Irina Yu. Yanina,1 Adrian Rühm,2 Michael V. Fedorov,2 Ronald Sroka,2 Valery V. Tuchin1,3,4
1Saratov State University, Russia
2LIFE-Center, Hospital of University of Munich, Munich, Germany
3Institute of Precise Mechanics and Control RAS, Russia
4University of Oulu, Finland
The optical properties of biological tissues (absorption coefficient μa, reduced scattering coefficient μ’s, anisotropy factor g, refractive index, etc.) help us to recognize light propagation through the tissue. The integrating sphere in combination with the Inverse Adding-Doubling (IAD) method, have become the standard experimental technique for determining the scattering and absorption coefficients of highly scattering media.
For this experiment, we used 11 phantoms. Besides, each phantom with different optical parameters of the measurement were conducted with sample thickness 0.5 mm, 1 mm and 2 mm. Size of each sample was 10 × 10cm in diameter. The phantoms consisted of silicon (n=1.55), with well-defined quantities of scattering material and absorbing pigment added for scattering and absorption, respectively.
The diffuse transmittance and reflectance and collimate transmittance measurements have been performed in the 350–1050 nm wavelength range using the commercially available Ocean Optics USB spectrophotometer with an integrating sphere. The inner diameter of the sphere is 101.6 mm, the size of the entrance port is 25.5×25.5mm and the diameter of the exit port is 16 mm. As a light source, a halogen lamp with filtering of the radiation in the studied spectral range has been used in the measurements. The diameter of incident light beam on the tissue sample is 3 mm. The scan rate is 1 nm s−1.
Measurements have been carried out in vitro with fresh lung samples taken from the pig.
All tissue samples were kept in saline at room temperature of about 20°C until spectroscopic measurements were carried out.
All the tissue samples were cut into pieces each with an area of about 50 × 50 mm2. For mechanical support, the tissue samples were sandwiched between two glass slides. Since compression of tissue may cause some an increase in the tissue absorption and scattering coefficients, in the measurements of the tissue the samples were sandwiched without (or with minimal) compression. The thickness of each tissue sample was measured with a micrometer in several points over the sample surface and averaged.
For processing the experimental data and determination of the optical properties of the phantoms and biological tissue, the inverse adding–doubling (IAD) method developed by Prahl et al has been used.
The overestimated value of absorption coefficient µa and underestimated value of reduced scattering coefficient µ’s parameters for all phantoms is observed. Loss of light through the sides of the sample and the sample holder may erroneously increase the calculated value of the absorption coefficient. Values of anisotropy factor are in the limits 0.45 to 0.6 for all samples. In spite of this, values of effective coefficient are in limits of an error of measurement.
The most approach to true optical parameters have phantoms samples ##3, 7, 9, 10,11. Errors in calculations are probably connected with discrepancy in experiment geometry.
Irina Yur'evna Yanina
Saratov State University, scientific researcher
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