Biomedical Engineering Reference
In-Depth Information
Figure 1.5
Another diode laser; in this case, the diodes are in the handpiece.
This confi guration allows for more direct application of laser light. However,
placement of diodes in the handpiece requires that “high voltage” and laser
cooling be attached to the handpiece by larger “umbilical” cord.
pulses) are not possible with these sources alone; they can,
however, be used to pump a Q-switched laser, and newer IPLs
have featured a Q-switched attachment (Fig. 1.6).
Basic parameters for any EMR procedure are power, time,
and spot size for continuous wave (CW) lasers, and for pulsed
lasers, the energy per pulse, pulse duration, spot size, fl uence,
repetition rate, and the total number of pulses (18). Energy is
measured in joules (J). The amount of energy delivered per
unit area is the fl uence, sometimes called the dose or radiant
exposure, is given in J/cm
2
. The rate of energy delivery is called
power, measured in watts (W). One watt is 1 joule per second
(W = J/s). The power delivered per unit area is called the irra-
diance or power density, usually given in W/cm
2
. Laser expo-
sure duration [called pulse width (pw) for pulsed lasers] is the
time over which energy is delivered. Dermatology uses EMR
exposures ranging from many seconds to nanoseconds. The
fl uence is equal to the irradiance times the exposure duration
(19). Power density is one of the most important parameters,
for it often determines the action mechanism in cutaneous
applications (Fig. 1.7). For example, a very low irradiance (typ-
ical range of 2-10 mW/cm
2
) does not markedly increase tissue
temperature and is associated with diagnostic applications,
photochemical processes, and biostimulation. On the other
extreme, a very short ns pulse can generate high peak power
densities associated with photomechanical injury and even
plasma formation (20). Other important factors are the laser
exposure spot size (which greatly affects intensity inside the
skin), whether the incident light is convergent, divergent, or
diffuse, and the uniformity of irradiance over the exposure
area (spatial beam profi le). The pulse profi le, that is, the char-
acter of the pulse shape in time (instantaneous power vs. time)
is another feature that can impact the tissue response (18).
When we train residents, one of the greatest challenges is
understanding all of these above terms within the context of a
specifi c EMR device. When a novice confronts a laser instru-
ment panel, there is often a grimace born of confusion. The
staggering array of options on some user interfaces can be
overwhelming (Fig. 1.8). Many physicians would prefer to
have fewer options and a simpler display, especially early on in
their use of a particular laser. However, like the driver who
feels confi ned by an automatic transmission, the experienced
Figure 1.6
Q-switched laser pumped by fl ashlamp in IPL device (from ALMA
Laser website). The laser rod is in the handpiece.
10
15
Photodisruption
Photoablation
10
12
Plasma-
induced
ablation
10
9
10
6
10
3
Thermal interaction
10
0
Photochemical interaction
10
-3
10
-15
10
-12
10
-9
10
-6
10
-3
10
0
10
3
Exposure time (s)
Figure 1.7
The relative mechanisms of action as a function of power density
ranges.
Source
: Modifi ed from Ref. 157.
laser surgeon usually wants the fl exibility in tweaking param-
eters. Many manufacturers have accommodated both types of
users, providing preset parameter sets (and “go-bys”) while
still allowing for experienced physicians to choose parameter
confi gurations of their liking.
