U.S. patent application number 10/241741 was filed with the patent office on 2005-01-27 for photoacoustic removal of occlusions from blood vessels.
This patent application is currently assigned to Endo Vasix, Inc.. Invention is credited to Celliers, Peter M., Da Silva, Luiz B., Esch, Victor C., London, Richard A., Maitland, Duncan J. IV, Visuri, Steven R..
Application Number | 20050021013 10/241741 |
Document ID | / |
Family ID | 25497457 |
Filed Date | 2005-01-27 |
United States Patent
Application |
20050021013 |
Kind Code |
A1 |
Visuri, Steven R. ; et
al. |
January 27, 2005 |
Photoacoustic removal of occlusions from blood vessels
Abstract
Partial or total occlusions of fluid passages within the human
body are removed by positioning an array of optical fibers in the
passage and directing treatment radiation pulses along the fibers,
one at a time, to generate a shock wave and hydrodynamic flows that
strike and emulsify the occlusions. A preferred application is the
removal of blood clots (thrombi and emboli) from small cerebral
vessels to reverse the effects of an ischemic stroke. The operating
parameters and techniques are chosen to minimize the amount of
heating of the fragile cerebral vessel walls occurring during this
photoacoustic treatment. One such technique is the optical
monitoring of the existence of hydrodynamic flow generating vapor
bubbles when they are expected to occur and stopping the heat
generating pulses propagated along an optical fiber that is not
generating such bubbles.
Inventors: |
Visuri, Steven R.;
(Livermore, CA) ; Da Silva, Luiz B.; (Danville,
CA) ; Celliers, Peter M.; (Berkelley, CA) ;
London, Richard A.; (Orinda, CA) ; Maitland, Duncan
J. IV; (Lafayette, CA) ; Esch, Victor C.; (San
Francisco, CA) |
Correspondence
Address: |
K. Alison de Runtz
Skjerven Morrill LLP
Three Embarcadero Center, 28th Floor
San Francisco
CA
94111
US
|
Assignee: |
Endo Vasix, Inc.
|
Family ID: |
25497457 |
Appl. No.: |
10/241741 |
Filed: |
September 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10241741 |
Sep 10, 2002 |
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09621244 |
Jul 21, 2000 |
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09621244 |
Jul 21, 2000 |
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09604454 |
Jun 27, 2000 |
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6428531 |
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09604454 |
Jun 27, 2000 |
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08955858 |
Oct 21, 1997 |
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Current U.S.
Class: |
606/15 ; 604/20;
606/10; 606/7 |
Current CPC
Class: |
A61B 18/245 20130101;
A61B 2018/20359 20170501; A61B 2018/2211 20130101 |
Class at
Publication: |
606/015 ;
606/007; 604/020; 606/010 |
International
Class: |
A61B 018/18 |
Goverment Interests
[0001] The United States Government has, rights in this invention
pursuant to Contract No. W-7405-ENG-48 between the United States
Department of Energy and the University of California for the
operation of Lawrence Livermore National Laboratory.
Claims
It is claimed:
1. A method of opening to the flow of blood a human cerebral blood
vessel that is blocked by a clot, comprising: positioning within
said cerebral vessel an array of optical fiber ends adjacent to and
extending in a direction across a surface of the clot, said optical
fibers individually having a core diameter less than 100 microns,
directing a sequence of one or more pulses of radiation along one
of the plurality of optical fibers and out of its end and then
repeating directing such a sequence of pulses along individual ones
of others of said plurality of optical fibers at a time, said
pulses individually containing less than 250 micro-Joules of energy
and having a duration less than 100 nanoseconds in order to
generate a shock wave followed by a bubble which together cause a
portion of the clot to be emulsified, introducing a flow of liquid
into the vessel adjacent the surface of the clot while radiation is
being directed out of said optical fibers, and while so directing
the radiation pulses and the liquid, advancing the catheter end
through the clot as the clot becomes emulsified until the blockage
to the flow of blood through the vessel is removed.
2. The method according to claim 1, wherein repeating directing the
sequence of pulses includes directing said pulses in sequence along
adjacent ones of said plurality o,f optical fibers.
3. The method according to claim 1, wherein repeating directing the
sequence of pulses includes directing said pulses in sequence along
said Plurality of optical fibers that are not adjacent to each
other.
4. The method according to claim 1, wherein directing the sequence
of pulses includes directing said pulses along individual ones of
said plurality of optical fibers in an order that, at any instant,
directs radiation against a coolest of a plurality of regions
across the clot that are illuminated by the individual fibers of
the array.
5. The method according to claim 1, wherein directing a sequence of
one or more pulses of radiation along individual ones of the
optical fibers includes directing a burst of a plurality of pulses
along the individual optical fibers with a repetition rate of one
kilo-Hertz or more.
6. The method according to any one of claims 1-5, wherein
positioning the array of optical fibers within the cerebral vessel
includes inserting a catheter containing the array of optical
fibers into a vessel of the human body a distance removed from said
cerebral vessel and advancing the catheter through various vessels
a distance of at least 50 centimeters to reach the cerebral vessel
clot.
7. The method according to claim 6, wherein inserting and advancing
the catheter includes using advancing a catheter having an outside
diameter of one-half millimeter or less at least for a portion of
its length positioned within cerebral vessels.
8. The method according to claim 7, wherein introducing a flow of
liquid into the cerebral vessel adjacent the clot includes passing
the liquid through a lumen of the catheter that extends along the
catheter length for a distance of at least 75 centimeters from the
optical fiber ends.
9. The method according to claim 8, wherein introducing a flow of
liquid into the cerebral vessel additionally includes passing
liquid through the catheter lumen at a rate of at least one-tenth a
cubic centimeter per minute.
10. The method according to claim 9, wherein an average power of
the radiation directed along the optical fibers to within said
cerebral vessel is less than one-half of one watt.
11. The method according to claim 10, wherein positioning the array
of optical fiber ends within said cerebral vessel includes
positioning optical fibers that individually have a core diameter
of 50 microns or less.
12. The method according to any one of claims 1-5, wherein an
average power of the radiation directed along the optical fibers to
within said cerebral vessel is less than one-half of one watt.
13. The method according to any one of claims 1-5, wherein the flow
of liquid introduced into the vessel is within a rate of from
one-tenth to five cubic centimeters per minute.
14. The method according to claim 13, wherein an average power of
the radiation directed along the optical fibers to within said
cerebral vessel is less than one-half watt.
15. The method according to any one of claims 3-5, wherein
positioning the array of optical fiber ends within said cerebral
vessel includes positioning optical fibers that individually have a
core diameter of 50 microns or less.
16. The method according to any one of claims 1-5, wherein the
generation of individual bubbles is optically monitored through the
same optical fibers that carry the pulses which generate the
bubbles, and, in response to a failure to detect the existence of a
bubble being generated by a pulse directed along one of the optical
fibers, suppressing subsequent pulses from being directed along
said one optical fiber.
