In this article, we demonstrated and validated a new detection method and analysis algorithm for spectral domain detection in LCS. We showed that this method can be used for measuring local µt
spectra in turbid media. The results of the sdLCS phantom measurement agreed well with the expected optical properties of the phantom and were comparable to the measured optical properties by tdLCS (). Whereas we did not explicitly show the localization aspect of our method by measuring on homogeneous media, previous studies with tdLCS proved that localized measurements of optical property spectra are possible within distinct tissue volumes [3
The reason to validate this new detection method, is its theoretical sensitivity advantage compared to time domain detection (Section 2.4). As a consequence, the acquisition speed of the measurement can be reduced with respect to time domain detection, without reducing the signal to noise ratio. The sdLCS system that was validated in this article was more than three times faster than the tdLCS system, but not more sensitive (Section 3.2). A sensitivity advantage for sdLCS could not be demonstrated, because the quantum efficiency of the spectrograph is not optimized for these experiments. However, if the quantum efficiency would be similar to that of our tdLCS detector, the sensitivity advantage would be 11 dB at equal acquisition times for tdLCS and sdLCS (Eq. (5)
An additional shortcoming of the used spectrograph is its relatively slow acquisition speed (167 spectra/sec.). Slow acquisition speeds result in a decrease of the modulation depth in iD(λ) (‘fringe wash-out’), due to the reference mirror movement induced change of ΔOPL within the integration time. As a consequence, an additional decrease in sensitivity can occur, accounting for approximately 0.5 dB in our sdLCS system. Hence, not only higher quantum efficiency, but also faster acquisition speed is required to further improve the performance of our sdLCS system. Since high quantum efficiency line scan cameras are available with line rates up to 140 kHz, enhanced sensitivity and/or acquisition speed are realizable for sdLCS.
As we showed before for tdLCS, it may be advantageous to support the selection of the volume for optical property determination with an image of the investigated tissue volume [3
]. Similar to tdLCS, this image can be reconstructed from the backscattered intensity as a function of depth. Our method for removal of the DC and modulation components in sdLCS also removes the ‘mirror image’ (complex ambiguity) when ΔOPL = 0 is situated inside the sample. As a consequence, this method for sdLCS can be used to reconstruct an image without cross-talk between iD
(ΔOPL) and iD
(–ΔOPL) within the investigated path length window inside the sample.
The only technique that is reported to have comparable performance to our sdLCS system in terms of localized measurements of optical properties, is dual window sOCT that was developed by Robles et al. [8
]. Whereas Robles et al. report that their method achieves higher resolution in both the spectral and the spatial domain by avoiding the inherent tradeoff between the two, their system may suffer from the effects of the sensitivity roll-off of the detecting spectrograph and the absence of focus tracking, which influence measurement depth. An advantage of our sdLCS system is that it does not rely on refractive index-dependent corrections for the unwanted signal attenuation due to these effects. This facilitates the exact determination of µs
contributions to the measured µt
. Since the current spatial and spectral resolution of our sdLCS system are comparable to the tdLCS system (), the resolution of our system is sufficient to obtain relevant determinations of local tissue chromophore concentrations, e.g. within the dermis, or epidermis only as shown in Ref. [3
In conclusion, we have demonstrated and validated a new approach for sdLCS, which has high potential to improve the accuracy and speed of localized optical property measurements by LCS. Undoubtedly, this will lead to improved clinical utility of the technique, e.g. for the non-invasive determination of blood composition (hemoglobin/bilirubin concentration and oxygen saturation).