U.S. patent application number 10/348656 was filed with the patent office on 2003-07-31 for ultra-long flashlamp-excited pulse dye laser for therapy and method therefor.
This patent application is currently assigned to Cynosure, Inc.. Invention is credited to Furumoto, Horace W..
Application Number | 20030144713 10/348656 |
Document ID | / |
Family ID | 26686934 |
Filed Date | 2003-07-31 |
United States Patent
Application |
20030144713 |
Kind Code |
A1 |
Furumoto, Horace W. |
July 31, 2003 |
Ultra-long flashlamp-excited pulse dye laser for therapy and method
therefor
Abstract
A flashlamp-excited dye laser generating light pulses for
therapy has a circulator which circulates a gain media through a
dye cell. A controller coordinates operation by triggering
flashlamps to excite the laser gain media while the circulator is
circulating the gain media. This operation enables the effective
generation of laser light pulses with a duration of at least one
millisecond. The laser pulse is formed from many subpulses. If the
flow velocity of dye solution is great enough such that the new
solution enters the resonant cavity before the solutions in the
cavity are substantially spent, subsequent subpulses are not
quenched, enabling the generation of ultra-long effective pulses
with high fluences. Specifically, longer effective pulses of up to
50 msec are attainable with energies of up to 50 Joules. These
energies enable reasonable spot sizes, which makes the invention
relevant to cutaneous as well as deep tissue therapy, for
example.
Inventors: |
Furumoto, Horace W.;
(Wellesley, MA) |
Correspondence
Address: |
HAMILTON, BROOK, SMITH & REYNOLDS, P.C.
530 VIRGINIA ROAD
P.O. BOX 9133
CONCORD
MA
01742-9133
US
|
Assignee: |
Cynosure, Inc.
Chelmsford
MA
|
Family ID: |
26686934 |
Appl. No.: |
10/348656 |
Filed: |
January 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10348656 |
Jan 21, 2003 |
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09501450 |
Feb 10, 2000 |
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6547781 |
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09501450 |
Feb 10, 2000 |
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08835012 |
Apr 8, 1997 |
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6273883 |
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08835012 |
Apr 8, 1997 |
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PCT/US97/05560 |
Apr 4, 1997 |
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60015082 |
Apr 9, 1996 |
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Current U.S.
Class: |
607/89 |
Current CPC
Class: |
H01S 3/092 20130101;
A61B 2017/00172 20130101; H01S 3/08072 20130101; A61B 2017/22085
20130101; H01S 3/1633 20130101; A61B 18/203 20130101; A61B
2018/00476 20130101; A61B 2018/00452 20130101; H01S 2301/08
20130101 |
Class at
Publication: |
607/89 |
International
Class: |
A61N 005/067 |
Claims
What is claimed is:
1. A flashlamp-excited pulse dye laser, comprising: an optical
system defining a resonant cavity and providing at least a portion
of light generated in the resonant cavity as an output laser light
pulse; a flashlamp for exciting the laser gain media contained in
the cavity; a circulator for circulating the gain media through the
cavity; and a controller for triggering the flashlamp to excite the
laser gain media while the circulator is circulating the gain media
through the cavity to generate a long effective laser light pulse,
comprising a series of shorter-duration subpulses, where the
circulator circulates the gain media such that at least half the
gain media in the cavity is replaced with new gain media during the
period defined by the beginning of one subpulse to the beginning of
the subsequent subpulse.
2. The laser of claim 1, wherein at least 90 percent of the gain
media is replaced within the period defined by the beginning of one
subpulse to the beginning of the subsequent subpulse.
3. A method of operation for a flashlamp-excited pulse dye laser,
the method comprising: periodically exciting original dye solution
in a resonant cavity with a flashlamp to generate a series of
subpulses over an effective pulse duration; replacing at least half
of the at least partially exhausted original dye solution in the
resonant cavity with new dye solution within the period defined by
the beginning of a subpulse to the beginning of the subsequent
subpulse to sustain lasing over the effective pulse duration.
4. The method of claim 3, wherein at least 90 percent of the at
least partially exhausted original dye solution in the resonant
cavity is replaced with new dye solution within the period defined
by the beginning of a subpulse to the beginning of the subsequent
subpulse.