17. The method according to claim 16, wherein suppressing
subsequent pulses includes suppressing a predetermined number of
pulses from being directed along said one optical fiber, after
which directing the pulses along said one optical fiber and the
monitoring of bubbles resumes.
18. The method according to claim 5, wherein the generation of
individual bubbles is optically monitored through the same optical
fibers that carry the pulses that generate the bubbles, and, in
response to a failure to detect the existence of a bubble being
generated by a first pulse of a burst of pulses being directed
along one of the optical fibers, suppressing those pulses after the
first pulse from being directed along said one optical fiber.
19. A method of opening to the flow of blood human cerebral blood
vessel that is blocked by a clot, comprising: directing
electromagnetic radiation through optical fiber transmission media
within the cerebral vessel toward said clot at different locations
across the clot in time sequence, simultaneously directing a
cooling liquid within the cerebral vessel in the vicinity of the
clot, and maintaining an average power level of the radiation
directed within the cerebral vessel at less that one-half of one
watt.
20. The method according claim 19, wherein the cooling liquid is
directed into the cerebral vessel at a rate of flow within a range
of from one-tenth to two cubic centimeters per minute.
21. The method according to either of claims 19 and 20, wherein
directing electromagnetic radiation includes directing said
radiation through a plurality of optical fibers, one at a time,
which individually have a core diameter less than 100 microns.
22. The method according to either of claims 19 and 20, wherein
directing electromagnetic radiation includes directing said
radiation in a manner to generate within the cerebral vessel a
succession of a combination of a shock wave and vapor bubble that
combine to emulsify the clot.
23. A method of opening to the flow of blood a human cerebral blood
vessel that is blocked by a clot, comprising: positioning an end of
a catheter into a blood vessel of the human a distance from the
cerebral blood vessel and advancing the catheter end at least 75
centimeters through various human vessels to within the cerebral
vessel in a manner to position an open end of a lumen and ends of a
plurality of optical fibers directed toward or imbedded in the
clot, said optical fibers individually having a core diameter
within a range of from 20 to 100 microns, directing radiation along
the plurality of optical fibers, one at a time in sequence, against
the clot while a liquid is being discharged at a rate within a
range of one tenth to five cubic centimeters per minute into the
vessel through said lumen open end, said radiation being directed
along each of the plurality of fibers in the form of a plurality of
pulses with a repetition rate within a range of from 1 to 20
kilo-Hertz, an amount of energy per pulse within a range of from 10
to 250 micro-Joules, and a duration of the individual pulses within
a range of from 1 to 100 nanoseconds, in a manner that the pulses
individually generate a shock wave followed by a bubble that
together cause a portion of the clot to be emulsified and the
average power delivered within the cerebral vessel is less than
one-half of one watt, and advancing the catheter end through the
clot as the clot becomes emulsified until the blockage to the flow
of blood through the vessel is removed.
24. A system for the removal of a clot from a blood vessel,
comprising: a catheter having a length in excess of 75 centimeters
between first and second ends thereof and an outside diameter less
than one-half of one millimeter alone at least a portion of the
length adjacent the first end, said catheter including a plurality
of optical fibers that individually have a core diameter less than
100 microns and a lumen extending along said length, said optical
fibers terminating in a spatial array across a first end of the
catheter, a source of liquid connected to supply cooling liquid to
the lumen at the second end of the catheter, the lumen having an
inside diameter and the source having a capacity such that the
liquid is discharged from the lumen at the first end of the
catheter with a rate of flow within a range of from one-tenth to
five cubic centimeters per minute, and a source of electromagnetic
radiation connected to the optical fibers at the second end of the
catheter in a manner to direct individual pulses of said radiation
along the optical fibers, one at a time in sequence, with the
pulses individually having a duration within a range of from one to
100 nanoseconds and containing an amount of energy within a range
of from 10 to 250 micro-Joules of energy, and with a maximum
average power of less than one-half of one watt being delivered
from the optical fibers at the first end of the catheter.
25. The system of claim 24, wherein said source of electromagnetic
radiation directs a burst of a plurality of pulses along one of the
optical fibers before switching to another of the optical fibers,
the burst of pulses being supplied at a frequency within a range of
from one to 50 kilo-Hertz.
26. The system of either one of claims 24 or 25, wherein said
source of electromagnetic radiation directs said pulses
sequentially along individual ones of the plurality of optical
fibers across the spatial array that are not adjacent one another.
Description
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to the removal of a partial
or total occlusion from a blood vessel by generating pressure waves
within the vessel through optical fiber media, and, more
specifically, to the removal of a blood clot from a vessel within
the human brain. The term "clot" is used herein to refer to a
thrombus, embolus or some other total occlusion of a vessel.
[0003] Medical procedures to open a partially or totally blocked
blood vessel are available. Angioplasty has long been used to
restore full blood flow in a coronary artery by mechanically
deforming deposits on the arterial walls but has been less
successful to open a totally occluded vessel. Laser techniques have
been proposed to directly ablate obstructing material from
arteries, such as plaque and certain types of clots, by inserting
optical fibers into the artery to the point of the obstructing
material but these techniques have enjoyed only limited success in
practice. Various uses of ultrasonic energy to generate acoustic
waves directed against plaque or a clot within an artery to
mechanically break up the obstructing material have also been
proposed but medical procedures utilizing these techniques have not
enjoyed widespread acceptance. Photoacoustic techniques have been
proposed for vasodilation and the break-up of plaque and clots in
arteries, wherein one or more optical fibers are inserted into the
vessel and pulses of radiation delivered to the vessel through the
fibers generate a pressure or acoustic wave directed against the
obstruction.
[0004] Major blood vessels within the brain are very small,
generally not exceeding three millimeters in diameter and being
much smaller than that in most places. Most cerebral blood vessels
decrease in diameter along their lengths until becoming
capillaries. Besides being small, the walls of cerebral vessels are
more fragile than those of vessels in other parts of the body and
are more loosely connected to surrounding tissue.
[0005] When a thrombus is formed or an embolus is lodged in a blood
vessel of the brain, an ischemic stroke results. The resulting
sudden cut off of the supply of fresh blood to cerebral vessels
terminates the supply of oxygen to these vessels and to the brain
tissue they supply. The seriousness of a stroke depends upon the
amount of brain tissue involved and its location. Generally, the
more serious strokes result when the larger cerebral vessels become
blocked, since they supply more volume of tissue than the smaller
vessels, but the blockage of vessels having a diameter of less than
one millimeter, or even one-half of one millimeter or less, can be
quite serious.
