Field of the invention
[0001] The present invention relates to the field of medical X-ray systems and more
particularly to radiology systems used for endovascular surgery of aneurysms of the
abdominal aorta.
Prior art
[0002]Aneurysms of the abdominal aorta can be treated by conventional open
surgery or by endovascular surgery. During an endovascular intervention, the
surgeon does not have direct access to the operating site. On the contrary, in an
endovascular operation, he makes an incision in the femoral artery at the groin
region and inserts a very flexible metal wire. By pushing this wire, he can then move
his tools, including the endoprosthesis, to the region of the aneurysm in the
abdominal aorta. When he considers that the endoprosthesis is situated at the
correct location, he can decide to release the endoprosthesis, which deploys
automatically.
[0003] The correct positioning of the endoprosthesis is therefore crucial. Indeed, this
prosthesis must be placed correctly and must not occlude arteries branching off from
the main artery. In the most complex cases, so-called fenestrated endoprostheses
comprise openings that have to be positioned precisely facing the ostia of the
secondary arteries. For example, these openings have to be positioned facing the
junctions between the aorta and the renal arteries, which issue from the latter. It is
therefore essential to position the endoprosthesis correctly, in order to avoid
obstructing these junctions and occluding the renal arteries.
[0004] In order to guide these manoeuvres, the surgeon uses a mobile interventional
radiology system in order to precisely position the endoprosthesis, which will redirect
the blood flow. These systems, also referred to as a mobile C-arm (or block amplifier),
allow the surgeon to acquire X-ray images during the intervention and to monitor the
2
position of the tools (catheter, prosthesis, etc.) in real time and in a minimally
invasive manner. The majority of these systems make it possible to obtain twodimensional images with a video image stream of up to thirty images per second.
The practitioner then uses these images to mentally reconstruct the tool and the
geometry of the artery, in order to validate its positioning in real time. This X-ray
imaging requires injection of contrast agent. This contrast agent is indispensable for
rendering the aorta visible to X-rays, but it has the disadvantage of being toxic. The
possible number of injections and of image captures is thus limited.
[0005] The step of fitting an endoprosthesis in place is very complex in some cases
and requires multiplying the image captures and consequently the irradiation of the
patient and the volume of contrast agent injected. The probability of post-operative
complications is also higher. In the short term, this is due to loss of blood and to the
poor irrigation of the zones obstructed by the surgical tools. In the medium and long
terms, imprecise positioning of the endoprosthesis leads to risks of leakage and
thrombosis. The deployment of endoprostheses in the aorta takes place in 3D. A
single 2D image may therefore prove inadequate for allowing the surgeon to clearly
assess the deployment of the endoprostheses. The possibility of having access to a
three-dimensional visualization of the operating site would therefore be a great
advantage to the surgeon, allowing him to position his tools quickly and precisely,
while reducing the number of angiographs.
[0006] There are currently several methods that permit this 3D visualization. However,
these methods remain based on taking multiple 2D images from different viewing
angles for the purpose of a 3D reconstruction. They therefore involve multiple
injections of contrast agent, and the operating time is also increased because of the
multiple images that have to be taken at different angles.
[0007] There is therefore a need for a tool that aids the surgical procedure, allowing
the surgeon to visualize his tools and their configuration in the aorta of the patient,
almost in real time and in 3D.
Summary of the invention
[0008] To this end, the invention relates to a method comprising: capture of a 2D
image of a vascular structure by X-ray; acquisition of a 3D model of the vascular
3
structure; acquisition of a model of an endoprosthesis in the vascular structure,
comprising a plurality of stents; determination from the 2D image, and for each stent,
of at least one position, at least one orientation, and at least one deployment value;
simulation of the deployment in the 3D model of the vascular structure, for each stent,
of a model of the stent representing the structure of the stent, said model of the stent
being initialized on the basis of the model of the endoprosthesis; display of the
deployed stent models.
[0009]Advantageously, the 2D image defines a 3D reference frame comprising a
vertical axis and a horizontal axis and a depth axis of the image; in the
endoprosthesis model, each stent is defined by: at least one position of at least one
characteristic point and an orientation, which are defined by at least 6 degrees of
freedom, including: the 3D position of a first characteristic point of the stent in the
reference frame; an angle of intrinsic rotation; the at least one deployment value of
the stent, defined about its central axis; the model of the stent is formed by a plurality
of interconnected beam elements; said method comprising: determination from the
2D image, and for each stent, of at least one position, at least one orientation, and at
least one deployment value; of the at least one characteristic point along the vertical
axis and horizontal axis and the at least one deployment value of said stent;
determination, for at least one stent, of its angle of intrinsic rotation.
[0010]Advantageously, the position and the orientation of each stent are defined by
the 3D positions of three characteristic points corresponding respectively to the
centre, the upper end and the lower end of the stent along its central axis, and the
intrinsic rotation of the stent about its central axis.
[0011]Advantageously, the determination, for at least one stent, of its angle of
intrinsic rotation comprises the determination of the angle of intrinsic rotation for
which at least one 3D position projection of at least one radiopaque marker on the
stent most closely matches to at least one image of the at least one marker on the
2D image.
