Key words
TMVI - TMVR - transcatheter mitral valve implantation - transcatheter mitral valve
replacement - computed tomography - CT
Introduction
Mitral valve regurgitation (MR) represents the most common valvular cardiac disease
in the western world. Prevalence is age-dependent and increases with age; above the
age of 75 years, it increases to as much as 10 % in the normal population [1].
The origin can be classified as either primary or secondary MR. Primary MR is due
to degenerative or destructive causes resulting in pathological changes in the valve
annulus and leaflets and ultimately to inadequate valve closure. The most common cause
is mitral valve prolapse [2]. Secondary insufficiency generally occurs in the course of annulus dilation due
to left atrial dilation (atrial mitral insufficiency) or alteration of left ventricular
geometry as consequence of primary or secondary cardiomyopathy [3]
[4].
According to the current European treatment guidelines, early surgical mitral valve
repair or surgical valve replacement is primarily recommended in patients with symptomatic
high-grade primary MR and left ventricular ejection fraction (LVEF) > 30 %. Optimization
of drug therapy is recommended first in patients with severely impaired LVEF (< 30 %).
In the absence of successful therapy, surgical or interventional therapy may be performed
after interdisciplinary decision making in the Heart Team. Interventional or surgical
treatment of secondary MR is the subject of ongoing debate. Thus, in current treatment
recommendations, surgical therapy is recommended only when bypass surgery is performed
simultaneously and LVEF > 30 % (class I, level of evidence C). Surgical mitral valve
therapy alone may be considered if LVEF < 30 % and operative risk is low (class IIa,
level of evidence C). If the operative risk is increased, interventional reconstruction/replacement
may be considered (class IIb, level of evidence C) [5].
Due to the increasing prevalence of MR in older age and the associated rise in surgical
risk, different interventional procedures have been developed as an alternative to
drug or surgical therapy alone. Various approaches are available for interventional
treatment of the mitral valve. A basic distinction can be made between transcatheter
mitral valve repair and transcatheter mitral valve replacement (TMVR). According to
their therapeutic approach, reconstructive interventional procedures can be divided
into ring annuloplasty (e. g. Cardioband), coaptation (e. g. MitraClip, PASCAL), and
neochord (e. g. NeoChord, Harpoon) procedures [6]. All procedures are subject to anatomical and morphological selection criteria.
If these are not met, TMVR is another treatment option.
In addition to treating MR, transcatheter mitral valve replacement (TMVR) can also
be used to treat calcified mitral valve stenosis or as part of a “valve-in-ring” or “valve-in-valve” procedure [7].
Specific measurements of the complex three-dimensional anatomy of the mitral valve
annulus and surrounding structures are necessary for pre-interventional planning of
a TMVR; a CT examination with ECG gating is a suitable method for this due to its
high spatial and adequate temporal resolution.
CT shows higher reliability compared to echocardiography with respect to the measurement
of the annulus and sizing of the prosthetic valve [8]. The key advantage of CT scanning over other non-invasive imaging modalities is
the ability to visualize the entire cardiac anatomy, including the coronary arteries
and the potential access routes as well as the estimation of the postinterventionally
altered left ventricular outflow tract (“neo-LVOT”) in a single examination within a few seconds.
Features of Mitral Valve Anatomy
Features of Mitral Valve Anatomy
The mitral valve consists of two connective tissue leaflets, an anterior (AML) and
posterior (PML) mitral valve leaflet, each of which can be subdivided into three segments
([Fig. 1]). Both valve leaflets merge at the medial and lateral commissure. In addition to
the leaflets, the functional unit of the mitral valve is completed by the annulus
as well as the holding mechanism. The anterior third of the annulus is formed by the
fibrous component, at the end of which there are two connective tissue-compacted areas:
the medial and lateral trigonum. These trigones, together with the AML and through
their direct relationship to the aortic valve and the left ventricular outflow tract
(LVOT), form what is known as the aortomitral continuity. The posterior two-thirds
form the muscular portion of the annulus. The subvalvular portion of the valve is
composed of the chordae tendineae and papillary muscles that connect the mitral valve
leaflets to the left ventricular myocardium [9]
[10]
[11].