[0006] If a cerebral vessel of a stroke victim can be unblocked
within about six hours after the blood flow is totally stopped, the
effects of the stroke on the oxygen starved brain tissue are often
largely reversed. If unblocked within this time, deterioration of
the walls of the blocked vessel to the point of hemorrhaging is
prevented. As a result, many have tried to develop techniques for
removing clots from cerebral vessels within a few hours after a
stroke has occurred.
[0007] One such technique is to position a catheter into the
blocked vessel to mechanically remove the clot. But this is very
difficult to do without causing further damage because the vessels
are so small, contain very sharp turns, are weakly constrained and
have fragile walls. Alternatively, a lytic drug is often applied
intravenously, in an attempt to dissolve the clot without having to
dislodge it mechanically. In an attempt to improve the rate of
success of the lytic drug, it has been introduced directly into the
blocked vessel through a catheter at the point of the blockage. But
none of these techniques have enjoyed a high rate of success.
[0008] Therefore, it is a primary object of the present invention
to provide techniques for reopening clotted blood vessels of the
human brain with an increased rate of success.
[0009] It is another important object of the present invention to
provide techniques to remove partial or total occlusions from other
parts of the body.
[0010] It is a further object of the present invention to provide
techniques for removing obstructions from the human body,
particularly clots from cerebral blood vessels, without causing
collateral damage to the vessel.
[0011] It is another object of the present invention to provide a
practical instrument and system to perform these functions.
SUMMARY OF THE INVENTION
[0012] These and other objects are accomplished by the various
aspects of the present invention, wherein, briefly and generally, a
catheter containing multiple small diameter optical fibers
terminating in a two-dimensional pattern is positioned adjacent the
occlusion and pulses of radiation are directed along the optical
fibers, one at a time in sequence, with the individual pulses
having a duration and amount of energy sufficient to generate a
shock wave and, from an expansion and collapse of a bubble, a
pressure wave, both of which are directed against the obstruction
in order to break it up and restore the flow of blood through the
vessel. Clots within either arteries or veins are emulsified in
this manner.
[0013] It has been found that the use of very small diameter
optical fibers allows the desired shock and pressure waves to be
generated with a relatively low amount of radiation pulse energy,
thereby keeping the amount of heat input to the vessel at a low
level. Proper thermal management according to the present invention
reduces the likelihood of damaging the walls of the blood vessel
adjacent the occlusion, which is especially important for the
relatively thin walled vessels of the brain. Further, it is
desirable that radiation pulses not being efficiently converted
into the desired pressure waves be terminated in order to prevent
inputting energy that heats the region without doing useful work.
In addition to keeping the power input low, a liquid coolant may be
introduced through the catheter to carry heat away from the region
of the occlusion during the treatment.
[0014] Additional objects, features and advantages of the various
aspects of the present invention will be better understood from the
following description of its preferred embodiments, which
description should be taken in conjunction with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 illustrates an application of the present invention
to remove a clot from a blood vessel in the brain, by use of a
multi-optical fiber delivery system;
[0016] FIG. 2 shows a catheter of the present invention positioned
in a blood vessel to emulsify a clot;
[0017] FIG. 3 is a perspective view of the catheter and delivery
system of FIGS. 1 and 2;
[0018] FIG. 4 is a cross-sectional view of an end of an optical
fiber used in the catheter of FIG. 3;
[0019] FIG. 5 show curves of the spectral radiation absorption by a
blood vessel wall and a thrombus;
[0020] FIG. 6 schematically illustrates the exposure of the clot by
radiation from the multiple optical fibers terminating in the end
the catheter of FIG. 3;
[0021] FIGS. 7A-E schematically illustrate in time sequence the
formation of shock and pressure waves by one of the optical fibers
of the catheter of FIG. 3;
[0022] FIG. 8 includes a family of curves showing the amount of
radiation pulse energy required to generate bubbles of various
sizes for various sized optical fibers of the catheter of FIG.
3;
[0023] FIG. 9 is a three-dimensional graph that provides a
comparison of a preferred range of parameters used in the present
invention to a typical set of parameters used in the prior art;
[0024] FIG. 10 is an electro-optical diagram of the instrument
shown in FIG. 1;
[0025] FIG. 11 is an electronic circuit block diagram of the system
control of the instrument system of FIG. 10;
[0026] FIGS. 12A-I form a timing diagram showing various signals of
the system control circuit of FIG. 11; and
[0027] FIGS. 13A-E show a portion of the timing diagram of FIG. 12
with an expanded scale.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0028] The present invention may, in general, be applied to the
removal of material forming a partial or total occlusion of any
human vessel but is particularly directed to opening a blood vessel
that is totally or substantially blocked to the flow of blood. More
specifically, the preferred embodiment of the present invention is
directed to the removal of a clot from a blood vessel in the brain
that has caused an ischemic stroke. If the flow of blood is,
restored in the vessel within a few hours of the onset of the
stroke, permanent damage to the blocked vessels is avoided.
[0029] Before applying the techniques of the present invention to a
patient with symptoms of a stroke, a physician first determines
whether the stroke has been caused by a hemorrhage or a blockage of
a cerebral vessel. This is usually determined by use of a standard
computed tomography (CT) x-ray test. If it is determined by the CT
test that the stroke has been caused by a blocked cerebral vessel,
the blockage is located by use of a standard angiography test. This
test may also be used to determine whether the blockage is a clot.
This test is performed by injecting an x-ray contrast liquid into
the vessels of at least that portion of the brain whose diminished
function is believed to be responsible for the stroke, while taking
x-rays of the brain. If a blockage exists in a vessel, a network of
vessels beyond the blockage will not appear in the x-ray since the
contrast liquid is prevented from flowing past the blockage. The
vessel and position of the clot or other obstruction within the
vessel is accurately determined in this manner.
[0030] A multiple optical fiber catheter having a lumen for
carrying a cooling liquid is then inserted into that vessel with
its end adjacent the blockage. One such insertion is illustrated in
FIGS. 1 and 2. An elongated catheter 11 is inserted into a femoral
artery 13 (FIG. 1) of a patient 15 and maneuvered through the
appropriate arteries until an end 39 of the catheter (FIG. 2) is
positioned within a blocked vessel 41, one of the cerebral vessels
17, adjacent a clot 43. The same insertion technique may be used
here as presently used to introduce a catheter to inject a lytic
drug directly into a clot in at attempt to dissolve it. A lytic
drug may optionally be supplied through the lumen of the catheter
11 in order to dissolve at least some of the small particles that
result from the emulsification of the clot by photoacoustic action.
But use of the drug alone has not been found to be particularly
effective.
