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© Springer International Publishing 2016
M.J. Budoff, J.S. Shinbane (eds.), Cardiac CT Imaging: Diagnosis of Cardiovascular Disease,
DOI 10.1007/978-3-319-28219-0_15
Transcatheter Aortic Valve
Implantation (TAVI)
Chesnal Dey Arepalli , Christopher Naoum , Philipp Blanke ,
and Jonathon A. Leipsic
Abstract
Computerized tomography (CT) plays a pivotal role in selection of patients’, appropriate
device size and pre-procedural guidance in successful outcome of transcatheter aortic valve
implantation (TAVI). CT based vascular and non-vascular evaluation and integration of
relevant measurements into TAVI work up has been shown to reduce morbidity and mortality. Post-procedural device assessment and complications could also be reliably evaluated
with CT. Utilization of CT is not just confi ned to TAVI but it is also increasingly being used
for any transcatheter valvular assessment.
Keywords
AS – Aortic Stenosis • AVC – Aortic Valvular Calcifi cations • AA – Aortic Annulus • SOV –
Sinus of Valsalva • Coronary Artery Ostium • PAR – Paravalvular Regurgitation • CT –
Computerized tomography • TTE – Transthoracic Echocardiography • TEE – Transesophageal
Echocardiography • TAVI – Transcatheter Aortic Valve Implantation
Introduction
Transcatheter aortic valve implantation (TAVI) is an alternative treatment option for symptomatic severe aortic stenosis
(AS) patients who are deemed inoperable or at high risk for
surgical aortic valve replacement (AVR). TAVI as the name
suggests is a procedure where in a bio-prosthetic device is
implanted without removing the diseased valve via a transcatheter approach.
The most common cause for AS is age related progressive
calcifi cation of a normal aortic valve (Fig. 15.1a–c ) [ 1 , 2 ].
Additionally, congenital causes like bicuspid aortic valve or
infl ammatory conditions like rheumatic heart disease are
also common causes of AS [ 1 ]. The prevalence of senile AS
is increasing as the mean age of Western societies is increasing [ 3 , 4 ]. Current data has documented the prevalence of AS
in the elderly population (age ≥ 75 years) as 12.4 % and
severe AS is 3.4 % [ 3 ]. Although senile or degenerative aor-
tic stenosis occurs primarily in the elderly population, with
mean age of 65–70 years, congenital bicuspid aortic valve
(BAV) with signifi cant aortic stenosis tends to occur slightly
earlier between 15 and 65 years of age (Fig. 15.1d ) [ 5 , 6 ]. In
one study it was observed that BAV was the commonest
cause of AS between the ages of 60 and 75 years (59 % of
cases) and those aged less than 60 years, BAV was a causative factor in 40 % [ 7 ]. BAV AS were found to require AVR
5 years before than those with a tricuspid valve [ 8 ].
Importantly, the natural clinical progression of symptomatic AS is rapid and associated with a poor clinical
C. D. Arepalli , MBBS, DNB • C. Naoum , MBBS, FRACP
Department of Radiology , St Paul’s Hospital UBC , Vancouver , BC ,
Canada
P. Blanke , MD
Department of Medicine , St Paul’s Hospital UBC , Vancouver , BC ,
Canada
J. A. Leipsic , MD, FRCPC, FSCCT (
*)
Department of Radiology , St. Paul’s Hospital ,
Room P2214 – 2nd Floor Providence Building 1081 Burrard
Street , Vancouver , BC V6Z 1Y6 , Canada
e-mail: jleipsic@providencehealth.bc.ca
1 5
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outcome with an average time of death shown to be within
5 years after onset of angina, 3 years after onset of syncope and within 1–2 years after onset of heart failure
symptoms [ 9 ].
Until the advent of TAVI, the sole effective treatment
option for symptomatic AS was surgical AVR. Medical treatment alone had very poor prognosis with a mortality rate of
50 % at 2 years. Unfortunately, many patients with severe
symptomatic AS are denied this surgery owing to associated
co-morbidities in the elderly population that make surgical
risk prohibitive [ 3 , 10 ].
