Why are computed tomography angiography (CTA) and perfusion computed tomography (CT) important imaging modalities for acute stroke patients?

Click here to delve into the clinical applications of CTA and perfusion CT for your acute stroke patients!
Last update11th Dec 2020

In this article, we will discuss two intravenous (IV) contrast computed tomography (CT) imaging techniques: CT angiography (CTA) and CT perfusion. Both CTA and CT perfusion imaging can help clinicians predict the benefit of intervention for acute stroke patients.

Computed tomography angiography (CTA) imaging

Computed tomography angiography requires rapid imaging and optimal timing after the injection of the IV contrast. This is because the CT imaging of the arteries in the brain must occur when the contrast first appears. This approach provides a surprisingly accurate rendition of the brain’s arteries and veins when performed properly!

For example, if the left middle cerebral artery (MCA) is occluded by an intravascular thrombus, the CTA image will show the patient’s normal right MCA opposite of an absent left MCA.

This example is considered a large vessel occlusion. Depending on the image timing relative to the onset of symptoms and other clinical factors, the patient could benefit from intraarterial thrombectomy.

Figure 1. Computed tomography angiography (CTA) showing an absent left middle cerebral artery (MCA), suggesting occlusion by an intravascular thrombus.

Computed tomography (CT) perfusion imaging

Computed tomography (CT) perfusion is another imaging technique that is dependent on the timing of arrival and distribution of the IV contrast within the brain. From this imaging data, calculated brain perfusion maps of regional blood flow and cerebral blood volume can be determined and displayed on images.

For acute infarcts, critical information about cerebral blood flow and time to maximal enhancement can be obtained using CT perfusion imaging.

How to use CT perfusion to weigh the benefit of an intraarterial treatment

Computed tomography (CT) perfusion is frequently used to predict the potential benefit of intervention based on the completed infarction size and brain tissue volume that is not yet infarcted (but is at-risk).

At-risk brain tissue (adjacent to the completed infarct) is called the penumbra and is marginally supplied by blood flow through the collateral arteries. Information about the infarction size and at-risk brain tissue volume, along with the patient’s history and physical exam, may be used to determine whether the benefit of an intraarterial treatment outweigh the risk.

For example, if there is evidence of a completed infarct but no evidence of other at-risk brain tissue in a patient beyond six hours from the onset of symptoms, it is unlikely that a catheter intervention will be of substantial benefit to the patient.

Figure 2. Calculated perfusion display in a patient with a left middle cerebral artery (MCA) occlusion showing the infarction (purple), and the at-risk brain tissue (green). The similarity in the surface area suggests that an intervention may not provide a substantial benefit to the patient.

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On the other hand, if the CT perfusion scan shows a relatively small and completed infarction compared to a much larger area of at-risk brain tissue, the patient might benefit from intraarterial treatment.

Figure 3. Calculated perfusion display in a patient with a left middle cerebral artery (MCA) occlusion showing the infarction (purple), and the at-risk brain tissue (green). The difference in the surface area suggests that an intervention might provide a substantial benefit to the patient.

Current approach to CT imaging of patients with acute neurological symptoms

When a patient presents with acute neurological symptoms suggestive of infarction, the current approach is to obtain a head CT alongside a CTA, and in some cases CT perfusion. Computed tomography perfusion is suggested for patients who present beyond the usual treatment time window.

In the case presented next, volume rendering of the patient’s CTA demonstrated a branch MCA occlusion. In this case, it was proven on digital subtraction angiography (DSA) and treated through a catheter which resulted in a complete restoration of flow to the M2 branch.

Figure 4. A middle cerebral artery (MCA) branch occlusion before and after treatment. The volume rendered computed tomography angiography (CTA) demonstrates the location of the occlusion, which is proven on digital subtraction angiography (DSA). The treatment resulted in a complete restoration of flow to the M2 branch.

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Recommended reading

  • Albers, GW, Marks, MP, Kemp, S, et al. 2018. Thrombectomy for stroke at 6 to 16 hours with selection by perfusion imaging. N Engl J Med378: 708–718. PMID: 29364767
  • Barber, PA, Demchuk, AM, Hudon, ME, et al. 2001. Hyperdense sylvian fissure MCA "dot" sign: A CT marker of acute ischemia. Stroke32: 84–88. PMID: 11136919
  • Jensen-Kondering, U, Riedel, C, and Jansen, O. 2010. Hyperdense artery sign on computed tomography in acute ischemic stroke. World J Radiol2: 354–357. PMID: 21160697

About the author

Alexander Mamourian, MD
Alex is a Professor Emeritus of Radiology at the University of Pennsylvania.
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