Biomedical Engineering Reference
In-Depth Information
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Figure 1.11 Scan pattern with two carbon dioxide lasers. Both patterns show
uniform coverage. Scanners appear to be working properly.
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beam are following the same optical path). This suggests that
the treatment beam is also sharp and the profi le is according
to the manufacturer's specifi cations. Also, burn paper can be
used—here the laser is used with a low energy and the spot is
checked for uniformity from beam edge to edge. With FIR
lasers [erbium (Er):yttrium-aluminum-garnet (YAG) and
CO 2 ], a tongue depressor can be used. This is especially useful
for Er:YAG and CO 2 lasers, where calibration is typically car-
ried out only internally. It follows that by checking the impact
pattern on wood, one can uncover damaged mirrors in the
knuckle of the articulated arm, or a bad focusing lens that ren-
ders the laser unstable or unsafe. Likewise, for scanners, it is
wise to check the tongue depressor with FIR lasers to ensure
that skin coverage will be uniform (Fig. 1.11).
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Microns, × 100
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Figure 1.12 Gaussian beam profi le of a popular carbon dioxide laser, as mea-
sured by author.
Pulse Profiles: Square vs. Spiky
The pulse profi le is the temporal shape of the laser pulse (21).
Many laser physicians assume that, for example, a 10-ms pulse
with a pulse dye laser comprises a single burst of energy. In
fact, in many pulsed laser applications, particularly the pulse
dye laser, the “macropulse” comprises several smaller pulses
(Fig. 1.13) (22). Depending on the application, the temporal
pulse profi le may impact the tissue effect. For example, we
evaluated various pulse durations in the treatment of leg veins
with a Nd:YAG laser. We found, for example, that in applying a
40-ms macropulse, where the micropulses were delivered in
four “installments” versus 14 installments, the purpura thresh-
old did not change. On the other hand, a very different
response was observed in the application of green-yellow
(GY) light. In this wavelength range, purpura thresholds are
much lower, making the tissue response more susceptible to
subtle changes in the pulse structure. For example, in our own
experience and that of others (23), even with longer pulse
PDLs, purpura (both inside the vessel and extending through-
out the spot) is more likely after PDL than KTP lasers. Although
both lasers generate pulse trains, the PDL delivers the energy
in very energetic narrow spikes, whereas the long-pulse KTP
laser delivers a smoother temporal pulse profi le (21). The
spiky pulse profi le is more likely to cause immediate thrombo-
sis and vessel rupture versus the smooth pulse profi le, which is
more likely to contract the vessel wall. We have observed that
even by extension of the macropulse duration with the PDL
(125-
Beam Profiles—Top Hat vs. Gaussian
Laser beam profi les can be of various shapes. A common pro-
fi le is the Gaussian profi le. This has the shape of a bell curve
and is the fundamental mode of most lasers (Fig. 1.12). One
often sees this shape when the beam has been delivered
through an articulated arm (with mirrors at the knuckles).
For some wavelengths, this is still the most effective way to
deliver energy (CO 2 and Er). The disadvantage of the arm is
the limited motion and the counterweight of the arm pulling
against the surgeon's hand (10). The Gaussian profi le is often
disparaged as an inferior profi le for lasers. In many applica-
tions, the criticism is well founded. For example, in treating a
lentigo with a typical Q-switched ruby laser, one will often
observe complete ablation of the epidermis at the center of the
“spot,” but only whitening at the periphery. On the other
hand, sometimes a bell-shaped profi le is desirable, for exam-
ple, when applying a small spot FIR beam with a scanner.
In this scenario, the wings of the beam allows for some overlap
without delivering “too much” energy at points of overlap (see
Fig. 1.11, note scan at the top of tongue depressor).
In most applications, a fl at top profi le is desirable, and with
many fi ber delivery systems, this is the case, as the beam is
mixed by the multiple internal refl ections within the fi ber.
Fibers have become increasingly popular with VIS light lasers,
NIR lasers, and MIR lasers. Some high peak power Q-switched
lasers exceed fi ber damage thresholds, such that articulated
arms are still required.
s micropulse width) up to 20 ms, fl uences suffi cient to
achieve vessel contraction or persistent bluing (two typical
endpoints for effective vessel reduction), there is still a risk for
immediate or delayed purpura. On the other hand, the IPL,
even when confi gured with pulse trains, the micropulses are
usually over 2-ms long (macropulse width about 20-30 ms),
μ
 
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