Note: Descriptions are shown in the official language in which they were submitted.
2198~26
LASER T~EATMENT OF TELANGIECTASIA
FIELD OF THE INVENTION
This invention relates to a laser treatment for
coagulating blood vessels and, in particular, to a la~er
treatment for leg telangiectasia.
BACKGROUND OF THE INVENTION
Telangiectasia derives from Latin roots meaning "dilated
end vessel". In clinical medicine, the term is used to
describe superficial cutaneous vessels which become visible
to the human eye. These vessels measure 0.1 millimetre (100
microns) to 1-3 millimetres in diameter. The dilated vessel
may involve a venule capillary or arteriole. Telangiectasia
arising from capillaries are initially of fine calibre and red
in colour, but may become larger and blue or purple in colour
because of increasing backflow from the venous side. Those
arising from venules are larger in calibre and blue in colour.
They often protrude and appear tortuous.
Facial telangiectasia respond well to laser treatment
which is now generally accepted as the standard of care.
Laser energy is targeted to the main chromophore in blood
which is oxyhemoglobin. Present technology utili2es yellow
light in the 577-585 nm wavelength to treat these small
vessels primarily of capillary origin. These shorter
wavelengths have limited penetration and are well abRorbed by
oxygenated haemoglobin in capillaries and arterioles but not
by deoxyhaemoglobin in venules. Therefore, this technology
is less effective against deeper, larger vessels and
particularly venous telangiectasia commonly seen in the lower
limbs.
Disfiguring or symptomatic telangiectasia of the leg can
occur in 29-40% of women and 5-15% of men. Although some
patients may have painful symptoms, moSt patients seek
treatment for cosmetic reasons. Any effective form of
--1--
i2198~26
treatment should therefore be relatively free from
complications, particularly pigmentary changes or scarring.
The general standard of care for leg telangiectasia is
sclerotherapy. Various studies suggest that the rate of
clearing is 60-70% but up to 30~ of patients can develop post-
treatment pigmentation and/or telangiectatic matting. The
hyperpigmentation is caused by the extravasation of red blood
cells after vessel injury.
The pulsed dye laser was developed to treat benign
vascular lesions. Numerous studies have shown it to be
effective in the treatment of small microvessels in the 50-300
micron range (up to 0.3 millimetre) such as occurs in port
wine stains, facial telangiectasia and fine leg
telangiectasia. A basic assumption is that a temperature
increase of 50~ C is generally required to coagulate these
vessels. It is well documented that a flashlamp pulsed dye
laser at 585 nm and 450 microsecond pulse width effectively
coagulates microvessels up to 0.3 millimetre without adverse
effects on the surrounding skin. However, the flashlamp
pulsed dye laser does not produce enough heating and
penetration for effective coagulation of vessels larger than
0.3 millimetre. The average diameter of blood vessels in the
deeper dermis is 0.4 millimetre and near subcutaneous tissue
vessels may be as large as 1-3 millimetres in diameter.
Various lasers have been used to treat leg telangiectasia
with limited success. Unwanted side effect~, such as
hyperpigmentation still occur at significant rates. Laser
treatment using a wavelength of 577-585 nm in submillisecond
pulses produces a photoacoustic shockwave or thermal shock
which results in extravascular purpura leading to post-
treatment hyperpigmentation. Therefore, current research is
directed towards utilizing longer wavelengths delivered in
longer pulses to achieve more gentle heating to avoid the
thermal shock of shorter pulsed duration. A flashlamp pulsed
dye laser is now available from the Candela Corporation with
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2198~2~
a longer pulse duration of 1.5 milliseconds. Other systems
include a non-coherent pulsed light source developed by Energy
Systems Corporation. Early reports indicate that conventional
treatment of leg telangiectasia using these systems is still
limited by hyperpigmentation and other unwanted side effect~.
In Selective Photothermolysis: Precise Microsurgery by
Selective Absorption of Pulsed Radiation, Science
1983:220:524-527, Anderson and Parrish proposed a scientific
model for the ideal vascular laser and developed the theory
of selective photothermolysis. They postulated that the use
of yellow light in pulses between 300 microseconds to 5
milliseconds would prevent non-selective thermal injury. The
most important criteria was that a wavelength which is
absorbed preferentially by the target chromophore was
delivered in a pulse whose duration did not exceed the thermal
relaxation time (TRT) of the target chromophore. The TRT of
a tissue is the time it takes to lose 50% of its heat after
irradiation.
