Quantitation and Visualization of Vasa Vasorum and Neointimal Development in Three Dimensions—High-resolution Microscopic Computed Tomography Analysis
Daniel J Pelchovitz University of Pennsylvania Cardiovascular Division, Hospital of the University of Pennsylvania , Robert L Wilensky University of Pennsylvania Cardiovascular Division, Hospital of the University of Pennsylvania
Vasa vasora are the blood supply of the artery itself, originating in the adventitia in response to the arterial wall’s nutritional requirements. The presence of a thicker tunica media or neointima requires the development of vasa vasorum to meet the metabolic demands of the arterial wall, since diffusion from the arterial lumen alone is insufficient. Over 40 years ago Wolinsky and Glagov demonstrated that arteries with a medial thickness exceeding 0.47mm required vasa vasorum for nutrition.1 Vasa vasora can be further categorized into first-order vasa, which run longitudinal to the vessel lumen, and second-order vasa, which originate from the first-order vasa and are circumferential around the artery.2
The vasa vasora are necessary to maintain normal vessel wall homeostasis and inadequate perfusion of the vessel wall as deficiencies in the vasa have been shown to lead to intimal hyperplasia.3 The initial hypothesis relating atherosclerosis to increased development of the vasa vasorum was made by Barger et al. in 1984.4 Since then, it has been shown that coronary vasa neovascularization takes place early after induction of experimental hypercholesterolemia, suggesting a role for neovascularization in atherogenesis.5 This neovascularization has been shown to favor secondorder vasa.2 Apolipoprotein E-deficient mice given the angiogenic factor vascular endothelial growth factor (VEGF), which leads to increased angiogenesis, show a subsequent increase in plaque area,6 while mice administered the anti-angiogenic factors endostatin and TNP-470 show decreased intimal hyperplasia.7 A decrease in intimal hyperplasia was also observed in a murine study following chronic endothelin receptor antagonism, which decreases VEGF expression and decreases vasa neovascularization.8 These studies provide evidence that neovascularization of the arterial wall is a crucial part of the atherosclerotic process. Abnormalities of the vasa vasorum have also been implicated in the development of neointimal hyperplasia after balloon angioplasty and stenting. In two animal models, local injury to the vascular wall stimulated intimal hyperplasia and adventitial neovascularization that was increased by VEGF and PR39 and tempered by the inhibition of VEGF and fibroblast growth factor, leading Khurana et al. to hypothesize that intimal hyperplasia has both angiogenesis-dependent and -independent phases.9 Indeed, it has previously been shown that following angioplasty injury, the number and density of adventitial microvessels increase in the initial three postprocedural days, then regress.10 Kwon et al. evaluated the spatial pattern of neovascularization, showing that although the number and diameter of the vasa increased after injury, the total vascular area was lower in injured vessels than in control vessels.11
Deployment of an intravascular stent leads to arterial wall compression and increased resistance within the vasa vasorum, resulting in vascular wall ischemia and subsequent neointimal proliferation.12 Further evidence for the importance of neovascularization on neointimal proliferation were studies showing that a tyrosine kinase inhibitor inhibited both neovascularization and neointimal proliferation after coronary stenting, and that the neointimal proliferation was proportional to the number of adventitial microvessels present.13 With the established importance of the coronary vasa vasorum on neointimal proliferation in atherosclerosis, angioplasty injury, and arterial stenting, accurate quantification of vasa vasora number and volume is a fruitful area of research. The traditional method of vasa vasorum quantification uses histology, but this approach requires staining for vasa vasora endothelial cells and suffers from difficulties such as cutting through metallic stents, inaccuracies due to unperfused vasa, and incorrect data due to a limited number of measured cross-sections. An in vivo human method is not plausible, as the coronary microcirculation begins at arterioles of 500?m in diameter and progressively branches into capillaries 5?m in diameter; these blood vessels are too small to visualize using currently available methods.12 Three-dimensional microscopic computed tomography (micro-CT) has emerged as an accurate and accessible method for quantification of the coronary vasa vasorum.
Microscopic Computed Tomography
Micro-CT was developed by Flannery et al. in 1987 as a high-resolution imaging technique with a resolution of 5–30?m per voxel, depending on the X-ray source and the number of two-dimensional images obtained.12,14–15 The technique provides high-resolution three-dimensional images that can be segmentally viewed and analyzed in any plane. Importantly, micro-CT allows for imaging of the entire microvasculature and its relationship to other sections of the growing atherosclerotic or intimal lesion. Quantitatively, the results obtained with micro-CT correlate with histological measures of vasa vasorum quantitation.11,16 It has been postulated that combining micro-CT measurements of the arterial wall with local pressure measurements may allow investigators to accurately estimate spatial patterns of compressive stresses, leading to the development of novel stents that have fewer adverse effects on the vasa vasorum17 and specialized stents for bifurcation or ostial lesions.
