ct interpretation course - cardiac and stroke networks in ... · in complete absence of blood flow,...
TRANSCRIPT
CT INTERPRETATION COURSE
Introductory Lecture on Basic Principles ASTRACAT
2012
Part One
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“Stroke” is a Clinical Diagnosis
A clinical syndrome characterised by rapidly developing clinical symptoms
and/or signs of focal loss of cerebral function lasting more than 24 hours.
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DEFINITIONS — Stroke is classified into two major types
• Brain Ischaemia - due to thrombosis, embolism, or systemic
hypoperfusion
• Brain Haemorrhage - due to intracerebral haemorrhage or
subarachnoid haemorrhage
– A stroke is the acute neurologic injury that occurs as a result
of one of these pathologic processes
– Approximately 80 percent of strokes are due to ischaemic
cerebral infarction and 20 percent to brain haemorrhage
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Pathophysiology of stroke • An infarcted brain is pale initially.
• Within hours to days, the gray matter becomes congested with engorged,
dilated blood vessels and minute petechial hemorrhages.
• When an embolus blocking a major vessel migrates, lyses, or disperses within
minutes to days, recirculation into the infarcted area can cause a
haemorrhagic infarction
• A primary intracerebral haemorrhage damages the brain directly at the site of
the haemorrhage and by compressing the surrounding tissue
• Thrombosis generally refers to local in situ obstruction of an artery
• Embolism refers to particles of debris originating elsewhere that block arterial
access to a particular brain region
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What’s the point of imaging?
•Exclude haemorrhage •Determine the mechanism/cause •Differentiate infarcted tissue from salvageable tissue •Identify intravascular thrombi •Patient selection for therapy •Assess risk of complications
• Haemorrhagic transformation • Hydrocephalus in posterior circulation infarction
•Assist with prognosis
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CT is the best test in the acute/subacute phase
•It confidently detects or excludes haemorrhage
•Confirms the diagnosis in most cases
•Quick & patient friendly
•Easy to interpret
•Readily available
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How to Recognise Haemorrhage on CT
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Dense white “blob”
Only seen after blood clots
Minimal oedema
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“Complex” haematoma
White “blob” not homogeneous
Thalamic haematoma
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“Complex” haematoma
“Blob” not homogeneous
More white matter oedema
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Venous haemorrhage – SSS thrombosis
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Unclotted blood same density as brain – clotted blood white
NB Normal white
Matter in a young person
NOT oedema
Acute on chronic SDH
Old
Newly clotted blood
Old & New
Mixed
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Blood clot/fluid level — anticoagulants (“complex” haematoma)
Beware the Resolving Haematoma
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Acute
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Fading, subacute ICH
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CE CT on FU
Without previous CT, could be taken for SOL
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Final FU – atrophy at haematoma site
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Post contrast medium CT— looks like SOL
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Non contrast CT – 2 weeks earlier
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Haematoma
(Infarct) ischaemic oedema
Normal Anatomy
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Optic tract
Substantia nigra
Red nucleus
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Insular Ribbon
CN
LN
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Th
Th = Thalamus
Insular Ribbon Int. Caps.
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Rostrum, corpus callosum
Splenium, corpus callosum
Insular
Ribbon
ic
ic
ic
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Head of
caudate
Lentiform
Internal
Capsule
Optic radiation
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Insular ribbon
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How to Recognise an Infarct
Actually — Ischaemic Oedema
Dead infarct (core) indistinguishable from salvageable ischaemic oedema
surrounding it (penumbra)
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•Low density
•Wedge shaped
•Grey & white matter
•Within known arterial
vascular territory
•Proportionally little mass
effect
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NB Grey matter not confined
to the cortex
CN
Ant. LN
Int. caps obliterated
Subcortical infarct
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Same rules apply to MRI
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Stroke oedema?
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Stroke oedema?
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Stroke oedema?
