Sample · RANZCR
Skull base foramina on CT and the circle of Willis on CT angiography
Radiological anatomy · 3 parts · 15 marks. Decide what you would write, then compare with the rubric and model answer. In the app you write it under time and it comes back marked.
Image A is a single axial computed tomography (CT) image of the skull base in bone window (window width 2000 Hounsfield units (HU), level 500 HU), 1 mm slice, at the level of the foramen ovale. The image is displayed in standard radiological convention (the patient's right on the viewer's left). Six arrows point to:
1. A large oval foramen in the greater wing of the sphenoid, anteromedial to a smaller round foramen, on the image's left side of the midline (the patient's right).
2. The small round foramen immediately posterolateral to structure 1, on the same side.
3. A canal running horizontally, anterior and slightly inferior to the floor of the sella, on the image's right side, opening anteriorly into the pterygopalatine fossa.
4. An irregular foramen at the junction of the petrous apex, basisphenoid and basiocciput, on the image's left side, partly closed by cartilage in life.
5. The bony canal coursing anteromedially through the petrous temporal bone towards the foramen lacerum of the same side, on the image's right side.
6. The air-filled space directly beneath the sella turcica in the midline, divided by an off-midline bony septum.
Image B is a maximum intensity projection (MIP) image from a CT angiogram of the circle of Willis, viewed from above (axial MIP, 20 mm slab).
(a) Label the structures indicated by arrows 1 to 6 on Image A. Include the side where relevant. 9 marks
Show the rubric and model answer
- 1: Right foramen ovale (side required for the mark; the arrow is on the image's left, which is the patient's right) (1.5)
- 2: Right foramen spinosum (1.5)
- 3: Left pterygoid (Vidian) canal (not "foramen rotundum", which lies superolateral and opens into the upper pterygopalatine fossa) (1.5)
- 4: Right foramen lacerum (1.5)
- 5: Left carotid canal (horizontal petrous segment of the internal carotid artery) (1.5)
- 6: Sphenoid sinus (midline structure; side not required, but noting the off-midline intersinus septum is acceptable) (1.5)
1. Right foramen ovale.
2. Right foramen spinosum.
3. Left pterygoid (Vidian) canal.
4. Right foramen lacerum.
5. Left carotid canal (petrous internal carotid artery canal).
6. Sphenoid sinus.
The image is in radiological convention, so arrows on the viewer's left are the patient's right. Paired structures without a side score zero.
(b) List the contents of the foramen ovale and the foramen spinosum. 3 marks
Show the rubric and model answer
- Foramen ovale: mandibular division of the trigeminal nerve (V3) (1)
- Foramen ovale: accessory meningeal artery, lesser petrosal nerve and emissary vein (from cavernous sinus to pterygoid venous plexus): any two (1)
- Foramen spinosum: middle meningeal artery and vein, and the meningeal (recurrent) branch of V3 (nervus spinosus) (1)
Foramen ovale: the mandibular division of the trigeminal nerve (V3), the accessory meningeal artery, the lesser petrosal nerve (variable; may pass through its own canaliculus) and an emissary vein connecting the cavernous sinus to the pterygoid venous plexus.
Foramen spinosum: the middle meningeal artery and its accompanying vein, and the meningeal branch of the mandibular nerve (nervus spinosus).
Image B shows the following on the patient's right: the precommunicating (P1) segment of the posterior cerebral artery is not seen, and a large vessel of similar calibre to the posterior cerebral artery arises from the right internal carotid artery terminus and continues posteriorly as the right posterior cerebral artery. The left side is normal.
(c) Name this variant and state one clinical implication. 3 marks
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- Right fetal (fetal-type) posterior cerebral artery: posterior communicating artery is the dominant supply to the right posterior cerebral artery, with an absent or hypoplastic right P1 segment (2)
- Implication, any one: right occipital lobe is supplied from the anterior (carotid) circulation, so a right internal carotid stenosis or embolus can cause a posterior cerebral artery territory infarct; occlusion of the basilar tip spares the right posterior cerebral artery territory; relevant when planning carotid or posterior communicating aneurysm treatment (1)
Right fetal-type posterior cerebral artery: the right posterior communicating artery is enlarged and supplies the right posterior cerebral artery, with an absent (or hypoplastic) right P1 segment.
Clinical implication: the right occipital lobe and other right posterior cerebral artery territory are fed from the right internal carotid artery, so emboli from right carotid disease can produce a posterior cerebral artery territory infarct, and a basilar occlusion will not affect that territory.
Where candidates lose marks here
- Omitting the side on paired structures: examiners withhold the label mark, and in the 2024 sitting this was singled out as a recurrent reason for lost marks.
- Reading arrows in anatomical rather than radiological convention, so every side is reversed.
- Confusing the pterygoid (Vidian) canal with the foramen rotundum: the Vidian canal is inferomedial and runs in the root of the pterygoid process; the foramen rotundum is superolateral.
- Calling foramen lacerum a passage for the internal carotid artery: the artery runs over its upper surface, not through it.
Original content written for Fellowship Ready, for exam preparation only, not clinical advice. Not affiliated with the Royal Australian and New Zealand College of Radiologists (RANZCR).