Neuroimaging of acute ischemic stroke

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All topics are updated as new evidence becomes available and our peer review process is complete.

Literature review current through: Jul 2016. | This topic last updated: Apr 04, 2016.

INTRODUCTION — Imaging studies are used to exclude hemorrhage in the acute stroke patient, to assess the degree of brain injury, and to identify the vascular lesion responsible for the ischemic deficit. Some advanced CT and MRI technologies are able to distinguish between brain tissue that is irreversibly infarcted and that which is potentially salvageable, thereby allowing better selection of patients likely to benefit from therapy. The use of this technology is dependent upon availability, and its role in guiding treatment decisions is still under study.

Neuroimaging during the acute phase (first few hours) of an ischemic stroke will be reviewed here. Other aspects of the acute evaluation of stroke, the clinical diagnosis of various types of stroke, and the subacute and long-term assessment of patients who have had a stroke are discussed separately. (See "Initial assessment and management of acute stroke" and "Clinical diagnosis of stroke subtypes" and "Overview of the evaluation of stroke".)

COMPUTED TOMOGRAPHY — The main advantages of CT are widespread access and speed of acquisition. In the hyperacute phase, a noncontrast CT (NCCT) scan is usually ordered to exclude or confirm hemorrhage; it is highly sensitive for this indication. A NCCT scan should be obtained as soon as the patient is medically stable. The presence of hemorrhage leads to very different management and concerns than a normal scan or one that shows infarction. Immediate CT scanning of all patients with suspected stroke is also the most cost-effective strategy when compared with alternate strategies such as scanning selected patients or delayed rather than immediate imaging [1].

The utility of CT for acute stroke has been enhanced by the advent of additional CT techniques including CT perfusion imaging (CTP) and CT angiography (CTA). Multimodal CT evaluation that employs the three techniques (NCCT, CTA, and CTP) combined shows improved detection of acute infarction when compared with NCCT evaluation alone [2-5]. In addition, multimodal evaluation that includes CTA and CTP may permit assessment of the site of vascular occlusion, infarct core, salvageable brain tissue and degree of collateral circulation [6,7].

Early signs of infarction on noncontrast CT — The sensitivity of standard noncontrast CT for brain ischemia increases after 24 hours. However, in a systematic review involving 15 studies where CT scans were performed within six hours of stroke onset, the prevalence of early CT signs of brain infarction was 61 percent (standard deviation +/- 21 percent) [8].

Early signs of infarction include the following [8-12]:

●Hypoattenuation involving one-third or more of the middle cerebral artery (MCA) territory

●Obscuration of the lentiform nucleus

●Cortical sulcal effacement

●Focal parenchymal hypoattenuation

●Loss of the insular ribbon or obscuration of the Sylvian fissure

●Hyperattenuation of large vessel (eg, "hyperdense MCA sign")

●Loss of gray-white matter differentiation in the basal ganglia

 

The presence of early CT signs of infarction implies a worse prognosis. In the systematic review, the presence of these signs was associated with an increased risk of poor functional outcome (odds ratio 3.11, 95% CI 2.77-3.49) [8].

Hyperdensity of the MCA, indicating the presence of thrombus inside the artery lumen (bright artery sign), can be visualized on noncontrast CT in 30 to 40 percent of patients with an MCA distribution stroke [11,13]. This finding is highly specific for MCA occlusion, although it may be less useful for predicting outcome than the other early CT signs.

While early CT signs of infarction are associated with a worse outcome, it remains unclear whether early infarction signs should be considered when deciding whether to use intravenous (IV) thrombolytic treatment for acute ischemic stroke [8]. An analysis from the NINDS trial found that early CT signs of infarction were not independently associated with increased risk of adverse outcome after IV alteplase (tPA) treatment, and patients treated with alteplase did better whether or not they had early CT signs [14]. (See "Reperfusion therapy for acute ischemic stroke", section on 'Intravenous thrombolysis'.)

Careful attention to the presence of these signs by experienced personnel is necessary; mistakes have occurred in up to 20 percent of cases in a controlled setting [15]. Studies that have examined the ability of neurologists, neuroradiologists, and general practitioners have found that early infarction can be very difficult to recognize on CT [16]. However, the importance of a truly normal head CT in acute stroke should not be underestimated; it excludes major ischemic damage with high specificity [17].

Standardized methods such as ASPECTS have been developed to aid recognition of early ischemia because of the known difficulty in detecting such changes. In addition, accentuating the contrast between normal and edematous (ischemic) brain tissue by variable window width and center level settings may improve detection of early ischemic change on noncontrast CT [18].

ASPECTS method of assessing ischemic changes — The Alberta stroke program early CT score (ASPECTS) was developed to provide a simple and reliable method of assessing ischemic changes on head CT scan in order to identify acute stroke patients unlikely to make an independent recovery despite thrombolytic treatment [19].

