Just as it’s important to be able to identify key anatomical features of the brain on computed tomography (CT) images, you should make it a habit to find the anatomical features of the skull base, since their absence can be a sign of disease.
Of course, you should always look at the calvarium in all cases, since many diseases such as multiple myeloma, or focal skull abnormalities like depressed skull fractures will present there!
Throughout this guide, you will frequently be shown abnormalities on both CT and magnetic resonance imaging (MRI). Viewing an abnormality such as bone destruction on an MRI often makes it easier to visualize when you look back at the CT scan.
Five important anatomic features in the skull base
The following five anatomic features found at the skull base are vital to identify on a CT scan, since their absence is considered abnormal:
- Carotid canals
- Foramen ovale
- Foramen spinosum
- Petrous apex
- Dorsum sellae
Identifying the carotid canals on brain CT
The bony carotid canals in the skull base are normally symmetric.
Sometimes on vascular imaging, a normal-sized carotid will be evident on one side but will appear narrow on the other. While this can be due to a carotid stenosis, you should also consider carotid hypoplasia.
Identifying the foramen ovale on brain CT
There are a pair of symmetrical openings in the skull base—the foramen ovale. The third division of the fifth cranial nerve carries sensory fibers from the face to the brainstem, that enter the skull through the foramen ovale.
If a patient’s CT scan shows a larger foramen ovale on one side, this could be due to a tumor! In this case, the tumor extended from the cavernous sinus through the foramen ovale which led to its enlargement (Fig. 6). This was much more evident on an MRI with contrast.
Identifying the foramen spinosum on brain CT
The small foramen just posterior to the foramen ovale is the foramen spinosum. The middle meningeal artery, a branch of the external carotid, enters the skull through this foramen. On a magnetic resonance angiogram (MRA), you may be able to see the small middle meningeal arteries entering the skull through the foramen spinosum.
Sometimes, the foramen spinosum may not be visible on a CT scan in its usual location posterior to the foramen ovale. This finding suggests either a vascular variant of the middle meningeal artery or an aberrant course of the carotid artery.
Identifying the petrous apex on brain CT
The petrous apex is the medial portion of the petrous bone that lies just posterior to the carotid canal. Infections, aneurysms, tumors, and cysts can involve the petrous apex.
On a CT scan, you can identify bony remodeling of one petrous apex by comparing it to the other. Remodeling of the petrous apex can be due to benign and malignant disease. Here, it was from a benign mass called a giant cholesterol cyst, which can be seen on MRI.
Identifying the dorsum sellae on brain CT
The sella turcica is a depression on the upper surface of the sphenoid bone that contains the pituitary gland. We often use the term sella to refer to this depression. On a normal brain CT, you can see the back wall of the sella turcica—the dorsum sellae.
On a CT scan, a normal adult sella contains low attenuation cerebrospinal fluid which sits on top of the pituitary gland. On computed tomography angiography (CTA) images, you can often see this normal low attenuation fluid anterior to the dorsum sellae.
A sellar and suprasellar mass is one reason you may see abnormally high attenuation inside the sella, anterior to the dorsum sellae on a CT scan.
Keep in mind that not all sellar enlargement arises from the pituitary gland. Sometimes you may be looking at an aneurysm, which may be more evident on a coronal reconstruction of a CTA image.
In this section, you have learned how to identify five important anatomic features in the skull base. You should look for these on every scan, so you can become familiar with variations that may occur with patient of different ages, as well as different angles of the scan reconstruction.
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- Guinto, FC Jr, Garrabrant, EC, and Radcliffe, WB. 1972. Radiology of the persistent stapedial artery. Radiology. 105: 365–369. PMID: 5079662
- Kim, YI, Ahn, KJ, Chung, YA, et al. 2009. A new reference line for the brain CT: the tuberculum sellae-occipital protuberance line is parallel to the anterior/posterior commissure line. AJNR Am J Neuroradiol. 30: 1704–1708. PMID: 19762457