Description
[0001] Most commonly in the context of vascular lesions, such as
portwine stains for example, hemoglobin of red blood cells within
the ectatic blood vessels serves as the laser light absorber, i.e.,
the chromophore. These cells absorb the energy of the laser light
and transfer this energy to the surrounding vessel as heat. If this
occurs quickly and with enough energy, the vessel reaches a
temperature to denature the constituents within the boundary of the
vessel. The fluence, Joules per square centimeter, to reach the
denaturation of a vessel and the contents is calculated to be that
necessary to raise the temperature of the targeted volume within
the vessel to about 70.degree. C. before a significant portion of
the absorbed laser energy can diffuse out of the vessel. The
fluence must, however, be limited so that the tissue surrounding
the vessel is not also denatured.
[0002] As suggested, simply selecting the necessary fluence is not
enough. The intensity and pulse duration of the laser light must
also be optimized for selectivity by both minimizing diffusion into
the surrounding tissue during the pulse while avoiding localized
vaporization. Boiling and vaporization lead to mechanical, rather
than chemical, damage--which can increase injury and hemorrhage in
the tissues that surround the lesion. This constraint suggests that
for the fluence necessary to denature the contents of the vessel,
the pulse duration should be long and at a low intensity to avoid
vaporization. It must also not be too long because of thermal
diffusion, however. Energy from the laser light pulse must be
deposited before heat dissipates into the tissue surrounding the
vessel. The situation becomes more complex if the chromophore is
the blood cell hemoglobin within the lesion blood vessels, since
the vessels are an order of magnitude larger than the blood cells.
Radiation must be added at low intensities so as to not vaporize
the small cells, yet long enough to heat the blood vessels by
thermal diffusion to the point of denaturation and then terminated
before tissue surrounding the blood vessels is damaged.
[0003] Conventionally, flashlamp-excited dye lasers have been used
as the laser light source. These lasers have the high spectral
brightness required for selective photothermolysis and can be tuned
to colors at which preferential absorption occurs. For example,
wavelengths in the range of 577 to 585 nanometers (nm) match the
alpha absorption band of hemoglobin and thus are absorbed well by
the red blood cells in the blood vessels. The absorption of
melanin, the principal pigment in the skin, is poor in this range,
yielding the necessary selectivity.
[0004] Flashlamp-excited dye lasers, however, present problems in
the pulse length obtainable by this type of laser. Theory dictates
that the length of the light pulse should be on the order of the
thermal relaxation time of the ectatic vessels or other dermal
target. Ectatic vessels of greater than 30 microns in diameter are
characteristic of cutaneous vascular lesions. These large vessels
have relaxation times of 0.5 milliseconds (msec) and longer and
thus require pulse durations of this length. Commercially available
flashlamp-excited dye lasers generally have maximum pulse lengths
that are shorter than 0.5 msec. Brute force excitation of the dye
gain medium can result in pulses as long as 1.5 milliseconds. As a
result, selective photothermolysis treatment of ectatic vessels
larger than 30 microns currently relies on higher than optimum
irradiance to compensate for the pulse duration limitations. This
leads to temporary discoloration of the skin, viz., purpura.
[0005] With shorter than desirable pulse durations, purpura, which
is a bluish lesion that appears as black and blue spots, forms at
the treated site. It is not medically harmful nor is it permanent,
and lasts but a couple of weeks. Patients prefer not to have this
cosmetically undesirable side effect. It is commonly believed that
pulses longer than 5 msec will reduce the formation of purpura.
[0006] Dierickx, et al., "Thermal Relaxation of Port Wine Stain
Vessels Probed In-Vivo: The Need for 1-10 Millisecond Laser Pulse
Treatments," J of Investigative Dermatology, 105, 709-714, (1995)
report the data and histologic assessment of the vessel injury
strongly suggest that pulse durations for ideal laser treatment are
in the 1-10 millisecond region and depend on vessel diameter. No
dermatologic laser presently used for port wine stain treatment
operates in this pulse width domain. Commercial medical dye lasers
with pulse durations of 1.5 msec are now available but these lasers
do not show the needed improvement in the treatment of ectatic
vessels. Moreover, the combination of two dye lasers was suggested
to generate 4.5 msec pulses according to U.S. Pat. No. 5,746,735
and the output used in leg vein treatment. The results showed
marginal improvement over pulses 1.5 msec long. See Alora M. B., et
al., "Comparison of the 595 nm Long Pulse (1.5 ms) and 595 nm Ultra
Long Pulse (4 ms) Laser in Treatment of Leg Veins," American
Society Laser Medicine 18th Annual Meeting Supplement 10, No. 158,
(1998). It is therefore desirable to get to 10 msec and longer.