[0012]Advantageously, the angle of intrinsic rotation is obtained by the execution of a
loop minimizing the distance between the at least one projection and the at least one
image of the at least one marker on the 2D image, as a function of the angle of
intrinsic rotation.
4
[0013]Advantageously, the determination, for at least one stent, of its angle of
intrinsic rotation comprises modelling the stent in the form of a beam element of a
finite element model, between a characteristic point representing the upper end of
the central axis of the stent and a characteristic point representing the lower end of
the central axis of the stent, in which the characteristic points are free of
displacements along the depth axis.
[0014]Advantageously, the endoprosthesis is represented in the form of a finite
element model in which: each stent is modelled by a beam element, the successive
stents being connected to each other by at least one beam element; the positions of
the characteristic points of the stents whose maximum diameter of deployment is
greater than or equal to the diameter of the vascular structure at the deployment
position are fixed along the vertical axis, horizontal axis and depth axis; the positions
of the characteristic points of the stents whose maximum diameter of deployment is
less than the diameter of the vascular structure at the deployment position are fixed
along the vertical axis and horizontal axis and are free in displacement along the
depth axis; the positions of the characteristic points of the stents whose maximum
diameter of deployment is less than the diameter of the vascular structure at the
deployment position along the depth axis are determined by the mechanical
equilibrium of the finite element model.
[0015] Advantageously, the display of the deployed stent models comprises the
display in superposition of the projection of the stent models on the 2D image of the
vascular structure.
[0016]Advantageously, the display of the deployed stent models comprises the 3D
display of the deployed stent models and of the 3D model of the vascular structure.
[0017]Advantageously, the 3D model of the vascular structure is a finite element
model representing the central line of the vascular structure by beam elements.
[0018] The invention also describes a computer program product comprising
computer code elements configured for executing a method according to one of the
embodiments of the invention.
[0019] The invention also describes a device comprising: at least one input port
configured to receive a 2D image of the vascular structure captured by X-ray; at least
5
one calculation unit configured to execute a method according to one of the
embodiments of the invention.
[0020] The method as a whole permits a both reliable and rapid calculation of the
deployment of an endoprosthesis in the aorta. For example, the method can be
carried out in about thirty seconds on standard calculation means, which allows the
deployment of the endoprosthesis to be modelled almost in real time.
[0021] The method of the invention provides the surgeon with real-time visualization
of the deployment of the endoprosthesis on the basis of its actual position. This
therefore makes it possible to considerably improve the precision of the treatment
performed by the surgeon.
[0022] The method of the invention requires a single image capture in order to
simulate the deployment of an endoprosthesis, which avoids the use of multiple
injections of contrast agent.
[0023] The method is applicable to prostheses that are deployed and also to
prostheses that are not deployed or that are partially deployed.
[0024] Other features, details and advantages of the invention will become clear from
reading the description provided with reference to the attached drawings which are
given as examples and in which:
[0025] [Fig.1] shows a medical imaging system in which the invention can be
implemented;
[0026] [Fig.2a] shows a perioperative 2D image showing an endoprosthesis in the
aorta, with injection of contrast agent;
[0027] [Fig.2b] shows a perioperative 2D image showing an endoprosthesis in the
aorta, without injection of contrast agent;
[0028] [Fig.2c] shows an endoprosthesis deployed in a mock-up aorta, according to a
set of implementations of the invention;
[0029] [Fig.3] shows a method of simulating the deployment of an endoprosthesis,
according to a set of implementations of the invention;
6
[0030] [Fig.4] shows a modelling of a stent within an endoprosthesis model,
according to a set of implementations of the invention;
[0031] [Fig.5] shows the deployment of a stent within the context of a model of the
endoprosthesis;
[0032] [Fig.6] shows the determination, on the basis of the 2D image, of the vertical
and horizontal positions of the characteristic points of the stents, and also the
deployment thereof;
[0033] [Fig.7a] shows a first example of modelling the deployment of a stent,
according to a set of implementations of the invention;
[0034] [Fig.7b] shows a second example of modelling the deployment of a stent,
according to a set of implementations of the invention;
[0035] [Fig.8] shows a superposition of the simulation of deployment of a stent on a
perioperative image, according to a set of implementations of the invention.
[0036] Figure 1 shows a medical imaging system in which the invention can be
implemented.
[0037] The system 100 is a mobile interventional radiology system.
[0038] It comprises an element called a C-arm 110, allowing X-ray images to be
taken of the body of a patient. The C-arm is able to turn about different axes in order
to capture images of a stationary patient at different angles. The C-arm comprises a
control interface allowing the medical team to control the orientation and the image
capture.
[0039] The C-arm 110 is connected to a calculation device, for example a computer
120, able to generate the display of the X-ray image, taken by the C-arm 110, on
display means, for example screens 121. For this purpose, the calculation device 120
comprises an inlet port for receiving the perioperative images. The calculation device
120 also comprises at least one calculation unit (for example a processor) for
processing the images taken by the C-arm. For example, the processor can perform
a pre-analysis of the image and can superpose, on the image, items of information
7
that may be of interest to the medical team (for example the time when the image
was captured, the temperature of the patient, his or her blood pressure, etc.).
