Intraarterial thrombectomy or thrombolysis computed tomography (CT) imaging essentials

Computed tomography (CT) scans can be useful for identifying complications after thrombectomy. So, let’s examine pre- and post-thrombectomy CT scans to illustrate the benefits of this type of imaging!

Let’s take a look at an example. A patient presented with acute onset of right hemiparesis and aphasia. Their brain CT showed an abnormally high attenuation in the left carotid artery, suggesting a blood clot or calcified embolus within the vessel.

A coronal reconstruction from a CT angiography (CTA) shows an absence of contrast enhancement in the distal left carotid artery (Fig. 1). You can see that the proximal anterior cerebral artery (ACA) and middle cerebral arteries (MCA) are also involved but are enhanced distally. This is most likely the result of filling by collateral flow or a small amount of contrast getting beyond the thrombus.

There is an occlusion of the distal internal carotid artery along with the proximal anterior and middle cerebral arteries, known as a T occlusion because it resembles that letter on a coronal or frontal projection.

Based on these findings, this patient is diagnosed with acute brain infarction from a large vessel occlusion, so decisions about treatment need to be made immediately.

For many patients who present immediately after the onset of symptoms, there is a medical treatment option using an intravenous drug used to dissolve the blood clot. But, in patients with large vessel occlusion and late presentation, it is usually best to restore flow and remove the thrombus with a catheter intervention called intraarterial thrombectomy or thrombolysis.

Intraarterial thrombectomy is appropriately named since current devices allow for the extraction of the clot within the artery. The data suggests that restoration of flow improves clinical outcomes after infarctions when performed in selected patients—even after 12 or more hours have elapsed since the time of symptom onset!

There are currently a variety of devices that can be delivered to the point of vascular occlusion through a long catheter inserted into an arm or groin artery that can be precisely positioned using x-ray fluoroscopy. These include a metal stent that can be placed into the clot and then recovered into the catheter and a small suction catheter that directly removes the clot.

For example, the digital subtraction angiography (DSA) image of a patient confirmed an occlusion of the distal carotid artery. After the placement of a treatment catheter into the cavernous carotid artery, an interventional device was used to reopen the distal internal carotid artery and MCA. This procedure, performed urgently, restored flow to the left MCA and the proximal ACA.

Let’s take a look at another example. After presenting with left-sided weakness, the patient in our next case had a CTA scan, which demonstrated a large vessel occlusion (Fig. 4). The patient was sent directly to the angiography suite for treatment of their complete occlusion of the right proximal MCA, called the M1 segment.

After intraarterial thrombectomy, blood flow was restored to the MCA. While catheter-based vascular intervention has been available for decades, recent improvements in hardware and techniques make it possible to open large intracranial arteries rapidly—frequently in 20 minutes or less from the start of the procedure!

Shortly after the patient’s thrombectomy, a CT scan was obtained. On the CT, a focal area of high attenuation was seen in the right basal ganglia. While alarming in appearance (since it resembles blood), high attenuation is frequently seen in the territory of an occluded vessel after an intervention.

This finding on CT post-thrombectomy is frequently due to contrast staining of the brain from repeated injections of contrast during the procedure. However, the question in this case remained as to whether this could represent a hemorrhage.

Repeat the scan 24 hours later

One way to tell the difference between high attenuation that’s due to contrast staining and high attenuation that’s due to a hemorrhage is by repeating the CT scan 24 hours later.

In the case of the patient who presented with left-sided weakness, the CT scan was repeated after 24 hours. The persistent high attenuation visible on this repeat scan indicates that there was some hemorrhaging in the region of the infarction, since contrast staining should have largely resolved at 24 hours.

Obtain a virtual non-contrast CT scan

Another way to tell whether the high attenuation on a CT scan after thrombectomy is due to a hemorrhage is by obtaining a virtual non-contrast CT scan. In our next case, a CT scan was obtained immediately after the patient’s left MCA occlusion was treated with intraarterial thrombectomy. This CT also demonstrated high attenuation in the basal ganglia.

The hospital had a dual-energy CT scanner available for imaging that was used to obtain the post-procedure CT scan. By imaging the brain simultaneously with x-rays of two different energies (low and high energy), it became possible to predict whether the high attenuation was due to contrast staining or hemorrhaging. This is based on the differences in attenuation of iodine and blood.

The dual-energy data set allows for the creation of a processed image called a virtual non-contrast scan on which iodine is effectively removed from the image. In this patient’s case, the high attenuation in the left basal ganglia was not visible on the virtual non-contrast image, indicating that it was due to contrast staining, and not a hemorrhage (Fig. 7).

The role of intraarterial thrombolysis or thrombectomy in the treatment of acute brain infarction with large vessel occlusion is expanding as more hospitals are offering this service to their patients. Keep in mind that both hemorrhage and contrast staining can be evident on a post-procedure CT scan.

When there is a question about hemorrhaging, repeat imaging with CT 24 hours later is helpful since contrast staining resolves more quickly than a hemorrhage. For a faster diagnosis, however, dual-energy CT can help make this distinction.

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