TAVI is a new alternate therapy that has been shown
through many registries, meta-analyses, and a number of
randomized trials to be an effective therapy for severe symptomatic AS who were deemed inoperable and high risk surgical patients [ 11 , 12 ]. TAVI was fi rst pioneered by Cribier
et al. in 2002 through transvenous transeptal approach [ 13 ].
However, over the past decade, alternative procedures have
a
cd
b
Fig. 15.1 Aortic valvular calcifi cation (AVC). AVC of varying
degrees – ( a ) mild, ( b ) moderate, ( c ) severe, ( d ) Bicuspid aortic
valve (BAV) with calcifi cation. Note the asymmetry of the cusps
with a larger non-coronary cusp and raphae between the right and
the left coronary cusps in keeping with a Type 1 A Bicuspid Valve
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been developed and at present retrograde transarterial transfemoral path, fi rst described by Webb and colleagues in
2005, is the most favored approach.
As opposed to surgical AVR where it is possible to directly
visualize and measure the size of the aortic root; TAVI relies
indirectly on imaging measurements before implantation.
Based on these measurements; device selection and the procedural approach are planned. Pre-procedural imaging plays
a critical role in ensuring the success of the procedure and
also to minimize the peri and post-procedural complications.
Imaging Modalities
Historically, TAVI imaging was reliant on measurements
derived form 2-dimensional echocardiography and invasive
angiography. However, multidetector computed tomography
(MDCT) has grown to become an essential tool for the
assessment of aortic root/annulus, evaluation of thoracic and
abdominal aorta and ilio-femoral vessels. Moreover, MDCT
is being used to predict optimal co-planar projection angles
for device deployment.
Current state of the art CT scanners acquire data volumetrically with isotropic spatial and high temporal resolution. Post processing of data provide multiplanar and curved
planar reformations (MPR and CPR), volume rendered technique images (VRT), minimum and maximum intensity projections (MIP
min
and MIP
max
respectively) which are essential
to accurately size the aortic annulus, aortic root and other
vascular access sites. Integration of CT data into sizing algorithms and work fl ow has consistently demonstrated signifi cant reduction in the incidence of paravalvular and vascular
complications [ 14 – 16 ]. Thus CT has become an accepted
integral part in the overall work fl ow of TAVI patients. Of
late, CT has also been shown to play an important additional
role in valve in valve procedures.
The role of MDCT for valvular intervention can be
broadly discussed under three headings:
(a) Pre-procedural,
(b) Peri-procedural and
(c) Post procedural.
Although the focus of this chapter is on pre-procedural
role of CT; it is equally pertinent to understand the peri and
post procedural complications as they play an important role
in understanding the dynamics of TAVI and thus indirectly
infl uence the pre-procedural work up.
Before one can proceed with CT based ‘Sizing of the aortic annulus’ and procedural planning in the work up of TAVI
patients, it is important to understand the complex aortic
valve and root anatomy. Knowledge of the 3 dimensional
(3 D) complex aortic root geometry and consideration of
various anatomical measurements are critical in the determination of patient eligibility, bioprosthesis size, procedural
planning and to anticipate procedural complications.
A n a t o m y
Aortic Root
The Aortic root is a further extension of the left ventricular
outfl ow tract (LVOT). As a result, the LVOT should be considered an anatomical segment between the left ventricle and
ascending aorta.
As the aortic root is a complex dynamic structure; understanding the morphology and kinesis of the aortic root subcomponents helps in better appreciation of interplay of different
sub-components with each other and also with adjacent structures. This not only helps in procedural success but also has the
potential in refi nement of the bio-prosthesis devices.
Aortic root sub-components include aortic annulus (AA),
aortic sinus and the valvular leafl ets, the coronary ostia and
the sinotubular junction.
