Before going into detail about how to read and interpret brain computed tomography (CT) images, first, let’s review how axial CT images are displayed. When you view an axial CT scan, the convention for left and right is established by imagining that you are looking at the scan while standing at the feet of the patient. This means that as you look at the axial CT images, your left is the patient’s right side.
High and low attenuation on a brain CT scan
The word attenuation is used to describe how tissues interact with the x-ray beam. X-rays easily pass through structures with low attenuation, and they are represented as such on the image with a shade of gray on a CT scan. Structures with high attenuation will limit x-ray passage and be displayed as shades of white. Fluid, fat, and air are dark because they have low attenuation. Bone and metal, on the other hand, are displayed on a CT image as white because of their high attenuation.
Using CT imaging for clinical diagnoses
While some may think the goal when viewing a CT is to look for a focal abnormality (like a hemorrhage or mass), it is just as important to look for normal structures. An absence of normal structures on brain CT, in many cases, will lead you to the correct diagnosis, especially when the abnormalities are symmetric or subtle.
As an analogy, if you were asked what is wrong with the image of a house after a hurricane, you would need to know that there used to be a chimney on the roof before you could recognize that it is missing. Similarly, it’s important to know what key anatomical features look like on a normal CT scan, so you can recognize when they are absent.
Five key anatomical structures to identify on a brain CT scan
So, before considering anything else, you should make it your routine to identify the following five anatomical structures on a patient’s CT scan. This is imperative, since noticing an absence of any one of these five structures could lead you to a correct diagnosis.
Here are the five anatomical structures you’ll want to identify when reading a CT scan:
- Fourth ventricle
- Ambient cistern
- Basal ganglia and thalami
- Pituitary gland
- Foramen magnum
Identifying the fourth ventricle on a brain CT scan
One of the five important structures to identify on a CT scan is the fourth ventricle. It should be in the midline and have low attenuation, since it is filled with fluid. The fourth ventricle should be visible on all normal CT scans.
The remaining contents of the posterior fossa are usually poorly visualized on most CT scans, because of artifacts from surrounding bones. A change in the normal shape and location of the fourth ventricle may be the only evidence of a tumor or subacute hemorrhage nearby.
For example, in this patient with a right-sided tumor, the fourth ventricle is visibly compressed and displaced to the patient’s left side (Fig. 4). By viewing magnetic resonance imaging (MRI) scans of the patient and then looking back at the CT, it becomes easier to identify the abnormality.
When reading a brain CT, you should also look within the fourth ventricle, since both benign and malignant tumors can appear there.
Sometimes, the fourth ventricle is not visible at all! This can be the result of compression or because it is filled with blood causing it to have a higher attenuation than normal (and appear gray like the surrounding structures).
Identifying the ambient cistern on brain CT
The second structure you should identify on all brain CT scans is the ambient cistern. This is a fluid-filled space lateral to the cerebral peduncles.
By looking for the ambient cistern on all CT scans you read, you will develop a sense of what is normal at different patient ages. In other words, don’t just look at the ambient cistern when you are looking for a midbrain mass.
An absence of a visible ambient cistern can be a sign of a brain herniation. A CT image may not look concerning at first glance, until you notice the absence of the ambient cisterns. This is frequently due to severe brain swelling that leads to a downward herniation of the cerebral hemispheres.
Identifying the basal ganglia and thalami on brain CT
The basal ganglia and thalami are symmetric, deep structures in the brain. Since they are frequently symmetrically abnormal, you’ll need to look for them in each brain scan you see so that you’re familiar with their normal appearance on CT.
The lentiform nucleus and the caudate are two components of the basal ganglia. Since these structures have a higher attenuation than white matter, they, along with the thalami, are normally defined along one border by the white matter of the internal capsule (which has lower attenuation).
On this CT scan of a patient who survived drowning, you can see the basal ganglia well (Fig. 9). But only because the basal ganglia are abnormally low in attenuation. Remember, they should have higher attenuation than the internal capsule. This finding indicates a bilateral infarction of the basal ganglia from prolonged hypoxia.
Identifying the pituitary gland on brain CT
In adults over 40 years of age, the pituitary gland sits inside the sella turcica and does not normally extend above the dorsum sellae (the posterior wall of the sella turcica). On a normal brain CT scan, you will see a fluid space in the region just anterior to the dosum sellae. Considered in relation to anatomical position, this cerebrospinal fluid (CSF)-filled space is sitting the above the pituitary gland. Loss of this fluid space can indicate that the pituitary gland is enlarged.
Sometimes, soft tissue—rather than fluid attenuation—appears anterior to the dorsum sellae. This may be due to a pituitary tumor. It is one abnormality that is frequently overlooked when only axial CT images are reviewed, so pay attention to this site on brain CT!
Identifying the foramen magnum on brain CT
The foramen magnum is at the bottom of the skull where the brain joins the spinal cord. Normally, you should be able to see the cerebellar tonsils and the medulla as separate structures within the foramen magnum.
On the other hand, mass effect from a tumor may cause the foramen magnum to appear featureless on a brain CT scan. Abnormalities at the foramen magnum are frequently missed on axial CT images.
The foramen magnum may also appear featureless due to a low cerebellar tonsil position called a Chiari malformation. A Chiari malformation is much easier to see on a CT sagittal reconstruction.
Don’t be lulled into thinking a head CT is easy to read because the brain is symmetric! Always look for these five normal anatomic features, since the most critical observation you make on a CT scan may be what you don’t see.
That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.
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