Abstract:
Introduction: Atrial fibrillation (AF) ablation requires a
precise reconstruction of the left atrium (LA) and pulmonary veins (PV).
Model-based FAM (m-FAM) is a novel module recently developed for the
CARTO system which applies machine-learning techniques to LA
reconstruction. We aimed to evaluate the feasibility and safety of a
m-FAM guided AF ablation as well as the accuracy of LA reconstruction
using the cardiac computed tomography angiography (CTA) of the same
patient LA as gold standard, in 32 patients referred for AF ablation.
Methods: Consecutive patients undergoing AF ablation. The m-FAM
reconstruction was performed with the ablation catheter (Group 1) or a
Pentaray catheter (Group 2). The reconstruction accuracy was confirmed
prior to the ablation by verification of pre-specified landmarks of the
LA and PVs by intracardiac echocardiogram (ICE) visualization and
fluoroscopy. A cardiac CTA performed before the ablation was used as
gold standard of LA anatomy. For each patient, the m-FAM reconstruction
was compared to his/her cardiac CTA.
Results: The m-FAM reconstruction was accurate in all patients
regardless the catheter used for mapping. In 12% re acquisition of the
LA landmarks was necessary to improve the accuracy. m-FAM time was
shorter in group 2 while the M-Fam fluoroscopy time was similar.
Pulmonary vein isolation was achieved in 100% of patients without major
complications. The m-FAM reconstructions accurately resemble the cardiac
CTA of the same patients.
Conclusions: The m-FAM module allows for rapid and precise
reconstruction of the LA and PV anatomy, which can be safely used to
guide AF ablation.
Introduction :
A precise reconstruction of the left atrium (LA), the pulmonary veins
(PV) and the left atrial appendage (LAA) is of critical importance in
order to achieve efficient and safe results during atrial fibrillation
(AF) ablation. The procedure is currently performed with the guidance of
three-dimensional mapping systems. In the CARTO 3 mapping (CARTO System,
Biosense Webster, Inc, Diamond Bar, CA) the LA reconstruction methods
most frequently used are the Fast Anatomical Mapping (FAM) and cardiac
computed tomography angiography (CTA) integration with the
electroanatomic mapping (CARTO Merge).1-6
The existing FAM algorithm currently used in CARTO constructs a
three-dimensional reconstruction of the atrial anatomy by applying a
standard ball-pivoting algorithm to the point cloud acquired by the
catheter. FAM mapping requires catheter manipulation by a skilled
operator and it is frequently time consuming for less experienced
operators. The result does not consider the underlying anatomy, provides
no information in areas where the catheter has not yet visited, and
contains anatomically incorrect artifacts due to deformation of the
atria by the catheter during the mapping procedure, such as merging of
the appendage and left superior pulmonary vein. Therefore, the raw
anatomy obtained with FAM requires post imaging refinement of the LA
surfaces, achieved by shaving the image with “sculpting tools” prior
to initiation of the ablation and throughout the procedure. In the CARTO
Merge, the CTA scan of the left atrium is integrated with the FAM
reconstruction. CARTO Merge provides a far better definition of the LA
anatomy, including the areas mentioned above, which are critical for a
successful ablation and provides a clear definition of anatomical
variants, such as PV common ostium, separated branches, or additional
PVs. The drawback of CARTO Merge approach is mainly related to the
exposure of the patients to contrast media and (low dose) radiation
during the cardiac CTA, plus the additional costs of the CTA scan, which
are added to the total cost of the procedure.