Aortic Annulus
Broadly, two terms defi ne the aortic annulus – (a) anatomic
annulus and (b) virtual basal ring. The Anatomical Aortic
annulus is more of a histological demarcation that is defi ned
as a fi brous structure that attaches the aortic root to the left
ventricle. It consists of three coronets like structures that
support the valvular leafl ets. The Virtual basal annulus is
defi ned by nadir (lowest depth) plane of each of the coronets
and it corresponds to the aortic-ventricular junction.
Although the term annulus suggests a circular shape,
with the use of 3-D MDCT imaging , it has been found to be
fairly consistently to be of a non-circular geometry with
often elliptical and sometimes oval shaped confi guration
[ 17 , 18 ].
It has been also observed that there are considerable
variations in shape, size and direction of the annulus during the cardiac cycle (Fig. 15.2 ). These are infl uenced by
deformation, stretch, compliance of the aortic root as well
as left ventricular geometry, mitral valve anatomy and
pathology [ 19 , 20 ]. Further, Nakai et al. [ 21 ] showed that
there is cranial displacement of aortic annulus during early
systole and in diastole whereas in rest of the systole and in
isovolumetric relaxation there is caudal displacement. It
has been observed in multiple studies, that the aortic annulus measurements in cohorts without and with aortic stenosis show largest size (area) and diameter in systole and
smallest measurements during diastole due to cyclical
changes [ 22 – 26 ].
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Imaging Modalities and Aortic Annulus
Assessment
Strengths and Limitations
of Echocardiography and MDCT
Accurate and reproducible pre-operative measurements of
annulus are of paramount importance that helps in patient
selection, device selection and size for TAVI procedure and
also forecasting the outcome of the geometrical confi guration of aortic annulus-prosthesis after implantation. In
addition, better understanding of the aortic root complex and
precise measurements would help in predicting and
mitigating peri and post procedural complications.
Owing to the complex 3D aortic root geometry, measurements in a single plane have been shown to lead to both
under or overestimation (depending upon the plane used)
and have therefore been shown to be limited value for TAVI
sizing. Two dimensional (2D) measurements as provided by
transthoracic echocardiography (TTE) and transesophageal
echocardiography (TEE) are routinely used. In fact, the current TAVI device sizes provided by vendors are based on 2D
echocardiography measurements. However, it was noted in
several studies that echocardiography measurements underestimated the true aortic annular size and were smaller in
size when compared to MDCT.
In a meta-analysis, MDCT aortic annulus diameter measurements on coronal view were 25.3 ± 0.52 mm which were
larger than sagittal view measurements by MDCT
(22.7 ± 0.37 mm), TTE (22.6 ± 0.28 mm), and TEE
(23.1 ± 0.32 mm) [ 27 ]. In a study by Mizia-Stec at el, the
mean aortic annulus diameter on TTE was 24 ± 3.6 mm,
26 ± 4.2 mm using TEE, and 26.9 ± 3.2 mm on MDCT
( P = 0.04 vs. TTE) [ 28 ]. Messika-Zeitoun and colleagues
[ 29 ] observed larger differences between CT and TTE
(1.22 ± 1.3 mm) or TEE (1.52 ± 1.1 mm) than the difference
between TTE and TEE (0.6 ± 0.8 mm; and p < 0.0001, respectively p = 0.03).
