J.F. de Boer

Two-dimensional birefringence imaging in biological tissue by polarization-sensitive optical coherence tomography.

Using a low-coherence Michelson interferometer, we measure two-dimensional images of optical birefringence in bovine tendon as a function of depth. Polarization-sensitive detection of the signal formed by interference of backscattered light from the sample and a mirror in the reference arm give the optical phase delay between light that is propagating along the fast and slow axes of the birefringent tendon. Images showing the change in birefringence in response to laser irradiation are presented. The technique permits rapid noncontact investigation of tissue structural properties through two-dimensional imaging of birefringence.

Determination of the depth-resolved Stokes parameters of light backscattered from turbid media by use of polarization-sensitive optical coherence tomography.

Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media. Using a low-coherence light source, one can determine the Stokes parameters of backscattered light as a function of optical path in turbid media. To demonstrate the application of this technique we determined the birefringence and the optical axis in fibrous tissue (rodent muscle) and in vivo rodent skin. PS-OCT has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials and their effects on the polarization state of backscattered light. This method may also find applications in material science for investigation of polarization properties (e.g., birefringence) in opaque media such as ceramics and crystals.

In vivo burn depth determination by high-speed fiber-based polarization sensitive optical coherence tomography.

We report the first application of high-speed fiber-based polarization sensitive optical coherence tomography (PS-OCT) to image burned tissue in vivo. Thermal injury denatures collagen in skin and PS-OCT can measure the reduction in collagen birefringence using depth resolved changes in the polarization state of light propagated in, and reflected from, the tissue. Stokes vectors were calculated for each point in a scan and birefringence relative to incident polarization determined using four incident polarization states. Using a high-speed fiber-based PS-OCT system on rat skin burned for varying periods of time, a correlation between birefringence and actual burn depth determined by histological analysis was established. In conclusion, PS-OCT has potential use for noninvasive assessment of burn depth.

Optical coherence tomography of the rat cochlea

Optical coherence tomography (OCT) was used to image the internal structure of a rat cochlea (ex vivo). Immediately following sacrifice, the temporal bone of a Sprague-Dawley rat was harvested. Axial OCT cross sectional images (over regions of interest, 1x1 mm-2x8 mm) were obtained with a spatial resolution of 10-15 microm. The osseous borders of the lateral membranous labyrinth overlying the cochlea and the scala vestibuli, media, and tympani, which were well demarcated by the modiolus, Reissners and the basilar membranes, were clearly identified. OCT can be used to image internal structures in the cochlea without violating the osseous labyrinth using simple surgical exposure of the promontory, and may potentially be used to diagnose inner ear pathology in vivo in both animal and human subjects labyrinth.

Polarization effects in optical coherence tomography of various biological tissues

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain spatially resolved ex vivo images of polarization changes in skeletal muscle, bone, skin and brain. Through coherent detection of two orthogonal polarization states of the signal formed by interference of light reflected from the biological sample and a mirror in the reference arm of a Michelson interferometer, the depth resolved change in polarization was measured. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique investigating tissue structural properties. In addition, the effects of single polarization state detection on OCT image formation is demonstrated.

Focus-extension by depth-encoded synthetic aperture in Optical Coherence Tomography

We present a novel method to extend the depth-of-focus of Optical Coherence Tomography (OCT). OCT is an interferometric imaging technique that provides depth-resolved scattering information. The axial resolution in OCT is provided by the coherence gate and is invariant over the full image depth. The lateral resolution is determined by the beam parameters such as wavelength and numerical aperture. The Rayleigh range determines the depth range over which the lateral resolution can be maintained. The lateral resolution is often sacrificed to maintain relatively long Rayleigh range. In this study, we propose to use a depth-encoded synthetic aperture detection scheme to extend the depth range over which a sharp focus can be maintained beyond the Rayleigh range. An annular phase plate is inserted into the light path in the sample arm, which gives rise to three separate images in a single B-scan, corresponding to three different optical path length encoded apertures. These three images are coherently summed after phase-manipulation to reconstruct a new image with a lateral resolution that is maintained over a five times larger depth range.

Determination of the depth resolved Stokes parameters of light backscattered from turbid media using polarization sensitive optical coherence tomography

Polarization-sensitive optical coherence tomography (PS-OCT) was used to characterize completely the polarization state of light backscattered from turbid media. Using a low-coherence light source, one can determine the Stokes parameters of backscattered light as a function of optical path in turbid media. To demonstrate the application of this technique we determined the birefringence and the optical axis in fibrous tissue (rodent muscle) and in vivo rodent skin. PS-OCT has potentially useful applications in biomedical optics by imaging simultaneously the structural properties of turbid biological materials and their effects on the polarization state of backscattered light. This method may also find applications in material science for investigation of polarization properties (e.g., birefringence) in opaque media such as ceramics and crystals.

Polarization sensitive optical frequency domain imaging system for endobronchial imaging

A polarization sensitive endoscopic optical frequency domain imaging (PS-OFDI) system with a motorized distal scanning catheter is demonstrated. It employs a passive polarization delay unit to multiplex two orthogonal probing polarization states in depth, and a polarization diverse detection unit to detect interference signal in two orthogonal polarization channels. Per depth location four electro-magnetic field components are measured that can be represented in a complex 2x2 field matrix. A Jones matrix of the sample is derived and the sample birefringence is extracted by eigenvalue decomposition. The condition of balanced detection and the polarization mode dispersion are quantified. A complex field averaging method based on the alignment of randomly pointing field phasors is developed to reduce speckle noise. The variation of the polarization states incident on the tissue due to the circular scanning and catheter sheath birefringence is investigated. With this system we demonstrated imaging of ex vivo chicken muscle, in vivo pig lung and ex vivo human lung specimens.

Coherent signal composition and global phase determination in signal multiplexed polarization sensitive optical coherence tomography

We present an analysis of the structural image information acquired with polarization sensitive optical coherence tomography (PS-OCT). In PS-OCT a total of four channels of data are acquired: two orthogonal polarization state components for each of two incident polarization states by which the sample is interrogated. Up to recently, the structural information of the sample was obtained by incoherent summation of these four channels. The four channels can be represented as a Jones matrix for each data point acquired from a sample. We show that the Signal to Noise ratio of the structural information can be improved by 2.3 dB by taking advantage of the structure of this Jones matrix, imposed by the propagation and scattering properties of the sample. We demonstrate that the Jones Matrices are all in the shape of an SU(2) matrix, which is key to understanding the coherent composition of the structural image in PS-OCT and the 2.3 dB SNR improvement. We also discuss a global phase of the Jones matrix in signal multiplexed PS-OCT.

Collecting optical coherence elastography depth profiles with a micromachined cantilever probe

We present an experimental setup that combines optical coherence elastography depth sensing with atomic force microscope indentation. The instrument relies on a miniaturized cantilever probe that compresses a sample with a small footprint force and simultaneously collects an optical coherence tomography (OCT) depth profile underneath the indenting point. The deflection of the cantilever can be monitored via optical fiber interferometry with a resolution of 2 nm. The OCT readout then provides depth profiles of the subsurface layer deformation with 15 nm resolution and depth range of a few millimeters.