STROKE IMAGING IS EASY TO UNDERSTAND , YET VERY IMPORTANT TO EVERY DOCTOR
A term that describes an acute episode of neurologic deficit.
80% of strokes are due to cerebral ischemia (embolic or thrombotic).
Transient ischemic attacks (TIAs) are focal neurologic events that resolve within 24 hours.
Those that resolve after 24 hours are called reversible ischemic neurologic deficits (RINDs).
|1) Cerebral infarction, 80%:||o Atherosclerosis-related occlusion of vessels, 60%
o Cardiac emboli, 15%
o Other, 5%
|2) Intracranial hemorrhage, 15%|
|3) Non-traumatic SAH, 5%|
|4) Venous occlusion, 1%|
- Atherosclerotic disease:
- Atherosclerosis represents the most common cause of cerebral ischemia/infarction.
- Carotid atherosclerosis causes embolic ischemia;
- Intracranial atherosclerosis causes in-situ thrombotic or distal embolic ischemia.
- ICA origin > distal basilar > carotid siphon, MCA.
- Critical carotid stenosis:
- Defined as a stenosis of >70% in luminal diameter.
- Patients with critical stenosis & symptoms have an é risk of stroke & benefit from carotid endarterectomy.
- Patients with stenosis < 70% or who are asymptomatic are usually treated medically.
- Imaging Features:
|Gray-scale imaging (B-scan) of carotid arteries:||o Evaluate plaque morphology/extent
o Determine severity of stenosis (residual lumen)
o Other features:
§ Slim sign à collapse of ICA above stenosis
§ Collateral circulation
|Doppler imaging of carotid arteries:
|o Severity of stenosis determined by measuring peak systolic velocity:
§ 50 – 70% à velocity 125 to 250 cm/sec
§ 70 – 90% à velocity 250 to 400 cm/sec
§ > 90% à velocity >400 cm/sec
o Stenoses > 95% may result in decreased velocity (<25 cm/sec)
o 90% accuracy for >50% stenoses
o Other measures used for quantifying stenosis:
§ End diastolic velocity (severe stenosis à >100 cm/sec)
§ ICA/CCA peak systolic velocity ratio (severe stenosis à >4)
§ ICA/CCA peak end-diastolic velocity ratio
o Innominate artery stenosis may cause right CCA/ICA parvus tardus
o CCA occlusion may result in reversal of flow in ECA
|Color Doppler flow imaging of carotid arteries:||o High-grade stenosis with minimal flow (string sign in angiography) is detected more reliably than with conventional Doppler US.|
|CT and MR angiography||Are used for confirmation of US diagnosis of carotid stenosis:
o On CTA, 1.0- to 1.5-mm residual lumen corresponds to 70%-90% stenosis.
o To determine complete occlusion versus a string sign (near but not complete occlusion), delayed images must be obtained immediately after the initial contrast images.
o At some institutions, carotid endarterectomy is performed on the basis of US and CTA/MRA if the results are concordant.
o Pitfalls of US and MRA in the diagnosis of carotid stenosis:
§ Near occlusions (may be over diagnosed as occluded)
§ Post-endarterectomy (complex flow, clip artifacts)
§ Ulcerated plaques (suboptimal detection)
§ Tandem lesions (easily missed)
|Carotid arteriography:||The gold standard is primarily used for:
o Post-endarterectomy patient
o Accurate evaluation of tandem lesions and collateral circulation
o Evaluation of aortic arch and great vessels
o Discordant MRA/CTA & US results
- Cerebral ischemia & infarction:
- Cerebral ischemia:
- Refers to a diminished blood supply to the brain.
