Technical considerations on the description of coronary arteries geometry
Nowadays, with the current imaging tools, the target of ongoing research is to directly correlate vascular geometry with the extent of atherosclerosis using CCTA or DSA. Whichever geometric method is to be used, it should be: easy, quick, easily incorporated into everyday clinical practice and widely accepted by CCTA reporting physicians and clinicians.
In an angiographic study of autopsy hearts, it was attempted to define coronary variations , using pairs of projection angiograms with focus on the left anterior descending artery (LAD) and first two major branches. The parameters studied included: i) angle LAD-left circumflex (LCx), ii) angles between LAD and the early diagonal and septal perforator branches, iii) distance between branch points and iv) tortuosity. Findings and definitions of this study can be found in table 2 (Fig. 2).
Zhu et al. used cine-angiography and reported that LAD exhibits highest curvature, torsion and tortuosity in its distal portion while in the right coronary artery (RCA), the same variables are lowest in its middle segment, with significantly higher torsion on the LAD than RCA. In another CCTA study, LM bifurcated in 87% of patients, trifurcated in 12% and quadrifurcated in 1%. Furthermore, all average bifurcation angles measured were significantly larger in systole and trifurcated systems had significantly wider angles and longer LM segments.
Continuous motion of coronary arteries is thought to influence both vessel wall mechanics and fluid dynamics, thus having implications on atherosclerosis development. Significant variation coefficients were noted using in vivo tracking of the coronary arteries motion. LAD showed greater variability of its torsion, compared with RCA but less displacement. The feature of torsion was described in this study as a factor reflecting the degree to which a curved line (or vessel) remains within one plane, or moves towards a certain direction, thus being part of a helix and showing a constant torsion along its length.
A 2003 study used biplane cineangiograms and intravascular US to assess the right coronary artery, measuring length, cyclic displacement, axial strain, curvature and torsion. Linear combinations of parameters predicted wall thickness with high confidence. Curvature and torsion and their excursion throughout the cardiac cycle were positively correlated with maximum wall thickness and cross-sectional asymmetry. In vivo CCTA was used to produce models for computational fluid dynamics (CFD) analysis, confirming that molecular viscosity was higher in systole, whereas endothelial shear stress was lower, in both normal and atheromatous vessels. The feature of curvature was higher in systole and atherosclerotic segments.
Association of coronary geometry with haemodynamic alterations and atherosclerosis lesions
Recent imaging research has highlighted the importance of coronary geometry regarding the onset and progress of atherosclerosis. Various geometric features (Table 3, Fig. 3) have been studied  and a notable correlation with haemodynamic alterations has been established. According to the “hemodynamic hypothesis”, local hemodynamics, such as low and oscillatory wall shear stress constitute a substantial factor for the onset and progression of atherosclerosis at coronary bifurcations.[13,14] In analogy to the Virchow’s triad, a multifactorial pathogenesis can be proposed as a triad (Fig. 4) or a more complicated interaction pattern (Fig. 5). Various experimental studies dated as early as the 70s, 80s have analyzed hemodynamic data in pulsatile flow through human vessels’ bifurcation casts, demonstrating variability of geometric features between different individuals. The aforementioned variability may be proposed as a potential source of atherosclerosis variability and the term “geometric risk factors” can be suggested.
The theory that coronary geometry affects atherosclerosis development is not new, given angiographic studies focused mainly on the length, angle and shape of the coronary arteries. Examples of measurements of such parameters are seen in Fig 6. Length of left main coronary artery (LMCA) was found significantly shorter in atherosclerotic LMCA [15,16] with the proximal segment mostly affected. Regarding the angle formed between branches, a small angle leads to proximal atherosclerotic disease, an observation stronger for left circumflex artery. Moreover, branches arising from parent vessels in a large angle demonstrate eccentric intimal thickening and thus, are more prone to atherosclerosis development. Regarding LMCA, its short length combined with a wide bifurcation angle results in an increased incidence of proximal segment plaques. Regarding right coronary artery (RCA), its shape is correlated with atherosclerotic lesions. C-shaped RCAs show an increased incidence of atherosclerotic plaques in their proximal and mid segments while S-shaped RCAs had a significantly higher TIMI frame count. (Fig. 7). The key associations are summarized in table 4 (Fig. 8).
Several studies highlighted the emerging role of CCTA as an exquisite modality for the evaluation of coronary geometry. Using patient-specific models, it provides meaningful data in order to perform CFD simulations. Using CFD simulations, the curvature radius of the vessel has been found to influence the production of helical flow and helicity intensity in both healthy and stenotic models, with an increased correlation in stenotic vessels. The smaller the value of the curvature, higher the exposure to low shear stress and lower exposure to oscillatory shear stress. Geometric features associated with haemodynamic alterations can be seen in table 5 (Fig. 9). The degree of stenosis, a CCTA well-studied geometric feature, has been positively correlated with maximum wall shear stress at the stenotic part of the coronary artery. The tortuosity index, another geometric factor, influences the low wall shear stress observed in the proximal segment of branches such as LAD. Other coronary artery geometric characteristics studied with CCTA which have been associated with atherosclerosis are high caliber arteries, high angles (e.g. LMS-LAD, LAD-LCx and LAD-septum) and high tortuosity. Regarding RCA in particular, wall shear stress extent in the middle and distal segments of the artery is strongly associated with the aforementioned features. CCTA can also be used as a quantifying tool of coronary geometry and its correlation with atherosclerosis. Coronary geometry was quantified by means of curvature and tortuosity with both of the features to be strongly correlated with significant stenosis at the per-segment level. Curvature was also associated with stenosis at per-artery level.
Characteristic examples and cases can be seen in figures 10-16.