Current State of Acute Stroke Imaging
2013; Lippincott Williams & Wilkins; Volume: 44; Issue: 11 Linguagem: Inglês
10.1161/strokeaha.113.003229
ISSN1524-4628
Autores Tópico(s)Cerebrovascular and Carotid Artery Diseases
ResumoHomeStrokeVol. 44, No. 11Current State of Acute Stroke Imaging Free AccessResearch ArticlePDF/EPUBAboutView PDFView EPUBSections ToolsAdd to favoritesDownload citationsTrack citationsPermissionsDownload Articles + Supplements ShareShare onFacebookTwitterLinked InMendeleyReddit Jump toSupplementary MaterialsFree AccessResearch ArticlePDF/EPUBCurrent State of Acute Stroke Imaging Ramón Gilberto González, MD, PhD Ramón Gilberto GonzálezRamón Gilberto González From the Neuroradiology Division, Massachusetts General Hospital, Harvard Medical School, Boston, MA. Originally published26 Sep 2013https://doi.org/10.1161/STROKEAHA.113.003229Stroke. 2013;44:3260–3264Other version(s) of this articleYou are viewing the most recent version of this article. Previous versions: January 1, 2013: Previous Version 1 IntroductionAcute ischemic stroke is common and often treatable. Imaging by computed tomography (CT) and MRI is valuable for stroke treatment, in addition, for diagnosis, and for identification of the pathogenesis. But how they are used also depends on practical considerations. The approach to imaging the patient with acute stroke used at the Massachusetts General Hospital (MGH) is described. It is a distillation of our experience and a critical review of the literature and was developed through collaborations among the Acute Stroke Service, the Neuroradiology Division, and the Neurointerventional Program at the MGH. The focus is on rapid diagnosis, the guidance of treatment using intravenously administered tissue-type plasminogen activator (tPA), and intra-arterial treatments (IATs).Patients With Ischemic StrokeStroke is highly heterogeneous, but it is useful to categorize patients into hemorrhagic and ischemic stroke and the latter into those with mild, moderate, and severe symptoms (Table 1). The data in Table 1 are from the Screening Technology and Outcomes Project in Stroke (STOPStroke) study, a prospective, multiyear, observational study of patients with consecutive ischemic stroke at MGH and University of California, San Francisco.1 Most patients diagnosed with ischemic stroke have mild symptoms (National Institutes of Health Stroke Scale [NIHSS], 85%, if a major artery is occluded, and therapy is not instituted rapidly.Table 1. Clinical Status, Treatment, and ImagingNIHSS% Stroke PopulationTreatmentKey FactorsImaging 1035IV tPATime, hemorrhageCTIATime, hemorrhage, core, penumbraCT+MR or MR aloneAdapted from González et al.1 CT indicates computed tomography; IV tPA, intravenously administered tissue-type plasminogen activator; and NIHSS, National Institutes of Health Stroke Scale.Most patients with a severe stroke syndrome (NIHSS, >10) have a major anterior circulation occlusion, most commonly of a middle cerebral artery. Although the NIHSS is imperfect with respect to dominant hemisphere and whether the occlusion involves the anterior or posterior circulation, it is a reliable predictor of outcomes in the absence of treatment.2 Outcomes in such patients may be improved with arterial recanalization with the most important factor for outcomes being final infarct size. In a study of 107 patients who underwent IAT at the MGH for anterior circulation occlusions, nearly half of the patients with final infarct volumes of ≤60 mL had good outcomes defined as a modified Rankin Scale of 0 to 2.3 Patients with larger final infarct volumes usually had poor outcomes (modified Rankin Scale, >2), and no one with a final infarct of >120 mL had a good outcome.MGH Stroke Imaging AlgorithmThe imaging algorithm that we have adopted to evaluate the patient with acute stroke is shown in Figure 1. It is based on the reliability of CT and MRI to measure critical physiological factors (Table 2). Clinical reliability classification was made using an experienced and evidence-based approach that has been described.4 The algorithm is applicable to all patients with stroke. A patient presenting with a stroke