The ASPECTS value is calculated from two standard axial CT cuts; one at the level of the thalamus and basal ganglia, and one just rostral to the basal ganglia (figure 1 and figure 2) [19,20].

●The score divides the MCA territory into 10 regions of interest.

 

●Subcortical structures are allotted three points (one each for caudate, lentiform nucleus, and internal capsule).

 

●MCA cortex is allotted seven points. Four of these points come from the axial CT cut at the level of the basal ganglia, with one point for insular cortex and one point each for M1, M2, and M3 regions (anterior, lateral, and posterior MCA cortex).

 

●Three points come from the CT cut just rostral to the basal ganglia, with one point each for M4, M5, and M6 regions (anterior, lateral, and posterior MCA cortex).

 

●One point is subtracted for an area of early ischemic change, such as focal swelling or parenchymal hypoattenuation, for each of the defined regions.

 

Therefore, a normal CT scan has an ASPECTS value of 10 points, while diffuse ischemic change throughout the MCA territory gives a value of 0.

Utility of ASPECTS — In the initial ASPECTS study, pretreatment noncontrast head CT scans from 156 patients with anterior circulation ischemia treated with intravenous alteplase (IV tPA) were prospectively scored with ASPECTS [19]. The following observations were made for baseline values.

●ASPECTS was inversely correlated with stroke severity.

 

●The median ASPECTS value was 8; a value of 7 or less was associated with a sharp increase in dependence and death at three months.

 

●ASPECTS predicted functional outcome and symptomatic intracerebral hemorrhage, with good sensitivity and specificity for functional outcome (0.78 and 0.96) and for intracerebral hemorrhage (ICH) (0.90 and 0.61).

 

●The inter- and intraobserver reliability was good to excellent; score reliability appears to be good when performed in real time by treating physicians as compared with expert readers [21].

 

In a prospective study of 100 patients with acute ischemic stroke, the ability to detect early ischemic changes by ASPECTS was similar on noncontrast CT and diffusion-weighted imaging (DWI) [22].

ASPECTS has been retrospectively applied to baseline and 24-hour CT scans for patients with middle cerebral artery occlusion who were randomized to intraarterial thrombolysis or placebo in the PROACT-II study [23]. Treated patients with a baseline ASPECTS >7 had a risk ratio (RR) of 3.2 (95% CI 1.2-9.1) for an independent functional outcome, while patients with ASPECTS ≤7 had a RR of 1.0 (95% CI 0.6-1.9).

Despite its promise, the available data suggest that ASPECTS analysis of noncontrast CT does not identify patients who may benefit from thrombolysis. The prospective CASES observational cohort study of 1135 patients treated with IV tPA found that each one point decrement in the baseline ASPECTS scores was associated with a lower probability of independent functional outcome (odds ratio 0.81, 95% CI 0.75-0.87) [24]. However, the ASPECTS score was not a predictor of symptomatic intracranial hemorrhage in patients treated within the standard three-hour time window.

Subsequent reports showed that the ASPECTS score of baseline noncontrast CT scans from the NINDS and ECASS-II tPA stroke studies was not associated with a statistically significant modification of tPA treatment effect [25,26]. This finding is in agreement with a report cited above from the NINDS cohort, which found that signs of early ischemic change on CT were not independently associated with increased risk of adverse outcome after IV tPA treatment [14]. (See 'Early signs of infarction on noncontrast CT' above.)

One problem with ASPECTS may be that the various types of parenchymal changes on noncontrast CT considered to represent early ischemic change may actually have different pathophysiologic mechanisms. In particular, there is evidence suggesting that hypoattenuation represents irreversible infarction, whereas focal swelling may represent penumbral tissue [27,28]. ASPECTS may have greater accuracy for detection of ischemic change and for identifying final infarct volume when used to analyze CTA-source images and the contrast CT images obtained from CTP than when used to analyze noncontrast CT images [29,30]. (See 'Utility of CT contrast dye' below and 'CT perfusion imaging' below.)

It is important to note that ASPECTS is not applicable to lacunar stroke, brainstem stroke, or any stroke outside of the middle cerebral artery territory.

Utility of CT contrast dye — Spiral (helical) CT and new generation multidetector CT scanners increase scan speed and allow CTA of both extracranial and intracranial cerebral arteries. The speed of these CT units also offers CTP capabilities. These scans can be performed immediately after conventional CT scanning, requiring only 5 to 10 minutes of additional time. In practice, one can perform both CTA and CTP during the same examination, with separate contrast boluses [30].

Advantages of these fast CT scans include the ability to rapidly identify patients with occlusion of the major vessels within the circle of Willis or extracranial cerebral arteries, as well as the ability to evaluate the perfusion status of the brain parenchyma. Additional information about brain perfusion can be obtained by post imaging analysis of the raw data (or source images) of CTA and CTP studies. (See 'CT angiography'below and 'CT perfusion imaging' below.)