The theory of selective photothermolysis requires that
the pulse duration be shorter than the thermal relaxation
time. For superficial cutaneous vessels, the thermal
relaxation time varies between 0.1 milliseconds to 10
milliseconds depending on the size and type of vessel. The
average thermal relaxation time for vessels in typical
vascular lesions is 1.2 milliseconds. The flashlamp pulsed
dye laser used to treat port wine stains and other vascular
lesions is pulsed at 0.45 milliseconds which is less than the
average thermal relaxation time of 1.2 milliseconds. However,
vessel size varies between arterioles which have thermal
relaxation times of hundreds of microseconds to larger venules
of adult port wine stains which have thermal relaxation times
of up to tens of milliseconds. Therefore, even in the typical
port wine stain, there are vessels with thermal relaxation
times ranging over three orders of magnitude and it i~
219882~
simplistic to define a single thermal relaxation time for the
target lesion.
In the treatment of leg telangiectasia where the target
vessel is generally of a uniform size, it is possible to have
a single thermal relaxation time for the target area. A
longer pulse duration can be utilized as long as the thermal
relaxation time is not exceeded. Pulsed durations longer than
1.5 milliseconds are not easily achieved using flashlamp
pulsed dye technology. For larger vessels above 300 microns,
longer pulse durations beyond five milliseconds would still
meet the requirements for selective photothermolysis and allow
the delivery of higher energies while reducing the thermal
shock as the pulse duration increased. The results show that
microhemorrhage related to thermal shock may be reduced with
the longer pulse duration but it is not eliminated. However,
technological limitations also make it unlikely that a longer
pulse duration will be attainable with conventional laser
systems.
There, therefore, remains a need for an effective method
of using a laser for coagulating vessels in vascular lesions,
telangiectasia and the like, having larger and/or deeper
vessels. In particular, there is a need for an effective
treatment for leg telangiectasia. As well, there is a need
for an effective treatment which reduces or eliminates
unwanted side effects such as hyperpigmentation and other
unwanted side effects.
SUMMARY OF THE INVENTION
The present invention overcomes the disadvantages of
prior assemblies and provides a method of using a laser to
coagulate vessels in vascular lesions, telangiectasia and the
like, having larger and deeper vessels and, in particular, leg
telangiectasia. The present invention also reduces the
occurrence of hyperpigmentation and mi~rohemorrhaging.
219882G
According to the present invention then, there is
provided a method of using a laser to coagulate vessels
comprising the steps of applying multiple pulses of a laser
to a target vessel area, in appropriate energy and pulse
duration and in which the pulse repetition rate is
approximately equal to or less than the inverse of the thermal
relaxation time of the target vessel to cause cumulative
heating resulting in coagulation of the vessel without
excessive heating of the surrounding tissue.
DETAILED DESCRIPTION
The present invention is a method of using a laser to
coagulate blood vessels. It may be used in several clinical
applications. For example, it may be used to treat
telangiectasia, venous malformations, hemangiomas, striae,
capillary malformations and, in particular, lesions resistant
to conventional treatment using a submillisecond flashlamp
pulsed dye laser. In particular, it is useful in the
treatment of leg telangiectasia. It can be used effectively
by dermatologists and other laser therapists involved in
cutaneous medicine treating larger sized vessels. This method
comprises applying multiple pulses of a laser to the blood
vessels in a target area wherein the wavelength, pulse
duration, energy, and repetition rate of the laser pulses are
determined primarily according to the vessel size and depth.
One surprising aspect of the present invention is the
effect of multiple pulses on larger vessels. Conventional
treatments have been largely unsuccessful on larger vessels
due in part to the inability to provide sufficient energy and
heating to the vessel to produce coagulation without unwanted
side effects like the scarring of the tissue. By applying
multiple pulses of a laser at a lower energy, sufficient
heating can be produced to coagulate larger vessels and yet
largely avoid unwanted side effects such as scarring. To
achieve efficient heating of the vessel from the multiple
--5--
2198~26
pulses, the pulses must be delivered at a repetition rate
which allows for a sequential increase in temperature from
each pulse. The multiple pulses are applied at a repetition
rate that is below the thermal relaxation time (TRT) of the
vessels in the target area and at a wavelength which can
penetrate to the depth of the vessel and be absorbed by the
vessel. The preferred repetition rate of the pulses measured
in units of Hertz is approximately equal to l/TRT.
