Transcript Pitfall of CT Angiography for Acute Stroke Imaging

Pitfalls of CT Angiography for
Acute Stroke Imaging
Assessment
MG Matheus, MD, V Jewells, DO, A Felix, MD, S Sen, MD,
MS, S Solander, MD, M Castillo, MD.
University of North Carolina-Chapel Hill
Introduction
CT is crucial in the workup of acute stroke patients. CT
angiography (CTA) provides information about tissue and
vascular anatomy, adding only a few minutes to overall
imaging time. Imaging assessment needs to be fast to
facilitate triage of appropriate candidates for thrombolytic
treatment. Size, lesion location and time from symptom
onset can guide management decisions.
CTA is highly accurate in detecting intracranial large
vessel occlusion. However, image assessment is laborious
and attention to technical details and knowledge of stroke
dynamic pathophysiology is needed to avoid image
misinterpretation.
Here, we address some technical and physiological
pitfalls related to image acquisition and interpretation of CTA
in acute stroke patients.
Approach
We retrospectively reviewed studies obtained in
the past 2 years in 133 patients with acute stroke
symptoms and found 16 patients in whom
technical/interpretative problems occurred. These
studies included:
- Non-contrast head CT and CTA
CTA consisted of axial 3 mm reconstructed source
images after contrast, MIP in three planes and 2
projections volume rendered (VR) images. All studies
were assessed for:
1. Possible technical problems with regards to
imaging acquisition/reconstruction
2. Clinico-pathological patterns of stroke that lead
to incorrect image interpretation
Results- Technical Pitfalls
• VR reconstruction showing vessel overlapping and
“kissing” artifacts
• Venous contamination causing vascular overlap
• VR images techniques masking bone/vessel interface
and intravascular densities
• Inappropriate window settings masking calcifications
and stenosis
• Previously VR reformatted images with no
visualization of distal vessels
• Previously reformatted 3D views without availability
of source images to confirm abnormalities
Results- Related to Stroke
Pathophysiology
• Intra-arterial dense material (clot and
calcifications) masking occlusions
• Primary and secondary collateral flow
masking obstruction and stenosis
• Stenosis at MCA bifurcation
• Anatomical variations
Representative Cases
Case # 1
Patient
presents
with
stroke symptom of less
than
2
hours.
Non
contrast head CT was
performed and shows a
left dense MCA (arrow).
Case # 1
Following the CT of the head, this CTA was performed :
Do you consider the left MCA to be occluded? This MIP was
interpreted as the MCA being patent.
Case # 1
Follow-u[ MRA shows that left ICA is occluded.
Case # 1
Catheter
angiogram
shows dissected left
ICA. There is cross
filling
from
right
injection to level of
occlusion (arrow). Pial
collaterals
supply
territory of left MCA
thus filling it with
contrast.
Case # 1- Teaching Point
On the CTA the dense clot-filled M1
segment of the left MCA appears isodense
to contrast filled arteries. Collateral filling
of the ipsilateral MCA branches to the
distal end of the clot resulted in a CTA that
gave the false appearance being normal.
Catheter angiography confirms these
findings. If CTA findings do not correspond
with patient’s symptoms, additional studies
using different techniques may be needed.
Case # 2
Patient
complained
of
left
sided
hemiplegia and left facial numbness
lasting approximately 1 hour. CTA was
performed, two MIP coronal views are
shown (next slide), no early ischemic
findings were observed. Vasculature and
brain parenchyma were symmetrical. Both
ICAs had calcifications.
Case # 2
Coronal MIPs show symmetrical filling of MCAs.
Case # 2
Immediately after the CT the patient underwent MRA which
shows occluded left ICA but cross filling of left sided
intracranial arteries via the circle of Willis.
Case # 2
Re-windowing the coronal and axial MIPs show calcification in the left
ICA (arrow) which confirms occluded artery as seen on MRA. Note that
with narrow window settings (left) the calcification is not appreciated.
Case # 2 – Teaching Point
Primary collateral blood flow created a
symmetrical vascular picture of the distal
brain vessels and the dense intra-arterial
calcification in the left ICA masked the
total vessel occlusion when the CTA was
viewed with narrow window settings. We
have seen similar findings in three other
patients. Wide windows should be used to
avoid this problem.
Case # 3
Patient presented with acute left MCA
stroke symptoms. CTA showed no
occlusions; VR images are shown (next
slide).
Case # 3
Both MCAs are patent and left A1 segment of the ACA is not
visualized, bone obscures visualization of the petrous portions of the
ICAs. The posterior circulation is not seen entirely.
Case # 3
Widening the window (right side image) allows one to
see that the petrous portion of the left ICA (arrow) is
narrowed when compared to the opposite side. This
finding is difficult to see with regular window (left image)
settings due to similar densities at vessel/bone interface.
Case # 3
Axial MIPs with wide window settings show narrowed petrous
(arrows) left ICA when compared to right ICA (arrowhead).
Case # 3- Teaching Point
With normal window settings,
distinguishing between adjacent bone and
opacified vessel may be difficult.
Separation of blood vessel/bone interface
necessitates wide window settings.
Case # 4
Patient had an acute right posterior
circulation infarct confirmed by non-contrast
head CT. CTA demonstrated diffuse
vascular irregularities and narrow intracranial
vessels. The basilar artery and both P1
segments were poorly visualized, VR images
are shown (next slide).
Case # 4
VRs of the circle of Willis show a narrowed basilar artery,
non visualization of the PCAs and adequate proximal
anterior circulation.
Case # 4
Axial MIPs show apparently complete circle of Willis,
noticed that, however vessel opacification is poor
suggesting stenosis (not seen) leading to poor blood flow
to these arteries.
Case # 4
MIP axial image shows occlusion of the right ICA.
