Turbulent Kinetic Energy Assessed by Multipoint 4-Dimensional Flow Magnetic Resonance Imaging Provides Additional Information Relative to Echocardiography for the Determination of Aortic Stenosis Severity

Abstract

A ortic stenosis (AS) is the most prevalent valvular heart disease in adults of advanced age and, if untreated, is associated with a high mortality when symptoms occur. 1,2 According to current guidelines, the diagnosis of severe AS is based on echocardiographic measures of mean pressure gradient (MPG) and aortic valve effective orifice area (AVA). 3,4 Class I indications for valve replacement are severe, symptomatic AS or severe AS with reduced left ventricular ejection fraction.3,4 However, gauging symptoms of AS is highly subjective and can be confounded by various other diseases: for example, coronary artery disease, pulmonary disease, or orthopedic disorders. In addition, correct quantification of AS severity by 2-dimensional (2D) echocardiography is challenging, and AS severity is misclassified in a non-negligible portion of the patient population. [5][6][7][8] This misclassification has in part been associated with the effect of pressure recovery and its dependence on valve morphology and ascending aortic (AAo) diameter, which is not accounted for in standard echocardiographic metrics.An echocardiographic approach to correct for pressure recovery is the energy loss index (ELI), 9 which represents Background-Turbulent kinetic energy (TKE), assessed by 4-dimensional (4D) flow magnetic resonance imaging, is a measure of energy loss in disturbed flow as it occurs, for instance, in aortic stenosis (AS). This work investigates the additional information provided by quantifying TKE for the assessment of AS severity in comparison to clinical echocardiographic measures. Methods and Results-Fifty-one patients with AS (67±15 years, 20 female) and 10 healthy age-matched controls (69±5 years, 5 female) were prospectively enrolled to undergo multipoint 4D flow magnetic resonance imaging. Patients were split into 2 groups (severe and mild/moderate AS) according to their echocardiographic mean pressure gradient. TKE values were integrated over the aortic arch to obtain peak TKE. Integrating over systole yielded total TKE sys and by normalizing for stroke volume, normalized TKE sys was obtained. Mean pressure gradient and TKE correlated only weakly (R 2 =0.26 for peak TKE and R 2 =0.32 for normalized TKE sys ) in the entire study population including control subjects, while no significant correlation was observed in the AS patient group. In the patient population with dilated ascending aorta, both peak TKE and total TKE sys were significantly elevated (P<0.01), whereas mean pressure gradient was significantly lower (P<0.05). Patients with bicuspid aortic valves also showed significantly increased TKE metrics (P<0.01), although no significant difference was found for mean pressure gradient. Conclusions-Elevated TKE levels imply higher energy losses associated with bicuspid aortic valves and dilated ascending aortic geometries that are not assessable by current echocardiographic measures. These findings indicate that TKE may provide complementary information to echocardiography, helping to distinguish within the heterogeneo...

Semi‐automated analysis of 4D flow MRI to assess the hemodynamic impact of intracranial atherosclerotic disease

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“…4D flow MRI sequence could be also modified for turbulence assessment by adding 6 extra non-orthogonal velocity encoding. 60 However, the phantom study was to examine the same 4D flow MRI sequence as applied for clinical scans. Our study was a retrospective analysis of 4D flow MRI data from a clinical scan where the scan time was an important concern.…”

Energetics of Blood Flow in Cardiovascular Disease

“…8 Direct measurement of viscous energy loss (laminar flow) or turbulent kinetic energy (turbulent flow) in patients with aortic valve disease or aortic dilatation also takes the hemodynamic impact of the abnormal, energy-consuming flow patterns in the ascending aorta and aortic arch into account and may therefore better reflect disease severity complementary to conventional parameters. 6,11 The definitive role of these and other new energetic markers of blood flow in clinical decision making in various types of cardiovascular disease is promising and subject to future studies.…”

Advanced cardiac MRI techniques for evaluation of left‐sided valvular heart disease

“…These discrepancies were assigned to pressure loss overestimation by Doppler echocardiography due to the pressure recovery phenomenon downstream of the aortic valve, and consequently overestimation of AS severity. 4D flow‐derived estimation of transvalvular pressure gradients based on TKE has been proposed as a supplementary method in the evaluation of AS, as it allows for functional analysis of energy efficiency and cardiac workload that echocardiography does not offer. The clinical value of 4D flow‐derived TKE is supported by good correlations between TKE values in the ascending aorta and poststenotic pressure loss in AS patients .…”