IMAGING IN STROKE
MODERATOR- DR C P AHIRWAR (MD)
PRESENTOR- DR NEELAM SONI
Stroke is an acute central nervous system injury with
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
It is a leading cause of morbidity and mortality in the
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
• Assess early signs of acute stroke, rule out hemorrhage
• Assess extracranial circulation (carotid and vertebral
arteries of the neck) and intracranial circulation for
evidence of intravascular thrombus
• Assess cerebral blood volume, cerebral blood flow, and
mean transit time
• Assess tissue at risk of dying if ischemia continues with
out re-canalization of intravascular thrombus
Overview of imaging modalities
• Can be performed quickly.
• Can help identify early signs of stroke, and can
help rule out hemorrhage.
CT angiography can depict intravascular
CT perfusion imaging can demonstrate
salvageable tissue which is indicated by a
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.
TISSUE AT RISK OR
Neuronal tissue is sensitive
for ischemia due to lack of
With complete absence of
flow neuronal viability is 2-3
In acute stroke ischemia is
always incomplete due to rich
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
The transition from ischemia to irreversible
infarction depends on both the severity and
the duration of the diminution of blood flow
Days and weeksTime
CT - ALTERED PARAMETERS IN PERFUSION
MR – PERFUSION DIFFUSION MISMATCH
PRESENCE OF PENUMBRA HAS SIGNIFICANT
IMPLICATIONS IN PT MANAGEMENT
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
Highly specific but sensitivity is poor.
HIGH HEMATOGRIT LEVEL
But in such cases the hyperattenuation is usually
Rarely, fat emboli appear hypoattenuated when compared
with attenuation in the contralateral vessel .
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 .
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
Axial unenhanced CT image,
in a 73-year-old woman 21⁄2 hours
the onset of left hemiparesis,
and obscuration of the posterior
part of the
right lentiform nucleus (white
arrow) and a loss
of gray matter–white matter
definition in the lateral
margins of the right insula (black
The latter feature is known as the
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%
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 the
apparent absence of the same vessel segment (arrows).
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
With perfusion studies we monitor the first pass of an
iodinated contrast agent bolus through the cerebral
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 corresponds
to 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.
CBV – VOLUME OF BLOOD PER UNIT OF BRAIN TISSUE
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
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
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
CBF = CBV / MTT
Normal CBF is 55 cc/ 100 gm tissue / minute
CBF 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 when
cerebral blood flow decreases to less than 10 mL/100 g/min.
INTERPRETATION OF PCT
SEVERELY DECREASED CBF (<30%) AND CBV (<40%)
MODERATELY DECREASED CBF (>60%)
INCREASED CBV (80-100% OR HIGHER)
MARKEDLY REDUCED CBF (>30%)
MODERATELY REDUCED CBV (>60%)
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.
SPIN ECHO IMAGES MORE SENSITIVE AND
SPECIFIC THAN CT IN ACUTE CVA
HYPER ON T2 AND FLAIR
LOSS OF GRAY WHITE MATTER
LOSS OF FLOW VOID IN T2WI IN VESSEL
BLOOMING IN GRE IF HRGE
LESS SENSITIVE THAN DWI IN FIRST FEW
Acute stroke in the left medial
temporal lobe in a 44-year-old man.
(a) Axial T2-weighted and (b)fluid attenuated
inversion recovery images
show areas with increased signal intensity.
(c) Gradient-echo image shows abnormal
low signal intensity in the same areas.
These findings are suggestive of hemorrhage
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.
The normal motion of
water molecules within
living tissues is random.
Acute stroke causes
excess intracellular water
“cytotoxic edema”, with
an overall decreased rate
of water molecular
diffusion within the
affected tissue. Brownian Motion
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 CVA
cytotoxic edema in the
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
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.
CT/ CONVENTIONAL MRI
SENSITIVITY AND SPECIFICITY < 50%
FALSE -VE DWI
LACUNAR INFARCTS OF BRAIN STEM
SMALL DEEP GREY MATTER INFARCTS
FALSE +VE DWI
CELLULAR TUMOURS LIKE LYMPHOMA
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
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.
LESION WHICH SHOWS CHANGES BOTH IN
DWI AND PERFUSION MR – INFARCT CORE
LESION WHICH SHOWS CHANGES ONLY IN
PERFUSION - PENUMBRA
weighted MR image
shows an area of mildly increased
signal intensity in the right parietal
lobe (arrows). The ADC values
in this region were decreased.
(b) Perfusion-weighted MR
image shows a larger area with
increased time to peak
enhancement (arrows) in the right
between the perfusion
and diffusion images is indicative
of a large penumbra.
Acute stroke in a 67-year-old woman with acute
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
So we have…
More sensitive for detection of hyperacute
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
OLD –VS- NEW
T1 T2 DIFFUSION
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
The volume difference between these two,
the PWI/DWI mismatch would be the
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
FDA-USA approved Rx of Ischemic Stroke.
Improved outcome within 3 hours in properly
Results are best within 90 minutes.
Results are better within 90 -180 minutes.
TPA reverses ischemic changes saving brain.
Thrombolytic Therapy - IV -t-PA
I. Inclusion Criteria
1. Within 3 hours of the stroke and patient not needing
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.
IA-TPA in selected pts. In < 6 hours due to MCA
& BA occlusion
In BA occlusion it can be given even after > 12
In future IA-TPA will be rewarding.
IV &IA(IMS Trial) showed 56% of recanalisation.
Current imaging techniques can be used
to identify hyperacute stroke and guide
Both CT and MR imaging are useful for
the comprehensive evaluation of acute
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
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.