Vol. 3 No. 3 - Sep 2017

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Vascular Deformation Mapping (VDM) of Thoracic Aortic Enlargement in Aneurysmal Disease and Dissection Nicholas S. Burris 1 , Benjamin A. Hoff 1,2 , Ella A. Kazerooni 1 , and Brian D. Ross 1,2,3 1 Department of Radiology; 2 Center for Molecular Imaging; and 3 Department of Biological Chemistry, University of Michigan, Ann Arbor, MI Corresponding Author: Nicholas Burris, MD Department of Radiology, University of Michigan, 1500 E. Medical Center Drive, TC B1-132, SPC-5030, Ann Arbor, MI 48109-5030; E-mail: Key Words: aortic aneurysm, spatial Jacobian, aortic dissection, 3D printing, aneurysmal enlargement, vascular deformation mapping Abbreviations: Vascular deformation mapping (VDM), abdominal aortic aneurysm (AAA), thoracic aortic aneurysm (TAA), electrocardiogram (ECG), computed tomography angiography (CTA), Hounsfield unit (HU), computed tomography (CT), matrix metalloproteinases (MMPs) Thoracic aortic aneurysm is a common and lethal disease that requires regular imaging surveillance to deter- mine timing of surgical repair and prevent major complications such as rupture. Current cross-sectional imag- ing surveillance techniques, largely based on computed tomography angiography, are focused on measure- ment of maximal aortic diameter, although this approach is limited to fixed anatomic positions and is prone to significant measurement error. Here we present preliminary results showing the feasibility of a novel tech- nique for assessing change in aortic dimensions, termed vascular deformation mapping (VDM). This tech- nique allows quantification of 3-dimensional changes in the aortic wall geometry through nonrigid coregistra- tion of computed tomography angiography images and spatial Jacobian analysis of aortic deformation. Through several illustrative cases we demonstrate that this method can be used to measure changes in the aortic wall geometry among patients with stable and enlarging thoracic aortic aneurysm and dissection. Furthermore, VDM results yield observations about the presence, distribution, and rate of aortic wall defor- mation that are not apparent by routine clinical evaluation. Finally, we show the feasibility of superposing patient-specific VDM results on a 3-dimensional aortic model using color 3D printing and discuss future direc- tions and potential applications for the VDM technique. INTRODUCTION The thoracic aorta is the largest blood vessel in the human body and is subject to most extreme hemodynamic forces. A healthy aorta is extremely durable, and it is able to absorb forces gen- erated by the heart owing to its thick walls and its elastic nature. Because of multiple factors (eg, hypertension, atherosclerosis, genetic aortic syndromes, infection), the structural integrity and elasticity of the aortic wall can deteriorate, leading to progres- sive dilation of the aortic lumen and formation of aortic aneu- rysm (1). Aortic dissection is a related form of aortic disease characterized by tearing of the inner layers of the aortic wall (ie, intima and media), leading to the creation of a false lumen— or channel—within the aortic wall itself, which is structurally com- promised and is subjected to high pressures. This results in aneurysm formation in ;60% of patients with chronic aortic dissection of the descending thoracic aorta (Stanford type B) (2). The incidence of aortic aneurysm is increasing in the US popu- lation, and mildly dilated aortas are being incidentally detected at higher rates owing to increased use of thoracic cross-sectional imaging for nonaortic indications (eg, lung cancer screening) (3). Recent data suggest that the prevalence of thoracic aortic dilation (.4 cm) is ;3% of individuals older than 55 years of age, which, on the basis of current US population estimates, means that ;2.7 million people in the USA would be recom- mended to undergo regular imaging of the thoracic aorta on the basis of the current American Heart Association guidelines for imaging surveillance (4-8). Imaging surveillance has a central role in the management of asymptomatic patients with aortic disease. The vast majority of patients with an aortic aneurysm, ;95%, are asymptomatic until the aneurysm ruptures, and only 40% of patients in whom the aneurysm ruptures reach the hospital alive (6, 9). Although the topic of aortic enlargement in abdominal aortic aneurysm (AAA) before and after endovascular repair has been the focus of significant research effort, the natural history and mechanisms of thoracic aortic aneurysm (TAA) progression remain poorly understood, and only a handful of studies have attempted to measure growth rates of the thoracic aorta (10-16). A major limitation in improving our understanding of TAAs is that the current clinical imaging surveillance techniques rely primarily on measurement of maximal aortic diameter. This parameter has been most widely studied and is shown to correlate with future RESEARCH ARTICLE ABSTRACT © 2017 The Authors. Published by Grapho Publications, LLC This is an open access article under the CC BY-NC-ND license ( ISSN 2379-1381 TOMOGRAPHY.ORG | VOLUME 3 NUMBER 3 | SEPTEMBER 2017 163

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