CT angiography — CTA is performed by administering a rapid bolus of standard intravenous CT contrast through a large bore intravenous line in the antecubital fossa. The helical CT scan is timed to capture the arrival of dye into the brain. Dye can be seen in the great vessels on the raw CT images; these serve as data for three dimensional computer reconstructions of the circle of Willis and extracranial cerebral arteries. Clot causes a filling defect in the vessel on CTA, which often can be seen on the raw images (also called source images). (See "Principles of computed tomography of the chest".)

For the detection of intracranial large vessel stenosis and occlusion, CTA in various studies had sensitivities of 92 to 100 percent and specificities of 82 to 100 percent when compared with conventional angiography [31]. The accuracy of CTA for the diagnosis of extracranial carotid stenosis is discussed separately. (See "Evaluation of carotid artery stenosis", section on 'CT angiography'.)

Recanalization rates for intravenous or intraarterial thrombolysis differ depending upon the site of arterial occlusion. CTA has become the standard of practice in our center to triage patients between intravenous thrombolysis, mechanical thrombectomy, and intra-arterial thrombolysis. It is also helpful in diagnosing stroke mimics. As an example, the patient with severe brainstem signs thought due to basilar thrombosis who has a normal basilar artery on CTA demands an alternative diagnosis. (See "Differential diagnosis of transient ischemic attack and stroke".)

The pial artery collateral vessels of the brain can be assessed using multiphase CTA, which acquires blood flow information in three phases after contrast injection; the first phase consists of conventional CTA with image acquisition from the aortic arch to skull vertex during the peak arterial phase; the second and third phases consist of image acquisition from the skull base to vertex during the mid-venous and late-venous phases [32]. Compared with perfusion CT, advantages of this method include whole brain coverage, reduced vulnerability to patient motion, no need for additional contrast or postprocessing, and more rapid determination of collateral status. In the ESCAPE trial, the presence of moderate-to-good pial collateral circulation, determined by multiphase CTA in a majority of subjects, was one of the criteria used to select patients for mechanical thrombectomy in the setting of acute ischemic stroke caused by a proximal intracranial artery occlusion in the anterior circulation [33]. (See "Reperfusion therapy for acute ischemic stroke", section on 'Mechanical thrombectomy'.)

CTA source images — CTA source images can provide an estimate of perfusion by taking advantage of the contrast enhancement in the brain vasculature that occurs during a CTA [34], potentially obviating the need for a separate CT perfusion study and a second contrast bolus. CTA source images typically cover the entire brain, in contrast to CT perfusion source images that are limited to a few brain slices.

During a CTA, contrast dye fills the brain microvasculature in the normal perfused tissue that is accessible to the blood pool and appears as increased signal intensity on the CTA source images. In distinction, contrast dye does not fill the microvasculature in ischemic brain regions that are less accessible to the blood pool and have poor collateral flow. These ischemic areas are easily seen as regions of hypoattenuation (low density or dark) on CTA source images (image 1) [35,36].

CTA source images are more sensitive than noncontrast CT scans for the detection of early brain infarction [31,37]. Hypoattenuation on CTA source images correlates with ischemic edema [36], and with the abnormality on diffusion-weighted MRI [38]. In this sense, CTA source images (or raw images of CT perfusion studies) can be considered as a surrogate for DWI. (See 'Diffusion-weighted imaging' below.)

CT perfusion imaging — Using an intravenous bolus of CT dye, a whole brain "perfused blood volume map" can be obtained by timing the scan to the passage of the contrast dye through the brain [39]. This can be obtained by continuing to scan the brain during a CT angiogram or by using a new bolus of contrast following the CTA. However, CTP requires repeatedly scanning the same portion of the brain parenchyma over the time required for the bolus to pass through the vasculature.

Similar to CTA-SI, the source images of the CTP (CTP-SI) are available for analysis. As with CTA-SI, areas of hypoattenuation on CTP-SI should correlate with ischemic brain regions. In addition, quantitative analysis of the kinetics of a bolus of CT dye passing through the brain enable estimation of cerebral blood flow (CBF), cerebral blood volume (CBV), and the mean transit time (MTT) that it takes blood to flow through the tissue [40]. Thresholds of CBF and CBV can be used to predict whether tissue will die or survive, but standardized, reliable, and validated thresholds have not been definitively established [41,42].

One study found that the ASPECTS method applied to CTP-SI or CBV maps was more accurate for identifying irreversible ischemia and clinical outcome than ASPECTS applied to noncontrast CT or CTA-SI [30]. In addition, ASPECTS applied to CBF maps or MTT appeared to identify the maximal extent of infarction in the absence of major reperfusion, and the difference between CTP-SI (or CBV) and CBF (or MTT) on ASPECTS appeared to identify ischemic tissue at risk for infarction. Thus, ASPECTS applied to CTP and its multiple parametric maps (CBV, CBV, MTT) holds promise for improving patient selection for intravenous thrombolysis of acute ischemic stroke, and for extending the time window beyond three hours [30,43]. However, this hypothesis should be confirmed in randomized clinical trials.

