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Description  |
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BACKGROUND OF THE INVENTION
Atherosclerosis, in which a vessel gets partially or fully blocked by
atheroma, is a common disease in the western world. Because of this there
is a large research activity for developing catheter methods to remove the
atheroma or expand the lumen of the artery. In laboratory experiments it
has been possible to obtain ablation of the atheroma using laser light.
This gives hopes for a method that can remove the atheroma even when there
is only a thin hole left or full blocking of the artery.
A critical problem with this application is the dosage and direction of the
laser light for accurate removal of the atheroma without damaging the
arterial wall. This invention relates to an intravascular catheter for
angioplasty using laser light for atheroma ablation and combining it with
ultrasonic imaging of the atheroma for guidance of the laser light to
obtain precise ablation without damaging the arterial wall. Ultrasonic
Doppler blood velocity measurements may also be used to monitor the change
in blood flow caused by the increased lumen. The invention also comprises
a complete laser beam delivery system for intra-arterial laser
angioplasty.
The laser light is delivered to the site using an optical fiber. At the tip
of the catheter an ultrasonic transducer is located, and the basic idea of
the invention is that the ultrasound transducer and the tip of the optical
fiber are mounted so that the directions of the ultrasonic beam and the
laser beam are related so that the ultrasound beam can be used to image
the atheroma and the arterial wall, and the laser beam can be directed in
any selected direction in the image, especially directions where an
atheroma is indicated, for controlled ablation of the atheroma. This can
be obtained by mounting the ultrasonic transducer and the fiber tip so
that the two beam directions coincide, or with different directions of the
two beams, the laser beam can be steered to a known direction in the image
generated by the ultrasonic beam. Coincident directions of the beams can
be obtained by
(i) The fibre tip penetrates the ultrasound transducer with such a small
hole that it has negligible effect on the ultrasound beam, and the hole is
large enough to feed through the laser light, so that the direction of the
laser light and the ultrasound beam substantially coincide. This is
schematically illustrated in FIG. 1.
(ii) The laser beam is bent off at an angle by a beam directing arrangement
using either a mirror, prism arrangement, bending of the fiber tip, or
combination of the three. The ultrasound transducer is mounted at a
distance from this arrangement and radiating towards said arrangement
which acts as a mirror for the ultrasound beam, so that the ultrasound
beam is reflected into the same direction as the laser beam. A typical
example of such an arrangement is schematically shown in FIG. 2.
An example of a method by which the beam directions are not coinciding, but
interrelated so that the image obtained by the ultrasound beam can be used
as a reference for guiding the laser beam, is schematically illustrated in
FIG. 3. Here the laser beam is mirrored in the opposite direction to the
ultrasound beam, and by rotating the mirror, the laser beam can be aligned
to a previous, well known direction of the ultrasound beam.
The ultrasound transducer can be used for pulse echo imaging in the
following modes
(i) A- or M-mode where range resolution is obtained visualizing the
different structures of the atheroma and the arterial wall, as illustrated
in FIG. 5 and FIG. 6 respectively.
(ii) The beam direction can be scanned in a plane to generate a
2-dimensional cross section image of the atheroma and the artery, as
illustrated in FIG. 7.
(iii) The 2-dimensional scan planes can be moved under position control by
moving the catheter along the vessel to generate 3-dimensional images of
the atheroma and the arterial wall, as illustrated in FIG. 8. This can be
obtained by mounting a longitudinal position sensor, for instance using an
optical grating, to the portion of the catheter which is outside the body.
The third scan dimension is then obtained by pulling the catheter out of
the artery, using the position indication to store 2-dimensional images at
defined sections.
By imaging we thus mean any kind of presentation of the backscattered
ultrasound which relates a portion of the signal to spatial location of
the scatterers in the region being sonified. By direction the laser beam
along the ultrasonic beam we can obtain a precise observation of both the
location of the atheroma to decide where to apply laser light, and
continuous monitoring of the effect of the laser light on the atheroma to
adaptively determine the energy levels to be applied and when to stop the
illumination to avoid damage to the arterial wall. By high energies of the
laser light, the ultrasound imaging might be affected by the gas or debris
from the ablation of the atheroma, so that it can be advantageous to apply
imaging and laser treatment in a time sequence using a spatial
interrelation between the beams, so that the laser beam can be directed in
a defined direction where an ultrasound image has been obtained. To obtain
this, the beam directions do not need to coincide, but they need to be
interrelated so that the laser beam can be directed towards a defined part
of a previously generated ultrasound image, and the ultrasound beam can be
directed towards the place where laser irradiation has occurred, to
monitor the effect of the irradiation.