[0007] In dye lasers, it has been observed that the premature
cessation of the lasing is caused primarily by the degradation of
the dye solution. Improved dye solution formulations can yield some
increases in pulse duration. Dye degradation, however, cannot be
totally eliminated and other steps must be taken if pulse durations
of 5 msec and longer and having the fluences for medical procedures
are to be achieved.
[0008] One attempt at lengthening the pulse duration utilizes a
flashlamp-excited dye lasers that has a dye cell that permits rapid
dye solution interchange during the laser excitation pulse.
Specifically, the dye in the dye cell is replaced while the
flashlamps are fired so that exhausted and degraded dye medium is
removed from the resonant cavity and replaced with fresh dye medium
during the excitation pulse, thereby facilitating the lengthening
of the laser pulse. The approach is similar to that used to
generate laser emission in cw dye lasers, albeit at the much higher
energies required for these medical applications.
SUMMARY OF THE INVENTION
[0009] The batch replacement of dye solution and subsequent
processing of the dye solution to lengthen the pulse duration in
dye lasers has met with some success. Nonetheless, still longer
pulses are required in some cases than currently appear practical
using this technique.
[0010] The problem that appears to limit the practicality of this
technique concerns the fact that it suboptimally uses the
flashlamps. The low peak current may not be high enough to excite
the dye gain medium well above threshold and the current below
threshold is wasted making the long continuous pulse dye laser very
inefficient. A preferred mode of operation for the dye laser to
generate long, effective laser pulses is to use a sequence of short
on and off flashlamp pulses. To generate long laser pulses without
exceeding the explosion point of the flashlamp, the current through
the lamp is limited to run safely without damaging the lamp. The
short current pulses have peak currents that are well above the
lasing threshold. The pulse duration of the individual pulse is
short enough so as to be well below the explosion point of the
flashlamp. If the time when the flashlamp is on and when it is off
is shorter than the thermal relaxation time of the target to be
heated, the heating effect is nearly the same as if the flashlamp
was continuously on.
[0011] The above technique allows heating of a target by a sequence
of on/off or pulse periodic pulses to be nearly the same as if the
flashlamp is continually on, but does not compensate for loss of
efficiency caused by degradation of the dye gain medium. But if the
dye gain medium that is degraded by the excitation pulse could be
extracted from the active gain volume before the next excitation
pulse arrives, the dye gain medium will be fresh and not contain
degraded dye solution that lessens the gain of the laser.
Efficiency of the laser is therefore doubly enhanced by periodic
pulsing, first by having flashlamp excitation pulse that is well
above threshold, and secondly by removal of degraded gain dye
solution when the flashlamp is not excited.
[0012] Consequently, the present invention is directed to a
technique for generating long effective pulses. The degraded dye is
removed during the long effective pulse. However, pulse periodic
heating technique is used to preserve the flashlamps. The long
effective pulse is optimal for therapeutic treatment, such as
selective photothermolysis. This long effective pulse is comprised
of much shorter subpulses across the duration of the effective
pulse. The flashlamps are fired for only these short, but
relatively intense pulses. In this way, the flashlamp useful life
is preserved, since the flashlamp will be driven for only a few
milliseconds to as short at microseconds. Moreover, overall pumping
efficiency is improved since a greater percentage of the generated
light is above the lasing threshold for the dye.
[0013] In some ways, this operation of a flashlamp-excited dye
laser is similar to that used previously in isotope separation.
Very high power laser beams were generated using very intense, but
short, flashlamp pulses, in which the dye media was replaced.
[0014] The difference, relative to the present invention,, is that
a pre-defined number of subpulses are created to yield a carefully
controlled effective pulse duration that will be therapeutically
efficacious. In contrast, the dye lasers used for isotope
separation, operated essentially continuously with flashlamps
pulsing at a rate of 100 to 1000 pulses per second. Moreover, in
the present invention, the heating effect of the subpulse is
cumulative on the target, and the total fluence of the effective
laser pulse can be carefully controlled to maximize damage to the
targeted structure, while minimizing collateral damage. In
contrast, in isotope separation, each pulse of the train of pulses
acted on new target material and the desired result for the isotope
separation process will be degraded if the target material was
irradiated more than once.
[0015] The pulse periodic operation is achieved by repeatedly
triggering the flashlamp(s) while a dye solution is being
circulated through the resonant cavity of the laser, typically a
dye cell. If the flow velocity of dye solution is great enough,
such that the new solution enters the cavity, and the next
flashlamp subpulse excites the new fresh dye gain medium,
ultra-long-effective pulses with high fluences are possible.
