Discrimination of Complex Activation Patterns in Near Infrared Optical Tomography with Artificial Neural Networks.

Near-infrared optical tomography (NIROT) has great promise for many clinical problems. Here we focus on the study of brain function. During NIROT image reconstruction of brain activity, an inverse problem has to be solved that is sensitive to small superficial perturbations on the head such as e.g. birthmarks on the skin and hair. To consider these perturbations, standard physical modeling is unpractical, since it requires the implementation of detailed information that is generally unavailable. The aim here was to test whether artificial neural networks (ANN) are able to handle such perturbations and thus detect brain activity correctly. For simplicity, we created a virtual test model, where we simulated a pattern of activated and resting brain regions, which was covered by skin features like hair or melanin. We compared the performance of this ANN approach with that of an inverse problem based on a Monte Carlo (MC) model for light propagation. We conclude that ANNs tolerate substantially higher levels of skin perturbations than MC models and consequently are more suitable for detecting brain activity.

A New Method Based on Virtual Fluence Detectors and Software Toolbox for Handheld Spectral Optoacoustic Tomography.

A minimal setup for optoacoustic (OA) imaging requires an ultrasound probe and a pulsed laser. Such a system is capable of imaging small blood vessels and is sensitive to variations in their oxygen saturation. However, absolute oxygenation values cannot be obtained without a proper correction for the varying light fluence resulting from the optical attenuation in the surrounding tissue. Other techniques, such as near-infrared optical tomography (NIROT) can be employed to assist OA imaging for fluence compensation. In this paper, we propose using blood vessels as virtual fluence detectors (VD), which serve as light detectors for NIROT image reconstructions. By avoiding the use of real photon detectors, a simpler system could be implemented in a hand-held device comparable in size with conventional ultrasound probes. Even for a low number of VDs it provides increased informational value which, in combination with a large number of light sources, results in precise reconstructions. We define a tomographic inverse problem based on ratios of OA signals measured at several wavelengths where optical properties of VDs, tumor and normal tissue can be reconstructed simultaneously. The use of ratio data effectively removes light source skin coupling errors for the case of emission in a single point, which is required for clinical applications. We have defined the mathematical structure of an inverse problem where chromophore concentrations for normal, tumor and embedded VDs are obtained simultaneously from this ratio data. To test the performance of our approach we show an image reconstruction on a virtual phantom with an embedded tumor in the vicinity of eight blood vessels. We conclude that this limited number of VDs, located in areas of maximum sensitivity result in high quality reconstructions. For the simplest case of a single blood vessel located in a homogeneous tissue, we present a graphical user interface based toolbox for conducting virtual experiments. The toolbox can be used to assist in the design and optimization of suitable hardware for different applications, among which imaging tumor oxygenation and ischemic lesions in the brain of preterm infants are of great clinical value.

Optical properties of mice's stool in 550 to 1000 nm wavelength range.

The aim of this work was to measure optical properties of stool of mice to provide this relevant wavelength-dependent behavior for optical imaging modalities such as fluorescent molecular tomography and near-infrared optical tomography. BALB/c nude female mice were studied and optical properties of the stool were determined by employing the inverse adding-doubling approach. The animals were kept on chlorophyll-free diet. Nine stool samples were measured. The wavelength-dependent behavior of absorption and scattering in 550 to 1000 nm range is presented. The reduced scattering spectrum is fitted to the Mie scattering approximation in the near-infrared (NIR) wavelength range and to the Mie + Rayleigh approximation in visible/NIR range with the fitting coefficients presented. The study revealed that the absorption spectrum of stool can lead to crosstalk with the spectrum of hemoglobin in the NIR range.