Image interpretation

Diffusion-weighted imaging

The major advantage of using MRI for acute stroke imaging is having a DWI sequence as the most sensitive imaging modality for early ischemia ( Fig. 1 ). DWI has level I evidence for assessment of ischemic core ^, and is now the recommended sequence in patient selection for late window thrombectomy.

Higher sensitivity of DWI versus CT for early acute stroke. A 55-year-old man who presented with left-sided weakness, approximately 1 hour prior to imaging onset. NIHSS: 21. Axial noncontrast CT images demonstrate subtle loss of gray-white differentiation in the right basal ganglia, insula, and frontal operculum, suggesting acute infarction. Subsequent MR imaging was obtained within 15 minutes of CT. Axial DWI and corresponding ADC maps are shown, demonstrating more clearly the extent of acute infraction, highlighting the increased sensitivity/accuracy of DWI compared with noncontrast CT in identifying early infarct core.

The advent of DWI has redefined stroke syndromes, such as acute ischemic infarction and transient ischemic attack (TIA), from an all-or-none process to a dynamic and evolving process. Approximately 40% of patients with a clinically defined TIA can have restricted diffusion indicative of ischemic infarction, a finding that has led to an ASA-endorsed proposal to revise the definition of TIA from time-based to tissue-based criteria. Another useful point of information obtained from DWI is that the pattern of the DWI abnormalities provides insight into the underlying etiology and stroke subtypes, such as lacunar or embolic etiologies.

Fluid-attenuated inversion recovery

The major applications of FLAIR in acute stroke imaging are (1) a complementary role to GRE for detection of intracranial hemorrhage ( Fig. 2 ) and, in particular, for identifying subtle subarachnoid hemorrhage, (2) FLAIR hyperintense vessel sign as a marker of slow flow and collaterals; in the setting of proximal arterial occlusion, FLAIR high signal intensities in the blood vessels within the subarachnoid spaces have been attributed to hemodynamic impairment, representing slow retrograde flow in leptomeningeal collateral ; and (3) determining the age of the infarction, which probably is the most important application. As a rule, the prevalence of lesion visibility on FLAIR increases as time passes from the stroke onset and up to 93% of acute stroke lesions could have positive FLAIR findings at greater than 6 hours. Following successful completion of the WAKE-UP (Efficacy and Safety of MRI-based Thrombolysis in Wake-up Stroke) trial FLAIR, in addition to DWI (DWI-FLAIR mismatch), now is recommended to identify acute so-called call wake-up stroke (discussed later). A faster variety of FLAIR imaging (EPI-FLAIR) can be obtained in one-third the acquisition time of conventional FLAIR but with similar diagnostic performance and has been used successfully for patient treatment decision making.

Detection of intracranial hemorrhage with MR imaging. A 68-year-old man who presented with headache, vomiting, and blurred vision. Stroke code was activated given presence of a dense left hemianopsia on neurologic examination. Axial FLAIR and GRE images are shown, demonstrating heterogenous signal intensity in the right occipital with multiple layering fluid levels and corresponding susceptibility signal on GRE, compatible with acute parenchymal hematoma.

Gradient-echo

The common applications of GRE are (1) detection of acute intracranial hemorrhage in pretreatment screening (see Fig. 2 ), where GRE alone or in combination with FLAIR has excellent diagnostic accuracy in detection of acute intracranial hemorrhage comparable to CT ^, ; (2) detection of intraarterial clot seen as increased local susceptibility signal, known as susceptibility vessel sign ^, —this is defined as a dark blooming artifact visible on T2∗-weighted GRE images because of the magnetic susceptibility effect of deoxygenated hemoglobin in the red blood cells trapped in an occluded artery ( Fig. 3 ); and (3) detection of hemorrhagic transformation in AIS patients treated with thrombolytic or mechanical revascularization therapies ( Fig. 4 ). Although CT is equally sensitive for detection of parenchymal hematomas, petechial hemorrhages are detected by GRE with significantly higher sensitivity. The clinical importance of these microbleeds when chronic is unknown and current AHA/ASA guidelines do not recommend MR imaging–GRE screening to withhold thrombolytic treatment. Faster variety of GRE imaging, such as EPI-GRE, provides similar diagnostic performance to conventional GRE but only a fraction of the acquisition time.

