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 °C over a range of 28–48 °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.
Highlights
Temperature-dependent changes in near-infrared (NIR) water absorption spectra have been well characterized (Collins 1925, Otal et al 2003)
As a result of the first bound water shift correction, Non-invasive tissue temperature measurements based on quantitative diffuse optical spectroscopy (DOS) of water (a) the water absorption spectrum was blue-shifted (‘first bw corrected’ in figure 3(a))
We have described a method for absolute temperature measurements based on resolving water vibrational frequency shifts due to macro-molecular binding
Summary
Temperature-dependent changes in near-infrared (NIR) water absorption spectra have been well characterized (Collins 1925, Otal et al 2003). We employ a type of DOS that combines broadband frequency-domain photon migration (FDPM) with steady-state (SS) reflectance spectroscopy (Tromberg et al 1993, Bevilacqua et al 2000, Cerussi et al 2006) This approach provides high resolution, quantitative absorption spectra, which are needed to characterize temperature-dependent changes associated with the water absorption features. By measuring scattering-corrected absorption changes in the 970 nm water peak, additional information regarding the disposition of tissue water can be obtained This includes the bound water fraction (Chung et al 2008) and, as we demonstrate in this paper, tissue temperature. The fraction of hydrogen-bound water molecules is reduced, causing the 970 nm water peak to increase in intensity, narrow in bandwidth and shift to higher energy (i.e. blue-shift), as shown in figure 1 (Kelly et al 1995, Libnau et al 1994, McCabe et al 1970, Merritt 2005). Once the bound water correction is determined, the best fit to the temperature-dependent pure water spectra yields the absolute temperature of the measured tissue volume
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