Assessment of Risk in Thoracic Aortopathy Using 18F-Sodium Fluoride

Bicuspid aortic valve, along with other congenital conditions affecting the aortic wall, has been associated with elastic fibre loss, decrease in size of elastic lamellae, increased collagen deposition and smooth muscle loss compared to healthy controls on both light microscopy and electron microscopy. Further, analysis of site specific regions of the ascending aorta using biopsy specimens demonstrated significant differences in the apoptosis rate and survival of smooth muscle cells favouring the lesser curvature over the greater curvature of the aorta (the area of greater wall shear stress).

Studies to date suggest vessel wall microcalcification (lesions<50μm) is a dynamic marker of this vessel wall pathology: the end-product of a pathological process involving vessel wall inflammation, apoptosis and necrosis of medial smooth muscle cells and subsequent elastic fibre degradation. Granular medial calcinosis, the deposition of microcalcification in the medial layer of resected aortic tissue, has been recognised as a significant feature of bicuspid aortic valve-related aortopathy when compared to aortic samples taken from patients with tricuspid valves.

Recent histological analysis of human and murine Marfan aortas demonstrated elastin breaks co- localising with areas of microcalcification in aortic media. Further, the areas of elastin break were significantly correlated to decreased distensibility and increased aortic diameter. Finally, the group demonstrated an elastin receptor complex-ERK1/2-ALP-mediated mechanism for increased microcalcification deposition. Although focused on Marfan syndrome, these results are relevant to bicuspid aortic valve-related aortopathy as they suggest microcalcification is associated with elastic fibre breakdown, a proposed mechanism of bicuspid aortic valve aneurysm formation at areas of high wall shear stress. The ability to demonstrate and quantify aortic microcalcification, both ex vivo and non-invasively in vivo in patients with bicuspid aortic valve-related aortopathy would be important step in improving our knowledge of the pathological process underpinning this condition.

Positron emission tomography (PET) utilises a radiotracer which provides functional information on tissues at the cellular or molecular level. Because of its limited spatial resolution, PET images are combined with computed tomography (CT) or magnetic resonance imaging (MRI) so that the functional PET images can be co-registered with the detailed anatomical and structural images afforded by CT or MRI. While CT or MRI alone has been utilised in a wide range of cardiovascular diseases and can identify larger areas of macrocalcification (>200 μm), PET/CT using 18F-sodium fluoride has been used to identify vessel wall microcalcification (calcific lesions <50 μm) that is not detectable on CT or MRI. 18F-Sodium fluoride has been extensively validated as a radiotracer of microcalcification utilising electron microscopy, immunohistochemical analysis, pharmacodynamic concentration-response curves, and ex vivo and in vivo micro-PET/CT on vessel wall sections. 18F-Sodium fluoride uptake is mediated through exchange of fluoride ions with hydroxyl groups of hydroxyapatite, the major constituent of vessel wall calcification. Positron emission tomography using 18F-sodium fluoride has demonstrated significant promise as identifying active vessel wall pathology in a number of cardiovascular diseases including carotid, coronary, aortic valve, and abdominal aortic aneurysm disease. The use of 18F-sodium fluoride PET imaging in thoracic aortic aneurysms is novel.

There is a correlation between areas of high wall stress and intimal tear location location. Further, aneurysmal aortic tissue has been demonstrated to have reduced delamination strength in both longitudinal and circumferential dimensions: evidence for an involvement of haemodynamic effects on precipitating dissection. Time resolved 3D blood flow captured on MRI, also known as 4D-flow MRI, is an evolving research tool which non-invasively captures patient-specific regionalised blood flow and velocity, allowing calculation of aortic wall sheer stress, and has been used to study bicuspid aortic valve-related aortopathy. A study pairing pre-operative 4D-flow MRI and post-operative biopsies of aortic tissue have demonstrated a decrease in elastin at sites of high aortic wall stress, suggesting a relationship between haemodynamics and the structural composition of the aneurysmal aortic wall. The ability to demonstrate a pathological process associated with aneurysmal weakness could be an important step to better identifying high risk lesions.