Biomedical Engineering Reference
In-Depth Information
achieved with cryogen spray, sapphire windows and refrigerated
air cooling also are adequate in most cases. Disadvantages of the
spray are the possibility of pigmentation changes (excessive
cooling) and the need for purchasing cryogen canisters. Also, no
bulk cooling is conferred with cryogen spray. Contact cooling
typically includes either a sapphire window or copper plate
(Fig. 2.5) where a temperature of about 5-15°C is maintained at
the skin surface (normally about 30°C). Contact cooling risks
include fogging and poor contact. Refrigerated air works well
but requires a second hand to hold near the surface or an acces-
sory that allows one-hand operation. Although ineffi cient and
sometimes irritating near the nose and mouth, the long applica-
tion times with refrigerated air provide for bulk cooling and
reasonable epidermal protection.
of PWS (7). Garden et al, for example, compared 20- and 360-µs
pulses and found superior lightening with the longer pulse.
In this study they used 2× the purpuric threshold dose (about
2 and 4 J/cm
2
, respectively, for the shorter and longer pulses).
Typical purpuric thresholds with modern day 1.5-ms lasers
in PWS are about 5-6 J/cm
2
, partly because of epidermal cool-
ing (8). Van Gemert et al. used a model to determine the best
wavelength for PWS treatment. They showed that for vessels
less than 14 µm in diameter, 577 nm is best, but for larger
vessels, 585 nm might be better. The authors concluded that a
number of wavelengths might be adequate for PWS. As PDL
technology matured, 585 nm and later 595 nm were adopted as
the optimal wavelengths; 585 nm has half the blood absorption
of 577 nm—the decreased blood absorption allows for deeper
penetration in the vessel and presumably deeper vessel clearing.
In one study 585 nm showed about twice the depth of clearing
(1.2 mm vs 700 µm) for PWS versus 577 nm. In theory 577
should be best for pink lesions where vessel thicknesses are less
than 150
particular devices
The PDL was the initial “test” for SPT. A review of the “older”
literature (1981-1988) is instructive and still relevant for the
modern practitioner. The early PDL evolved from very short
pulses (300 ns and 1 µs) to 450 µs. Much of this was based on
early experiences, where very short microsecond pulses showed
steam bubble formation inside the erythrocytes, whereas longer
pulses showed gentler heating of HgB and superior lightening
m. Two manufacturers produce the PDL in the USA.
Both permit a range or pulse durations (0.45-40 ms). The Can-
dela laser deploys eight micropulses (Fig. 2.6) for durations
exceeding 3 ms and the Cynosure laser counters with six micro-
pulses. The use of micropulsing allows PDLs to create longer
pulse durations (>3 ms). More micropulses have been shown to
increase the purpura threshold as a gentler heating of the vessel
is achieved (9). Although one study showed differences in beam
profi les between the two lasers (10), the most recent versions
from both manufacturers create almost fl at-topped beam pro-
fi les. The optimal wavelength for PWS treatment has evolved
over time. Generally longer wavelengths favor deeper treat-
ment, but the associated lesser absorption coeffi cients for blood
require higher fl uences. Unfortunately commercially available
PDLs today use only 595 nm. Some older versions allowed user
selectively between 585 and 595 nm (11).
μ
Potassium Titanyl Phosphate Laser
The term potassium titanyl phosphate (KTP) “laser” is not
technically correct, as the laser is a frequency-doubled Nd:YAG
laser; however, we use the term for the remainder of the chapter
as its usage has become commonplace in esthetic dermatology.
The KTP laser has been a workhorse in treatment of vascular
Figure 2.5
Note copper tip with popular neodymium:yttrium-aluminum-
garnet laser.
Classic PDL technology
4 pulses, dissimilar distribution
NEW PDL technology
8 pulses, equal energy
Purpura threshold
Time (ms)
Figure 2.6
Schematic shows multiple pulsing and relationship to purpura threshold.
Abbreviation
: PDL, pulsed dye laser.
