Ulf L. Oesterberg

Near-infrared diffuse tomography combined with a priori MRI structural information: testing a hybrid image reconstruction methodology with functional imaging of the rat cranium

The ability to combine MRI and NIR information into a single imaging modality can potentially provide high-resolution images of absolute hemoglobin concentration and oxygen saturation from non-invasive simultaneous measurement. In order to take advantage of the MRI information, a finite element based imaging algorithm has been used to include both a priori structural information and spatial constraints during NIR image reconstruction. A case study of the optical changes in a rat cranium in response to variations in the inhaled oxygen supply has been used to test this imaging algorithm. The ability to introduce structure from MRI into NW image reconstruction resulted in significant improvements in the discrimination of hemoglobin concentration changes from oxygen saturation changes in the brain and muscle tissues.

Laser diode to single-mode fiber coupling using an out-of-plane misalignment model

The emergence of widespread fiber-optic local-area networks (LANs) for multimedia applications has presented industry with new challenges regarding the design of efficient cornmunication links. Telecommunication lines typically involve many users per link, while data/video transmission will likely have far fewer users per link. Since the latter case requires more transceivers and connectors, one would like to reduce the component cost. However, the requirement for low coupling losses must not be compromised in the process. Bandwidth-distance requirements will likely favor singlemode technology over multimode for long-haul transmissions. Bit error rates (BERs) of 10—15 would then be achievable for high data rates. Unfortunately, it is well known that very small misalignments can have dramatic effects on the coupling efficiency in single-mode systems. Single-mode technology places severe mechanical constraints on connectors because the small core size makes accurate alignment difficult. Components must be manufactured to not only sat-

Image reconstruction of continuously varying objects and simulated breast cancer lesions

Numerical simulations of near-infrared diffusion tomography are used to reconstruct absorption and scattering images of continuously varying objects and simulated breast cancer lesions. The breast cancer lesions are modeled using chromophore levels derived from typical breast anatomy and an approximation for breast cancer lesions based on contemporary investigations of tissue/tumor optical properties. Images of hemoglobin concentration and oxygenation are derived using least squares regressions on multiple wavelength near-infrared absorption images. The simulations are performed using finite element based iterative reconstructions. The results are compared with in vivo measurements of breast cancer patients acquired using near-infrared diffusion tomography.

Measurement of cross talk in order-packed image fiber bundles

This paper describes a novel technique to quantitatively measure cross talk in order-packed image fiber bundles using a calibrated optical video system. At the input end of the fiber bundle, by carefully matching numerical aperture of the focusing lens, light is coupled into one single fiber. At the output end, a large area around the excited fiber is monitored using a video CCD array. The light intensity in integrated and the ratio between output light in adjacent fibers and output light from the excited fiber constitutes the percentage cross talk. Additionally, static modulation transfer function (MTF) measurements together with visual inspection was used to correlate the cross talk to the imaging properties of an optical fiber bundle. Comparison between different samples results in the following observations: typical cross talk varies between 1-15% for good versus bad imaging fiber bundles; image fiber bundles with less cross talk show higher static MTF at low spatial frequencies; different defects in the fiber bundle manifest themselves by specific signatures, both in the static MTF and the cross talk; when the cross talk is less 5%, the spatial cut-off frequency is not altered; cross talk decreases when reducing the numerical aperture of the coupling system.

Preliminary study of near-infrared tomographic imaging of heterogeneous media: simulations and images of excised breast tissue

A preliminary study of the effect of a spatially heterogeneous background upon near-infrared tomographic imaging is presented using computer simulations and phantom experiments. In simulations, increasing heterogeneity is added to a sample region and the reconstructed images are analyzed. A finite-element reconstruction algorithm based on the frequency-domain diffusion equation is used. A phantom study for a heterogeneous object was performed by embedding an object in excised breast tissue. The ability to distinguish and quantify a heterogeneous inclusion in a heterogeneous background is discussed. In this case, a 20 mm diameter object in a 100 mm background is characterized with up to 75% added heterogeneity in simulation. A 30 mm cube of pork embedded in a 86 mm sample of excised breast tissue is localized and recovered to within 25% of its expected contrast.

