Optical coherence tomography for investigation of the pancreatico-biliary system: still experimental?

Optical coherence tomography (OCT) is an optical imaging modality introduced in 1991 [1] that performs high-resolution, cross-sectional, subsurface tomographic imaging of the internal microstructure in materials and biologic systems by measuring backscattered or backreflected infrared light. The physical principle of OCT is similar to that of B-mode ultrasound imaging, except that the intensity of infrared light, rather than sound waves, is measured. Wavelenghts of the infrared light used in OCT are one to two orders of magnitude higher than ultrasound wavelenght, so OCT technology can yield a lateral and axial spatial resolution of about 10 micron, which is 10- to 25-fold better than that of available high-frequency ultrasound imaging. The spatial resolution of OCT images is nearly equivalent to that of histologic sections. The depth of penetration of OCT imaging is approximately 1-3 mm, depending upon tissue structure, depth of focus of the probe used, and pressure applied to the tissue surface. Although the progressive increase in ultrasound resolution is accompanied by a corresponding decease in depth of penetration, a similar trade-off between resolution and depth of penetration does not occur in OCT imaging. In contrast to magnification endoscopy, OCT has depth. Several in vitro studies demonstrated the feasibility of OCT in the gastrointestinal (GI) tract: in these studies the GI tract wall was identified as a multiple layer structure chraracterized by a sequence of hyper- and hypo-reflective layers, with a variable homogenicity of the back-scattered signal [2, 3, 4]. Neoplastic and normal tissue also showed different light backscattering patterns [5]. However, the optical properties of nonliving tissues are different from tissue in vivo. Subsequent studies were therefore performed in ex-vivo tissue specimens and aimed at comparing OCT imaging with histology, to assess the realiability of the OCT technique to identify and recognize the GI wall structure. OCT was shown to clearly differentiate the layers’ structure of the GI wall [6]. In the last decade, OCT technology has evolved from an experimental laboratory tool to a new diagnostic imaging modality with a wide spectrum of clinical applications in medical practice, including the gastrointestinal tract and pancreatico-biliary ductal system.