CME INDIA Presentation by Dr. K. Mugundhan, MD., DM (Neuro)., FRCP (Glasg, Lond, Edin & Ire), FACP, FICP, Professor of Neurology and Head, Stanley Medical College, Chennai.

Based on a presentation at APICON 2024, New Delhi.

Time sensitivity is paramount

Time sensitivity is paramount in treating patients with acute stroke. Given its accessibility, ease of use, and rapid acquisition, physicians should prioritize CT as the primary imaging tool in these cases.

Noncontrast head CT and CT angiography (CTA) of the head and neck are indispensable modalities in acute stroke patients, aiding in diagnosis, prognosis determination, and identification of candidates for acute stroke therapy. Advanced techniques like multiphase CTA or CT perfusion (CTP) offer further support in identifying salvageable brain tissue and evaluating treatment risks. Acute stroke management demands a blend of art and science, necessitating swift assessment of clinical and imaging data to make timely clinical decisions.

  • In effectively managing acute stroke patients, swift clinical assessment and rapid interpretation of imaging are of utmost importance.
  • Imaging serves as a pivotal tool in acute stroke management, facilitating accurate diagnosis, predicting outcomes, and informing the selection of appropriate treatment strategies.

Imaging

  • Noncontrast CT.
  • CT angiography.
  • Magnetic­ resonance imaging (MRI), and magnetic resonance angiography (MRA).
  • Perfusion imaging (CT and MRI).

CT is the workhorse of acute stroke imaging

  • Speed and ease of acquisition.
  • 24/7 availability.
  • Lower cost.
  • Relative absence of contraindications when compared to MRI.

On CT the mean HU value of ischemic stroke is-

Experts calculated the image contrast for each scan by comparing the mean Hounsfield Unit (HU) values of 1–5 consecutive voxels (the contrast window width) belonging to the ischemic lesion with those of the normal surrounding parenchyma.

Normal ≥ 29 HU

Hyperacute~ 25-29 HU

Acute 23-26 HU

Subacute 20-23 HU

Chronic ≤ 20 HU

Neuroimaging in Acute Stroke - What Physicians Must Know?

Courtsey: http://www.neuroradiologycases.com/2011/11/superior-cerebellar-territory-infarct.html

Imaging Features on Noncontrast CT to Rapidly Assess Patients Presenting with Acute Stroke

  • Any intracerebral, intraventricular, subarachnoid, subdural, or extradural hemorrhage.
  • Eye deviation.
  • Hyperdense artery sign.
  • Early ischemic changes (obscuration of lentiform nucleus, insular ribbon sign, loss of gray-white differentiation of surface cortex).
  • Alberta Stroke Program Early CT Score (good 8-10, intermediate 5-7, poor 0-4).
  • The primary purpose of non-contrast CT in patients with acute stroke is to rule out a hemorrhagic stroke and to identify imaging features that may suggest the presence of an ischemic stroke.
  • It provides images of the skull, brain parenchyma, ventricles, and adjacent structures.

Ischemia  

  • Severe cerebral ischemia (blood flow<10mL/100 g/min) causes brain tissue water to shift from the extracellular to the intracellular space, thus resulting in hypoattenuation on CT.
  • With severe ischemia, gray matter becomes edematous, loses signal, and starts to become darker on CT.
  • This results in blurring and then loss of normal gray-white matter differentiation.
  • These changes, called early ischemic changes, can be seen as early as 1 hour after stroke onset.
  •  These changes are often considered as evidence of irreversibly infarcted brain.

The classic radiologic signs of early ischemic change seen on CT

  • Obscuration of the lentiform nucleus.
  • Insular ribbon sign (loss of gray-white matter differentiation at the insula).
  • Cortical ribbon sign (loss of gray-white matter differentiation at the surface cortex).
  • Hyperdense artery sign.

What is this?

Neuroimaging in Acute Stroke - What Physicians Must Know?
  • Lentiform Nucleus Obscuration Insular Ribbon Sign.

Hyperdense Artery Sign.

  • A thrombus within the intracranial arterial tree is often seen on the noncontrast CT as the hyperdense artery sign.
  • A thrombus that is rich in red blood cells is likely hyperdense.
  • The sign has high specificity (95%) but modest sensitivity (50%) for detecting intraarterial thrombus when assessed later by catheter angiography.
  • False positives occur in patients with raised hematocrit (due to dehydration or smoking).