Specifically, longer effective pulse duration of up to 50 msec, and
longer, can be achieved with energies of up to 50-100 Joules, and
greater. These high energies enable treatment with reasonable spot
sizes, which makes the invention relevant to medical therapy.
[0016] According to one aspect, the invention features a
flashlamp-excited dye laser generating light pulses at a color and
pulse duration required for selective photothermolysis. This laser
preferably comprises a cell containing a laser gain media located
in a resonant cavity. Dye solutions are typical examples of such
gain media. At least one flashlamp is provided to excite the gain
media in the cavity, typically contained in the cell. A circulator
is used to circulate the gain media through the cavity. Finally, a
controller coordinates operation by triggering the flashlamp to
excite the laser gain media, while the circulator is circulating
the gain media through the cell. Laser light subpulses are
generated with a duration of a few hundred microseconds with a low
energy content so as not to create unwanted side effects such as
purpura. Though each subpulse has low energy, the thermal effect of
the subpulses are cumulative, and the heating effect of the long
effective laser pulse is about the same as if the effective laser
pulse was on continuously.
[0017] For some applications, the effective duration of the output
laser light pulse containing the subpulses is preferably at least
five milliseconds. Generally, the cumulative energy of the
subpulses is about twenty Joules, but can be as large as 50 Joules,
which may be necessary for large targets.
[0018] In specific embodiments, the circulator replaces gain media
in the dye cell with new gain media between the generation of
subpulses so that enough new gain media is within the cell to
enable the generation of the subsequent subpulse. This operation
ensures that the laser output will not be quenched by accumulation
of exhausted dye solutions.
[0019] Different configurations for the gain media flow through the
dye cell can be implemented. In one embodiment, the flow is
transverse to the laser axis; in another, the flow is longitudinal,
or parallel, to the axis. Preferably, if the longitudinal
configurations are implemented, a plurality of media input ports
are provided along the cell. A plurality of media output ports are
also useful to allow flow out of the cell. The dye cell segments
between the adjacent inlet and outlet ports is ideally short so
that the residence time of the flowing gain media through the dye
cell segment is less than the period between subpulses.
[0020] In the transverse flow embodiment, the gain media flows
between two parallel or nearly parallel transparent cell walls,
which allows the excitation light to enter the dye cell. The
transparent cell walls are long in the direction of the flashlamps
and laser resonator axis and shorter in the direction of the flow.
The gain media flows perpendicular to the long axis of the dye cell
and is contained within allow excitation light from the flashlamp
to enter the dye cell and within another set of allow the laser
light to reflect between mirrors that comprise the laser
resonator.
[0021] According to another aspect, the invention can also be
characterized in the context of a method of operation for a
flashlamp-excited dye laser. Such a method comprises exciting the
dye solution in the resonant cavity with a flashlamp and then
generating a laser light output subpulse from the resonant cavity
with the excited dye solution. The excitation at least partially
exhausts the dye solution. To counteract this effect, some of the
at least partially exhausted dye solution is replaced in the
resonant cavity with new dye solution before the generation of the
next subpulse, within the duration of the longer effective pulse.
The number and cumulative fluence of the subpulses is defined such
that the effective pulse duration is appropriate to treat the
targeted tissue, while minimizing the detrimental impact on the
matrix surrounding the targeted structures.
[0022] The above and other features of the invention including
various novel details of construction and combinations of parts,
and other advantages, will now be more particularly described with
reference to the accompanying drawings and pointed out in the
claims. It will be understood that the particular method and device
embodying the invention is shown by way of illustration and not as
a limitation of the invention. The principles and features of this
invention may be employed in various and numerous embodiments
without the departing from the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] In the accompanying drawings, like reference characters
refer to the same parts throughout the different views. The
drawings are not necessarily to scale, emphasis instead being
placed upon illustrating the principles of the invention. Of the
drawings:
[0024] FIG. 1 schematically shows a selective photothermolysis
treatment system of the invention;
[0025] FIG. 2 is a schematic perspective view of a first embodiment
of the flashlamp-excited pulse dye laser 1 of the present
invention;
[0026] FIGS. 3A, 3B, 3C, and 3D are a timing diagrams showing the
relationship between the trigger signal from the controller 160,
the flashlamp driving current, the laser pulse amplitude of the dye
laser, the tissue heating effect of the subpulses.
