1	Basic Brain Imaging focus on stroke Adam Guttentag M.D.
2	Stroke Acute change in neurological status. Symptoms persist longer than 24 hours. Resolution in less than 24 hours = TIA Ischemia 80% Embolic Thrombotic Hemorrhage 20%
3	Stroke = “Brain Attack” Rapid clinical and imaging evaluation. “Stroke team” needed Thrombolytic therapy for appropriate patients <3 hours from onset of Sx No contraindications to tPA (long list) Involvement of large area of brain a contraindication Risk of hemorrhage from tPA Assessed by imaging and clinical evaluation CT usually negative in the first few hours
4	Acute stroke Initial imaging CT without IV contrast Rapid Reliable to exclude hemorrhage High sensitivity for acute blood Acute blood appears bright on CT relative to brain tissue. IV contrast only if suspicion of another etiology, e.g. tumor or AVM
5	Normal 40 year old
6	Normal 65 year old
7	Ischemic Stroke CT 1^st 6 hours: 60% normal Vague hypodensity in ischemic area. Insular ribbon sign Sulcal effacement from slight swelling Loss of grey/white interface 12-24 hours More apparent hypodensity Minimal mass effect
8	Ischemic Stroke CT After 24 hours Well circumscribed hypodensity 3-5 days Peak mass effect Mass effect gone by 2-4 weeks Long term Ex vacuo dilatation of ventricles Encephalomalacia in infarcted area
9	Acute right sided weakness
10	24 hours later
11	Insular ribbon sign
12	Acute MCA Infarct - Insular ribbon sign CT
13	Dense MCA sign CT Thrombus in vessel is hyperdense relative to flowing blood
14	Basilar artery thrombus CT Equivalent to dense MCA sign
15	Acute left hemiplegia
16	Acute Right MCA infarct CT Minimal mass effect Well circumscribed infarct Cytotoxic edema pattern
17	Differentiating cytotoxic from vasogenic edema Cytotoxic Grey and white matter Wedge shape Infarct Vasogenic Almost exclusively in white matter tracts Finger-like projections Infection, XRT, tumor
18	Diffuse cerebral edema from hanging
19	Acute ΔMS: New stroke?
20	Ischemic Stroke CT Contrast enhancement Not usually indicated unless there’s suspicion of a different process Enhancement in infarcted area begins after about three days Enhancement more than 6-8 weeks after ictus suggests another etiology New role: “Perfusion” scans of brain with rapid IV contrast infusion.
21	Ischemic Stroke CT– limitations Low sensitivity for acute ischemic infarct Decision to use thrombolytic based on absence of CT findings Potential use of thrombolytic in patients without thrombotic or embolic infarct Arterial dissection Postictal state Infection or tumor Up to 40% of acute strokes are not from causes amenable to thrombolysis 20% without defect seen on angiogram 10-20% from small vessel disease or arterial dissection
22	Acute stroke: Initial imaging MRI Diffusion weighted images take only ~2 minutes highly sensitive for acute ischemia Contraindications Pacer, AICD, aneurysm clip, etc Full imaging takes ~30 minutes Special MR compatible life support and monitoring equipment needed
23	Ischemic Stroke MRI Glossary - types of pulse sequences T1WI- images where signal depends on T1 of tissues T2WI- images where signal depends on T2 of tissues FLAIR = FLuid Attenuated Inversion Recovery Images with pathology bright and CSF dark DWI = Diffusion Weighted Imaging Fast acquisition where signal depends on ability of water to diffuse in all directions
24	Ischemic Stroke MRI Acute stroke appears as Normal to low signal on T1WI High signal on T2WI High signal on FLAIR High signal on DWI FLAIR and DWI most sensitive
25	Ischemic Stroke MRI Abnormal area becomes bright on DWI within 30 minutes of onset of ischemia High signal visible on T2WI in about 8 hours T1WI image becomes abnormal after ~16 hours
26	Ischemic Stroke MRI Infarcted area remains bright on T2WI forever. As encephalomalacia occurs, infarct becomes progressively darker on T1WI until it matches signal of CSF
27	Ischemic Stroke Diffusion Weighted Imaging Requires high field magnet with rapid gradients In ischemic tissue, diffusion of water molecules in 3 dimensions is restricted Magically, these areas appear as bright areas on DWI Ask a physicist how it works…
28	Ischemic Stroke Diffusion Weighted Imaging Area of ischemia visible within 30 minutes 10-14 days later, diffusion is “normal” Brain becomes isodense Late, with encephalomalacia, area shows low signal T2WI becomes progressively brighter with respect to DWI as infarct ages crossover at about 3-7 days
29	Ischemic Stroke Diffusion Weighted Imaging
30	Ischemic Stroke Diffusion Weighted Imaging Super-sensitive Not super-specific False positives: Hemorrhage MS Abscess Lymphoma and other tumors In uncertain situations, repeat in 2 weeks Infarct should normalize on DWI
31	Hyperacute Left MCA infarct MRI
32	Subacute Infarct MRI Dark on T1WI, bright on T2WI and FLAIR, very bright on DWI
33	Acute R ICA occlusion
34	MRA in acute stroke? Good for seeing larger intracerebral vessels only. Not generally useful unless intervention is planned very early. In most strokes, treatment is not usually affected by identification of an occluded vessel. Ask your neurologist if the information is useful.