Stroke secondary to atherosclerotic disease. A 67-year-old man who presented with aphasia and right-sided weakness, approximately 4 hours prior to imaging. Baseline NIHSS: 9. DWI demonstrates acute infarction in the left frontal lobe, including Broca area, with larger region of Tmax perfusion delay in the left MCA territory. GRE demonstrates focal susceptibility signal at the left M1/M2 junction suggestive of thrombus ( white arrowhead ). Axial multiplanar reformat from CE-MRA shows a focal superior division M2 occlusion corresponding to the region of thrombus ( red arrowhead ). Coronal multiplanar reformat from CE-MRA of the neck demonstrates high-grade, severe stenosis of the left proximal internal carotid artery ( arrow ); the etiology of the MCA infarct was presumed as artery-to-artery embolus from ICA atherosclerotic disease.

Favorable diffusion-perfusion mismatch profile in patient selection for mechanical thrombectomy. A 62-year-old woman who presented with right-sided hemiparesis; time from onset to imaging of 9 hours; NIHSS: 13. Aligned axial DWI, ADC, FLAIR, and Tmax maps from perfusion are shown from baseline MR imaging. There is acute infarct with mild FLAIR signal hyperintensity in the left basal ganglia (volume: 15 mL) and a larger region of Tmax greater than 6 perfusion delay in the left MCA territory (volume: 100 mL), with mismatch volume of 85 mL and mismatch ratio of 6.7. CE-MRA demonstrated left M1 occlusion ( arrowhead ). Given the favorable mismatch profile, despite presentation greater than 6 hours since last known well, patient underwent mechanical thrombectomy, with recanalization of the left M1 (TICI [thrombolysis in cerebral infarction] grade 2b). Post-thrombectomy MR imaging (DWI and GRE) 6 hours later shows mild petechial bleeding within the left basal ganglia infarct, without significant infarct expansion.

MR angiography

Vascular imaging is an essential component of any stroke imaging protocol. In potential candidates for EVT, a noninvasive vascular study of intracranial circulation (level Ia) and extracranial arteries (level IIa) is recommended during the initial imaging evaluation. Inclusion of neck and extracranial vascular tree provides useful additional information about the aortic arch anatomy, tortuosity of the supra-aortic arteries, and possibility of tandem proximal stenosis, to facilitate treatment planning and achieving safer and faster reperfusion in patients with acute stroke (see Fig. 3 , Fig. 5 ). In AIS patients with suspicious of arterial dissection, fat-saturated T1-weighted images can be obtained to look for intramural hematoma (see Fig. 5 ).

Stroke secondary to common carotid artery dissection. A 52-year-old woman who presented with acute-onset left hemiplegia and dysarthria approximately 5 hours from the onset. Axial DWI ( A ) demonstrates areas of acute infarct in the right MCA territory, involving the basal ganglia and right parietal lobe. Multiplanar reformats from CE-MRA ( B , C ) of the brain and neck demonstrate abrupt right M1 occlusion ( arrowhead [ B ]) and a smooth segmental stenosis of the right common carotid artery ( arrow [ C ]). Axial T1 image with fat suppression ( D ) demonstrates crescentic T1 hyperintensity adjacent to the right common carotid artery ( arrowhead [ D ]) compatible with intramural hematoma in setting of arterial dissection; the intramural hematoma is not clearly visualized on T1 images without fat suppression ( E ), due to surrounding fat tissue in the carotid space.

The commonly used MRA techniques in stroke imaging include noncontrast time-of-flight (TOF) technique and CE-MRA. The sensitivity of CT angiography (CTA) and MRA in comparison to the gold standard of digital subtraction angiography ranges from 87% to 100%, with CTA having a slight edge over MRA. ^, In the setting of acute stroke imaging, CE-MRA is advantageous over TOF-MRA because it provides fast and efficient delineation of intracranial arteries and collateral circulation beyond the point of occlusion afforded by T1 shortening effect of gadolinium injection and with similar performance to CTA. TOF-MRA can be limited by longer acquisition times and inability to assess distal arterial stenosis due to spin saturation and phase dispersion secondary to slow, in-plane, or turbulent flow. ^, Recent studies have shown higher diagnostic accuracy for CE-MRA than TOF-MRA in detection of arterial stenosis and occlusion in patents with AIS. ^, With the advent of fast imaging tools, CE-MRA of the entire supraoptic arteries now can be obtained during a 20-second acquisition time. Combining CE-MRA and dynamic susceptibility contrast (DSC) perfusion has been shown clinically feasible, ^, to provide results similar to those accomplished by CTA and CT perfusion (CTP).