[0031] The system of which the catheter 11 is a part is also shown
generally in FIG. 1. A manifold 19 connects the lumen of the
catheter 11 to a hose 21 having a connector 23 for removably
connecting the hose 21 to a supply of liquid. The supply shown in
FIG. 1 includes a pump 25 and reservoir 27. The primary purpose of
delivering the liquid through the catheter is to remove heat from
the region of the cerebral vessel that is being deposited by the
emulsification process described below. This is part of the overall
control of the heat flow in the region that has a purpose of
avoiding thermal damage to the thin cerebral vessel walls. These
small vessels are particularly susceptible to such damage if the
photoacoustic process is not properly controlled. The flow of
liquid into this region also helps in carrying away small particles
of the clot that result from its emulsification, and keeps the ends
of the optical fibers free of debris. The liquid may be isotonic
saline or water, or some other biocompatable cooling agent of a
type commonly used in medicine. Optionally, as mentioned above, a
lytic drug can be included in the liquid to assist in dissolving
these small particles. A lytic drug is more effective to dissolve
the particles of the clot than in dissolving the clot itself since
there is a greatly increased surface area to absorb the drug.
[0032] The manifold 19 also extends the optical fibers of the
catheter 11 as a bundle 29 to a multi-fiber connector 31 that is
removably connected to an instrument 33. This instrument contains
the optics and electronics required to perform the medical
procedure. Included on its face are various control switches or a
keypad 35 and a display 37.
[0033] Referring to FIG. 3, the delivery system of FIG. 1 is shown
by itself. This delivery system is replaceable, since it detaches
from both the liquid pump 25 and instrument 33, and is optionally
disposable after one use. Six optical fibers 45-50 terminating at
one end in the connector 31 are shown to terminate at their other
ends at the catheter end 39 by surrounding an opening of a lumen 51
that forms a fluid passage between the catheter end 39 and the hose
21. The optical fibers are attached to an inside surface of an
outer flexible shell 53 of the catheter 11. Optionally, the lumen
51 is formed by a cylindrically shaped shell 55 on an inside of the
optical fibers 45-50. The shell 55 may be omitted and the optical
fibers need not be attached along the entire length of the catheter
11 in the same manner shown in FIG. 3 at its end 39. Although six
optical fibers are shown for the purpose of this description, and
will generally be the least number of fibers used, fewer or more
fibers may be included. Generally, the ends of the optical fibers
are spaced equidistant around the circumference of a circle at the
end 39 of the catheter 11 around the lumen 51 but some other
arrangement could be used.
[0034] Many of the cerebral vessels 17 (FIG. 1) in which it is
desired to remove blockages are less than one millimeter in inside
diameter (ID), and even as small as one-half of one millimeter, and
seldom larger that three millimeters in inside diameter, as shown
for the vessel 41 in FIG. 2. Therefore, an outside diameter (OD) of
the catheter 11 (FIGS. 2 and 3), at least for a portion of its
length adjacent its end 39 that passes through the cerebral
vessels, must be small enough to maneuver the sharp turns of those
very small vessels. The flexibility of at least that end length of
the catheter must also be adequate to allow it to travel through
the sharp turns of the small vessels, while at the same time being
strong enough along its length to permit it to be pushed from
outside of the patient. The distance that the end 39 of the
catheter 11 travels from its insertion into the femoral artery 13
until reaching a clot within the brain is at least 50 or 75
centimeters for a child or small adult, averaging about 90
centimeters for an average sized adult. Therefore, the length of
the catheter 11, between the manifold 19 and its end 39, is
preferably at least 90 cm. for use with adults but can be as short
as 50 or 75 cm. for use children or small adults. A usual length
will generally be about 190 cm. in order to accommodate the
additional use of manifolds for other purposes, such as to
introduce a contrast fluid, when performing the procedures being
described herein.
[0035] This combination of flexibility to bending and strength
along its length can be accommodated in a catheter having an
outside diameter (OD) of less than one-half a millimeter, a
diameter within a range of 300 to 450 microns being preferred, with
a 350 micron diameter being typical. A number of different designs
for a long catheter of this diameter can be employed in order to
provide the desired combination of flexibility and longitudinal
strength. This includes the choices of materials and their
thicknesses that are made, whether the lumen tube 55 is used,
whether the optical fibers are attached to the catheter structure
along the catheter other than at the end 39, and similar factors.
Flexibility is improved when the optical fibers 45-50 are
unattached along most of the length of the catheter within the
lumen 51 but this can have the effect of restricting the amount of
flow of liquid that is practical through the lumen and the
longitudinal strength is not as great. The outside diameter and
number of optical fibers used also affects these issues. A proper
balance of these competing goals is achieved in a useful catheter
assembly. The same considerations apply if a larger catheter is
used, which could have up to a one millimeter outside diameter for
cerebral or other vessels.
[0036] Each of the optical fibers 45-50 is chosen to be very small
in diameter for reasons given below but this also contributes to
the flexibility of the catheter 11 A cross-sectional view of a
short length of an end of each of these fibers is shown in FIG. 4.
A cylindrically shaped glass core 61 is surrounded by a glass
cladding 63, which in turn is covered by a plastic sheat 65.
Alternatively, one or both of the core and At cladding may be
plastic. In one specific embodiment, the diameter of the core 61 is
50 microns, with an overall outside diameter of 65 microns. Even
smaller optical fibers are contemplated. A difference in the
refractive indices of the materials of the core 61 and 35 cladding
63, as is well known, defines the numeric aperture of the fiber. An
angle a that defines the shape of a spreading cone 67 of radiation
leaving the fiber end increases as this refractive index difference
increases. This difference is chosen to be high for another reason,
however. That is to increase the internal reflection within the
core when the fiber is bent. This reduces radiation losses through
the cladding when the fiber is bent through the sharp turns of the
cerebral vessels in which it is positioned. For this purpose,
materials are chosen for the core and cladding that have a
difference in their refractive indices that is as high as
practical. As the refractive index difference is increased, so is
the difficulty and cost of making the optical fiber. Refractive
index values that result in a numerical aperture in excess of 0.20
are practical, such as a numerical aperture of 0.22 or even
0.29.
[0037] With reference to FIG. 2, an introduction to how the
catheter 11 is used to remove or provide an opening through the
clot 43 is given. Radiation directed along each of the optical
fibers 45-50 is converted into pressure waves within the vessel by
a photoacoustic process. These pressure waves mechanically break
the clot 43 apart by an emulsification process, with resulting
small particles harmlessly traveling away from the site through the
blood in the vessel 41. If the end 39 of the catheter 11 is spaced
a distance away from the end surface of the clot 43 as is shown in
FIG. 2, the repetitive pulses of radiation from the optical fibers
are converted into pressure waves in liquid within the vessel 41,
which is usually some combination of blood and cooling liquid along
with small particles of the clot. The absorption of the radiation
in this liquid depends upon the relative proportions of these three
constituents. The cooling liquid is usually non-absorptive, and the
blood and clot particles have similar spectral absorption
characteristics. The higher the absorption, the stronger the
pressure waves generated in the liquid for a given amplitude of
radiation pulses.