Early experiences with the integration of CT not only
documented differences in annular measurements between
Fig. 15.2 Dynamic nature of Aortic Annulus over the course of
cardiac cycle (75–25 %) ( a – f ) and corresponding measurements
( g – l ). Note the variation in the shape and size of the aortic annulus
from diastole (65–75 %) through systole (25–35 %). Measurement
of aortic annulus is usually performed in systole. The Geometrical
shape of the aortic annulus changes from elliptical in diastole
(75 %) to near circular in systole (25 %). The perimeter size
increased from 74 to 79 mm (~5 mm difference – ~6 %) variation,
whereas area size increased from 4.24 to 4.84 cm
2
(~0.6 cm 2 differ-
ence – ~variation of 13 %)
abc
def
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MDCT and echocardiography but also differences in potential sizing recommendations. Koos et al. evaluated the
impact of 3D imaging methods (Cardiac magnetic resonance – CMR and dual source CT – DSCT) versus 2D TEE
methods on TAVI selection [ 30 ]. In their cohort of 58
patients, different measurements would have changed TAVI
strategy in 22–24 % patients. DSCT coronal measurements
when compared to 2D TEE would have changed strategy in
16 patients. Further in 14/16 of those patients, the prosthesis size would have been larger and in the rest of the two, it
was too large to implant. In a related similar study of 45
patients comparing 2D TTE and TEE and MSCT measurements, Messika-Zeitoun et al. observed that a decision
based on MSCT would have modifi ed TAVI strategy in
40–42 % [ 29 ].
The limitations of 2-D imaging largely refl ect the complex
almost uniformly non-circular confi guration of the annulus. It
is not that 2-D diameters derived from echocardiography or
MSCT are inaccurate but it is simply a geometrical reality
that the complex morphology of a non-circular structure cannot be accurately characterized with a single 2-D measurement. The growing appreciation of the limitations of 2-D
imaging has driven and increased focus on the utilization of
3D modalities in measurement of aortic root including aortic
annulus and left ventricular outfl ow tract.
Ng et al. evaluated 2D circular, 3D circular and 3D planimetered annular and LVOT areas by TEE and compared
with MSCT planimetered areas [ 23 ]. In their cohort, the
mean annular area by MSCT planimetry was 4.65 ± 0.82 cm 2
whereas by 2D TEE circular (3.89 ± 0.74 cm 2 , P < 0.001), 3D
TEE circular (4.06 ± 0.79 cm 2 , P < 0.001), and 3D TEE planimetered annular areas (4.22 ± 0.77 cm 2 , P < 0.001). They
observed 3D TEE derived planimetered annular areas had
the narrowest limits of agreement and least bias when compared to MDCT. Although the circular geometric assumptions made by 2D and 3D circular measurements were
overcome by 3D TEE; Ng et al. observed 3D TEE when
compared to MDCT still underestimated annular areas by up
to 10 %.
MSCT has also been shown to be highly discriminatory of
those patients that historically experienced paravalvular
regurgitations owing to undersizing secondary to device sizing on the basis of 2-dimensional echocardiography. Willson
and colleagues noted that the annular area in particular on
MDCT was highly discriminatory of PAR [ 31 ]. This was
important knowledge as it has been well established the
ghi
jkl
Fig. 15.2 (continued)
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paravalvular regurgitation is associated with increased morbidity and mortality [ 28 – 30 , 32 , 33 ].
Building upon this retrospective knowledge multiple
groups have shown that the integration of MDCT in device
selection allows for the reduction in the burden of
PAR. Jilaihawi et al, prospective integration of CT guided
algorithm reduced worse than mild paravalvular regurgitation (PAR) from 21.9 to 7.5 % when compared to 2D TEE
[ 24 ]. Building upon this single center data Binder et al. in a
prospective, multicenter controlled trial, evaluated the effects
of the integration of an area based sizing algorithm on the
clinical outcomes post TAVI [ 34 ]. With a non-randomized
trial design half of the subjects (n = 133) underwent TAVI
with the integration of MDCT measurements and sizing
recommendations and the other half underwent TAVI without
the integration of MDCT. They observed more than mild
PAR (primary endpoint) was present in 5.3 % (7 of 133) of
the MDCT group and in 12.8 % (17 of 133) in the control
group (p = 0.032). The combined secondary endpoint (composite of in-hospital death, aortic annulus rupture, and severe
PAR) occurred in 3.8 % (5 of 133) of the MDCT group and
in 11.3 % (15 of 133) of the control group (p = 0.02). Finally
the recently published large multicenter high risk CoreValve
(Medtronic, Minneapolis, MN) trial showed excellent clinical outcomes as compared to surgical aortic valve replacement with the THV selection supported almost exclusively
by MDCT perimeter measurements [ 35 ].
