ABSTRACT
When a pulse of light enters a dižusive medium such as tissue, it undergoes a number of changes. Most obviously, its intensity is attenuated due to the scattering and absorption properties of tissue. ™e measurement of this attenuation forms the basis of continuous wave (CW) imaging, as described in Chapter 19. However, the pulse is also delayed and broadened. If this broadening is also measured, we can obtain additional information about the tissue’s optical properties. ™is concept forms the basis of time-domain imaging, which is in some senses analogous to frequency-domain imaging also described in Chapter 19. Formally, frequency-domain data are the Fourier transform of time-domain data, expressed at a particular modulation frequency. Time-domain data are therefore somewhat more general than the equivalent frequency-domain data, as they include information from all modulation frequencies. ™ey also contain information about higher moments of the time distribution of the detected light, and can reveal information about measurement errors such as poor contact and unintentional re¦ections or room light. Furthermore, measuring time-domain data by recording the ¦ight times of individual photons ažords the opportunity of maximizing sensitivity, thereby allowing measurements across larger volumes of tissue than other methods.