In its broader aspect, the intra-arterial laser angioplasty delivery system
according to the invention comprises:
a combined laser and ultrasound catheter comprised of
means for emitting a laser beam in an artery towards an atheroma for
ablation of the atheroma,
optical fiber means for feeding laser light from a laser to said emitting
means,
an ultrasound transducer for intra-arterial imaging of tissue structures
like the atheroma, the vessel wall and surrounding tissue, by emitting a
pulsed ultrasound beam towards said tissue structures and also arranged to
receive backscattered ultrasound from said tissue structures, and
beam directing means arranged to direct the ultrasound beam towards said
tissue structures and also arranged so that the laser beam can be brought
into for practical purposes any of the directions the ultrasound beam can
assume for imaging, so that the laser beam can be steered towards a
portion of the ultrasound image indicating a portion of an atheroma for
ablation of said portion of the atheroma,
receive circuits adapted to receive and process the backscattered
ultrasound for imaging of the tissue structures, and
means for analysing said ultrasound image, either manually or
automatically, to determine the presence of atheroma so that said image
can be used to direct the laser beam towards the regions in the ultrasound
image which have been determined to represent atheroma and determine the
intensity and the dosage of the laser beam for accurate ablation of said
atheroma.
Another aspect of this invention relates to an intra-arterial laser
angioplasty catheter comprising:
means for emitting a laser beam in an artery towards an atheroma for
ablation of the atheroma,
optical fiber means for feeding laser light from a laser to said emitting
means,
an ultrasound transducer for intra-arterial imaging of tissue structures
like the atheroma, the vessel wall and surrounding tissue, by emitting a
pulsed ultrasound beam towards said tissue structures and also arranged to
receive backscattered ultrasound from said tissue structures, and
beam directing means arranged to direct the ultrasound beam towards said
tissue structures and also arranged so that the laser beam can be brought
into substantially any of the directions the ultrasound beam can assume
for imaging, so that the laser beam can be steered towards a portion of
the ultrasound image indicating a portion of an atheroma to hit said
portion of the atheroma, so that the ultrasound image can be used to guide
the laser ablation of the atheroma.
SUMMARY OF THE DRAWINGS
FIGS. 1a and b both schematically show the tip of a catheter according to
the invention where the optical fiber tip penetrates the ultrasound
transducer so that the ultrasound beam direction and the laser beam
direction substantially coincide.
FIG. 2 schematically shows another embodiment of the tip of a catheter
according to the invention where the ultrasound and laser beam directing
means is composed of a mirror to reflect the ultrasound beam off at an
angle, and a mirror and/or prism arrangement and/or bending of the fiber
tip for refracting the laser beam so that the two beams coincide after
this.
FIG. 3 schematically shows the tip of still another catheter according to
the invention, where the ultrasound and laser beam directing means is
composed of a mirror to reflect the ultrasound beam off at an angle and
reflection of the laser beam in the opposite direction with the same
arrangement so that a defined interrelation between the two beam
directions is obtained.
FIG. 4 shows a complete block diagram of a combined ultrasound imaging and
laser light delivery system.
FIG. 5 shows an A-mode display of the atheroma and arterial wall from the
reflections of an ultrasonic pulse.
FIG. 6 shows a M-mode display of the atheroma and the arterial wall from
the reflections of an ultrasonic pulse.
FIG. 7 shows a 2-dimensional cross sectional ultrasound image of a vessel
with atheroma obtained by rotating the beam directing means of the
catheter tip in FIGS. 2 and 3.
FIG. 8 indicates how a 3-dimensional display of the vessel and the atheroma
can be obtained from 2-dimensional cross sections at defined positions
longitudinally along the axis of an artery.
FIG. 9 shows an example of a system for rotating the beam directing
assembly manually or by a motor, together with a lens for coupling of the
laser light into the optical fiber. A longitudinal position sensor for
measuring longitudinal changes in the position of the catheter tip for
3-dimensional imaging of the vessel and the atheroma is also indicated.
