Acute Ischemic Appearances (General Guidelines)

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  < 6 hours: no change in appearance

  
  
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  or

  
  
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  > 1.5 hours: loss of gray–white differentiation

  
  
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  > 3 hours: hypodensity

  
  
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  > 6 hours: swelling

  
  
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  > weeks: ex-vacuo changes

Window Width and Level (WW/WL) for Early CT

The standard brain view on CT is set around 90/40. A setting of 40/40 may give high contrast of brain parenchyma to demonstrate the early ischemic signs more easily.

ASPECTS (Alberta Stroke Programme Early CT Score)

ASPECTS is scoring system that increases the reliability of detecting early CT changes in MCA territory infarction in the first few hours of ischemic stroke (Figure 4.1).⁵

Non-contrast head CT scans are interpreted at two levels:

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   1. at the level of the basal ganglia and thalamus

   
   
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   2. at the level just rostral to the basal ganglia

Figure 4.1 ASPECTS template.

Source: Barber PA, Demchuk AM, Zhang J, Buchan AM. Validity and reliability of a quantitative computed tomography score in predicting outcome of hyperacute stroke before thrombolytic therapy. ASPECTS Study Group. Alberta Stroke Programme Early CT Score. Lancet 2000; 355: 1670–1674.⁵ Reproduced with permission from Elsevier

Acute Hemorrhage (Extravasated Blood)

This appears hyperdense (bright) on non-contrast CT at 40–60 Hounsfield units (HU). In the first few hours, the intensity may increase to 60–80 HUs. Intensity attenuates with time at a rate of ~0.7–1.5 HU/day.

The severity of hemorrhagic transformation into an ischemic stroke is classified according the ECASS trials as hemorrhagic infarction types 1 and 2 (HI-1, HI-2) and parenchymal hemorrhage types 1 and 2 (PH-1, PH-2) (Figures 4.2 and 7.5).^6–8

Figure 4.2 Hemorrhagic infarction and parenchymal hemorrhage.

Source: Álvarez-Sabín J, Maisterra O, Santamarina E, Kase CS. Factors influencing haemorrhagic transformation in ischaemic stroke. Lancet Neurol 2013; 12: 689–705.⁹ Reproduced with permission from Elsevier.

CT Angiography

CT angiography (CTA) of the head and neck with contrast is the mainstay for evaluating most acute stroke patients for the presence of a large-vessel occlusion (LVO) who may be candidates for EVT with mechanical thrombectomy. By current guidelines, all patients with a clinical presentation consistent with acute ischemic stroke with a premorbid modified Rankin scale (mRS) score of 0–1 (little or no disability at baseline), NIHSS of ≥ 6, who present within 6 hours of symptom onset, and an ASPECTS score of ≥ 6 should be considered potential EVT candidates and should therefore have CTA done at the same time as NCCT.¹⁰ Performing CTA, however, should not delay the administration of IV thrombolysis, and therefore systems should be in place to initiate an IV tPA bolus and drip in the CT scanner suite while the CT technician sets up the contrast injection for CTA.

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  Involves IV bolus of contrast, and imaging the arteries quickly during first pass of contrast.

  
  
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  Allows visualization of vessels or lack thereof (occlusion, stenosis, AVM, aneurysms).

  
  
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  Requirements:

  
  
  
  
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     ▪ adequate renal function because the contrast bolus is larger than usual

     
     
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     ▪ good IV access, usually 18 gauge (you don’t want contrast in soft tissues!)

  
  
  
  
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  CT viewing window width/level at 800/100 may be best to visualize the arteries next to bones.

  
  
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  Now done along with NCCT to identify arterial occlusion of major arteries and to plan endovascular therapies.

Standard Head CT with Contrast

Allows evaluation for stroke mimics by detecting blood–brain barrier breakdown:

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  Tumor, infection, inflammation, etc.

  
  
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  Rarely used in isolation in current clinical practice.

CT Perfusion

Using an additional contrast bolus, CT perfusion (CTP) demonstrates which areas of the brain are receiving inadequate blood flow at the time of an acute ischemic stroke. CTP maps are increasingly being utilized in the acute setting to distinguish salvageable brain tissue (the “ischemic penumbra”) from the irreversibly damaged tissue (the “core infarct”). The use of CTP to select patients who may be amenable to reperfusion with EVT in an extended time window is discussed in Chapter 6.

The most commonly used CTP parameters are:

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  CBV (cerebral blood volume) – volume of blood per 100 g of brain in an imaging voxel. If the CBV is low, the brain tissue is considered to be irreversibly damaged (“core”). If the CBV is normal or high then this brain tissue is thought to be salvageable (“penumbra”).

  
  
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  CBF (cerebral blood flow) – volume of blood passing through 100 g of brain per minute. If the CBF is very low the brain tissue is irreversibly damaged. CBF may be slightly reduced in the penumbra.

  
  
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  MTT (mean transit time) – average time it takes contrast bolus to move through a volume of brain in seconds. Both the core infarct and ischemic penumbra will have prolonged MTT in the setting of acute ischemic stroke.