Fig. 1 Segments of the mitral valve. Short-axis view through the mitral valve (in systole)
(a). The three anterior (A1–A3) and posterior (P1–P3) valvular segments can be identified
in the CT as shown above. Additionally, the anatomic structure of the mitral valve
is schematically depicted in b (modified from Capoulade et al.
[37]). (AML = anterior mitral leaflet, PML = posterior mitral leaflet, LCC = left coronary
cusp of the aortic valve, NCC = non-coronary cusp of the aortic valve).
The complexity of the mitral valve anatomy is based on the saddle-shaped three-dimensional
dynamic form of the annulus ([Fig. 2a, b]). This saddle-shaped annulus has its highest point in the middle of the anterior
third and extends to the level of the aortic valve, more specifically to the insertion
of the left coronary and the acoronary cusp. Another, smaller elevation is found at
the insertion of the posterior leaflet. The lowest points of the annulus are formed
by the trigones [12]
[13].
Fig. 2 Mitral valve annulus in saddle-shape (a, b) and in D-shape (c, d). The anterior part of the mitral valve annulus is included in the saddle-shape (blue
contour in a, b). Therefore, AP (black lines in b, d) is larger in saddle-shape than in D-shape configuration (blue contour in c, d). In the D-shaped annulus, the anterior part (orange contour in c, d) is not included. The white stars in b, d mark the trigones. (AP = anterior-posterior diameter).
Available Devices
Challenges to secure placement and anchoring of the prosthesis during TMVR include
the complex anatomy of the mitral valve, annulus dynamics and aortomitral continuity.
For example, the annulus increases in size by up to 20 % during systole [14]. In addition, implantation of the prosthesis carries the risk of obstruction because
of the close spatial relationship of the mitral valve annulus to the LVOT.
The various conceivable solutions for taking these anatomical and physiological conditions
into account result in a variety of devices which differ, for example, by the anchoring
mechanism and in their position relative to the annulus ([Table 1, ]
[Fig. 3]).
Table 1
Overview of the most common devices for TMVR
|
Device
(manufacturer)
|
Rate of successful implantations (%)
|
LVOT-obstruction
(%)
|
Access route
|
Anchoring mechanisms
|
Relevant
sizing-parameters
|
|
Tendyne
(Abbott)
|
93.3
|
0
|
transapical
|
“apical tether”
|
AP; LM
|
|
Tiara
(Neovasc)
|
84.2
|
0
|
transapical
|
Capture of trigona
|
Perimeter;
LM
|
|
Intrepid
(Medtronic)
|
96
|
0
|
transapical
|
Radial force
|
Perimeter; AP; LM
|
|
Sapien III
(Edwards)
|
76.7
|
N/A
|
transseptal-transfemoral
|
Radial force
|
Area, Perimeter, max. Diameter
|
Overview of commonly used valve prostheses for TMVR, their complications rates and
important parameters regarding implantation/planning (modified from Ranganath et al.
[10] (TVMR = transcatheter mitral valve replacement, LVOT = left ventricular outflow
tract, AP = Anterior-posterior diameter, LM = Lateromedial diameter, N/A = not available).
Fig. 3 Peri-interventional fluoroscopic representation of various TMVR devices. a, b The TIARA device is configured in the shape of a crown. The anchoring takes place
by capturing the trigones using the myocardial shelf. c, d The Tendyne device is configured conically. It is anchored via an “apical tether”.
e, f Sapien 3 in mitral position (here as valve-in-valve implantation) in a previously
implanted biological mitral valve prosthesis. In contrast to the other devices, the
Sapien 3 is expanded via balloon (f). (TMVR = transcatheter mitral valve replacement).
