Key Words
cardiac magnetic resonance imaging, feature tracking, aortic valve
annulus, strain analysis
The aortic valve annulus has become a more prominent feature for
analysis as more attention is paid to percutaneous aortic valve
replacement, surgical aortic valve repair, and homograft aortic valve
replacement. The aortic valve annulus has been
studied1 – especially where annular contribution to
aortic insufficiency is now better understood and where aortic
annuloplasty accompanying aortic valve repair is in greater use with
growing evidence that it can improve aortic valve repair
durability.2,3,4,5
Holst and colleagues6 are to be congratulated for a
timely pilot study of the aortic valve annulus. The aortic annulus
consists of a muscular component and a fibrous component. The authors
show that aortic annular dilation and adverse deformation occurs at the
muscular component of the aortic annulus. In their study, CMR imaging
and CMR feature-tracking strain analysis is used to characterize aortic
annular regional longitudinal strain (RLS) in humans with regurgitant
aortic valves (n=4) compared to humans with normal aortic valves (n=7).
Previous CT-based studies of aortic annular deformation analysis have
been published;1,5 however, using CT-based imaging, it
is not possible to obtain feature tracking at the muscular portion of
the aortic annulus. The novel modality, as reported in this study,
produces increased sensitivity for evaluation of aortic annular
deformation. This is accomplished by modifying CMR imaging software –
originally intended to be used to obtain LV global longitudinal strain
– and obtaining regional longitudinal strain (RLS) data from the aortic
valve annulus and computationally converting the data to arrive at
circumferential aortic annular strain data.7
It is important to define abnormal aortic annular deformation and normal
aortic annular deformation. Adverse systolic aortic annular deformation
in patients with severe aortic insufficiency results in muscular annular
dilation, (positive strain value); normal aortic annular deformation in
patients with normal aortic valves results in muscular annular
constriction, (negative strain values). Despite an exceedingly small
patient sample size, the data obtained in the study are sufficiently
robust to preliminarily support the hypotheses and conclusion. The
hypotheses and evidence supporting the hypotheses are as follows:
In patients with severe aortic regurgitation, adverse aortic annular
deformation is located at the muscular portion of the aortic annulus.
The muscular portion of aortic annular RLS of patients with severe
aortic regurgitation is significantly different compared to patients
with normal aortic valves, (median RLS: 4.18% in patients with severe
AR vs -10.41% in well-functioning AVs, p=0.024; at RLS muscular
annulus; segments II-IV).
There is no contribution of adverse aortic annular deformation at the
fibrous portion of the aortic annulus, (median RLS: -2.66% in aortic
regurgitation patients vs -3.86% in healthy controls, p=0.788; at RLS
fibrous annulus; segments I, V-VII).
(The authors use median (IQR) instead of mean (SD) because the sample
size is too small to approximate a normal distribution.)
It is significant to observe that, in the muscular section of the aortic
annulus, the direction of the median RLS in the aortic regurgitation
group is opposite from the direction of the median RLS in the normal
aortic valve group, (segments II-IV). The median RLS difference between
groups achieves statistical significance because the RLS differences are
in opposite directions. If this data holds up in a future study with a
sufficient sample size, then this finding is of major physiological
significance. The authors speculate on the implications regarding
adverse annular remodeling; including right aortic leaflet prolapse in
patients with adverse muscular annular deformation and dilation.
It is immensely helpful to pay attention to the one excellent figure in
the manuscript. A CMR image of the aortic valve is used to correlate
surrounding anatomic structures. The figure also shows anatomic
annotated overlays of the CMR-strain generated imaging – correlated
with longitudinal aortic valve annular strain (with anatomically
associated segments I-VII), and with comparison of the muscular
component to fibrous component. (The use of aortic annular segments
I-VII appears to be a novel addition to the author’s imaging analysis.)
A significant objective for aortic valve surgery is improvement of
aortic valve repair reproducibility and
durability.3,4,5 The use of a variety if aortic valve
annuloplasty techniques is undergoing a resurgence since first proposed
in 1966.9 Is there value in using CMR-based strain
analysis to study bicuspid aortic valve pathology and bicuspid aortic
valve repair? TAVR for treatment of aortic regurgitation is
significantly underdeveloped. Certainly, there may be a place for
CMR-based strain analysis to better understand and develop improved TAVR
technology. This pilot study adds to a series of ongoing studies where
we can track the valuable contributions of this group and other groups
of scientists in their quest to obtain a better understanding of cardiac
physiology using novel computational imaging and strain methods.