- Refers to brain damage, being the result of ischemia.
|Large vessel occlusion, 50%|
|Small vessel occlusion (lacunar infarcts), 20%|
|Emboli:||o Cardiac, 15%:
§ Arrhythmia, atrial fibrillation
§ Atrial myxoma
§ MI (anterior infarction)
§ Left ventricular aneurysm
§ Fat, air embolism
o Polyarteritis nodosa
|o Hypo-perfusion (border-zone or watershed infarcts)
o Vasospasm: ruptured aneurysm, SAH
o Hematologic abnormalities:
§ Hypercoagulable states
§ Hb abnormalities (CO poisoning, sickle cell)
o Venous occlusion
o Moyamoya disease
- Imaging Features:
|Angiographic signs of cerebral infarction:||o Vessel occlusion, 50%
o Slow antegrade flow, delayed arterial emptying, 15%
o Collateral filling, 20%
o Non-perfused areas, 5%
o Vascular blush (luxury perfusion), 20%
o AV shunting, 10%
o Mass effect, 40%
|Cross-sectional imaging:||Ø CT à the 1.st study of choice in acute stroke in order to exclude:
ü Intracranial hemorrhage.
ü Underlying mass/AVM.
Ø Most CT examinations are normal in early stroke.
Ø Early CT signs of cerebral infarction include:
Ø Edema (maximum edema occurs 3 – 5 days after infarction):
§ Cytotoxic edema àdevelops within 6 hours (detectable by MRI).
§ Vasogenic edema àdevelops later (first detectable by CT at 12 – 24 hours).
Ø Characteristic differences () distributions of infarcts:
§ Embolic à periphery, wedge shaped
§ Hypo-perfusion in watershed areas of ACA/ MCA and MCA/PCA
§ Border-zone infarcts
§ Basal ganglia infarcts
§ Generalized cortical laminar necrosis
Ø Reperfusion hemorrhage is not uncommon after 48 hours:
§ MRI much more sensitive than CT in detection
§ Most hemorrhages are petechial or gyral.
Ø Mass effect in acute infarction:
ü Sulcal effacement
ü Ventricular compression
Ø Sub-acute infarcts:
ü Hemorrhagic component, 40%
ü Gyral or patchy contrast enhancement (1 – 3 weeks)
ü GWM edema
Ø Chronic infarcts:
ü Focal tissue loss à atrophy, porencephaly, cavitation, focal ventricular dilatation
ü Wallerian degeneration à distal axonal breakdown along white-matter tracks
CT & MRI appearances of infarcts:
|Factor||1st Day||1st Week||1st Month||> 1 Month|
|Stage:||Acute||Early subacute||Late subacute||Chronic|
|CT density:||Subtle decrease||Decrease||Hypodense||Hypodense|
|MRI:||T2W: edema||T2W: edema||Varied||T1W dark, T2W bright|
|Hemorrhage:||No||Most likely here||Variable||MRI detectable|
|Enhancement:||No||Yes; maximum at 2-3 weeks||Decreasing||No|
- Cerebral infarcts cannot be excluded on the basis of a negative CT.
- MRI with diffusion weighted imaging (DWI) & perfusion weighted imaging (PWI) should be performed immediately if an acute infarct is suspected.
- CM administration is àreserved for clinical problem cases & should not be routinely given, particularly on the first examination.
- Luxury perfusion refers to hyperemia of an ischemic area:
- The increased blood flow is thought to be due to compensatory vasodilatation 2.ry to parenchymal lactic acidosis.
- Cerebral infarcts have à a peripheral rim of viable but ischemic tissue (penumbra).
|§ Acute cerebral ischemia may result in a central irreversibly infarcted tissue core surrounded by peripheral region of stunned cells that is called àa penumbra
§ These cells have ceased to function, but this region is potentially salvageable with early re-canalization.
- Thrombotic & embolic infarcts occur in vascular distributions àe., MCA, ACA, PCA, etc.
- MR perfusion/diffusion studies are àimaging studies of choice in acute stroke:
- DWI detects reduced diffusion coefficient in acute infarction, which is thought to reflect cytotoxic edema.
- In patients with multiple T2W signal abnormalities from a variety of causes, DWI can identify those signal abnormalities that arise from acute infarction.
