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*"What the current imaging methods can and cannot measure in AAA's*"

By Barry Katzen, MD

DR. KATZEN: Thank you. I am going to try and add briefly to what Jock
has already said, and I apologize for any redundancy, and where there
is, I will just try and skip through things.

But I will try and give you a little insight into what I think we can
and can\'t do with imaging, and a little bit \-- and perhaps the future
\-- in the context of understanding aneurysms at least, and where some
of the future may lie. Next slide, please.

(Brief Pause.)

DR. KATZEN: Well, Jock has already talked a little bit about what we
need to know, and I think the presumption is that we know what we need
to know, and I think one of the things that we have learned certainly
over the past decade of experience is that we probably don\'t really
know what we need to know.

And part of that is that we are learning and exploring as we go. I think
he has highlighted some of the critical areas that we have been at least
assessing. Next slide. I am going to skip through some of this.

There are a variety of imaging modalities available for interrogating
aneurysms and trying to understand them. I think that all of you are
aware of these modalities, and how they are used in clinical practice.

While I think there was a lot of hope for ultrasound, it certainly has
limited information, and we certainly learned about the pitfalls in
conventional abdominal CT, because it really does not provide volumetric
data in very thick slice thicknesses. Next, please.

And spiral CT has been the principal data source that we have been using
to interrogate and understand aneurysms because it provides high
resolution volumetric data, from which we can provide \-- we can get
accurate measurements.

The problem is that in the 2-D formatting of volumetric data there is
the same pitfalls of a sort of lesser imaging modality. So if the
volumetric basis of CT scanning is not used, then really we are settling
back to a more basic 2-D types of information. Next.

The pitfalls of 2-D imaging include the compression of 3-D data into a
flat display, and I think Jock has alluded to this, and the need for us
to think and understand more three dimensionally, and the elimination of
curves and angles which can be dramatic, and affect a lot of very
critical measurements.

And it can all lead to inadequate measurements, including angles,
diameters, and compressed lengths. Next. Some of the reformatting
options that we use and that are available to us include maximum
intensity projection types of images, surface renderings, which include
an outer look at the aneurysm morphology, multiplanar reconstructions,
which allows us to reconstruct images along the plane of the vessel, or
a pre-defined vessel such as curve multiplanar reconstructions.

And then you can get sophisticated combinations of these, including some
of the proprietary services that are available. Next, please.

I want to mention just briefly MR, because we are not going to talk a
lot about, and in talking about the future, I think it is important to
discuss it. MRA is being widely used in Europe now as an imaging
modality for patient selection.

It offers the benefit of non-ionizing radiation, and no use of renal
toxic agents. It has a fairly high-spatial resolution, less than CT, but
you can see that you can get very good quality images with modern fast
MR scanners, and the use of MR contrast media, and the image quality is
continually improving.

And you can obtain the volumetric types of data that we are used to
using more and more for AAA analysis. Next.

And here is an example in another application of one advantage that MR
may offer us over CT, and that is the use of dynamic imaging. And it
relates to some extent to the physiologic parameters that Jock is
looking at.

If there was some way that we could actually measure physiologically
what is going on, and where stress is occurring, and where maximum
diameter changes are occurring, and begin to look at the aorta itself in
a dynamic fashion, it offers perhaps some opportunity for us in the
future to understand where maximum stresses are going to be.

MR also lends itself to many of the sophisticated software analyses,
including virtual endoscopy, and many of the reformats that you are used
to seeing in this particular case, negotiating up through a coar and
looking at the suplavian artery. Next.

So I think that in terms of future imaging that this is going to be
something certainly that we are going to be interested in looking at.
And as I said, it is being used more and more certainly among our
European colleagues.

Well, how do you think in 3-D dimension, as Jock likes to say. Well, one
way is to spend three years in a radiology residency, and you learn over
time how to think in 3-D when you are looking at flat images.

But the other is to use these powerful tools to make imagery simpler for
all of us to look at and understand. Next.

Well, you have heard and seen some of the disadvantages of looking at
flat images, and here is an example of a patient with an aortic aneurysm
that has a sort of globular looking lumenagram on angio.

And ordinarily most of us would look at this and think of it as a tear
drop, round shape, but when you look at the axial image, you can see
here that actually it is a very flat type of shape.

Now, does this have any real significance? It may in terms of the device
that we choose. It may in terms of the deployment methods that we have
to employ and so on. Next.

The presence or absence of calcium is something that cross-sectional
imaging, particularly CT, provides over angiography. And by the way,
this is one of the disadvantages of MR, is the ability to detect
calcification, and in which it is limited.

But the presence of calcium is very important as Jock mentioned.
Tortuosity in and of itself may not be an adverse predictor, or an
adverse stress point.

Whereas, tortuosity and the presence of calcium, will have a totally
different affect both on the endograft and the ability to deploy it.
Next.

A similar example here, with an underestimation of calcium by
traditional angiographic techniques. Next.

Now, angiography \-- I guess the question is whether there is a role for
angiography, and I think that varies among investigators. To some
extent, for angiographers, you will hear them say that this is a little
bit like driving a race track before the race.

So there is some value if the angiographer is the same person who is
going to be the implanting physician. But the angiography, per se, as an
imaging modality, is probably having less and less value, depending on
the quality of information you can derive from cross-sectional imaging.