DETAILED DESCRIPTION OF THE INVENTION
FIGS. 1, 2, and 3 are schematic schetches of basic ideas of the invention,
namely how a pulsed ultrasound beam can be used to obtain information of
how to steer laser irradition for ablation of the atheroma. The drawings
are schematic, illustrating basic principles, and in practical
implementations different variations can be used such as sending the beams
forward in a cone instead of at right angles. The arrangement will be
suitably mounted in a fully or partially closed catheter with
transluminant windows for the ultrasound and laser beams, so that any part
of the tissue is kept away from contact with the moving parts of the beam
directing system. For simplicity the cover of the catheter is not shown
since it is not part of the essentials of the invention.
FIGS. 1a and b both show a schematic illustration of an ultrasonic
transducer element 101 mounted at the tip of a catheter emitting an
ultrasonic beam 105 in the front of the catheter tip. A laser beam is
guided through an optical fiber, 102, which passes through a hole, 103, in
the ultrasound transducer element so that a laser beam, 104, is emitted
along the same direction as the ultrasound beam. In FIG. 1a the two beams
point axially along the artery, hitting an atheroma 106 which partially
blocks the artery defined by the arterial wall 107. In FIG. 1b the tip of
the catheter is bent so that the two beams point in the radial direction
of the artery hitting an atheroma 106b which is seated on the arterial
wall 107b.
The ultrasonic beam is pulsed to obtain range resolution to resolve the
distance to the surface of the atheroma and the arterial wall. In a
typical application the two beams will hit atheroma, 106. The ultrasonic
beam will be partially reflected, but some of the energy will traverse
into the tissue and be partially reflected from within the tissue and
tissue interfaces to give an image of structures inside the atheroma for
determination of the calcification or imaging of the arterial wall. Like
in FIG. 1b some of the energy 105A will traverse the atheroma and hit the
arterial wall 107B and be partially reflected and part of this energy,
105B, will be transmitted further into the tissue. The laser beam, 104,
will, with a proper selection of the wavelength of the laser light, be
absorbed at the surface of the atheroma 106B so that ablation of the
atheroma is obtained. Since the laser beam coincides with the ultrasound
beam direction, the ultrasound image will give an instantaneous depiction
of the effect of the laser light, which can be used as a control of the
laser irradiation.
Using a well known A-mode display of the backscattered ultrasound from a
short transmitted pulse, the atheroma and the arterial or vessel wall can
be depicted as schematically shown in FIG. 5. This can be used to align
the laser beam against the atheroma, observe the effect of the
irradiation, and make sure that damaging of the arterial wall is avoided.
The well known M-mode display of the ultrasound echos can also be used, as
shown in FIG. 6. This gives a better indication of the temporal effect of
the laser irradiation.
In the configuration of FIG. 1a the ultrasound transducer can also be used
for ultrasonic Doppler measurements of the blood velocity in the artery by
which we can monitor the haemodynamic effect of the ablation.
FIGS. 2 and 3 illustrate embodiments where mirrors are used to direct the
ultrasound beam. In FIG. 2 a beam directing means, composed of for
instance a mirror and/or prism arrangement, is used to direct the laser
beam in the same direction as the ultrasound beam, and in FIG. 3 the laser
beam is mirrored in the opposite direction of the ultrasound beam.
In FIG. 2 an ultrasonic transducer, 201, is illustrated, emitting an
ultrasonic beam 202. This beam is reflected by the arrangement 203 which
acts as an ultrasonic mirror, and the beam is bent off at a direction 204.
A laser beam is guided through an optical fiber, 205, and is bent off at
the beam directing arrangement 203 so that the laser beam direction, 207,
after this bending coincides with that of the ultrasonic beam, 204.
In a typical application the two beams will be used simultaneously and hit
the atheroma 208. An arterial wall 209 is also shown. In the same way as
in FIG. 1 an A-mode and M-mode ultrasound image can be generated and used
for guidance of the laser irradiation. By rotating the beam directing
assembly 206, the two beams can be scanned in a plane transverse to the
catheter direction. The ultrasound can then be used to form a
2-dimensional image of the atheroma and the arterial wall as illustrated
in FIG. 7, in which 701 illustrates a cross section of the catheter
arrangement, 702 illustrates the image of the atheroma, 208, and 703
illustrates the image of the arterial wall, 209. This 2-dimensional image
is then used to find atheroma and for control of the laser beam intensity
and pulsing so that atheroma portions of the image are irradiated by laser
light.
