Johannes F. de Boer

Modelling light distributions of homogeneous versus discrete absorbers in light irradiated turbid media

Laser treatment of port wine stains has often been modelled assuming that blood is distributed homogeneously over the dermal volume, instead of enclosed within discrete vessels. The purpose of this paper is to analyse the consequences of this assumption. Due to strong light absorption by blood, fluence rate near the centre of the vessel is much lower than at the periphery. Red blood cells near the centre of the vessel therefore absorb less light than those at the periphery. Effectively, when distributed homogeneously over the dermis, fewer red blood cells would produce the same absorption as the actual number of red blood cells distributed in discrete vessels. We quantified this effect by defining a correction factor for the effective absorbing blood volume of a single vessel. For a dermis with multiple vessels, we used this factor to define an effective homogeneous blood concentration. This was used in Monte Carlo computations of the fluence rate in a homogeneous skin model, and compared with fluence rate distributions using discrete blood vessels with equal dermal blood concentration. For realistic values of skin parameters the homogeneous model with corrected blood concentration accurately represents fluence rates in the model with discrete blood vessels. In conclusion, the correction procedure simplifies the calculation of fluence rate distributions in turbid media with discrete absorbers. This will allow future Monte Carlo computations of, for example, colour perception and optimization of vascular damage by laser treatment of port wine stain models with realistic vessel anatomy.

Two-dimensional birefringence imaging in biological tissue using phase and polarization sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) was used to obtain images of optical birefringence in biological tissues. Through simultaneous detection of two orthogonal polarization states of the signal formed by interference of light backscattered from the biological sample and a mirror in the reference arm of a Michelson interferometer, the optical phase delay between light propagating along the fast and slow axes of birefringence was measured. Simultaneous detection of both polarizations also permits reconstruction of the electro-magnetic wave backscattered from the sample. Inasmuch as any fibrous structure will influence the polarization of light, PS-OCT is a potentially powerful technique in the field of biomedical imaging. It allows rapid non-contact investigation of tissue structural properties through spatially resolved imaging of birefringence.

Burn depth determination by high-speed fiber-based polarization-sensitive optical coherence tomography at 1.3 μm

Burn depth determination is a critical factor in the treatment of thermal injury. We have developed a technique, polarization sensitive optical coherence tomography (PS- OCT), to assess burn depth non-invasively. Thermal injury denatures collagen in human skin. PS-OCT is able to measure the resulting reduction in collagen birefringence using depth resolved changes in the polarization of light propagated and reflected from the sample. In a previous study, we used a free space PS-OCT system at 850 nm to image in vivo the skin of rats burned for various amounts of time. Using a high-speed system at 1.3 micrometers has the advantages of greater depth penetration and reduction of motion artifacts due to breathing and small movements of the animal. Stokes vectors were calculated for each point in the scans and the relative birefringence was determined using different incident polarization states. Birefringence was correlated with actual burn depth determined by histological analysis. Our results show a marked difference between normal tissue and even the slightest burn, and a consistent trend for various degrees of burns.

Intensity and field correlations in multiple scattered light

Abstract We discuss the influence of interference in multiple scattering of light in strongly scattering media on field (amplitude) and intensity correlations. Results of experimental measurements on the dependence of the recently found long-range intensity fluctuations on the spatial profile of the incoming beam are presented. Our results are consistent with the recently introduced concept of an energy velocity, which is substantially lower than the phase velocity. We demonstrate that an important relation exists between the redshifts introduced by Wolf and weak localization of light.

Automatic estimation of retinal nerve fiber bundle orientation in SD-OCT images using a structure-oriented smoothing filter

Optical coherence tomography (OCT) yields high-resolution, three-dimensional images of the retina. A better understanding of retinal nerve fiber bundle (RNFB) trajectories in combination with visual field data may be used for future diagnosis and monitoring of glaucoma. However, manual tracing of these bundles is a tedious task. In this work, we present an automatic technique to estimate the orientation of RNFBs from volumetric OCT scans. Our method consists of several steps, starting from automatic segmentation of the RNFL. Then, a stack of en face images around the posterior nerve fiber layer interface was extracted. The image showing the best visibility of RNFB trajectories was selected for further processing. After denoising the selected en face image, a semblance structure-oriented filter was applied to probe the strength of local linear structure in a discrete set of orientations creating an orientation space. Gaussian filtering along the orientation axis in this space is used to find the dominant orientation. Next, a confidence map was created to supplement the estimated orientation. This confidence map was used as pixel weight in normalized convolution to regularize the semblance filter response after which a new orientation estimate can be obtained. Finally, after several iterations an orientation field corresponding to the strongest local orientation was obtained. The RNFB orientations of six macular scans from three subjects were estimated. For all scans, visual inspection shows a good agreement between the estimated orientation fields and the RNFB trajectories in the en face images. Additionally, a good correlation between the orientation fields of two scans of the same subject was observed. Our method was also applied to a larger field of view around the macula. Manual tracing of the RNFB trajectories shows a good agreement with the automatically obtained streamlines obtained by fiber tracking.