MAGNETIC RESONANCE IMAGING — Advanced MRI imaging techniques have the potential for further defining stroke subgroup populations that may benefit from intravenous thrombolysis or interventional vascular treatments [44]. In addition, MRI sequences using high susceptibility methods, such as gradient echo (GRE) pulse sequences, are equivalent to CT for the detection of acute intracerebral hemorrhage (ICH) and better than CT for the detection of chronic hemorrhage [45-47]. ICH can be diagnosed by MRI with up to 100 percent sensitivity and accuracy by experienced readers [46]. (See "Spontaneous intracerebral hemorrhage: Pathogenesis, clinical features, and diagnosis", section on 'Hemorrhage appearance'.)

Brain MRI protocols that combine conventional T1 and T2 sequences with diffusion-weighted imaging (DWI), perfusion-weighted imaging (PWI), and GRE can reliably diagnose both acute ischemic stroke and acute hemorrhagic stroke in emergency settings. These MRI techniques may obviate the need for emergent CT in centers where brain MRI is readily available. As an example, one specialized stroke center found that routine use of these MRI sequences to screen patients prior to intravenous thrombolysis for suspected ischemic stroke was practical and safe [48]. Furthermore, MRI screening did not cause excessive treatment delays or lead to worse outcomes. On the other hand, MRI-specific selection criteria for acute thrombolysis of ischemic stroke have not been validated, and no randomized studies have compared CT and MRI screening in this setting.

Newer ultrafast MRI imaging protocols can reduce acquisition times from the 15 to 20 minutes required by conventional MRI to five minutes or less, but the utility of these newer methods is not yet established [49,50].

Diffusion-weighted imaging — DWI is based upon the capacity of fast MRI to detect a signal related to the movement of water molecules between two closely spaced radiofrequency pulses. This technique can detect abnormalities due to ischemia within 3 to 30 minutes of onset, [51-53], when conventional MRI and CT images would still appear normal.

In acute stroke, swelling of the ischemic brain parenchymal cells follows failure of the energy-dependent Na-K-ATPase pumps and is believed to increase the ratio of intracellular to extracellular volume fractions [54].

DWI contains an additional component of T2 effect, and increased T2 signal due to vasogenic edema can "shine through" on DWI images, making it difficult to distinguish vasogenic from cytotoxic edema on these images. This problem can be overcome by use of the apparent diffusion coefficient (ADC). The ADC provides a quantitative measure of the water diffusion. In acute ischemic stroke with cytotoxic edema, decreased water diffusion in infarcted tissue causes increased (hyperintense) DWI signal and a decreased ADC, visualized as hypointense signal on ADC maps of the brain. In contrast, vasogenic edema may cause increased DWI signal may occur due to T2 shine through, but water diffusion is increased, and increased ADC is seen as hyperintense signal on ADC maps.

The decrease in ADC in the region of the infarct is a necessary transition on the way to infarction. The decrease in diffusion in the infarct is transient, lasting one to two weeks. It then actually reverses, passing through a phase of pseudonormalization and later becoming elevated and bright on ADC maps [55]. DWI abnormalities last somewhat longer due to the prominent T2 effect, but chronic infarction is not bright on DWI.

In a study comparing CT, DWI, and standard MRI, abnormal DWI was a sensitive and specific indicator of ischemic stroke in patients presenting within six hours of symptom onset [56]. Others have confirmed these results [57-61]. However, occasional patients with acute ischemic deficits have a normal DWI; in these cases, follow-up MRI or CT may confirm an infarct [62,63]. In some of these patients, the stroke was a small brainstem lacune; in others, ischemia was seen on perfusion MRI in regions that had not yet become abnormal on DWI [62].

MR and DWI utilizing higher magnetic field strengths of 3 Tesla (T) units are increasingly available in clinical settings. However, there is only limited and conflicting evidence regarding whether DWI obtained using 3 T MRI scanners is better for the detection of early (≤6 hours) and small infarcts compared with standard 1.5 T MRI [64,65]. Although seemingly advantageous because of improved signal-to-noise ratios, higher magnetic field strengths also introduce increased imaging artifacts and geometric distortions [66], and these artifacts may obscure early ischemic changes, particularly in regions of brain near the skull base [65]. Thus, further refinement of higher field strength DWI imaging is needed to determine if such imaging is useful in acute ischemic stroke.

Clinical utility of DWI — A systematic review published in 2010 from the American Academy of Neurology (AAN) concluded that DWI is superior to noncontrast CT for the diagnosis of acute ischemic stroke in patients presenting within 12 hours of symptom onset [67]. Subsequent large single-center case series found that the sensitivity of DWI for acute ischemic stroke is approximately 90 percent [60,61], although the sensitivity may be much lower for patients with minor, nondisabling stroke [68].