An essential concept of the treatment of the present
~ 10 invention is the matching of the pulse duration and repetition
rate of the laser to the vessel diameter. Although vessel
size and, therefore, the thermal relaxation times vary between
arterioles and larger venules, target selectivity is possible
in selective photothermolysis by selecting an appropriate
pulse duration. In port wine stains, the larger ectatic
vessels are the targets and it is their thermal relaxation
time which should not be exceeded. Therefore, a pulse
duration of up to five milliseconds would still be
appropriate. When the pulse duration exceeds the thermal
relaxation time of the target, heating becomes inefficient
since the vessel is already cooling before heating is
complete. It also provides the basis for non-selective
scarring due to the conduction of heat to the surrounding
tissue. Laser pulse duration and therefore the exposure time
of the vessel less than the thermal relaxation time confine
the heat to the target chromophore and increase selectivity.
The 0.45 millisecond pulse duration of the flashlamp pulsed
dye laser spares small capillaries in the 0.01 to 0.05
millimetre range since they have cooled significantly during
the delivery of laser energy.
The multiple pulse protocol of the present invention is
the best avenue for treating larger vessels. The present
invention shows that multiple pulse techniques reduce the risk
of microhemorrhage for any technology and can produce
cumulative heating to attain the required change in
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2198~2~
temperature for threshold coagulation of a target vessel. One
step in the present invention is the calculation of the
optimal laser pulse energy required for the target vessel. The
optimal energy is determined by relating the vessel size and
depth to the oxyhemoglobin absorption rate using industry
standard calculations. The standard of comparison for the net
energy required to coagulate a given vessel at a given depth
is calculated using a clinical model of a 300 micron vessel
at 0.5 millimetre depth treated by a flashlamp pulsed dye
laser. Optimal treatment parameters may be selected from the
following table:
TABLE 1
VESSEL SIZE/DEPT~ NM. PULSE ENERGY 8KIN SKIN/v~SS
WIDTH- ~/CM2 TEMP. TEMP. RATIO
MSEC. CHANGE
300 microns/0.5 mm. 585 0.45 8 12.33 0.2~
580-7805 26 12.25 0.24
500 microns/0.5 mm. 585 0.45 13 19.0 0.38
580-780 15 28 10.0 0.20
1 mm./0.5 mm. 585 0.45 24 48.5 0.97
580-780 30 34 21 0.42
1 mm./l.O mm. 780- 100 36 11 0.22
1000
Ideally, treatment parameters that produce the lowest
change in epidermal temperature with a low skin/vessel
temperature ratio are preferable. From Table 1, skinivessel
temperature ratios of 0.24 or less suggest the preferred
wavelengths. It is obvious from the table that a flashlamp
pulsed dye laser at 585 nm with a pulse duration of 0.45
milliseconds cannot treat larger, deeper vessels. The energy
required to coagulate these vessels produces too much
epidermal heating as shown by the high ratios 0.38 and 0.97
for vessels 0.5-1.0 millimetre in diameter resulting in
scarring and other side effects.
--7--
21988~6
As previously stated, a flashlamp pulsed dye laser at 585
nm and 450 microseconds pulse width coagulates microvessels
up to 0.3 millimetre in diameter without adverse effects on
the surrounding skin. The required energy using this
combination of wavelength and pulse duration is 6-8 joules/cm2
for a vessel having a diameter of 0.3 millimetre at a depth
of 0.5 millimetre. Using these treatment parameters, there
is a change in skin temperature of 12.3~C and a skin/vessel
ratio of 0.24. Theoretically, treatment parameters which
achieve the lowest skin/vessel temperature ratio and effective
coagulation are considered to be the most ideal. The
limitation of a 585 nm x 450 microsecond pulsed dye laser is
illustrated by analyzing the calculations for treatment
parameters of a vessel measuring 0.5 millimetre at the same
depth of 0.5 millimetre. In this situation, at 585 nm, the
energy required to achieve threshold heating would be 12.6
joules/cm2. However, the significant concern is that the
elevation in skin temperature would be 19~C with the
skin/vessel temperature ~atio rising to 0.38. This means that
a substantial increase in epidermal heating would occur which
would likely result in clinical scarring. The higher peak
powers resulting from using the higher fluence delivered in
the same pulse duration also results in significant thermal
shock and microhemorrhage. Using a longer pulse duration to
deliver the required energy is the basis for the development
of a flashlamp pulse dye laser with a longer pulse duration
at 1.5 milliseconds. Increasing the wavelength increases the
depth of penetration which makes longer wavelengths more
useful for deeper vessels between 0.5 and 1.0 centimetre
depth.
