Neuroimaging of Ischemic Stroke With CT and MRI

Ischemic Stroke Classification and Outcomes Major or Minor stroke based on whether a proximal cerebral artery is occluded. Minority of strokes (~25%) which present with large cerebral vessel occlusion and are classified as major strokes are responsible for more than 70% of poor outcomes (death, lengthy hospital stays and discharge to in-patient rehabilitation facilities).

Key questions Is there a hemorrhage? Is a major artery occluded? What part of the brain is irreversibly injured (core)? Is there an ischemic penumbra?

Is There Hemorrhage? CT and MRI can detect clinically significant parenchymal hemorrhages; however CT remains superior in detecting acute subarachnoid hemorrhage

Hypodensidity hypodensity on CT within 3 h of symptom onset, 31% of patients with stroke had early ischemic signs, loss of the gray-white matter distinction (27%) diffuse areas of hypoattenuation (9%). Patients with such signs had a high probability of having an infarct on follow-up (positive predictive value [PPV] 87%)

ECASS-II-up to 6 h after onset, the incidence of CT hypoattenuation (47%) was higher than in the NINDS tPA trial, and the PPV for brain infarction was 96% patients without early CT changes were less severely affected, developed smaller infarcts, had fewer intracranial hemorrhagic events, and had a better clinical outcome at 30 days than did patients with early changes.

In the first few hours after onset of ischemia, CT hypoattenuation is most difficult to detect when it occurs in the posterior fossa, or when the patient has chronic ischemic changes or areas of leukomalacia Focal and diffuse brain swelling can be seen as sulcal effacement and compression of the cerebrospinal fluid (CSF) spaces. (In the NINDS tPA study, 14% of patients displayed this effect)

the hyperdense artery sign On noncontrast CT, a hyperdensity within the proximal intracranial arteries—the hyperdense artery sign— represents acute thromboembolism. This sign can be detected in the middle cerebral artery (MCA), as well as in the posterior cerebral and basilar arteries. A thrombus in the Sylvian branches of the MCA can appear as a bright dot. The absence of hyperdense artery signs does not imply that a thrombus is not present. False positives can result from atherosclerotic calcifications, although in such instances the hyperdense vessels are usually bilateral.

hyperdense MCA sign A hyperdense MCA sign (HMCAS) is found in 40-50% of patients with an MCA infarction. 36 h after treatment with intravenous alteplase for such strokes, the HMCAS disappears in almost 50% of patients, and these individuals have the best outcomes. After 2 weeks, the HMCAS is no longer seen in 95% of patients, even when thrombolytics are not used.

Is a Major Artery Occluded? CTA more reliably produces high-quality angiographic information of both the head and neck vessels in less than a minute using modern multidector CT instruments

How Large Is the Irreversibly Injured Core? Patients presenting with cerebral infarct volumes of greater than 70-100 ml in the anterior circulation, or involving bilateral aspects of the pons or thalamus in the posterior circulation are likely to have poor outcomes despite therapeutic intervention. the signal abnormality observed on diffusion MRI is the best early marker of the infarction core A clearly visible hypoattenuation on the noncontrast CT scan rarely reverses. Early in ischemic stroke, the findings on noncontrast CT are usually normal Probably the best CT-based marker for the infarct core is a low-blood-volume abnormality identified in cerebral blood volume maps derived from CT perfusion data

Is There a Clinically Significant Ischemic Penumbra? CT and MR provide information of similar quality, however, MRI can evaluate a larger proportion of the brain and for that reason it is the superior method at this time

Evidence-based Recommendations for Use of tPA For eligible patients, intravenous tPA should be administered intravenously with a dose of 0.9 mg/kg (maximum of 90 mg), with 10% of the total dose administered as an initial bolus and the remainder infused over 60 min, provided that treatment is initiated within 3 h of clearly defined symptom onset The overall maximal benefit of tPA is unlikely to exceed more than one out of 100 patients. The maximal theoretical impact of tPA is thus quite small

this drug had a clear statistically significant benefit only in patients presenting with NIHSS scores of 6-10. These are patients with moderate neurological deficits, and generally result from occlusions of smaller arterial branches. only patients with lower NIHSS scores and the absence of proximal, major artery occlusions benefited with tPA. intravenous tPA is less effective in recanalizing occlusions of major cerebral arteries that produce major neurological symptoms

intravenous tPA administration resulted in recanalization of approximately 10% of distal internal carotid artery occlusions and approximately 25% of M1 occlusions. Occlusions at the level of M2 were recanalized at a rate of less than 40%. All available data indicate that intravenous tPA is not very effective in patients with proximal artery occlusions and patients who have major neurological deficits and are at high risk for death or poor outcomes. In sum, the available evidence indicates that intravenous tPA can be expected to benefit perhaps one out of 100 patients with ischemic stroke, and most commonly in patients with less-severe strokes

Intra-arterial Thrombolysis IAT may be of benefit within the first 6 h of onset in the treatment of carefully selected acute ischemic stroke patients secondary to occlusion of the MCA and are not eligible to receive intravenous thrombolysis.

Mechanical Thrombolysis

Advanced Neuroimaging to Guide Stroke Therapy The size of the core may be estimated with high probability with diffusion MRI and also with CT perfusion, albeit with more difficulty. A clinically useful approximation of the clinically relevant ischemic penumbra is available with perfusion imaging using both CT and MRI

Patients without a significant penumbra are unlikely to benefit from therapy designed to restore blood flow, even if the patient is within the time window for such therapy. Great caution should be exercised before recanalization treatment is implemented in a patient who has evidence of a large core (on the order of 100 ml or larger) especially by diffusion-weighted imaging. Finally, while treatment may benefit some, patients with a small core/penumbra abnormality are unlikely to benefit from treatment designed for revascularization of an occluded artery

Ischemic Stroke Triage to treat patients with mild-to-moderate symptoms (NIHSS ≤ 10) in the traditional fashion with tPA following a noncontrast CT that shows no hemorrhage, if they meet the other inclusion criteria for this treatment Level II centers would have expertise in the administration of intravenous tPA in the emergency department, along with 24 h availability of CT