  
  
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  TTP (time to peak) – time in seconds from start of contrast injection to maximal enhancement of brain tissue.

  
  
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  T_max (maximum of the tissue residue function) – similar to TTP, time in seconds for contrast bolus to perfuse brain tissue.¹¹

Normal perfusion parameters are:

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  gray matter

  
  
  
  
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     ▪ CBF: 60 mL/100 g/min

     
     
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     ▪ CBV: 4 mL/100 g

     
     
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     ▪ MTT: 4 s

  
  
  
  
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  white matter

  
  
  
  
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     ▪ CBF: 25 mL/100 g/min

     
     
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     ▪ CBV: 2 mL/100 g

     
     
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     ▪ MTT: 4.8 s

  
  

RAPID (Rapid Processing of Perfusion and Diffusion) Software (Figure 4.3)

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   ▪ CBF < 30% volume = critically hypoperfused, estimated as “core infarct.”

   
   
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   ▪ T_max > 6 seconds = severely hypoperfused, estimated as “ischemic penumbra.”

   
   
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   ▪ Mismatch ratio = ratio of volume of “penumbra” (T_max > 6 seconds) shown in green to volume of “core” (CBF < 30%) shown in pink, with a ratio of 1.8 found to indicate a favorable target for reperfusion.

   
   
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   ▪ Used in DEFUSE 3 and DAWN trials to select extended window endovascular patients.^{12, 13}

Figure 4.3 RAPID map. (a) Cerebral blood flow (CBF) < 30% (shown in pink on the left) represents a core infarct of 11 mL in a patient with a right middle cerebral artery (MCA) occlusion in the 6–24-hour window. The maximum of the tissue residue function (T_max) > 6 seconds (shown in green on the right) represents an ischemic penumbra of 161 mL. The mismatch volume is 150 mL, and the mismatch ratio is 14.6. (b) From left to right, the cerebral blood volume (CBV), CBF, mean transit time (MTT), and T_max are shown, demonstrating low blood volume in a small area of the right MCA territory with a much larger area at risk for infarction. For further information, see: Albers GW. Use of imaging to select patients for late window endovascular therapy. Stroke 2018; 49: 2256–2260.¹⁴

How to Try to Prevent Contrast-Induced Acute Kidney Injury

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  Hydration is the key to prevention of contrast-induced acute kidney injury.

  
  
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  For high-risk patients (glomerular filtration rate [GFR] < 60, age > 75, sepsis, heart failure) consider intravenous 0.9% sodium chloride at a rate of 1 mL/kg/h for 12 hours before and after giving the contrast, whenever possible.

  
  
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  Avoid nephrotoxic agents (NSAIDs, aminoglycosides).^{15, 16}

  
  
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  Results of the Prevention of Serious Adverse Events Following Angiography (PRESERVE) trial demonstrated no benefit of periprocedural intravenous sodium bicarbonate or oral acetylcysteine over intravenous isotonic sodium chloride in prevention of contrast-induced acute kidney injury.¹⁷

Excluding Stroke Mimics

MRI is used as the primary screening modality for acute stroke treatment decision-making in the ED in only a small number of centers worldwide. However, MRI can be helpful in excluding stroke mimics such as peripheral vertigo and migraine with aura by demonstrating restricted diffusion consistent with acute infarct on diffusion-weighted imaging (DWI) sequences.

Wake-up Stroke

MRI brain can be used to select patients whose stroke symptoms are noted upon waking, and who may be eligible for tPA treatment beyond 4.5 hours from their last known normal time. Abnormal signals on FLAIR reflecting irreversible infarction begin to appear about 4–5 hours after stroke onset. In the WAKE-UP trial, patients who had restricted diffusion on DWI but no FLAIR sequence changes were potentially eligible for tPA treatment.¹⁸ There seems to be a benefit of tPA treatment in selected wake-up stroke patients based on imaging, with a number needed to treat (NNT) of 9 (see Chapter 5).

Determining Stroke Etiology

Although MRI of the brain is not generally feasible to perform in the hyperacute evaluation phase of a stroke patient, it can be helpful in determining etiology for both ischemic and hemorrhagic stroke patients.

In ischemic stroke, a cardioembolic etiology may be suggested by acute infarcts in multiple vascular territories or a cortical wedge-shaped infarction.¹⁹

Moreover, small infarcts that occur at the border of two vascular territories, so-called “borderzone” or “watershed” infarcts, may suggest a large-artery stenosis and stroke resulting from relative cerebral hypoperfusion through that narrowed artery.

In hemorrhagic stroke, the presence of cortical superficial siderosis and cortical microbleeds, along with a lobar intracerebral hemorrhage, would be suggestive of cerebral amyloid angiopathy as the etiology. Use of the modified Boston criteria may be helpful in categorizing the probability of cerebral amyloid angiopathy (CAA) as ICH etiology, based on neuroimaging findings.²⁰

Magnetic resonance angiography (MRA) of the head and neck using a time-of-flight technique without contrast can be utilized instead of CTA in patients who cannot have a CT-based study performed, such as those who are pregnant, have a true contrast allergy, or cannot safely receive IV contrast because of advanced kidney disease.