Case # 4- Continuation
Angiography confirmed the severe basilar
stenosis and right ICA occlusion. Most of
the arterial supply to the right cerebral
hemisphere was via right ophthalmic
artery and right PCA and not via the
anterior
communicating
artery
as
suspected from the CTA.
Case # 4
Right external carotid artery injection
shows opacification of right MCA
territory.
Lateral view of ECA injection
shows opacification of right
MCA territory.
Case # 4
Left ICA injection shows poor opacification of the right MCA territory
implying inadequate cross filling through ACommA.
Left vertebral artery injection
shows opacification of right
MCA territory.
Case # 4- Teaching Point
The status of the circle of Willis suggested
by the CTA was misinterpreted because of
patient’s low arterial input of contrast and
non-visualization of the collateral supply
by the right ophthalmic and right posterior
communicator artery. The degree of
narrowing of the basilar artery was
overestimated on CT. Hemodynamic
alterations were thought to be responsible
for the patient’s symptoms.
Case # 5
Patient presented with acute stroke
symptoms suggesting involvement of left
posterior circulation. CTA showed left
occipital hypodensity. Axial MIPs are
shown (next slide).
Case # 5
The transition between left P1 and P2 segments is not well visualized, but
small distal PCA branches show opacification implying that these arteries
are patent (click for sequential MIPs from CTA).
Case # 5
VR images show normal basilar artery. The right vertebral artery is
dominant while there is a vessel in the region of the left sided one. A
discrepant finding with respect to the MIPS is that both PCAs are not seen
past their proximal segments on these images probably due to the fact that
they were excluded from the reformations.
Case # 5
Injection into the right subclavian artery shows occlusion of
proximal vertebral artery with recanalization cephalad by
collaterals.
Case # 5
The right vertebral artery
filled via muscular collaterals
and there was slow flow to
the basilar artery. The left
PCA is occluded (arrow) past
its P2 segment while the right
sided one is patent.
Case # 5
Injection into left vertebral artery shows that it ends in PICA
thus the vessel seen on the CTA cannot be the vertebral
artery but is probably a vein draining into the marginal
sinus.
Case # 5- Teaching Point
Initially, there were discrepant findings between
the MIPs and VR images, the latter showing
occlusion of both PCAs. Catheter angiogram
showed occluded left PCA. Despite visualization
of the presumed left vertebral artery on CTA,
angiogram showed it be occluded. Moreover,
the right vertebral was proximally occluded and
recanalized distally. The static nature of CTA
does not allow one to visualize delay circulation
times which may have been related to patient’s
symptoms.
Case # 6
Patient presented to the hospital after a
peripheral interventional procedure with
signs of a right MCA infarct. Embolic
infarct was suspected. CTA is shown in
next slide.
Case 6
Sequential axial MIPs (on click) showing normal appearing
vessels.
Case # 6
Coronal MIPs show left MCA fenestration (circle) and
incompletely seen right M1 segment but with good
opacification of the ipsilateral sylvian branches.
Case # 6
VR images confirm left
MCA fenestration (circle)
and adequate filling of
right
MCA
despite
symptoms corresponding
to that side.
Case # 6
Angiogram confirms left fenestration (circle). On the right, there is a similar
fenestration but its superior limb is occluded (arrow) explaining the patients
symptoms.
Case # 6- Teaching Point
CTA showed patent right MCA. This
artery was however fenestrated and the
superior limb of the fenestration was
occluded
resulting
in
a
basal
ganglia/capsular infarction. The fact that
the inferior limb of the fenestration was
patent gave the false impression that the
entire left MCA was patent. This was
suspected and lead to catheter angiogram
and attempted thrombolysis.
Case # 7
Patient presented with posterior circulation
infarct symptoms and CTA showed an
unusual configuration of the top of the
basilar artery.
Case # 6
Sagittal MIP (left) shows irregular basilar artery termination (arrow). This
finding cannot be confirmed on the VR image (right) as the basilar artery
apex is inseparable from adjacent bone.
Case # 6
Catheter angiogram shows clot occluding distal basilar
artery. The definitive diagnosis could be made on CTA and
required this study.
Case # 6- Teaching Point
Contrast and/or clot may be of similar
density to bone and inseparable from it on
VR images. This is dependent on window
settings and time of study acquisition.
Some times, changing window setting may
solve this problem but others times the
problem may persist. Suspected defects
seen
on
MIPs
may
necessitate
confirmation by catheter angiography.
Discussion
Stroke is the end product of a dynamic cascade of
events that culminates with tissue death.
CTA information is only a snapshot of entire
process.
CTA may reveal distinct phases of disease process
or patient characteristics that serve as confounding
factors in imaging, such as




recanalization of prior occlusion
intra-arterial clot that is as dense as IV contrast
collateral flow that may be primary or secondary
symmetrical collateral flow that may be insufficient under
hypoperfusion situations.
Discussion
Technical factors such as slice thickness , type
of reconstructions, suitable window settings and
MIP/VR interactive assessment at the work
station may improve
assessment of distal
branch occlusion and intra-vascular densities.
Keep in mind, when assessing a patient with
acute stroke symptoms, that there is a high
likelihood that chronic findings and/or unusual
flow patterns may be related to the patient’s
symptoms.
Suggested Image Assessment
•
•
•
•
•
•
Assess all acquired imaging settings
Alter window level and center when assessing MIPs
and VRs to find calcifications, clots, dissections and
stenoses that may be either concealed or
overestimated
Assess 3D images dynamically, changing vessel
bifurcations angles
Keep in mind that you are dealing with a dynamic
disease with possible associated chronic findings;
Keep in mind that venous and arterial systems may be
contrasted and overlapping
Look for possible collateral flow