requiring a considerable amount of time and computational capacity.[5,56] These studies found
that hemodynamic and geometrical parameters can be linked with (early) plaque development, but conclusions were drawn based on modeling or static approaches of the coronary arteries. A striking advantage of this study is the use of dedicated, commercially available software on
cCTAdatasets, which makes this method immediately clinically applicable. The association of dynamic vessel geometry with plaque development can thus be investigated, and more evidence can be obtained about the process of atherosclerosis that may lead to identification of potential high-risk sites or patients.
Based onECG-gatedCTdatasets,40% and90% of theRRinterval were the most frequently identified phases for evaluation of the coronary arteries in end-systole and end-diastole, re- spectively. Although this may seem late in the cardiac cycle, these percentages correspond with the results of Juergenset al.,[57] who foundESandEDphases ranging from respectively
20-35% and75-90% of theRRinterval. Furthermore, some selections ofEDphase turned out to be at110% of theRRinterval. This may possibly imply incorrect registration of theECGwith the reconstructions. Since we maintained consistently the phase with minimal and maximal filling of the left ventricle with less possible variation in phase, our results are robust enough to represent the actualESandEDphases, as we expect regression to the mean and lessening of the associations if the selected phases were actually notESandED.
Geometry changed in terms of curvature and tortuosity on both artery and segment level, but there were no significant differences for all arteries. However, it could be interesting to observe these geometric parameters for each individual segment. We combined the measurement of all these segments to increase our sample size, since some segments were frequently hard to assess. Besides, the left main artery (segment5) and proximalLAD(segment6) are often shorter than20 mm, making them more sensitive for curvature and tortuosity measurements, increasing the risk of upward peaks in case of incorrect parts of the centerlines. With sufficiently large sample sizes it could be interesting to investigate whether segments more closely adjacent to the myocardium experience larger geometric differences than segments closer to the coronary ostia. For example, Zhuet al. measured geometry of the proximal, mid, and distal segments separately of the RCA andLADin one phase and found for instance a two-fold difference in maximum curvature in
LADsegments between individuals.[45] They state that a particular geometrical feature must
exhibit large variability between vascular regions or indivuals, if it should be considered as an atherosclerotic risk factor. They also suggested to add a dynamic dimension in addition to their work, which can create even more variability. In their study, biplane cineangiography was used for the quantifications, which is invasive in contrast toCTand so only applicable in selected patients at high suspicion ofCAD. Furthermore, this involves two projection directions, requiring additional reconstruction algorithms to obtain three-dimensional information.[47]
A particular strength of this study resides in the fact that coronary arteries were geometrically quantified in four dimensions based oncCTAdata involving multiple phases of the cardiac
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cycle. The only patient study somewhat comparable to ours was conducted by O’Loughlinet al.,[52] but they only investigated the length of segments, and only included a few segments of
the coronary tree. They found that the ratio of length in end-systole divided by the length in end-diastole was correlated with the location ofCAD, but only for the first two segments of the
LADandLCx. Our method involves more detailed and three-dimensional geometric information in terms of curvature, tortuosity, and inflection of both segments and (multiple) arteries. Based on the anatomical properties significant differences in vessel and segment lengths betweenES
andEDare not expected. Our vessel length measurements support this assumption. Therefore,
the method used to perform the segmentation of arteries and segments can be classified as a valid and robust method. As a results, geometric information of segments and arteries can be used to investigate possible associations with stenosis sites.
Like more arterial geometry studies, we performed measurements based on a semi-automatically extracted centerline from theCTdatasets. The centerline creation did not always yield an accu- rate result. Especially extraction of severely diseased coronary arteries was frequently incorrect, thus requiring manual adaptation of the centerline to achieve a better match with the vessel trajectory. This could have affected our geometric measurements. Corresponding segments from the same patient were equivalently adapted in both phases to minimize intra- and inter-observer variability, but this is still a possible shortcoming of the study since variability was introduced. Because of our method selecting segments where we take in consideration different anatomical configuration between individuals, length measurements are less likely to differ between phases, in contrast to the previous mentioned study of O’Loughlinet al.[52] They included only the
segments with highest chances of a visualized bifurcation (i.e. first two segments of theLAD
andLCx), to acquire more reliable results when it comes to coronary segment length. Since plaques may develop throughout the coronary artery system, we decided to investigate as many segments larger than1.5mm as possible. Although not significant for all arteries or segments, path length was smaller inES, suggesting slight compaction of the endothelial cells in the vessel wall.[58] This also corresponds with previous studies that related compression of coronary
segments with significant stenosis.[52,59]
Our method of describing the coronary geometry was mainly based on the curvature and tortuosity parameters derived from a spatial representation of the vessel (i.e. vessel centerline or vessel medial line). Tortuosity in coronary arteries has previously been assessed based on the number of bends in the vessel.[16,24,43,60] The measurements of Liet al. for example are
therefore substantially differing from ours, resulting in a more tortuousLADthanRCAbased on their tortuosity definition.[24] We implemented tortuosity as ratio of path length (P) divided by
the straight distance (D), which has also been applied in larger blood vessels.[38–40] However,
this ratio was initially implemented to study tortuous iliac arteries, which are compared to coronary arteries mainly oriented in the anatomic coronal plane. Due to the curved course of the coronary arteries, the straight distance directly traverses the heart, making this parameter less relevant. Ideally, we should have used the great circle distance with respect to the surface