Even in patients with subacute ischemic stroke who delay seeking medical attention, DWI may add clinically useful information to standard MRI. In a prospective observational study of 300 patients with suspected stroke or transient ischemic attack (TIA) and a median delay of 17 days from symptom onset, DWI compared with T2 provided additional clinical information imaging for 108 patients (36 percent) such as clarification of diagnosis or vascular territory; this was considered likely to change management in 42 patients (14 percent) [69].

In the evaluation of acute ischemic stroke or TIA, the presence of multiple DWI lesions on the baseline MRI scan is associated with an increased risk of early lesion recurrence [70-72]. Furthermore, the presence of multiple DWI lesions of varying ages, as determined by the ADC value, is an independent predictor of future ischemic events [73].

Perfusion-weighted imaging — Diffusion-weighted imaging reveals evidence of ischemic injury, not ischemia itself. In contrast, perfusion-weighted imaging (PWI) uses fast MRI techniques to quantify the amount of MR contrast agent reaching the brain tissue after a fast intravenous bolus. Integration of the amount of gadolinium entering the brain on first pass allows construction of maps of cerebral blood volume. Analysis that also includes the time course of arrival and washout permits the construction of maps of relative cerebral blood flow and mean transit time. The latter sensitively identifies the ischemic zone.

PWI can be performed with standard MRI and MR angiography, requiring a total imaging time of less than 15 minutes. Access to MRI is usually the limiting factor.

Another method of MRI perfusion imaging is continuous arterial spin labeling (CASL). Instead of using an intravascular contrast agent, CASL magnetically labels the blood entering the brain. CASL imaging within 24 hours of stroke symptom onset can depict perfusion defects and diffusion-perfusion mismatches [74]. In addition, cerebral blood flow asymmetry on CASL appears to correlate with stroke severity and outcome.

Guidelines from the AAN published in 2010 concluded that the baseline lesion volume on PWI may predict baseline stroke severity, but found that evidence was insufficient to support or refute the utility of PWI for use in diagnosing acute ischemic stroke [67].

Identifying reversible ischemia — Accurate identification of patients with reversible ischemic injury in the brain is important for selecting those patients most likely to benefit and least likely to be harmed by reperfusion and neuroprotective therapy. In patients with acute stroke, there are often areas that are ischemic but do not yet appear abnormal by DWI or ADC maps on early scans. Regions with decreased cerebral blood volume are usually involved in the final infarct, while regions with normal cerebral blood volume but low cerebral blood flow and increased mean transit time may or may not survive the ischemic insult.

The expectation that PWI and DWI could reliably define the ischemic penumbra and infarct core in acute stroke is still unrealized [75]. Although PWI can reveal the ischemic zone, the thresholds of PWI derived cerebral blood flow and volume that might discriminate the ischemic penumbra from infarct core have not been definitively established [76]. And while DWI can often reveal irreversibly infarcted tissue, it is now clear that some DWI lesions represent injured but still viable tissue [77,78]. In addition, while some cases manifest with a "classic" mismatch pattern where the ischemic core on DWI is embedded within a hypoperfused penumbral brain region on PWI (image 2), others show a "nonclassic" fragmented mismatch pattern in which part or all of the ischemic region on DWI is dissociated from the hypoperfused region on PWI (image 3) [79-81].

Consensus guidelines from the American Heart Association published in 2003 concluded that no recommendation could be given to employ PWI either to guide the use of thrombolysis or predict resulting complications such as post-thrombolytic hemorrhage [82]. A subsequent review evaluated MRI methods for selecting patients for thrombolysis and concluded that although DWI and PWI thresholds can delineate areas of brain with higher probability of infarction or salvage, their precise role in acute stroke management is not yet settled [83].

Despite these limitations, DWI and PWI have clear utility.

●Severe perfusion defects in areas with a diffusion-perfusion (DWI/PWI) mismatch may be a risk factor for lesion enlargement [84,85].

 

●Patients with an occluded artery are at a higher risk for lesion enlargement by growth of infarction into areas of perfusion deficit, implying that early recanalization (either spontaneously or with thrombolytic agents) may prevent lesion growth [86].

 

●Abnormal volumes on DWI and PWI during an acute stroke correlate well with initial NIH stroke scale scores, chronic scores, and final lesion volume, and also may predict early neurologic deterioration [87,88].

 

●Significant correction of focal brain hypoperfusion on PWI after tPA can predict excellent outcome at three months in ischemic stroke [89].

 

●Patients with acute ischemic stroke may in theory be selected for thrombolytic therapy based on DWI/PWI mismatch, thereby allowing extension of the conventional 3 hour window of opportunity for acute stroke thrombolysis [90,91]. (See "Reperfusion therapy for acute ischemic stroke", section on 'Intravenous thrombolysis'.)

 

The technique of cerebrospinal fluid-suppressed apparent diffusion coefficient (ADC) measurements can reduce the false elevation of ADC that results from cerebrospinal fluid (CSF) artifact and allow for a more accurate identification of ischemic tissue at risk for infarction [92]. This information may ultimately be most useful in identifying risk groups of patients who would benefit from various therapies such as thrombolysis in the acute setting.

