How to identify early signs of acute infarction on computed tomography (CT) scans

The term stroke is used to describe the acute onset of neurological symptoms, such as sudden difficulty with language or unilateral weakness. However, it is important to note that these symptoms do not always mean the patient has had an acute brain infarction. In some cases, a hemorrhage or brain tumor can cause similar symptoms!

Since the diagnosis of acute infarction cannot be made by physical symptoms alone, one of the goals for the initial computed tomography (CT) evaluation is to determine if the patient has evidence of a brain hemorrhage; intracranial blood can be the result of an underlying vascular lesion, a venous thrombosis, or a brain tumor.

While hemorrhage can be secondary to an acute brain infarction, this is rarely the case when a CT scan is obtained immediately after the onset of symptoms.

There are six findings on the initial brain CT that support the diagnosis of an acute infarction:

1. Dense middle cerebral artery (MCA) sign
2. Dot sign
3. Insular ribbon sign
4. Basal ganglia asymmetry
5. Loss of gray-white boundary
6. Low attenuation of the cortex

Keep in mind, you must know the nature of the patient’s symptoms so that you know where in the brain to look for these findings since they are frequently subtle on CT. For example, knowing that a patient is experiencing right-hand weakness should lead you to pay particular attention to any subtle brain CT findings on the patient’s left.

An early sign of acute infarction is a thrombus in the MCA, which appears unusually white on CT because clotted blood has higher attenuation than flowing blood.

Why is this sign associated with the MCA in particular? Because it requires comparison with an artery on the other side (which is possible with the MCA) to be a reliable indicator of infarction.

For example, if a patient has acute aphasia, that function is usually on the left side of the brain, so the right MCA can be used as a normal reference. Since we have a single basilar artery, diagnosis based on its attenuation is less reliable.

On a patient with acute left-sided hemiparesis, the right MCA may appear to have a slightly higher attenuation than the left when viewed with a standard 5 mm thick reconstruction of the CT scan data. However, most modern CT scanners can acquire data using detector collimation of 1 mm or less.

When 1 mm axial CT images are reconstructed from the CT data of the patient, the high attenuation of the patient’s right MCA is more evident (Fig. 2). This finding is called the dense MCA sign. You will have a higher chance of making this diagnosis on CT when images are reconstructed at thin sections and displayed using a narrow window and level of about 40 Hounsfield units (HU).

The dot sign is a variation of the dense MCA sign. It indicates a thrombus in the more distal middle cerebral artery branches within the Sylvian fissure where they turn perpendicular to the image slice.

Medial to the Sylvian fissure there should be a thin, high attenuation zone of normal cortex. In the appropriate clinical setting, when this is absent, it is an early sign of an acute MCA occlusion that is called the insular ribbon sign.

The insular ribbon sign is subtle and only hints at what is to come. Often, subsequent scans show the true size of the infarction. This should serve as a reminder that the detection of acute infarcts on CT depend on the recognition of subtle changes in tissue attenuation!

Asymmetry in the basal ganglia is another early sign of an MCA territory infarction since the small arteries that supply blood flow to the basal ganglia come from the proximal middle cerebral artery. For example, you may be able to notice that the putamen is harder to see on the affected side.

Focal loss of the normal borderline between the gray matter and white matter on brain CT is another sign of an acute infarction. When observing this finding, look carefully at both sides of the brain for comparison.

In a normal patient, you should be able to follow the relatively high attenuation of the cortical ribbon around the hemispheres on a good-quality brain CT scan. Loss of the gray-white matter interface is a subtle finding, and your chances of seeing it increase if you know where to look based on a patient’s symptoms. As well, you may want to order a follow-up CT scan since infarctions often become more evident over time.

Focal low attenuation in the cortex is an important finding to recognize in patients with suspected infarctions. Keep in mind that this finding is often inapparent or due to an artifact—especially if there is age-related atrophy of the cortex.

Small, acute cortical infarctions may vary in their ease of detection between patients. Knowing details about a patient’s symptoms can help with detection of infarcts.

For example, if you know that a patient’s symptoms include the acute onset of right-handed weakness, you are more likely to find a small left-sided cortical infarction. Pay particular attention to areas of low attenuation evident in the left precentral gyrus since this is where hand motor function resides.

Don’t assume low attenuation in the basal ganglia is an old lacunar infarction

Do not routinely attribute low attenuation in the basal ganglia to an old lacunar infarction unless you have a prior CT or magnetic resonance imaging (MRI) scan that shows it! An area of low attenuation may initially be thought to indicate an old lacunar infarction, but it’s important to consider the patient’s symptoms.

If the patient has acute weakness on one side, an MRI with diffusion images may prove helpful, as it did for this patient with acute left-handed weakness (Fig. 9). The MRI demonstrated restricted diffusion in the same location, which is characteristic of acute infarction. Again, patient history was crucial for the correct interpretation of CT in this stroke patient.

Always remember to use appropriately windowed images that improve soft-tissue contrast with thin sections when needed, and do not try to interpret the CT scan without knowing the patient's symptoms. The findings are so subtle that reading a brain CT scan without a history in this setting can lead to both false positive and false negative interpretations.

Now that you’re aware of these six early findings for an acute infarction on brain CT, you should be significantly more effective in detecting brain infarctions!

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