Pitfall of CT Angiography for Acute Stroke Imaging PowerPoint Presentation

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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

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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.

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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

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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

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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

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Representative Cases

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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).

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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

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Case # 1 Follow-u[ MRA shows that left ICA is occluded.

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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.

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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.

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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.

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Coronal MIPs show symmetrical filling of MCAs. Case # 2

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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.

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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

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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.

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Case # 3 Patient presented with acute left MCA stroke symptoms. CTA showed no occlusions; VR images are shown (next slide).

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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.

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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.

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Case # 3 Axial MIPs with wide window settings show narrowed petrous (arrows) left ICA when compared to right ICA (arrowhead).

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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.

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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).

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VRs of the circle of Willis show a narrowed basilar artery, non visualization of the PCAs and adequate proximal anterior circulation. Case # 4

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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.

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Case # 4 MIP axial image shows occlusion of the right ICA.

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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.

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Case # 4 Right external carotid artery injection shows opacification of right MCA territory. Lateral view of ECA injection shows opacification of right MCA territory.

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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

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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.

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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).

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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).

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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

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Case # 5 Injection into the right subclavian artery shows occlusion of proximal vertebral artery with recanalization cephalad by collaterals.

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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.

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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.

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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.

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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.

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Case 6 Sequential axial MIPs (on click) showing normal appearing vessels.

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Case # 6 Coronal MIPs show left MCA fenestration (circle) and incompletely seen right M1 segment but with good opacification of the ipsilateral sylvian branches.

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Case # 6 VR images confirm left MCA fenestration (circle) and adequate filling of right MCA despite symptoms corresponding to that side.

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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.

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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.

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Case # 7 Patient presented with posterior circulation infarct symptoms and CTA showed an unusual configuration of the top of the basilar artery.

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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.

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Case # 6 Catheter angiogram shows clot occluding distal basilar artery. The definitive diagnosis could be made on CTA and required this study.

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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.

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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.

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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.

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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