Acute Stroke Imaging

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Chapter 3 Acute Stroke Imaging

There was a time, not too long ago, when acute brain imaging in patients with suspected stroke was thought to be useful only to exclude hemorrhage or obvious stroke mimickers, such as tumors. The introduction of effective acute stroke therapies changed this conception completely, however. Today emergency brain imaging is essential for the management of acute stroke patients. We have learned that computed tomography (CT) scans can offer valuable information even when obtained within the first few hours of the ischemic event (dispelling the notion that CT scans are not useful for ischemic strokes until 1 or 2 days after onset). New CT-based protocols, including CT perfusion (CTP) scans and CT angiograms, are rapidly gaining ground in clinical practice. Diffusion-weighted and perfusion-weighted (DWI and PWI) magnetic resonance imaging (MRI) provide the ability to depict the penumbra and promise expansion of the therapeutic window for vessel opening on individual cases based on the subsistence of salvageable tissue. Conventional angiography has been transformed from a purely diagnostic test into a means for therapeutic intervention. Even transcranial Doppler may have an important role in the emergent evaluation and management of acute ischemic stroke, providing proof of large intracranial vessel occlusion and possibly improving the chances of recanalization with thrombolysis when continuous insonation is employed.

The uses of various neuroimaging techniques in acute stroke are multiple and continue to expand. The most common current indications and purposes of acute neuroimaging in stroke patients are listed in Table 3-1.

TABLE 3-1 Indications and purposes of emergency neuroimaging in patients with suspected acute ischemic stroke

Indication/purpose Imaging modality
Confirmation of diagnosis (TIA vs. stroke vs. stroke mimics) CT/MRI
Differentiation of ischemia vs. hemorrhage CT/MRI
Visualization of established infarction (as contraindication for thrombolysis) CT
Localization of ischemia/stroke pattern (which may guide evaluation of stroke mechanism) CT/MRI
Evaluation of penumbra (which may extend therapeutic window for acute revascularization) DWI-PWI/CTP
Identification of early prognostic markers (e.g., HDMCA sign, extensive high ASPECTS score, large volume of DWI restriction) CT/MRI
Visualization of arterial site of occlusion MRA/CTA/catheter angiography
Documentation of recanalization MRA/CTA/catheter angiography/TCD
US-assisted intravenous thrombolysis TCD
Access and information to make endovascular treatment possible Catheter angiography

ASPECTS, Alberta Stroke Program Early CT Score; CT, computed tomography; CTA, CT angiography; CTP, CT perfusion; DWI, diffusion-weighted imaging; HDMCA, hyperdense middle cerebral artery; MRA, magnetic resonance angiography; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging; TCD, transcranial Doppler; TIA, transient ischemic attack; US, ultrasound.

This chapter illustrates and summarizes the multiple values of brain imaging in the acute phase of ischemic stroke and concludes with a succinct discussion on the radiological features of subacute and chronic infarctions that allow timing of ischemic strokes.

