Synopsis
Valvular
and vascular diseases are accompanied by abnormal flow that results in energy
losses. These energy losses can provide valuable information about the impact
of the disease on cardiac afterload and thereby stress on the myocardium. Recent
advances in PC-MRI have given the opportunity to quantify several measures of
energy loss that could be valuable for diagnosis purposes. This presentation
will give an overview of current MRI-derived measures of energy loss relevant for
assessing valve performance and cardiac function. The limitations and potential
sources of error of these measures will also be discussed.
Target audience
Those
with an interest in non-invasive quantification of energy losses related to valvular
and vascular diseases.
Objectives
-
To
review the concept of energy loss when applied to evaluate valvular diseases
and cardiac function.
-
To
give an overview of current PC-MRI-based approaches for non-invasive quantification
of energy losses.
-
To
review the limitations and potential sources of error in the previous
approaches.
Purpose
Several
valvular and vascular diseases, such as aortic stenosis and aortic coarctation,
are accompanied by energy losses. Blood flow over a stenosis is dominated by
jet flow, characterized by a high velocity jet followed by an area with
turbulent velocity fluctuations and small eddies. Kinetic energy is dissipated
into heat mainly due to turbulence and, to a lesser extent, viscosity. This
represents an irreversible loss of energy, causing an increase in cardiac
afterload and hence stress on the myocardium [1]. The aim of this
presentation is to review current MRI-derived measures of energy loss relevant
for assessing valve performance and cardiac function, discussing their
limitations and potential sources of error.Methods
Phase-contrast magnetic resonance
imaging (PC-MRI) has gained increasing importance in quantifying blood flow in
the heart, heart valves and great vessels. With three-dimensional (3D) cine (time-resolved)
phase-contrast MRI with three-directional velocity-encoding (4D flow MRI), it is possible
to determine velocity fields over a volume of interest in clinically feasible
scan times [2]. More importantly,
a 4D Flow MRI acquisition provides information to quantify several measures
related to energy loss, which could provide additional information for assessing
valve performance and cardiac function.Results
The gold standard for
obtaining the left ventricular load associated with aortic stenosis is measuring
the transstenotic pressure gradient by invasive catheterization. In the clinical
setting, this gradient is estimated from the simplified Bernoulli equation
using Doppler measurements of the velocity at the valve [3]. However, this
approximation is known to overestimate the actual pressure gradient across the
stenosis [4]. Modifications of
the simplified Bernoulli equation have been proposed, which take geometric
factors into consideration [5].
Data from a conventional 4D
Flow MRI acquisition allow estimation of several energy loss measurements. These include turbulent kinetic energy (TKE),
which is a measure of turbulence intensity [6], and viscous energy losses [1]. More recently,
other authors have developed approaches to quantify the total turbulent
production of the flow using 4D Flow MRI [7]. Several studies
have investigated the accuracy and the dependence on image settings of these
parameters [8,9].Discussion and Conclusions
The methods discussed here
provide different measures of energy loss that can be applicable to several
valvular and vascular pathologies. Since these measures are non-invasive, as
opposed to the gold standard measurement of pressure gradients, they could possibly
enhance diagnosis and improve understanding of the impact of these pathologies
on ventricular function.Acknowledgements
No acknowledgement found.References
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