- 50% of patients with TIA have DWI abnormality.
- MRI in acute stroke:
- On PD/T2WI and FLAIR infarction is seen as àhigh
- These sequences detect 80% of infarctions before à24 hours.
- They may be negative up to à2 – 4hours post-ictus!
- High signal on conventional MR-sequences is comparable to àhypo-density on CT.
- It is the result of irreversible injury with cell death.
- So hyper-intensity means BAD news à dead brain.
|Hyper-intensity on MR = irreversible ischemic brain damage|
- Comprehensive Stroke Protocol: MR
|MRI:||o Sag T1, Ax T2, Ax DWI|
|MRA:||o TOF head & neck|
|MRA:||o elliptic-centric arch /carotids|
|Perfusion:||ü Single dose Gad
ü Automated arterial input function
ü Parameter maps: CBV, MTT, CBF
|Post Gad:||o ax T1, coronal T2 FLAIR|
- Imaging features:
|Loss of flow voids:||o Artery or vein.|
|Loss of Flow-Related Enhancement|
|Intravascular enhancement:||o DD of intra-vscular enhancement:
ü Sturge-Weber syndrome
|Parenchymal Enhancement:||o Rule of 3’s à starts 3days, peaks 3 weeks, gone 3 months.
o Parenchymal MR Imaging in Stroke:
ü Confirm diagnosis
ü Ischemic vs. hemorrhagic
ü Underlying cause
ü Risk of progression:
ü Mass effect / herniation
ü Hemorrhagic transformation
- MRI is better than CT for Hemorrhage:
- MRI may be as accurate as CT for the detection of acute hemorrhage in patients presenting with acute focal stroke symptoms.
- MRI is more accurate than CT for the detection of chronic intra-cerebral hemorrhage.
- Diffusion & perfusion imaging in stroke:
- Standard diffusion protocol includes a DWI & an apparent diffusion coefficient (ADC) image àusually interpreted side by side:
|DWI:||o Summation of diffusion + T2 effects.
o Abnormalities appear as à high signal.
|ADC:||o Diffusion effects only.
o Abnormalities appear as àlow signal.
- Performed using the susceptibility effects of a rapid bolus injection of gadolinium administered intravenously.
- Rapid continuous scanning during this injection allows the signal changes associated with the gadolinium to be plotted over time for a selected brain volume.
- These time-signal plots can be processed to yield several possible parameters relating to cerebral perfusion.
- Vascular (perfusion) parameters:
|Mean Transit Time: (MTT)||§ Measured in àseconds.
§ A measure of how long it takes blood to reach the particular region of the brain.
|Cerebral Blood Volume: (CBV)||§ Measured in àrelative units &
§ Correlates to the total volume of circulating blood in the voxel.
|Cerebral Blood Flow: (CBF)||§ Measured in àrelative units &
§ Correlates to the flow of blood in the voxel.
- Stroke Evolution on MRI:
(> 6 hours)
(Days to weeks)
|DWI||High||High||High (ê with time)||Iso-intense to bright|
|ADC||Low||Low||Low to iso-intense||Iso-intense to bright|
|TW2/Flair||Iso-intense||Slightly bright to bright||Bright||Bright|
- A typical infarct is DWI bright & ADC dark.
- Gliosis appears DWI bright due to T2 shine-through but is also bright on ADC.
- Very sensitive for detecting disease àwill pick up infarcts from about 30 minutes onward.
- But is non-specific & will also detect non-ischemic disease.
- Less sensitive than DWI.
- But dark signal is fairly specific for restricted diffusion, which usually means ischemia.
- Significance of a DWI-bright, ADC-dark lesion:
- This tissue will almost certainly go on to infarct & full necrosis.
- Rare instances of reversible lesions have been reported àvenous thrombosis, seizures, hemiplegic migraine and hyper-acute arterial thrombosis.