At this time, there are more than 30 devices available, some of which are summarized
in [Table 1]
[10]
[15]
[16]
[17]
[18]. Currently the most common one is the Tendyne device (Abbott Cardiovascular, Plymouth,
USA), which is the only apparatus with CE certification for use in routine clinical
practice. In addition, off-label use with a prosthesis for transcatheter aortic valve
implantation, e. g., the Sapien 3 (Edwards Lifesciences, Inc., Irvine, CA, USA), is
also possible [19]. Other devices are currently undergoing clinical trials.
The chosen access route should be as perpendicular as possible to the annulus in order
to achieve optimal deployment and anchorage of the device, thus reducing the risk
of LVOT obstruction or paravalvular leak. On the one hand, a transapical approach
can be selected, since the imaginary central axis between annulus and apex of the
left ventricle (LV) can usually be projected well and determined by CT ([Fig. 4]) [20]
[21]
[22]
[23]. On the other hand, the less invasive transfemoral-transseptal access is also an
option for prosthesis implantation, which does not require opening of the thorax by
mini-thoracotomy as in the transapical approach, thus allowing a purely endovascular
procedure [24]. In the context of transseptal puncture, CT can provide helpful additional information
for visualizing the best possible procedure regarding access to the left atrium and
mitral valve [10].
Fig. 4 Planning for transapical approach. a The annulus-apex distance (red line) is determined in a two-chamber view and corresponds
to the distance from the middle of the mitral valve plane (blue line) to the LV apex.
b If the annulus-apex-line is elongated until it intersects the thoracic wall (blue
arrow), the optimal trajectory for a transapical approach can be planned. The use
of a scan of the complete thorax is recommend for identification and counting of the
correct intercostal space (ICR). In the depicted case, the optimal transapical access
route is located in the 5. ICR. c This can be visualized as 3D-reconstruction.
In a 2017 meta-analysis by del Val et al., the proportion of successful implantations in the reviewed studies was approximately
92 % of the total procedures performed. LVOT obstruction was observed in only 0.4 %
of patients [25].
CT Scanning Protocol
A CT scanner with the highest possible temporal resolution is advantageous due to
the need to visualize heart structures as artifact-free as possible throughout the
cardiac cycle.
With only a few exceptions, a helical scan of the heart with retrospective ECG gating
should be employed in order to allow measurement in multiple cardiac phases and to
create dynamic reconstructions of mitral valve motion as a moving image series over
the cardiac cycle. Supplemental cardiac calcium scoring should be considered to plan
the cardiac scan window and positively identify mitral valve calcifications [26].
Additionally, a depiction of the anticipated access route is useful. If a transapical
approach is anticipated, supplementary imaging or reconstruction of the thorax may
be helpful.
Optimally, imaging of cardiac anatomy should be performed as a helical scan of the
heart using retrospective ECG gating immediatly followed by an untriggered scan utilizing
the same contrast bolus for imaging of the access pathway. In order to achieve sufficient
contrast, it is recommended to adapt the amount of contrast medium to the scan time
and, if necessary, the patient’s cardiac output. The typically flattened contrast
kinetics in patients with high-grade MR can be counteracted with an appropriately
high flow rate (> 5 ml/s).
Cardiac scan reconstruction should be performed in 5 % increments with respect to
the RR interval and the use of the smallest possible slice thickness and increment
is recommended.
To shorten the switching time, it may be useful to vary the scan direction so that
the helical scan of the heart is performed in caudocranial scan direction and remaining
volume is scanned in the opposite direction.
Using a scan mode with retrospective ECG gating provides a comparatively high probability
for possibly diagnostic assessment of the coronary arteries since correction of arrhythmias
can be achieved via manual editing of the ECG-based reconstruction parameters [27]. Furthermore, even with higher heart rates or arrhythmia, diagnostic coronary imaging
is more likely to be possible in this scan mode compared to using prospective ECG
triggering [28]. Therefore, evaluation of a CT scan for planning TMVR should include assessment
of the coronary arteries with respect to anomalies and possible stenosis.