[0040] The separation of the system into two devices, respectively the C-arm 110 for
taking the images, and the calculation device 120 for processing and displaying them,
permits great flexibility in the use of the C-arm 110, which can remain mobile even if
the position of the display screens 121 remains fixed.
[0041] In a set of embodiments of the invention, this device 120 also makes it
possible to generate a visualization of the deployment of an endoprosthesis and to
generate its display in 2D or 3D, as will be explained below.
[0042] The system 100 is given solely as an example of a system in which the
invention could be implemented. The invention could be implemented in numerous
other medical systems. For example, according to different image capture modes,
the processing and the display of the images can be performed in a single device. It
is also possible to capture an image at one location and send the image for
processing and display thereof to another site, for example on a remote server.
[0043] Figures 2a, 2b and 2c show respectively a perioperative 2D image showing an
endoprosthesis in the aorta, with and without injection of contrast agent, and an
endoprosthesis deployed in a mock-up aorta according to a set of implementations of
the invention.
[0044] Figure 2a shows a perioperative image of the aorta taken by X-ray, for
example by a device such as the device 100, and taken with contrast agent. This
image permits visualization of the boundaries of the aorta. In particular, it makes it
possible to know the diameter of the aorta at each point and to visualize from where
the adjacent veins branch off.
[0045] Figure 2b shows a perioperative image 200b of the aorta taken by X-ray, for
example by a device such as the device 100, taken without contrast agent and before
deployment of the stents. Without contrast agent, the boundaries of the aorta are no
longer discernible, but the stents 211b, 212b, 213b, 214b, 215b, 216b, 217b and
218b, 219b, 220b of the endoprosthesis are apparent. The stents 211b to 218b are
not deployed, the stent 219b is partially deployed, and the stent 220b is completely
deployed. The radiopaque markers, such as the markers 230b and 231b, are also
8
apparent. Perioperative imaging also makes it possible to locate certain critical
characteristic points which may, for example, be visualized by a marker such as the
markers 230b and 231b. These markers can be positioned at particularly important
points on the endoprosthesis in order to allow the surgeon to locate them on the
perioperative image.
[0046] When introducing the endoprosthesis, the surgeon can capture a first image
with injection of contrast agent in order to visualize the boundaries of the aorta, then
one or more images without contrast agent, in order to visualize the position of the
stents and the radiopaque markers. These two views can be displayed in
superposition in order to visualize the position of the stents within the aorta.
[0047]At this stage, the endoprosthesis may be non-deployed, partially deployed or
fully deployed. However, an image such as the one in Figure 2b allows the positions
of the stents to be determined, as will be explained below.
[0048] The perioperative 2D images of this type thus allow the surgeon to have a
general view of the position and deployment of endoprostheses in an artery.
However, a 2D view of this kind may often prove inadequate for assessing a
deployment which, by its very nature, takes place in 3D.
[0049] Figure 2c shows an image of an endoprosthesis deployed in a mock-up. This
image is similar to what a surgeon sees by superposition of a perioperative image
with contrast agent and of a perioperative image without contrast agent, that is to say
visualizing both the endoprosthesis and the space in which the latter deploys. In the
example of Figure 2c, the endoprosthesis is completely deployed. This image shows
an endoprosthesis deployed in a mock-up of the aorta 220. The endoprosthesis is
formed by a plurality of stents 211, 212, 213, 214, 215, 216, 217, 218. The
perioperative image thus permits visualization, in 2D, of the position and deployment
of the stents within an artery.
[0050]As will be explained below, the invention makes it possible, on the basis of a
2D image, to obtain a 3D simulation and visualization of the deployment of an
endoprosthesis. In the remainder of the description, the 3D deployment will be
expressed according to a reference frame Rim, based on three axes x, y and z, which
correspond respectively to a vertical axis, a horizontal axis and a depth axis. The
plane formed by the vertical axis x and horizontal axis y is called the plane of
9
projection or the plane of the 2D image. However, this reference frame is given solely
by way of example, and it will be possible for a person skilled in the art, on reading
the present disclosure, to choose any 3D reference frame meeting his needs.
[0051] Figure 3 shows a method of simulating the deployment of an endoprosthesis
according to a set of implementations of the invention.
[0052] The aim of the method 300 is to simulate the deployment of an endoprosthesis
and to allow a surgeon to visualize the deployment of an endoprosthesis in an artery,
if appropriate in 3D.
[0053] The method 300 comprises a step 310 of capturing a 2D image 311 of a
vascular structure by X-ray. This 2D image is a perioperative image and can be taken,
for example, by a device of the C-arm type, such as the device 100. The images in
Figures 2a and 2b are two examples of such an image. As has been explained above,
the capture of such an image by X-ray can comprise an injection of contrast agent
into the vascular structure. It should be noted that, in the remainder of the description,
the method will be illustrated by examples relating to the simulation of an
endoprosthesis in the aorta. However, these examples are given as non-limiting
examples, and the method may be applied to the simulation of endoprosthesis
deployment in any vascular structure. There is no fundamental difference between
the deployment of an endoprosthesis in the aorta and in another vascular structure,
and therefore the method may be applied directly to other vascular structures. The
method is also applicable to the simulation of deployment of the endoprosthesis in
surgical mock-ups.
