Biomedical Engineering Reference
In-Depth Information
2
Laser treatment of vascular lesions
E. Victor Ross and Andrew C. Krakowski
One of the fi rst lesions to be treated using selective photo-
thermolysis (SPT) was the port-wine stain (PWS), and vascu-
lar lesions were the fi rst “test” case for SPT. Although Goldman
accurately predicted that vascular lesions could be selectively
heated, the pulsed dye laser (PDL) was the fi rst laser to show
that this selectivity was practical. The spectrum of oxy- and
deoxy-hemoglobin (HgB) should be memorized by the laser
surgeon (see Fig. 1.15 in chap. 1). Generally laser parameters
should be applied that exploit the absorption peaks for oxy-
HgB (roughly 70% of the total HgB, the remainder being
mostly deoxy-HgB and met-HgB).
The fi rst lasers to treat vascular lesions were CO
2
and argon
lasers. The argon laser at 488 and 515 nm enjoyed a high
absorption coeffi cient for HgB, but the pulse durations [con-
tinuous wave (CW)] were longer than the thermal relaxation
time of the targeted blood vessels, and in the absence of sur-
face/epidermal cooling, the high absorption by epidermal
melanin resulted in a high risk of hypopigmentation and scar-
ring (1). Scanners were adapted to limit the laser dwell time to
50 ms but even then collateral nonspecifi c thermal damage
was observed. Although the importance of spatial and tempo-
ral selectivity had been known prior to the advent of the PDL,
Anderson formulated a formal defi nition of SPT (2). The fi rst
real test of this theory was the use of the PDL in treatment of
vascular lesions. The fi rst PDLs were slow (0.5 Hz or less),
equipped with only small diameter spot sizes (3-5 mm), lacked
cooling, and used a 577 nm wavelength near one of the peaks
of oxy-HgB absorption. Over the years from 1981 to 1990 the
wavelength was changed to 585 nm; the rationale for the lon-
ger wavelength was deeper penetration of the laser beam (into
the skin and the vessel) and enhanced dye life. Later, 7, 10, and
12 mm spots were introduced, and repetition rates approached
2 Hz. By the late 1990s, surface cooling devices were added and
the laser wavelength was again increased to 595 nm to further
enhance epidermal/vascular penetration. Epidermal cooling
allowed treatment of darker skin and the use of higher laser
energies. Larger spot size increased the number of photons
that could penetrate deeper and increase the range of applica-
tions from discrete lesions to larger regions of the body where
diffuse redness (i.e., neck and chest) was often observed.
In general, vascular laser technologies (where vessels are spe-
cifi cally targeted) can be divided into three spectral ranges. The
fi rst group comprises green-yellow (GY) light sources, such as
PDL and frequency doubled neodymium:yttrium-aluminum-
garnet (Nd:YAG) lasers (532 nm). The second group includes
the 800 nm diode laser and alexandrite lasers. The third group,
comprised of near infrared radiation (NIR) lasers with a smaller
ratio of melanin to HgB absorption and deeper penetration
(940, 980, 1064 nm), complete the triad. In general, smaller
lesions in lighter skin are treated with GY light sources and
larger lesions in darker skin are treated with NIR lasers.
Intense pulsed light (IPL) sources cover all of the HgB peaks
and cannot be “boxed” into one of the three aforementioned
categories. However, if one were to analyze the thermal con-
tribution from the range of wavelengths, small vessels in
particular are coagulated by the GY light portion of the output
spectrum.
So long as a lesion is a darker red than the surrounding
tissue, a case can be made for HgB-selective technologies. Non-
selective laser technologies can still be useful in vascular lesions
[i.e., CO
2
laser in a pyogenic granuloma (PG)]; however, with-
out a contrast agent, they require damage to the surface (a top-
to-bottom injury) to adequately damage the underlying vessels.
In designing optimal treatment algorithms, the microanatomy
of the vascular lesion should always be considered.
laser parameters of importance
in the treatment of vascular lesions
Wavelength
More than any other parameter, wavelength determines the
cutaneous tissue effect (see Fig. 1.15 in chap. 1). Ideally the
ratio of absorption of the vascular target versus normal sur-
rounding skin should exceed 10:1. The O
2
saturation of cutane-
ous blood ranges from 50% to 80% (3). The peaks of oxy-HgB
absorption are 418, 542, and 577 nm with a smaller peak at
940 nm (2). However, all wavelengths from blue to NIR enjoy
preferential heating of blood vessels over bloodless dermis
(see Fig. 1.16 in chap. 1). Because epidermal pigment overlies
the vessel, wavelength selections should be based on optimal
ratios of vascular to pigment destruction. For example, the
ruby laser would prove adequate (although one of the least
selective choices) in treating a PWS in a vitiligo patient but
would prove the poorest choice for vascular lesion destruction
in any pigmented patient. Goldman did use the long pulsed
ruby laser in a very fair-skinned patient with a PWS with only
mild scarring.
A simple analysis of vascular to pigment absorption coeffi -
cient ratios, however, does not adequately characterize the
laser-tissue interaction. For example, blue light, solely in terms
of absorption coeffi cients, would be the best choice for vessels,
even within the context of its high melanin absorption. How-
ever, because of scattering, blue light will almost invariably
damage the epidermis at light doses suffi cient to damage an
underlying vessel (Fig. 2.1).
Pulse duration is another important parameter. Early
PDLs generated very short pulses (as short as 1 µs) (2). The
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