Common pitfalls of interpreting brain computed tomography (CT) scans

There are several artifacts that can appear on computed tomography (CT) scans that may be mistaken for significant disease. When that happens, the patient undergoes unnecessary follow-up imaging, a longer stay in the emergency room, and even unwarranted treatment.

Let’s review the six extremely common pitfalls that you will likely encounter within the first twenty CT scans you view:

1. Beam hardening
2. Focal high attenuation in parenchyma
3. Low attenuation fluid in meningeal spaces
4. Low attenuation in parenchyma
5. Air in unexpected spaces
6. Isodense subdural collections

Beam hardening is due to the alteration in the mean energy of a CT x-ray beam as it passes through the patient’s bone and soft tissue. Since the bone and tissue reduces the number of low energy x-rays that exit the head on the other side, there is an increase in the mean energy of the x-ray beam. As a result, the CT reconstruction software may introduce an artifact in the reconstruction since there seems to be too many x-rays arriving at the detector.

Beam hardening causes low attenuation artifacts along the beam path during reconstruction. This low attenuation frequently resembles a chronic subdural hematoma along the inner table of the skull.

You can usually recognize this as an artifact because it has an odd, straight medial border and usually appears in a depression along the inner table of the skull. Another clue is that an acute traumatic hemorrhage will have high attenuation, not low attenuation, in most patients.

Not all patients with focal high attenuation in the brain parenchyma on CT have a traumatic hemorrhage. A high attenuation lesion could also be a cavernoma! Magnetic resonance imaging (MRI) can be used to identify the presence of a dark hemosiderin ring characteristic of a cavernoma.

Enlarged, low attenuation fluid spaces around the brain can indicate a resolving subdural hemorrhage, brain atrophy that leads to the appearance of a wide subarachnoid space, or a traumatic subdural hygroma caused by a tear in the arachnoid membrane.

It is important to determine which of these three possibilities is the correct diagnosis. It can be helpful to look at the position of the cortical veins, and how they relate to the brain surface. If they are displaced away from the inner table of the skull and lie on the brain (within cortical sulci), it is more likely that the patient has a subdural hematoma.

But if the patient’s cortical veins lie along the inner table of the skull, you can suspect your patient has wide subarachnoid spaces due to atrophy.

Not all low attenuation in the brain parenchyma seen in trauma patients represents a traumatic contusion! In some patients, this can indicate a low-grade cortical brain tumor, which can be confirmed on an MRI.

Let’s take a look at a clinical example. A patient was found in her car with deployed airbags after an unwitnessed single-car crash. The CT scan in the emergency room demonstrated some low attenuation in the brain that could be confused for a contusion, but the overlying cortex was normal in appearance.

The patient was sent for an MRI with contrast. That exam demonstrated a frontal lobe abnormality that proved to be a metastatic lesion from her previously-treated breast cancer. The cortical lesion may have resulted in a seizure that led to the accident.

On CT, air can be seen within veins in the neck and head in unexpected places—especially in trauma patients. When misinterpreted, it can lead to unnecessary procedures and follow-up scans.

Air surrounding the trachea on a CT suggests the possibility of a traumatic airway injury. However, sometimes air may appear to be constrained with mostly linear boundaries. If there is no evidence of an anterior neck injury and the findings resolve on subsequent CT scans, the findings are likely due to air trapped within neck veins.

Air in the left and right cavernous sinuses from intravenous air is a common finding on trauma head CT scans. Air can also be seen in many other places on head CT and is a source of undue concern! Air in the superior ophthalmic vein is one example.

The finding of air in veins is almost always the result of inadvertently injecting air into peripheral veins during an infusion of intravenous fluids and not the result of a penetrating injury or fracture of an air-containing structure.

Since subdural hematomas start out with higher attenuation than the normal brain, and some weeks later show lower attenuation than the brain, there is a short period of time when the brain and the resolving hematoma have the same attenuation.

These isodense subdural hematomas can be very difficult to perceive on non-contrast CT but can be detected by focal increase in the thickness of the cortex and asymmetry of the cortical sulci. A magnetic resonance fluid-attenuated inversion recovery (FLAIR) scan can better demonstrate the subdural collection.

Let’s consider a patient that presented with left-sided weakness. The coronal image from their CT scan showed a shift of the midline structures in the brain from right to left. The septum pellucidum (which lies between the frontal horns of the ventricles) is a good landmark to measure the magnitude of this shift.

Notably, the right ventricle appeared compressed in comparison to the left ventricle on CT. While it is reasonable to consider that this shift was due to a mass in the right hemisphere, you should consider the possibility of an isodense subdural hematoma if there is a loss of sulci and thickening of the cortex.

In the patient mentioned above, their right-sided subdural hematoma became more apparent on a CT scan two days later. As a result of the evolution of blood products, the subdural was then a lower attenuation than the brain.

Keeping these six pitfalls in mind when reviewing CT scans will help you make accurate diagnoses, reduce interpretation errors, and reduce unnecessary follow-up imaging for patients who have experienced head trauma!

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