Aortic root measurements in standardized cine CMR sequences are comparable to state-of-the-art TTE measurements in a contemporary, prospectively enrolled, middle-aged sample from the general population. The SoV and STJ showed a high degree of agreement, whereas it was only fair for the AoAn. Comparability improved if CMR measurements were compared to II instead of LL TTE measurements.
Aortic root dimensions can be interchangeably measured by CMR or TTE
Few studies with only small cohorts compared aortic root measurements by TTE and CMR: In retrospective analyses with a maximum of 140 patients, diameters of the aortic root measured by CMR and TTE strongly correlated18,19. However, in those studies measurements were performed in specifically acquired sequences for measurements of the aortic root through the true cross sectional aortic valve plane18. To the best of our knowledge, there is only one retrospective study with a limited number of patients with suspected Marfan syndrome comparing standardized cine CMR measurements of the aortic root with TTE17. Hence our large prospective study in a general population is the first study to demonstrate that measurements assessed by LAX and LVOT cine CMR are comparable to TTE measurements. This is of clinical relevance as it supports not only the application of established TTE measurements but also of measurements derived from CMR orientations which are included in almost every CMR protocol for screening for aortic root diseases.
However, in CMR, the LAX view is part of routine clinical protocols whereas the LVOT view is not routinely included8. As LAX and LVOT view measurements for reliably assessing aortic root dimensions both demonstrated high correlations with TTE, the necessity for an additional LVOT view, at least regarding aortic root measurements, might be questionable.
Furthermore, the highest agreements were found for TTE according to LL convention with CMR rather than TTE II measurements and CMR9. This is in contrast to our results from our large cohort, which demonstrate comparable correlations, but a systematic overestimation of diameters measured by LL convention compared to CMR II measurements.
Diameters measured according to LL convention include the outer wall of one side of the aortic root. Therefore, an overestimation compared to diameters acquired by II convention, even though from different imaging modalities, seems comprehensible. Up to now, there is an ongoing discussion about which convention to use for echocardiographic measurements of the aortic root. While the ASE guidelines recommend to use the LL convention, the pediatric guidelines, supported by the 2010 American College of Cardiology and American Heart Association guidelines, support the II convention14,20,21. In clinical routine, as most reference values are based on the LL convention, TTE measurements are primarily performed using the LL convention22. However, in recent years technical upgrades and digital post-processing have led to major improvements in spatial resolution of transthoracic ultrasound. Thereby, the depiction of the thin aortic wall does not any longer limit the reproducibility of measurements23. Our results suggest a reconsideration of the LL-convention in TTE for improving comparability of SoV and STJ measurements with CMR.
Summarising, measurements of the SoV and AoAn highly correlated between TTE and CMR. Although we assessed a cohort with a low burden of aortic root dilatation, our data support an interchangeable clinical application of TTE and CMR for screening of incidental aortic root dilatation. However, CMR is still limited by lower availability, higher costs and more time-consuming examinations compared to TTE.
CMR and TTE measurements of the aortic root are highly reproducible
Echocardiographic evaluation of the aortic root is a well-established method in clinical routine and recommended in patients with suspected aortic root disease14,15. Cine CMR is widely and increasingly adapted in routine clinical practice allowing for accurate measurements of the aortic root as well. However, data on reproducibility are derived from small cohorts or measurements limited to a single diameter of the ascending thoracic aorta1,8. In our population-based study, derived from 741 subjects, both methods demonstrated a very high reproducibility, with CMR even exceeding TTE, for measuring aortic root diameters especially for SoV and STJ.
The complex structure of the AoAn limits reliable measurements and comparability
In contrast to measurements of SoV and STJ, diameters of the AoAn are routinely assessed according to II convention. However, both reproducibility and correlation of CMR and TTE measurements are reduced compared to measurements of SoV and STJ. The AoAn is an entity without a visible anatomic structure only virtually defined by the hinge-points of the three aortic valve leaflets14,24. This complex anatomy of the AoAn with an ellipsoid structure, in contrast to the more circular SoV and STJ, makes the exact measurement of the AoAn a challenging and error-prone process25. Only small alterations of the imaging plane, result in major differences of the measured diameter8. Individual alterations in dimension during the cardiac cycle further impair the reproducibility of AoAn assessment. Hence, our data suggest that measurements of the aortic annulus should only be performed in 3D CMR or computed tomography (CT) datasets with specifically angulated CMR planes.
Limitations
As the study sample origins from the general population of Hamburg, most subjects are of Caucasian ascend and represent a predominantly healthy population. Hence, the translation of our findings into other populations, especially the typical patient collective in aortic root surgery, is limited.
Another limitation of this study is that 3D reconstructed CT scans, as the recommended standard of reference for the measurement of aortic root dimensions, were not performed5. No conclusions can be drawn about the accuracy of each the modalities. Nevertheless, the aim of our study was to investigate whether measurements of the aortic root in standard planes of a regular cine-CMR are comparable to state-of-the-art TTE measurements. By showing the high correlation of both imaging techniques, these additional CMR measurements show high validity when compared to TTE and could be implemented in clinical routine for screening purposes and prevent or trigger further imaging.
The ascending aorta was not measured, hence no conclusions can be drawn regarding dilatations above the aortic root. Two further limitations have to be considered: First, since no blood pressure measurements were performed during TTE and CMR examinations, a possible impact of blood pressure fluctuations on aortic root dimensions, e.g. a raised blood pressure during CMR examination, was not considered. Second, TTE and CMR examinations were not performed on the same day. However, the median time interval between the two examinations was only 28 days. Consequently, it is highly unlikely that during this short period, aortic root diameters significantly changed.
