Accurate follow-up of aortic dilation is pivotal for timely prevention of fatal complications such as aneurysm formation, aortic dissection, and rupture. Standardized measurements and careful side-by-side comparisons of serial examinations are needed to minimize errors in the assessment of aortic dimensions. For all imaging techniques, appropriate electrocardiographic gating, consistent determination of the edges of the aortic wall (inner edge–to–inner edge, leading edge–to–leading edge, and outer edge–to–outer edge diameters) and measurement in the same cardiac phase (systole or diastole) are recommended to improve the reproducibility of the measurements. Currently, magnetic resonance imaging (MRI) and computed tomography (CT) are regarded as the standards of reference to assess all segments of the aorta. However, these imaging techniques are not widely available and are associated with important costs. In addition, CT is associated with nonnegligible radiation exposure risks (particularly when repeated imaging is required to follow-up aortic abnormalities), and the image quality of MRI may be reduced when cardiac or aortic devices are implanted (MRI-compatible pacemakers or endovascular prostheses). Furthermore, image acquisition and analysis with CT and MRI are not standardized. Two-dimensional (2D) transthoracic echocardiography (TTE) is the imaging technique of first choice for screening patients at increased risk for aortic root dilation and thoracic aneurysms of the tubular ascending aorta and for evaluation of associated aortic valve disease. In addition, TTE is an excellent imaging technique for serial measurements of the maximal aortic root diameters and for determining the timing of elective surgery for thoracic aortic aneurysms of the ascending aorta. In contrast to CT and MRI, transthoracic echocardiographic assessment of the thoracic aorta has been standardized for years. Current recommendations advocate for the measurement of the aortic diameters using a leading edge–to–leading edge method at end-diastole and perpendicular to the long-axis of the aorta.
The lack of standardization of image acquisition and analysis of the thoracic aorta across the various imaging modalities has been highlighted recently in landmark articles. From the National Registry of Genetically Triggered Thoracic Aortic Aneurysms and Cardiovascular Conditions, which included 2,046 individuals with various genetic conditions predisposing to aortic disease, 965 studies (TTE, CT, and MRI) in 930 patients from six leading centers were reanalyzed by an imaging core laboratory in a standardized manner (at end-systole if electrocardiographic gating was available, using the inner edge–to–inner edge algorithm and at prespecified locations of the thoracic aorta). The measurements obtained by the imaging core laboratory were compared with nonstandardized measurements performed at each center. Interestingly, the study showed that the methodology used to assess the aortic dimensions on TTE was highly consistent across the centers, following the American Society of Echocardiography recommendations, whereas the methodology used to analyze computed tomographic and MRI data was variable (systole vs diastole, inner edge to inner edge vs leading edge to leading edge). Compared with the imaging core laboratory measurements, TTE showed excellent agreement for proximal segments of the thoracic aorta (intraclass correlation coefficients [ICCs] ranged from 0.84 to 0.92) and good agreement for the aortic arch and descending aorta (ICCs of 0.70–0.71). MRI measurements showed the best agreement between the centers and the imaging core laboratory for all aortic segments (ICCs of 0.82–0.95). In contrast, computed tomographic measurements showed lower ICCs for all aortic segments, with large differences between the centers and the imaging core laboratory, and yielded closer agreement only when data were acquired with electrocardiographic gating.
This important topic of intra- and intermodality variability in aortic measurements is further highlighted in the present issue of JASE with a prospective study by Rodríguez-Palomares et al. , enrolling 140 patients with severe aortic valve disease or aortic dilation who were evaluated with 2D TTE, electrocardiographically gated CT, and MRI. The measurements were performed at three prespecified levels (sinus of Valsalva, sinotubular junction, and proximal ascending aorta), and standardized protocols were used for each imaging modality: on TTE, the diameters were measured at end-diastole using the inner edge–to–inner edge, outer edge–to–outer edge, and leading edge–to–leading edge conventions, whereas on CT and MRI, diameters were measured at end-diastole (or at 75% of the RR interval on electrocardiographically gated CT) and using the inner edge–to–inner edge and outer edge–to–outer edge conventions. Rodríguez-Palomares et al. elegantly show that the measurements performed on TTE using the leading edge–to–leading edge convention yielded the best agreement with the measurements performed on CT and MRI using the inner edge–to–inner edge convention. In contrast, the use of the inner edge–to–inner edge convention on TTE significantly underestimated the dimensions obtained with CT and MRI. The majority of the studies comparing the various methodologies to assess the aortic dimensions with 2D TTE in healthy individuals have shown that the leading edge–to–leading edge convention yields larger dimensions than the inner edge–to–inner edge convention ( Table 1 ), and the study by Rodríguez-Palomares et al. is the first to prove this in comparison with CT and MRI.
|Study||No.||Location||Methods||Main findings||Mean difference in measurements|
|Son et al.||112||AA, SV, STJ, Asc Ao||Leading edge to leading edge resulted in larger dimensions than inner edge to inner edge at the levels of AA, SV, and STJ||<1.9 mm|
|Muraru et al.||218||SV, STJ, Asc Ao||Leading edge to leading edge resulted in larger dimensions than inner edge to inner edge at the levels of SV, STJ, and Asc Ao||2–4 mm|
|Fitzgerald et al.||512||SV, STJ, Asc Ao, Ao Arch||Leading edge to leading edge resulted in larger dimensions than inner edge to inner edge at the levels of STJ and Asc Ao||1.5 mm|
|Bossone et al.||1,148||AA, SV, STJ, Asc Ao||Leading edge to leading edge resulted in larger dimensions than inner edge to inner edge at the levels of SV and STJ||<1 mm|
|Saura et al.||704||AA, SV, STJ, Asc Ao||Leading edge to leading edge resulted in larger dimensions than inner edge to inner edge at all aortic levels||2 mm|