You can get very a very interesting and accurate sense of diameters, as
well as length measurements, though they are probably more accurate than
traditional CT. Next.

Now, the advantages of reformat that I alluded to, here is an example of
an aneurysm sac that has a very unusual looking limb that has migrated
in this case, and assumed a very peculiar kind of orientation within the
sac, and which is shown to be a kink in the reformatting.

So reformatting allows us to see and understand things quite a bit
easier than in the axial images. Next.

Surface renderings, which are a little bit less valuable to some extent,
can be very valuable in sort of reproducing the angiogram, the
appearance of an angiogram in a volumetric way.

And it allows us to very sensitively see branch arteries, and in this
case, an atonic pelvic kidney, or identify branches that are occurring
from other areas, and actually recreate an angiogram. Next.

Surface renderings also give us a very good visualization of angulation
and allow us to do the calculations of angulation, and try and
understand what the angulation is going to mean for deployment.

What imaging cannot tell us is what the stresses are going to be
relative to the angulation, and what the dynamics of the curve point are
from a shear stress point of view. Next.

And by the way the shear stress models, you know, by and large, are
generally done in straight tubes as well. So they suffer the same
problem. If we could somehow transfer the imaging data and mix the
physiologic data \-- and that\'s why MR may offer some potential for
that, and we might get better information.

Again, as Jock said, imaging cannot tell us what the makeup of luminal
filling defect is. Maybe someday it might be able to, but at least with
current technologies it can\'t. Next.

The complex sac anatomy is another critical event that may \-- a
critical issue that may create both initial and long term problems.
Next.

In this particular case, you can see that there is a so-called empty sac
with a flow divider in it. The question is how do you understand whether
this is going to be a significant barrier, or whether it is a dissection
that may be extending through the whole sac.

And that may actually create to an acute conversion, or lead to an acute
conversion. And this is where the volumetric information becomes
important. Next.

And in the final couple of minutes, I am not going to spend a lot of
time on this, because I think that Jock made the simple point, which is
that the measurement of arteries in an off-axial way introduces error.
And this has been demonstrated in a lot of different ways as you see
here. Next.

And here in this image that you have already seen before, we have become
much more sensitive to the need to get true axial measurements. Next.

In the iliac artery, it is the same type of thing, and that is that
measuring a circle with an off-axial image is going to introduce error,
and depending on the device, this may be critical to both deployment and
long term outcome. Next.

Angulation is very critical, and there are numbers that can be
accurately measured by imaging, and we have been able to use these as
inclusion and exclusion criteria. Next.

The other important thing to understand about measurements is the
pitfalls involved in length measurements. These are less important in
modular devices where there is some operator flexibility, and you can
shorten up devices.

And so to one extent, this is somewhat device specific type of
information, but as you see in the image on your right, you can see that
if you just took all the axial images, and just added them up, you would
probably be about 15 percent short here on device length. Next.

Angulation of the neck can be compensated for and all of this
information can be determined in advance. Next. And this translates into
real world problems, in terms of device failure. Next slide, please.

Or as you can see here, if you \-- this is a device that was deployed in
the AP or frontal projection, and when you do the correct angulation,
you can see that actually there was an abundant amount of room still in
the proximal neck for deployment. Next.

Obviously these kinds of technologies can allow us through not only
preclinical testing, but we might be able to actually create virtual
computer models for testing as well. Next. And not just for
implantation.

The final thing that imaging provides us is the ability to look at
volumes, which may be important in ways that we really don\'t
understand. I just want to mention it for completeness.

And certainly for following performance of the device as we believe it
is important, but is it important for inclusion and exclusion criteria
is another thing that we may need to look at. Next.

Finally, I just want to introduce a concept. We are tending to lump all
the things and want all these issues in one barrel, and you might have
an Indy car that is a great car, or a Formula One car, both of which are
great performance models, but would not perform at the highest level in
the same environments if they were both put in each other\'s
environment.

And I think it is probably time as our state of knowledge has become
sophisticated enough to begin to start looking at these things, and
begin to think about different devices, and different specifications,
and it may even require different testing.

Self-expanding devices under current recommendations have greater
latitude in measurements of diameter, or at least we are using them with
greater latitude, in terms of oversizing and how we deploy these.

Whereas, unsupported devices have tended to be more precisely measured,
and more precisely implanted in terms of the size tolerances. The same
thing may be true in terms of ultimately response to stress, and
response to curves, and so on. Next.

And so the effect or the interpretation of imaging findings themselves
may be device dependent. The effect of curves, and the effect of
angulation as you see here, and here is a patient that had a fairly
angulated neck, and is now seven years post-implantation, with quite an
excellent result.

And here is a patient with an angulated neck that ultimately had a
device migration. Next. Is it angulation, or is it device, or is it a
combination of angulation and device. I think we need to begin to start
looking at these things in a more compartmentalized way.

And so in conclusion, endografts require precision measurement for
accurate deployment. Volumetric imaging is critical to optimal treatment
planning and understanding.

And I think it is important for us to think and image in three
dimensions, and probably to understand that there are limitations to
what imaging can show us, particularly about the nature of the arterial
wall and the stresses that are going on in the vessels over curves.
Thank you.