Oftentimes, in the first few hours after a brain infarct, there are no corresponding abnormalities visible on computed tomography (CT). While acute infarctions may be difficult to initially find on CT, they tend to become more evident over time.
Using a series of brain CT scans from ten patients, you’ll learn about the appearance of several types of infarcts (pre- and post-craniectomy), and how acute infarctions evolve over time.
Acute infarction case 1: Right hemispheric infarction
A CT scan was obtained from a patient who presented to the emergency room after developing an acute onset of left-sided facial weakness, suggesting a right hemispheric infarction. On the initial CT scan, there was some blurring of the lateral border of the basal ganglia, but the right insular cortex appeared to be normal.
On a CT scan 3 days after the incident, the patient’s right-sided infarction became clearly evident, with a well-defined zone of low attenuation involving the patient’s right hemispheric cortex and white matter. The influx of fluid into the damaged brain cells lead to cytotoxic edema, which is visible on CT as regional low attenuation.
Cytotoxic edema accounts—in part—for the difficulties encountered in the medical treatment of infarctions. The swelling that initially occurs after a brain infarction is largely intra-cellular. Intra-cellular edema is not as responsive to steroids as vasogenic edema, which is extra-cellular and accompanies most primary and metastatic tumors.
By 5 days after the incident, the full extent of the infarction became visible as low attenuation in the cortical and subcortical white matter when compared to the other side of the brain. The infarction-induced swelling results in the loss of the cortical sulci and the Sylvian fissure when compared to the patient’s unaffected side.
But 11 days after the incident, the patient’s infarction became less evident. While it would be nice to think that this indicates that the brain has recovered due to treatment, this is one pattern of infarction evolution that is deceptive and infrequently encountered. The apparent improvement is due to a phenomenon called fogging.
Like the fog that rolls in from the sea and makes it hard to see the landscape, the fogging of brain CT in this circumstance is caused by a combination of revascularization, inflammatory cells, and hemorrhaging in the infarcted tissues.
This results in a small increase in the attenuation of the infarcted cortex. Since the infarct is normally lower in attenuation than the normal brain, the infarction will then appear similar to the normal brain.
After a few more days, the infarction will reappear and then eventually involute and contract so that the infarction territory will have an attenuation that resembles fluid on CT. This late phase of a chronic infarction is usually called encephalomalacia.
Acute infarction case 2: Perinatal infarction
The next case demonstrates the late effect of perinatal infarctions on the brain of a child. In this patient, there were multiple areas of fluid attenuation in the cortex and central structures that can be described as macrocystic encephalomalacia (Fig. 3).
Acute infarction case 3: Acute left-sided hemiparesis without aphasia
A CT scan of a patient with acute left-sided hemiparesis without aphasia demonstrated several of the subtle changes expected with an early middle cerebral artery (MCA) territory infarction. This includes a dense MCA sign.
At a higher image level, asymmetry of the basal ganglia can be seen with the normal left lentiform nucleus visible since it has a higher attenuation than the surrounding white matter. However, the patient’s right lentiform nucleus is poorly seen. Early changes due to ischemia cause it to resemble white matter.
A CT scan taken 2 days later better demonstrated the right basal ganglia infarction. You will note that there is sparing of the right thalamus, since it is supplied by arteries arising from the posterior circulation, and not the MCA branches that supply the basal ganglia.
Acute infarction case 4: Small left MCA territory infarction
This patient’s early CT scan demonstrated a small left MCA territory infarction (Fig. 5). On a CT scan 2 weeks later, linear high attenuation in the cortex was visible in the same distribution, but with no mass effect on the cortical sulci.
High attenuation in the cortex in the days to weeks after an infarction is common and often due to fine petechial hemorrhages. It does not carry the same treatment implications as a true hemorrhagic infarction.
Acute infarction cases 5 and 6: Hemorrhagic infarcts
The next cases show CT scans from two different patients, both illustrating the appearance of hemorrhagic infarctions (Fig. 6). Hemorrhagic infarctions are associated with a larger area of high attenuation along with the loss of cortical sulci, indicating some mass effect. The use of anticoagulation and revascularization therapies may increase the likelihood of bleeding into an infarction, but significant hemorrhaging occurs in less than 10% of infarctions.
Acute infarction case 7: A small infarction escalating to a large hemorrhage with mass effect and shift
The next case involves a relatively small infarction, visible on CT by its focal loss of the gray-white border and cortical sulci. However, just 6 hours later, this patient showed worsening of symptoms and a second CT scan was obtained (Fig. 7).
The second CT scan, performed at the same level, shows a large hemorrhage in the region of the infarction with increased mass effect and shift, indicated by the displaced septum pellucidum relative to the midline of the skull. This focal hemorrhage is an important finding and urgent notification of clinical services is required since the finding alters subsequent treatment.
Acute infarction case 8: Right-sided MCA territory infarction
This patient case involves a large right-sided MCA territory infarction, with diffuse swelling and loss of the cortical sulci over the entire right hemisphere when compared to the normal left side (Fig. 8). The CT scan showed displacement of the ventricle and midline structures due to a large area of cytotoxic edema.
Acute infarction cases 9 and 10: Large vascular territory infarctions
Because of the limitations of medical treatment for infarctions and expected swelling within the first week post-infarction, patients with large vascular territory infarctions (typically involving the MCA) may require removal of the skull for survival. This procedure is called a craniectomy. The patient’s skull flap is frozen and is not replaced until the infarct swelling subsides.
When the swelling is severe in the first week after an infarction, it can result in brain herniation and death. This is certainly a risk with occlusion of vessels that supply a large volume of the brain. Infarcts with brain swelling and substantial midline shift are sometimes called malignant infarctions, because of the poor prognosis for the patient.
The next case example is a patient who survived a right-sided carotid occlusion with infarction after treatment with a craniectomy. At 3 weeks after the infarction, the swelling has subsided and there is very little mass effect evident. While there is evidence of a prior right-sided infarct, there is no midline shift.
After craniectomy and resolution of brain swelling, the bone flap can then be reattached to the skull.
While acute infarctions are frequently difficult to initially detect on CT, they appear more visible upon subsequent scans over the days and weeks to follow.
With the knowledge gained from these case studies, you will find it easier to recognize the evolution of infarcts on brain CT scans over time.
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