Recent technology advancements in photonics have spurred a prodigious progress toward the development of innovative methods and systems for clinical functional optical imaging, laser surgery, and phototherapy.^1,2 Development of optical biomedical methods and techniques has stimulated a great interest in the study of optical properties of human tissues, which define the efficacy of tissue optical probing and light action on tissue and when are known (measured) give an opportunity to predict precise photon propagation trajectories and fluence rate distribution within irradiated tissues.

This study presents refractive index measurements of human and porcine adipose tissues and lipid droplet content in the visible and near-infrared. The coefficients of the Sellmeier formula were calculated for approximation of tissue dispersion. For the first time, to the best of our knowledge, the phase transition temperatures and temperature increments $dn/dT$ of adipose tissue were quantified for a wide wavelength range from 480 to 1550 nm and from room temperature up to 50°C. For human abdominal adipose tissue, the refractive index increment averaged across all wavelengths is $dn/dT=-(3.54\pm 0.15)\times {10}^{-4}°{\mathrm{C}}^{-1}$, for porcine tissue $dn/dT=-7.92(0.74)\times {10}^{-4}°{\mathrm{C}}^{-1}$, and for porcine lipid droplet $dn/dT=-6.01(0.29)\times {10}^{-4}°{\mathrm{C}}^{-1}$. Data available in literature for refractive indices of adipose tissues measured by different techniques are summarized and compared with the received data.

Histological slices of skin samples with the subcutaneous adipose tissue after photothermal/photodynamic treatment are analyzed. In the case of subcutaneous indocyanine green injection and 808-nm diode laser exposure of the rat skin site in vivo, the greatest changes in tissue condition were observed. Processes were characterized by dystrophy, necrosis, and desquamation of the epithelial cells, swelling and necrosis of the connective tissue, and widespread necrosis of the subcutaneous adipose tissue. The obtained data are useful for safe layer-by-layer dosimetry of laser illumination of ICG-stained adipose tissue for treatment of obesity and cellulite.

Observation of temperature-mediated phase transitions between lipid components of the adipose tissues has been performed by combined use of the Abbe refractometry and optical coherence tomography. The phase transitions of the lipid components were clearly observed in the range of temperatures from 24°C to 60°C, and assessed by quantitatively monitoring the changes of the refractive index of 1- to 2-mm-thick porcine fat tissue slices. The developed approach has a great potential as an alternative method for obtaining accurate information on the processes occurring during thermal lipolysis.

Morphological changes of the adipose tissue at phototreatment are studied in vitro using optical coherence tomography. The 200 to 600 μm fat tissue slices are used in the experiments. The observed change in the tissue structure was associated with fat cell lipolysis and destruction caused by the photodynamic effect. It is found that overall heating of a sample from room to physiological temperature leads to deeper and faster morphology tissue changes if other processing conditions are kept constant. These data support the hypothesis that photodynamic/photothermal treatment induces fat cell lipolysis during some period after treatment.

The Journal of Biomedical Optics (JBO) publishes peer-reviewed papers on the use of novel optical systems and techniques for improved health care and biomedical research.