MR angiography — MR angiography (MRA) to detect vascular stenosis or occlusion is done at many centers as part of a fast MRI protocol for acute ischemic stroke. Results from a case series showed that the combined use of DWI with MRA within 24 hours of hospitalization substantially improved the early diagnostic accuracy of ischemic stroke subtypes [93].

Contrast-enhanced MRA shows promise for improved imaging of intracranial large vessels compared with the more established time-of-flight technique [94]. For the detection of intracranial large vessel stenosis and occlusion, contrast-enhanced MRA in various studies had sensitivities of 86 to 97 percent and specificities of 62 to 91 percent when compared with conventional angiography [31]. The accuracy of MRA for the diagnosis of extracranial carotid stenosis is discussed separately. (See "Evaluation of carotid artery stenosis", section on 'MR angiography'.)

High susceptibility sequences — Increasing evidence supports the utility of high susceptibility MRI sequences (ie, GRE or T2* weighted images) for the early detection of acute thrombosis and occlusion involving the middle cerebral artery (MCA) or internal carotid artery (ICA) [95,96]. Acute thrombotic occlusion may appear on high susceptibility MRI as a hypointense (dark) signal within the MCA or ICA, often in a curvilinear shape; the diameter of the hypointense signal is larger than that of the contralateral unaffected vessel. This finding is called the susceptibility sign, and it is analogous to the hyperdense MCA sign described for CT imaging. (See 'Computed tomography' above.)

In a retrospective report of 42 patients with stroke in the MCA territory who had MR imaging 95 to 360 minutes from stroke onset, a positive susceptibility sign corresponding to MCA or ICA occlusion was found in 30 (71 percent) [96]. The specificity of the sign was 100 percent. The overall sensitivity was 83 percent compared with MR angiography but varied widely depending on location, from 38 percent for occlusions distal to the MCA bifurcation to 97 percent for occlusions proximal to the MCA trunk. Patients who had positive susceptibility signs had significantly higher NIHSS scores (table 1) compared with patients who did not have the sign, but no significant differences were found for infarct volume.

High susceptibility MRI sequences are also useful for the detection of acute intraparenchymal hemorrhage, especially if this is a concern after intra-arterial therapy, a situation where retained contrast is not easily distinguished from blood on CT [97]. (See "Spontaneous intracerebral hemorrhage: Pathogenesis, clinical features, and diagnosis", section on 'Hemorrhage appearance'.)

CT VERSUS MRI TECHNIQUES IN HYPERACUTE STROKE — The goals of very early neuroimaging are to exclude hemorrhage or stroke mimics, detect signs of early infarction, depict the infarct core and extent of perfusion deficit, reveal the status of large cervical and intracranial arteries, and guide treatment decisions [98]. As already noted, diffusion-weighted MRI (DWI) is more sensitive than CT for the early detection of acute ischemia, and high susceptibility MRI sequences such as gradient echo (GRE) are now known to be as good as CT for the detection of acute hemorrhage. (See 'Magnetic resonance imaging' above.)

These points are illustrated by a prospective single-center study that evaluated 356 patients referred because of suspicion for acute stroke irrespective of time from symptom onset [58]. Of these, 217 had a final clinical diagnosis of acute stroke. All 356 patients had both brain MRI (employing DWI and GRE) and head CT, with median times from symptom onset to scanning of 6.1 and 6.5 hours, respectively. Assessment of all brain images was blinded to clinical information.

The following observations were reported [58]:

●Acute ischemic stroke was detected in more patients by MRI than by CT (46 versus 10 percent), a difference that was statistically significant

 

●Acute intracranial hemorrhage detection was similar with MRI and CT (6 versus 7 percent)

 

●The sensitivity for the detection of any acute stroke was much greater for MRI than for CT (83 versus 26 percent), while specificity was similar (98 versus 97 percent)

 

●Contraindications to MRI (eg, electronic implants, patient intolerance, or medical instability) led to the exclusion of about 11 percent of the 450 patients screened for this study

 

These results suggest that MRI can be used as the only imaging method for patients with suspected acute ischemic or hemorrhagic stroke who have no MRI contraindications. In addition, a few reports have demonstrated that it is possible to use MRI routinely as the sole neuroimaging screening method prior to intravenous thrombolytic therapy [99,100]. In one such study of 135 patients screened with MRI and treated with intravenous tPA, quality improvement processes led to reduced door-to-needle times of ≤60 minutes [99].

Limited evidence suggests that the utility of head CT, when performed with CT perfusion imaging (CTP), may be equal to that of MRI in hyperacute stroke evaluation, as found in a study of 22 patients who were evaluated using both CT and MRI techniques within six hours of stroke onset (average time interval of 2.33 hours for CT and 3.0 hours for MRI) [101]. The following results were noted:

●Perfusion lesion volumes derived by CTP did not differ from those derived by perfusion-weighted imaging (PWI) for both time to peak maps and cerebral blood volume maps.

