Refractive index of adipose tissue and lipid droplet measured in wide spectral and temperature ranges

Abstract

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±0.15)×10°C, for porcine tissue dn/dT=-7.92(0.74)×10°C, and for porcine lipid droplet dn/dT=-6.01(0.29)×10°C. Data available in literature for refractive indices of adipose tissues measured by different techniques are summarized and compared with the received data.

Measurement of tissue optical properties in the context of tissue optical clearing

“…where n 0 is the RI for temperature of 0°C; dn∕dT is the temperature increment,°C −1 ; and T is the current temperature,°C . 111,112,114,115,119 The average across all wavelengths from 480 to 1550 nm RI temperature increment dn∕dT equals to −ð3.54 AE 0.15Þ × 10 −4°C−1 with n 0 ¼ 1.4833 (see Table 3) that correlates well with the slope measured for oleic acid, −3.8 × 10 −4°C−1 with n 0 ¼ 1.467. 117 This can be explained by the high content of oleic acid in human fat (46%) and its low-melting temperature (16°C).…”

“…118 Therefore, it may easily and intensively leak out from the adipose cells and accumulate at the interface between the sample and measuring prism of the refractometer. 114 Table 4 shows mean value and SD of phase transition characteristic temperatures averaged over 10 human and 10 porcine fat samples studied using Abbe refractometer. 115 For human fat, the very low temperature (24.1°C) and lowtemperature transitions (31°C to 35°C) could be associated with the fusible free fatty acid (FFA) of a fat droplet like oleic acid; the middle temperature (39.1°C)-with cell membrane phospholipids, and the high temperature (48.0°C)-with less fusible free fatty triglyceride (FFT) of a fat droplet like palmitic acid.…”

“…Since blood contains hemoglobin, one of the major chromophores of soft tissues in the visible/NIR region, it is of great importance to know the optical properties of both whole blood and its components. 113,114,[123][124][125] Measurement of RI of blood is of interest for many fields of optical biomedical diagnostics, including optical tomography and biopsy. 82 In the recent pilot study, 112…”

“…where n H 2 O is the RI of distilled water; C is the hemoglobin concentration; α is the specific refraction increment (see Table 6); 92,113,119 and dispersion is well described by Sellmeier equation: 113,114,126 E Q -T A R G E T ; t e m p : i n t r a l i n k -; e 0 0 3 ; 3 2 6 ; 1 3 0…”

“…110 The refractometric properties of biological tissues and their individual components, such as human, porcine, and rat AT, porcine muscle tissue, rat brain tissue, and the main blood proteins-hemoglobin and albumin, have been studied in a wide range of wavelengths and temperatures. [111][112][113][114][115][116][117][118] The temperature dependences of RIs were analyzed for the presence of critical points corresponding to phase transitions. Figure 4 shows temperature dependence for the mean RI for abdominal human AT measured using multiwavelength Abbe refractometer DR-M2/1550 (Atago, Japan) at 930 nm and Table 1 The approximations for wavelength dependence of reduced scattering coefficient in the spectral range from 400 to 1300 nm for selected tissues.…”