[0038] Alternative to the radiation pulses being absorbed by the
liquid, they may be absorbed by the clot 43 (FIG. 2) if the end 39
of the catheter 11 is positioned in the clot. The photoacoustic
process then takes place within the clot. This will more often
result when the clot 43 is a thrombus since a thrombus is generally
soft. It is not difficult for the attending physician to urge the
catheter a distance into the clot. In practice, the physician moves
the catheter around in the vessel 41 during treatment, both across
the face of the clot 43 and back and forth along the length of the
vessel 41. The radiation pulses are then absorbed by the liquid
some of the time and by the clot some of the time. As the clot is
emulsified, the end 39 of the catheter 11 is moved by the physician
against a disintegrating face of the clot until the end 39 has
moved completely through it. This process does take some time since
the clot 43 can have a substantial length. If the clot is a
thrombus, it typically will be from one to four or more centimeters
in length along the vessel 41.
[0039] Although it is desired that the clot 43 be highly absorptive
of the radiation pulses, it is also desired that the wall of the
blood vessel have a low absorption since it is difficult to prevent
the radiation pulses being directed against the vessel wall, at
least for an instant, as the catheter 11 is manipulated by the
physician. The prevention of damage to the vessel wall is an
important goal of the present invention. Fortunately as shown in
the curves of FIG. 5, a typical thrombus is much more absorptive
than the vessel walls to electromagnetic radiation within the
visible portion of the spectrum. A maximum difference in
absorption, as can be seen from FIG. 5, Occurs at a radiation
wavelength of about 415 nanometers. In a practical instrument,
however, a wavelength of about 532 nanometers (green) is used
because lasers generating that wavelength are readily available,
small in size, economical, trouble free and easy to use. A
frequency doubled Q-switched laser with a neodymium (Nd.sup.3+)
doped host material provides the treatment pulses of radiation
having a wavelength of about 532 nanometers, depending upon the
host material. YAG (yttrium aluminum garnet) or YLF (yttrium
lithium fluoride) are examples of suitable hosts.
[0040] Referring to FIG. 6, the relative positions of an end of the
clot 43 being treated and the end 39 of the catheter 11 are shown
schematically. Dotted circles 45'-50' generally illustrate regions
of the strongest portions of interaction of the shock and pressure
waves that are generated by radiation pulses from respective
optical fibers 45-50. In general, when operating within the
parameter ranges described below, the individual regions 45'-50'
have about twice the diameter of the cores of the respective
optical fibers 45-50. It is useful if the regions 45'-50'
substantially meet but the effects of any gaps in regions of
emulsification of the clot 43 are overcome by the physician moving
the catheter 11 around during the treatment. Such movement is
necessary, in any event, since the catheter end 39 is smaller than
the clot 43, and emulsification across the entire surface is
desired. Indeed, the end 39 of the Catheter 11 can be eccentrically
shaped so that rotation of the catheter by the physician causes the
end 39 to move across the surface of the clot 43.
[0041] Several of the specific techniques of the present invention
have a purpose of minimizing the elevation of the temperature of
the vessel wall in order to avoid damaging the wall. One such
technique is to direct; radiation pulses along only one of the
multiple fibers at a time. Another is to limit the number of
successive radiation pulses from a single fiber, before switching
to another, in order to avoid creating a "hot spot" that heats the
vessel wall by conduction or convention. A single pulse from each
fiber in sequence minimizes any hot spots but is not as effective
in emulsifying the clot. The best emulsifying action occurs when
the shock and pressure waves repeatedly impinge upon a common area
of the exposed clot end surface at a high rate. In addition to
maintaining a beneficial turbulence initiated by a set of shock and
pressure waves, an extended series of such waves results an more
finely emulsifying larger particles that are initially broken away
from the clot before they drift too far away from the clot along
the vessel. The smaller the particles resulting from the
emulsification, the less risk that a particle will lodge somewhere
else to block the same or another vessel.
[0042] Yet another thermal management technique involves directing
successive bursts of pulses along fibers that are removed from one
another in order to spread out the heat that is generated. For
example, pulses from the fiber 46 can follow those from the fiber
49, followed by pulses from the fiber 47, then from the fiber 50,
and so on, generally in a star pattern. Whatever specific sequence
is used, it is usually desirable to have one fiber in between two
fibers that carry radiation pulses in successive periods of time.
In general, with reference to FIG. 6, one of the fibers 45-50
chosen to deliver the radiation at any instant time is that which
illuminates the coolest of the respective regions 45'-50' across
the clot 43. If two or more of the regions are about the same
temperature, then the region to receive radiation is randomly
selected from the two or more coolest regions. The relative
temperature of each region is dependent upon the amount of time
since it and adjacent regions have been exposed, because each
region receives heat-by conduction from its adjacent regions as
well as by absorbing radiation incident upon it. A particular
sequence of illumination of the fibers can be intuitively
established or determined by mathematically modeling the heat
absorption and transfer characteristics for the clot and/or liquid
being illuminated, in order to minimize the temperature rise within
the vessel.
[0043] Although such skipping techniques can be the best for
thermal management, it is not always the best for efficient
emulsification. Especially when each burst of pulses through one
fiber is only a few, or even just one pulse, before moving to the
next fiber, it can be more efficient to direct such bursts through
adjacent fibers so that the turbulence created from one fiber is
built upon by pulses from the adjacent fiber, rather that moving to
a fiber so far away that a momentum of emulsification must be
started all over again. This also operates to allow pulses from one
fiber to more finely emulsify at least some of any larger particles
earlier broken away from the clot by pulses from an adjacent
fiber.
[0044] The ultimate goal is to remove the clot with the least
amount of heat being generated. When one set of radiation pulses is
not as efficiently emulsifying the clot as another set, it will
take more pulses overall to remove the clot and thus deliver a
greater amount of heat in the process. There is thus a balance that
is desired to be achieved between the direct reduction of heat
input to the region of the clot from a particular spatial pattern
of exposure to radiation pulses and a reduction of the amount of
heat generated when the radiation pulses are used more efficiently.
It may even be of some advantage to direct radiation pulses out of
two or more of the optical fibers at one time but this is not
preferred. Whatever pulse sequence is implemented, it is controlled
by the electro-optical system within the instrument 33.
[0045] Referring to FIGS. 7A-E, the effect believed to result from
one radiation pulse being directed out of a single optical fiber
core 71 against an exposed face of a clot 73 is explained. In this
example, the radiation is absorbed by the liquid in front of the
clot 73. The effects will be similar if the absorption is in the
clot itself. In either case, radiation is absorbed according to an
absorption coefficient of the material in which the radiation is
directed, and this absorbed energy superheats water within the
material. According to the present invention, each pulse contains a
small amount of energy, in order to minimize the amount of heat
generated in the region, but is delivered by a pulse having a very
short duration. This increases the efficiency of the process, which
is expressed in terms of the mass of the clot that is emulsified
per unit of laser pulse energy delivered to the treatment site
within the vessel.
