Sensitivity of Transmission Raman Spectroscopy Signals to Temperature of Biological Tissues

Abstract

Optical properties of biological tissues can be influenced by their temperature, thus affecting light transport inside the sample. This could potentially be exploited to deliver more photons inside large biological samples, when compared with experiments at room temperature, overcoming some of difficulties due to highly scattering nature of the tissue. Here we report a change in light transmitted inside biological tissue with temperature elevation from 20 to 40 °C, indicating a considerable enhancement of photons collected by the detector in transmission geometry. The measurement of Raman signals in porcine tissue samples, as large as 40 mm in thickness, indicates a considerable increase in signal ranging from 1.3 to 2 fold, subject to biological variability. The enhancements observed are ascribed to phase transitions of lipids in biological samples. This indicates that: 1) experiments performed on tissue at room temperature can lead to an underestimation of signals that would be obtained at depth in the body in vivo and 2) that experiments at room temperature could be modified to increase detection limits by elevating the temperature of the material of interest.

Figure 1

Schematics of the Transmission Raman setup with heated sample holder.

Figure 2

Broadband light transmission spectra of 40 mm tissue (with 120 mg HAP inside the tissue). The marked vertical bands are laser excitation wavelength (blue) and corresponding Raman band of HAP at ~960 cm⁻¹ (red). Note the change from 20–30 ^oC is more significant that the change observed from 30–40 ^oC.

Figure 3

TRS spectra of 40 mm thick slab of porcine tissue at four representative temperatures. The blue band indicates the position of the 960 cm⁻¹ Raman peak of HAP, when excitation wavelength is 808 nm. The red highlighted strip indicates 1120 cm⁻¹ position, which is the location of a Raman band shown in Fig. 4 to vary with temperature.

Figure 4

Normalised transmission Raman spectra of the porcine adipose tissue measured with the low dispersion grating and 50 µm slit.

Figure 5

(a) Spectral difference of Raman spectra of porcine tissue Raman spectra of porcine tissue with HAP (120 mg) and without HAP at several temperature values (b) overlapped plots of spectral difference of Raman spectra of porcine tissue Raman spectra of porcine tissue/120 mg HAP 20 °C and 40 °C.