Correction of phase-error for phase-resolved k-clocked optical frequency domain imaging

Phase-resolved optical frequency domain imaging (OFDI) has emerged as a promising technique for blood flow measurement in human tissues. Phase stability is essential for this technique to achieve high accuracy in flow velocity measurement. In OFDI systems that use k-clocking for the data acquisition, phase-error occurs due to jitter in the data acquisition electronics. We presented a statistical analysis of jitter represented as point shifts of the k-clocked spectrum. We demonstrated a real-time phase-error correction algorithm for phase-resolved OFDI. A 50 KHz wavelength-swept laser (Axsun Technologies) based balanced-detection OFDI system was developed centered at 1310 nm. To evaluate the performance of this algorithm, a stationary gold mirror was employed as sample for phase analysis. Furthermore, we implemented this algorithm for imaging of human skin. Good-quality skin structure and Doppler image can be observed in real-time after phase-error correction. The results show that the algorithm can effectively correct the jitter-induced phase error in OFDI system.

Stokes parameters imaging of light reflected from biological tissue using polarization-sensitive optical coherence tomography

Polarization sensitive optical coherence tomography (PS-OCT) was used to determine the depth resolved Stokes parameters of light backscattered from highly scattering biological samples. Through simultaneous detection of the amplitude and relative phase of signal fringes in orthogonal polarization states formed by interference of light backscattered from turbid media and a mirror in the reference arm of a Michelson interferometer, changes in the Stokes parameters due to the optical phase delay between light propagating along the fast and slow axes of birefringent media were measured. Inasmuch as fibrous structures in many biological tissues influence the polarization state of light backscattered, PS-OCT is a potentially useful technique to image the structural properties of turbid biological materials. The method can also be applied to the investigation of birefringent properties in highly scattering materials such as ceramics and crystals.

Preliminary Experiments on the Transmission of Light Through Non-Linear Disordered Media

Since the discovery of enhanced backscatter (weak localization) [1], light scattering in disordered media has drawn a lot of attention. In the pursuit of strong localization, a lot of interesting phenomena were discovered, such as fluctuations on the transmission of light through disordered slabs [2–7]. Recently, a new class of disordered media have come into focus, “non-linear disordered media.” By non-linear in this context, a non-linear response of the constituting particles of the disordered medium to the electro-magnetic field is meant, e.g., particles with a second-harmonic susceptibility. Agranovich and Kravtsov predicted interesting phenomena that can be observed in non-linear disordered media [8]. They stated that enhanced backscatter can also be observed in the second-harmonic light generated in a non-linear disordered medium, although this is not yet experimentally verified.

Towards guided biopsy using endoscopic NIR fluorescence imaging and OCT by cancer-specific fluorescent antibodies (Conference Presentation)

Determining cancer phenotypes early in the development of the disease is fundamental to increase the efficacy of therapy. To this end, genetic profiling of bioptic tissue has become an indispensable tool; however, erroneous sampling of the tissue leads to a considerable number of false negatives. To address this shortcoming, we developed a motorized endoscope (outer diameter 1.35mm) combining near-infrared fluorescence (NIRF) imaging and optical coherence tomography (OCT) at a frame rate of 52 images per second. Here, NIRF imaging is used to guide biopsies by targeting the tumor with fluorescently labeled monoclonal antibodies that attach to receptors on the membrane of cancer cells. Furthermore, the biopsy forceps severely alter the structure of the tissue and OCT can help in reconstructing the morphology thanks to its 10 µm resolution. nWe demonstrated the device on a mouse model bearing human colon cancer, by administering a sub-therapeutic dose of a monoclonal antibody labeled with both a PET tracer and a near infrared molecule that is already used in several clinical trials (89Zr-Labetuzumab-IRdye800CW). In addition to successfully imaging 1 mm tumors with our endoscope, we developed a NIRF-OCT handheld scanner that allows mapping of the distribution of the antibody with high resolution. Finally, we also performed PET in order to quantify the bio-distribution of the drug in the mouse.

NEUROPATHOLOGICAL HALLMARKS OF ALZHEIMER’S DISEASE IN POSTMORTEM AD RETINAS

Figure 1. Proc PFA and dissec ridian. Section staining is show tinal nerve fib layer, INL1⁄4in nuclear layer, Jurre den Haan, Tjado H. J. Morrema, Jacoline B. ten Brink, Frank Verbraak, Johannes de Boer, Philip Scheltens, Annemieke M. Rozemuller, Arthur A. B. Bergen, Femke H. Bouwman, Jeroen J. M. Hoozemans, Neuroscience Campus Amsterdam, Netherlands, Amsterdam, Netherlands; Alzheimer Center and Department of Neurology, VU University Medical Center, Amsterdam, Netherlands; VU University Medical Center, Department of Pathology, Amsterdam Neuroscience, Amsterdam, Netherlands; Department of Ophthalmogenetics, Department of Ophthalmology, Academic Medical Center, University of Amsterdam, Amsterdam, Netherlands; OphthalmologyDepartment VUUniversityMedical Center, Amsterdam, Netherlands; Department of Physics, VU University, BioLaserLab, Amsterdam, Netherlands; VU University Medical Center, Amsterdam, Netherlands; Alzheimer Center and Department of Neurology, Amsterdam Neuroscience, VU University Medical Center, Amsterdam, Netherlands; VU University Medical Center, Amsterdam Neuroscience, Amsterdam, Netherlands. Contact e-mail: j.denhaan1@vumc.nl