The Eye Deviation Sign

  • Look at the patient’s eyes to see if they are symmetrically deviated toward the suspected ischemic cerebral hemisphere.
  • False negatives occur when the CT slices are thick (5 mm or thicker).

Imaging features on noncontrast CT play a vital role in rapidly assessing patients presenting with acute stroke. These features include:

  • Presence of any intracerebral, intraventricular, subarachnoid, subdural, or extradural hemorrhage.
  • Eye deviation.
  • Hyperdense artery sign.
  • Early ischemic changes, such as obscuration of lentiform nucleus, insular ribbon sign, and loss of gray-white differentiation of surface cortex.
  • Alberta Stroke Program Early CT Score (ASPECTS), which categorizes the extent of early ischemic changes as good (8-10), intermediate (5-7), or poor (0-4).

Alberta stroke programme early CT score (ASPECTS)

  • Segmental assessment of the MCA vascular territory is made and 1 point is deducted from the initial score of 10 for every region involved:
Lentiform Nucleus
Insula
Caudate
Internal Capsule
M1: “anterior MCA cortex,” corresponding to frontal operculum
M2: “MCA cortex lateral to insular ribbon” corresponding to anterior temporal lobe
M3: “posterior MCA cortex” corresponding to posterior temporal lobe
M4: “anterior MCA territory immediately superior to M1”
M5: “lateral MCA territory immediately superior to M2”
M6: “posterior MCA territory immediately superior to M3“

ASPECT Score

  • ASPECT score may be used to predict the risk of complications in stroke patients.
  • ASPECT < than or equal to 7 may be indicative of high risk and patient may be referred to higher center.
  • ASPECTS, more than 7, proceed to thrombolysis, if no contraindications.

Anatomic Classification of Intracranial Hemorrhage and Corresponding Etiology

Location                           Etiology  
Intraparenchymal LobarAmyloid angiopathy, bleeding diathesis, hypertension, vascular malformations, tumors, venous thrombosis, vasculitis  
Intraparenchymal DeepHypertension, tumors, moyamoya syndrome Intraventricular Hypertension, tumors  
SubarachnoidAneurysms, amyloid angiopathy, trauma  
Subdural Trauma, bleeding diathesis  
Extradural  Trauma

 Volume of the hemorrhage/ (A x B x C)/2

  • A is the longest measurement of hemorrhage in the axial plane.
  • B is the longest measurement of hemorrhage diameter perpendicular to A in the same or different axial plane.
  • C is longest measurement of hemorrhage in the vertical plane (multiplying the number of CT slices with   hemorrhage by the slice thickness).
  • A baseline hemorrhage volume of 60 mL or higher is often considered as being associated with poor prognosis.

Markers of poor prognosis with intracerebral hemorrhage seen on noncontrast CT

  • BLEND SIGN (a hemorrhage with hyperdense and hypodense regions).
  • ISLAND SIGN (at least three scattered small hemorrhages separate from the main hemorrhage) white arrow.
  • BLACK HOLE SIGN -A hypodense region surrounded by a hyperdense region not connected with the adjacent brain tissue.

When to do MRI?

  • The MR correlate of the CT dense vessel sign is focal “blooming” at the level of intraluminal thrombus (marked hypointensity that often extends beyond the lumen of the vessel) on GrE or SWI.
  • In Hyperacute infarction Parenchymal signal changes on MRI, T1 isointense and T2 isointense to mildly hyperintense. T2 hyperintensity is best appreciated on FLAIR (sometimes only in retrospect).
  • In the first 24 hours, FLAIR hyperintensity – 80%.
  • In First 6 hours – less than two thirds of cases.
  • DWI – 90%the sensitivity (<6 hours period).
  • DWI hyperintensity with ADC map hypointensity – within minutes of the onset of ischemia.
  • MRP allows for identification of areas of hypoperfusion that might be the target of thrombolytic therapy.
  • Important to detect the penumbra-Usually correlated with DWI(PWI/DWI)). If PWI/DWI mismatch>25% patient applicable to thrombolytic therapy.
  • Hypointensity on susceptibility-weighted sequences (GrE and SWI) may be present, indicative of hemorrhage.
  • The presence of microhemorrhages implies vascular fragility and carries an increased risk of hemorrhagic transformation of infarction and an increased risk of future infarction.

