IMAGING IN STROKE

MODERATOR- DR C P AHIRWAR (MD)PRESENTOR- DR NEELAM SONI

STROKE

Stroke is an acute central nervous system injury with abrupt onset.

This can occur following ischemia caused by blockage (thrombosis, arterial embolism) or a hemorrhage of CNS or intracranial blood-vessels.

Approximately 80% of all strokes are due to acute ischemia .

It is a leading cause of morbidity and mortality in the developed world.

GOALS OF IMAGING

To establish the diagnosis as early as possible.

Give accurate information about intracranial vasculature and brain perfusion for guidance in selecting the appropriate therapy.

Imaging should target assessment of 4 P’s:

• Parenchyma: • Assess early signs of acute stroke, rule out hemorrhage

• Pipes • Assess extracranial circulation (carotid and vertebral arteries

of the neck) and intracranial circulation for evidence of intravascular thrombus

• Perfusion • Assess cerebral blood volume, cerebral blood flow, and mean

transit time• Penumbra

• Assess tissue at risk of dying if ischemia continues with out re-canalization of intravascular thrombus

Overview of imaging modalities

Unenhanced CT• Can be performed quickly.• Can help identify early signs of stroke, and can

help rule out hemorrhage. CT angiography can depict intravascular

thrombi CT perfusion imaging can demonstrate

salvageable tissue which is indicated by a penumbra.

Acute infarcts may be seen early on conventional MR images, but diffusion-weighted MR imaging is more sensitive for detection of hyperacute ischemia.

Gradient-echo MR sequences can be helpful for detecting a hemorrhage.

MR Angiography – To evaluate the status of neck and intracranial vessels

DWI AND PWI - A mismatch between findings on diffusion and perfusion MR images may be used to predict the presence of a penumbra.

PENUMBRA

TISSUE AT RISK OR SALVAGEABLE TISSUE.

Pathophysiology - Neuronal tissue is sensitive

for ischemia due to lack of stored energy

With complete absence of flow neuronal viability is 2-3 minutes

In acute stroke ischemia is always incomplete due to rich collateral supply

ACUTE STROKE

Acute cerebral ischemia may result in a central irreversibly infarcted tissue core surrounded by a peripheral region of stunned cells that is called as a penumbra

This region is potentially salvageable with early recanalization

The transition from ischemia to irreversible infarction depends on both the severity and the duration of the diminution of blood flow

Minutes

Days and weeksTime

Hours

ISCHEMIC PENUMBRA

IDENTIFIED BY CT - ALTERED PARAMETERS IN PERFUSION MR – PERFUSION DIFFUSION MISMATCH

PRESENCE OF PENUMBRA HAS SIGNIFICANT IMPLICATIONS IN PT MANAGEMENT

CT

KEY TECHNIQUES NECT CTA PERFUSION CT

NECT Widely available. Can be done quickly. It not only can help identify a hemorrhage (a

contraindication to thrombolytic therapy), but it also can help detect early-stage acute ischemia by depicting features such as –

1. THE HYPERDENSE VESSEL SIGN. 2. THE INSULAR RIBBON SIGN. 3. OBSCURATION OF THE LENTIFORM NUCLEUS

HYPERDENSE VESSEL SIGN

Acute thrombus has high attenuation value this feature is referred to as the hyperdense vessel sign.

Highly specific but sensitivity is poor. FALSE POSITIVE

HIGH HEMATOGRIT LEVEL MCA CALCIFICATIONBut in such cases the hyperattenuation is usually bilateral!!!

Rarely, fat emboli appear hypoattenuated when compared with attenuation in the contralateral vessel .

HYPERDENSE MCA

OBSCURATION OF LENTIFORM NUCLEUS

Lentiform nucleus appears hypoattenuated because of acute ischemia of the lenticulostriate territory , resulting in obscuration of the lentiform nucleous.

This feature may be seen on CT images within 2 hours after the onset of a stroke .

OBSCURATION OF LENTIFORM NUCLEOUS

INSULAR RIBBON SIGN

It is the local hypoattenuation of the insular cortex region due to Cytotoxic edema as this region is susceptible to early and irreversible ischemic damage.

INSULAR RIBBON

Axial unenhanced CT image, obtainedin a 73-year-old woman 21⁄2

hours afterthe onset of left hemiparesis, shows hypoattenuationand obscuration of the posterior part of theright lentiform nucleus (white arrow) and a lossof gray matter–white matter definition in the lateralmargins of the right insula (black arrows).The latter feature is known as the insular ribbonsign.