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Stroke oedema? This small
haematoma
caused the
“stroke”
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Stroke oedema? Complex SOL
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Stroke? — MRI — Same rules
Hyperdense MCA
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Dense ICA
Plus fleck of
Calcification in
Vessel wall
Dense MCA
Hyperdense MCA with Fragmentation
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More Subtle Examples of Early Infarction
Basic neuroanatomy to support early
diagnosis
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Insular Ribbon
CN
LN
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Insular Ribbon Int. Caps.
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Rostrum, corpus callosum
Splenium, corpus callosum
Insular
Ribbon
ic
ic
ic
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Head of caudate
Lentiform
Internal
Capsule
Optic radiation
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Insular ribbon
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Insular Ribbon & BG signs
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Early Infarction & FU
(Insular ribbon & BG signs)
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Insular Ribbon, BG & CN– obliterated on Right. Normal on Left. Blue stars = Insula. Yellow star = BG
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CN
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Reduced attenuation (low density)
obliterates grey/white differentiation
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Next day
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Insular Ribbon Int. Caps.
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Rostrum, corpus callosum
Splenium, corpus callosum
Insular
Ribbon
ic
ic
ic
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Ext caps
CN
LN
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Day 1
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Day 2
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Day 4
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Haemorrhagic Infarction
and how to tell it from a primary
intracerebral haemorrhage
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New
H’gic
infarct
Old
Infarct
Why?
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Pt with SBE throwing off multiple emboli
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Plain CT – haemorrhagic infarction
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ICH
Contrast Enhancement in Infarction
Luxury perfusion and the blood brain
barrier
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Plain CT
CE CT
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NC CE
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NC CE
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CE CE
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“Special” Infarcts
or unusual consequences
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BG (Pallidal) Infarction – CO Poisoning
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Bilateral BG/Ext Caps Infarction
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Post Cardiac Arrest Infarction Cortical Mantle
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Post Arrest Plain CT
Post CE — Acute Cortical Laminar Necrosis
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Acute Occlusion ICA > ACA/MCA Infarction > Malignant Oedema >
Herniation > Venous Congestion & Haemorrhage
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Raised ICP with h’age often mistaken for SAH
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Normal CT
Then Effect of Sudden Rise in ICP
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Intradural venous congestion over tent looks like SAH
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“Watershed” Infarcts ie Border Zone
©Radiology Assistant
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How to Recognise the Different Vascular Territories
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lat midline
from top from below
Red = MCA
Green = ACA
Purple = PCA
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Circle of Willis rarely
a true circle
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No circle at all
No Post.Comm. Arteries
Ant & Post circulation isolated
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Dominant Post Comm
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ICA occlusion – ACA & MCA infarcts
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MCA & PCA infarcts because of dominant
Post Comm Art on Right
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Carotid Artery Dissection
No infarct
Circle of Willis protects brain
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Polo Mint – thrombus in arterial wall, end on Thrombus en face
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C of W protects
No infarct
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C of W protects via Ant Comm
No infarct
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Vascular Territories (contd) - PCA
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PCA
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How to Select Patients for thrombolysis?
• Clinical — NIHSS
• Infarct size — ASPECT Score
• Distinguishing dead tissue from living, but stunned, brain —
CT Perfusion
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How to Select Patients for thrombolysis?
• Clinical — NIHSS
• Infarct size — ASPECT Score
• Distinguishing dead tissue from living, but stunned, brain —
CT Perfusion
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ASPECT Scoring System
•A = ACA; P = PCA; M = MCA
•MCA territory (10 points is Normal)
• Subtract one point for each:
• M1, M2, M3
• M4, M5, M6
• Caudate, Int Caps, LN, Insula
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c LN
ic In
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Level with foramina of Monro
M1
M2
M3
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Sylvian fissure
Level with 3rd V
M4
M5
M6
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(VRS)
Level with top of Lat Vs
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ASPECTS
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Examples from paper
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How to Select Patients for thrombolysis?
