Optical Biopsy

This article reviews the application of various types of optical spectroscopy and metrology to minimally invasive medical diagnostics. The promises and hopes, as well as the difficulties, of these developing techniques are discussed. The term ‘‘optical biopsy’’ has entered into common usage among researchers in the field of biomedical optics. Although it is inherently an inaccurate term—it is perhaps something of an oxymoron because ‘‘biopsy’’ refers specifically to the removal of tissue, whereas the implication of ‘‘optical’’ is that tissue is not removed— it is nonetheless commonly understood to represent the use of some form of optical measurement, often a type of spectroscopy, to noninvasively (or minimally invasively) perform a tissue diagnosis, in situ, in vivo, and in real-time. The motivation is to reduce the need for surgical removal of biopsy tissue samples; rather, some form of spectral analysis of the tissue is recorded in vivo by an imaging system or with an optical probe placed on or near the surface of the tissue in question. The measurement is frequently mediated by optical fibers, and a diagnosis of the tissue is then attempted based on the optical measurements. For a number of endoscopic applications, for which ‘‘random’’ biopsies are often taken in an attempt to find premalignant or early malignant conditions, an instant optical measurement could enable ‘‘guided biopsy,’’ with increased probability for sampling a diseased site, while reducing the number of tissue samples. Thus, additional motivation is provided by the potential for reduced healthcare costs as a consequence of eliminating unnecessary histology. Moreover, the immediacy of diagnostic information can reduce the emotional trauma to the patient awaiting an answer. For diseases of the gastrointestinal (GI) tract, for example, the potential benefits of optical tissue diagnosis can be significant. Several disorders of the GI tract are correlated with a predisposition for cancer, including colitis, colon polyps, and Barrett’s esophagus. Typically, these diseases are followed with annual (or more frequent) endoscopic examination accompanied by tissue biopsies. As many as 20–30 ‘‘random’’ biopsies may be taken in one session. This is a time-consuming (and therefore expensive) procedure, which entails some degree of risk for the patient. For each conventional biopsy, the biopsy tool must be withdrawn from the endoscope and the specimen removed before the tool can be reinserted for the next biopsy. In contrast, an optical diagnostic probe could be moved from site to site in succession, with each measurement being recorded in a fraction of a second, by simply moving the location of the probe tip. When a diseased site is found, the surgical biopsy can be performed at that particular site.