Spatial refractive index measurement of porcine artery using differential phase optical coherence microscopy.

Biomedical Engineering Department, University of Texas of Austin, TexasBackground and Objectives: We describe a methodol-ogytorecordspatialvariation ofrefractiveindexofporcinerenal artery using differential phase optical coherencemicroscopy (DP-OCM).Study Design/Materials and Methods: The DP-OCMprovidesquantitativemeasurementofthinspecimenphaseretardationandrefractiveindexbymeasuringopticalpath-lengthchangesontheorderofafewnanometersandwithalateralresolutionof3mm.TheDP-OCMinstrumentationisan all-ﬁber, dual-channel Michelson interferometer con-structed using a polarization maintaining (PM) ﬁber.Results: Two-dimensional en face dual-channel phaseimages are taken over a 150 200 mm region on amicroscopic slide, and the images are reconstructed byplotting a two-dimensional refractive index map as theOCM beam is moved across the sample.Conclusions: Because the DP-OCM can record transientchangesintheopticalpath-length,thesystemmaybeusedto record quantitative optical path-length alterations oftissue in response to various stimuli. A ﬁber-based DP-OCM may have the potential to substantially improve invivo imaging of individual cells for a variety of clinicaldiagnostics, and monitoring applications. Lasers Surg.Med. 38:955–959, 2006. 2006 Wiley-Liss, Inc.Key words: phase sensitive optical coherence micro-scopy; refractive index; relative phase retardationINTRODUCTIONOptical coherence tomography (OCT) has emerged as animportantopticalimagingmodalityinnoninvasivemedicaldiagnostics. OCT generates high-resolution images byutilizing the cross correlation between light backscatteredfrom the sample and the reference reﬂector. Varioustechnical approaches have been developed to improveOCT spatial resolution, imaging acquisition rate [1,2],andimagequality[3,4].Mostofthesetechniquesusefringeamplitude or a combination of fringe amplitude andpolarization information in reﬂected light [5]. Magnitudeof back-reﬂected light is dependent on the local refractiveindex gradients of the structural components comprisingthe test specimen. Polarization information basicallyreveals different phase retardation, caused by a tissue,between two orthogonal polarization states of light.Knowledge of local refractive index variations in biologi-cal tissue is not only important in tissue optics but also amajor factor that determines fringe signal variation inconventionalOCT.Averagerefractiveindexoverrelativelyhomogeneous tissue medium has been measured byTearney et al.[6] and Knu¨ttel et al.[7]. The basic approachpresented by Tearney et al. [6] was to adjust the referencemirror for maximum signal at a particular focus positionwithinatissuewhereasKnu¨tteletal.[7]adaptedamovingﬁber tip/collimating lens in the sample path in order totrace maximum signal variation. Two-dimensional bire-fringence mapping of porcine myocardium tissue wasreported [6]. The DP-OCM is capable of measuring opticalpath-length changes on the order of a few nanometers ( 5nm) [8] so that it can be applied to image two-dimensionallocal refractive index variation of thin specimens.This article presents a method to image refractive indexdistribution over a tissue specimen processed by standardhistology sectioning methods and ﬁxated on a microscopeslide.MATERIALS AND METHODSDP-OCM Instrumentation and Signal ProcessingDetailsoftheDP-OCMusedforrefractiveindexproﬁlingcan be found in references [8,9]. A simpliﬁed schematicdiagram is shown in Figure 1. The DP-OCM system isessentially a dual-channel Michelson interferometer withtwo independent channels corresponding to orthogonalpolarization modes of a polarization maintaining (PM)optical ﬁber. At the interferometer input, light emittedfrom a broadband source (l