syndrome undergoes neurological evaluation, including the NIHSS followed by a noncontrast CT (NCCT). If the patient is within the appropriate time window and the NCCT excludes hemorrhage and a large hypodensity, then tPA is given if there is no contraindication for it. It is prepared while a CT angiography (CTA) is performed, and tPA infusion commences when prepared. If the CTA reveals a distal internal carotid artery (ICA) occlusion, proximal middle cerebral artery (MCA) occlusion or both, a diffusion MRI is acquired. If the low diffusivity lesion volume is small (<70 mL), the patient proceeds to intra-arterial therapy if all other clinical and medical criteria are met. Perfusion imaging with CT or MRI may be performed if the patient is not eligible for MRI or for intra-arterial therapy.Table 2. Reliable Identification of Key Factors in StrokeHemorrhageMajor Artery OcclusionLate InfarctEarly Infarct CorePenumbraCT/CTA+++−−CTP−−−−+MRI/MRA++++−MRP−−−−+Neurological/NIHSS−−−−+Adapted from González et al.3 CTA indicates computed tomography angiography; CTP, CT perfusion; MRP, magnetic resonance perfusion; and NIHSS, National Institutes of Health Stroke Scale.Download figureDownload PowerPointFigure 1. Massachusetts General Hospital (MGH) acute ischemic stroke imaging algorithm. Patients with a new neurological deficit undergo noncontrast computed tomography (NCCT) scan to assess for hemorrhage and for the presence of a large completed infarct. This is followed immediately by CT angiography (CTA) to assess for accessible proximal artery occlusion. During this time, tissue-type plasminogen activator is prepared if the patient meets all relevant criteria, and it is administered if indicated. If the patient is a candidate for intra-arterial treatment (IAT) and there are no contradictions, a diffusion-weighted MRI scan (DWI) is acquired. If there is a small diffusion abnormality (<70 mL in an anterior circulation stroke), then the patient proceeds to endovascular therapy if the patient meets all other criteria for such treatment. If endovascular therapy is not indicated, if the diffusion abnormality is large, or if there is no large artery occlusion, then MRI perfusion may be considered. CT perfusion may be performed in patients who are not able to undergo MRI.Imaging of Patients With Stroke Not Eligible for Thrombolytic TherapyPatients presenting with mild stroke symptoms (NIHSS, 5) if the time since stroke onset is ≤4.5 hours. Patients with NIHSS 70 to 100 mL in anterior circulation occlusions, are usually not treated with IAT because such patients have a low probability of a good outcome3,6 and because the risk of reperfusion hemorrhage increases with pretreatment infarct size, especially when >100 mL.7 Because of the importance of core infarct size in predicting IAT outcomes,3,6 estimating the size of the core is critical.Vascular ImagingAfter establishing a significant neurological deficit in a patient, identifying the responsible arterial occlusion is the next most valuable piece of information. This may be accomplished with CT or MR angiography. Because patients with stroke typically first undergo NCCT imaging, it is efficient to acquire a CTA immediately after review of the NCCT images. With modern multidetector CT technology, the arterial system can be visualized from the aortic arch to the vertex in less than a minute. The reliability of CTA is very high,2,8–12 with a reported sensitivity of 98.4 and specificity of 98.1 using cerebral angiography as the gold standard.8 The rapid reconstruction and presentation of CTA images for review are important. Thick slab (30 mm), overlapping (5-mm slice interval) maximal intensity projection images in the 3 cardinal planes, may be created at the CT console immediately after data acquisition.13 Vascular imaging using MRI is also reliable, although less so than CTA. Three-dimensional time-of-flight MR angiography identifies proximal occlusions of major intracranial arteries with sensitivity of 84% to 87% and specificity of 85% to 98%.9,14Imaging the Infarct CoreThe volume of irreversibly injured brain is another important factor in addition to the severity of the