Fig. 9.1
ASPECT scoring system. The middle cerebral artery territory is divided into seven ganglionic level regions (caudate, internal capsule, insula, and lentiform nucleus, M1–3) and three supraganglionic level regions (M4–6) (a). For posterior circulation stroke, two points each are allocated to midbrain and pons, and one point is allocated to the thalamus, posterior cerebral artery (PCA) territory, and cerebellar hemisphere on each side (b). ASPECTS indicates Alberta stroke program early CT score
Another method for assessing EIC was assessing whether >1/3 of the MCA territory was involved or not. The European Cooperative Acute Stroke Study (ECASS-1) used “one-third” of MCA distribution as patient selection criteria. This trial showed that in patients with NCCT hypoattenuation affecting one-third or less of the MCA distribution, administration of intravenous alteplase resulted in patients achieving good outcome at 90 days (odds ratio [OR] 3.43; 95% confidence interval [CI] 1.61–7.33). This was in contrast to patients with hypoattenuation more than one-third of the MCA distribution who did not achieve statistically significant good outcomes when compared to patients who did not receive intravenous alteplase (OR 1.27; 95% CI 0.82–1.95). Of note, however, estimating EIC extent using the one-third rule had modest to poor reliability (κ-value of 0.39; 95% CI 0.29–0.49).
9.1.2 Posterior Circulation Alberta Stroke Program Early CT Score (pc-ASPECTS)
The ASPECTS was developed for anterior circulation ischemic strokes. A posterior circulation ASPECTS was developed in 2007 for posterior circulation ischemic strokes on CT angiography-source images. This choice of modality was because non-contrast CT was not sensitive enough to detect EIC in brain parenchyma supplied by the posterior circulation. Similar to ASPECTS, pc-ASPECTS has a total of ten points. Two points each are allocated to midbrain and pons, and one point is allocated to the thalamus, posterior cerebral artery (PCA) territory, and cerebellar hemisphere on each side (Fig. 9.1b). Dichotomizing pc-ASPECTS at >7 vs <7 was able to predict patients who had a favorable functional outcome, using CTA-source images [2].
9.1.2.1 Hyperdense Artery Sign
In about a third of intraluminal MCA thrombus NCCT scans, a hyperdense MCA sign (HDMCAS) can be seen. In patients with higher hematocrit levels, dense MCA might be seen even within normal vessels; therefore, comparison of both MCAs is crucial. MCA “dot” sign can be identified in the Sylvian fissure affecting the M2/M3 branches of the MCA there (Fig. 9.2a). HDMCAS and MCA “dot” sign are relatively insensitive, with sensitivity of around 50% and 40%, respectively. However, they are both specific with their specificities reported to be 95% and 100%, respectively. Thin slice non-contrast CT scans (slice thickness <2.5 mm) in comparison have higher sensitivity and specificity for detecting intraluminal thrombus than non-contrast CT scans with average slice thickness >2.5 mm.
Fig. 9.2
Hyperdense MCA sign. Hyperdense MCA sign or MCA “dot” sign can be identified in the Sylvian fissure (white arrow in a) affecting the territory of M2/M3 branches of the MCA (arrowheads in b). MCA indicates middle cerebral artery
Of note, the hyperdense artery sign can be detected in the ICA, basilar artery, vertebral artery, and the posterior cerebral artery. The ability of thin slice non-contrast CT to detect proximal intraluminal thrombi with high sensitivity and specificity makes this imaging modality an important tool when deciding if a patient should be triaged for endovascular therapy [3].
9.2 CTA
Six recent randomized controlled trials (MR CLEAN, ESCAPE, Extend-IA, SWIFT-PRIME, REVASCAT, and THRACE) have shown that endovascular thrombectomy (with or without intravenous tPA) is superior to current medical treatment [4, 5]. All of these trials used vascular imaging to determine patients’ eligibility, with the majority undergoing CTA [6]. The CTA can be used to assess the following characteristics.
9.2.1 Intravascular Thrombus
Vascular imaging with CTA (or MRA) is the gold standard in detecting intravascular thrombi. The presence, extent, and location of the thrombi have diagnostic, therapeutic, and prognostic implications in almost every patient with AIS. Detecting the presence of an intravascular thrombus is the first step in assessing an AIS patient with a CTA. Patients with intravascular thrombi visualized on CTA have more severe clinical presentation (higher NIHSS). They are also more likely to benefit with thrombolysis or thrombectomy. The absence of intracranial thrombus on a CTA head however does not rule out ischemia [7].
A proximal thrombus, affecting the internal carotid artery (ICA), or the first segment of the MCA (M1), has a lower rate of recanalization with intravenous tPA in contrast to a more distal thrombus. These patients with proximal thrombi are more likely to benefit from endovascular thrombectomy (EVT). A recent meta-analysis shows that EVT is efficacious in patients with proximal intracranial occlusions when an arterial puncture is initiated within 7.3 h from onset of symptoms [8].
The length of the thrombus on CTA is an independent predictor of recanalization following tPA treatment. Thrombi >15 mm in length are less likely to reanalyze early with intravenous tPA in comparison to shorter thrombi. Length or extent of the clot can be measured semiquantitatively using the clot burden score (CBS). Similar to ASPECTS, the CBS is a 10-point scoring system with a score of 10 indicating absence of thrombi from ICA, M1, M2, and anterior cerebral artery (ACA). CBS of 0 would indicate a thrombus extending from involving all the aforementioned vessels (Fig. 9.3a). Patients with low CBS are less likely to benefit with intravenous tPA and therefore are better treated with EVT [9]. Intracranial thrombi that are porous (permeable) are more likely to dissolve early with intravenous tPA. Permeability of the thrombus can be assessed on CTA by looking for the presence of contrastation through the thrombus. A helpful technique to assess permeable thrombi is shown in Fig. 9.3b [10].
Fig. 9.3
Assessment of clot burden. The clot burden score is a 10-point scoring system with a score of 10 indicating absence of thrombi from ICA, M1, M2, and anterior cerebral artery (ACA) and a score of 0 indicating a thrombus extending from involving all the aforementioned vessels (a). Permeability of the thrombus can be assessed on CTA by looking for the presence of contrastation through the thrombus (b). ICA indicates internal carotid artery, CTA computed tomography angiography, HU Hounsfield unit
9.2.2 Collateral Status
Collaterals supply the brain parenchyma through smaller arteriolar connections from the pial blood supply (leptomeningeal) or through larger arteriolar/arterial connections within the circle of Willis (Willisian) to the blood vessels supplying the brain. These collaterals provide retrograde filling to the blood vessels distal to the occluded segment of the intracranial artery. There is sufficient evidence that the presence of good collateral supply is associated with better clinical and radiological outcomes in AIS. Collateral imaging was used for patient selection in the ESCAPE trial; patients with poor collaterals on CTA were excluded (Fig. 9.4a) [11].