MR perfusion

Due to fast acquisition time and relative ease of postprocessing, bolus DSC has been used as a robust and widely accepted technique to measure cerebral perfusion in patients with acute stroke. The major applications are as follows:

• 1.
  

  Defining ischemic penumbra: in patients with AIS and following a cerebral arterial occlusion, there is a developing ischemic core and surrounding hypoperfused brain tissue that potentially is at risk of infarction in the absence of reperfusion. This potentially salvageable tissue is referred to as ischemic penumbra. Timely revascularization, on the other hand, can salvage the ischemic penumbra from infarction. Multiple parametric maps can be obtained from DSC perfusion to assess hemodynamic compromise, including mean-transit time, time-to-maximum (Tmax), cerebral blood flow, and cerebral blood volume (CBV). Among these, Tmax has been used consistently for assessment of ischemic penumbra and volume of Tmax greater than 6 seconds is now considered a measure of tissue-at-risk and a target for thrombectomy therapies (see Fig. 4 ).

  
  
• 2.
  

  Identifying DWI-negative TIA patients: DWI-negative patients, who account for greater than 50% of all TIA patients, remain a source of uncertainty and great clinical challenge. Many of these patients may have transient cerebral ischemia, where the degree of hemodynamic compromise did not reach the severity required to cause restricted diffusion. Perfusion imaging can be used to detect regions of hypoperfusion or postischemic hyperperfusion in approximately 25% to 30% of these patients, providing additional evidence to confirm a footprint of ischemia that otherwise would go undetected.

Thrombolytic treatment

For AIS patients presenting less than 4.5 hours from onset, in absence of contraindication, thrombolytic treatment can be administered if there is (1) no intracranial hemorrhage and (2) no large ischemic core. The latter is defined as lack of clearly hypoattenuating region on initial CT rather than subtle hypodensities on noncontrast CT that can contribute toward a lower ASPECTS (Alberta Stroke Program Early CT Score) in the most updated AHA/ASA guidelines. There is no APECTS cutoff for exclusion of patients from receiving thrombolytic treatment. If using MR imaging for initial assessment of AIS patients who present less than 4.5 hours from the onset, IV tissue plasminogen activator can be administered safely in the absence of large infarction core on diffusion MR imaging and intracranial hemorrhage.

For patients presenting with unknown onset or wake-up strokes, there likely is a potential role for MR imaging for thrombolytic treatment selection. Unwitnessed AIS is relatively common, accounting for approximately 15%–25% of all AIS patients, who before 2019 would not have received reperfusion therapies. Many of these patients, however, presumed to be after 4.5 hours from onset, still have salvageable brain tissue that is maintained by collateral circulation. With the recent positive WAKE UP trial, which used DWI-FLAIR mismatch to select patients for extending the time window for IV thrombolysis, use of MR imaging (FLAIR-DWI mismatch) now is recommended (level IIa) to identify unwitnessed AIS patients who may benefit from thrombolytic treatment ( Fig. 6 ). Also, perfusion imaging has been used for extending the thrombolytic window up to 9 hours according to recently published results of Extending the Time for Thrombolysis in Emergency Neurologic Deficits trail.

Only gold members can continue reading. Log In or Register to continue

Share this:

• Click to share on Twitter (Opens in new window)
• Click to share on Facebook (Opens in new window)

Related posts:

Management of Unruptured Intracranial Aneurysms Subarachnoid Hemorrhage of Unknown Cause High-Resolution Magnetic Resonance Vessel Wall Imaging for the Evaluation of Intracranial Vascular Pathology Computed Tomography–Based Imaging Algorithms for Patient Selection in Acute Ischemic Stroke Imaging for Treated Aneurysms (Including Clipping, Coiling, Stents, Flow Diverters) Brain Arteriovenous Malformations

Stay updated, free articles. Join our Telegram channel

Join