Role of Beam Spot Size in Heating Targets at Depth

December 2015 | Volume 14 | Issue 12 | Original Article | 1437 | Copyright © December 2015


E. Victor Ross MDa and James Childs PhDb

aScripps Clinic, La Jolla, CA
bCynosure Inc., Westford, MA

Abstract
BACKGROUND AND OBJECTIVE:   Wavelength, fluence and pulse width are primary device parameters for the treatment of skin and hair conditions. Wavelength selection is based on tissue scatter and target chromophores. Pulse width is chosen to optimize target heating. Energy absorbed by a target is determined by fluence and spot size of the light source as well as the depth of the target. We conducted an in vitro skin study and simulations to compare heating of a target at a particular depth versus spot size.
STUDY DESIGN/MATERIALS AND METHODS: Porcine skin and fat tissue were prepared and separated to form a 2mm skin layer above a 1 cm thick fat layer. A 50μm thermocouple was placed between the layers and centered beneath a 23 x 38 mm treatment window of an 805 nm diode laser device (Vectus, Cynosure, Westford, MA). Apertures provided various incident beam spot sizes and the temperature rise of the thermocouple was measured for a fixed fluence.
RESULTS: The 2mm deep target's temperature rise versus treatment area showed two regimes with different positive slopes. The first regime up to approximately 1 cm2 area has a greater temperature rise versus area than that for the regime greater than 1 cm2. The slope in the second regime is nonetheless appreciable and provides a fluence reduction factor for skin safety. The same temperature rise in a target at 2 mm depth (typical hair bulb depth in some areas) is realized by increasing the area from 1 to 4 cm2 while reducing the fluence by half.
CONCLUSIONS: The role of spot size and in situ beam divergence is an important consideration to determine optimum fluence settings that increase skin safety when treating deeper targets.

J Drugs Dermatol. 2015;14(12):1437-1442.

INTRODUCTION

The role of spot size is an important consideration to determine optimum fluence settings for efficacy and safety. Spot size is often underestimated as a factor in laser tissue interactions. The major reason most providers favor larger spots is speed, as larger spots allow for larger coverage rates (in cm2/s), at the same repetition rate. Larger spots also can provide for greater subsurface target heating at equivalent device fluences. This enhanced deeper heating is important in laser hair reduction and vascular lesion applications. Subsurface heating can be further increased by photon recycling.
In this study, we examined the role of larger vs. small spots in a Monte Carlo (MC) model using an 805 nm laser. The Monte Carlo model is a statistical model that sends a large number of photons into the skin and maps their progress based on known absorption and scattering coefficients (values that determine how likely a photon is to be absorbed or scattered as a function of path length in tissue).
Then, an ex vivo experiment was performed, and the results were reported and compared to the MC model findings. In the experiment, we compared multiple spot sizes and measured the heating at 2 mm depth in the skin. We looked at a range of spot areas and examined the temperature (T) increase to develop a scale that would report the influence of spot size over this range of diameters.

Ex Vivo Materials & Methods

When light travels from the skin surface into epidermis and dermis, a certain fraction is scattered back toward the surface. Back-scatter results in a greater subsurface energy density compared to that with a beam in a purely absorbing medium that is simply attenuated exponentially as a function of depth. To quantify the actual subsurface fluence enhancement based on backscatter, the back-scatter from lower layers of tissue was evaluated in an ex vivo porcine skin (Yucatan white) model. A 2 mm thick skin sample was separated from the subcutaneous fat and placed either directly on a transparent window (where there would be no back scatter from deeper tissue) or on a 10 mm thick fat layer on the window (which should produce some backscatter from the deeper tissue). A 23x38 mm rectangular-shaped 805 nm diode-based hand piece was placed on the skin and centered over a radiometer consisting of a 50 μm thermocouple located at the skin-window or skin-fat interface (Figures 1a and 1b). This thermocouple absorbs the laser light and is basically a probe whose temperature rise was a measure of the radiation density, i.e. photon energy density, locally in the tissue.
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