[0027] FIG. 4 is a circuit diagram of the flashlamp driver 162 of
the present invention;
[0028] FIGS. 5A and 5B show the differences between longitudinal
and transverse dye flow, respectively, through the resonant cavity
of a laser;
[0029] FIG. 6 schematically shows a dye cell 105 configured for
longitudinal dye flow through the dye cell; and
[0030] FIG. 7 schematically shows a dye cell 105 configured for
longitudinal dye flow and having multiple input 610-614 and output
ports 620-624 to reduce the residence time of dye solution in the
dye cell 105.
DETAILED DESCRIPTION OF THE INVENTION
[0031] Turning now to the drawings, FIG. 1 shows a selective
photothermolysis treatment system 10, which has been constructed
according to the principles of the present invention.
[0032] A flashlamp-excited pulse dye laser 1 for the system 10
generates an output laser light pulse 120. The output laser light
pulse 120 is coupled into a medical delivery system 20, such as a
single optical fiber, and transported to the skin 50 or other
tissue of a patient. The output laser light pulse 120 achieves
substantial penetration to treat the targeted dermal structure 60,
such as a vascular lesion.
[0033] The targeted structure 60 is one of many different types of
lesion, depending on the application, such as portwine stain
birthmarks, hemangiomas, telangiectasia, idiopathic vulvodynia, and
leg veins. Further, the targeted structures, in other applications,
are vessels in simple wrinkles, caused by age or sun exposure,
blood vessels in scar tissue, or hair follicles. In this last
application, the target is thepapilla, which the pulse permanently
damages in order to yield permanent or semi permanent hair removal
in a region of the patient's dermis.
[0034] The effective pulse duration of the output laser light pulse
120 are matched to the thermal relaxation time of the targeted
structure. Generally, this requires durations greater than 0.2
msec. For vessels of 30 microns in diameter and larger, as are
present in portwine stains of adult patients, the duration should
ideally exceed 0.5 msec, whereas pulse durations of 1 msec to 10
msec or longer to 20-30 msec should be selected when the vessels
are larger than 100 microns or for hair removal applications.
[0035] FIG. 2 is a schematic diagram illustrating the
flashlamp-excited pulse dye laser 1 in more detail. As is generally
common among most such lasers, a dye cell 105 for containing a
liquid laser gain media, specifically a dye solution, extends
longitudinally along a center axis 108 of the laser 1. A front
window 130 and a rear window 132 define the longitudinal extent of
the dye cell 105. Both windows 130 and 132 are transparent. The dye
cell 105 is located in a resonant cavity 110, the ends of which are
defined by a first mirror 112 and a second mirror 114. A tuning
element 113, typically a birefringent filter, allows selection of
the appropriate wavelength for therapeutic or cosmetic treatment.
Usually, the cavity supports multiple spatial and longitudinal
modes, to yield a top-hat output beam spatial profile, instead of a
more Gaussian distribution.
[0036] While the second mirror 114 is fully reflective, the first
mirror 112 is partially reflective and partially transmissive,
defining an output aperture 116. As a result, a portion of the
light generated in the resonant cavity 110 passes through this
first mirror 112 as the output beam 120 of the laser 1.
[0037] The dye solution in the dye cell 105 is optically pumped by
flashlamps 124a and 124b. Exterior to a light-transmissive left
side wall 122a of the dye cell 105 is a left flashlamp 124b. A
right flashlamp 124a is on an exterior side of a right side wall
122b, which is also transmissive to light. These flashlamps 124a,
124b generate broadband light that excites the dye solution
contained in the dye cell 105. This results in the stimulated
emission of light from the excited dye solution. Right and left
reflectors 126a and 126b surround the respective flashlamps 124a
and 124b to maximize the light injected into the dye cell 105.
These reflectors are typically elliptical or diffuse.
[0038] The flashlamps 124a and 124b used in the present invention
preferably have higher pulse energy capacity than typically found
in short pulse dye lasers. During the generation of an output laser
light pulse of 5 msecs, the total pumping energy injected into the
dye solution by the flashlamps is approximately 2000 Joules.
[0039] A dye circulator functions to circulate dye solution through
the dye cell 105.
[0040] Specifically, the dye circulator 150 circulates dye solution
through the dye cell 105 such that subpulses are generated across
the effective, operator-selected pulse duration. Typically, this is
achieved by replacing enough of the dye solution between subpulses
such that the lasing action will not be quenched in the subsequent
subpulse to thereby allow its generation. Typically, the circulator
circulates dye solution such that at least half of the dye solution
is replaced within the period defined by the beginning of one
subpulse to the beginning of the subsequent subpulse. Preferably,
at least 90% of the dye solution is replaced between subpulses in
one embodiment.
