1996

The assessment of aortic regurgitation using phase contrast MRI is influenced by complex flow
Frida Truedsson1, Christian L Polte2, Odd Bech-Hanssen2, Åse A Johnsson3, and Kerstin M Lagerstrand1

1Institute of Clinical Scientist, Gothenburg, Sweden, 2Institute of Medicine, Gothenburg, Sweden, 3Institute of Clinical Sciences, Gothenburg, Sweden

Synopsis

Assessment of aortic regurgitation (AR) severity by cardiovascular MRI can be obtained directly by phase-contrast MRI. For some AR-patients, especially those with aortic dilatation and bicuspid aortic valve, the flow profile can be highly complex displaying asymmetric outflow jets, helical vortex flow and systolic backward flow. This study showed that such complex flow influences the accuracy of the AR assessment and needs to be taken into account in clinical practice as it may compromise the decision-making and timing of surgery.

Purpose

Assessment of aortic regurgitation (AR) severity by cardiovascular MRI can be obtained directly by through-plane phase contrast MRI (PC-MRI) in the ascending aorta.1 For some AR-patients, especially those with aortic dilatation and bicuspid aortic valve (BAV), the flow profile can be highly complex displaying asymmetric outflow jets, helical vortex flow,2,3 and systolic backward flow.4 We hypothesize that such complex flow affects the accuracy of the AR assessment as the PC-MRI only measures velocity components orthogonal to the imaging slice.

Accordingly, the aim of this study was to investigate the influence of complex flow on the accuracy of PC-MRI in the assessment of chronic AR severity.

Methods

Patients with moderate (n=15) and severe (n=28) chronic AR were included in the study. PC-MRI was performed at the level of the sinotubular (ST)-junction (Ao1) and 1 cm above the ST-junction (Ao2) (Figure 1).

To investigate the influence of complex flow on the assessment of AR using PC-MRI, a reference method (left ventricular stroke volume - pulmonary stroke volume), unaffected by complex flow, was used for comparisons of regurgitation volumes (RVol) (Figure 1).

Detailed flow analysis using an offline research tool (Segment v1.9 R2046 5) was performed to characterize the complexity of the flow at the position of the PC-MRI measurement. The backward flow volume (BFV) during systole and the forward flow volume (FFV) during systole were calculated. Net, forward and backward flow curves, as well as velocity profiles describing the velocity distribution over the vessel area were plotted for all phases of the cardiac cycle (Figure 2).

Based on the detailed flow analysis, patients were then grouped into presence of complex flow (CF) or non-complex flow (NCF). Patients with presence of CF fulfilled at least one of the following conditions: 1) Backward flow present during the majority of the systolic cardiac phases, and/or 2) BFV ≥ 10 ml (Figure 2).

Results

Patient characteristics are shown in Table 1. Patients with CF had significantly larger ascending aortas and more systolic backward flow than patients with NCF. Also, patients with CF had significantly smaller RVols compared to patients with NCF, and BAV was more common in patients with CF than in patients with NCF. No significant differences were found regarding systolic FFV, age, gender and BSA.

Overall, the PC-MRI method measured significantly smaller RVols than the reference method (Table 2). The underestimation was lager in patients with CF than in patient with NCF (Bland-Altman analysis, Figure 3, Table 2). The relative difference in RVol between the methods was significantly larger for patients with CF than for patients with NCF at Ao2 and had a tendency to be larger at Ao1 (Table 2).

Aortic dimensions were significantly greater at the distal measurement position Ao2 (38±8 mm) than at the level of the ST-junction Ao1 (35±7 mm, P<0.001). Moreover, the grade of flow complexity, as indicated by systolic BFV, was shown to increase with increasing aortic dimension (R=0.8, P<0.001).

Discussion

This study investigated if complex flow, associated with asymmetric outflow jets and helical vortex flow, compromises the assessment of AR using PC-MRI. The comparison between the PC-MRI and reference method confirmed a systematic offset in RVol between the methods 1,6-11, which was shown to increase with increased flow complexity. We also showed that the accuracy of the PC-MRI measurements decreases with increasing aortic dimension, associated with higher flow complexity.

Hence, this study shows that complex flow indeed influences the assessment of AR using PC-MRI and that larger aortas, such as dilated aortas, are more prone to these effects. This needs to be taken into account in clinical practice as it may compromise the decision-making and timing of surgery.

Acknowledgements

The authors acknowledge support from; C4I center, Sahlgrenska University Hospital, Gothenburg, Sweden.

References

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  3. Hope MD, Hope TA, Meadows AK, et al. Bicuspid Aortic Valve: Four-dimensional MR Evaluation of Ascending Aortic Systolic Flow Patterns. Radiology. 2010;255:53-61.
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Figures

Figure 1. A) Left-ventricular outflow tract viewed in end-diastole, illustrating the image plane position at the sinotubular (ST)-junction (Ao1: red line) and 1 cm above the ST-junction (Ao2: red dashed line), and corresponding PC images (B, C) for the quantification of aortic flow. D) Right-ventricular outflow tract viewed in end-diastole, illustrating the image plane position for PC imaging at the pulmonary trunk level (white line), and the corresponding PC image (E) for the quantification of pulmonary flow (white arrow). F) Delineation of the left ventricular endocardial contour (white dashed lines) in short-axis view to determine ventricular volumes.

Figure 2. Examples of net (grey), forward (orange), and backward (blue) flow rate curves obtained from PC-MRI measurements in AR patients with non-complex flow (NCF) (A) and complex flow (CF) (B), including the corresponding velocity profiles (C, D). For NCF patients, regurgitant flow was visible as backward flow during diastole. For CF patients, backward flow was also visible during systole. In the velocity profiles, forward flow is orange and backward flow is dark blue. For NCF patients the velocity profiles showed homogeneous distribution of velocities over the vessel area, and for CF patients the velocity profiles were more eccentric.

Figure 3. Bland-Altman plots of the RVol between the reference method (RVol=left ventricular stroke volume - pulmonary stroke volume) and PC-MRI at Ao1 (A, B; top panel; solid lines) and Ao2 (C, D; bottom panel; dashed lines) for patients with NCF (A, C; blue panels to the left), and patients with CF (B, D; red panels to the right; NCF: n=16, CF (Ao1/Ao2): n=27/25). The colored lines represent the mean relative difference (MD) and 95% limits of agreement (LoA), and the grey line represents zero relative difference. SD, standard deviation.

Table 1. Clinical characteristics of patients with non-complex and complex flow in the ascending aorta

Table 2. Comparison between the reference method and the PC-MRI method to obtain the regurgitant volume

Proc. Intl. Soc. Mag. Reson. Med. 27 (2019)
1996