Biomedical Engineering Reference
In-Depth Information
such that the CO
2
laser is more useful for global skin
improvement (fi ne or moderate wrinkle without
severe contour defects) on the face. The thresholds for
ablation for CO
2
and Er lasers vary inversely with
their optical penetration depths in tissue (20 and
1
40.0°C
35
m, respectively). This assumes thermal confi ne-
ment. It follows that less surface fl uence is required for
ablation with the Er laser. With the CO
2
laser, we are
operating
at ablation threshold
in typical resurfac-
ing applications, so a large fraction of energy is in-
vested in tissue heating. This results in low ablation
effi ciency, and only a small mass of dermal tissue is
ablated. In contrast, the Er laser operates well above
threshold (approximately 8-10× for a fl uence of
5 J/cm
2
), resulting in greater ablation and less ther-
mal denaturation. The CO
2
laser at typical operating
parameters performs self-limited controlled heating
of the skin, whereas the Er laser operates in an almost
purely ablative regime.
μ
30
25
20
(
A
)
15
10
(
B
)
5.0°C
Radio Frequency Technology
RF-skin interactions are fundamentally different from the
optical ones. Rather than “optical” fl uence and absorption
coeffi cients, local heat generation depends on the local resis-
tance and local current density. With most RF systems, there is
a rapidly alternating current which, given the impedance of
the skin, generates heat. The distribution of the current den-
sity is determined by the confi guration of the electrodes rela-
tive to the skin anatomy. Depending on the type of surface
cooling, one can create various zones of heating under the
skin. There are two types of electrode deployments. In one sce-
nario, bipolar electrodes are combined with either a diode
laser or an IPL device (Fig. 1.28). In this confi guration, there is
so-called synergy between the two applications (75). With the
bipolar electrode confi guration, electrical fi eld density is
intrinsically confi ned fairly superfi cially (the fi eld intensity
reaches about as deep as one-half the distance between the
electrodes).
In monopolar confi gurations, the dispersive electrode is
located at a distant point on the body. Monopolar skin rejuve-
nation systems tend to create large volumes of heating. They
disperse the electrical energy over the breadth of the electrode
through a concept known as capacitive coupling. This type of
coupling helps to prevent the natural accumulation of electri-
cal energy at the electrode edge (76). The fi rst nonablative RF
device (Therma Cool TC, Thermage, Hayward, California,
USA) uses cryogen spray cooling (CSC), where the spray is
started before the RF current. With a large monopolar elec-
trode (>1 × 1 cm), for example, current is deposited diffusely
in the dermis and the effects tend to be deep; that is, large vol-
ume of skin is heated. The goal is uniform heating of the deep
dermis and fat. Depending on the dose, electrode confi gura-
tion, time, and local skin structure, one observes various
immediate ultrastructural changes (76).
On the other hand, if both positive and negative electrodes
are placed in the contact tip (bipolar electrode), current density
tends to fl ow superfi cially (path of least resistance from elec-
trode to electrode, and therefore temperature elevation is con-
fi ned to superfi cial skin). By placing the electrodes further apart,
the current density depth will increase. Otherwise, control of
(
C
)
Figure 1.27
Heating bands with a mid-infrared heat lamp (Titan, Cutera,
California, USA). By fi ltering out water, the depth and the thickness of the
heated slab increase.
Despite the various modalities used in selective
dermal heating, the degree of rhytid improvement
typically seen after Er:YAG and CO
2
LSR has not been
observed (69) Even where fi broplasia is observed
histologically, clinical improvement is often modest
(70). Several reasons for this apparent discrepancy
are suggested: (
i
) Although selective dermal heating
can be achieved with deeply penetratingMIR lasers
combined with topical cooling, the zone of heating
in the dermis will always be broader and deeper than
the fi ne band of basophilic staining denaturation
observed with Er:YAG and FIR lasers. The wounds
are quite dissimilar, and if one creates a subepider-
mal zone of injury with the complete denaturation
characteristics of the CO
2
laser, pitted scarring will
be most likely. Thus in MIR-based NSR, one is sub-
stituting a larger volume of gentler heating for the
precise and complete denaturation observed after
typical short-pulsed CO
2
LSR.
7.
FIR systems
: the major lasers are the CO
2
and Er:YAG
lasers. Using models, as well as experiments, one can
determine the relative rates of ablation and heating
(30,71,72). Overall, the ratio of ablation to heating
is much higher with the Er:YAG laser. However, one
can extend the thermal fi eld of the Er:YAG laser by
extending the pulse or increasing the repetition rate,
and likewise one can decrease the thermal fi eld of
CO
2
laser by decreasing pw or increasing fl uence
(73,74). It follows that for applications where preci-
sion is required in ablation, Er:YAG is preferred. On
the other hand, depending on settings, the CO
2
laser
combines an enviable blend of ablation and heat-
ing. The depth of RTD is typically more uniform
with CO
2
than the depth of
ablation
with Er:YAG,