[0041] In the embodiment shown, this circulator includes a dye pump
150 which receives new dye solution from a supply reservoir 152.
The dye is pumped into a supply manifold 154 (shown here in
phantom), which distributes the dye solution flow along the
longitudinal axis 108 of the dye laser 1. The dye solution flows
through the dye cell 105, and thus the resonant cavity 110, in a
direction transverse to the axis 108 of the laser 1. A collection
manifold 156 (in phantom) collects the dye solution after it has
passed through the dye cell 105 and directs it to a depleted dye
reservoir 158.
[0042] A separate supply reservoir 152 and depleted dye reservoir
158 are not strictly necessary. Recirculation and filtration
systems are possible. U.S. patent application Ser. No. 08/165,331,
filed on Dec. 10, 1993, entitled Method and Apparatus for
Replenishing Dye Solution in a Dye Laser, which is incorporated
herein by this reference, is directed a system in which by-products
from the lasing process are filtered out and the dye solution
reused.
[0043] A controller 160 coordinates the operation of the dye pump
150 and the flashlamp driver 162. Specifically, in one embodiment,
the system operator first selects a desired effective laser pulse
duration. This determination is typically based on the observed
size of the targeted structures. The controller then determines the
number of flashlamp subpulses that are to be generated to achieve
the selected laser pulse duration. Thereafter, the controller 160
first establishes a steady state flow of dye solution through the
dye cell 105 by activating the dye pump 150. When the dye solution
is flowing through the dye cell 105, the controller 160 then sends
a trigger signals to a flashlamp driver 162 for each subpulse. The
trigger signal defines the subpulse durations and causes the
flashlamp driver 162 to supply a driving current to the flashlamps
124a and 124b. Light from the flashlamps excites the dye solution
to lase and produce the output laser light 120 having the selected
effect pulse duration.
[0044] Constant amplitude output laser light subpulses are produced
with an intensity detector 164 that senses the intensity of the
output laser light 120 and provides feedback to the flashlamp
driver 162 in one embodiment. Typically, the detector is a diode or
other photodetector that generates an intensity signal indicative
of the amplitude of the output laser light. This signal is received
by the flashlamp driver 162. There, the feedback signal is combined
with the trigger signal. This allows the flashlamp driver to
adaptively modify the level of the driving current to the
flashlamps 124a, 124b in response to the instantaneous intensity of
the output laser light. If the gain medium contains a dynamic
concentration of depleted dye, a modulated excitation is required
to maintain constant output. If depleted dye can be removed
quickly, the excitation pulse will remain nearly constant,
however.
[0045] Usually, some exhausted dye solution tends to accumulate in
the dye cell 105 over the course of the long effective pulse. In
fact, even with fast circulation, the percentage of new,
unexhausted, dye is never as large as during the first subpulse. At
least some of the light generated in the dye cell 105 is absorbed
by this exhausted dye solution and this effect tends to increase
the threshold level of excitation needed for subsequent subpulses.
The intensity detector 164 detects any reduction in output light
amplitude and causes the flashlamp to be driven harder to maintain
constant output levels. Thus, the driving current is varied to
maintain a constant amplitude series of subpulses in the output
light amplitude.
[0046] The amount of dye degraded in each subpulse depends on the
excitation energy level of the flashlamp subpulse. The higher the
excitation energy, the more dye molecules are degraded. And, the
amount of degraded dye left in the dye cell excitation volume
before the next excitation subpulse depends on the flow velocity of
the dye solution through the cell and length of the cell in the
flow direction. Low viscosity solution such as those made from
alcohol solvents can achieve velocities up to about 10 meters per
sec with the use of a pump of practical size. With solvents such as
ethylene glycol, often used because it is less of a fire hazard,
flow velocities through the dye cell may be limited to 5 meters per
second. In transverse flow flashlamp excited dye lasers, the height
of the excitation zone parallel to the flow direction is limited by
the height of the window or the image of the flashlamp if focusing
specular reflectors are used to transfer light from the flashlamp
to the dye cell. In either approach, the excitation zone height is
about 1 cm. If the flow is 5 meters per second, the dye solution is
interchanged in {fraction (1/500)} of a second using plug flow
calculation. If the flashlamp light is off for 2 msec, one plug
flow of degraded dye is removed before the next excitation pulse.
If the thermal relaxation time of the target is much longer than 2
msec, successive subpulses will have a cumulative thermal effect on
the target.