[0046] Very shortly after the pulse has been delivered, as shown in
FIG. 7A, a volume 75 of liquid immediately adjacent the end face of
the fiber core 71 is superheated in a manner to generate a shock
wave 77 that is directed against the clot surface 73. A shock wave
is characterized by traveling at a speed greater than the speed of
sound in the same medium. The shock wave does not contain a great
deal of energy but is believed to be quite useful because of a very
sharp change in pressure that occurs. In order to generate the
shock wave, the radiation energy is deposited into the volume 75 in
a time that is shorter than this volume can expand to relieve the
increased pressure. Therefore, the radiation pulse is made to have
a very short duration.
[0047] A short time later, as shown in FIG. 7B, a bubble 79 has
started to form and a hydrodynamic effect takes place that includes
a pressure wave and mass flow 81 being directed against the clot 73
by the bubble's growth. This flow travels at less than the speed of
sound but contains considerably more energy than the shock wave.
Depending upon the amount of radiation energy deposited, the bubble
79 is a "vapor" (higher energy) or a "cavitation" (lower energy)
type of bubble. At a subsequent instant of time shown in FIG. 7C,
the bubble 79 is of a maximum size, and then, as shown in FIG. 7D,
begins to collapse as its interior pressure drops below the
pressure of the surrounding material and the ambient pressure
overcomes the kinetic energy of the hydrodynamic flow. This
collapse causes a hydrodynamic effect including mass flow and a
pressure wave 83 moving in a direction opposite to that of the
initial hydrodynamic flow 81. If the bubble is symmetrically formed
as shown in FIG. 7C, another shock wave also results from this
collapse. This is a complex dynamic process where, in very
simplified terms, the bubble expands from the energy input to the
fluid, then cools and collapses after the energy input pulse is
terminated.
[0048] It has been found that the efficiency of the emulsification
process is-improved when both of the shock wave and hydrodynamic
effect are generated by individual radiation pulses but only one or
the other of them may be satisfactory for some applications and/or
circumstances. At a later time of FIG. 7E, equilibrium again exists
but only after some of the clot surface 73 has been broken away in
response to the shock wave(s), hydrodynamic flow and liquid
turbulence created by the mechanical motion of the flow. This
process is repeated by each of subsequent radiation pulses.
[0049] Preferred Process Parameters
[0050] In order to remove a clot without creating thermal effects
that have a potential of damaging a vessel wall, certain ranges of
relative parameters have been discovered to work best. As mentioned
above, it is a goal to have an efficient process. This minimizes
the amount of laser energy required, and thus the cost and
complexity of the laser source used in the instrument, and also
minimizes the amount of time required to remove a clot. Maximizing
the efficiency is possibly most important in minimizing the heat
imparted to the treatment site in the course of removing a given
volume of the clot, thus reducing the chance for tissue damage,
particularly to the thin blood vessel walls.
[0051] A first parameter of interest is the size of the individual
optical fibers 45-50, which are preferably made to be the same. It
has been found that efficiency is increased by using smaller
fibers, contrary to what one might initially think. Optical fibers
with a core diameter of 200 microns or less can be used but those
with a core diameter of 100 microns or less are preferred. The
fiber cores must be large enough, however, to withstand the
destructive effects on the fiber of the shock wave and hydrodynamic
flow being generated at its tip. Depending upon the other
parameters, the smallest core diameter that is practical is about
20 microns. Another factor that affects the minimum size of the
optical fiber is commercial availability and cost. Optical fibers
with the 50 micron core diameters mentioned above are available,
and 25 micron core fibers may soon be available at a reasonable
cost
[0052] It has been found, as illustrated by the family of curves of
FIG. 8, that the size of the bubble generated, and thus the
intensity of the pressure waves generated by its expansion and
collapse, is controlled by more than the fiber size and can be made
much bigger than the size of the fiber by use of appropriate levels
of energy per pulse. In a specific example of a fiber with a 50
micron core diameter, a low energy of 100 micro-Joules per pulse
generates a bubble having a maximum diameter (FIG. 7C) of 120
microns. This is read from the 50 micron curve of FIG. 8. It will
also be noted from FIG. 8 that if it is desired to generate a
certain bubble diameter, increasing the size c)f the optical fiber
also requires increasing the amount of energy per pulse. Thus, the
same amount of work can be performed by a bubble generated through
a smaller fiber with a lower level of energy. The lower level of
energy means that the heat deposited into the vessel in the region
of the clot (treatment site) is also reduced, thus contributing to
the goal of increased efficiency.
[0053] The amount of radiation energy delivered from the end of a
single optical fiber by each of the individual pulses is chosen
from the curve of FIG. 8 for the diameter of the core of the
optical fiber being used. (Of course, other curves can-be added for
other 75 than the 50, 100 and 200 micron core diameters shown.) A
lower limit is that which will generate both the initial shock wave
(FIG. 7A) and the bubble induced hydrodynamic flow (FIGS. 7B-D)
since it has been found most efficient to use both in the
emulsification process. This lower limit is about 10 micro-Joules
for very small optical fibers and 50 micro-Joules for others, with
100 micro-Joules being usable with a 50 micron core diameter fiber,
for example. In general, it is desired to provide as much energy as
possible in each radiation pulse since it takes a substantial
amount of base energy to raise the temperature of the material
adjacent the optical fiber end to the boiling point of water and
then further supply the heat of vaporization. Additional amounts of
energy supplied above this base energy level are then more
efficiently converted to useful work in emulsifying the clot by
increasing the intensity of the shock wave and the size of the
bubble. However, the amount of energy per pulse is kept below that
which causes damage to the end of the optical fiber. For the small
optical fibers described herein, the energy level is kept below
about 250 micro-Joules per pulse per fiber.
[0054] The width of each radiation pulse is made relatively short
in order to generate the initial shock wave That is, the shock wave
is generated as a result of a small volume of material at the end
of the optical fiber (FIG. 7A) being heated very rapidly. This
requires depositing the energy of the pulse in a very short period
of time. A pulse width range of 1-100 nano-seconds has been found
satisfactory. The "width" of a radiation pulse is defined for the
purposes herein to be its duration at one-half its peak amplitude
(known as "FWHM"--Full Width Half Max). In a specific
implementation, a 20 nano-second pulse width is used with 100
micro-Joules of energy per pulse delivered through an optical fiber
having a 50 micron core diameter.