Another step in the method of the present invention is
the selection o~ an appropriate wavelength and pulse duration.
One feature of the present invention is the relationship
between the wavelength and pulse duration which gives the
required energy to increase the temperature of the target
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2193~2~
vessel above the threshold for coagulation while maint~;n;ng
the optimal skin to vessel temperature ratios. Utilizing a
pulse duration shorter than the thermal relaxation time
reduces the risk of scarring by minimizing the transfer of
heat conducted to surrounding tissues. It also provides for
more efficient heating since active heating occurs before the
effects of passive cooling can be felt. A laser system which
delivers the required wavelength and approximate pulse
duration is therefore optimal.
The appropriate wavelength to be used in the treatment
of a particular vessel is determined based upon a number of
factors. These factors include, among others, the type of
vessel, for example whether it is a capillary, artery or vein;
degree of oxygenation; the size and depth of the vessel; the
thickness of the vessel wall; and haemodynamic factors such
as flow and viscosity. As an example, one important factor
used to determine the wavelength is the absorption curves for-
deoxyhemoglobin for venous telangiectasia and oxyhemoglobin
for arterial and capillary telangiectasia. The absorption
peak at 585 nm for oxyhemoglobin indicates that wavelengths
near this peak are more optimal for capillary or arterial
telangiectasia. These vessels appear pinkish or more red.
However, the 585 nm wavelength has limited penetration. Using
longer wavelengths in the 590-615 nm range provides for
adequate absorption and increased penetration to deeper
capillary or arterial vessels. To treat venous
telangiectasia, wavelengths in the range of 585 -1000 nm may
be used although the longer wavelengths are preferred. For
example, a relatively well oxygenated superficial capillary
type telangiectasia of small calibre, for example less than
500 microns, will respond well to wavelengths in the 585-615
nm range. A suitable laser would be a long pulse (pulse
duration of 1.5 milliseconds) flashlamp pulsed dye laser. For
vessels in the deeper dermis of medium calibre (0.5-1.0
millimetre), a longer wavelength is preferable. If the vessel
_g_
2198~26
is more venous in nature, then the longer wavelength i5 well
absorbed by deoxyhemoglobin and is still well enough absorbed
by oxyhemoglobin to treat capillary type telangiectasia.
Wavelengths around 620-700 nm are useful and a long pulse Ruby
laser (2-3 milliseconds) is suitable. For deeper vessels near
subcutaneous tissue, an Alexandrite laser at 750 nm in a long
pulse system (1-2 milliseconds) is appropriate.
A further step is to select the repetition rate best
suited for the vessel diameter and thermal relaxation time to
achieve efficient cumulative heating of the target vessel.
The thermal relaxation time ( TRT) of a vessel varies with a
number of factors including diameter and shape. For any given
thickness, spheres cool faster than cylinders which cool
faster than planes. Vessels are cylindrical but the tissue
layers are planar. Complicated formulas can be used to
calculate the TRT but a simple rule of thumb provides a
reliable approximation. The TRT in seconds is roughly equal
to the square of the target dimension in millimetres.