[0047] Photothermolysis treatment of larger ectatic vessels, for
example, require the longer pulse durations obtainable by the
present invention. Vessels of 100 and 200 micrometers in diameter
have thermal relaxation times of 4.8 and 19.0 msec, respectively,
and require similar pulse durations for optimally effective
therapy. A flushing time of 2 msec is adequate to remove degraded
dye for these size and larger blood vessels and still allow heating
of the vessels by the cumulative effect of the subpulses. Energies
required in the effective laser pulse to treat these size vessels
are usually from 1 to 20 Joules, but fifty Joules may be required
in hair removal applications.
[0048] FIGS. 3A-3D show trigger signal voltage, the flashlamp
excitation current in Amperes, the laser pulse amplitude 120 as a
function of time during the pulse generation, and the cumulative
heating effect, respectively. Specifically, the controller 160
first engages the dye pump 150 to establish steady state dye flow
through the dye cell 105 prior to the beginning of the laser pulse.
The controller 160 then sends the first subpulse trigger signal
206. This yields the first flashlamp excitation current function
207 with capacitive and inductive distortion. The result is the
first laser light sub-pulse 208. For simplicity, in this
embodiment, the negative feedback constant intensity control is not
activated and constant gate drive is used to control the switching
transistors.
[0049] Thereafter, additional subpulse trigger signals are
generated. In the specific embodiment, there are a total of five
sub-pulse triggers, each being one half millisecond in length.
There is a resting time of two milliseconds between each trigger
signal. There are five on subpulses and four periods when the
flashlamp(s) are off. As a result, the effective pulse duration is
ten and one half milliseconds long. A feature of note, however, is
the fact that this long effective pulse duration is obtained from a
flashlamp with easily obtainable one half millisecond long
subpulses.
[0050] In another embodiment, FIG. 4 shows a circuit diagram of the
flashlamp driver 162 shown in FIG. 2 that actively controls the
level of driving of the flashlamps in response to the intensity of
the generated laser light. Specifically, the flashlamp driver 162
receives the trigger signal from the controller 160 via conductor
305. This subpulse trigger signal defines the time for which the
flashlamps will be driven for the subpulse. The length of the laser
light pulse is tunable by changing the number of subpulse trigger
signals generated, assuming a constant subpulse-to-subpulse
period.
[0051] Specifically, in the typical application, the laser operator
selects a desired pulse duration. Typically, the operator makes
this decision based upon the specific application. For example,
when small veins are the target, typically shorter pulse durations
of 1-20 milliseconds are optimum. For very large ectatic vessels,
pulse durations of 10-100 milliseconds may be necessary. When the
target is the hair papilla, pulse durations of 10-50 milliseconds
are preferred. In any case, the user enters the desired pulse
duration.
[0052] The controller 160 then determines the subpulse duration.
For example, in the illustrated example of FIG. 3A, 0.5
milliseconds subpulses are used. Typically, the subpulses must be
long enough to not vaporize the chromophore. As a result, subpulses
are typically longer than 0.1 milliseconds. With dye lasers, the
subpulses, however, typically must be shorter than two milliseconds
for efficient flashlamp operation and longevity. Then, the
controller defines the time period between successive pulses. In
the illustrated example of FIG. 3A, a resting time of two
milliseconds is provided between each subpulse. As a general rule,
the time between subpulses must be less than the thermal relaxation
time of the targeted structure for selectivity. Finally, the
controller determines the total number of subpulses across the
effective pulse duration. Again, in the example illustrated in FIG.
3A, five subpulses are generated to yield an effective pulse
duration of about 10.5 milliseconds. Longer effective pulse
durations are obtained simply by programming a long pulse duration
into the controller, which then simply generates a longer series of
subpulses.
[0053] In the constant intensity mode of operation, the trigger
signal is received at a summing node 310 through a resistor R1. The
feedback signal, which is indicative of the intensity of the output
laser light 120, is received from the intensity detector 164
through a resistor R2 also at the summing node 310. The voltage of
the summing node is biased by third resistor R3 that is connected
between the summing node 310 and the supply voltage Vcc. In the
particular embodiment shown, the trigger signal is a low level
active signal which pulls the voltage of the summing node 310 below
ground. A comparator 315 compares the voltage of the summing node
to the ground potential. Thus, in response to a receipt of the
trigger signal the comparator 315 turns a power transistor such as
an insulated gate breakdown transistor (IGBT) or power Darlington
320 on, rendering the transistor conductive. This event places the
voltage of a high voltage power supply 325 across the flashlamp,
which generates a driving current to the flashlamps 124a and 124b.
