Optical imaging of other tissues

The application of optical imaging to other areas of the body is restricted by the limited penetration of light across large thicknesses of tissue. Muscle tissue has been widely investigated using NIRS, although relatively few optical imaging studies have been reported. The forearm muscle has probably been the most frequently studied, the first time by Maris et al. (1994) using a frequency-domain optical topography system to map differences in the hemoglobin oxygenation in finger extensor muscles during exercise. Graber et al. (2000) have used a CW optical tomography system for a series of real-time dynamic arm imaging experiments, and Araraki et al. (2000) and Hillman et al. (2001) have used time-domain systems to measure changes in tissue absorption in response to finger flexion exercises at two wavelengths from which cross-sectional images of haemoglobin concentration were reconstructed.

Another relatively advanced application of diffuse optical imaging is the examination of the finger for rheumatoid arthritis. Hielscher et al. (2004) have produced images by scanning a single source and detector along the finger, in which contrast is provided by an increase in blood volume due to inflammation and changes in the optical properties of the synovial fluid. Because the synovial fluid is low scattering and the diameter of the finger is small, the diffusion approximation may not be appropriate for image reconstruction (Hielscher et al. 1999). Similar work is being carried out by Xu et al. (2001, 2002) who have generated 3D images using the diffusion approximation from continuous wave data.

A related application, which could prove to be clinically significant but is not yet an imaging technique, is the optical examination of the heel bone, which is routinely examined by x-ray in middle-aged women to detect osteoporosis. Pifferi et al. (2004) have obtained preliminary transmission spectra through the heel and found that the bone mineral density (determined from the absorption spectrum of bone) decreased with age in seven volunteers. Ugryumova et al. (2004) have proposed a similar application based on optical coherence tomography.