[0055] A repetition rate of pulses directed against the same or
adjacent regions of the clot should be high enough to keep the clot
surface in a dynamic state and assure that any large particles are
further emulsified before drifting away from the region. A pulse
rate of about one kilo-Hertz or more is enough for this. The main
consideration for an upper limit is to allow the bubble from one
pulse to be fully formed and collapsed (FIGS. 7B-D) before the next
pulse hits. A pulse rate of about 20 kilo-Hertz or less allows this
to occur, although rates up to 50 kilo-Hertz may be possible in
certain circumstances. A pulse repetition rate of 5 kilo-Hertz has
been used with the other parameters of the specific implementation
given above.
[0056] The average power delivered to the vessel and clot is
maintained as low as possible in order to minimize thermal load of
the treatment site within the vessel in a way that avoids damaging
the vessel. A maximum average nominal operating power of 0.5 watt
is desirably maintained over the time of the treatment, and
preferably less than 300 milli-watts. The achievement of this low
power level can require, in some cases, that the treatment be
performed with a duty cycle of less than one, such as 0.6 or 0.8.
That is, no radiation pulses are directed into the vessel during
periodically occurring intervals such that the pulses are generated
60% or 80% of the time. The maximum power level that can be used
without causing damage also depends upon whether a cooling liquid
is discharged through the lumen 51, and if so, the rate of its
flow. A liquid flow rate as little as 0.1 cubic centimeters per
minute provides beneficial cooling results. A flow rate in excess
of two cc./min. will seldom be necessary, and a rate in excess of
five cc./min. is not contemplated. A rate of one cc./min. has been
used with the other parameters given above for the specific
implementation. The flow rate is chosen so as not to overburden the
vascular system but yet provide sufficient cooling. The amount of
teat generated, and thus the average power input to the blood
vessel, is independent of the number of optical fibers that are
used in the preferred case where pulses are directed through only
one of the fibers at a
[0057] A comparison is illustrated in a three-dimensional graph of
FIG. 9 of the combination of parameters used in the present
invention with those typically used by others for a range of
applications similar to what is being described herein but not
specifically addressed to cerebral vessels. The three axes of the
graph are energy per pulse per fiber, pulse duration and pulse
repetition rate. The scaling of the graph is logarithmic. A point
87 indicates the combination of parameters given above for the
specific implementation of the present invention. A point 89 shows
those of a typical prior system, although specific different
systems do have parameters that vary substantially from the values
of the point 89. However, the present invention clearly utilizes
much lower levels of energy per pulse (by a factor of approximately
500), much shorter pulses (by a factor of approximately 200) and
much higher repetition rates (by a factor of approximately 100)
than generally used before.
[0058] The Opto-Electronic Instrument
[0059] The structure and function of the instrument 33 (FIG. 1) is
illustrated by FIG. 10. A treatment radiation source 91, preferably
the Q-switched, frequency doubled Nd:YAG laser mentioned above,
emits radiation pulses of a fixed frequency that is set to
correspond to the desired pulse repetition rate discussed above. An
input control signal 104 effectively turns the laser 91 on and off.
The pulses are reflected from a dichroic mirror 93, then from
another mirror 95 through an optical system 99 that focuses the
laser output beam through an aperture of a mirror 101 onto the
optical fiber connector 31. This beam is scanned in sequence across
a line of the individual fibers 45-50 by a galvanometer 97 that
controllable tilts its mirror 95 in response to a control signal
from a controller 103.
[0060] The galvanometer 97 preferably holds the beam a on a single
optical fiber for a time to direct a burst of a given number of one
to many pulses into that one fiber before moving the beam to
another fiber. A drive signal 106 supplies the proper positioning
voltage to the galvanometer, depending upon which optical fiber is
to receive the output pulses of the laser 91. Movement from one
fiber to another necessarily takes some time, during which none of
the optical fibers receives a pulse it will usually be preferable
to reduce or eliminate this gap in delivering radiation pulses to
the fibers. This can be done by substituting an acousto-optic
modulator for the galvanometer 97 and mirror 95, to controllably
scan the beam from the laser 91 across the ends of the optical
fibers 45-50 held in the connector 31.
[0061] As mentioned above, part of the thermal management of the
clot removal process preferably also includes monitoring whether
bubbles are being generated by each of the optical fibers. If not,
delivery of radiation pulses along that fiber is terminated, at
least temporarily, since those pulses are likely delivering only
heat to the affected blood vessel without performing any
emulsification. This bubble monitoring and radiation pulse control
is accomplished by the system shown in FIG. 10.
[0062] A second laser 105 is provided to monitor the existence of a
bubble. It can be a simple continuous wave (cw) laser with an
output within the visible portion of the radiation spectrum. Its
output beam is chosen to have a sufficiently different wavelength
from that (of the treatment laser 91 to enable the two laser beams
to be optically separated from each other. A helium-neon laser is
appropriate, as is a simpler diode laser with an appropriate
wavelength.
[0063] An output beam of the monitoring laser 105 is directed
through the dichroic mirror 93 to strike the mirror 95 coaxially
with the beam from the treatment laser 91. The monitoring beam is
then scanned across the optical fibers 45-50 together and coaxially
with the treatment beam. If the galvanometer 97 and mirror 95 are
replaced with an acousto-optical modulator for scanning the
treatment beam, another such modulator is used for the monitoring
beam.
[0064] When a bubble is present at the end of an optical fiber
receiving both of the treatment and monitoring beams, as shown in
FIG. 7C, there is a reflection of the monitoring beam from an end
surface of the fiber that has an interface with the inside of the
bubble. When no bubble is present, as in FIG. 7E, there is a
reflection of the monitoring beam from the fiber end surface which
now interfaces with the liquid within the vessel or the clot
itself. The amount of the intensity of the monitoring beam that is
reflected is much different in each of these two cases because of
the much different refractive indices of water vapor, in one case,
and liquid or clot material, in the other case. The monitoring
beam, which has been reflected from the fiber end at the bubble and
then transmitted back through the fiber, emerges out of the end of
the optical filer, is reflected by the mirror 101 and focused by
appropriate optics 107 onto a photodetector 109 which has an
electrical output 110. This reflected monitoring beam is passed
through a linear polarizer 111 to reject radiation reflected from
the end of the optical fiber within the connector 31. A filter 113
is also placed in the path of the reflected monitoring beam in
order to prevent reflected radiation from the treatment laser 91
from reaching the photodetector 109.
[0065] A block electronic circuit diagram for the system control
103 of FIG. 10 is given in FIG. 11, witty several of its signals
being given in the timing diagrams of FIGS. 12A-I and 13A-E. The
signal 110 (FIGS. 12C and 13C) from the photodetector 109 is
converted from a current to a voltage signal by a circuit 121. This
voltage "bubble" signal is connected to two sample-and-hold
circuits 123 and 125, the outputs of which are connected to two
inputs of a comparator 131. In order for the comparator 131 to
operate properly, the output voltage of the sample-and-hold circuit
123 is shifted in voltage level by a constant voltage bias 124.
