Thoracic aorta measurements performed in “bright blood” unenhanced sequences correlated better with contrast enhanced 3D-MRA than measurements performed in “dark blood” sequences both in TAV and BAV cohorts, especially in the ascending aorta. Comparing aortic measurements between US and MRI unenhanced sequences measurements - "bright blood" sequences had the strongest correlation in TAV group as well. Our results suggest that possibly there is no strict need for intravenous contrast administration and multiplanar reconstructions for routine MRI evaluation of maximal diameters of thoracic aorta except for evaluation of aortic sinuses in BAV.
Recent literature [8, 9] encourages the use of contrast unenhanced segmental analysis for measurement of diameters in aorta, but the data are partially controversial and there is no unified and wide accepted contrast unenhanced aortic magnetic resonance imaging protocol. Possibility for measurement of aortic diameters without contrast media due to the good tissue contrast is one of the MRI strengths compared to other imaging modalities. Studies [10–12] have shown that, without using intravenous contrast in MRI studies, qualitative and quantitative parameters of thoracic aorta diameters do not underperform comparing to contrast enhanced MRA and other imaging methods or even surpass them in evaluating aortic wall changes. We aimed to reduce need for aortic multiplanar reconstructions, especially where such competence is not readily available.
On the basis of our study results, we have found that measurements of the aortic segments in patients with TAV using "bright blood" sequences are non-inferior in terms of reproducibility to the "gold standard" – 3D magnetic resonance angiography with contrast media [12, 13]. Additionally, measurements performed in “bright blood” unenhanced sequences correlated better with contrast enhanced 3D-MRA than measurements performed in “dark blood” sequences both in TAV and BAV cohorts. Our findings correspond to Krishnam and co. performed studies with 50 patients [14], when no significant difference was observed in the measurements of aortic segments between MRA with contrast media and "bright blood" sequences. Although the Koktzoglou study [9] found that the precision of measurement in the aortic sinus region in the "bright blood" sequence was lower than in MRA with contrast, but using our 2D "bright blood" sequences, we obtained good reproducibility results in all segments. Meanwhile, using the measurements of 2D "dark blood" sequences, the weakest correlation was found in the aortic sinuses (as in the Koktzoglou study) and diaphragmatic segments: in the aortic sinus region R = 0.88 and in the diaphragmatic aortic part R = 0.87 (p <0.05).
We used routine measurements in axial slices as most robust method both to radiologists and cardiologists, because this approach is simpler to use than multiplanar reconstruction (MPR), which requires special training and software. Other anatomical planes were not used, because they are more difficult to track segments for non-experienced user. It should also be noted that measurements of the maximum segmental diameters of aorta in the normal axial body planes (as we suggest in our methodology) are simpler for less experienced image reader and suitable for a wide range of routine applications, and are statistically insignificantly different from the "golden standard" (3D-MRA with contrasting media. From the results obtained, it can be seen that although the "dark blood" and "bright blood" sequences are statistically reliably and strongly correlated with each other, but slightly better results are observed in "bright blood" sequences, therefore, when measuring aortic segmental diameters in the case of TAV, we recommend to use “bright blood” sequences.
The second important finding of this study is the fact, that in the TAV group the strongest correlation was found between US (derived aortic root and ascending aorta measurements) and "bright blood" sequences, as compared with measurements performed in “dark blood” sequences. The "dark blood" sequence is usually used as an additional assessment of the morphology of the aortic wall, but with pulsation of the wall, we have found that it is more difficult to estimate it’s true wall position than in "bright blood” sequences - this could be one of the main reasons, why in our results, in most cases, better results were obtained with "bright blood" sequences. Since this trend has been established both between different MRI sequences (comparing sequences with and without c/m) and between different methods (US vs. MRI), therefore, in the measurement of aortic segmental diameters in TAV, we recommend using 2D “bright blood” sequences without contrast media.
Our results for the US measurements of aorta varied more than in the MRI. We believe that this is mainly due to the fact that the US measurements were performed by 2 cardiologists and is known to be more operator-dependent [15, 16], which results in a greater variation between cardiologists' US experience, methodology and the results of the measurements. Statistically significant but much weaker correlation between US derived aortic root and ascending aorta measurements and measurements in "bright blood" sequences in BAV group we think could be explained by both the physiological pulsation of the aorta root (especially if ECG synchronization is not applicable) and a significant discrepancy between US and MRI measurements due to the asymmetry of the BAV sinus. After deeper analysis, it was found that the difference is most likely due to the difference in measurement methodology. The MRI maximum diameters of aorta sinuses were determined by evaluating the image of a short-axis reconstructed by MPR, while the US plane is routed to AS from a 3-chamber image whose plane generally coincides with the smaller AS-dimension. Having detected the disadvantages of US methodology, our specialists in congenital and acquired heart disease took into account these differences in methodologies and started (in the case of BAV) measuring US diameters of the AS in a short axis image corresponding to the MRI reconstruction plane.
We see some limitation of research work presented: 1) quite small sample size, 2) aorta 3D-MRA with contrast media was performed without ECG synchronization, 3) no 3D phase contrast aorta imaging data for comparison has been acquired due to technical limitations.
In conclusion BB correlated better with ce3D-MRA than measurements performed in DB both in TAV and BAV cohorts, especially in the AA. Our results suggest that there is no strict need for intravenous contrast administration and MPR for routine MRI evaluation of thoracic aorta’s maximal diameters, except for AS in BAV due to common asymmetry of the AS.