In vivo velocity and flow errors quantification by phase-contrast magnetic resonance imaging

Discrepancies between cardiovascular magnetic resonance and Doppler echocardiography in the measurement of transvalvular gradient in aortic stenosis: the effect of flow vorticity

BACKGROUND

Valve effective orifice area EOA and transvalvular mean pressure gradient (MPG) are the most frequently used parameters to assess aortic stenosis (AS) severity. However, MPG measured by cardiovascular magnetic resonance (CMR) may differ from the one measured by transthoracic Doppler-echocardiography (TTE). The objectives of this study were: 1) to identify the factors responsible for the MPG measurement discrepancies by CMR versus TTE in AS patients; 2) to investigate the effect of flow vorticity on AS severity assessment by CMR; and 3) to evaluate two models reconciling MPG discrepancies between CMR/TTE measurements.

METHODS

Eight healthy subjects and 60 patients with AS underwent TTE and CMR. Strouhal number (St), energy loss (EL), and vorticity were computed from CMR. Two correction models were evaluated: 1) based on the Gorlin equation (MPG(CMR-Gorlin)); 2) based on a multivariate regression model (MPG(CMR-Predicted)).

RESULTS

MPGCMR underestimated MPGTTE (bias = -6.5 mmHg, limits of agreement from -18.3 to 5.2 mmHg). On multivariate regression analysis, St (p = 0.002), EL (p = 0.001), and mean systolic vorticity (p < 0.001) were independently associated with larger MPG discrepancies between CMR and TTE. MPG(CMR-Gorlin) and MPGTTE correlation and agreement were r = 0.7; bias = -2.8 mmHg, limits of agreement from -18.4 to 12.9 mmHg. MPG(CMR-Predicted) model showed better correlation and agreement with MPGTTE (r = 0.82; bias = 0.5 mmHg, limits of agreement from -9.1 to 10.2 mmHg) than measured MPGCMR and MPG(CMR-Gorlin).

CONCLUSION

Flow vorticity is one of the main factors responsible for MPG discrepancies between CMR and TTE.

The clinical anatomy and pathology of the human arterial valves: implications for repair or replacement

A thorough understanding of valvar anatomy is essential for design engineers and clinicians in the development and/or employment of improved technologies or therapies for treating valvar pathologies. There are two arterial valves in the human heart--pulmonary and aortic valves. Both are complex structures whose normal anatomical components can vary greatly between individuals. We discuss the anatomy, pathology, and challenges relating to transcatheter and surgical repair/replacement of the arterial valves in a translational manner. The high prevalence of aortic valvar pathologies in the burgeoning elderly population, coupled with poor clinical outcomes for patients who go untreated, has resulted in prolific spending in the research and development of more effective and less traumatic therapies. The accelerated development of therapies for treating arterial valves has been guided by anatomical information gathered from high-resolution imaging technologies, which have focused attention on the need for complete understanding of arterial valvar clinical anatomies. This article is part of a JCTR special issue on Cardiac Anatomy.

Comparison between cardiovascular magnetic resonance and transthoracic Doppler echocardiography for the estimation of effective orifice area in aortic stenosis

BACKGROUND

The effective orifice area (EOA) estimated by transthoracic Doppler echocardiography (TTE) via the continuity equation is commonly used to determine the severity of aortic stenosis (AS). However, there are often discrepancies between TTE-derived EOA and invasive indices of stenosis, thus raising uncertainty about actual definite severity. Cardiovascular magnetic resonance (CMR) has emerged as an alternative method for non-invasive estimation of valve EOA. The objective of this study was to assess the concordance between TTE and CMR for the estimation of valve EOA.

METHODS AND RESULTS

31 patients with mild to severe AS (EOA range: 0.72 to 1.73 cm2) and seven (7) healthy control subjects with normal transvalvular flow rate underwent TTE and velocity-encoded CMR. Valve EOA was calculated by the continuity equation. CMR revealed that the left ventricular outflow tract (LVOT) cross-section is typically oval and not circular. As a consequence, TTE underestimated the LVOT cross-sectional area (ALVOT, 3.84 ± 0.80 cm2) compared to CMR (4.78 ± 1.05 cm2). On the other hand, TTE overestimated the LVOT velocity-time integral (VTILVOT: 21 ± 4 vs. 15 ± 4 cm). Good concordance was observed between TTE and CMR for estimation of aortic jet VTI (61 ± 22 vs. 57 ± 20 cm). Overall, there was a good correlation and concordance between TTE-derived and CMR-derived EOAs (1.53 ± 0.67 vs. 1.59 ± 0.73 cm2, r = 0.92, bias = 0.06 ± 0.29 cm2). The intra- and inter- observer variability of TTE-derived EOA was 5 ± 5% and 9 ± 5%, respectively, compared to 2 ± 1% and 7 ± 5% for CMR-derived EOA.

CONCLUSION

Underestimation of ALVOT by TTE is compensated by overestimation of VTILVOT, thereby resulting in a good concordance between TTE and CMR for estimation of aortic valve EOA. CMR was associated with less intra- and inter- observer measurement variability compared to TTE. CMR provides a non-invasive and reliable alternative to Doppler-echocardiography for the quantification of AS severity.

Silicone rubber trileaflet valve assessment using cardiovascular magnetic resonance imaging

Hydrodynamic valve performance is usually evaluated by effective orifice area (EOA), geometric orifice area (GOA) and transvalvular pressure gradients (PG). Magnetic resonance imaging (MRI) is a very efficient tool for assessing these parameters. In this study, we evaluated a silicone rubber trileaflet valve using MRI. EOA, GOA and PG were evaluated under steady (12 l/min - 20 l/min) and pulsatile (5 l/min) flow conditions using continuity equation, planimetry and simplified Bernoulli's equation respectively; the contraction coefficient (Cc) and Reynolds number (Re) were also evaluated as hydrodynamic parameters. The highest EOAs were 0.75 cm(2) for steady flow conditions and 0.98 cm(2) for pulsatile flow. For pulsatile flow conditions greater GOA by planimetry was 1.43 cm(2). For both flow conditions the highest PGs were > 40 mmHg. Cc was on range from 0.8 to 0.86 under steady flow conditions and 0.68 for pulsatile flow at peak sytole. The Re was much greater than 2200 for all flow conditions. This study allowed the non invasive evaluation of a silicone rubber trileaflet valve under steady and pulsatile conditions with MRI in order to understand valve opening.