This bias voltage is added to the reference measurement made prior
to generation of the bubble. The circuits 123 and 125 store the
value of the photodetector voltage signal at different times in
response to the falling edges of respective one-shot multivibrator
circuit 127 and 129 outputs (FIGS. 12D,E and 13D,E respectively).
The one-shots 127 and 129 receive a timing signal (FIGS. 12A and
13A) in a circuit 136 from a timing signal generator 137.
[0066] An output of the comparator 131 (FIG. 12F) is inverted by an
inverter 132, the output of which is connected to one input of an
AND-gate 133. A second input to the AND-gate 133 is the output of
the one-shot 129, after being inverted by an inverter 130. In
practice, the inverted output usually may be obtained from the
one-shot circuit 129 itself. An output of the AND-gate 133 goes
high when the outputs of both the one-shot 129 and comparator 131
are low, an occurrence that takes place only when an expected
reflection from a bubble is not being detected by the photodetector
109. This occurrence resets a latch 134 whose state (FIG. 12G)
becomes one input to a two input AND-gate 135. The others Input to
the AND-gate 135 is the timing signal (FIGS. 12A and 13A) of the
signal generator 137. The latch 134 is set by a rising edge of the
output of the one-shot 127. The laser controlling signal 104 (FIG.
12H) is the output of the AND-gate 135. The drive signal 106 (FIG.
12I) that positions the mirror 95 of the galvanometer 97 is
developed by a circuit 139 which is also synchronized with the
timing signal (FIGS. 12A and 13A) from the signal generator
137.
[0067] The operation of the system shown in FIGS. 10 and 11 can be
further understood with reference to its timing diagram of FIGS.
12A-I and 13A-E. One cycle of operation is indicated between times
t0 and t2, when laser pulses are directed by the galvanometer 97
into the optical fiber 45. The next cycle occurs between times t2
and t4, when the pulses are directed into the fiber 46. Between
times t4 and t6, in a next cycle, any laser pulses are directed to
the fiber 47, and between times t6 and t8, laser pulses are
directed to the fiber 48. Not shown are the operating cycles that
follow to sequentially direct pulses along the remaining two
optical fibers 49 and 50 of the example given. Once all of the
fibers have received a train of pulses, the sequence is started
over again and continues until the clot has been removed. Of
course, the order in which the fibers carry the pulses may be
something different, as previously discussed.
[0068] The timing signal of FIGS. 12A and 13A is clock, driven,
repetitively enabling (when high) and disabling (when low) the
laser because it provides one of the inputs to the AND-gate 135.
The specific form of timing signal illustrated imposes a duty cycle
on the operation of the treatment laser 91 but this is not
necessary in every application. By turning off the laser for a time
(such as between times t1 and t2) after delivering a burst of
pulses (such as between times t0 and t1) to individual ones of the
optical fibers, the amount of heat delivered to the treatment site
within the blocked blood vessel is reduced. This is another way to
control the amount of average power that is delivered to the
treatment site. In the example shown, pulses are delivered sixty
percent of the time, so it is said that it is operating with a 60%
duty cycle, but this is easily changed by changing the timing
signal of FIGS. 12A and 13A.
[0069] In the example being given, a bubble is detected to be
generated at the end of the fibers 45, 46 and 48 but, is not so
detected at the end of the fiber 47. That is, when a bubble is
present, the photodetector signal 110 (FIGS. 12C and 13C) includes
a pulse from light reflected from the monitoring laser 105
immediately after each pulse (FIGS. 12B and 13B) from the treatment
laser 91. This is best shown in FIGS. 13B-C, wherein a reflected
radiation pulse 143 occurs immediately after a treatment radiation
pulse 145. The existence of the reflected pulse 143 is detected
comparing values of the photodetector signal 110 just before and
just after the treatment laser pulse.
[0070] The trailing edge 147 of the output of the one-shot. 127 is
caused to -occur just prior to the treatment laser pulse 145. This
is controlled by the length of the output pulse of the one-shot 127
and the rising edge of the timing signal of FIG. 13A. The rising
edge of the timing signal of FIG. 13A causes both the one-shot
pulse to begin and the Q-switch of the treatment laser 91 to be
turned on. The Q-switch of the laser 91 is set, for the laser 91 to
emit its first pulse 145 at a set time after the rising edge of the
timing signal of FIG. 13A. The result is to store in the
sample-and-hold circuit 123 the value of the photodetector signal
before the treatment pulse, as a reference. A trailing edge 149 of
the output of the one-shot 129 is timed to occur immediately after
the treatment laser pulse 145, when the pulse 143 occurs if a
bubble has been generated by the just ended treatment pulse. The
one-shot signal edge 149 causes the value of the photodetector
output signal at that instant to be stored in the sample-and-hold
circuit 125.
[0071] If there is a difference in the voltage levels stored in the
sample-and-hold circuits 127 and 129, as adjusted by the voltage
bias 124, which exceeds a preset amount, the output of the
comparator 131 goes high, resulting in the latch 134 remaining in
its set state. But if there is not at least this difference in the
voltages stored in the sample-and-hold circuits 123 and 125, then
the output of the comparator 131 goes low and this causes the latch
134 to be reset at the trailing edge of the pulse output of the
one-shot 129. This combination of events is shown to occur at 151
in FIG. 12G when a bubble is not detected.
[0072] It will be noted that the existence or non-existence of the
bubble is detected only after the first treatment laser pulse of
each burst. If none is detected, as for the fiber 47 in this
example, no further treatment pulses of that burst are allowed to
occur. Further pulses are prevented by the latch 134 being reset at
151 (FIG. 12G) by the comparator 131. The treatment laser is then
reenabled at 153 by the latch 134 being set from the rising edge of
the output pulse of the one-shot 127 Also, the next time pulses are
directed to the fiber 47, the same process occurs, namely the
transmission of the first pulse of a burst. If a bubble is detected
after that pulse, then the entire burst will occur. Thus, the
existence of a bubble is examined each time a new optical fiber
becomes addressed.
[0073] Of course, this is only one of many specific arrangements
and timing that can be implemented. For example, the existence or
non-existence of a bubble can be determined after each treatment
laser pulse. Further, the lack of the detection of a bubble can be
used to disable that fiber for more than one cycle, and perhaps for
the entire treatment. In the case where only one or a very few
pulses are contained in each burst, the detection of the absence of
a bubble at the end of one fiber can be used to disable the system
from sending treatment radiation pulses down that fiber for a
certain number of cycles and then trying again.
[0074] Although the various aspects of the present invention have
been described with respect to their preferred embodiments, it will
be understood that the invention is entitled to protection within
the full scope of the appended claims.
* * * * *