COMPUTED TOMOGRAPHY

CT Signs of Acute Ischemia

It is important to discriminate signs of brain edema, such as loss of insular ribbon (Figure 3-3), obscuration of lenticular nucleus (Figure 3-4), loss of gray–white matter differentiation, and sulcal effacement (Figure 3-5) from areas of hypoattenuation, because only the latter represents irreversible damage (established infarction).3,4
Interobserver agreement for the recognition of early ischemic changes is only fair when performed without following a formal method and without considering the clinical information.5,6 Thus it is crucial to know the expected location of the ischemic insult on the basis of the information provided by the history and physical examination and to follow a methodical approach to maximize the yield of CT scan interpretation. Modifications of window settings may also increase the sensitivity of CT scanning to detect early ischemic changes (Figure 3-6).7
Visual identification of early signs of ischemia (particularly hypoattenuation) involving more than one third of the estimated middle cerebral artery (MCA) territory was considered an exclusion criterion for enrollment in several thrombolysis trials, most notably those conducted by the European Acute Stroke Study (ECASS) investigators,8,9 on the basis of a reasonable but unproved assumption that patients with early signs of extensive ischemia would have higher risk of bleeding after thrombolysis. However, this preconception was not validated by the analysis of the radiological data from the National Institute of Neurological Disorders and Stroke (NINDS) rt-PA study, which did not include this radiological exclusion criterion.10 In this trial, ischemic changes on baseline CT scan were observed in 31% of patients. They correlated with greater severity of initial clinical deficits and with longer time from symptom onset but were not independently associated with functional outcome after controlling for other baseline variables. Early ischemic changes were not associated with clinical deterioration within the first 24 hours or symptomatic intracranial hemorrhage within the first 36 hours in the adjusted analysis.11
Moreover, although there is some evidence that extensive early ischemic changes may portend higher risk of intracerebral hemorrhage,12 there is no proof that the extension of early ischemic changes significantly affects the chances of functional recovery after thrombolysis.12,13
Nonetheless, most current acute stroke management guidelines include extensive early signs of ischemia as a contraindication for thrombolysis. The guidelines sponsored by the American Heart Association indicate that thrombolysis should not be used if the baseline CT scan shows multilobar hypodensity involving more than one third of the cerebral hemisphere.1 This carefully crafted recommendation appears prudent. It is important to notice that it intentionally indicates hypodensity (as opposed to other early signs that may represent only tissue swelling and are more difficult to identify) and eliminates the need to estimate the MCA territory as a parameter to define the extension of the changes. On the basis of current evidence, withholding thrombolysis in patients with early signs of tissue edema but no large hypodensities is not justified.
The extension of ischemic changes in the territory of the middle cerebral artery can be quantified using the Alberta Stroke Program Early CT Score (ASPECTS) (Figures 3-7 and 3-8), a 10-point topographic scoring system that has been shown to be easy to use in real time with moderately good interrater reliability.2,14,15 A cutoff score of less than 7 points is most useful to determine ischemia involving more than one third of the MCA territory.15 Notice that this score can only be used in cases of middle cerebral artery ischemia.
The hyperdense MCA sign on baseline CT scan has been found to be associated with poor prognosis17,18 and a higher risk of hemorrhage after thrombolysis.19 The combination of hyperdense MCA sign and extensive sulcal effacement predicts massive swelling and brain herniation.20 Conversely, early resolution of the hyperdensity in the MCA indicates successful reperfusion and is associated with favorable outcome after thrombolysis.
Intravenous thrombolysis can be beneficial in patients presenting with the hyperdense MCA sign.18 However, when the hyperdense signal appears to involve the carotid terminus in a patient with signs suspicious for carotid bifurcation occlusion (depressed arousal, severe leg weakness), it may be more effective to pursue intra-arterial therapy directly if this is a pragmatically feasible option.21
Distal thrombi can sometimes be visualized generating the Sylvian fissure “dot” sign (Figure 3-9, C).22 Its sensitivity is modest (close to 40%), but when identified, it is very specific in predicting M2 or M3 branch occlusion.23 The dot sign is associated with better outcome than more proximal hyperdense vessel signals.22

TABLE 3-2 Early signs of ischemic stroke on brain CT scan.

Sign Significance
Hyperdense vessel sign Intraluminal thrombus
Loss of insular ribbon Focal tissue edema
Obscuration of the lenticular nucleus Focal tissue edema
Loss of gray–white matter distinction Focal tissue edema
Sulcal effacement Focal tissue edema
Areas of hypoattenuation Tissue infarction

CT Perfusion

There is growing interest in the application of CT protocols using multimodal CT scanning (CT scan, CT perfusion, and CT angiogram) for the emergency diagnosis and management of ischemic stroke.3,25 The most attractive features of CT perfusion imaging are its potential for widespread availability (it can be performed on any standard helical CT scanner) and the short time required for the acquisition of data (with adequate training, CT perfusion and CT angiogram can be acquired in 15–20 minutes, and images can be processed and interpreted in 10 minutes or less).26,27
Prolonged relative MTT and delayed relative TTP are the most sensitive physiological parameters to detect hypoperfusion.28 These measures correlate well with MRI abnormalities on PWI and accurately predict final infarct volume in patients who have persistent arterial occlusions (Figure 3-10).29,30 Meanwhile, reduced absolute CBV (or reduced relative CBV by visual inspection) is the best indicator of established infarction; it correlates well with DWI lesions on MRI and with final infarct size in patients who recanalize.2830 The product CBF × CBV may have greater diagnostic accuracy than CBV alone, but this requires the use of quantitative measures.31

TABLE 3-3 Relative advantages of CT perfusion and DWI-PWI MRI for the assessment of ischemic penumbra.

CT perfusion
Easier access
Rapid acquisition of images
Robust quantitative physiological measurements
Feasible in patients with contraindication for MRI
DWI-PWI MRI
May be easier to visualize the penumbra
Depiction of cellular edema
Greater spatial resolution (whole brain imaging)
Does not require iodine contrast

CT, computed tomography; DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging.