[0054] The capture of the 2D image defines a 3D reference frame which will be used
in the rest of the method. This 3D reference frame comprises a vertical axis, a
horizontal axis and a depth axis of the image capture. In the example in Figure 2,
these axes are labelled x, y and z, respectively. However, this nomenclature is given
solely by way of example, and it will be a simple matter for a person skilled in the art
to provide the axes with the names and orientations that meet his needs.
[0055] The method 300 also comprises the acquisition of a 3D model 321 of the aorta.
This 3D model 321 makes it possible to define the contours and the volume of the
aorta in 3D. The aorta can be defined, for example, by a set of 3D polygons. The
10
boundaries of the aorta can thus be expressed in the reference frame defined by the
2D imaging.
[0056]According to different embodiments of the invention, the 3D model can be
obtained in different ways. For example, it may have been created beforehand for the
patient by 3D imaging, obtained beforehand and then registered in real time during
the operation, or it may be created from the perioperative 2D images.
[0057] In a set of embodiments of the invention, the 3D model of the aorta 321 is a
finite element model, comprising a representation of the central line of the aorta and
of the surface thereof.
[0058] In a set of implementations of the invention, the 3D model of the aorta is
obtained from a perioperative 2D image, for example an image obtained from a Carm, by a method of non-rigid registration of a previous model of the aorta.
[0059]As has been indicated above, the method of non-rigid registration of the aorta
is based on a finite element model of the central line of the artery. The perioperative
information items contained in the plane of projection of the perioperative 2D image
serve as boundary conditions on the finite element model. The deformations outside
the plane are then calculated by the mechanical model.
[0060] In a set of embodiments of the invention, the registration method has the
following steps:
- the central line of the artery is extracted using the Voronoi diagram method,
introduced for example by Antiga, L. (2002). Patient-specific modeling of geometry
and blood flow in large arteries. Politecnico di Milano;
- the central line is implemented in a finite element model based on beam elements.
This can be done in different ways. Duriez, C. (2013). Real-time haptic simulation of
medical procedures involving deformations and device-tissue interactions (Doctoral
dissertation, Université des Sciences et Technologie de Lille-Lille I) provides an
example of implementation of the central line of the aorta via a finite element model;
- a first 2D/2D non-rigid registration is performed between the perioperative images
and the projection of the central line extracted in the preceding step. This non-rigid
registration is composed of a rigid registration then an interpolation;
11
- the 2D information items concerning the position of the aorta or of the tools during
surgery are then implemented in the finite element model. The matrix of projection of
the perioperative images is assumed to be known. By this means, it is possible to
calculate the back-projection lines of each of the characteristic points identified on
the 2D images. These information items are then implemented in the mechanical
model in the form of boundary conditions, the points of the 3D model being forced to
move along the back-projection lines. Thus, the mechanical model calculates the
deformations that are not contained in the plane of projection of the perioperative 2D
image in order to reach a state of equilibrium;
- the volume of the aorta is then recreated around the updated central line.
[0061] In a set of embodiments of the invention, the 3D model of the aorta is
superposed on the perioperative images.
[0062] The method 300 also comprises the acquisition of a model 331 of the
endoprosthesis. In this model, each stent is defined by at least one position and one
orientation of the stent, which are defined by at least 6 degrees of freedom, and a
deployment of the stent.
[0063] The degrees of freedom representative of the position and orientation of the
stent comprise at least one 3D position of at least one characteristic point of the stent,
and an intrinsic rotation rx’ of the stent about its central axis. According to different
embodiments, the 6D position of the stent can be defined in different ways. For
example, it can be defined by the 3D position of at least two characteristic points of
the stent and an angle of intrinsic rotation, or by the 3D position of a single
characteristic point of the stent and three angles of rotation. A person skilled in the
art will be easily able to define the type of stent modelling that meets his needs.
[0064] Figure 4 shows a modelling of a stent within an endoprosthesis model
according to a set of implementations of the invention.
[0065] Figure 4 shows the modelling 400 of a stent within an endoprosthesis model
331.
[0066] In this example, the stent is associated with its own reference frame Rstent
comprising 3 axes (x’, y’, z’) initially aligned with the axes (x, y, z) of the reference
12
frame Rim. The intrinsic rotation of the stent rx’ is defined about the central axis (which
can also be called the vertical axis) x’. This simplified model comprises the 3D
position of three characteristic points P0, P1, P2, corresponding respectively to the
centre, the upper end and the lower end of the stent along its central axis x’. In this
example, the position and the orientation of the stent are thus defined by 10 degrees
of freedom (9 degrees for the three 3D positions, and one degree for the rotation).