Since the two beams are used simultaneously with the same direction, the
ultrasound image will be an instantaneous guide for controlling the
intensity and pulsing of the laser beam, and will also give an
instantaneous monitoring of the effect of the laser irradiation. By high
energy levels of the laser light, debris from the ablation can disturb the
ultrasound imaging. It can then be advantageous to not shoot the laser
light along the same direction at the same time as the ultrasound beam, to
avoid the disturbance in the image.
This can be done in several ways:
(i) With stationary direction of the ultrasound beam and A- or M-mode
imaging, the ultrasound beam and the laser beam can be activated
separately in a time sequence.
(ii) With scanning of the ultrasound beam for 2-dimensional imaging a
similar time sequence can be used by first doing an ultrasound scan to
generate an image, and then moving the beam directing means so that the
laser is pointed towards a selected portion of the ultrasound image for
irradiating atheroma for a selected portion of the time, and then doing
another ultrasound scan to monitor the effect of the laser irradiation.
(iii) The beam directing means can point the ultrasound and laser beams in
different directions in a plane and the beams are scanned and active
simultaneously but at different directions in the plane. The direction of
the laser beam within the ultrasound image is known at any time, so that
the image can be used for guiding the intensity and pulsing of the laser
beam, and the ultrasound beam will shortly afterwards sweep over areas
that have been irradiated by the laser light to monitor the effect of the
laser irradiation.
An arrangement which gives an implementation of the last method is shown in
FIG. 3. In this figure an ultrasonic transducer, 301, is illustrated,
emitting an ultrasonic beam 302. This beam is reflected by the arrangement
303 which acts as an ultrasonic mirror, and the beam is bent off at a
direction 304. A laser beam is guided through the optical fiber, 305, and
is reflected at an angle by the mirror 303 so that the laser beam
direction, 307, after this reflection is opposite to that of the
ultrasonic beam. An atheroma 308 and an arterial wall 309 are also shown.
In a typical application the assembly 306 is rotated to generate a cross
sectional ultrasound image of the artery like in FIG. 7. In the image the
laser beam will then at any instant be in the opposite direction of the
ultrasound beam.
In FIG. 2 and FIG. 3 a second ultrasound transducer 200, respectively 300,
is mounted to the same, acoustically isolating holder 210, respectively
310, as the first transducer 201, respectively 301. This transducer
radiates in the oposite direction of the former, and will thus give a beam
along the artery which can be used for ultrasound Doppler measurements to
monitor the blood velocity downstream from the obstruction for assessment
of the hemodynamic effect of the ablation.
By moving the catheter along the vessel, two-dimensional images of the
artery can be obtained at defined cross sections. This can for example be
done by mounting a longitudinal position sensor on the catheter outside
the body and pulling the catheter out under position control. Cross
sectional images are then stored for defined positions, and these images
can then be used for three-dimensional reconstruction of the vessel and
the atheroma in a manner known per se, and as illustrated in FIG. 8, where
one of the cross sections is indicated at 800.
An example of a system for rotational steering of the beam directing system
together with a third dimension position sensing is shown in FIG. 9. The
optical fiber 901 is enclosed in a bendable steel tubing 902 which
connects the rotor 903 of the motor and the beam directing means 904. As
described earlier the rotating assembly will be covered by the catheter
tube to avoid contact between the moving parts in the catheter and the
tissue. This cover is for the sake of simplicity not shown in the figure.
The stator 905 of the motor is mounted to a suspension (not shown) which
is fixed relative to the incoming laser beam 906 which shines through a
lens 907 in the housing so that the beam is focussed onto the optical
fiber tip 908. A rotary position sensor 909 is connected to the steel
tubing to provide the rotary position of the beam directing system.
Because of the length of the steel tube there can be error in the
monitoring of the angular position of the beam directing arrangement
caused by twisting of the steel tube. This error can be reduced by
mounting a position sensor close to the beam directing system at the far
end tip of the catheter.
The motor and the position sensor can be used in a servo loop for precise
positioning of the beam directing means 904. The beam directing system can
also be rotated manually by for instance turning the wheel 910. An optical
position sensor 911 which reads gratings 912 in the catheter to determine
changes in the longitudinal position of the catheter is schematically
indicated in the figure. The position sensor is mounted outside the
patient body, and the catheter tip 904 can be moved longitudinally within
the artery by pulling the catheter in and out. Two-dimensional cross
sectional images at different longitudinal positions can then be stored
and used for reconstructing a three-dimensional image of the atheroma and
artery. In the figure the catheter is indicated to be straight, but
generally bends of the catheter must exist. For instance when moving the
catheter in and out of the artery through the longitudinal position
sensor, we could move the motor assembly, but with a U-bend of the
catheter between the position sensor and the motor a limited pulling of
the catheter out of the artery can be obtained without moving the motor
assembly.