neurological deficit and the presence a major arterial occlusion in gauging the potential benefit of IAT. Imaging can provide this information, but it must be accurate. If the core volume threshold is 70 mL, imaging must provide a measurement that is accurate within 10 to 20 mL.Diffusion MRIDiffusion MRI is the best available method for the early detection of the infarct core.2,15–17 Acute infarction produces a high contrast abnormality on diffusion-weighted imaging (DWI), the volume of which is relatively simple to quantify.18 The high contrast/noise ratio of DWI makes it accurate. DWI abnormalities sometimes reverse,19 but this is rare,20 and when it occurs, it usually involves only a small part of the lesion.6 In addition, a DWI reversal is often a pseudoreversal in that such tissue proceeds to infarction, despite apparent temporary normalization of the DWI signal abnormality.6Studies have shown that a DWI abnormality volume of >70 mL is highly specific for a poor outcome,21,22 and this threshold volume is useful in selecting patients for endovascular intervention.3 This threshold was also used in the Diffusion and Perfusion Imaging Evaluation for Understanding Stroke Evolution Study II (DEFUSE II) trial.6 The use of early infarct core identification for triage decisions is supported by the observations that the final infarct volume is the single best predictor of good outcome at 90 days.23,24 As shown by Yoo et al,23 good outcomes are observed in nearly half such patients when the final infarct volume was ≤60 mL. The rate of good outcomes rapidly declines with infarcts that are larger.Noncontrast CTCT is a frontline imaging method in acute stroke because it is reliable for detecting hemorrhage. Moreover, CTA may be subsequently acquired. However, NCCT is unreliable for detecting the early infarct core. NCCT is highly specific for infarction when a hypodensity is clearly visible, but such changes occur late.CT PerfusionMuch research has been devoted to developing CT perfusion (CTP) techniques for identification and quantification of the early infarct core. However, it is not sufficiently reliable. This is because it is a method that has inherently low signal/noise and contrast/noise ratios (CNR) and produces noisy images with high measurement error (Figure 2). Proponents of CTP may have been misled by correlation and regression studies of CTP-derived parameters in comparison with DWI or another gold standard. These studies typically show statistically significant correlations (Figure 3). A recent evidence-based analysis of diffusion and perfusion imaging in stroke by the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology found that diffusion MR was a Level A/Class I method but found insufficient evidence to even classify perfusion imaging.17Download figureDownload PowerPointFigure 2. Contrast/noise ratios (CNRs) and infarct volume detection on diffusion-weighted imaging (DWI) and computed tomography perfusion (CTP)–derived cerebral blood flow (CBF) images. Images are from a patient with acute stroke with a left middle cerebral artery stem occlusion. DWI is at top left, whereas the others are the same CTP-derived CBF image with different window settings. The top right image is a color-coded CBF image at a wide window. The bottom left CBF image is displayed with a narrow window with the center set at a level of 15% of the mean signal within a normal-appearing region. The bottom right CBF image is displayed with a narrow window with the center set at a level of 45% of the mean signal within a normal-appearing region. Mean signal intensities and signal SDs were obtained from regions-of-interest within the DWI hyperintense areas and from similar regions in the contralateral hemisphere. These values were used to calculate the CNRs. A similar procedure was done on the CBF image. CNR is >8 for the DWI-identified core, whereas it is <1 in the same region on the CBF image. The much higher clarity of the infarct border on the DWI compared with the CBF images is easily appreciated.Download figureDownload PowerPointFigure 3. Infarct volume estimates by diffusion MRI and computed tomography perfusion (CTP). The data were obtained from 19 patients with documented middle cerebral artery and distal internal carotid artery occlusions in which both diffusion MRI and CTP were performed at close time intervals. There is a significant correlation between the 2 measurements (r=0.6451; P 20 mL.There is no consensus on how to best apply CTP. A variety of acquisition parameters have been used, as well as many different data processing methods. In addition, different parameters, for example, cerebral blood volume (CBV) or cerebral blood flow (CBF) thresholds, have been proposed for defining infarcted tissue.25 The variability in data processing has been shown to produce different results for the same patient.26 There are differences in opinion whether CBV or CBF is the best parameter to define the infarct core.27 Moreover, it has been shown that 2 different commercial software packages result in large differences in estimates of infarct size using the same patient data.28 It is thought that standardization and validation will make CTP viable.29However, CTP is unlikely to become a reliable method to measure the core.4,30 Theory informs us that CBV may be elevated or depressed in core tissue and thus it is not useful. This has been empirically confirmed.31 CBF is more capable of estimating the infarct core. The reason is that below a certain CBF threshold, brain tissue is likely to be viable only for a short period of time. But there are problems that are related to the underlying imaging physics: the CNR of infarct cores on CTP-derived CBF images is very low (Figure 2). The figure shows the DWI and CTP-derived CBF images from a patient with a documented left MCA stem occlusion. The CNR of the core on DWI is >8, whereas the CNR on the CBF images is 10 in the presence of a DWI lesion of <70 mL).Time, Imaging, and Opportunities for Expanding Stroke TherapyOne of the most interesting observations by MRI of patients with stroke is the finding that many patients have a small cores and large penumbras even many hours after ictus. This suggests that there may be many patients who may be treatable beyond the currently accepted time windows. Early studies found significant diffusion/perfusion mismatches after 1037 to 24 hours38 after stroke onset. Ribo et al33 reported that 43 of 56 patients with stroke (77%) presenting 3 to 6 hours after stroke onset had a DWI/perfusion-weighted imaging mismatch of ≥50%.In a study of 109 patients with anterior circulation that had diffusion/perfusion MRI within 24 hours of stroke onset, more than half had a DWI/mean transit time mismatch volume of ≥160%.39 A large mismatch was most common in patients with occlusions involving the distal ICA and the proximal MCA. Notably, there was no time dependence: 69% of patients who were scanned for 8 hours have core infarcts below this threshold.ConclusionsCT and MRI of the patients with acute ischemic stroke provide valuable diagnostic and prognostic information. Neuroimaging can inform on the presence of hemorrhage, vessel occlusion, irreversible injury, and tissue at risk, which helps in making optimal patient management decisions. CT and MRI provide complementary information, and the most comprehensive understanding of the state of the brain in the patient with a stroke syndrome may require both. Much progress has been made in treating stroke, and new insights on stroke physiology provided by imaging suggest that there are major opportunities to treat many more patients effectively.DisclosuresNone.FootnotesCorrespondence to Ramón Gilberto González, MD, PhD, Neuroradiology Division, Massachusetts General Hospital, Harvard Medical School, Boston, MA 02114. E-mail [email protected]References1. Gonzalez R, Furie K, Goldmacher G, Smith W, Kamalian S, Payabvash S, et al. Good outcome rate of 35% in IV-tPA treated patients with computed tomography angiography confirmed severe anterior circulation occlusive stroke.Stroke. 2013; 44:3109–3113.LinkGoogle Scholar2. Jauch EC, Saver JL, Adams HP, Bruno A, Connors JJ, Demaerschalk BM, et al;American Heart Association Stroke Council; Council on Cardiovascular Nursing; Council on Peripheral Vascular Disease; Council on Clinical Cardiology. Guidelines for the early management of patients with acute ischemic stroke: a guideline for healthcare professionals from the American Heart Association/American Stroke Association.Stroke. 