Stroke and its imaging
Presently the whole world is engulfed in the Covid-19 pandemic. It is still in increasing trend in India with more than fifty thousand new cases being added on a daily basis. Covid-19 is being presented in many different ways, and one of the manifestations is its presentation with an acute ischemic stroke.
When patient is presented with an acute ischemic stroke, he/she is subjected to a CT scan or an MRI imaging. There is a lot of debate on whether a CT scan or an MRI should be performed. However, both the imaging modalities are good enough to decide a future treatment protocol.
If a patient goes for a CT scan then a plain CT and a CT angiography with a delayed scan is performed. If a patient goes for an MRI then a DWI, T2 FLAIR, T2* gradient echo and TOF MRA sequences are performed. It is possible to predict composition of the clot in both the CT scan and the MRI.
The density of a clot in a plain CT scan and Blooming Artifact caused by clot in gradient-echo MRI gives insight into composition of the clot.
Stroke Imaging Predict clot composition
When clots retrieved via a mechanical thrombectomy, were examined in histopathology lab, it was found to consist of RBCs, WBCs and fibrin/platelets. Accordingly clots can be categorized into three types:
- RBC rich/dominant,
- Fibrin/platelets rich/dominant, and
- Mixed type
In an RBC rich clot, RBCs were outnumbered by >15% than fibrin/platelets. In fibrin/platelets rich clot, fibrin/platelets outnumbered by >15% than RBC. In mixed type, both were not the case.
Now coming back to imaging, a clot may appear hyperdense in a plain CT scan, which is commonly known as Hyperdense Middle Cerebral Artery Sign (HMCAS). Such clots which appear hyperdense in a plain CT scan were found to be rich in RBCs (after they were removed by a mechanical thrombectomy and sent for Histopathology). The mean percent of RBC composition was higher in these clots (47% Vs 22%). This can be attributed to Fe present in Haemoglobin in the RBCs, which can result in increased density on a CT scan.
In an MRI, a clot may show a Blooming Artifact (BA) on a gradient-echo imaging. When the clot showing significant BA on the MRI is removed and sent for Histopathological examination, it was found to be rich in RBCs. The mean percent RBC was greater with BA (42% Vs 23%). This can be explained by magnetic-susceptibility artifacts caused by Fe in hemoglobin present in the RBCs.
Predict etiology of stroke
Stroke can either be Cardioembolic, Thrombotic, or due to undetermined causes. Cardioembolic stroke occurs when a small clot is dislodged from the heart or greater vessels and it gets lodged in an intracranial artery like middle cerebral artery or basilar artery. Thrombotic stroke occurs due to formation of thrombus in intracranial artery. In such cases, the underlying artery is already diseased with changes of atherosclerosis.
Clot composition can help in determining whether a stroke is cardio embolic or thrombotic. It is found that a clot in a cardio embolic stroke is rich in RBCs. The area occupied by RBC is:
- 38% in cardio embolic stroke,
- 23% in thrombotic stroke and
- 26% in undetermined causes
Accordingly, a clot in a thrombotic stroke is fibrin/platelet rich. The area occupied by fibrin/platelets is:
- 71% in thrombotic stroke,
- 59% in cardio embolic stroke and
- 68% in undetermined causes.
Thus indirectly, HMCAS and BA signs which suggest that clot is rich in RBCs, also suggest that it is cardio embolic.
According to new stroke guidelines, mechanical thrombectomy is first line of treatment in large vessel occlusion in an acute ischemic stroke. Recanalization rates range from 70-80%. However, even after several attempts, recanalization fails in 20-30%. It is found that clots rich in RBCs tend to come out easily than those rich in fibrin/platelets.
Clot composition in successful recanalization is found to be 37% RBCs and 60% fibrin/platelets.
Clot composition in unsuccessful recanalization is found to be 20% RBCs and 74% fibrin/platelets.
So, coming to the imaging part, HMCAS and BA signs which suggest an RBC rich clot, can indirectly predict which clot will come out easily.
Also, when we compare clot composition with the technique of mechanical thrombectomy, we find some difference in the composition of the clot removed by suction and by stent retrievers.
Clots removed by suction are rich in RBCs (35% RBCs, 61% fibrin/platelets), while those removed by stent retrievers are rich in fibrin/platelets (24% RBCs, 72% fibrin/platelets).
To sum up, we can say that HMCAS sign on a plain CT scan and BA signs on a gradient-echo MRI can suggest that the clot is rich in RBCs. This indirectly tends to suggest that this is more of a cardio embolic type and it’s more likely for a recanalization after a mechanical thrombectomy, especially by suction.
However, there is no random trial on this. Sometimes HMCAS and BA can be present in fibrin rich clots and thrombotic occlusion. Also, the rate of recanalization in fibrin rich clots is also high enough for a mechanical thrombectomy to be consider as its first line of treatment in a large vessel occlusion.
- Liebeskind, D. S., Et al. (2010). CT and MRI Early Vessel Signs Reflect Clot Composition in Acute Stroke. AHA Journal,1238-1243. doi:10.1161/STROKEAHA.110.605576
- Shin JW, Jeong HS, Kwon H-J, Song KS, Kim J (2018) High red blood cell composition in clots is associated with successful recanalization during intra-arterial thrombectomy. PLoS ONE 13 (5): e0197492. https://doi.org/10.1371/journal. pone.0197492