[0067] This use of three characteristic points makes it possible to further improve the
determination of the position and the orientation of the stent.
[0068] However, this modelling of the position and orientation of a stent is given
solely as a non-limiting example, and the position and orientation of a stent could be
represented in other ways. For example, they could be represented by the position of
the characteristic point P0 and 3 rotations, or the positions of the characteristic points
P1 and P2 and the intrinsic rotation about the axis x’.
[0069] Figure 5 shows the deployment of a stent in the context of a model of the
endoprosthesis.
[0070] In a set of embodiments of the invention, the deployment of the stent is
defined by a single value. This value can be, for example, the diameter of the stent, a
percentage or a ratio of deployment.
[0071] The diagram 500 shows the simplified deployment of several stents. The axis
510 shows the deployment of the stent in mm. The axis 520 shows the position and
the length of each stent.
[0072] It is also possible, according to different embodiments of the invention, to
represent the deployment of the stent according to a limited number of values
representing the deployment of the stent at different characteristic points about its
central axis. For example, the deployment of the stent can be represented by a
deployment value at the upper end of the stent and a deployment value at its lower
end.
[0073] The representation of the deployment of the stent by at least one value
representative of a level of deployment of the stent about its central axis thus makes
it possible, in combination with the position and orientation values, to represent the
13
deployment of each stent within the endoprosthesis in a highly synthetic manner, with
the aid of a limited number of parameters, of which most of the values can be
extracted from a 2D perioperative image.
[0074] The model 331 of the endoprosthesis thus permits a simplified modelling of
the endoprosthesis, making it possible to represent the overall positioning of the
endoprosthesis with a limited number of degrees of freedom.
[0075] Returning to Figure 3, the method 300 comprises a step 330 of determining
from the 2D image, and for each stent, the at least one position of the at least one
characteristic point along the vertical axis (x) and horizontal axis (y) and the at least
one deployment value of said stent.
[0076] This step involves identifying all the values of the endoprosthesis model that
can be obtained directly from the 2D image.
[0077] Figure 6 shows the determination, from the 2D image, of the vertical and
horizontal positions of the characteristic points of the stents, and of the deployment of
said stents.
[0078] More precisely, Figure 6 shows the image of Figure 2c, on which different
parameters for modelling the deployment of the endoprosthesis in 3D are highlighted.
Figure 6 thus shows the detection of certain values on the perioperative image of the
aorta, on a 2D image similar to the ones that a surgeon could visualize in real time.
Although Figure 2c and Figure 6 show an endoprosthesis deployed in a mock-up of
the aorta, the elements presented in Figure 6 are also applicable to an image
showing the aorta, highlighted by virtue of the contrast agent, in superposition with an
image of the endoprosthesis, and also to images of the endoprosthesis when not
deployed or when partially deployed.
[0079] The three vignettes 610, 620 and 630 respectively show three examples of
determination, on the image 200, of the positions of the characteristic points of the
stents, the deployment of the stents, and the association with the central line of the
aorta.
14
[0080] In particular, the 2D image makes it possible to obtain directly, as shown in
Figure 2b, the positions of each of the characteristic points of the stents along the x
and y axes of the reference frame Rim. The reading can be done manually from the
information items shown on the screen, or semi-automatically from processing in
which the position of the centres of gravity of the stents is detected. In the example
shown in Figure 6, the stent 211 is associated with three characteristic points 211-P1,
211-P0 and 211-P2, according to the model shown in Figure 4. The analysis of the
image 200b thus makes it possible to deduce directly the positions of the
characteristic points 211-P0, 211-P1 and 211-P2 along the x and y axes, for each
stent. For example, these points can correspond to the points 211b-0, 211b-1 and
211b-2 of the image 200b, the position of which can be determined directly by
analysis of the image 200b. According to different embodiments of the invention, the
positions x, y of the characteristic points can be determined with respect to a single
origin of the reference frame Rim, or with respect to a central point of the aorta. The
stents are then positioned in the simulation by virtue of the positions of the
characteristic points that are thus determined.
[0081] The vignette 620 shows the determination, for each stent, of at least one
deployment value of said stent about its central axis. In the example of Figure 6,
each stent is associated with a deployment value of its upper end and a deployment
value of its lower end. For example, the stent 211 is associated with a deployment
value 211-D1 at its upper end and with a deployment value 211-D2 at its lower end.
Here once again, the deployment values of the stent can be obtained directly by
analysis of the image 200. In the example of the vignette 620, the stents are
deployed, and the deployment value can be obtained directly by measuring the
deployment of the stents on the image, manually or automatically by image analysis.
[0082] In embodiments in which the endoprosthesis is not deployed or not fully
deployed, the deployment value can be determined as the diameter of the aorta at
the point of deployment, if the maximum diameter of deployment of the
endoprosthesis is greater than or equal to said diameter of the aorta at the point of
deployment. In the opposite case, the deployment value corresponds to the
maximum deployment of the endoprosthesis. Indeed, the final deployment diameter
of the endoprosthesis at this point will in this case be constrained either by the
diameter of the aorta or by the diameter of the stent.
