Anatomy for the Royal College of Radiologists Fellowship. Malcolm Sperrin
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auditory canal leads to the tympanic membrane—this is tethered at its superior margin to a bony promontory called the scutum (marked with a red asterisk). Within the middle ear are the ossicles—the malleus, incus and stapes. The base of the stapes attaches to the oval window of the inner ear, which comprises the shell-shaped cochlea and three semi-circular canals.
The roof of the middle ear is separated from the meninges and brain by a thin ceiling of bone called the tegmen tympani. A canal called the additus ad antrum opens from the superior middle ear into an air chamber within the petrous bone called the maxillary antrum.
Several important structures run through the petrous bone. The facial nerve enters the skull base through the internal auditory meatus. It travels through the petrous part of the temporal bone in the facial canal, which has a question-mark-shaped course, before leaving the skull at the stylomastoid foramen. The internal carotid artery also passes through the petrous temporal bone in the carotid canal.
Exam tips:
The incus and malleus form an ice-cream cone configuration—with the head of the malleus forming the ice cream, and the body of the incus forming the cone.
Unusual views of common structures are an exam favourite. If (when!) you find that you have no idea what the arrow’s pointing at, try to orientate yourself by identifying surrounding structures. For example, for question 2, you could identify the odontoid process of C2 poking up in the midline from the bottom of the image. That would make the two bony structures on either side the lateral masses of C1—which articulate superiorly with the skull at the atlanto-occiptal joints.
Q1.4 Axial slice from a CT cisternogram study with intrathecal contrast
1 Name the arrowed structure.
2 Name the arrowed structure.
3 Name the arrowed structure.
4 Name the arrowed structure.
5 Name the arrowed structure.
Answers
1 Left gyrus rectus (straight gyrus).
2 Pituitary infundibulum (pituitary stalk).
3 Interpeduncular cistern.
4 Cerebral aqueduct (aqueduct of Sylvius).
5 Quadrigeminal cistern.
Comments:
The basal cisterns are a network of cerebrospinal fluid (CSF)—filled spaces which communicate with the ventricular system via the Foramen of Magendie (also called the median or medial aperture) and the two lateral Foramina of Luschka (lateral apertures)—these drain into the cisterna magna, which sits at the base of the skull between the medulla and cerebellum.
Cisterns that you should be able to identify include the suprasellar cistern, which sits above the pituitary fossa (sella turcica) and contains important structures including the Circle of Willis and the optic chiasm. The suprasellar cistern has a star-shaped configuration, with the posterior point of the star formed by the interpeduncular fossa, which lies between the two cerebral peduncles of the midbrain. The two posteriolateral points of the star are the ambient cisterns which are lateral to the brain stem; these join posteriorly behind the midbrain to form the quadrigeminal cistern. The anterolateral points of the star are the Sylvian fissures (lateral sulci), which separate the frontal and parietal lobes from the temporal lobes, and contain the middle cerebral arteries.
Other cisterns to recognise are the prepontine and premedullary cisterns, which are best seen on sagittal images and lie in front of the pons (with its’ belly) and the medulla, respectively.
Exam tips:
The quadrigeminal cistern can be mistaken for the fourth ventricle. The quadrigeminal cistern is seen at the level of the midbrain (which has ‘Mickey Mouse ears’)—the fourth ventricle is seen at the level of the pons. An additional clue in question 5 is that the cerebral aqueduct, which drains from the third to the fourth ventricle, is also visualised; therefore this cannot be the fourth ventricle.
Question 2 is difficult because it is an obscure view of a common structure. The key here is to orientate yourself—it is a midline structure in the suprasellar cistern, lying just behind the optic chiasm.
Q1.5 Midline sagittal section from a CT venogram with intravenous contrast
1 Name the arrowed structure.
2 Name the arrowed structure.
3 Name the arrowed structure.
4 Name the arrowed structure.
5 Name the arrowed structure.
Answers
1 Straight sinus.
2 Posterior arch of C1 (atlas).
3 Tectorial membrane.
4 Basilar artery.
5 Internal cerebral vein.
Comments:
The superior sagittal sinus meets the right and left transverse sinuses at the confluence of the sinuses (torcular herophili). The transverse sinuses become the sigmoid sinuses, which leave the skull base through the jugular foramen to form the internal jugular veins. The sigmoid sinus becomes the internal jugular vein when it is joined by the inferior petrosal sinus in the jugular foramen.
The right and left basal veins of Rosenthal join with the internal cerebral veins to form the vein of Galen, which is short and runs within the quadrigeminal cistern. This then combines with the inferior sagittal sinus to form the straight sinus, which drains into the confluence of the sinuses.
The tectorial membrane is continuous with the posterior longitudinal ligament of the spine, and passes from the posterior body of C2, along the posterior odontoid process, to the clivus.
Exam tips:
Remember that even though the phase of contrast is venous, arterial structures (such as the basilar artery in this case) may still be discernible, and vice versa.
In most cases in the exam, if the structure shown is paired, then you should include left or right in your answer. In some cases, however, it isn’t possible to tell (as with the internal cerebral vein here)—in that case, just give the name of the structure without including laterality.
Q1.6 Axial slice from a T2-weighted MRI of the brain
1 Name the arrowed structure.
2 Name the arrowed structure.
3 Name the arrowed structure.
4 Name