Spectroscopy and Imaging with Diffusing Light

V opaque media are ubiquitous in nature. While some materials are opaque because they strongly absorb visible light, others, such as foam, white paint, biological tissue and milk, are opaque because photons traveling within them are predominantly scattered rather than absorbed. A vanishingly small number of photons travel straight through such substances. Instead, light is transported through these materials in a diffusion (figure 1). In the physics community there has been substantial recent interest in such diffusing photons. This has led, for example, to the development of dynamic light-scattering probes of turbid complex fluids. In the biophysics and medical communities, diffusing photons are now used to view body function and structure. This is made possible by a spectral window that exists within tissues in the 700-900-nm region, in which photon transport is dominated by scattering rather than absorption (figure 2). Thus, to a very good approximation, near-infrared photons diffuse through human tissue. Two biomedical applications for diffusing near-infrared light probes closely parallel the application of nuclear magnetic resonance to tissue study. Generally, the categories can be termed spectroscopy and imaging. The medical utility of either approach ultimately depends on whether the measured tissue contrast provides enough information to differentiate normal from abnormal body function. Spectroscopy is useful for the measurement of timedependent variations in the absorption and scattering of large tissue volumes. For example, brain oxymetry (hemoglobin spectroscopy) of the frontal, parietal or occipital regions can reveal internal bleeding caused by head injury. Imaging is important when a localized heterogeneity of tissue is involved—for example, an early breast or brain tumor, a small amount of bleeding in the brain or an early aneurysm. Here images enable one to identify the site of the Diffusing near-infrared light provides new mechanisms for clinical diagnosis of tissue structure and function.