WINDOW SETTING Detection of early acute ischemic stroke on unenhanced

CT images may be improved by using variable window width and center level settings to accentuate the contrast between normal and edematous tissue

STANDARD WINDOW SETTING (W80 C 20) – SENSITIVITY 57% SPECIFICITY 100%

STROKE WINDOW SETTING (W8 C 32) SENSITIVITY 71% SPECIFICITY 100%

CT ANGIOGRAPHY

CT angiography typically involves a volumetric helical acquisition that extends from the aortic arch to the circle of Willis. The examination is performed by using a time-optimized

bolus of contrast material for vessel enhancement. CT angiographic demonstration of a significant thrombus

burden can guide appropriate therapy in the form of intraarterial or mechanical thrombolysis.

Identification of carotid artery disease and visualization of the aortic arch may provide clues to the cause of the ischemic event and guidance for the interventional neuroradiologist

(a) Unenhanced CT image in a 72-year-old woman with acute right hemiplegia shows hyperattenuation in a proximal segment of the left MCA (arrows).(b, c) Axial (b) and coronal (c) reformatted images from CT angiography show theapparent absence of the same vessel segment (arrows).

CTA

CT Perfusion (CTP):

Basic concept….. With CT and MR-diffusion we can get a good impression of

the area that is infarcted. But, we cannot preclude a large ischemic penumbra (tissue at

risk). With perfusion studies we monitor the first pass of an

iodinated contrast agent bolus through the cerebral vasculature.

Areas of decreased perfusion will tell us which area is at risk.

CT perfusion maps of cerebral blood volume (a) and cerebral blood flow (b) show, in the left hemisphere, a region of decreased blood volume (white oval) that correspondsto the ischemic core and a larger region of decreased blood flow (black oval in b) that includes the ischemic core and a peripheral region of salvageable tissue. The difference between the two maps (black oval white oval) is the penumbra.

CT PERFUSION PARAMETERS ASSESSED

CBV – VOLUME OF BLOOD PER UNIT OF BRAIN TISSUE (N 4-5ML/100GM)

CBF – VOLUME OF BLOOD FLOW PER UNIT OF BRAIN TISSUE PER MINUTE (N 50-60ML/100GM/MINUTE)

MTT – TIME DIFFERENCE BETWEEN THE ARTERIAL INFLOW AND VENOUS OUTFLOW

TIME TO PEAK ENHANCEMENT – TIME FROM THE BEGINNING OF CONTRAST INJECTION TO MAXIMUM CONTRAST CONCENTRATION IN A ROI

CTP TECHNIQUES

DYNAMIC CONTRAST ENHANCED CT

BASED ON MULTI COMPARTMENT TRACER KINETIC MODEL PERFORMED ON MDCT 2-4 SECTIONS ARE OBTAINED AND ONE OF THE SECTIONS

PASS THROUGH BASAL GANGLIA

PERFUSED-BLOOD-VOLUME MAPPING- LESS COMMONLY USED

Dynamic Contrast Enhanced CT

Performed by monitoring a first pass of contrast bolus through the cerebral circulation

The transient increase in attenuation generates time-attenuation curves for an arterial and venous ROI

Mathematical modeling can be then used to calculate perfusion parameters and generate color coded perfusion maps (deconvolution analysis)

CBF = CBV / MTTCBF

MTT

Normal CBF is 55 cc/ 100 gm tissue / minuteCBF below this refers to penumbra or tissue at risk

EFFECT OF REDUCTION IN CBF

Diagram shows the evolution of events at a microscopic level with decreasing cerebral perfusion (from right to left). Irreversible cell death generally occurs whencerebral blood flow decreases to less than 10 mL/100 g/min.

INTERPRETATION OF PCT

INFARCTED AREA SEVERELY DECREASED CBF (<30%) AND CBV (<40%) PROLONGED MTT

PENUMBRA INCREASED MTT MODERATELY DECREASED CBF (>60%) INCREASED CBV (80-100% OR HIGHER)

OR INCREASED MTT MARKEDLY REDUCED CBF (>30%) MODERATELY REDUCED CBV (>60%)

CT PERFUSION PROTOCOL

Acute stroke in a 65-year-old man with left hemiparesis. CT perfusion maps of cerebral blood volume (a), cerebral blood flow (b), and mean transit time (c) show mismatched abnormalities (arrows) that imply the presence of a penumbra. The area with decreased blood volume represents the ischemic core, and that with normal blood volume but decreased blood flow and increased mean transit time is the penumbra.