Dynamism of the Aortic Annulus
It is to be noted that aortic annulus is a dynamic structure
with variation during systole and diastole phases of the cardiac cycle. The aortic annulus is often elliptical and assumes
more circular shape during systole (Fig. 15.2 ). Further, it has
been observed that there is increased asymmetrical deformation during diastole which especially affects the right coronary cusp portion of the annulus [ 36 ]. The ellipticity index
(EI) defi ned as ratio of maximum to minimum diameters
substantially decreased from diastole to systole due to signifi cant increase in antero-posterior minor diameter [ 25 ].
Signifi cant changes in area and radius were observed in both
non-diseased annulus and stenotic annulus [ 37 ]. This has
been attributed to annular reshaping than stretch with corresponding increase in systolic area [ 25 ].
Although cross-sectional area (CSA) derived measurements varied during cardiac cycle, it was found perimeter
based diameter measurements show negligible increase in
patients with calcifi ed valves (0.56 ± 0.85 %; p < 0.001) and
very small changes in normal subjects (2.2 ± 2.2 %, p = 0.01)
[ 25 ]. In another study by Aspern et al., no signifi cant differ-
ence was observed with perimeter- derived effective diameters (ED) (mean difference: 0.2 ± 0.4; p = 0.07), however,
area derived ED showed a signifi cant mean difference
(0.4 ± 0.6 mm; p = 0.009), thus reconfi rming infl uence of the
cardiac cycle on aortic annulus area measurements [ 38 ]. It is
likely that the diseased valves/tissues leave little scope for
stretch and therefore perimeter derived aortic annulus measurements are least subjected to variation in cardiac cycle
(Fig. 15.2 ). However, it is being noted smoothing algorithms
used for perimeter derived measurements are inconsistent
across work station platforms and might cause measurement
errors. Although area based ED measurements are smaller
than perimeter derived ED, both approaches showed good
agreement [ 38 ].
Whether perimeter or area is chosen for transcatheter
heart valve (THV) selection it is essential to understand the
impact that the geometrical variable (area or perimeter
derived measurements) may have on the prosthesis size that
would be selected. Blanke et al. showed assuming a perfect
circle with ellipticity index of 1, with increase in nominal
diameter, area increases exponentially whereas perimeter
increases proportionally [ 39 ]. Thus, for e.g. 10 % diameter
oversizing would translate to 10 % increase in perimeter
whereas it would increase area by 21 %. These geometrical
truths are essential to understand and of clinical importance
when certain degree of annular oversizing is contemplated
during TAVI.
In summation, 3D MDCT measurements play a critical
role in annular sizing and THV selection. MDCT measurements are accurate, reproducible and choice of area or perimeter derived measurements need to be tailored depending
upon the choice of prosthesis devices, allowing for dynamism of the aortic annulus and based on the available tools
(software) available at the sites.
MDCT Measurement of the Aortic Annulus
Approach to Aortic Annulus Measurement
We recommend using the phase of the cardiac cycle with best
image quality and in systolic phases (25–45 %) to allow for
consistent assessment of the annulus when it is the largest.
Aortic annulus measurements are performed orthogonally in
relation to the plane of 3 hinge points on a dedicated work
station. Manual multiplanar reformats of the annulus is preferred as it helps in better understanding of the annulus and
the annular plane rather than relying on automated tools
which should ideally only be used by highly experienced
operators. Briefl y, the steps to achieve ‘optimal’ sizing of the
aortic annulus are mentioned below (Fig. 15.3 ), however,
reader should look into expert consensus documents/recommendations for further in-depth information [ 15 , 40 ].
