D’Arcy Little, MD, CCFP, FRCPC
Former Family Physician, University of Toronto
Staff Radiologist, Orillia Soldiers’ Memorial Hospital,
Etobicoke General Hospital and Humber River Regional Hospital
Research Fellow, Diagnostic Imaging, University of Toronto
Saving the Brain; 4th Annual Collaborative Stroke Network Conference,
OCFP, Toronto, March 5th, 2010.
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The goals of an imaging evaluation for acute
stroke are to:
◦ establish a diagnosis as early as possible
◦ guide appropriate treatment
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Assess location and size of territory involved
Rule out stroke mimics
Rule out hemorrhage
Obtain accurate information about the intracranial
vasculature and brain perfusion for guidance in
selecting the appropriate therapy.
• 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
• In the absence of blood flow,
available energy can maintain
neuronal viability for 2-3 minutes
• In the brain, ischemia is
incomplete, with collateral supply
• Cerebral ischemia = central
irreversibly infarcted tissue core
surrounded by peripheral region of
stunned cells, the ‘penumbra’
• The penumbra is potentially
salvageable with early
recannalization
Srinivasan, et al. State-of-the-Art Imaging of Acute Stroke. 26 (supp 1). Radiographics 2006;
26:S75-S95.
Figure 1b. (a) Schematic of brain involvement in acute stroke shows a core of irreversibly
infarcted tissue surrounded by a peripheral region of ischemic but salvageable tissue referred to
as a penumbra
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
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NCCT
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Performed quickly
Identifies early signs of stroke
R/O hemorrhage
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CTA
◦ Depict IV thrombi
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CTP
◦ Depict salvageable tissue, or “penumbra”
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?MRI
Figure 2a. Dense MCA sign suggestive of intravascular thrombus.
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 3. Obscuration of the Lentiform Nucleus
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 4. Insular Ribbon Sign
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 5a. What do you see here?
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 5b. Lentiform nucleus sign; “Stroke Windows”
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 6. Schematic shows the 10 regions of the MCA distribution, each of which accounts for
one point in the ASPECTS system: M1, M2, M3, M4, M5, M6, the caudate nucleus (C), the
lentiform nucleus (L), the internal capsule (IC), and the insular cortex (I)
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 7a. Unenhanced CT images in a 56-year-old man with right hemiparesis (a at a lower level
than b) demonstrate involvement of the M1 region, insular cortex (I), and lentiform nucleus (L)
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 7b. Unenhanced CT images in a 56-year-old man with right hemiparesis (a at a lower
level than b) demonstrate involvement of the M1 region, insular cortex (I), and lentiform nucleus
(L)
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
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The Alberta Stroke Programe Early CT Score (ASPECTS) is a 10point quantitative topographic CT scan score used in patients
with middle cerebral artery (MCA) stroke.
Segmental assessment of MCA territory is made and 1 point is
removed from the initial score of 10 if there is evidence of
infarction in that region.
caudate
putamen
internal capsule*
insular cortex
M1: "anterior MCA cortex," corresponding to frontal operculum
M2: "MCA cortex lateral to insular ribbon" corresponding to
anterior temporal lobe
M3: "posterior MCA cortex" corresponding to posterior temporal
lobe
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M4: "anterior MCA territory immediately superior to M1"
M5: "lateral MCA territory immediately superior to M2"
M6: "posterior MCA territory immediately superior to
M3"
(M1 to M3 are at the level of the basal ganglia and M4
to M6 are at the level of the ventricles immediately
above the basal ganglia)
An ASPECTS score less than or equal to 7 predicts worse
functional outcome at 3 months as well as symptomatic
haemorrhage.
*in the initial paper, it specifically refers to the posterior
limb, however subsequent articles indicate any portion
of the internal capsule may be counted
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For assessment of
intracranial and
extracranial circulation
Can demonstrate
thrombi
Timed bolus of
contrast for vessel
enhancement
Can guide decision
regarding intraarterial
or mechanical
thrombolysis by
quantifying clot
burden
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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.
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Used to measure the following parameters:
◦ Cerebral Blood Flow (CBF)
 The volume of blood per unit brain tissue (N = 4-5
mL/100g)
◦ Cerebral Blood Volume (CBV)
 The volume of blood flow per unit of brain tissue per
minute (N = 50-60 mL/100g/min in gray matter)
◦ Mean Transit Time (MTT)
 Time difference between arterial inflow and venous outflow
◦ (Time to Peak Enhancement:
 Time from beginning of contrast injection to max
concentration in ROI)
(CBF
)
MTT
Normal CBF is 55 cc/ 100 gm tissue / minute
CBF below this refers to penumbra or tissue at risk
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Performed by monitoring a first
pass of contrast bolus through
the cerebral circulation
The transient increase in
attenuation generates timeattenuation 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)
CBV
CBF
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Infarct shows:
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Penumbra shows either:
◦ Severely decreased CBF (< 30%) and CBV (<40%)
with increased MTT
◦ Increased MTT, moderately decreased CBF and
normal or increased CBV
◦ Increased MTT, markedly decreased CBF and
moderately reduced CBV
Acute stroke in 65M with left
hemiparesis…….
CBV
CBF
CORE = area with decreased CBV
PENUMBRA = CBF or MTT-CBV
MTT
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term introduced by Hachinski et al to designate periventricular
or subcortical (centrum semiovale) areas of hypodensity on
computed tomography (CT)
or hyperintensity on T2-weighted magnetic resonance imaging
(MRI).
a common condition
epidemiological studies demonstrating a high prevalence in
subjects over 65 years of age, as evaluated by CT or MRI.
probably caused by chronic cerebral ischemia but the
pathogenesis and its clinical significance are not completely
understood.
Some individuals remain asymptomatic for prolonged periods,
while others develop gait disturbance, cognitive impairment,
mood disorders, disability, and even dementia.
LA increases overall morbidity and mortality and also the risk of
stroke.
Further understanding of the pathogenesis of LA is essential
because it is potentially preventable and modifiable .
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DWI
◦ More sensitive for
detection of
hyperacute ischemia
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GRE
◦ Detects hemorrhage
Figure 12a. Acute stroke in the left medial temporal lobe in a 44-year-old man
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 12b. Acute stroke in the left medial temporal lobe in a 44-year-old man
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 12c. Acute stroke in the left medial temporal lobe in a 44-year-old man
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Figure 13a. Intravascular thrombus
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
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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
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Tissues with a higher rate
of diffusion undergo a
greater loss of signal in a
given period of time than
do tissues with a lower
diffusion rate.
Therefore, areas of
cytotoxic edema, in which
the motion of water
molecules is restricted,
appear brighter on
diffusion-weighted images
because of lesser signal
losses.
Figure 19. Flow chart shows an acute stroke imaging protocol
Srinivasan A et al. Radiographics 2006;26:S75-S95
©2006 by Radiological Society of North America
Radiographics. 2003;23:565-592.
Radiographics. 2006;26:S75-95.
Questions and Comments:
darcy.little@sympatico.ca