- Match and mismatch:
- In the acute stroke setting, a region that shows àmatched both diffusion and perfusion abnormalities is thought to represent irreversibly infarcted tissue,
- While a region that shows àmismatched perfusion abnormalities and diffusion likely represents viable ischemic tissue, or a penumbra
|EXP: (exponential)||o The map that “subtracts” the T2 effect.
o In equivocal cases, use EXP map as àa problem solver:
ü If it stays bright on the EXP map, then it is àtrue restricted diffusion.
|MTT:||o Highly sensitive for disturbances in perfusion.
o But not good for prediction of later events.
ü For example à an asymptomatic carotid occlusion would have a dramatically abnormal MTT, without the patient being distressed.
|CBV:||o A parameter that changes late in the ischemic cascade, &
o Usually reduced CBV is also accompanied by restricted diffusion..
o Reduced CBV + restricted diffusion àcorrelate well with tissue that goes on to infarction.
|CBF:||o In experimental setting:
ü Can be used to predict the likelihood of brain tissue infarcting.
o In current clinical practice:
ü If a CBF abnormality exceeding the DWI abnormality (diffusion- perfusion mismatch):
§ This implies that there is brain at risk that has not infracted yet.
§ This brain at risk is the target of therapeutic interventions.
Role of CT/CTA in acute stroke:
|1) Important in early stages of stroke evaluation to facilitate thrombolytic therapy.
§ Demonstrates the anatomic details of the neuro-vasculature from the great vessel origins at the aortic arch to their intracranial termination.
§ Highly accurate in the identification of proximal large vessel circle of Willis occlusions &
§ Therefore in the rapid triage of patients to intra-arterial (IA) or intravenous (IV) thrombolytic therapy.
CT Early signs of ischemia:
|Hypo attenuating brain tissue:||o The reason we see ischemia on CT is that in ischemia cytotoxic edema develops as a result of failure of the ion-pumps.
o These fail due to an inadequate supply of ATP.
o An increase of brain water content by 1% will result in a CT attenuation decrease of 2.5 HU.
o Hypo-attenuation on CT is highly specific for irreversible ischemic brain damage if it is detected within first 6 hours.
o Patients who present with symptoms of stroke and who demonstrate hypo-density on CT within first 6 hours were proven to have larger infarct volumes, more severe symptoms, less favorable clinical courses and they even have a higher risk of hemorrhage.
o Therefore whenever you see hypo-density in a patient with stroke this means bad news.
o No hypodensity on CT is a good sign.
|Obscuration of the lentiform nucleus:||o Obscuration of the lentiform nucleus, also called blurred basal ganglia, is an important sign of infarction.
o It is seen in MCA infarction & is one of the earliest & most frequently seen signs.
o The basal ganglia are almost always involved in MCA infarction.
|Insular Ribbon sign:||o This refers to hypo-density & swelling of insular cortex.
o It is a very indicative & subtle early CT-sign of infarction in the territory of MCA.
o This region is very sensitive to ischemia because it is the furthest removed from collateral flow.
o It has to be differentiated from herpes encephalitis.
|Dense MCA sign:||o This is a result of thrombus or embolus in the MCA.
o On CT-angiography occlusion of the MCA is visible.
|Loss of sulcal effacement|
- Stroke protocol (CT/CTP/CTA):
o To exclude hemorrhage àan absolute contraindication to thrombolytic therapy.
o To detect irreversible “core” of infarction (>1⁄3 of a vascular territory) à a relative contraindication to thrombolysis.