15
[0083] The vignette 630 shows the association between the characteristic points of
the stents and the central line of the aorta.
[0084] The central line of the aorta can be obtained by projection of a previously
obtained 3D central line, or by a method of skeletonization of the 2D image
(numerous methods of skeletonization of the 2D image are known, for example the
method described by Couprie, M., Coeurjolly, D., & Zrour, R. (2007). Discrete
bisector function and Euclidean skeleton in 2D and 3D. Image and Vision Computing,
25(10), 1543-1556). In a set of embodiments of the invention, the position of the
characteristic points of the stent along the z axis is initialized on the central line of the
aorta.
[0085]As is shown in Figure 6, the step 330 of determination of the horizontal and
vertical positions and of the deployments of the stents can be done by direct analysis
of a perioperative 2D image, either manually or automatically by image analysis.
[0086] This determination is thus carried out very quickly. The step 330 can thus be
effected in real time once the 2D image has been captured. Moreover, this step
permits great precision, to less than a millimetre, on the values determined.
[0087] Certain characteristics of the model of the endoprosthesis cannot be
determined directly by the direct analysis of the image. For example, the position of
the characteristic points on the depth axis z and the intrinsic rotation of the stents
cannot be determined directly. These values can be initialized to default values. For
example, the positions of the points on the depth axis can be initialized such that the
points are situated on the central line of the aorta, and the intrinsic rotation of the
stent about the axis x’ can be initialized to 0.
[0088] In a set of embodiments of the invention, the endoprosthesis assembly is
modelled in the form of a finite element model in which each stent is modelled by a
beam element, and the stents are connected to each other by small beam elements.
[0089] In a set of embodiments of the invention, a plurality of connecting beam
elements (for example five) are arranged in series between the ends of the beam
elements representing the stents. These connecting beam elements can be
associated with stiffness matrices, of which the mechanical characteristics are
different from those of the stiffness matrices of the beam elements representing the
stents. Thus, these connecting beam elements will have a less rigid behaviour. This
16
makes it possible to precisely model the mechanical equilibrium between the
successive stents.
[0090] The stents whose maximum diameter of deployment is greater than or equal
to the diameter of the aorta, at the site of deployment, are considered as constrained:
the position of their characteristic points is fixed along the axes x, y and z. Indeed,
their position will be completely constrained by the volume of the aorta at the time of
their deployment. By contrast, those whose maximum diameter of deployment is
smaller than the diameter of the aorta are considered “free”: their characteristic
points are constrained in displacement along the horizontal axis z and vertical axis y
but free in displacement along the z axis. Their position along the depth axis z is then
defined by the mechanical equilibrium of the finite element model.
[0091] This makes it possible to obtain a precise 3D position of the stents whose
position is not completely constrained by the volume of the aorta, while taking into
account the equilibrium of the mechanical forces within the aorta.
[0092] The positioning of the characteristic points of the stents on the central line of
the aorta provides a good compromise between the calculation time and the
precision of the method.
[0093] Returning to Figure 3, the method 300 comprises a step 340 of determination
of the intrinsic rotation angle of at least one stent of the endoprosthesis.
[0094]According to different embodiments of the invention, this step, performed on
each stent separately, can be performed on all or some of the stents of the
endoprosthesis. For example, it can be performed on each stent of the
endoprosthesis. It can also be performed on certain critical stents only, for example
stents comprising a window that is to be positioned facing an ostium. The surgeon
can also manually select the stents on which to perform this step according to his
needs.
[0095] In a set of embodiments of the invention, the determination, for at least one
stent, of its angle of intrinsic rotation rx’ comprises the determination of the angle of
intrinsic rotation rx’ for which the projection of the position of at least one radiopaque
marker most closely matches to the image of this marker on the 2D image. For
example, in the image 200 also represented in vignettes 610, 620 and 630 in figure 6,
17
the stent 211 comprises a certain number of radiopaque markers, such as the
markers 230 and 231. The position of these radiopaque markers is known in the
simplified model of the stent. Thus, knowing the deployment values of the stent, it is
possible, for each value of the angle of intrinsic rotation rx’, to project the 3D position
of the radiopaque marker onto the 2D image and to check whether it tallies with the
position of the radiopaque marker on the 2D imaging. This comparison can be
carried out for one or several markers.
[0096] The determination of rx’ can thus be effected in different ways. For example,
the angle of intrinsic rotation rx’ can be obtained by executing a loop minimizing the
distance between the projections of the 3D positions and the images of the at least
one marker on the 2D image, as a function of the angle of intrinsic rotation rx’.
[0097] Thus, the angle rx’ can be modified iteratively, and at each iteration the position
difference, for each marker, between the projection of its 3D position in the model on
the 2D image and its image taken by X-ray can be calculated. If several markers are
used, the absolute values of the position differences can be added together or taken
separately. Thus, the values of the angles rx’ can be determined iteratively in order to
minimize the position differences between the projections of the 3D positions and the
images of the one or more markers on the 2D imaging, for example by way of a
gradient descent algorithm in stochastic methods, or any other algorithm by which it
is possible to identify a global minimum of the differences between the projections of
the 3D positions of the markers and their images, as a function of the intrinsic
rotation rx’. According to different embodiments of the invention, such algorithms can
be initialized with a single point of departure, or with multiple points of departure in
order to avoid convergence towards a local minimum. Any minimization algorithm
can be used here.
