Download Imaging of Stroke

Imaging of Stroke
Andrew Richards
PPH Llanelli
Medical interventions in
ischaemic disease of the brain
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Primary prevention
Secondary prevention
Treatment of acute stroke
Treatment of chronic stroke
Imaging in secondary
prevention
• 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
occlusion
MRI/MRA
• Advantages: Does not always require
contrast agent, rapid acquisition, can
image arteries or veins
• Disadvantages: Inaccessible, limited by
motion and artifact, implants (e.g.
pacemaker)
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
limitations
• All three combined gives overall
accuracy at 94% (=catheter
angiography)
Imaging in the management
of acute stroke
• Potentially available techniques are CT and
MRI
• 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,
1998)
• 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
hours
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
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Insular ribbon sign
Lentiform nucleus obscured
Subtle mass effect
Hyperdense, occluded arteries (100%
specific, 30% sensitive)
MRI
• Advantages: Diffusion weighted imaging
(DWI) extremely sensitive to acute
infarction, conventional sequences also
useful
• Disadvantages: Inaccessible, artifacts,
implants
Selecting patients for
thrombolysis
• Imaging of stroke pathophysiology
required in the light of the evidence
accrued
• 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
• CBFxMTT=CBV
CT Perfusion
• Advantages: Widely available, rapid
acquisition, accepted technique (FDA
approved)
• 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)
Summary
• Imaging in secondary prevention here and
now
• 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