Identifying the early CT findings of infarctions

In this video, we'll review the six findings on CT that support the diagnosis of an acute infarction, the three things you must do when looking at a CT scan, and the super-early clues that predict the future onset of an infarction.

Alexander Mamourian, MD
Alexander Mamourian, MD
29th Sep 2020 • 5m read

"Stroke" symptoms can often be caused by a hemorrhage or a brain tumour. So how do you know when your patient has had an acute brain infarction? In this video, from our Brain CT Essentials course, we'll review the six findings on CT that support the diagnosis of an acute infarction, the three things you must do when looking at a CT scan, and the super-early clues that predict the future onset of an infarction.

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Video transcript

The term stroke is used to describe the acute onset of neurologic symptoms, such as difficulty with language or unilateral weakness. But it does not mean that in all cases, the patient has had an acute brain infarction. In some cases hemorrhage or brain tumor can cause similar symptoms. So one of the goals in the initial CT evaluation is to determine if the patient has evidence of a brain hemorrhage, like the one seen in this patient. intracranial blood can be the result of an underlying vascular lesion, venous thrombosis or brain tumor.

And while hemorrhage can be secondary to brain infarction, that is rarely the case. When hemorrhage is evident on the CT scan obtained immediately after onset of symptoms. In this Medmastery lesson, I will review six findings on CT that support the diagnosis of an acute infarction. But you must know the nature of the patient's symptoms so that you know where in the brain to look, since these are frequently subtle findings on CT.

When there's thrombus in the middle cerebral artery, it will appear unusually white on CT, because clotted blood has a higher attenuation than flowing blood. Why is this sign associated with the middle cerebral artery? That's because it requires comparison with an artery on the other side. For example, if the 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, however, diagnosis based on its attenuation is less reliable because there's no comparison. To optimize your ability to make the diagnosis of an early infarction, you should do three things when viewing the CT scan: first, know the symptoms so you know which side to look for this sign, two, use the other side as a reference and three, look at the thin reconstructions and window the soft tissue CT images appropriately.

On this scan of a patient with an acute left sided hemiparesis, the right middle cerebral artery appeared to have slightly higher attenuation than the left. This image is a standard five millimeter thick reconstruction of the patient's CT scan data. However, since most modern CT scanners can acquire data using one millimeter or less detector collimation, this one millimeter axial CT image could then be reconstructed from the same CT data of the patient.

This reconstruction clearly shows that the patient's right middle cerebral artery is unusually high in attenuation, and this is called the dense MCA sign, you'll have a better 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.

This scan demonstrates the dot sign, a variation of the dense middle cerebral artery sign. It indicates a thrombus in the distal middle cerebral artery branches that lie within the sylvian fissure, as indicated by the blue arrow, where they turn perpendicular to the slice. Medial to the sylvian fissure, there should be a thin white line of cortex seen here on the patient's left.

But when this line is absent, and in the appropriate clinical setting, it's an early sign of an acute middle cerebral artery occlusion that is called the insular ribbon sign. Note the change in appearance in this patient CT scan just one day later. That early insular ribbon sign was just a hint of what was to come, and this scan demonstrates the true size of the infarction.

It should serve as a reminder that the detection of acute infarcts on CT depends on recognition of subtle changes in tissue attenuation. In this patient with an acute left-sided weakness, notice that the right putamen that lies between the blue arrows is harder to see than the normal putamen on the patient's left.

Asymmetry in the basal ganglia is another early sign of the middle cerebral artery territory infarction. Since the small arteries that supply blood to the basal ganglia arise from the proximal middle cerebral artery. On this image, the arrows indicate focal loss of the normal borderline between the gray matter and the white matter. Look carefully at the other side to see what that interface should look like.

In a normal patient, you should be able to follow the relatively high attenuation cortical ribbon all the way around the hemispheres on a good quality CT scan. But there's no question this is a subtle finding, and your chances of seeing it will go up if you know where to look for it based on the patient's symptoms.

The infarct is now more clearly seen on the CT scan obtained one day later. Focal low attenuation in the cortex is an important finding to recognize in patients with suspected infarcts. A cerebellar infarct was evident on this patient CT scan that proved to be secondary to a right vertebral dissection on vascular imaging.

But this finding is often inapparent or artifactual, especially if there's age-related atrophy of the cortex. These two scans are of different patients and both demonstrates small acute cortical infarcts. While the infarction is quite evident on the left image, it's difficult to perceive on the right image. But once you know that the patient had symptoms of right-hand weakness, you're more likely to find it.

The low attenuation evident in the cortex in this patient proved to be reproducible on multiplanar reconstruction and lies within the left pre-central gyrus, precisely where hand motor function resides. And do not routinely attribute low attenuation in the basal ganglia to an old lacunar infarction unless you have prior CT or MR scans that show it.

This finding on CT was initially thought to indicate an old lacunar infarction. But, because of the patient's acute left-sided weakness, a magnetic resonance scan with diffusion images was obtained the same day. That study demonstrated restricted diffusion in the same location that is characteristic of an acute infarction.

Again, history was crucial for correct interpretation of CT in this stroke patient. Now you know what to look for, you should be more effective in detecting early findings on CT of brain infarction. And remember to use appropriately windowed images that improve soft tissue contrast, thin sections when needed, and do not try to interpret the CT scan without knowing the patient's symptoms.

The findings are sufficiently subtle that reading a CT scan without history in this setting can lead to both false positive and false negative interpretations.