 

●CTA-SI ischemic lesion volumes did not differ from DWI ischemic lesion volumes.

 

●Lesion volumes on CTP cerebral blood flow maps significantly correlated with lesion volumes on follow-up non-contrast CT.

 

Other studies have shown that the CTP derived map of cerebral blood volume (CBV) correlates with the MRI DWI lesion size [38] and is predictive of infarcted brain tissue that is not salvageable despite reperfusion [35].

Many more patients considered for the study were eligible for contrast-enhanced CT than for MRI (93 versus 58 percent) [101]. This reflects the well-known problem that MRI in practice is more limited by patient contraindications or intolerance than CT. In addition, MRI is less widely available than CT outside of major stroke centers.

ULTRASOUND METHODS — Carotid Duplex ultrasound (CDUS) and transcranial Doppler (TCD) ultrasound are noninvasive methods for neurovascular evaluation of the extracranial and intracranial large vessels. Carotid and vertebral Duplex and TCD have traditionally been used independently in an elective fashion to evaluate patients with transient ischemic attack (TIA) and ischemic stroke of possible large artery origin.

Although both methods may help to establish the source of an embolic stroke, they have rarely been used acutely for this purpose. However, accumulating evidence suggests that both Duplex and TCD can be used urgently at the bedside to select patients for interventional thrombolytic or endovascular treatment [102-105]. (See 'Combined duplex and TCD' below.)

Carotid and vertebral duplex — Color flow guided duplex ultrasound is well established as a noninvasive examination to evaluate extracranial atherosclerotic disease. This topic is discussed separately. (See "Evaluation of carotid artery stenosis".)

Transcranial Doppler — TCD ultrasound uses low frequency (2 MHz) pulsed sound to penetrate bony windows and visualize intracranial vessels of the circle of Willis. Its use has gained wide acceptance in stroke and neurologic intensive care units as a noninvasive means of assessing the patency of intracranial vessels.

In patients with acute stroke, TCD is able to detect intracranial stenosis, identify collateral pathways, detect emboli on a real-time basis, and monitor reperfusion after thrombolysis [106-108]. Major drawbacks include examiner-dependence, poor patient windows (unable to insonate a flow signal in 15 percent of cases), and low sensitivity in the vertebrobasilar system.

Combined duplex and TCD — The combination of urgent duplex and TCD appears to have high utility when performed by skilled ultrasonographers, although the available data come mainly from small studies. As an example, a study of 150 patients found that the detection of arterial lesions amenable to interventional treatment (LAITs) by combined duplex and TCD (n=150) at mean time of 128 minutes after stroke or TIA onset was 100 percent sensitive and specific compared with digital subtraction angiography (DSA, n=30) [109]. The combination of duplex and TCD detected LAITs in 96 percent of patients eligible for thrombolysis. Accuracy of the individual components was lower but still good; duplex ultrasound had a sensitivity and specificity of 96 and 90 percent compared with DSA, while that of TCD was 96 and 75 percent. About 10 percent of patients had incomplete TCD studies because of inadequate temporal windows.

A major limitation of this approach is that most centers are unable to perform examinations acutely because they lack sufficient numbers of experienced ultrasonographers.

CONVENTIONAL ANGIOGRAPHY — Digital subtraction angiography, the most widely used method of conventional catheter-based angiography, remains the gold standard for evaluating the cerebral vessels with regard to determining the degree of arterial stenosis and the presence of dissection, vasculopathy, vasculitis, or occult lesions such as vascular malformations [31]. In addition, it provides information about collateral flow and perfusion status.

Nevertheless, diagnostic conventional angiography is rarely performed in the acute setting for two main reasons. One is the availability of the noninvasive techniques, such as CT angiography, MR angiography, duplex ultrasonography, and transcranial Doppler ultrasound, to rapidly visualize intracranial and extracranial arterial disease. The other is the risk of stroke, albeit low, associated with conventional angiography.

The major exception is suspected large vessel occlusion; angiography is more sensitive than noninvasive methods in these cases and offers the potential for "in-situ" treatment. In addition, angiography shows promise when combined with neurointerventional techniques for acute intraarterial thrombolysis and angioplasty.

The main drawback to conventional cerebral angiography is the risk of stroke (0.14 to 1 percent) and transient ischemia (0.4 to 3 percent) [110-115]. The risk of neurologic complications appears to be higher in patients ≥55 years of age, in patients with atherosclerotic cerebrovascular disease or cardiovascular disease, and with fluoroscopic time ≥10 minutes [114,115]. Clinically silent embolism as detected by diffusion-weighted magnetic resonance imaging (DWI) may occur in up to 25 percent of cerebral angiographic procedures [116,117]. The rate of clinically silent embolism may be reduced by use of air filters and heparin [118], but it is unclear if such methods reduce the more important clinical parameter of ischemic stroke.