CONVENTIONAL MRI

SPIN ECHO IMAGES MORE SENSITIVE AND SPECIFIC THAN CT IN ACUTE CVA

SEQUENCES T1 T2 FLAIR GRE

ACUTE CVA

HYPER ON T2 AND FLAIR LOSS OF GRAY WHITE MATTER

DIFFERENTIATION SULCAL EFFACEMENT MASS EFFECT LOSS OF FLOW VOID IN T2WI IN VESSEL BLOOMING IN GRE IF HRGE LESS SENSITIVE THAN DWI IN FIRST FEW

HOURS

Acute stroke in the left medialtemporal lobe in a 44-year-old man.(a) Axial T2-weighted and (b)fluid attenuatedinversion recovery imagesshow areas with increased signal intensity.(c) Gradient-echo image shows abnormallow signal intensity in the same areas.These findings are suggestive of hemorrhage

MR ANGIOGRAPHY Sensitive for intravascular thrombus. MR angiograms in two patients with acute stroke symptoms

reveal flow gaps in the left proximal middle cerebral artery (arrow in a) and the basilar artery (arrows in b). Both findings were due to intravascular thrombi.

Diffusion-Weighted Imaging Brownian motion The normal motion of

water molecules within living tissues is random.

Acute stroke causes excess intracellular water accumulation or “cytotoxic edema”, with an overall decreased rate of water molecular diffusion within the affected tissue.

Brownian Motion

DWI

AREAS OF CYTOTOXIC EDEMA WITH RESTRICTED WATER MOLECULE DIFFUSION IN ACUTE STROKE APPEAR BRIGHTER COMPARED TO NORMAL TISSUE

TAKES FEW SECS TO 2 MINUTES

DWI ACUTE CVAAcute stroke–induced cytotoxic edema in the right cerebellar hemisphere. Diffusion-weighted MR image shows areas of signal intensity increase due to the restricted mobility of water molecules

Acute stroke of the posterior circulation in a 77-year-old man. (a) Diffusion weighted MR image shows bilateral areas of increased signal intensity (arrows) in the thalami and occipital lobes. (b) ADC map shows decreased ADC values in the same areas (arrows). These findings are indicative of acute ischemia.

CLINICAL APP OF DWI CHANGES IN DWI OCCUR WITH IN 30MIN OF ONSET

OF ISCHEMIA WITH CORRESPONDING REDUCTION IN ADC AND SEEN UP TO 5 DAYS

MILD HYPERINTENSE DWI WITH PSEUDONORMAL ADC FROM 1 -4WKS

AFTER SEVERAL WKS DWI SIGNAL VARIES (T2 EFFECT) WITH INCREASED ADC

DWI ALONE CANNOT BE USED AND SHOULD ALWAYS BE COMPARED WITH ADC TO ASSESS THE AGE OF INFARCT

CHRONIC INFARCT

Chronic infarcts in a 71-year-old man with a remote history of multiple strokes. (a) Diffusion weighted MR image shows areas of decreased signal intensity in the left frontal lobe. (b) ADC map shows increased ADC values in the white matter of the right frontal lobe. These features are suggestive of chronic infarction.

ACCURACY CT/ CONVENTIONAL MRI

SENSITIVITY AND SPECIFICITY < 50% DWI

SENSITIVITY 88-100% SPECIFICITY 86-100%

FALSE -VE DWI LACUNAR INFARCTS OF BRAIN STEM SMALL DEEP GREY MATTER INFARCTS

FALSE +VE DWI ABSCESS CELLULAR TUMOURS LIKE LYMPHOMA

MR PERFUSION The passage of an intravascular MR contrast

agent through the brain capillaries causes a transient loss of signal because of the T2* effects of the contrast agent.

The dynamic contrast-enhanced MR perfusion imaging technique involves tracking of the tissue signal changes caused by susceptibility (T2*) effects to create a hemodynamic time–signal intensity curve,as in dynamic CT perfusion imaging.

Perfusion maps of cerebral blood volume and mean transit time can be calculated from this curve by using a deconvolution technique.