Model-based FAM (m-FAM) is a novel module recently developed for the
CARTO 3 system (Biosense Webster). 7 8 The scope of
the mFAM algorithm is generating an improved, anatomically correct
reconstruction of left atrial anatomy, while at the same time requiring
fewer samples than are currently needed. This is carried out by defining
a parametric model representing a shape of a portion of a heart, and
constructing a statistical prior of the shape from a dataset of other
instances of the portion. The method is further carried out by fitting
the parametric model to the point clouds data and statistical prior to
produce an isosurface of the portion of the heart of the subject, which
is then display in CARTO. m-FAM applies machine-learning techniques to
LA reconstruction using an adaptive model trained from over 300 LA
anatomies obtained from CTA scans. Training was performed to define
statics of relevant proportions and sizes to represent a “realistic”
and anatomically-accurate left atria. Examples of these features are the
angle between the PVs, distances from atria center to the valve and PV
ostium or the relations of those values to the volume. Having the
geometrical primitives associated with any of the left atria anatomical
structures, as well as keeping the proportions and sizes trained by a
large left atria population, provides a way to represent the most
probable left atria even with very limited information. The algorithm
selects the model that best fits all the available information: points
locations, contact force magnitude and direction, user tagging of
anatomical parts, and a statistical score measuring anatomical
correctness of the model. The reconstruction adapts smoothly to new
points as they are mapped by the catheter, and allows assignment of
special points where the catheter is in contact with the LA surface
(“magnets”), such as ablation points, that constrain the surface while
retaining overall anatomical correctness. The model deforms to best
align with those locations and model reconstruction.
Mathematically, the algorithm can be defined as an optimization problem
in which model parameters that best fit the data and prior statistical
knowledge are estimated in an iterative process. The model is a result
of fitting a parametric shape model to the point cloud acquired by
catheter positions. The model includes a geometrical primitive for each
of the left atria parts, main chamber, pulmonary veins, appendage, and
valve. The geometrical primitives are blended to form the atria
structure. Once the mapping catheter or the ablation catheter introduced
in the LA engage the PVs, their location is tagged on the software and
model fitting begins. A few additional force- and respiration-gated
points are required to further refine the surface and volume of the LA
at key locations, such as the roof, back wall and anterior LA, this
tagged points are called magnet points. After acquisition of enough
magnet points, the software produces a reconstruction of the LA that
includes the LAA ostium and PVs. The model continually self-adjusts with
added ablation tags during the procedure. Importantly, the model is
trained to adjust for non-classical left atrial and pulmonary veins
anatomy as for common ostium or additional side branches. In case of
side branches, these branches are incorporated in the PVs of the closest
major PV. The potential advantages of the m-FAM include: reduction of
the mapping time required to achieve an entire LA reconstruction and
better shaping of the LA anatomy compared to FAM.
In view of the fact that the m-FAM is a totally new feature of the CARTO
3 system, we decided to investigate two different workflows to obtain a
m-FAM reconstruction of the LA using either the ablation catheter (3.5mm
Smart-Touch or 3.5mm ST-SF ablation catheters, Biosense Webster) or a
Pentaray catheter (Biosense Webster) to collect the location tags
required by the software. We opted for a mapping using either the
ablation catheter or a Pentaray catheter because these are the most
commonly catheters used to create the initial LA reconstruction. Of
note, m-FAM reconstruction can also be obtained with a Lasso NAV
catheter (Biosense Webster) if preferred. The accuracy of the m-FAM
reconstruction was confirmed by confirmation of the tagged magnet points
both with fluoroscopy and intra-cardiac echocardiography (ICE) at the
drop-points of the pulmonary veins to precisely define the pulmonary
vein antrum and avoid ablations either too distal or too proximal to the
pulmonary veins as well as the roof, anterior and posterior LA.
Moreover, the m-FAM reconstruction was compared to a cardiac computed
tomography angiography (CTA) of the LA which is routinely performed in
our center prior to ablation of AF. Importantly, for the purpose of this
study, the operator remained blinded to the CTA scan imaging of the
patient during the m-FAM mapping reconstruction and reconstruction
confirmation with ICE and fluoroscopy.
The aim of the present study was to evaluated the feasibility and safety
of the ablation procedure when guided by the m-FAM reconstruction and
the time required to achieve the m-FAM reconstruction and fluoroscopy
time. We also compared the LA reconstruction obtained with m-FAM
obtained with different catheters to the patient’s cardiac CTA.