INFORMATION FOR PATIENTS — UpToDate offers two types of patient education materials, "The Basics" and "Beyond the Basics." The Basics patient education pieces are written in plain language, at the 5th to 6th grade reading level, and they answer the four or five key questions a patient might have about a given condition. These articles are best for patients who want a general overview and who prefer short, easy-to-read materials. Beyond the Basics patient education pieces are longer, more sophisticated, and more detailed. These articles are written at the 10th to 12th grade reading level and are best for patients who want in-depth information and are comfortable with some medical jargon.

Here are the patient education articles that are relevant to this topic. We encourage you to print or e-mail these topics to your patients. (You can also locate patient education articles on a variety of subjects by searching on "patient info" and the keyword(s) of interest.) 

●Basics topics (see "Patient information: Stroke (The Basics)")

 

SUMMARY AND RECOMMENDATIONS — Our recommendations are based upon the available literature, consensus guidelines [31,119], and clinical experience.

Brain imaging plays a vital role in acute stroke by:

●Delineating ischemia from hemorrhage

●Estimating tissue at risk for infarction

●Excluding some stroke mimics, such as tumor

 

Head CT is the preferred imaging study at most centers because of widespread availability, rapid scan times, and ease of detecting intracranial hemorrhage. MRI has an advantage in the very early detection of ischemia with DWI imaging, and it reliably detects hyperacute hemorrhage with proper sequences including high susceptibility images.

Advances in the use of CT angiography (CTA) source images and CT perfusion imaging (CTP) suggest that CT techniques are increasingly able to provide crucial information regarding early ischemia and perfusion lesions in hyperacute stroke assessment. CT remains indispensable in the frequent circumstance where there are contraindications to MRI such as pacemakers and patient intolerance due to anxiety or motion.

●Brain imaging and a comprehensive neurovascular evaluation should be obtained for most patients suspected of having acute ischemic stroke or transient ischemic attack. Neurovascular imaging is important in acute stroke to determine the potential sources of embolism or low flow in ischemic stroke and to detect possible aneurysms or vessel malformations in hemorrhagic stroke.

 

●Brain imaging is required to guide the selection of acute interventions to treat patients with stroke. (See "Initial assessment and management of acute stroke" and "Reperfusion therapy for acute ischemic stroke" and "Antithrombotic treatment of acute ischemic stroke and transient ischemic attack".)

 

●Brain imaging in acute ischemic stroke can be obtained with either noncontrast head CT or conventional MRI. Both imaging techniques can be used to exclude acute intracerebral hemorrhage. Brain MRI with diffusion imaging is superior to noncontrast CT for the detection of acute ischemia and the exclusion of some stroke mimics. However, at present there are no data to show that MRI is superior to CT for selecting patients who could be treated with intravenous recombinant tissue plasminogen activator (alteplase [tPA]). Thus, MRI should be used rather than CT only if it does not unduly delay treatment with intravenous alteplase in an eligible patient.

 

●Where available, assessment of ischemic brain injury and brain perfusion status with either diffusion and perfusion MRI or with contrast CT source images and perfusion CT should be performed if the findings are likely to influence treatment decisions, such as acute thrombolysis or endovascular interventions.

 

●Neurovascular imaging should assess the extracranial (internal carotid and vertebral) and intracranial (internal carotid, vertebral, basilar, and Circle of Willis) large vessels. Noninvasive methods are preferred unless urgent endovascular therapy is planned. MR angiography (MRA), CTA, or the combination of ultrasound methods (Duplex and transcranial Doppler [TCD]) can be used. Conventional angiography is usually reserved for situations where acute intraarterial thrombolysis is being considered and for follow-up when noninvasive studies are inconclusive. Availability and expertise at individual centers is a major factor in the choice of the initial noninvasive neurovascular studies. A detailed comparison of the advantages and disadvantages of these studies for the diagnosis of carotid artery disease is found separately. (See "Evaluation of carotid artery stenosis".)

 

●Vascular imaging should not delay treatment with intravenous tPA (alteplase) for eligible patients with acute ischemic stroke. (See "Reperfusion therapy for acute ischemic stroke".)

 

●In select patients, neurovascular imaging of the aortic arch, vertebral artery origins, and common carotid arteries should be considered if clinical suspicion for a proximal large artery source of stroke or transient ischemic attack (TIA) is high. Useful methods include CTA, time-of-flight MRA, and contrast enhanced MRA. In addition, transesophageal echocardiography is useful for assessing the aortic arch.

 

●The brain imaging and neurovascular studies should not be considered in isolation, but rather as one part of the acute stroke evaluation. (See "Initial assessment and management of acute stroke".)

 

ACKNOWLEDGMENT — The editorial staff at UpToDate would like to acknowledge Walter Koroshetz, MD, who contributed to an earlier version of this topic review.

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