The Infrared Spectral Signatures of Disease: Extracting the Distinguishing Spectral Features between Normal and Diseased States

O ver the past decade, new medical diagnostic methods have been developed by several research groups worldwide, based on infrared microspectroscopy and microscopic imaging (see, for example, the compiled references in a number of recent books). These methods can be applied both to tissue sections and individual exfoliated cells. The success of these methods in differentiating cancerous from normal tissues, as well as individual cancerous, precancerous, and normal cells, is due to two major factors. First, infrared microspectroscopy monitors, in one measurement, a snapshot of the overall biochemical composition of an individual cell. This composition varies with a number of well-understood cell-biological processes; thus, the cell’s division cycle, its maturation and differentiation, as well as a transition from normal to cancerous states can be monitored via a wellunderstood spectral measurement. This differs significantly from the standard cytopathological methodology, which relies on a visual inspection of cell morphology and tissue architecture and is, therefore, subjective in nature. The second factor for the success of spectral diagnoses is the fact that data can be acquired fairly rapidly: it takes about 500 ms to collect a good infrared micro-spectrum from a voxel of biological material. The size of such a voxel is typically about 12 3 12 3 5 lm in the x, y, and z directions, where the lateral (x,y) dimension is determined by the diffraction limit and the z direction is determined by the thickness of the tissue section or the thickness of a cell. In the case of infrared micro-spectral imaging of human tissues, up to 100 000 individual voxel spectra are collected to create huge hyperspectral data sets, where the term ‘‘hyperspectral’’ implies spatially resolved data with distinct x and y coordinates, and spectral information from each x,y point. The analysis of the hyperspectral dataset is carried out by methods of chemometrics, which detect small, but recurring differences,