• Clinical — NIHSS
• Infarct size — ASPECT Score
• Distinguishing dead tissue from living, but stunned, brain —
CT Perfusion
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Penumbra
Unlike muscle, brain tissue exquisitely sensitive to ischaemia
Absence of neuronal energy stores
In complete absence of blood flow, available energy can sustain neuronal
viability for 2-3 minutes
In acute stroke, ischaemia incomplete
Collateral blood supply from uninjured arterial & leptomeningeal territories
Results in central infarcted tissue surrounded by peripheral
“stunned” cells (penumbra)
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Imaging of the Penumbra
CT
Discrepancy in perfusion parameters
MRI
Mismatch between diffusion & perfusion parameters
(DWI/PWI)
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CT Perfusion Imaging
CBF = CBV/MTT
CBV = area under parenchymal curve/area under arterial
curve
MTT calculated from time difference between arterial inflow
& venous outflow + time to peak enhancement
Penumbra -v-Dead tissue
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Pericallosal artery Sigmoid (venous) sinus
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CBV CBF MTT
CTP
Normal CT
CTP – perfusion defect
CTA – ICA dissection
© radiology assistant
CTA
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Normal CTA
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Hypodensity Right Insula — CTA
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Taken from “Radiographics, October 2006”
Goals of Acute Stroke Imaging The Four Ps
Parenchyma Assess early signs of acute stroke Rule out haemorrhage
Pipes – look for intravascular thrombus Extracranial circulation (neck) Intracranial circulation
Perfusion Cerebral blood volume Cerebral blood flow Mean transit time
Penumbra Tissue at risk
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Conclusions
Stroke is a clinical diagnosis
CT is best and will suffice in majority
MR for CT neg. stroke or
for definitive diagnosis when suggested clinically eg
dissection or
where CT suggests alternative diagnosis eg SOL
Advanced MR techniques best left to specialist units
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DDX of Arterial Infarction
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Not all Infarction is Arterial
Venous Infarction
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Venous sinus thrombosis
•Low incidence [<1 in 10,000 persons]
•Risks factors
• Tissue damage and stasis
• Haematological disorders
• Malignancies
• Collagen vascular disorders
• Pregnancy
• Medications
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Venous sinus thrombosis
•CT brain shows non-arterial distribution infarcts in the white matter and/or cortical white matter junction, often associated with haemorrhage
•Empty delta sign on contrast-enhanced CT scan
•Occasionally, the cortical veins can be seen with fresh thrombus within.
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MR and MR Venography
•Advantages • sensitive to blood and
parenchymal changes • Cortical vein thrombosis and
sinus thrombosis can be identified with multiple appropriate sequences
•Disadvantages • Difficult to recognise thrombus
in first few days on conventional MRI
• Flow and susceptibility artefacts and saturation effects in TOF can make interpretation difficult
• Difficult in unwell patients • Contraindications to MRI • Expensive and availability
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CT Venogram
• Advantages
• Can be added as an adjunct to unenhanced CT - Non invasive
• Not expensive, reliable and easy to interpret
• Fast acquisition reduces motion artefacts
• Monitoring of critically ill patients is easier.
• Can differentiate slow flow from thrombus, which is a problem with MRI.
• Disadvantages
• Radiation (Mean effective dose <2.2 mSV)
• Contrast media flow dynamics not seen as well as in DSA
• Metallic artefacts may impair visualisation of venous structures
• Contraindicated in pregnancy
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Normal CTV Anatomy
For interest only
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Normal radiological cerebral venous anatomy
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Normal radiological cerebral venous anatomy
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Deep veins
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Outflow tracts
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Normal Variants
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Asymmetric transverse sinus
Venous thrombosis
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Empty Delta Sign (SSS = sup sag sinus & TS = transverse sinus)
Venous sinus thrombosis
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Arrows point to Empty Delta Sign
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Example:
Plain T1W MRI
Bilat. Parasagittal H’age
Thrombus in SSS
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MRV showing non-filling of SSS and SS