Retrospectively (2012–2013) the quantitative flow in the ascending aorta was analyzed in 16 patients (6 females) with bicuspid aortic valve (BAV) disease and in 18 (7 females) healthy volunteers (HV).
Patients > 7 years, no past congenital heart defect other than BAV, no arterial hypertension, no thorax deformations, a diameter of the ascending aorta < 4.5 cm in adults and < 2.2 cm/m2 in children, no significant aortic valve regurgitation, and a flow velocity through the aortic valve of < 250 cm/s by echocardiography no absolute contraindications to CMR imaging were included. Healthy volunteers were matched to the BAV group by sex and age (± 2 years) to serve as a control group. Individuals who did not meet the aforementioned criteria were excluded.
All subjects were studied on a standard cardiac 1.5 Tesla MRI-scanner, and a standard cardiac 12-channel coil was used for all patients (MAGNETOM Avanto, Siemens Healthcare, Erlangen, Germany). A written informed consent was obtained from all patients. At the beginning of the study, localizers were obtained in three orthogonal slices for registration of the thoracic anatomy in order to plan the following sequences.
Through plane two-dimensional phase contrast velocity encoding flow measurement
Standard through plane two-dimensional phase contrast velocity encoding imaging of the ascending aorta was performed, as previously described [9, 10]. Parameters in the 2D phase contrast velocity encoding imaging were free breathing (typical acquisition times: around 3 min), retrospective ECG gating, and the velocity encoding set to 200 cm/s.
Slice thickness was 5 mm, repetition time 36.7 ms, echo time 3.09 ms, flip angle 30°, averages 3, segmentation 3, rectangular field of view 260 to 330 × 330 mm, matrix 256 × 256. Data were reconstructed to provide 30 magnitude and phase (velocity-mapped) images per cardiac cycle.
The imaged vessel’s region of interest was always positioned at iso-center of the magnet to maximize gradient fidelity. Furthermore, the ECG was continuously observed during acquisition. The running acquisition was always aborted when more than three extra systoles were noted [9, 10].
Flow measurement of the ascending aorta was performed at the level of the pulmonary artery bifurcation. Post processing was done by manual contour delineation using the Argus software tool (Siemens®) by cardiac MRI experts. In all results depicted, aortic flow was defined as net forward aortic flow.
Four-dimensional flow measurement
Quantitative flow measurement of the ascending aorta by the four-dimensional flow sequence was performed using a time-resolved three-dimensional phase-contrast sequence with three-directional velocity encoding as previously described [11]. No contrast agent was used, and respiratory gating was achieved by a respiratory navigator. All data were measured in a sagittal-oblique volume that included the entire thoracic aorta. Velocity encoding Vx, Vy, and Vz set to 200 cm/s similar to the 2D phase contrast velocity encoding.
Assessment of flow was performed using the Fraunhofer-MEVIS flow software (Fraunhofer MEVIS—Institute for Medical Image Computing). Once ROI was fully defined for each time step throughout the cardiac cycle, the acquired data were fully processed for final quantification (Figs. 1 and 2). In all results depicted, aortic flow was defined as net forward aortic flow. Flow measurement of the ascending aorta was performed at the level of the pulmonary artery bifurcation (Fig. 1).
Interobserver variability
A second investigator, blinded to the first assessment, using the same software tools, performed a second assessment of the flow. Contour drawing in the ascending aorta was performed by two experienced members of the cardiac MRI team (A.K. and K.B.) who were familiar with the Mevis software flow tool. In the four-dimensional flow measurement of the ascending aorta, iso-surface images of the aorta are produced. The post-processing manual contour delineation was performed at the level of the pulmonary artery bifurcation (Fig. 1).
Statistical analysis
Linear regression plots (Fig. 3) were used to describe the correlation (Pearson correlation and coefficient of determination) of net flow within the different techniques (two-dimensional versus four-dimensional phase contrast velocity encoding).
Bland–Altman plots (Fig. 4) were used to describe the agreement of ascending aorta net flow within the different techniques (two- versus four-dimensional phase contrast velocity encoding) as well as the interobserver variability of the four-dimensional net flow.
The individual percentage difference was defined as the individual difference between two measurements divided by their mean and multiplied by 100.
Differences between two- and four-dimensional flow analyses by each observer and interobserver differences were tested by Student’s t test for paired variables. Statistical significance was set at p < 0.05.