Therefore, for a vessel measuring 0.1 millimetre (100
microns), the TRT iS approximately 10 milliseconds. For
vessels up to 0.50 millimetre or 500 microns the TRT is up to
250 milliseconds or 0.25 seconds. Therefore, it is apparent
that smaller vessels cool more quickly. To achieve efficient
heating from multiple pulses, the pulses must be delivered at
a repetition rate which allows for a sequential increase in
temperature from each pulse. For small vessels, the
repetition rate needs to be rapid. For example, a 300 micron
vessel with a 90 millisecond TRT cannot be efficiently heated
by multiple pulses using the present technology which only
achieves repetition rates in the order of 1-2 Hz or 1-2 pulses
every second. However, these small vessels do not require
multiple pulses as they are effectively coagulated using a
standard flashlamp pulsed dye laser. For larger vessels,
repetition rates of 1-2 Hz can be effective in producing
cumulative heating. For example, a vessel of 1.0 millimetre
--10--
21988~5
diameter has a TRT of one second and a vessel of 0.5
millimetre has TRT of 0.25 seconds. At a repetition rate of
1 Hz, the smaller vessel would have lost most of its heat
within the one second time frame. At 2 Hz, the smaller vessel
would lose most of its heat but there would be some
incremental increase in temperature after each pulse. The
larger vessel would have lost 50% of its heat after one second
and sequential pulses would be more effective in producing
cumulative heating.
A number of experiments were conducted to confirm the
concept of using multiple pulses to treat larger calibre
vessels. The first objective was to investigate whether
multiple pulses could achieve effective heating with a lower
degree of thermal shock or vessel rupture.
TABLE 2
wavelength (nm) 585 585
pulse duration (microseconds) 450 450
number of pulses 1 4
energy (joules/cm2) 6 4
repetition rate (Hz) -- 1
Five subjects with untreated port wine stains were studied.
Adjacent areas measuring two square inches were outlined in
each subject, labelled appropriately and photographed. Each
subject was treated with a Candela Corporation flashlamp
pulsed dye laser utilizing a wavelength of 585 nm, and a pulse
duration of 450 microseconds. A standard single pulse
technique using a fluence of 6.0 joules/cm2 was used to treat
one area and a multiple pulse technique using 4 joules/cm2
delivered in four pulses at a repetition rate of 1 Hz was used
to treat the adjacent area. At one hour and 24 hour intervals
after treatment, each subject underwent skin biopsies. Each
subject was seen at three months to assess the degree of
fading. The skin biopsies were reviewed by an independent
219882~ -
dermatopathologist and assessed for a variety of histologic
findings including the degree of subendothelial injury, edema
in the perivascular cuff and the degree of microhemorrhage or
extravasation.
The results confirmed that multiple pulses produced
effective coagulation as there was no significant difference
in the degree of fading between each treatment method. There
was a significant difference in the degree of microhemorrhage
seen at both one and 24 hours. There was little, if any,
microhemorrhage at 24 hours in the multiple pulse sites as
compared with florid microhemorrhage seen at the single pulse
sites.
The study described above was repeated using a flashlamp
pulsed dye laser with a pulse duration of 1.5 milliseconds
with another five subjects with untreated port wine stains.
Once again, there was no significant difference in the degree
of fading obtained but microhemorrhage were less evident with
multiple pulses and lower fluences as compared to a higher
fluence delivered in a single pulse. A longer pulse duration
decreased the degree of thermal shock but required the use of
higher fluences.
A further clinical study was carried out in ten subjects
with leg telangiectasia.
TABLE 3
wavelength (nm) 590 and 595
number of pulses 1 1 4 5 6
energy (joules/cm2) lS 20 8 8 8
repetition rate (Hz) -- -- .8 .8 .8
- The study had two arms: one using a 590 nm wavelength and
another using a 595 nm wavelength. At each wavelength, three
sites were treated using a single pulse technique at fluences
of 15 joules/cm2 and 20 joules/cm2. In the same patient8,
three adjacent sites were treated using a multiple pulse
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219882~
technique of 8.0 joules/cm2 at four, five, and six pulses
delivered at a repetition rate of 0.8 Hz. Patient~ were seen
at 3, 6 and 12 weeks after treatment. Rates of clearing and
side effects were assessed. Each method of treatment achieved
a 60% rate of fading. However, the degree of post-treatment
hyperpigmentation was reduced from 80~ in the single pulse
sites to 10% in the multiple sites.
These experiments confirm that the energy required for
irreversible vascular injury can be delivered in multiple
sequential pulses at a critical repetition rate instead of a
single pulse without any loss of efficacy. As well
microhemorrhage which is an unwanted side effect of single
pulses can be significantly reduced by using multiple pulses
at appropriate parameters.
