Optical coherence tomography imaging in developmental biology.

Optical coherence tomography (OCT) is an attractive imaging technique for developmental biology because it permits the imaging of tissue microstructure in situ, yielding micron-scale image resolution without the need for excision of a specimen and tissue processing. OCT enables repeated imaging studies to be performed on the same specimen in order to track developmental changes. OCT is analogous to ultrasound B mode imaging except that it uses low-coherence light rather than sound and performs cross-sectional imaging by measuring the backscattered intensity of light from structures in tissue (1). The principles of OCTimaging are shown schematically in Fig. I. The OCT image is a gray-scale or false-color two-dimensional (2-D) representation of backscattered light intensity in a cross-sectional plane. The OCT image represents the differential backscattering contrast between different tissue types on a micron scale. Because OCT performs imaging using light, it has a oneto two-order-of-magnitude higher spatial resolution than ultrasound and does not require specimen contact. OCT was originally developed an9 demonstrated in ophthalmology for high-resolution tomographic imaging of the retina and anterior eye (2-4). Because the eye is transparent and is easily optically accessible, it is well suited for diagnostic OCT imaging. OCT is promising for the diagnosis of retinal disease because it can provide images of retinal pathology with 10-~ resolution, almost one order of magnitude higher than previously possible using ultrasound. Clinical studies have been performed to assess the application of OCT for a number of macular diseases (3,4). OCT is especially promising for the diagnosis and monitoring of glaucoma and macular edema associated with diabetic retinopathy because it permits the quantitative measurement of changes in the retinal or retinal-nerve fiber layer thickness. Because morphological changes often occur before the onset of physical symptoms, OCT can provide a powerful approach for the early detection of these diseases. Recently, OCT has been applied for imaging in a wide range of nontransparent tissues (5-9). In tissues other than the eye, the imaging depth is limited by optical attenuation resulting from scattering and absorption. Ophthalmic imaging is typically performed at 800-nm wavelengths. However, because optical scattering decreases with