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Acute Stroke Imaging

Acute Stroke Imaging. Ellen G. Hoeffner, MD Neuroradiology Division Department of Radiology University of Michigan Health System. Goals and Objectives. Provide an update on the role of imaging in acute stroke patients

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Acute Stroke Imaging

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  1. Acute Stroke Imaging Ellen G. Hoeffner, MD Neuroradiology Division Department of Radiology University of Michigan Health System

  2. Goals and Objectives • Provide an update on the role of imaging in acute stroke patients • Describe the indications for CT/CTA/CTP and MRI/MRA/MRP in patients presenting with acute stroke symptoms • Recognize the findings of acute ischemia on CT/CTA/CTP and MRI/MRA/MRP

  3. Role of Imaging in Acute Ischemic Stroke • Exclude intracranial hemorrhage and other stroke mimics • Identify intravascular thrombus that can be treated with thrombolysis or thrombectomy • Identify the presence and size of a core of irreversibly infarcted tissue • Identify hypoperfused tissue at risk for infarction if adequate perfusion is not restored

  4. Imaging the Parenchyma • Patients who present up to 3-4.5 hours after sx onset and are IV tPA candidates should get a non-enhanced head CT (NECT) (Stroke 2013;44:870-947) • Determine if early ischemic signs are present • Size, location and vascular distribution • Frank hypodensity on CT involving > 1/3 MCA territory is relative contraindication to rx • Goal - CT obtained in 25 min of arrival in ER and interpreted within an additional 20 min • Can also use MRI to r/o hemorrhage and determine if ischemic signs present per AHA/ASA guidelines

  5. Hemorrhage and Stroke Mimics

  6. Early Ischemic Signs (EIS)- Sulcal Effacement • Usually seen in conjunction with hypodensity • Rarely seen without hypodensity • May reflect elevated cerebral blood volume in vasodilatedcollateral vessels

  7. Early Ischemic Signs (EIS) - Hypodensity • Hypodensity corresponding to territory of affected vessel • Increased water in affected area with cytotoxic edema • Typical locations • Insular ribbon – loss of insular ribbon • Basal ganglia – obscuration of basal ganglia • Lobar cortex – loss of cortical ribbon

  8. Early Ischemic Signs (EIS) - Hypodensity • Factors affecting detection of EIS • Stroke severity • Time from symptom onset • More rapidly the hypodensity becomes evident, more profound the ischemia • Detection rate of EIS within 3 hours 31-67% • Increases up to 82% by 8 hours • At best moderate agreement among a variety of physicians in detecting and quantifying EIS on NCCT (Stroke 1999; 30: 1528-33)

  9. Improves detection of EIS (Radiology 1999;213:150-155) Standard WW – 80 HU CL – 20 HU Variable WW – 8 HU CL – 32 HU Variable Window Width and Center Level

  10. Alberta Stroke Program Early CT Score (ASPECTS) Lancet 2000;355:1670- 74 Normal = 10 Entire MCA territory = 0 NeuroimagClin N Am 2011;21:407-23

  11. What is > 1/3 MCA Territory? • ASPECTS value < 4 or 5 ~ > 1/3 MCA territory (Int J Stroke 2009;354-64 and Stroke 2006;37: 973–78) • Lesion volume > 70-100 cm3 ~ > 1/3 MCA territory • Can estimate volume using ABC/2 rule (Neurology 2009;72:2104–2110) • A and B are largest axial dimensions • C is number of axial slices the lesion appears on measured by slice thickness A B ABC/2 = [5.6 cm x 2.2 cm x (5 x 0.5 cm)]/2 = 15.4 cm3

  12. Early Ischemic Signs (EIS) - Hyperdense Vessel • Hyperdense Middle Cerebral Artery Sign (HMCAS) • Intravascular blood clot in M1 segment of MCA • Often associated with large infarcts, severe clinical course, and poor response to IV PA • Clot length > 8 mm little recanalization potential with IV tPA(Stroke 2011;42:1775-7)

  13. Early Ischemic Signs (EIS) - Hyperdense Vessel • MCA dot sign • Hyperdensity of small arteries in Sylvian fissure, seen in cross section • Clot in M2 and M3 branches • Strokes less severe than with HMCAS

  14. DWI • Time course of restricted diffusion • Appears within minutes after arterial occlusion • Nadir at 1-4 days • Persists up to 2 weeks • Sensitive (73 – 100%) in detecting acute ischemic stroke • More sensitive than CT, especially early after symptom onset • Specific (86 -100%) in differentiating infarctions from other lesions • False negative cases - small strokes clinically localized to brainstem or deep gray nuclei (AJNR 1998;19:1061-6 )

  15. DWI • DWI exclusion criteria used by some clinicians • Lesion volume > 70 – 100 mL (1/3 MCA territory) • Volume > 70 mL generally poor outcome regardless of treatment and recanalization status (J Cereb Blood Flow Metab 2010;30:1214–25 and Stroke 2009;40:2046–54) • Lesion > 100 mL increased risk hemorrhagic transformation (Ann Neurol 2008;63:52–60)

  16. DWI ADC

  17. MRI for Hemorrhage Detection • MRI as accurate as CT for detection of acute hemorrhage (JAMA 2004;292:1823-1830) • T2*-weighted GRE or EPI-SWI sequences must be used • Hemorrhage, acute and chronic, seen as area of hypointensity

  18. Vascular Imaging – Intracranial • Strongly recommended for acute stroke patients if IA fibrinolysis or mechanical thrombectomy is being considered (Stroke 2013;44:870-947) • Should not delay use of IV tPA • May help triage patient to best therapy (IV vs IA/endovascular)

