Arterial Luminal Curvature and Fibrous Cap Thickness Affects Critical Stresses Within Atherosclerotic Plaques: An in Vivo MRI-Based Finite Element Method Simulation Study
Zhongzhao Teng¹, Umar Sadat¹, Zhiyong Li¹², Chengcheng Zhu¹, Victoria Young¹, Martin John Graves¹, Jonathan H. Gillard^1
1University Department of Radiology, University of Cambridge, Cambridge, United Kingdom; ²School of Biological Science & Medical Engineering, Southeast University, Nanjing, Jiangsu, China

It has been widely accepted that the plaque rupture is the result of the loading due to blood pressure and flow exceeds the material strength of the fibrous cap (FC) and the site with thin FC is regarded as the vulnerable site. Considerable research has been done to discover the correlation between FC thickness and critical stress conditions, however, the relationship of arterial luminal curvature remains unexplored. We found that stress value taken from the thinnest location will significantly over-estimate the plaque stability. For a better plaque risk assessment, stress at the sites with maximum lumen curvature should be included.

Impact of the Age of Plaque Haemorrhage on Plaque Stress in Patients with Symptomatic Carotid Artery Disease- A Patient Specific Magnetic Resonance Imaging-Based Finite Element Method Simulation Study
Umar Sadat¹, Zhongzhao Z. Teng², Zhi Yong Li², Cheng Cheng Zhu², Victoria E. Young², Martin J. Graves², Jonathan H. Gillard^2
1University Department of Radiology , University of Cambridge, Cambridge, United Kingdom; ²University Department of Radiology, University of Cambridge, Cambridge, United Kingdom

Patients suffering from a transient ischemic attack (TIA) are at high risk of recurrent TIAs, particularly within the first 4 weeks. The risk of recurrent thromboembolic events gradually decreases afterwards. The United Kingdom National Stroke Strategy warrants emergency management of high-risk patients. High resolution magnetic resonance can assist us to identify high-risk plaques and assess the morphological and biomechanical changes within plaques using computational simulations, thereby refining our risk stratification criteria for management of high-risk patients. In this study we assess the impact of age of plaque haemorrhage on plaque stress in patients suffering from TIAs.

Carotid Artery Plaque Burden as Measured by Magnetic Resonance Imaging: A Potential Imaging Indicator for Acute Cerebral Ischemic Lesion Volume
Huilin Zhao¹, Xihai Zhao², Ye Cao¹, Jinnan Wang³, Chun Yuan², Xiangyang Ma⁴, Jianrong Xu^1
1Radiology, Renji Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China; ²Radiology, University of Washington, Seattle, WA, United States; ³Philips Research North America, Briarcliff Manor, NY, United States; ⁴Philips Research Asia, Shanghai, China

Carotid atherosclerosis has been demonstrated to be associated with cerebrovascular events (TIA or stroke). Thus, atherosclerotic disease in carotid arteries may be an effective indicator for the severity and outcomes of stroke, such as cerebral infarct volumes. This study sought to determine the association between carotid plaque burden and cerebral ischemic lesion volume by MRI in 43 symptomatic patients. We found a strong correlation of left carotid artery plaque burden with ispilateral cerebral hemisphere ischemic lesion volumes. Our findings suggest that carotid plaque burden may be a potential imaging indicator for acute cerebral ischemic lesion volume.

Plaque Burden Measurement by Black-Blood MR Imaging Technique in Intracranial and Extracranial Carotid Arteries in Acute Stroke Patients
Huilin Zhao¹, Xihai Zhao², Ye Cao¹, Jinnan Wang³, Chun Yuan², Xiangyang Ma⁴, Jianrong Xu^1
1Radiology, Renji Hospital, Shanghai Jiaotong University School of Medicine, Shanghai, China; ²Radiology, University of Washington, Seattle, WA, United States; ³Philips Research North America, Briarcliff Manor, NY, United States; ⁴Philips Research Asia, Shanghai, China

Atherosclerosis is a systemic disease frequently involving multiple vascular territories, such as carotid artery and cerebral arteries, which are related to cerebrovascular events. Thus, atherosclerotic disease in one vascular bed may be an indicator for the other vasculatures. This study sought to determine the association of atherosclerotic plaque burden between carotid arteries and M1 segment of middle cerebral arteries using MR black-blood vessel wall imaging in 31 symptomatic patients. We found that development of atherosclerosis has been shown to be parallel in intracranial and extracranial cerebrovascular system in stokes. Our findings suggest that atherosclerotic disease in carotid artery may be an indicator of intracranial cerebrovascular atheroma, or vice versa.

Cerebrovascular Reactivity Within Perfusion-Territories in Patients with an ICA Occlusion
Reinoud Pieter Harmen Bokkers¹, Matthias J. van Osch², C. J. Klijn³, L Jaap Kappelle³, Willem P. Mali¹, Jeroen Hendrikse^1
1Department of Radiology, UMCU, Utrecht, Netherlands; ²Department of Radiology, LUMC, Leiden, Netherlands; ³Department of Neurology, UMCU, Utrecht, Netherlands

Patients with a symptomatic occlusion of the internal carotid artery (ICA) and hemodynamic compromise of the brain may benefit from bypass surgery. Our objective was to investigate cerebrovascular reactivity in the perfusion-territories of the cerebral arteries at brain tissue level in patients with an ICA occlusion using arterial spin labeling MRI, and determine whether cerebrovascular reactivity varies within the perfusion-territory of the remaining ICA. Our results show that ASL-MRI can visualize brain tissue with impaired cerebrovascular reactivity. The brain tissue on the side of the occlusion, supplied through collaterals originating from the unaffected ICA, was the most impaired.