[0098] This provides a simple and quick way of determining the intrinsic rotation rx’ of
a stent.
[0099] The method 300 then comprises the simulation of the deployment in the 3D
model of the aorta 311, for each stent, of a model of the stent 351 formed by a
plurality of beam elements and initialized on the basis of the model 331 of the
endoprosthesis 210.
18
[0100] Once the values of the parameters of the model 331 of the endoprosthesis
have been determined, in which each stent is represented in a simplified manner, this
step involves using these parameter values to initialize a more complex model of
each stent and to simulate its deployment in the 3D model of the aorta.
[0101] In cases where the endoprosthesis is already deployed, this makes it possible
to model the actual deployment of the endoprosthesis in 3D. In cases where the
endoprosthesis is not deployed or not completely deployed, this allows the surgeon
to visualize how the deployment of the endoprosthesis will look if he decides to
deploy it in its current position.
[0102] Figures 7a and 7b show two examples of modelling of the deployment of a
stent according to a set of implementations of the invention.
[0103] The model 351 of a stent is formed by a plurality of beam elements modelling
the stents. The number of beam elements may be variable depending on the desired
complexity. A model 351 of a stent can be formed, for example, from a hundred or so
beam elements.
[0104] Figure 7a shows the initialization of a model 700a (corresponding to an
example of the model 351) in the 3D model of the aorta 330. As has been explained
above, the steps 330 and 340 make it possible to determine, for each stent,
characteristics of a simplified model: position of the characteristic points, rotation
about the central axis, etc. These values permit initialization of the position and
orientation of the beam elements of the model 351, 700a representing the structure
of the stent.
[0105] In the example in Figure 7a, the stent is not yet deployed. In order to simulate
the deployment of the stent, it then suffices to simulate the extension of the beam
elements from the initialization position to the limits of the 3D model 330 of the aorta.
[0106] Figure 7b shows a stent model deployed in the 3D model of the aorta 330. As
has been explained above, the model 700b is formed by a plurality of beam elements,
for example the elements 710b, 711b, 712b, 713b. The stent can thus be deployed
either to its maximum extension or until the beam elements reach the limits of the
aorta, for example at the points 720b, 721b.
19
[0107] The finite element model makes it possible to take into consideration at the
same time the constraints within the stent and the interaction between the stent and
the aorta.
[0108]Since the finite element model representing the structure of the stent is
initialized on the basis of the simplified model calculated beforehand, the simple
simulation of the deployment of the stent can be effected rapidly, while at the same
time very precisely modelling the deployment of the stent.
[0109] The method 300 as a whole thus permits a both reliable and rapid calculation
of the deployment of an endoprosthesis in the aorta. For example, the method can be
carried out in about thirty seconds on standard calculation means, which allows the
deployment of the endoprosthesis to be modelled almost in real time.
[0110] This result is permitted by the fact that the complete model modelling the
structure of each stent (model 351, 700a, 700b), which comprises numerous
elements, is initialized by the preceding steps and need only be deployed. Although it
is possible to directly model the deployment of a complete model representing the
structure of each stent of the endoprosthesis without prior hypothesis, this would take
a considerable amount of time incompatible with the performance of an operation in
real time.
[0111] Returning to Figure 3, the method 300 comprises a step 360 of displaying the
deployed model of the stents. This display can be either in 2D, by superposing the
projection of the deployment of the stents on a perioperative 2D image such as the
image 200, or in 3D, by displaying the deployment of the stents in 3D and the model
of the aorta 330. The display can be shown, for example, on the screens 121 of a
medical imaging device.
[0112]Provided that the method permits simulation of the deployment of the
endoprosthesis in a very precise manner and in real time, the display 360 allows the
surgeon real-time visualization of the deployment of the endoprosthesis on the basis
of its actual position. This therefore makes it possible to considerably improve the
precision of the treatment performed by the surgeon.
[0113] In the case where the method is applied to a deployed endoprosthesis, the
surgeon can visualize the deployment in 3D. In the case where the method is applied
20
to an endoprosthesis that is not deployed, the surgeon can visualize how the
deployment of the endoprosthesis will look, in 3D, if he decides to deploy it on the
basis of the current position.
[0114] Moreover, this method requires a single image capture in order to simulate the
deployment of an endoprosthesis, which avoids the use of multiple injections of
contrast agent.
[0115] Figure 8 shows superpositioning of the simulation of deployment of a stent on
a perioperative image according to a set of implementations of the invention.
[0116] The image 800 shows the superpositioning of the stimulation of the
deployment of the endoprosthesis on the 2D imaging obtained from a C-arm. As has
been explained above, the invention also permits 3D representation of the simulation
of the deployment of the endoprosthesis on the 3D model of the aorta. In the case of
3D visualization, the surgeon can manipulate the representation in order to visualize
the prediction of the deployment at different viewing angles.