MR PERFUSION

LESION WHICH SHOWS CHANGES BOTH IN DWI AND PERFUSION MR – INFARCT CORE

LESION WHICH SHOWS CHANGES ONLY IN PERFUSION - PENUMBRA

(a) Diffusion-weighted MR imageshows an area of mildly increased signal intensity in the right parietal lobe (arrows). The ADC valuesin this region were decreased.

(b) Perfusion-weighted MRimage shows a larger area with increased time to peak enhancement (arrows) in the right cerebral hemisphere.

The mismatchbetween the perfusionand diffusion images is indicativeof a large penumbra.

Acute stroke in a 67-year-old woman with acuteleft hemiplegia

CLINICAL APPLICATION

Unenhanced CT: rule out hemorrhage Not very good to detect ischemia

T1 or T2 weighted MRI Good for detecting ischemia Cannot differentiate between acute versus chronic

ischemia So we have…

Diffusion-weighted MR

More sensitive for detection of hyperacute ischemia

becomes abnormal within 30 minutes Distinguish b/w old and new stroke

New stroke: bright on DWI Old stroke: Low SI on DWI

It detects irreversible infarcted tissue

MRIOLD –VS- NEW

ISCHEMIC INFARCT

T1 T2 DIFFUSION

Perfusion-Weighted imaging Allows the measurement of capillary perfusion

of the brain Uses a MR contrast agent The contrast bolus passage causes a nonlinear

signal decrease in proportion to the perfusion cerebral blood volume

Meaning, it can identify areas of hypoperfusion, the reversible ischemia, as well (unlike DWI)

Comparison of PWI and DWI

DWI Depicts irreversibly damaged infarct PWI Reflects the complete area of

hypoperfusion The volume difference between these two,

the PWI/DWI mismatch would be the PENUMBRA!

If there is no difference in PWI and DWI, no penumbra is present

Significance of PWI/DWI mismatch

IV thrombolytic treatment is not typically administered to patients with acute stroke beyond 3-hrs period Risk of hemorrhage

However, recent studies have shown that IV thrombolytic therapy may benefit patients who are carefully selected according to PWI/DWI mismatch, beyond 3-hrs window

CT VS MRI

ACUTE CVA IMAGING PROTOCOL

IV-TPA-WONDERFUL THERAPY.

FDA-USA approved Rx of Ischemic Stroke.

Improved outcome within 3 hours in properly selected patients. Results are best within 90 minutes. Results are better within 90 -180 minutes.

TPA reverses ischemic changes saving brain.

Thrombolytic Therapy - IV -t-PAI. Inclusion Criteria

1. Within 3 hours of the stroke and patient not needing ventilator. 2. CT Scan head Normal or < 1/3 MCA hypo density. II. Exclusion Criteria

1. BP > 185/110 mm on admission. 2. Use of Oral Anticoagulants. 3. Major surgery preceding FOURTEEN days. 4. Head injury - LAST THREE MONTHS. 5. Prior Intracranial hemorrhage/Recent GI bleed. 6. Prolonged PT / aPTT / INR / low Platelet count.

Intraarterial TPA.

IA-TPA in selected pts. In < 6 hours due to MCA & BA occlusion In BA occlusion it can be given even after > 12 hours.In future IA-TPA will be rewarding. IV &IA(IMS Trial) showed 56% of recanalisation.

CONCLUSION

Current imaging techniques can be used to identify hyperacute stroke and guide therapy

Both CT and MR imaging are useful for the comprehensive evaluation of acute stroke

THE FUTURE Selection of patients for thrombolytic

therapy may be made more effectively by performing appropriate imaging studies rather than relying on the time of onset as the sole determinant of selection.

New emerging technique MR permeability image used to predict microvascular permeability and quantification of BBB – Pts with defective BBB are more prone for bleeding complication following thrombolytic therapy.

QUIZ

CT Signs in Early MCA Ischemia

Hyperdense MCA Insular Ribbon Lentiform Nucleus

MCA Infarct

MCA

ACA Infarct

PCA Infarct

PCA

What do you see here? What do you expect on CTA?

Q. Imaging should target assessment of 4 P’s ??????

MRI in Stroke Intervention“The 4 P’s”

Pipes Perfusion Parenchyma

MRA Perfusion MR Diffusion MR

“Penumbra”Rowley AJNR 22(4); 599-601, 2001

THANK YOU