APPENDIX
(Summarised from Pontinen's 1995 textbook)
OPTIMUM LASER IRRADIATION DOSE
The irradiation dose is the most important parameter for LLLT. It is more important than the type of laser used (visible v invisible; pulsed v unpulsed). The dose is measured in joules (J) per treated point (J/point) or per square centimetre (J/cm²). Both types of dose calculation (J/point and J/cm²) are needed, as LLLT is sometimes applied to specific points (TPs, AhShi points, APs, local points etc) and sometimes to larger areas (wounds, ulcers sprained areas etc).
The following are essential for successful results with LLLT:
1. For optimal biostimulatory effect (to treat wounds, burns, bruises etc), the irradiation dose has a lower and upper limit, with an optimum in the middle. If the dose is too low, it may induce no measurable effect. If the dose is too high, it may induce no effect, or may have a negative effect.
2. The biostimulatory effect is cumulative: repeated doses, at suitable, relatively short intervals, give an added response. Repeated low doses, at intervals of 1-7 days, induce stronger effects than the same total dose given in one treatment. The optimal weekly irradiation dose for HeNe LLLT is about 1 J/cm². With a laser emitting a mean output power (MOP) of 3 or 60 mW, this would take 333 or 16.5 seconds/cm² respectively. The dose for a GaAs laser on fibroblasts is lower than that for HeNe laser.
3. For optimal effect on the AP points, doses recommended in the former Soviet literature are about 0.1 J/AP point. With a laser emitting a MOP of 3 or 60 mW, this would take 33 or 1.65 seconds/AP point respectively.
CALCULATION OF THE IRRADIATION DOSE
One joule (J, unit of energy) is equal to one wattsecond (Ws), i.e. the energy which is generated when 1 watt (W) of power flows for 1 second (J = Ws).
The irradiation dose is the amount of energy which is conducted into the tissue. It is of great importance whether this energy has to be conducted through a small point (say 1 mm²) or through areas of several cm2. Therefore, in treating surfaces such as wounds, ulcers etc, it is better to express the dose as an energy density, i.e. as J/cm².
Because 1 J = 1 Ws, the irradiation dose (D) can be calculated as follows:
P (W) x t (s)
D (J/cm²) = ------------- Equation 1,
A (cm²)
where
D = Laser dose (J/cm²),
P = Laser power conducted to the tissue (W, or MOP mW/1000),
t = time of irradiation (s)
A = Area of treated surface (cm²)
This can be transformed to calculate the time needed for treatment:
D (J/cm²) x A (cm²)
t (s) = ------------------- Equation 2.
P (W)
To calculate the exposure time needed at a target area (A), the MOP of the laser must be converted to W: for example a laser of MOP 15 mW emits 15/1000 = .015 W. As 1 J = 1 Ws, 1 W = 1 J/s. Therefore, if a laser has e.g. a MOP of 15 mW, it emits laser energy of 0.015 W = 0.015 J/s. In 10 s the emission is 10 x 0.015 = 0.15 J etc.
The following table shows the emission dose/second and /minute respectively from lasers with MOP in the range 3 to 60 mW and the emission time needed to give a total irradiation dose of 1 and 2 J respectively.
Mean Emission dose Emission time needed to output per second per minute deliver a target dose of power 1 J or 2 J mW mJ J mJ J min-sec min-sec 3 3 .003 180 .18 5-34 11-08 6 6 .006 360 .36 2-47 5-34 8 8 .008 480 .48 2-05 4-19 10 10 .010 600 .60 1-40 3-20 12 12 .012 720 .72 1-23 2-46 15 15 .015 900 .90 1-07 2-13 20 20 .020 1200 1.20 0-50 1-40 25 25 .025 1500 1.50 0-40 1-20 30 30 .030 1800 1.80 0-33 1-07 40 40 .040 2400 2.40 0-25 0-50 60 60 .060 3600 3.60 0-16.5 0-33
The Table shows that a Class 3B laser, emitting a MOP of 60 mW, can deliver a target dose of 2 J in 33 seconds, whereas a Class A laser, emitting a MOP of 3 mW would need twenty times longer (11 minutes and 8 seconds) to deliver the same dose (2 J). There is a practical advantage in using lasers in the upper end of Class 3B. They cut down dramatically on treatment time/session.
If GaAs lasers are made to work in a single pulsed mode with low frequencies, their MOP is very low. In order to allow direct comparison of different models of pulsed lasers, their emitted energy output (ìJ)/pulse (Ep) and the pulse frequency/second (Hz) (F) should be certified. Typical measured values for energy/pulse range from 0.1-5.0 ìJ and typical pulse frequencies range from 10-10000 Hz.
The MOP of a single pulsed laser depends on its frequency (F) and on the emitted energy/pulse (Ep) as shown in the following Table.
MOP (in mW) is calculated as (Ep x F / 1000). For example, if a laser pulses at 10000 hz and emits 5 ìJ/pulse, its MOP is (10000 x 5 / 1000) mW = 50 mW.
Mean output power (MOP) for a single pulsed GaAs laser with different frequencies (F) and varying pulse energy (Ep).
Pulse Freq. Pulse energy (Ep)
(F) in Hz 0.1 ìJ 0.3 ìJ 1 ìJ 3 ìJ 5 ìJ
Mean output power (MOP) IN mW
10 0.001 0.003 0.01 0.03 0.05
100 0.01 0.03 0.1 0.3 0.5
1000 0.1 0.3 1.0 3.0 5.0
10000 1.0 3.0 10.0 30.0 50.0
The table shows that a single pulsed laser is unlikely to be effective if the pulse frequency is less than 1000 Hz. For example, a laser with a pulse energy (Ep) of 1 ìJ and a pulse frequency of 1 KHz (=1000 Hz) has a MOP of only 0.1 mW. If a (5 x 5) cm2 area needs a laser dose of 1 J/cm², the exposure time (for a 0.1 mW MOP laser) is calculated as follows (as in Equation 2):
Desired dose (D): 1 J/cm² MOP (P): 0.1 mW = 0.0001 W Target area (A): 25 cm² D x A 1 x 25 t = --------, i.e. t = ---------- = 250000 s P 0.0001
thus, t = 4167 min = c. 70 hours. This says that a laser with a MOP of 0.1 mW is of no practical use for LLLT. It also shows that one needs to know the MOP (or the mean pulse frequency and power/pulse) of the laser and how to calculate roughly the irradiation dose needed for effective LLLT.
Thousands of LLLT sessions have been given with irradiation doses far below the clinically effective range, mainly due to ignorance of those critical parameters.
The MOP is not completely dependant on the pulse frequency; both pulse frequency (Hz) and pulse energy (ìJ/pulse) are important in deciding MOP. These calculations and the previous Table demonstrate the advantage of high frequency pulse-train modulated, high pulse energy GaAs lasers over a single pulse laser.