODS: We analysed 163 patients without intracranial disease who had undergone CT scanning. These comprised 82 men and 81 women with a mean age of 39 years (range, 4|73 years). HRCT was performed, using a GE 9800 scanner. All CT scans were obtained 6-7 slices at the base of the skull, with 1.5 mm collimation at 1.5 mm intervals parallel to the infraorbital line. We analysed the foraminae by closesly correlating imaging findings and established anatomic knowledge. RESULTS: In 45 cases (27.6 %) the foramen ovale was 5-10 mm in diameter and asymmetrical. Deficiency of the medial bony wall including persistent foramen lacerum medius was seen in five cases (3.1 %). Confluence of the foramen ovale and the foramen spinosum was seen in 13 cases (8 %) and confluence of the foramen ovale and the foramen of Vesalius in 23 (14.1 %). Posterolateral groove for the accessory meningeal artery was observed in 36 cases (22 %). The foramen spinosum was asymmetrical in 42 cases (25.8 %). A small or absent foramen spinosum with a larger ipsilateral foramen ovale was observed in 11 cases (6.7 %). Medial bony defect was seen in 16 cases (9.8%). The foramen spinosum was absent in four cases (2.5 %). In 74 cases (45.4 %), the foramen of Vesalius was absent ; it was present unilaterally and bilaterally in 55 (33.7 %) and 34 cases (20.9 %), respectively. Five cases showed duplicated foramina. Canaliculus innominatus was seen in 14 cases (8.9 %) and was present bilaterally in three (1.8 %). CONCLUSION: HRCT clearly delineates bony structure and is well able to display the rich spectrum of anatomic variation found in the base of the skull. The recognition of these normal variants will result in a better understanding of skull base neurovascular anatomy and diminish speculation as to their true nature during the interpretation of CT images.