2013; 44:870–947.LinkGoogle Scholar3. Yoo AJ, Verduzco LA, Schaefer PW, Hirsch JA, Rabinov JD, González RG. MRI-based selection for intra-arterial stroke therapy: value of pretreatment diffusion-weighted imaging lesion volume in selecting patients with acute stroke who will benefit from early recanalization.Stroke. 2009; 40:2046–2054.LinkGoogle Scholar4. González RG, Copen WA, Schaefer PW, Lev MH, Pomerantz SR, Rapalino O, et al. The Massachusetts General Hospital acute stroke imaging algorithm: an experience and evidence based approach.J Neurointerv Surg. 2013; 5suppl 1i7–i12.CrossrefMedlineGoogle Scholar5. Urra X, Ariño H, Llull L, Amaro S, Obach V, Cervera Á, et al. The outcome of patients with mild stroke improves after treatment with systemic thrombolysis.PLoS One. 2013; 8:e59420.CrossrefMedlineGoogle Scholar6. Lansberg MG, Straka M, Kemp S, Mlynash M, Wechsler LR, Jovin TG, et al;DEFUSE 2 Study Investigators. MRI profile and response to endovascular reperfusion after stroke (DEFUSE 2): a prospective cohort study.Lancet Neurol. 2012; 11:860–867.CrossrefMedlineGoogle Scholar7. Lansberg MG, Thijs VN, Bammer R, Kemp S, Wijman CA, Marks MP, et al;DEFUSE Investigators. Risk factors of symptomatic intracerebral hemorrhage after tPA therapy for acute stroke.Stroke. 2007; 38:2275–2278.LinkGoogle Scholar8. Lev MH, Farkas J, Rodriguez VR, Schwamm LH, Hunter GJ, Putman CM, et al. CT angiography in the rapid triage of patients with hyperacute stroke to intraarterial thrombolysis: accuracy in the detection of large vessel thrombus.J Comput Assist Tomogr. 2001; 25:520–528.CrossrefMedlineGoogle Scholar9. Bash S, Villablanca JP, Jahan R, Duckwiler G, Tillis M, Kidwell C, et al. Intracranial vascular stenosis and occlusive disease: evaluation with CT angiography, MR angiography, and digital subtraction angiography.AJNR Am J Neuroradiol. 2005; 26:1012–1021.MedlineGoogle Scholar10. Torres-Mozqueda F, He J, Yeh IB, Schwamm LH, Lev MH, Schaefer PW, et al. An acute ischemic stroke classification instrument that includes CT or MR angiography: the Boston Acute Stroke Imaging Scale.AJNR Am J Neuroradiol. 2008; 29:1111–1117.CrossrefMedlineGoogle Scholar11. Cipriano LE, Steinberg ML, Gazelle GS, González RG. Comparing and predicting the costs and outcomes of patients with major and minor stroke using the Boston Acute Stroke Imaging Scale neuroimaging classification system.AJNR Am J Neuroradiol. 2009; 30:703–709.CrossrefMedlineGoogle Scholar12. González RG, Lev MH, Goldmacher GV, Smith WS, Payabvash S, Harris GJ, et al. Improved outcome prediction using CT angiography in addition to standard ischemic stroke assessment: results from the STOPStroke study.PLoS One. 2012; 7:e30352.CrossrefMedlineGoogle Scholar13. Pomerantz SR, Harris GJ, Desai HJ, Lev MH. Computed tomography angiography and computed tomography perfusion in ischemic stroke: A step-by-step approach to image acquisition and three-dimensional postprocessing.Semin Ultrasound CT MR. 2006; 27:243–270.CrossrefMedlineGoogle Scholar14. Tomanek AI, Coutts SB, Demchuk AM, Hudon ME, Morrish WE, Sevick RJ, et al. MR angiography compared to conventional selective angiography in acute stroke.Can J Neurol Sci. 2006; 33:58–62.CrossrefMedlineGoogle Scholar15. González RG, Schaefer PW, Buonanno FS, Schwamm LH, Budzik RF, Rordorf G, et al. Diffusion-weighted MR imaging: diagnostic accuracy in patients imaged within 6 hours of stroke symptom onset.Radiology. 1999; 210:155–162.CrossrefMedlineGoogle Scholar16. Mullins ME, Schaefer PW, Sorensen AG, Halpern EF, Ay H, He J, et al. CT and conventional and diffusion-weighted MR imaging in acute stroke: study in 691 patients at presentation to the emergency department.Radiology. 2002; 224:353–360.CrossrefMedlineGoogle Scholar17. Schellinger PD, Bryan RN, Caplan LR, Detre JA, Edelman RR, Jaigobin C, et al;Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology. Evidence-based guideline: the role of diffusion and perfusion MRI for the diagnosis of acute ischemic stroke: report of the Therapeutics and Technology Assessment Subcommittee of the American Academy of Neurology.Neurology. 