[0117] In both cases, this allows the surgeon to visualize the simulation of the
deployment of the endoprosthesis in real time.
[0118] The above examples demonstrate the capacity of the invention to permit
determination of the deployment of an endoprosthesis. However, they are given
solely by way of example and do not in any way limit the scope of the invention as
defined in the claims below.

CLAIMS
1. Method (300) comprising:
- a capture (310) of a 2D image (311) of a vascular structure by X-ray;
- an acquisition of a 3D model of the vascular structure (321);
- an acquisition of a model (331) of an endoprosthesis (210) in the
vascular structure, comprising a plurality of stents;
- a determination (330, 340) from the 2D image, and for each stent, of at
least one position, of an angle of intrinsic rotation (rx’), and at least one
deployment value;
- a simulation of the deployment (350) in the 3D model of the vascular
structure (311), for each stent, of a model of the stent (351)
representing the structure of the stent, said model of the stent being
initialized on the basis of the model (331) of the endoprosthesis (210);
- a display (360) of the deployed stent models;
said method being characterized in that the determination (340), for at least one
stent, of its angle of intrinsic rotation (rx’) comprises the determination of the angle of
intrinsic rotation (rx’) for which at least one 3D position projection of at least one
radiopaque marker on the stent most closely matches to at least one image of the at
least one marker on the 2D image.
2. Method according to Claim 1, in which:
- the 2D image (311) defines a 3D reference frame (Rim) comprising a
vertical axis (x) and a horizontal axis (y) and a depth axis (z) of the
image;
- in the endoprosthesis model, each stent is defined by:
- at least one position of at least one characteristic point (P0, P1,
P2) and an orientation, which are defined by at least 6 degrees
of freedom, including:
22
- the 3D position of a first characteristic point (P0) of the
stent in the reference frame;
- the angle of intrinsic rotation (rx’);
- the at least one deployment value of the stent, defined about its
central axis;
- the model of the stent is formed by a plurality of interconnected beam
elements;
said method comprising:
- determination (330, 340) from the 2D image, and for each stent, of at
least one position, at least one orientation, and at least one deployment
value; of the at least one characteristic point along the vertical axis (x)
and horizontal axis (y) and the at least one deployment value of said
stent;
- determination (340), for at least one stent, of its angle of intrinsic
rotation (rx’).
3. Method according to either of Claims 1 and 2, in which the position and the
orientation of each stent are defined by the 3D positions of three characteristic
points (P0, P1, P2) corresponding respectively to the centre, the upper end
and the lower end of the stent along its central axis (x’), and the angle of
intrinsic rotation of the stent about its central axis (x’).
4. Method according to one of Claims 1 to 3, in which the angle of intrinsic
rotation (rx’) is obtained by the execution of a loop minimizing the distance
between the at least one projection and the at least one image of the at least
one marker on the 2D image, as a function of the angle of intrinsic rotation (rx’).
5. Method according to one of Claims 1 to 4, in which the determination (340), for
at least one stent, of its angle of intrinsic rotation (rx’) comprises modelling the
stent in the form of a beam element of a finite element model, between a
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characteristic point representing the upper end of the central axis of the stent
(P1) and a characteristic point representing the lower end of the central axis of
the stent (P2), in which the characteristic points are free of displacements
along the depth axis (z).
6. Method according to Claim 5, in which:
- the endoprosthesis is represented in the form of a finite element model
in which:
- each stent is modelled by a beam element, the successive
stents being connected to each other by at least one beam
element;
- the positions of the characteristic points of the stents whose
maximum diameter of deployment is greater than or equal to the
diameter of the vascular structure at the deployment position are
fixed along the vertical axis (x), horizontal axis (y) and depth axis
(z);
- the positions of the characteristic points of the stents whose
maximum diameter of deployment is less than the diameter of
the vascular structure at the deployment position are fixed along
the vertical axis (x) and horizontal axis (y) and are free in
displacement along the depth axis (z);
- the positions of the characteristic points of the stents whose maximum
diameter of deployment is less than the diameter of the vascular
structure at the deployment position along the depth axis (z) are
determined by the mechanical equilibrium of the finite element model.
7. Method according to one of Claims 1 to 6, in which the display (360) of the
deployed stent models comprises the superposed display of the projection of
the stent models (351) on the 2D image (311) of the vascular structure.
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8. Method according to one of Claims 1 to 6, in which the display (360) of the
deployed stent models comprises the 3D display of the deployed stent models
and of the 3D model of the vascular structure (321).
9. Method according to one of Claims 1 to 8, in which the 3D model of the
vascular structure (321) is a finite element model representing the central line
of the vascular structure by beam elements.
10.Computer program product comprising computer code elements configured to
execute a method according to one of Claims 1 to 9 when said program is
executed on a calculation unit of a calculation device.
11.Device (120) comprising:
- at least one input port configured to receive a 2D image (311) of the
vascular structure captured by X-ray;
- at least one calculation unit configured to execute a method according
to one of Claims 1 to 9