Saddle vs. D-Shape
Accurate measurement of the mitral valve annulus is the focus of preinterventional
CT planning. The anatomically saddle-shaped form of the annulus makes it difficult
to establish a two-dimensional plane that can be easily measured. Therefore, it is
advantageous to simplify the complex 3-dimensional shape of the annulus to a 2-dimensional
contour by projecting suitable landmarks. This projection takes the simplified form
of a virtual “D” ([Fig. 2c, d]). With this “D-shaped” configuration the trigona fibrosa are connected by a virtual line and the anterior
horn, which is not contained in the two-dimensional plane, is neglected [11]
[12]. Looking at the shape of the most commonly used devices, such as Tiara or Tendyne,
the cross-section at the level of the landing zone also corresponds most closely to
a “D-shaped” configuration. Likewise, use of the D-shape is advantageous with respect to the correct
evaluation of a possible LVOT obstruction. Neglecting the anterior horn reduces the
probability of an oversized prosthesis, while assuming an approximately tubular deployment
of the prosthesis allows for a more realistic representation of the actual position
of the implanted device, especially with regard to the positional relationship to
the LVOT or the assumed configuration of the “neo-LVOT” after implantation [29].
Step-by-Step Instructions
Step-by-Step Instructions
To set the correct annulus plane, it is useful to have dedicated postprocessing software
that allows the preparation of multiplanar reconstructions (MPR) in several planes.
However, this can also be done with most radiological image storage and communication
systems (PACS). For this purpose, it is recommended to link the related planes and
visualize their alignment using a crosshair.
[Fig. 5] illustrates an image-by-image representation of the previously described steps.
Fig. 5 Step-by-step instructions for setting the mitral valve annulus using CT. All available
planes should be considered simultaneously for each setting step, employing postprocessing
software with the possibility of creating multiplanar reconstructions.
Step 1 (a–c): Rough approach to the mitral valve plane. The crosshairs are centered on the middle
of the mitral valve plane (a). Subsequently, the second and third planes (b, c) are aligned along the AV valve plane. A rough orientation to the LV apex is helpful
for alignment in basal-apical direction.
Step 2 (d–f): Setting the left trigonum. In the short-axis view, the location of the left trigonum
is identified and put in the crosshairs are (white arrow in d, e, f). When set correctly, it is shown as a triangularly configured, clearly defined structure.
Step 3 (g–i): Setting the right trigonum. By rotating the second plane (i), the right trigonum fibrosum (white arrow in g, h, i) is also adjusted in short axis view (g) until it can be seen as a clearly delineated triangular structure. Once this is
achieved, TT and thus the septal contour of the mitral valve annulus are correctly
set.
Step 4 (j–l): Setting the lateral annulus contour. In the short axis (j), the crosshairs are set to the TT. The lateral annulus contour (purple arrows in
j) is now adjusted in the second plane (k, l) until a D-shape is seen in short axis view (j) and the lateral contour is completely occupied by the mitral valve annulus. (TT = inter-trigonal
distance, AV valve plane = atrioventricular valve plane).
This created sectional plane now allows the determination of the relevant measurement
values for sizing of the valve prosthesis ([Fig. 6]), which should be performed in an analogous manner in systole and diastole.
Fig. 6 Mitral valve annulus in D-Shape: The central measured values are TT, LM and AP (TT = inter-trigonal
distance, LM = lateral-medial diameter, AP = anterior-posterior diameter).
The most common measured values to be determined are shown in [Fig. 6] and summarized in List 1 [29].
Risk Factors of LVOT Obstruction
Risk Factors of LVOT Obstruction
Proper selection of prosthesis type and size is important, since LVOT obstruction
is a potential and relevant complication of TMVR.
