Biomedical Engineering Reference
In-Depth Information
effective fl uence is not diminished. The benefi t of continuous
contact cooling is its simplicity. The disadvantage is that the
cooling effect continues throughout the time that the device-
crystal is in contact with the skin. This results in a variable
degree and depth of cooling determined by the length of time
the cold housing is in contact with the skin. This nonselective
and variable depth and temperature of cooling may necessitate
additional treatment energy so that the cooled vessel will heat
up suffi ciently for thermocoagulation.
Another method of cooling is contact precooling. In this
approach, the cooling device contacts the epidermis adjacent to
the laser aperture. The epidermis is precooled and then treated as
the handpiece glides along the treatment area. Because the cool-
ing surface is not in the beam path, no optical window is required
and better thermal contact can be made between the cooling
device and the epidermis. The drawback is the nonreproducibil-
ity of cooling levels and degrees that are based on the speed and
pressure at which the surgeon uses the contact cooling device.
Yet another method for cooling the skin is to deliver a cold
spray of refrigerant to the skin that is timed to precool the
skin before laser penetration and also to postcool the skin to
minimize thermal backscattering from the laser-generated
heat in the target vessel. The authors have termed this latter
effect “thermal quenching” (Fig. 11.5). This method reproduc-
ibly protects the epidermis and superfi cial nerve endings. In
addition, it acts to decrease the perception of thermal laser
epidermal pain by providing another sensation (cold) to the
sensory nerves. Finally, it allows an effi cient use of laser energy
because of the relative selectivity of the cooling spray that can
be limited to the epidermis. The millisecond control of the
cryogen spray prevents cooling of the deeper vascular targets
and is given in varying amounts so that epidermal absorption
of heat is counteracted by exposure to cryogen.
Since the target vessel poorly absorbs 1064-nm wavelength, a
much higher fl uence is necessary to cause thermocoagulation.
Although a fl uence of 10-20 J/cm
2
is suffi cient to thermocoag-
ulate blood vessels when delivered at 532 or 585 nm, a fl uence
of 70-150 J/cm
2
is required to generate suffi cient heat absorp-
tion at 1064 nm. Various 1064-nm lasers are currently available
that meet the criteria for selectively thermocoagulating blood
vessels. Variabilities include the spot size, laser output (both in
fl uence and in how the extended time of the laser pulse is
generated), pulse duration, and epidermal cooling (Fig. 11.6).
In addition, although many claims are made by the laser
(
A
)
(
B
)
Figure 11.5
Thermal quenching through the application of dynamic cooling. (
A
) Laser pulse penetrates through the epidermis and dermis to be absorbed by the
vascular target. (
B
) After absorption by the blood vessel, a pulse of cooling selectively protects the epidermis and quenches the heat rising from the thermocoagu-
lated vessel.
Source
: From Ref. 28.
(
A
)
(
B
)
Figure 11.6
(
A
) Feeding reticular vein with distal telangiectasia before treatment. (
B
) 80% improvement, 3 months apart after one treatment with the Vasculight
6-mm spot, 120 J/cm
2
at 16-ms pulse duration. In comparison with sclero, would also require a second treatment to get to 95% clearance, concerning that there is
some matting but no pigmentation.