CT Angiogram

CT angiography has been shown to be safe.27 Renal complications related to contrast administration are exceptional and almost uniformly reversible.
The source images of CT angiography have been used to estimate perfusion deficits (Figure 3-12 serves as example).35 The advantages of this method over dynamic CT perfusion imaging include visualization of the whole brain and use of a single bolus of contrast material.3 However, this application has not been validated, and its sensitivity is likely to be poorer than those of CT perfusion or DWI-PWI.

MAGNETIC RESONANCE IMAGING

Although MRI scans provide better anatomical definition for the recognition of ischemic lesions than CT (particularly for small infarctions and strokes in the brainstem and posterior fossa), the added expense of MRI was deemed unjustified in the acute stroke setting until the introduction of physiological sequences: diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI). These new sequences are extremely valuable tools for the acute diagnosis of ischemic stroke and offer promise to expand the therapeutic window for recanalization. Equally important are the advances in our understanding of stroke pathophysiology facilitated by the information afforded by these imaging techniques.

Furthermore, MRI (with DWI and susceptibility weighted sequence) has been proved superior to CT scanning for the detection of acute ischemia and chronic hemorrhage and at least comparable to CT for the diagnosis of acute hemorrhage.36 Thus solid arguments support the use of MRI as the primary imaging modality for the emergency evaluation of acute stroke patients if the study can be performed without delay.

Diffusion-Weighted and Perfusion-Weighted Imaging

Areas with restricted diffusion due to cellular edema are hyperintense on the DWI sequence and hypointense on the apparent diffusion coefficient (ADC) map (Figure 3-13). The ADC value quantifies diffusion; the lower the value, the greater the restriction of motion of water molecules. Conversely, high ADC values are observed in areas of vasogenic edema and chronic infarction in which water molecules have freedom of motion.
Table 3-4 lists the practical values of DWI in the evaluation of acute stroke patients.

TABLE 3-4 Main practical uses of DWI in patients with acute stroke presentation.

Hyperacute and acute diagnostic confirmation of ischemic stroke
Differentiation of acute vs. subacute vs. chronic ischemic lesions
Assessment of ischemic penumbra (in combination with PWI)
Acute differential diagnosis between TIA and minor stroke with reversible neurological deficits
Distinction of cytotoxic and vasogenic edema (in conditions such as eclampsia or hyperperfusion syndrome)
Identification of patients at risk of severe reperfusion hemorrhage

DWI, diffusion-weighted imaging; MRI, magnetic resonance imaging; PWI, perfusion-weighted imaging; TIA, transient ischemic attack.

PWI-DWI Mismatch

A TTP delay greater than 4 seconds relative to the contralateral hemisphere appears to be the best marker of the penumbra.48 Nonetheless, this measure may overestimate the size of penumbra in some cases.48 CBF maps most closely identify the final infarct volume.49,50
The ischemic penumbra is represented on MRI by the perfusion-diffusion (PWI-DWI) mismatch (Figures 3-11, 3-16, and 3-17). The PWI lesion corresponds to the area of hypoperfusion and the DWI lesion to the ischemic core.
Unfortunately, there is no validated definition of PWI-DWI mismatch.52 Different definitions have been used in published studies, and visual estimates are most commonly used in practice for acute decision making.
In theory, the DWI lesion can expand to reach the size of the initial PWI deficit unless reperfusion occurs. However, confirmation of this hypothesis has proved elusive at times. In fact, some studies have shown that mismatch volume may fail to correlate with DWI lesion expansion.50,53 Possible explanations for this lack of correlation are that areas of ischemia are highly heterogeneous,54 DWI lesions may be reversible (normalization of ADC values has been noted in some patients after thrombolysis),55 and PWI lesions actually incorporate regions of true penumbra and regions of reversible oligemia.56
Determining whether PWI-DWI mismatch can be reliably used to identify salvageable tissue is of major practical importance. Because DWI expansion can be prevented by early reperfusion regardless of the presence of PWI-DWI mismatch,53 there is no indication for MRI before administering intravenous thrombolysis within 3 hours of symptom onset (there are some data suggesting greater safety for thrombolysis within 3 hours in patients selected with MRI vs. those only evaluated with CT scan,57 but not enough solid evidence to change current practice guidelines). However, all other acute revascularization treatments are not the standard of care, and there is considerable interest in developing imaging protocols to guide their application (penumbra-based protocols

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