The native LVOT is located between the basal interventricular septum and the aortomitral
continuity ([Fig. 7a]). Due to protrusion of the prosthesis into the left ventricle, the resulting elongation
of the native LVOT and interaction with the AML result in the formation of a so-called
“neo-LVOT” ([Fig. 7b, c]).
Fig. 7 Left ventricular outflow tract bevor and after TMVR and aortomitral angulation. CT-plains
in 3-chamber (a, c, d) and short axis view (a, d) before (a, d) and after TMVR (b, c). a Native LVOT (blue hatched area) and well visible AML and PML. b, c The native LVOT (blue hatched area) remains unchanged in the distal part even after
TMVR. However, after TMVR the outflow tract anatomy changes and the lateral contour
of the neo-LVOT (red bordered area in b and red hatched area in c) is deformed by the valve prothesis and significantly narrowed compared to the native
LVOT. d The aortomitral angulation results from the intersection of the orthogonals to the
center of the aortic valve plane (green arrow) and the mitral valve plane (blue arrow).
In this example, the measured angle is 49°.
The risk of obstruction grows with increasing size and protrusion of the device. According
to the current state of knowledge, limit values for the area of the “neo-LVOT” are 1.7–1.9 cm2 and are device-specific [30].
Patient-specific characteristics included aortomitral angulation ([Fig. 7 d]), left ventricular dimension and basal septal thickness ([Table 1]).
Aortomitral angulation influences the subsequent position and alignment of the prosthesis
and describes the angle between the respective orthogonals to the mitral and aortic
valve planes ([Fig. 7 d]). Theoretically, an approximately parallel alignment of both axes would be optimal,
whereas the risk of obstruction increases significantly if this approaches or exceeds
90° [29]
[31].
Hypertrophy of the basal interventricular septum (> 15 mm) may also lead to narrowing
of the LVOT (or “neo-LVOT”). In addition, a small left ventricular cavity is another risk factor in this regard.
In patients with secondary MR, the LV is usually clearly dilated, whereas in patients
with primary MR the mostly hyperdynamic LV function leads to a systolic reduction
in the LV cavity [12].
Landing Zone
CT also provides information on the so-called landing zone of the device, which is
influenced by numerous factors such as the presence/distribution of annular calcification
or the development of a myocardial shelf.
Myocardial shelf
The anatomical difference between the landing zone in primary and secondary MR is
important in pre-interventional planning. Secondary MR is often associated with regional
wall movement disorders and dilation of the LV, which leads to displacement of the
papillary muscles as well as ultimately to annulus dilation. Furthermore, remodeling
of the basal myocardium also disrupts the actual linear connection between the left
atrial and left ventricular myocardium, resulting in the so-called “myocardial shelf”. The size of the “myocardial shelf” depends on several factors; typically, it is largest in the presence of LV dilation
combined with posterior wall infarction. Importantly, the “myocardial shelf” is also subject to dynamic size change and may even disappear altogether during systole.
Determining the exact extent of the myocardial shelf is important because some self-expanding
systems require a persistent shelf for anchorage. In patients with primary MR, there
is typically no “myocardial shelf” present [29]
[32]
[33].
Mitral valve calcification
Calcification of the mitral valve annulus, trigona fibrosa or valve leaflets may also
influence intervention planning and can be well visualized by CT ([Fig. 8]). Generally, calcifications are found in the posterior portion of the annulus fibrosus
and are particularly present in elderly patients [34]
[35].
Fig. 8 Extensive calcifications of the mitral valve annulus. The short-axis view and the
three-chamber view show pronounced calcifications of the mitral valve annulus (blue
arrows). These are most prominent along the segments P2 and P3. In this case, the
specific configuration of the annular calcification are consistent with “caseating
calcification” (blue arrow in b).
Computed tomographic imaging in short-axis view ([Fig. 8a]) is particularly suitable for describing calcifications. According to Guerrero et al.
