Laser catheter delivery system for controlled atheroma ablation combining laser angioplasty and intra-arterial ultrasonic imagining

Laser catheter delivery system for controlled ablation of atheroma, combining laser angioplasty and intra-arterial ultrasonic imaging. In laser ablation of atheroma, dosage and directing of the laser light beam represent critical problems, since accurate removal of the atheroma is required without damaging of the arterial wall. The present system comprise a combined laser and ultrasound catheter composed of PA0 apparatus for emitting a laser beam in an artery towards an atheroma for ablation of the atheroma PA0 an optical fiber assembly for feeding laser light from a laser to the emitting apparatus PA0 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 the tissue structures and also arranged to receive backscattered ultrasound from the tissue structures PA0 beam directing apparatus arranged to direct the ultrasound beam towards the 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 to ablate that portion of the atheroma PA1 receive circuits adapted to receive and process the backscattered ultrasound for imaging of the tissue structures, and PA1 apparatus for analysing the ultrasound image, either manually or automatically, to determine the presence of atheroma so that the image can be used to direct the laser beam towards the regions in the ultrasound image which have been determined to represent a portion of an atheroma and determine the intensity and the dosage of the laser beam for accurate ablation of that portion of the atheroma.

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.

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.