  19. Vascular Imaging - Extracranial • Imaging soon after sx onset may be useful (Stroke 2013;44:870-947) • Helps determine stroke mechanism and potentially prevent a recurrence • Necessary to assess vessels prior to emergent endovascular treatment or emergent endarterectomy

  20. CTA • Disadvantages • Iodinated contrast • Radiation • Streak artifact • Atherosclerotic calcifications produce streak artifact and can result in overestimation of stenosis • Flow artifacts on 64 slice scanners • Advantages • Widely available, fast • Spatial resolution 2X that of MRA • 98 % sensitivity and specificity for detecting acute proximal intracranial occlusive thrombus • 97% sensitivity and 99 % specificity for detecting extracranial occlusion • 85 % sensitivity and 93% specificity for detecting >70% extracranial stenosis

  21. MRA • Disadvantages • Less practical in emergency setting than CT in most institutions • Gadolinium needed for CE MRA • More hampered by artifacts • Inferior sensitivity for intracranial occlusion (80-90%) and stenoses (60-85%)compared to CTA • Advantages • No need for contrast with 2D and 3D TOF techniques • Comparable to CTA for detection of extracranial disease

  22. Vascular Imaging • Findings that help guide and plan therapy • Proximal occlusion - IV tPA less effective in ICA and M1 occlusions • Clot burden - larger CTA clot burden associated with … • Larger infarct volume • Lower recanalization rates • Greater risk of intraparenchymal hematoma and death • Less likely to have independent functional outcome (Int J Stroke 2008;3:230-6) • Collateral circulation - better the collateral supply, smaller the infarct volume (AJNR 2009 30: 525-31) • Severe stenosis proximal to occlusion can alter treatment

  23. Perfusion Imaging • Perfusion imaging may help in selecting patients for acute reperfusion therapy beyond IV tPA time window (Stroke 2013;44:870-947) • Identify patients with potentially salvageable tissue (penumbra) from those with extensive infarct (core) • Provide additional information regarding diagnosis, mechanism and severity of ischemic stroke • CBV, CBF and MTT most common parameters measured • CBF = CBV/MTT

  24. Infarct core – irreversibly infarcted and nonviable • Energy and ion pump failure with neurologic sx • Ischemic penumbra – at risk for infarction, but still salvageable • Failure of normal electrical activity with neurologic sx • Patients with large penumbra and small core may benefit from revascularization CORE PENUMBRA

  25. CT Perfusion • Disadvantages • No DWI equivalent • Quantitative values vary among different software programs • Limited coverage • Uses ionizing radiation and iodinated contrast • Advantages • Availability and speed • Lower cost • Easier quantification

  26. CT Perfusion • Qualitative assessment • Salvageable tissue - MTT, / CBV, CBF • Infarct - MTT, CBV, CBF • Area of decreased CBV considered infarct core • Area of decreased CBV smaller than MTT abnormality (mismatch) - suggests penumbra present • MTT abnormality = core + penumbra • Decreased CBV = core

  27. . MTT CBV CBF Findings compatible with left MCA infarct with penumbra CBV MTT CBF

  28. MTT CBF CBV Findings compatible with right PCA infarct w/ penumbra CBV MTT CBF

  29. MR Perfusion • Disadvantages • Less practical • Lack of linearity between signal intensity and contrast concentration • No consensus as to which perfusion parameters are most accurate • No consensus in defining core, penumbra, mismatch ratio • Advantages • Can combine with DWI, best method for defining core • Whole brain coverage • No ionizing radiation • Use of diffusion-perfusion mismatch for selecting patients for rx validated in small trails

  30. MR Perfusion • DWI abnormality smaller than PWI abnormality (mismatch) suggests penumbra present • PWI abnormality = core + penumbra • DWI abnormality = core • Qualitative assessment • No consensus as to which perfusion parameter best defines the penumbra • CBF reduction and MTT prolongation are more sensitive indicators of potentially viable tissue (penumbra) • CBV reduction marker of infarct core

  31. DWI MTT CBV

  32. MR Perfusion • Diffusion-perfusion mismatch is a strong indicator of infarct growth • In patients with diffusion-perfusion mismatch, infarct growth and final infarct size significantly smaller in those treated with t-PA vs. controls (Ann Neurol 2002;51:28–37)

  33. DWI TTP

  34. Initial PWI Initial DWI DWI 3 days after IV t-PA Ann Neurol 2002;51:28–37

  35. MR Perfusion • In setting of acute ischemia/infarction may have normal diffusion, but abnormal perfusion • Up to 25 % of patients with hyperacute stroke may have abnormalities on perfusion imaging only • This is group of patients who may benefit most from perfusion imaging

  36. Optimum diffusion-perfusion mismatch ratio as a target for therapy has not been defined • Initially, arbitrarily set as PWI abnormality 20% larger than DWI abnormality • More recent studies suggest PWI abnormality may need to be significantly larger than DWI (possibly 2-4 times larger) for best chance of favorable outcome (Stroke 2011; 42: S12-S15)

  37. Summary • 0-3 or 4.5 hours • NECT +/- CTA • EIS; hemorrhage or non-stroke cause for sx • Large infarct (> 70-100 cc) worse outcome • Proximal, large clot worse outcome with IV tPA • MRI +/- MRA suitable alternative

  38. Summary • 3 or 4.5 hours – 6 hours or more • NECT, CTA +/- CTP or MRI, MRA +/- MRP • Large infarct (> 70-100 cc) worse outcome • Thrombus amenable to IA rx • Maybe large penumbra

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