With continuous lasers, dermatologists did not have control over the
duration that the patient's skin was exposed to the laser to the extent that
scarring could easily be avoided (except minimally in the case of superpulsed
CO2 laser). They simply were not fast enough to
deliver a pulse that was short enough to not scar while powerful enough to
penetrate the skin and affect the tattoo ink. However, Q-switching
technology brought that crucial control and raised the technology of tattoo
removal to a new level. Each of the Q-switched lasers currently available
should be examined in their own right and compared to one another. At this
point, I would like to extend a special thanks to Stephen May of Spectrum
Medical Technologies, Inc., who supplied me with information and
diagrams about Q-Switched lasers.
Instead of using a partially reflecting mirror that would allow the
continuous emission of a coherent beam of light, the Q-switch was invented by
R.W. Hellworth45
[as seen in figure 6.1].
Acting on the same principle as a
camera shutter, the flashlamp releases light into the laser chamber. This
prevents the atoms from quickly returning to their ground state. The photons
are contained until high peak powers have been reached and then the Q-switch
dumps the energy in a short, 5-10 nsec pulse of very high intensity.46
This method was pioneered in the realm of tattoo removal by W. H. Reid
and his associates.47 The object was to heat specific pigments without damaging
surrounding tissue. This concept is further explained by the idea theory of
selective phototheolysis. This theory works on two ideas: the wavelength of
the laser light matches the absorption spectrum of the chromophore in the
skin and the exposure time of the laser light is limited to the thermal
relaxation time of the target itself (thermal relaxation time being the time
it takes for heat to disseminate beyond its intended target and into its
surroundings).48 The thermal relaxation time for most chromophores in the
skin is totally dependant upon its size; chemical composition is
irrelevant.49 The damage is confined to the target without damaging
surrounding tissue. The heat of the laser destroys and breaks up the
pigment into particles small enough to allow macrophage activity to get
rid of the tattoo ink. This is why Q-switched lasers cause almost no
scarring while continuous lasers almost always do.
Tattoo inks, melanin, and oxyhemoglobin absorb different wavelengths
of laser light better than others. In
figure 6.2, which wavelengths melanin
and oxyhemoglobin absorb best are revealed. The higher the wavelength,
the lower the absorption coefficient is for melanin. This leads to a
lower risk of hypopigmentation, or removal of color. Oxyhemoglobin is
affected least by ruby lasers, causing the least amount of damage to
surrounding blood vessels.
Figure 6.3 shows which tattoo
inks absorb certain colors best. Where absorption is high, the laser light
is not reflected by the ink particles. Laser light the same color as the ink
particles are reflected more readily and absorbed less. This is a simple
principle of optics that states that light contains all colors of the
spectrum, but only the same color as that light is reflected. The higher the
wavelength is, the lower the absorption rate is by melanin
[again, figure 6.2] and the deeper the penetration
[figure 6.4]. According to Stephen May, this is
due to collagen scatter, which is the skin's ability to make weaker laser
light have less penetration. Increasing the spot size of the laser can
increase penetration. However, if the wavelength is too high, or the
spot size is too big, there could be a risk of overtreating where
penetration has gone past the epidermal and dermal layers.
The first Q-switched laser was the ruby laser at 694 nm. Using this
laser, like all Q-switched lasers, leaves little if any scarring
[see
figure 6.5]. Melanin (the body's natural pigment) and green and black inks
are absorbed by this light very well.50 Because of the fact that melanin
also absorbs this light well, transient hypopigmentation (a lack of coloring)
occurs after treatment, but usually returns within a few months. However,
this is a concern with darker skinned patients, where noticeable
hypopigmentation may indeed be permanent.51 While this laser is excellent
for the removal of darker colors, it works very poorly with lighter colors,
especially yellow and red. This is because lighter colors, especially red,
are more inclined to reflect the laser light than absorb it. One adverse
side effect noted in the use of the Q-switched ruby laser (QSRL) is the
"breaking of the skin" and sometimes "frank bleeding" and "tissue
splatter".52 Occuring at higher fluences (greater than 6 J/cm2), this is
a concern to dermatologists because of hepatitus and HIV.
The Nd:YAG laser at 1064 nm is yet another tool used for the removal
of tattoos
[figure 6.6].
This laser also comes with a double switching option, giving it
a wavelength of 532 nm, a green laser which is superior to any other laser
in the removal of red ink53. While this laser is not as
effective on black and green inks as the QSRL, its larger wavelength gives
it deeper penetration into the skin, with less collagen scatter that causes
laser light to only be effective up to a certain depth. Also, this longer
wavelength means that it has very little effect or inteference from the
surrounding melanin, causing almost no hypopigmentation when used.54
However, the Nd:YAG has been known to generate a textural change in the
skin and even a hypertrophic scar in rare cases.55 The Q-switched Nd:YAG
has also been known to cause tissue splattering and bleeding. Advantages
the Nd:YAG has over the QSRL are its higher pulse rate, which helps speed
up the treatment process, less post-operative pain, and shorter healing
times.56
The Q-switched Alexandrite laser is the last of the Q-switched
lasers to be approved by the FDA. Working at a wavelength of 755 nm, it
has a deeper penetration than the ruby laser, but still affects melanin in
the form of hypopigmentation, but the melanin regenerates in a short amount
of time.57 The adverse effect of punctate bleeding and tissue splatter does
not occur at therapeutic fluences.58 Where the QSRL is delivered in a series
of mirrors and has variable intensity (hot spots) within the beam, the
Alexandrite laser is delivered through a fiber optic cable, giving it a
homogeneous result with less variation from pulse to pulse (the Nd:YAG is delivered
through mirrors and has good beam quality with peak energy at the center of
the beam).59 Excluding these differences, the Alexandrite laser has the
same benefits, results, and complications as the QSRL (except that it is not
as effective on red ink
[figure 6.7]).60
6.1 Introduction
6.2 Defining Q-Switching
6.3 Ruby Laser
6.4 Nd:YAG Laser
6.5 Alexandrite Laser