An arrangement of necessary functional blocks or electronic equipment units
for obtaining a more complete delivery system for intra-arterial laser
angioplasty, is also shown in connection with the catheter in FIG. 4. The
system is set up and controlled through the main controller unit 415,
which can be set up by an operator through a keyboard 416. The main
controller sets up the rest of the system through a controller bus 417
which is composed of multibit address and data signals together with
necessary analog control signals. The bus is assumed to be bidirectional
so that information can flow both from the main controller to the subunits
as well as from the subunits to the main controller.
In the figure a motor 411 is used to move the beam directing system 403.
The motor is driven by the motor controller 412 which feeds a drive signal
413 to the motor. The signal 414 indicates the position of the beam
directing system and is fed back to the motor controller so that the beam
directions can be steered by a servo-like method. For precise adjustment
of the beam directions manual adjustment can also be used either by direct
manual movement of the mechanical linkage to the beam directing system or
via the servo system using the motor as a drive.
The laser is shown as a block 427 attached to the motor. The reason for
this is that the optical coupling into the fiber and the drive link
between the motor and the beam directing system is an integral design. In
a typical situation the laser light is fed from the laser cavity resonator
to the fiber coupling, either through a lens and mirror/prism system or
through a fiber. The light is then focused onto the catheter fiber end by
a mechanical design in conjunction with the linkage between the motor and
the fiber system so that the light enters the fiber even when the beam
directing system is moving. An example of such a coupling has allready
been described in relation to FIG. 9.
In imaging mode the transducer 401 is connected to the pulser 418 and the
receiver 420 via the select switch 419 which is set up by the main
controller 415 via the control bus. The beam direction position signal 414
is also fed back to the main controller 415. Depending on the position of
the beam directing system and the mode selection of the system, the main
controller triggers pulses via the pulser 418 which is converted to an
ultrasound pulse by the transducer 401. These pulses are deflected into
the direction 404 by the beam directing system 403 which acts as an
ultrasonic mirror. The pulses are reflected by the atheroma 408, partially
transmitted into the atheroma and reflected by inhomogeneities in the
atheroma and the arterial wall 409. The reflected signal is also mirrored
by 403 and is picked up by the same transducer and converted to an
electrical signal which is amplified by the receiver 420, and undergoes
further processing by the signal analyzer block 421. This block also
generates display signals 422 which are fed to a display monitor 423.
A typical signal analysis to be done in block 421 is compression and
envelope detection of the received signal which is standard for ultrasound
imaging systems. In a two dimensional imaging mode described in relation
to FIGS. 2 and 3, a typical display on the screen would be as shown in
FIG. 7, while for stationary positions of the beam directing system, an A-
or M-mode display as shown in FIGS. 5 and 6 could be used.
The signal is then further analysed to determine whether the ultrasonic
beam is intersecting atheroma or which portion of the image is atheroma.
This can be done manually by pattern recognition from observing the image
on the screen and then using a cursor to outline the atheroma region, or
automatically through tissue characterisation based on for instance
relative appearance of the signal intensity in the image. The
backscattered signal intensity will increase from any presence of
calcification in the atheroma, so that the intensity can be used to
differentiate calcified atheroma from the normal tissue. Geometrical
deviation of the arterial cross section from the normal circular shape can
also be used to distinguish atheroma from normal tissue.
Having thus, based on the backscattered ultrasound, differentiated between
atheroma and normal tissue, either manually or automatically, a signal 424
is fed to the laser controller 425 which, linked to the direction of the
laser beam relative to what have been determined to represent atheroma in
the image, determines the laser irradiation of the atheroma both by pulse
rate and pulse intensity through a signal 426 to the laser 427, for
precise ablation of the atheroma.
For Doppler measurements the transducer 400 is by the selector 419
connected to the pulser 418 and receiver 420. The signal analysis is then
a standard Doppler signal analysis for determining the blood velocities in
front of the catheter. These velocity tracings are then displayed on the
display monitor 423. The blood velocity measurements can be combined in a
well known way with the imaging of the arterial cross section to determine
volume flow of blood.
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