The fi rst step is to lock the orthogonal planes at 90° to
each other. The cross hairs in the coronal and sagittal images
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a
d
ghi
ef
bc
Fig. 15.3 Steps to achieve optimal sizing of aortic annulus. Step 1:
Volumetric data is loaded onto the workstation and in a standard 3D format ( panel 1 ), the views are transverse ( a ), Coronal ( b ) and sagittal ( c ).
The fi rst step is to lock the orthogonal planes at 90° to each other ( a – c ).
Step 2: The cross hairs in the coronal and sagittal images are moved to the
level of aortic valves and adjusted so that the cross hairs on the transverse
plane are positioned at the level of aortic valve ( panel 2 – d – f ). Step 3:
Then the cross hairs on the coronal and sagittal views are rotated and
adjusted so as to correspond to the aortic annulus plane ( panel 3 ). Now the
corresponding views are oblique set of images, for e.g. transverse/coronal/
sagittal oblique ( g – i ). Step 4: The cross hairs on the transverse oblique are
rotated either clockwise or anticlockwise ( panel 3 g ) so as to check
whether the cross hairs correspond to aortic annular plane i.e. nadir point
of the coronary cusps ( panel 4 – j – l ). In this panel views, the cross hairs
cuts across the cusp rather than located at the hinge/nadir point ( panel 4 –
k ). Step 5: The cross hairs are further adjusted by placing the cross hairs at
the aortic annular plane ( panel 5 – m – o ). Once again the cross hair on the
transverse oblique view is rotated and again investigated to see the cross
hairs on the rest of the two oblique planes correspond to aortic annulus
plane ( panel 6 – p – r ). Any adjustments, if necessary, are made by follow-
ing the above steps. Step 6: Once a satisfactory plane is achieved ( panel
7 – s – u ), it is further confi rmed by toggling the transverse oblique set of
images cranially and caudally across the aortic root ( panel 7 – s and panel
8 – v , red arrows ). If a true aortic annular plane has been achieved, then
one would observe that the valve hinge points would disappear or appear
symmetrically all at once ( panels 7 – s – u and 8 – v – x ). Utmost focus is
given so that cross hairs are just touching the nadir points (
panel 7 – s – u ).
If this plane is not achieved, then the coronal ( panel 7 – t ) and sagittal
oblique ( panel 7 – u ) planes needs to be further fi ne-tuned so as to achieve
desired plane and again cross checked by rotating the cross hairs on the
transverse oblique view ( panel 7 – s ). Step 7: Once an optimal view is
achieved then the cross hairs are at the true aortic annular plane ( panel
9 – y , z , A ). Then a ROI trace is made along the circumference of the aortic
annulus to calculate the major and minor diameters, mean diameters, area
and circumference of the aortic annulus ( panel 9 – y ). Corresponding
coronal and sagittal oblique planes are shown ( panel 9 – z , a )
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jkl
mno
pqr
Fig. 15.3 (continued)
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should be moved to the level of aortic valves and adjusted so
that the transverse plane is positioned at the level of aortic
valve. This would correspond to coronal and sagittal oblique
planes respectively. Next the transverse oblique plane is
moved up and down across the aortic root so as to visualize
the valve hinge points. The valve hinge points should disappear or appear symmetrically all at once, if not, coronal and
sagittal oblique planes need to be further fi ne-tuned so as to
achieve desired plane. Again, the cross hairs are rotated
across the transverse oblique so that to visualize the nadir
point of the valves in coronal oblique and sagittal oblique
planes. Utmost focus is given so that cross hairs are just
touching the nadir points and they disappear equally in all
planes. Once this is confi rmed, the specifi c position of aortic
annulus in the transverse oblique plane is identifi ed and a
ROI trace is made along the circumference of the aortic
annulus. Majority of the present day work stations have automatic tools that would calculate diameters (D) – Dmin,
s
v
yz
wx
tu
Fig. 15.3 (continued)
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