2010; 75:177–185.CrossrefMedlineGoogle Scholar18. Sims JR, Gharai LR, Schaefer PW, Vangel M, Rosenthal ES, Lev MH, et al. ABC/2 for rapid clinical estimate of infarct, perfusion, and mismatch volumes.Neurology. 2009; 72:2104–2110.CrossrefMedlineGoogle Scholar19. Campbell BC, Purushotham A, Christensen S, Desmond PM, Nagakane Y, Parsons MW, et al;EPITHET–DEFUSE Investigators. The infarct core is well represented by the acute diffusion lesion: sustained reversal is infrequent.J Cereb Blood Flow Metab. 2012; 32:50–56.CrossrefMedlineGoogle Scholar20. Grant PE, He J, Halpern EF, Wu O, Schaefer PW, Schwamm LH, et al. Frequency and clinical context of decreased apparent diffusion coefficient reversal in the human brain.Radiology. 2001; 221:43–50.CrossrefMedlineGoogle Scholar21. Sanák D, Nosál' V, Horák D, Bártková A, Zelenák K, Herzig R, et al. Impact of diffusion-weighted MRI-measured initial cerebral infarction volume on clinical outcome in acute stroke patients with middle cerebral artery occlusion treated by thrombolysis.Neuroradiology. 2006; 48:632–639.CrossrefMedlineGoogle Scholar22. Yoo AJ, Barak ER, Copen WA, Kamalian S, Gharai LR, Pervez MA, et al. Combining acute diffusion-weighted imaging and mean transmit time lesion volumes with National Institutes of Health Stroke Scale Score improves the prediction of acute stroke outcome.Stroke. 2010; 41:1728–1735.LinkGoogle Scholar23. Yoo AJ, Chaudhry ZA, Nogueira RG, Lev MH, Schaefer PW, Schwamm LH, et al. Infarct volume is a pivotal biomarker after intra-arterial stroke therapy.Stroke. 2012; 43:1323–1330.LinkGoogle Scholar24. Zaidi SF, Aghaebrahim A, Urra X, Jumaa MA, Jankowitz B, Hammer M, et al. Final infarct volume is a stronger predictor of outcome than recanalization in patients with proximal middle cerebral artery occlusion treated with endovascular therapy.Stroke. 2012; 43:3238–3244.LinkGoogle Scholar25. Dani KA, Thomas RG, Chappell FM, Shuler K, MacLeod MJ, Muir KW, et al;Translational Medicine Research Collaboration Multicentre Acute Stroke Imaging Study. Computed tomography and magnetic resonance perfusion imaging in ischemic stroke: definitions and thresholds.Ann Neurol. 2011; 70:384–401.CrossrefMedlineGoogle Scholar26. Konstas AA, Goldmakher GV, Lee TY, Lev MH. Theoretic basis and technical implementations of CT perfusion in acute ischemic stroke, part 2: technical implementations.AJNR Am J Neuroradiol. 2009; 30:885–892.CrossrefMedlineGoogle Scholar27. Kamalian S, Kamalian S, Maas MB, Goldmacher GV, Payabvash S, Akbar A, et al. CT cerebral blood flow maps optimally correlate with admission diffusion-weighted imaging in acute stroke but thresholds vary by postprocessing platform.Stroke. 2011; 42:1923–1928.LinkGoogle Scholar28. Fahmi F, Marquering HA, Streekstra GJ, Beenen LF, Velthuis BK, VanBavel E, et al. Differences in CT perfusion summary maps for patients with acute ischemic stroke generated by 2 software packages.AJNR Am J Neuroradiol. 2012; 33:2074–2080.CrossrefMedlineGoogle Scholar29. Wintermark M, Albers GW, Alexandrov AV, Alger JR, Bammer R, Baron JC, et al. Acute stroke imaging research roadmap.AJNR Am J Neuroradiol. 2008; 29:e23–e30.MedlineGoogle Scholar30. González RG. Low signal, high noise and large uncertainty make CT perfusion unsuitable for acute ischemic stroke patient selection for endovascular therapy.J Neurointerv Surg. 2012; 4:242–245.CrossrefMedlineGoogle Scholar31. Deipolyi AR, Wu O, Macklin EA, Schaefer PW, Schwamm LH, Gilberto Gonzalez R, et al. Reliability of cerebral blood volume maps as a substitute for diffusion-weighted imaging in acute ischemic stroke.J Magn Reson Imaging. 2012; 36:1083–1087.CrossrefMedlineGoogle Scholar32. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement.Lancet. 1986; 1:307–310.CrossrefMedlineGoogle Scholar33. Ribo M, Molina CA, Rovira A, Quintana M, Delgado P, Montaner J, et al. Safety and efficacy of intravenous tissue plasminogen activator stroke treatment in the 3- to 6-hour w
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