35	Not all ischemic damage is due to acute thrombosis or embolism: Vasculopathies: Diabetic Hypertensive Giant cell (temporal) arteritis Takayasu’s arteritis PAN Sarcoid Collagen vascular diseases esp. SLE Wegener’s granulomatosis drugs Larger vessel disease Moyamoya Fibromuscular dysplasia Infectious disease Syphilis TB Herpes Nonvascular Global hypoxia CO poisoning
36	Hemorrhagic stroke CT CT best for finding acute bleeding Normal white matter = 30-34 HU Normal grey matter = 37-41 HU Attenuation of blood depends on hemoglobin conc. Blood with Hct of 45% = 56 HU Acutely extravasated blood will be denser than brain and easily visible Clotted blood, e.g. subdural hematoma, will be even denser.
37	By the way, what are HU? Hounsfield Units Measure of CT density H₂0 = 0 HU
38	Intracranial hemorrhage - possible locations Intraparenchymal Intraventricular Subarachnoid Subdural / epidural
39	Hemorrhagic stroke CT Acute bleed in anemic patient will be harder to detect. With Hct <30%, nonclotted blood may be isodense with brain. Highest clot density at about 72 hrs after bleed High density disappears after several weeks
40	Intraparenchymal Bleeding location, location, location… Hypertensive bleed - most common (80%) Basal ganglia Thalamus Pons Cerebellum
41	Don’t be fooled:
42	Intraparenchymal Bleeding Atypical locations like cerebral hemispheres AVM Aneurysm Berry and mycotic Trauma Amyloid angiopathy Tumor Vascular mets, primary tumors Vasculitis, cocaine, amphetamine, bleeding diathesis, anticoagulation, etc
43	Intraparenchymal bleed CT — natural history Accompanied by vasogenic edema, some mass effect Consider enhanced scan for mass effect out of proportion to size of hematoma Possibility of underlying tumor Later, some peripheral enhancement may be seen Confusion with other etiologies Much later, thin slit of hypodensity is seen at site of bleed.
44	Subarachnoid hemorrhage Berry aneurysm Trauma
45	Subarachnoid hemorrhage Berry aneurysm 80-90% of subarachnoid bleeds Prevalence 1-5%? Familial vs. sporadic Adult polycystic kidney disease (10% of pts) Marfan’s, Ehlers-Danlos Fibromuscular dysplasia Multiple in 15-20%
46	Subarachnoid bleeding Berry aneurysm Mortality 10-15% Often re-bleed in first day 50% will re-bleed within 6 months Angiogram and treatment early Before onset of arterial spasm leading to infarction MRA or CTA? Rapid (2-3 minutes) No need for specialized call personnel Endovascular therapy vs. surgery
47	Subdural and Epidural Hematoma Usually traumatic in origin Subdural Venous bleed Low pressure Slow growth Epidural Arterial bleed High pressure Rapid growth Both may be life-threatening from mass effect and herniation
48	Acute subdural hematoma CT
49	Intraventricular Hemorrhage Trauma Hypertension Aneurysm
50	Hemorrhagic stroke MRI Signal from blood depends on pulse sequence, age of clot Hemoglobin contains iron Strong paramagnetic effects on signal from nearby protons
51	Hemorrhagic stroke MRI Natural history of hemoglobin in clot
52	MRI in acute ΔMS Standard T1 and T2 weighted images are insensitive for small acute hemorrhage Need CT first Diffusion weighted images are highly sensitive for acute ischemia Area of acute infarction visible within 30 minutes of onset of ischemia
53	Subacute subdural hematomas
54	Take home messages: Bleed or not? CT for initial evaluation of acute ΔMS MR more rapidly positive for acute ischemia DWI positive within 20 minutes. But: early identification of presence and size of infarct not usually necessary. Only if thrombolysis is being considered
55	Take home messages: Old or new stroke? CT: look for water density and dilatation of underlying ventricle = old stroke. MRI: water signal on T1 and T2 and low signal on DWI = old stroke Cause of bleed? Know typical locations of hypertensive hemorrhages: Basal ganglia, cerebellum, pons
56	Review Questions
57	Concerning acute stroke: (T or F) Patients are candidates for tPA thrombolysis up to 24 hours after onset of symptoms. MRI is the modality of choice for initial evaluation of acute stroke symptoms. Diffusion weighted imaging shows acute infarction earlier than any other modality. 50% are due to acute hemorrhage.
58	Hypertensive hemorrhage is common in all but: Basal ganglia Temporal lobe Cerebellum Brainstem
59	Intravenous contrast: all are true except: Contrast enhanced CT is not generally useful in early stroke diagnosis Persistence of contrast enhancement after 6-8 weeks suggests underlying tumor. Contrast enhanced MRI is more useful than enhanced CT in early stroke. Perfusion CT may help diagnose major stroke as rapidly as MRI
60	Additional reading Beauchamp N et al. Imaging of acute cerebral ischemia. Radiology:1999;212:307-324 Von Kummer R et al. Acute stroke: usefulness of early CT findings before thrombolytic therapy Radiology 1997; 205: 327-333. Hoeffner EG et al. Cerebral Perfusion CT: Technique and Clinical Applications Radiology 2004 231: 632-644 Provenzale JP et al. Assessment of the Patient with Hyperacute Stroke: Imaging and Therapy Radiology 2003;229:347-359.
61	The End