PENETRATION DEPTHS OF 830nm DIODE LASER IRRADIATION IN THE HEAD AND NECK ASSESSED USING A RADIOGRAPHIC PHANTOM MODEL AND WAVELENGTH-SPECIFIC IMAGING FILM

Abstract

One of the major claims laid against low reactive-level laser therapy (LLLT) is its purported lack of penetration in biologic tissues, particularly inhomogeneous turbid soft tissue and bone. In order to assess the actual penetration depth of 830 nm laser energy in the head and neck region, a commercially-available radiographic ‘phantom’ model was used (Rand Phantom, Rand Laboratories, U.S.A.). This model consists of dried human skeletal bones (skull and cervical vertebrae) cast inside a proprietary urethane compound having the same effective atomic number and optical density as soft tissue, thus simulating a mix of soft tissue types with randomly distributed fat at radiographic wavelengths and exposure doses. The phantom is sliced horizontally at 2.5 cm intervals allowing the insertion of X-ray sensitive film for training of radiographers or assessment of penetration of X-rays in the head and neck. In this study the authors used wavelength specific imaging film for 830 nm radiation (Konika Medical Laser Imaging Film, LP820H). The phantom is designed to replicate absorption of tissue at X-ray wavelengths. X-ray photons have a very high frequency (≅ 3 x 1018 Hz), approximately three orders of magnitude higher than near infrared (IR) photons at 830 nm (frequency ≅ 3 x 1013 Hz). It was therefore theorized that the penetration of near IR laser energy would be minimized by the high optical density of the phantom’s ‘soft tissue’ thereby providing an accurate penetration model. In a preliminary experiment, penetration of 830 nm diode laser energy (OhLase-3D1, Proli Japan, GaAlAs, continuous wave, 60 mW, 1∼20 sec) was assessed in a skin model (MIX-DP, Saisei Medical, Japan) at thicknesses from 2 mm to 15 mm. The optical density of the ‘skin’ slices was assessed using densinometry (X-RITE Co., Inc., USA), and then the laser penetration was assessed for each skin model thickness with the medical imaging film placed under the ‘skin’, developed, and the esposure areas plotted graphically. It was found that thepenetration of 830 nm laser energy decreased with the increase in optical density, although in a nonlinear fashion, but that extending the exposure time increased the penetration. In the subsequent experiment the laser was irradiated at various points of the phantom head and neck, maintaining the angle of the probe to the head at about 15°, with 830 nm sensitive imaging film inserted between the slices. Contrary to the results of the previous experiment, at exposure times of 180 sec and 300 sec, penetration of laser energy at 830 nm into the head and neck model as assessed by exposure patterns seen in the 830 nm-sensitive film, was in the range of several centimeters rather than millimetres. It is thought that the presence of bone possibly has an echo effect on incident laser energy at 830 nm thereby giving greater penetration than in the skin alone model. It is also possible that the bone structure may also help to focus the laser energy. The implications for applications of LLLT are discussed.