§ Scanning parameters:
o 140 kV, 170 mA, pitch = “high quality” (3: 1)
o Table speed à 7.5 mm/sec.
o Scanning from à skull base to vertex.
o Slice thickness à 5 mm
o Window width ànarrow settings, with a center level of about 30 HU (width of 5 to 30 HU):
v This facilitates the detection of early, subtle, ischemic changes contiguous with normal parenchyma.
|CTP:||§ Scanning parameters:
o IV contrast bolus ~ 50 cc
o Dynamic scanning ~ 45 seconds
o 4-8 slices
o 5 mm
o Calculate perfusion parameters:
v Cerebral blood volume CBV
v Mean transit time MTT
v Cerebral blood flow CBF
|CTA:||§ Scanning parameters:
Initial phase scan parameters:
o IV contrast (non-ionic, non-osmolar CM) bolus 100 ml (90-120 ml) à 4 cc/sec
o Scan delay of à 25 sec.
v A longer delay may be needed for patients with à compromised cardiac function & atrial fibrillation.
o Scanning from à skull base to the vertex
o Used parameters are as per the NCCT scan.
Second phase scan parameters:
o Performed immediately after initial scan, with minimal possible delay.
o Scanning from the aortic arch to the skull base.
o Similar scan parameters except for àan é in the table speed to 15 mm/sec.
§ Note that:
o Major advantages of first scanning the intracranial circulation include:
v obtaining the most important data first, which can be reviewed during subsequent acquisition;
v allowing time for clearance of dense IV contrast from the subclavian, axillary, and other veins at the thoracic inlet, reducing streak artifact
o On a >16-slice scanner, CTA may be performed from the vertex to the aortic arch in one pass,
v This gets rid of the loss of contrast enhancement in the neck CTA usually seen with a two-stack protocol
- Lacunar infarcts:
- Lacunar infarcts:
- Account for à20% of all strokes.
- The term refers to à occlusion of penetrating cerebral arterioles
- Commonly affected are:
|§ Thalamoperforators (thalamus)
§ Lenticulostriates (caudate, putamen, internal capsule)
§ Brainstem perforator (pons)
- Usually cause characteristic clinical syndromes:
- Pure motor hemiparesis, pure hemisensory deficit, hemiparetic ataxia, or dysarthria-hand deficit.
- Imaging Features:
|o MRI is àthe imaging study of choice.
o Small ovoid lesion (< 1 cm):
§ Hyper-intense on T2W & proton density–weighted (PDW) image.
o Location of lesions is very helpful in DDx:
ü Dilated peri-vascular or Virchow-Robin (VR) space:
§ Can be large (giant VR space), can cause mass effect & can have surrounding gliosis
§ More elongated appearance on coronal images
- Within 3 hours of stroke onset à IV thrombolysis + recombinant tissue plasminogen activator (r-tPA)
- If > 3 hours of stroke onset à no IV thrombolysis given (é probability of intracranial hemorrhage)
- The time window for treatment with IA agents is:
- Twice as long for the anterior circulation &
- Indefinite for the posterior circulation.
- For thrombosis localized to the posterior circulation:
- The time window for treatment may be extended beyond 6 hours due to the extreme consequences of loss of blood flow to the brainstem, despite the risk of hemorrhage.
- Advanced CTA/CTP imaging of acute stroke:
- Has the potential to not only help exclude patients at high risk for hemorrhage from thrombolysis,
- But also identify those patients most likely to benefit from thrombolysis.
- Even without hemorrhage, treatment failure with thrombolytics is not uncommon.
- The choice between IA & IV thrombolysis depends on a variety of factors, including:
|o the time post ictus,
o the clinical status of the patient, &
o whether the clot is proximal (IA) or distal (IV).
- When typical findings of occlusive thrombus on CTA & ê tissue enhancement on CTP are not present, the DDx include:
|o Lacunar infarct.
o Early small distal embolic infarct.
o Transient ischemic attack.
o Complex migraine headaches.
- Golden points:
|What you like to know?||§ Is there hemorrhage?
§ How extensive is the edema?
|§ Where’s the occlusion?
§ What’s the blood flow?
§ How much brain is dead?
§ How much brain is at risk?
|4 P:||o PARENCHYMA:
§ Exclude Hge.
§ Early signs of stroke.
§ Intra-cranial circulation.
§ Extra-cranial circulation
o PERFUSION à CBV –CBF –MTT
o PENUMBRA : Tissue at risk of dying