The following examples illustrate some of the preferred
parameters for application of the present invention to target
vessels in telangiectasia.
For the application of the present invention to vessels
in the superficial dermis having a depth of up to
approximately 0.5 millimetre, the following criteria may be
used.
TABLE 4
vessel depth 0.5 mm.
vessel size 300 microns
wavelength (nm) 585 - 595
pulse duration (milliseconds) 1.5 1.5
number of pulses 1 3-5
energy (joules/cm2) 15-20 8-12
repetition rate (Hz) -- 5-10
For vessels having an approximate diameter of 300 microns,
wavelengths in the 585-595 nm range are preferred. A
flashlamp pulsed dye laser in the long pulse mode (1.5
-13-
21g8~2~
milliseconds) has been found to be effective. In one example,
twenty subjects were treated with a single pulse using 15-20
joules/cm2 in a 7.0 millimetre elliptical spotsize. A total
of 120 treatment sites were assessed. A 60% rate of clearing
was obtained after two treatments and the incidence of post-
treatment hyperpigmentation was 75%.
To decrease the risk of post-treatment hyperpigmentation,
the multiple pulse technique of the present invention may be
used. In one example, a multiple pulse energy of 8-12
joules/cm2 with a pulse width of 1.5 milliseconds was applied
to the target area. The pulses were repeated 3-5 times at a
repetition rate of 5-10 Hz. A 300 micron vessel has a thermal
relaxation rate of approximately 100 milliseconds or 0.1 of
a second. Since a vessel heated by one pulse loses 50% of its
heat in approximately 0.1 seconds, a repetition rate of 10 Hz
produces more efficient cumulative heating than a rate of 5
Hz. Present technology requires extensive modifications to
achieve these higher repetition rates. Because of this
limited availability to provide the preferred repetition rate
of 5-10 Hz, the applicability of the present invention to
small calibre vessels is limited.
For treating vessels with a diameter of 0.5-2.0
millimetres, lower repetition rates may be used. A vessel
with a diameter of 0.5 millimetre has a thermal relaxation
time of approximately 250 milliseconds or longer. Therefore,
a laser with a repetition rate of 1-4 Hz can produce
cumulative heating and higher repetition rates would produce
greater efficiency. At larger diameters, the thermal
relaxation time increases. A vessel with a diameter of 1.0
millimetre may have a thermal relaxation time approaching one
second and at 2.0 millimetres the thermal relaxation time may
approach four seconds. Therefore, with increasing vessel
diameter, lower repetition rates become more effective. For
a vessel of 1 millimetre, a repetition rate of even 0.5 Hz
will be able to produce cumulative heating.
-14-
219~826
For vessels within the superficial dermis, wavelengths
as low as 585 nm will achieve sufficient penetration. For
oxyhemoglobin, the 585 nm wavelength produces the best
absorption although longer wavelengths can be useful. For
venous telangiectasia, where oxygen tensions are lower,
deoxyhemoglobin is an efficient target chromophore for longer
wavelengths between 600-1000 nm. Therefore, to treat
telangiectasia of this size and depth, a variety of laser
systems producing a range of wavelengths may be useful. These
systems include a flashlamp pulsed dye laser (585-595 nm x 1.5
milliseconds), a long pulse Ruby laser (690 nm x 3
milliseconds) or an Alexandrite laser (700-1000 nm x 1-3
milliseconds). All of these systems may be equally useful
although for venous type telangiectasia the longer wavelengths
are preferable. All of these systems are used in the present
invention in conjunction with some method of epidermal
cooling, such as chilled Vigilon, or proprietary systems as
are available with the long pulse Ruby laser.
TABLE 5
vessel size 0.5 mm. - 2
mm.
wavelength (nm) 585 - 595
pulse duration (milliseconds) 1.5
number of pulses 5 - 10
energy (joules/cm2) 10 - 20
repetition rate (Hz) 0.5 - l
The present invention was applied to these larger vessels
using a wavelength of 585-595 nm with a pulse duration of 1.5
milliseconds. The energy used was 10-20 joules/cm2 at a
repetition rate of 0.5 - 1 Hz. The number of pulses was 5-10.
The precise energy level and the number of pulses will be
determined by a skilled clinician based on factors commonly
used in conventional treatments to determine such parameters.
