The choice of wavelengths to use for NIR studies is a complex one. The NIR window used for tissue optics is bounded roughly between 650 nm and 850 nm. At lower wavelengths, absorption by haemoglobin limits penetration in tissue, while at higher wavelengths, absorption by water dominates. NIRS studies of the brain have typically employed wavelengths either side of the isobestic point of haemoglobin at 800 nm, where the specific extinction coefficients of HbO and HHb are equal. The actual wavelengths used for a given study are often determined in an ad hoc way and are often dictated by the availability of appropriate light sources (Cope 1991). Recently, Yamashita et al. (2001), Strangman et al. (2003), and Boas et al. (2004) have shown experimentally and theoretically that a pair of wavelengths at 660-760 nm and 830 nm provides superior separation between HHb and HbO than the more commonly used 780 and 830 nm.
Pifferi et al. (2003) and Taroni et al. (2004) have investigated the additional information obtained by using a wider range of wavelengths. They used four wavelengths (683 nm, 785 nm, 912 nm, and 975 nm) to image the breast, with the wavelengths selected empirically to optimise distinction between HbO, HHb, water and lipids. They also found that using four shorter wavelengths (637 nm, 656 nm, 683 nm, and 785 nm) improved the distinction between tumours and cysts.
The first systematic evaluation of the optimal wavelengths for NIR imaging was carried out by Corlu et al. (2003) who determined the wavelengths which minimised crosstalk between chromophores by maximising the uniqueness with which different chromophores can be distinguished. They simulated spatially varying distributions of HbO, HHb, water, and scattering coefficient, and determined the four wavelengths which minimised cross-talk between the chromophores. These did not correlate with the wavelengths which had been assumed to be optimal for NIRS studies. Such theoretical studies are of growing importance as advances in laser diode technology allow a much greater choice of wavelengths. This work also introduced the concept of reconstructing for chromophore concentration directly, rather than the more usual method of post-processing images generated at different wavelengths.
Uludag et al. (2004) used both theoretical and experimental methods to investigate the way in which cross-talk between calculated HHb and HbO concentrations ([HHb] and [HbO]) from dual wavelength measurements is affected by noise and error in the measurement. Their results agreed with those of Yamashita et al. (2001) and Strangman et al. (2003), i.e. that one of the selected wavelengths should be much shorter than 780 nm. They also showed that non-optimal wavelengths can lead to cross-talk which not only reduces the quantitative accuracy with which the changes can be determined but also changes the shape of the timecourse of the signal.
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