Improved contrast in clinical imaging

Abstract

Optical coherence tomography (OCT) is a standard tool for detailed twoand three-dimensional noninvasive analysis of retinal diseases in ophthalmology. It is also a promising research instrument, permitting early tumor diagnosis in skin and bladder tissue. OCT provides high-resolution images of biological tissue using the interference between incident and returning (reflected or backscattered) light1 to obtain structural information that can be used to distinguish between a skin tumor and healthy skin. A particularly important use is to distinguish benign and malignant tumors by means of ‘basement membrane invasion.’ If the tumor has spread beyond the epithelium (outer layer) of a structure in the body through the basement membrane (middle layer), and is intruding into the underlying connective tissue, it must be cut out. However, the structural contrast is not always sufficient to result in a meaningful diagnosis. So-called exogenous contrast agents are frequently used to increase the structural contrast in OCT images. Microspheres, nanoshells, nanorods, microbubbles, or other contrast agents are inserted into the tissue and increase the image intensity within specific regions of interest.2 They are, however, difficult to administer and possibly toxic, and so different approaches are needed.3 A common alternative to contrast agents relies on analyzing the frequency dependence of backscattered light. This can improve differentiation of tissue structures, but this ‘spectroscopic-OCT’ method requires computationally expensive time-frequency analysis,3 making it too slow for significant use with patients. Furthermore, this technique analyzes the wavelength dependence of the sample only within one distinct spectral regime. Higher wavelength dependences can be exploited by simultaneously imaging in two bands4 and using color to represent the difference between the two resulting images. This is computationally inexpensive and this method of generating Figure 1. Experimental setup. FC: Fiber collimator. BS: Beamsplitter. G: Diffraction grating. InGaAs: Indium gallium arsenide. Si: Silicon.