A capacitor C1 stores charge to assist in driving the flashlamps
124a, 124b. A simmer supply 340 is also connected across the
flashlamps 124a and 124b to provide a simmer current to maintain a
stable voltage across the lamp prior to the main excitation pulse.
Without the simmer, operation is erratic. This simmer current is
evident from portion 205 of the flashlamp excitation plot in FIG.
3.
[0054] FIGS. 5A and 5B illustrate the differences between a
longitudinal flow dye laser and the transverse flow configuration.
The first embodiment of FIG. 1 corresponds to the transverse flow
type of FIG. 5B. These configurations generally provide shorter
residence time of the dye solution in the dye cell 105. The dye
solution must merely move across the width of the resonant cavity
110. The longitudinal flow configuration of FIG. 5A offers an
alternative. But, since the dye solution moves along the length of
the dye cell, resident time is longer for the same flow
velocity.
[0055] FIG. 6 illustrates a second embodiment of the dye cell 505
in which the dye solution travels longitudinally along the length
of the dye cell 505, parallel to the laser axis 530. The dye
solution is circulated through an input port 510 by a pump 150. The
dye travels the length l of the dye cell 505 and exits an output
port 515. First and second mirrors 112, 114 define the resonant
cavity 520 in which the dye cell 505 is located as described in
connection with FIG. 1.
[0056] The second embodiment configuration places certain limits on
the dye cell 505 construction. A given cross-section of fluid 550
should traverse the length of the dye cell 505 in approximately 2.5
msec. This is a good estimate for the useable lifetime of dye
solutions during lasing. But, velocity is limited by the pressure
the dye cell 505 can withstand. A rule of thumb is that a flow of
10 meters per second is the maximum speed for pumps operating below
100 pound per square inch (psi) for alcohol solvent and 5 meters
per second for ethylene glycol solvents. These factors limit the
length of the dye cell 505 to approximately one half to one inch in
length.
[0057] FIG. 7 shows a third embodiment based upon a modification of
the second embodiment of FIG. 6. Here, a plurality of dye input
ports 610, 612, 614 are placed longitudinally along the length of
dye cell 605. An input manifold 625 of the circulator supplies dye
to each of these ports from a pump 650. Output ports 620, 622, 624
are placed between the input ports 610-614 on the opposite side of
the dye cell 105. An output manifold 632 collects dye solution
exiting the dye cell 605 through these ports. In this
configuration, dye flowing through any one of the input ports
610-614 is divided and passes out both of the nearest output ports
620-624, again flowing parallel to the laser axis 630. If the
longitudinal distance between an input port and the closest output
port is approximately 25 mm, 50 mm between adjacent input ports, a
flow velocity of 10 m per sec is sufficient to limit the residence
time of the dye solution to 2.5 msec. This allows the dye solution
to be interchanged twice in a 5 msec effective laser pulse duration
or four times in a 10 msec effective pulse.
[0058] Dye lasers having a transverse flow of dye gain media
through the resonant cavity have been developed in the past in a
number of different contexts for different applications. Continuous
wave (cw) dye lasers have even been developed. The dye in these
lasers is pumped by another laser. This laser is focused on a small
spot on a curtain of the flowing dye solution. Thus, volume of dye
excited in this device is very small. Only the small portion of the
dye curtain in the path of the beam from the focused pumping laser
is excited, and therefore generates light by stimulated emission.
Even though this type of laser-excited dye laser generates a
continuous wave output, it can not produce the kilowatts of average
power required by medical applications.
[0059] Very high pulse rate transverse flow dye lasers have been
developed for isotope separation applications. The intent of these
designs is to produce output energies of approximately one Joule in
a few microseconds. Thermal distortion, which limited firing rates
were avoided by replacing the excited dye in the resonant cavity
from a previous pulse with new dye and then triggering the
flashlamp. Such devices have been shown to generate pulse
frequencies of almost one kilohertz. In these industrial
applications, the peak and average output powers far exceed those
required for medical procedures where longer pulse durations,
moderate peak and average powers at lower frequencies are
preferred. Average power close to a kilowatt has been generated
using transverse flow dye lasers. For medical application, average
power of at most a few tens of watts is required. Moreover,
specifically defined effective pulse durations are a necessity.
[0060] While this invention has been particularly shown and
described with references to preferred embodiments thereof, it will
be understood by those skilled in the art that various changes in
form and details may be made therein without departing from the
spirit and scope of the invention as defined by the appended
claims.
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