Quantitative MR Perfusion and Ischemic Stroke: Improved Discrimination Between Ischemic and Presumed Penumbra Using QCBF Over Tmax or MTT
Christopher S. Eddleman¹, Maulin Shah², Omar M. Arnaout¹, Richard Bernstein³, Bernard R. Bendok¹, Hunt H. Batjer¹, Timothy J. Carroll^4
1Neurological Surgery, Northwestern University, Chicago, IL, United States; ²Biomedical Engineering, Pennsylvania State University, State College, PA, United States; ³Neurology, Northwestern University, Chicago, IL, United States; ⁴Radiology, Northwestern University, Chicago, IL, United States

Time-based indicators of cerebral blood flow, e.g., Tmax and MTT, are often used to grade stroke severity in both MR and CT perfusion studies. However, these measures often overestimate the infarcted territory, thus underestimating salvagable brain. We show that quantitative MR perfusion is superior to time-based measures in distinguishing normally perfused from ischemic brain tissue.

Is Reduced CBV a Reliable Surrogate Marker for Infarct Core and Can It Be Used to Identify Lesion Mismatch?
Matus Straka¹, Jun Lee², Maarten G. Lansberg², Michael Mlynash², Gregory W. Albers², Roland Bammer^1
1Radiology, Stanford University, Stanford, CA, United States; ²Stroke Center, Stanford University Medical Center, Stanford, CA, United States

Mismatch between stroke core and penumbra can be used used to identify patients that could benefit from reperfusion therapies. Hyperintense DWI in MRI or hypointense CBV in CT can be used to identify stroke core, and equivalence of CT-CBV and DWI lesion volumes was tested. DSC-MRI CBV was used as a surrogate for CT-CBV and 59 patients were analyzed. Results indicate that only large lesions (>10ml) can be identified on CBV and accuracy and reliability of CBV-based mismatch is lower then of DWI. CBV-based stroke core identification yeilded generally smaller lesions and correlation with DWI was low.

Model-Based Permeability Estimates Are Preferable to Model-Free Initial Area Under the Curve (IAUC) Measures in the Identification of Hemorrhagic Transformation in Acute Ischemic Stroke
Andrea Kassner^1,2, Rebecca E. Thornhill^1,2, Swati Matta¹, Fang Liu¹, David J. Mikulis^1,3
1Medical Imaging, University of Toronto, Toronto, Ontario, Canada; ²Physiology and Experimental Medicine, Hospital for Sick Children, Toronto, Ontario, Canada; ³Medical Imaging, Toronto Western Hospital, Toronto, Ontario, Canada

Thrombolytic therapy is known to increase the risk of hemorrhagic transformation (HT) in acute ischemic stroke (AIS).  Accurate and robust methods for predicting HT are required for improving treatment guidance.  Model-based permeability estimation with dynamic contrast-enhanced MRI can predict HT, but the estimates (KPS coefficients) are sensitive to noise and require an arterial input function. However, studies of tumors suggest that a model-free measure, the initial area under the contrast-concentration curve (IAUC) is more robust. We evaluated both KPS and IAUC in AIS patients and found that only KPS successfully delineated HT. Model-based estimates are recommended over IAUC in AIS.

Pulsed Arterial Spin Labeling Perfusion MRI Correlates with Clinical Severity in Patients with Vertebrobasilar Artery Stenoses
Bradley J. MacIntosh^1,2, Lars Marquardt³, Ursula G. Schulz³, Peter M. Rothwell³, Peter Jezzard^2
1Imaging & Brain Sciences, Sunnybrook Health Sciences Centre, Toronto, ON, Canada; ²Clinical Neurology, FMRIB Centre, Oxford, OXON, United Kingdom; ³Clinical Neurology, Stroke Prevention Research Unit, Oxford, OXON, United Kingdom

Arterial spin labeling is a versatile perfusion MRI technique and recent studies have shown clinical merit. One clinical arena that is under-investigated is perfusion profiles in patients with vertebral or basilar artery (VBA) stenosis. The arrival of the magnetic spin tracer is expected to be delayed in these patients therefore a multiple inflow 3D-GRASE-PASL implementation is used to estimate cerebral blood flow (CBF) and the arterial arrival time (AAT).  Patients with presumed severe VBA disease (N=4), on the basis of their clinical history, showed significantly prolonged AAT (P<0.01) and reduced CBF (P=0.08) when compared to patient with presumed no VBA disease (N=10).

MR Elastography of Stroke: A Feasibility Study
Sebastian Hirsch¹, Kaspar Josche Streitberger¹, Jan Rodrigo Hoffmann², Randolf Klingebiel³, Dieter Klatt¹, Sebastian Papazoglou¹, Jürgen Braun⁴, Ingolf Sack^1
1Institute of Radiology, Charité - University Medicine Berlin, Berlin, Germany; ²Institute of Neurology, Charité - University Medicine Berlin, Berlin, Germany; ³Institute of Neuroradiology, Charité - University Medicine Berlin, Berlin, Germany; ⁴Institute of Medical Informatics, Charité - University Medicine Berlin, Berlin, Germany

The characterization of neuronal tissue inside an infarcted region is still a subject of intense research. MR elastography (MRE) is capable of measuring the mechanical connectivity of soft tissue in vivo. This feasibility study aims to assess the potential of MRE for the characterization of tissue regeneration after stroke. The hypothesis was that stroke-related changes of the biomechanical properties of neuronal tissue are detectable by MRE. The results demonstrate through both a decrease in the complex shear modulus and an increase in shear wave amplitudes that tissue integrity is degraded inside a stroke region.