Indirect optical image reconstruction with a cw He-Ne laser for breast cancer detection

We demonstrate successful image reconstruction based on experimental data obtained with a cw HeNe laser system in a laboratory phantom having two optically distinct regions. The experimental system which presently supplies intensity only measurements exploits a tomographic type of data collection scheme that provides information from which a spatially variable optical diffusion coefficient map is deduced. Different types of boundary constraints have been considered and the images produced indicate that the zero average intensity boundary constraint results in better imaging quality. While the cw optical system used in these experiments is crude, and the physical phantom imaged is highly idealized, the results suggest that tomographic data collection coupled to a finite element image formation algorithm offers a potentially powerful approach for biologically based optical imaging.

Optical image reconstruction using DC data: enhanced experimental results with tissue-like phantoms

An experimental study of detectability in steady-state optical image reconstruction through varying the single target size (15 mm - 4 mm in diameters) at three different locations (center, middle and near boundary surface) and a through evaluation of our finite element based image reconstruction algorithm to distinguish multiple targets are performed in this paper. The multi-target experiments consisted of several interested geometric and contrast combinations including two targets with the same contrast (2:1 between the target and the background) at three different separation distances, two targets with different contrasts (2:1 and 4:1) at two different separation distances and three targets with the same and different sizes (20 mm, 15 mm and 8 mm in diameters) and contrasts (2:1, 4:1 and 8:1) respectively. The reconstruction algorithm used along with a few imaging enhancement methods including total variation minimization, dual meshing and spatial low pass filtering are discussed. Quantitative measures of image quality including the size, location and shape of the heterogeneity are used to quantify the analysis. The results show that near 22:1 ratio (tissue thickness relative to detectable anomaly size) can be obtained and multiple targets can be correctly resolved using dc data with an 86 mm diameter circular tissue-like phantom.

Cross talk study of flexible multimode fibers

The multimode fiber is one of the important components in optical imaging and multichannel sensing systems. The critical issue in the design of such systems is the level of the crosstalk ratio (light leakage between adjacent fibers or interchannels). The system performance degrades whenever crosstalk leads to a transfer of power from one fiber to another (or one channel from another). This paper develops a unique approach to derive and compute the crosstalk ratio, effective coupling coefficient and field distribution of flexible multimode fibers. Our results also demonstrate that Gaussian approximation can be used to compute the crosstalk ratio successfully. Furthermore, by taking advantage of the Gaussian approximation, we can obtain quantitative correlation between the crosstalk ratio and fiber parameters in order to reduce crosstalk. This may lead to improved contrast and sensitivity of fiber imaging and multichannel sensing systems.

Applications of time-frequency bases to multiple access fiber optic communication systems

In this paper we present background material concerning the propagation of pulses through fiber optics, and current transmission schemes in fiber optics. We discuss the value of using different signal processing techniques, and time-frequency bases in fiber optic transmission. Finally we will outline methods which allow one to send optical pulses with arbitrary shapes through fiber optics.

Use of the asymmetric photoionization model to explain the length and time dependence of second-harmonic generation in fibers

The growth of Second Harmonic Generation (SHG) in fibers has been observed to saturate with time as well as with length and can actually decrease after prolonged exposure to fundamental light. We have measured the length dependence of the SHG as a function of time during preparation, saturation and subsequent exposure to IR light. We have also measured the time dependence of erasure of the (chi) (2) grating with exposure to various amounts of green light. Based on these measurements we have been able to determine the parameters of the asymmetric photoionization model, specifically the current and two photon absorption rate. We have performed preliminary experiments using planar waveguides to measure the asymmetric photocurrent.