The need for understanding the propagation of pulses and photon density waves in turbid media arises from the advantages that time-resolved techniques bring to the field of biomedical optical diagnostics. The most significant of these advantages is the ability of time-resolved techniques to discriminate between photons traveling different optical paths in the medium. When an adequate physical model is used to process this information, absorption and scattering contributions, as well as spatial in homogeneities in the medium, may be resolved with better accuracy than that of continuous-wave (CW) methods. Another consideration of practical importance is that a given amount of information about the medium can be collected with a smaller number of emitter–detector channels in a time-resolved experiment than in a CW one. This feature can be exemplified by a reflectance measurement done on a homogeneous semi-infinite medium at a single emitter–detector separation: the full set of optical properties can, in principle, be derived from time- or frequency-resolved data, whereas a single CW reflectance value is not sufficient for the purpose. Decreasing the number of source–detector channels helps to reduce the probability of errors related to optode/medium coupling and system calibration. Furthermore, in many measurement schemes, the intensity measurements may be eliminated completely and purely temporal characteristics, such as average arrival time (in the time domain) or phase shift (in the frequency domain), used instead. This approach may increase the signal-to-noise ratio and the overall stability of the system. These advantages come with a price: first, the time resolved equipment is typically more complex and expensive than that of a CW technique, and second, methods used for interpretation of the data are usually more complicated. In addition, calibration of a time-domain instrument may require precise determination of the time instant when the incident pulse enters the medium. Nevertheless, it appears that for certain clinical applications, the advantages of the time-resolved techniques may outweigh their drawbacks.
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