Non-invasive tissue temperature measurements based on quantitative diffuse optical spectroscopy (DOS) of water

Abstract

We describe the development of a non-invasive method for quantitative tissue temperature measurements using Broadband diffuse optical spectroscopy (DOS). Our approach is based on well-characterized opposing shifts in near-infrared (NIR) water absorption spectra that appear with temperature and macromolecular binding state. Unlike conventional reflectance methods, DOS is used to generate scattering-corrected tissue water absorption spectra. This allows us to separate the macromolecular bound water contribution from the thermally induced spectral shift using the temperature isosbestic point at 996 nm. The method was validated in intralipid tissue phantoms by correlating DOS with thermistor measurements (R=0.96) with a difference of 1.1+/-0.91 degrees C over a range of 28-48 degrees C. Once validated, thermal and hemodynamic (i.e. oxy- and deoxy-hemoglobin concentration) changes were measured simultaneously and continuously in human subjects (forearm) during mild cold stress. DOS-measured arm temperatures were consistent with previously reported invasive deep tissue temperature studies. These results suggest that DOS can be used for non-invasive, co-registered measurements of absolute temperature and hemoglobin parameters in thick tissues, a potentially important approach for optimizing thermal diagnostics and therapeutics.

Figure 1

Pure water absorption spectra measured as a function of temperature (15–65 °C). The water peak around 970 nm increases in intensity, narrows in width and blue-shifts in wavelength as temperature increases. An isosbestic point at 996 nm is observed.

Figure 2

The relationship between the 996 nm water absorption value and the bound water effect correction. The water absorption spectra (left: normalized, bound water shift (bws) corrected; right: fully corrected for 996 nm isosbestic point) are from intralipid phantoms.

Figure 3

(a) Before and after first (fine dashed line) and second (coarse dashed line) bound water effect corrections. (b) Bound water corrected tissue water spectrum fit to a pure water spectrum at a specified temperature. The temperature of the best-fit pure water spectrum is the temperature of the measured tissue volume.

Figure 4

Comparison between DOS and thermistor temperature measurements during phantom heating and cooling. A high correlation (R = 0.96) is observed with a slope of 0.94.

Figure 5

Non-invasive in vivo measurements of thermal and hemodynamic changes in forearm muscles in a human subject.