-15-
21~8~2g~
These factors include the patient's skin type and target
vessel size and type. Generally, for type II and type III
skin, lower energy levels give a greater margin of safety
since epidermal absorption by melanin is a greater problem
with these skin types. For venous type telangiectasia, a
longer wavelength is favoured and would be the preferred
option for a vessel at the deeper levels of the upper dermis.
With an increased wavelength, slightly increased energy leve~s
are generally required to compensate for a slightly lower
absorption rate.
TABLE 6
wavelength (nm) 690
pulse duration ~milliseconds) 1 - 3
number of pulses 5 - 10
energy (joules/cm2) 10 - 20
repetition rate (Hz) 0.5 - 1
A further trial used a wavelength of 690 nm, a pulse
duration of 1-3 milliseconds, energy of 10-20 joules/cm2, a
repetition rate of O.S-1 Hz, and 5-10 pulses. This longer
wavelength may be more useful for venous type telangiectasia
and deeper, thicker or larger calibre vessels. Again, the
clinician will select specific energy levels and repetition
rates depending upon skin type and vessel size. A larger
vessel can be heated with lower repetition rates although
there is an optimal rate for each vessel size. The number of
pulses required will increase with an increase in vessel
diameter. However, increasing the number of pulses can also
be used to compensate for lower energy levels used in skin
types II and III.
-16-
21988~
TABLE 7
wavelength (nm) 750
pulse duration (milliseconds) 1.2
number of pulses 5 - 10
energy (joules/cmZ) 10 - 12
repetition rate (Hz) 1 - 2
A further trial used a longer wavelength of 750 nm, with
a pulse duration of 1-2 milliseconds. The energy used was 10-
12 joules/cm2, with a 2 millimetre spotsize. The repetition
rate was 1-2 Hz with 5-10 pulses. The longer wavelength of
the Alexandrite laser used in this trial is more useful for
venous telangiectasia and larger, thicker vessels.
The energy required to achieve coagulation temperature
(an increase in temperature of approximately 50~C) can be
delivered in any combination of energy and number of pulses.
The exact combination will vary with skin type, vessel size,
type of telangiectasia, and other characteristics. At 750 nm,
a vessel of O.S millimetre diameter at 0.5 millimetre depth
requires about 30 joules/cm2 to achieve the required
temperature change. This energy can not be delivered in one
pulse since epidermal heating and thermal shock would be
excessive. As well, present technology cannot deliver the
required peak power to emit this amount of energy in one
pulse. With the present invention, using the principle of
cumulative heating at a repetition rate appropriate for a
given vessel diameter allows for the energy required to be
delivered in multiple pulses until visible coagulation is
achieved. For the 0.5 millimetre vessel, the combination of
treatment parameters may be 10 joules/cm2 x 2.0 millimetres
spotsize x pulse duration of 1 millisecond x repetition rate
2 Hz x 10 pulses. For a larger vessel of 1 millimetre, the
combination of treatment parameters may be 10 joules/cm2 x 2.0
millimetres spotsize x pulse duration of 2 milliseconds x
-17-
2198~26
repetition rate 1 Hz x 5 pulses. The shorter pulse durationin the first set of treatment parameters allows for a higher
repetition rate. The lower repetition rate in the second set
of treatment parameters achieves efficient heating since the
thermal relaxation time of a 1.0 millimetre vessel approaches
1 second. Therefore, at a repetition rate of 1 Hz, the
radiated vessel has lost only 50% of its heat when the second
pulse is delivered. A lower number of pulses is required for
the larger vessel since the relationship of vessel diameter
to attained repetition rate is more optimal. For the smaller
vessel of approximately 0.5 millimetre, the thermal relaxation
time is of the order of 0.25 seconds. With the repetition rate
of 2 Hz which is within the capability of available
technology, the vessel has lost more than 50% of its heat by
the time the next pulse arrives and the heating is less
efficient. Application of the present invention at a
repetition rate of 4 Hz would be more ideal but it is not yet
attainable with present technology.
The above-described embodiments of the present invention
are meant to be illustrative of preferred embodiments of the
present invention and are not intended to limit the scope of
the present invention. Various modifications, which would be
readily apparent to one skilled in the art, are intended to
be within the scope of the present invention. The only
limitations to the scope of the present invention are set out
in the following appended claims.
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