Hyperacute Hemorrhage

Early clot formation without clot retraction typically consists of predominantly diamagnetic oxyhemoglobin. On imaging:

  • T1-weighted MRI sequences may show mild hyperintensity, hypointensity, or isointensity. The presence of protein tends to decrease T1 signal, while edema or water content increases T1 signal.
  • T2-weighted MRI sequences typically exhibit hyperintensity.
  • Susceptibility-weighted imaging (SWI) commonly reveals peripheral hypointensity with central hyperintensity. This pattern arises from the presence of red blood cells at the periphery of the hematoma, containing hemoglobin that has begun to desaturate (deoxyhemoglobin).

Acute Hemorrhage (4 to 72 Hours)

  • Conversion of oxyhemoglobin to deoxyhemoglobin.
  • Clot formation and retraction leads to decreased water content.
  • T1 hypointense.
  • T2 and GRE hypointensity that begins at the periphery and extends to the center of the hematoma.
  • Hypointensity is more marked than earlier.

CT Angiography

  • The primary modality used to image blood vessels supplying the brain is the CTA.
  • It is best to acquire a head and neck CTA (aortic arch to vertex) to visualize all extracranial and intracranial arteries supplying the brain.
  • Iodinated contrast agent used for CTA results in a small risk of contrast-induced nephropathy (25% increase in serum creatinine from baseline or a 0.5-mg/dL increase over 48 to 72 hours).
  • The risk is so minimal in most patients and the benefits of vascular imaging so great that most stroke centers perform CTA immediately after noncontrast CT without waiting for a serum creatinine.
  • The only exception is in patients with known severe renal insufficiency not on dialysis.
  • If patients are on metformin, it is recommended to withhold that drug for at least 48 hours after CTA.

CTA in Hemorrhage

  • CTA is a useful tool to help understand the etiology of any intracranial hemorrhage and to identify the underlying pathologies.
  • Intracranial aneurysms.
  • Arteriovenous malformations.
  • Dural arteriovenous fistulas.
  • Any other vascular malformations.
  • A head CTA is recommended in patients with intracranial hemorrhage.
  • Young.
  • Lobar or infratentorial location of hemorrhage.
  •  Do not have hypertension or impaired coagulation.
  • On CTA, the spot sign is a serpiginous or linear contrast density located within the parenchymal hemorrhage.
  • The presence of a spot sign suggests hemorrhage that is likely to grow over time.

Spot Sign Mimics

  • Choroidal calcifications, Tumors.
  • Moyamoya disease, Aneurysms.
  • Micro-arteriovenous malformations.
  • If the spot sign is seen in the early arterial phase of a multiphase CTA, it is likely to be associated with greater hemorrhage growth than when the sign is only seen in the venous phase of a multiphase CTA.
  • The rate of increase in size of the spot sign may also be a marker of hemorrhage growth.
  • The spot sign, a marker of intracerebral hemorrhage growth, seen in the three phases of a multiphase CT angiogram.
Neuroimaging in Acute Stroke - What Physicians Must Know?

CTA in Ischemia

  • CTA helps to detect thrombi within arteries and their extent, collateral status beyond occlusive thrombus, and any other associated pathologies.
  • The tool also helps in determining the risk of recurrent strokes and in planning acute endovascular treatment and surgical management of carotid stenosis

Presence And Location of Thrombus

  • Identifying thrombi in large proximal intracranial arteries (also called large vessel occlusions [LVOs]).
  • Patients with LVOs can benefit from acute endovascular treatment.
  • Determining the location of thrombus within the intracranial arterial tree (eg, internal carotid artery versus M1 segment of MCA versus more distal arteries) helps identify the likelihood that a thrombus will respond to IV thrombolysis.
  • Occlusive thrombus within the internal carotid artery is less likely to dissolve with IV thrombolytics when compared to thrombus in more distal arteries.
  • Assessing thrombus burden (the extent of thrombus within the intracranial arterial tree) helps to determine whether acute ischemic stroke treatment (endovascular therapy or IV thrombolysis) is likely to be successful or not.
  • A large thrombus burden is associated with reduced efficacy of IV thrombolytics and potentially with longer endovascular treatment times.
  • In patients with minor strokes or transient ischemic attacks, detecting thrombus within the arterial vasculature supplying the brain identifies patients at risk of recurrent ischemic strokes.
  • Presence of acute stroke on noncontrast CT.
  • Circle of willis intracranial vessel occlusion or >50% stenosis.
  • Extracranial occlusion or >50% stenosis.
  • Identifies patients at high risk of recurrent ischemic stroke.

