Imaging of Stroke

Imaging of Stroke
Andrew Richards
PPH Llanelli
Medical interventions in
ischaemic disease of the brain
Primary prevention
Secondary prevention
Treatment of acute stroke
Treatment of chronic stroke
Imaging in secondary
• Extent of existing ischaemic disease in
the brain and to exclude haemorrhage
and other diseases
• Extent of existing atherosclerotic
disease in the head and neck vessels
• To guide medical or surgical treatment
CT (Angiography)
• Advantages: Accessible, rapid
acquisition and reconstruction, can
image arteries and veins
• Disadvantages: Requires iodinated
contrast, limited by bone (e.g. skull
base, shoulders), uses ionising radiation
CT/CTA image interpretation
and analysis
• Overall accuracy of CT angiography
(CTA) for detecting thromboses and
stenoses of large intracranial and
extracranial arteries is 95-99%
• Comparable to conventional
angiography for total and near total
• Advantages: Does not always require
contrast agent, rapid acquisition, can
image arteries or veins
• Disadvantages: Inaccessible, limited by
motion and artifact, implants (e.g.
MRA Techniques
• Time-of-flight (TOF) 2D and 3D
techniques. Flow sensitive.
Compliments U/S
• Phase contrast (subtraction) method
• Contrast MRA-similar to CTA, but overall
accuracy slightly lower. Adequate
diagnostic test before surgery
Choice of technique (1)
• Duplex ultrasonography (DUS)-requires
institutional validation, QA and good
neurological history
• MRA combined with DUS similar to
catheter angiography
• CTA combined with DUS similar to
catheter angiography
Choice of technique (2)
• DUS, MRA, CTA all similar (80%
sensitivity, 90% specificity)
• DUS operator dependent
• MRA requires knowledge of technical
• All three combined gives overall
accuracy at 94% (=catheter
Imaging in the management
of acute stroke
• Potentially available techniques are CT and
• Exclusion of haemorrhage
• Exclusion of other pathologies that may
mimic acute stroke
• Determination of the ischaemic penumbra
(‘time is brain’)
• Selection of patients for timely thrombolysis
with rTPA
NINDS Trial (1995)
• Patients received intravenous thrombolysis
within three hours of the onset of symptoms
• Simple NECT used to exclude haemorrhage
• Benefit at long term follow-up in treated
group (partly offset by 10 fold increase in
symptomatic haemorrhage)
• rTPA subsequently received FDA approval for
treatment of MCA stroke
ECASS and ECASS II (1995,
• Patients received intravenous thrombolysis
within six hours of the onset of symptoms
• Simple NECT used to exclude haemorrhage
• No benefit to thrombolysis in either trial
(ECASS II used lower dose of rTPA)
• Better patient selection needed (no more
than 1/3 of MCA territory to be affected by
completed infarction)
• Most patients need to be treated within three
Non-enhanced CT (NECT)
• Advantages: Accessible, rapid
acquisition, high sensitivity for the
detection of haemorrhage
• Disadvantages: Less sensitive to
hyperacute infarct than magnetic
resonance diffusion weighted imaging
Role of NECT
• Exclude haemorrhage
• Detect early signs of stroke
• Exclude other causes
Early signs of stroke on NECT
Insular ribbon sign
Lentiform nucleus obscured
Subtle mass effect
Hyperdense, occluded arteries (100%
specific, 30% sensitive)
• Advantages: Diffusion weighted imaging
(DWI) extremely sensitive to acute
infarction, conventional sequences also
• Disadvantages: Inaccessible, artifacts,
Selecting patients for
• Imaging of stroke pathophysiology
required in the light of the evidence
• Available techniques are CT and/or MRI
perfusion imaging
• Both techniques based on indicatordilution method
Cerebral blood volume (CBV)
• Normal value is 4-5ml/100g
• When reduced to <40% of normal, is
likely to indicate irreversible cell death
• Reflects completed infarct volume on
CBV maps
Cerebral blood flow (CBF)
• Normal value is 50-60ml/100g/min
• When reduced to <20% normal, is
likely to indicate irreversible cell death
• When area of reduced CBF is larger
than area of reduced CBV on respective
maps, is likely to indicate ischaemic
penumbra or salvageable brain
Indicator dilution method
• Tight bolus injection of contrast agent
• Wash-in, wash-out analysed
continuously over time
• Time-intensity curve generated
Derived parameters from TIC
• Mean transit time (MTT) and time to
peak (TTP). Very sensitive to
haemodynamic impairment
• CBF and CBV. Absolute values are not
accurate, but compared with normal,
contralateral hemisphere. Better predict
outcome of an ischaemic lesion
CT Perfusion
• Advantages: Widely available, rapid
acquisition, accepted technique (FDA
• Disadvantages: Limited to MCA
territory, bone artifacts, only semiquantitative, ionising radiation
MR Perfusion imaging
• Advantages: Covers entire brain, fast
acquisition, can be performed with DWI
• Disadvantages: Gadolinium contrast
agents not licensed, semi-quantitative,
limited access, patient motion, artifacts,
implants (pacemakers)
• Imaging in secondary prevention here and
• Imaging in acute stroke requires investment
(MRI unlikely to be available out-of-hours)
• Evidence base for thrombolysis in acute
stroke is thin
• Only a small number of patients are likely to
benefit from thrombolysis in acute stroke
• An effective stroke thrombolysis service
represents a significant challenge

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