Tricuspid Regurgitant Velocity Evaluation using 2D Slice-following Flow in Patients: Comparison to Echocardiography

Background: Transthoracic echocardiography (TTE) is used to measure tricuspid regurgitant (TR) velocities as an estimate of right heart pressure via the simplified Bernoulli’s equation. Measurement of TR velocity has immense diagnostic importance, and it is the TTE only diastolic parameter (1) which cannot be measured by MRI. Several groups have shown that cardiac MRI (CMR) can adequately evaluate diastolic function, except for this missing metric (2-4). We hypothesize that MRI 2D flow can measure the velocity of these regurgitant jets, using a slice plane that moves with the valve plane.

Methods: Our current slice-following 2D flow method (5) uses a breath-hold (~18 seconds) and retrospective ECG-gating with 2D short-axis view, with spatial resolution of 1.5 x 1.5 x 6mm, VENC= adjustable, with flow-encoding in the through plane direction. The slice plane dynamically moves with the valve during the cardiac cycle, and is planned just above the tricuspid valve, based on a 4Ch cine and an RV 2Ch cine (see Figure 1 & 2). The automated slice-following is performed using deep learning network TVnet (6). Ten patients (age 34 ±14, 4 female) with recent echocardiograms and with measured TR jets were imaged on a 1.5T Siemens, using slice-following and static-slice 2D flow. The study was IRB-approved, and the participants provided written informed consent.

Results: MRI and TTE agreed in 7 of 10 subjects. Of the 10 subjects, five patients had TR jets by TTE stable across multiple RRs, with two that were well visualized by MRI (Figures 1 and 2), with better jet visualization using the dynamic slice. Figure 1A-B shows examples of the flow maps in one subject, where careful inspection shows that the slice is following the valve throughout the cardiac cycle. Figure 1C shows that in this case, the jet velocity by MRI and TTE agreed. Figure 2 shows another example, where there was greater agreement to TTE, using a dynamic slice vs. static slice. With static slice flow, the maximum velocities in mid-systole were lost, as the imaging plane moved out of the jet. Two of the five significant jets were not well visualized, due to imperfectly planned slices (Figure 3A). One of the five significant jets was not visualized by MRI but was also not well imaged by TTE (Figure 3C). The remaining five patients had jets which spiked at early systole, and were detected by MRI as spikes of lower velocity (Figure 3B).

Conclusion: Automated slice-following 2D flow holds promise for evaluation of tricuspid regurgitant velocities and is demonstrated to quantify TR velocity accurately in some cases. However, there are likely additional causes of disagreement between MRI and TTE, beyond valve motion such as jet variability and slice planning that to be explored. In the future we hope this research will be able to an additional reliable non-invasive measure of right heart pressure.

Figure 1: MRI and TTE show agreement regarding velocity in a tricuspid jet. A) Image plane is placed at the vavle in begin systole, with an evident jet observed. B) Dynamic slice tracking portrays the jet throughout systole. C) Comparison of MRI phase-contrast vs. TTE with dynamic phase-contrast, showing strong agreement.
Figure 2: MRI and TTE show agreement regarding velocity in a tricuspid jet. A) Image plane is placed at the vavle in begin systole. B) Dynamic slice tracking shows the jet throughout systole. C) Comparison of MRI phase-contrast vs. TTE with static and dynamic slices, showing good agreement and better capture of the valve using a dynamic slice.
Figure 3: (A-B) Examples of TR jets missed by MRI. A) The slice plane is observed to be too high above the jet. B) The jet was missed by MRI and not well imaged by TTE. C) Patient with spike at begin systole which appears as a spike of lower velocity by MRI.