Nature of Thrombus

  • CTA can be used to detect intracranial thrombus that is permeable to IV contrast and therefore to blood flow.
  • The arterial silhouette on the CTA is used to identify contrast permeation within the site of intracranial thrombus.
  • This construct is called residual flow on CTA.

Residual flow on CTA Grades

  • Grade 0—no contrast permeation of the thrombus.
  • Grade 1—contrast permeating diffusely through thrombus.
  • Grade 2—tiny hairline lumen or a streak of well-defined contrast within the thrombus extending either through its entire length or through part of the thrombus.
  • Intracranial thrombi with grade 1 or 2 residual flow on CTA are more likely to dissolve early with IV rtPA than thrombi with no residual flow.

Thrombus Attenuation Increase

  • Subtracting the mean Hounsfield units of the thrombus on noncontrast CT from the mean Hounsfield units of the thrombus on CTA.
  •  A difference of 10 units or more is suggestive of a permeable thrombus that is more amenable to lysis with thrombolytics.

Collateral Status

  • Multiphase CTA.
  • On the three time-resolved phases of the multiphase CTA, arteries distal to the blocked artery are assessed for:
    • Extent of arterial contrast
    • Delay in filling of contrast
    • Impaired washout of contrast when compared to arteries on the contralateral side.

Identifying Other Pathologies

The CTA also helps identify other relevant pathologies

  • Disease of the aorta (thrombus, plaque, and dissection.
  • Extracranial atherosclerotic disease.
  • Arterial dissections.
  • Carotid webs.
  • Intracranial arterial narrowing (focal vasculopathy or vasculitis).

Planning Neurointervention

  • Assessment of aortic arch and large artery anatomy helps in choosing the type of catheter to be used during the procedure.
  • The location and extent of thrombus within the arterial tree also helps determine the device type and profile used for mechanical thrombectomy.

CT Perfusion

  • CT perfusion (CTP) involves:
    • Acquiring multiple scans of the brain over time; summating this time resolved images of contrast filling in and washing out from brain using mathematical formulas.
    • Generating estimates of cerebral blood flow, blood volume, and transit time within brain tissue.

Predicting Ischemic Infarct and Salvageable Brain

  • Current CTP techniques are prediction tools. They help predict the probability of brain tissue being dead or alive by estimating the degree of blood flow within that tissue.
  • Brain that has a reasonably high probability of being dead even after reperfusion is called the ischemic core.
  • Brain that is not functioning but is salvageable after reperfusion is called the penumbra.

Predicting Risk of Intracerebral Hemorrhage After Acute Stroke Treatment

  • Brain tissue with very low blood flow on CTP is likely to infarct early.
  •  By detecting regions of very low blood flow or regions of brain with increased blood-brain barrier permeability, CTP can help predict brain regions with increased risk of hemorrhage after acute stroke treatment.
  • Subacute-appearing changes on non-contrast CT or regions of the brain with changes seen on DWI and FLAIR images (no DWI-FLAIR mismatch>>> predict the risk of hemorrhage with acute treatment.)

Patient Selection for Acute Stroke Therapy

  • The goal is to assess imaging to:
    • Determine type of stroke.Assess markers of prognosis.Treatment risk such as the extent and severity of ischemia or hemorrhage.Identify treatment targets such as the presence of a large vessel occlusion in ischemic stroke.
    • Spot sign in patients with hemorrhage and make decisions.

CME INDIA Quick Take-Aways

  • Time is BRAIN.
  • Given its ease of use, availability and speed of acquisition, physicians should use CT as primary imaging tool.
  • NCCT, CTA are essential modalities in acute stroke, to diagnose stroke, determine prognosis and identify patients who might benefit from acute stroke treatment.
  • Acute stroke medicine is an art and science involving rapidly assessing relevant clinical and imaging information and making clinical decision in a timely manner.

References:

  1. Gomolka RS, Chrzan RM, Urbanik A, Kazmierski R, Grzanka AD, Nowinski WL. Quantification of image contrast of infarcts on computed tomography scans. Neuroradiol J. 2017 Feb;30(1):15-22. doi: 10.1177/1971400916678226. Epub 2017 Jan 6. PMID: 28059673; PMCID: PMC5564335.
  2. https://cdn.mednet.co.il/2020/04/Neuroimaging-in-Acute-Stroke.pdf


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