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The objective of this work is the investigation of intense pulsed light (IPL) photoepilation using Monte Carlo simulation to model the effect of the output dosimetry with millisecond exposure used by typical commercial IPL systems. The temporal pulse shape is an important parameter, which may affect the biological tissue response in terms of efficacy and adverse reactions. This study investigates the effect that IPL pulse structures, namely free discharge, square pulse, close, and spaced pulse stacking, has on hair removal. The relationship between radiant exposure distribution during the IPL pulse and chromophore heating is explored and modeled for hair follicles and the epidermis using a custom Monte Carlo computer simulation. Consistent square pulse and close pulse stacking delivery of radiant exposure across the IPL pulse is shown to generate the most efficient specific heating of the target chromophore, whilst sparing the epidermis, compared to free discharge and pulse stacking pulse delivery. Free discharge systems produced the highest epidermal temperature in the model. This study presents modeled thermal data of a hair follicle in situ, indicating that square pulse IPL technology may be the most efficient and the safest method for photoepilation. The investigation also suggests that the square pulse system design is the most efficient, as energy is not wasted during pulse exposure or lost through interpulse delay times of stacked pulses.
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Mathematical Modelling of the
Optimum Pulse Structure for Safe
and Effective Photo Epilation Using
Broadband Pulsed Light
Caerwyn Ash PhD1, Kelvin Donne PhD2, Gwenaelle Daniel PhD2, Godfrey
Town2, Marc Clement PhD1
1. School of Medicine, Swansea University, Swansea SA2 8PP
2. University of Wales, Cardiff, CF10 3NS
2010
Statement of Disclosure
The following potential conflict of interest relationships
are germane to our presentation:
Salary and test equipment loan: CyDen Ltd., Wales
Travel grant: Swansea University, Wales
2010
IPL Pulse Structure Categories (time vs. wavelength)
Free Discharge
Square Pulse
Close Pulse Stacking
Spaced Pulse Stacking
Ash C, Town G, Bjerring P, (2008), Relevance of the Structure of Time Resolved Spectral Output to Light Tissue Interaction Using Intense Pulsed Light (IPL),
Lasers in Surgery and Medicine 40:83–92
Reference sources for Monte Carlo model parameters
P Bjerring, K Donne, M Clement, M Kiernan, The Importance of Temporal Profile in Selective Non-Ablative Wrinkle
Reduction using 585nm Light, 2002, White Paper
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and thermal relaxation of tissue. Phys Med Biol 1996; 41(8):1381–1399.
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homogeneous versus discrete absorbers in light irradiated turbid media. Phys Med Biol 1997;42(1):51–65.
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boundary voxelization. Comput Meth Prog Bio 2008;89:14– 23.
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Laser Physics Vol 11, Vol 1, 2000, P 146-153.
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Treatment of Vascular Malformations and Hemangioma with Pulsed Dye Laser, Lasers Med Sci Volume 22, Number 2 /
June, 2007.
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Investigated by Mathematical Modeling, Lasers in Surgery and Medicine, Volume 39 Issue 2, Pages 132 – 139.
F Sun, A Chaney, R Anderson, G Aguilar, Thermal Modelling and Experimental Validation of Human Hair and Skin Heated
by Broadband Light, Lasers in Surgery and Medicine 41:161–169 (2009)
Materials and Methods
• 5µm cell size
• Cartesian coordinate grid
• Values of tissue
properties taken from
abundance of peer
reviewed literature
• 1×107 photons
• Spectrum equals 10J/cm2
Physical Constants used for the various tissue layers
Materials and Methods
) 10 (1.7 )(µ -3.4812(melanin)a
We modelled the IPL spectrum as 600 differently coloured lasers (500 to 1,100nm) of different
intensities of 1nm bandwidth and fluence simultaneously.
Absorption by tissue chromophores taken from literature
SL Jacques. Origins of tissue optical properties in the UVA, visible and NR regions. In: Alfano RR, Fujimoto JG,
editors. Advances in optical imaging and photon migration, Vol. 2.Washington, DC:OSA; 1996:364–370.
Materials and Methods
Where T (x,y,z,t) is the local temperature in the tissue (K),
α is the thermal diffusivity of the tissue = [k/ρc]
c is the specific heat (J kg-1 K-1),
ρ is the mass density, (kg m-3),
k is the thermal conductivity of the tissue, (W m-1 K-1),
Q (x,y,z,t) is the volumetric thermal source term, (W m-3)
t is the time (seconds),
Following establishment of the photon distribution using the Monte Carlo technique, the
temperature distribution at any instant may be calculated by numerically solving the time-
dependent thermal diffusion equations. The temperature of the skin is governed by the
following bio-heat equation
Results
• Free discharge, Square Pulse and Close Pulse Stacking system attain temperatures above 70ºC
• Spaced pulse stacking system suggests that the device would need to deliver a greater fluence to attain
similar results than the other systems as much energy is lost during the prolonged off time between pulses.
• Spaced pulse stacking systems may require more internal water-cooling and active or parallel skin surface
cooling to generate and deliver energy safely.
Rate of modeled follicular temperature increase against time for different pulse structures
Results
• Longer pulse structures allow the epidermal layer to cool during the pulse
• Skin temperatures for all four systems indicating the peak temperature for the free discharge system
produced the highest absorbed temperatures thus probably causing greater patient discomfort. Such
systems probably use active or parallel cooling of the skin to prevent adverse reactions such as erythema,
hyperpigmentation.
Rate of epidermal temperature increase against time for different pulse structures
Discussion
• Systems were compared in this study with equal fluence of 10J/cm2
• Controlled repeatable and consistent delivery of optical energy was shown by square pulse
and close pulse stacking temporal profiles This controlled delivery will also provide repeatable
clinical results shot-to-shot. Whereas the free discharge systems, although simple in their
technology, inherently vary shot-to-shot and may cause disparity during treatment and during
the lifetime of the device*.
• The Monte Carlo model used for this study is two dimensional (2D) in its representation of the
light-tissue interaction. However, in reality the interaction of light with hair follicles is in three
dimensions (3D) This may explain the retained temperatures within the temperature modelling
and during exposure. The model is representative for a broad field as approximately the same
amount of photons jump into as out of the azimuthal plane.
• It has long been assumed that the optical properties of the various tissue layers do not
change during exposure to light. This may not be the case as during exposure absorption of
optical energy by chromophores cause heating and mechanically modify some biological
targets.
• Spectral Jitter during the pulse duration has not been considered
• Assumed constant treatment area for all devices
Project future Improvements
• Further development of Computer simulation will make possible open access to software to
predict system performance based on measured output IPL parameters
• Joint Collaboration with School of Computer Science, department of bioengineering and access
to IBM Blue Ice supercomputer to model tissue in 3D with a 1um cell size using parallel
processing to increase speed of results.
IBM Blue Ice
Conclusion
• The value of comparing the physics of light-tissue interaction against different
parameters is of significance to clinicians and system designers.
• The mathematical model assists in identifying positive theoretical options and avoids
experimental repetition. Mathematical modeling also facilitates viewing photon
deposition through skin and final depth in tissue.
• Computer modeling of different pulse profiles indicates square pulse and close pulse
stacking of sub-pulses are the most efficient in delivering optimal light doses to achieve
sufficient thermal transient in the follicle for effective hair reduction.
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