[36] an association was found between minor calcifications of the mitral valve and an
increased likelihood of migration of the TMVR device; a score was used for quantification
and standardized recording of the calcifications. This score refers to the thickness
of annular calcifications, their extension in relation to the circumference, the involvement
of the trigona and the valve leaflets [36].
Annulus-Apex Connection
The orientation and length of the annulus-apex line represent ([Fig. 4a]) additional relevant information for TMVR planning. To achieve this, an orthogonal
connecting line is drawn from the center of the mitral valve plane to the epicardial
contour of the LV apex ([Fig. 4a]) [31]. This facilitates the planning of a transapical access route, since the necessary
length and orientation of the delivery system can be assessed here ([Fig. 4]).
The location of the LV apex with respect to the center of the mitral valve plane may
vary from patient to patient [31]. Visualization of the actual anatomy can provide information on whether deviating
from a strictly transapical access path will allow better alignment of the delivery
system.
In extension of the annulus apex line, the corresponding intercostal space should
be indicated where it intersects the thoracic wall ([Fig. 4b]). This may be helpful in planning the exact transapical access route. A 3 D reconstruction
is particularly suitable for illustration ([Fig. 4c]).
Report and Documentation
Due to the complexity of the evaluation, the use of a standardized report template
and uniform image documentation is recommended to increase clarity and comprehensiveness
of the report.
On the one hand, MPRs in short-axis view and in 2- and 3-chamber view should be prepared
for general overview. On the other hand, moving image series over the cardiac cycle
along the mitral valve annulus and in 2-chamber and 3-chamber view should also be
obtained to visualize the dynamic and anatomical changes within the cardiac cycle.
It is recommended that measurement of the mitral valve annulus should be performed
using the standardized mitral valve plane setting in “D-Shape”, as described previously.. An overview of relevant measured values can be found in
List 1; these should also be documented as image findings.
List 1: Relevant parameters for standardized reporting of TMVR-planning-CT
Mitral annulus (diastolic and systolic measurements)
-
TT, AP, LM
-
perimeter and area
-
leaflet morphology
Additional parameters
LVOT (diastolic and systolic measurements)
Landing zone
Access route
-
transapical: annulus-apex distance, ICS for optimal trajectory
-
transfemoral-transseptal: course of Vv. femorales, Vv. Iliacales and V. cava inferior
Other aspects
Coronary arteries
Thorax/abdomen
(TMVR = transcatheter mitral valve replacement, TT = inter-trigonal distance, AP = anterior-posterior
diameter, LM = lateral-medial diameter, ICS = intercostal space, LV = left ventricle,
LVOT = left ventricular outflow tract, CAD = coronary artery disease).
Summary
CT represents the central imaging modality for TMVR planning. With this imaging modality,
both a visualization of the anatomical conditions and an individual and patient-centered
interventional planning and prosthesis selection are reliably possible.
The mitral valve annulus should be set in a D-shape configuration and using definite
anatomical landmarks for reproducibility of two-dimensional sizing. Calcifications
of the annulus can be estimated very well using computed tomography and can be easily
described with regard to location and distribution.
Furthermore, CT allows detailed visualization of the size and configuration of the
LVOT and thus allows estimation of the likelihood of postinterventional LVOT obstruction.
Appropriate visualization capabilities are available for both a transapical and a
transfemoral-transseptal approach, allowing planning of the optimal interventional
procedure.
The collected measured values and qualitative statements on intervention planning
should be recorded in a standardized report.
CT Planning prior to Transcatheter Mitral Valve Replacement (TMVR)
Heiser L, Gohmann RF, Noack T et al. CT Planning prior to Transcatheter Mitral Valve
Replacement (TMVR).
Fortschr Röntgenstr 2022; 194: 373–383
Correction on page 374, 375. The correct sentence is: This saddle-shaped annulus has
its highest point in the middle of the anterior third and extends to the level of
the aortic valve, more specifically to the insertion of the left coronary and the
acoronary cusp.
