Other approaches to optical imaging

The predominant factor which reduces the image quality in diffuse optical imaging is scatter, and many of the advances in instrumentation, theory, and experimental techniques have been designed to reduce its effect. We will briefly mention two techniques which have been developed to improve the spatial resolution of diffuse optical imaging by combining the functional sensitivity of NIR measurements with the spatial resolution of ultrasound.

Ultrasound is an acoustic wave which can be used to modulate the amplitude of light travelling through a compressible object. This was exploited by Wang et al. (1995), who used a focussed ultrasound beam to modulate single channel optical measurements recorded across a cuvette. The amplitude of the modulated optical signal carried information about the optical properties at the focus of the ultrasound beam and was used to produce high-resolution images of the optical properties. Li et al. (2002) developed the technique further by imaging the output field with a CCD camera and analysing the laser speckle, which correlated with the ultrasonic modulation. They successfully imaged objects embedded in 25 mm thickness of tissue.

Photoacoustic imaging offers a further approach to combining the advantages of ultrasound and optical methods. When NIR light passes through tissue, it is absorbed, heating the tissue. The heated tissue expands and produces an acoustic wave which can be measured using conventional ultrasound detectors at the surface. An image of the sources of the ultrasound signal reveals the regions of highest optical absorption within the tissue, with the spatial resolution of ultrasound. Wang et al. (2003) have used photoacoustic techniques to produce anatomical and functional images of the intact rat brain with a spatial resolution of 200 um.