Showing papers by "Simon R. Arridge published in 2002"

Detection and modeling of non-Gaussian apparent diffusion coefficient profiles in human brain data.

Abstract: This work details the observation of non-Gaussian apparent diffusion coefficient (ADC) profiles in multi-direction, diffusion-weighted MR data acquired with easily achievable imaging parameters (b 1000 s/mm2). A technique is described for modeling the profile of the ADC over the sphere, which can capture non-Gaussian effects that can occur at, for example, intersections of different tissue types or white matter fiber tracts. When these effects are significant, the common diffusion tensor model is inappropriate, since it is based on the assumption of a simple underlying diffusion process, which can be described by a Gaussian probability density function. A sequence of models of increasing complexity is obtained by truncating the spherical harmonic (SH) expansion of the ADC measurements at several orders. Further, a method is described for selection of the most appropriate of these models, in order to describe the data adequately but without overfitting. The combined procedure is used to classify the profile at each voxel as isotropic, anisotropic Gaussian, or non-Gaussian, each with reference to the underlying probability density function of displacement of water molecules. We use it to show that non-Gaussian profiles arise consistently in various regions of the human brain where complex tissue structure is known to exist, and can be observed in data typical of clinical scanners. The performance of the procedure developed is characterized using synthetic data in order to demonstrate that the observed effects are genuine. This characterization validates the use of our method as an indicator of pathology that affects tissue structure, which will tend to reduce the complexity of the selected model.

Three-dimensional optical tomography of the premature infant brain

Abstract: For the first time, three-dimensional images of the newborn infant brain have been generated using measurements of transmitted light A 32-channel time-resolved imaging system was employed, and data were acquired using custom-made helmets which couple source fibres and detector bundles to the infant head Images have been reconstructed using measurements of mean flight time relative to those acquired on a homogeneous reference phantom, and using a head-shaped 3D finite-element-based forward model with an external boundary constrained to match the measured positions of the sources and detectors Results are presented for a premature infant with a cerebral haemorrhage predominantly located within the left ventricle Images representing the distribution of absorption at 780 nm and 815 nm reveal an asymmetry consistent with the haemorrhage, and corresponding maps of blood volume and fractional oxygen saturation are generally within expected physiological values

Quantification of spinal cord atrophy from magnetic resonance images via a B‐spline active surface model

Abstract: A method is presented that aims at segmenting and measuring the surface of the spinal cord from MR images in order to detect and quantify atrophy. A semiautomatic segmentation with very little intervention from an operator is proposed. It is based on the optimization of a B-spline active surface. The method allows for the computation of orthogonal cross-sections at any level along the cord, from which measurements are derived, such as cross-sectional area or curvature. An evaluation of the accuracy and reproducibility of the method is presented.

System and method for enabling simultaneous calibration and imaging of a medium

Abstract: The methods and systems are provided that alleviate the impact of experimental systematic errors. These calibration methods and systems can be based on the discovery that by including source and detector calibration factors as part of the inverse calculation for image reconstruction, image artifacts can be significantly reduced. The novel methods and systems enhance contrast in images of the distribution of the radioactive properties of a medium, and enable improved detection of, for example, spatial variation in optical properties within highly scattering media, such as human or animal tissue. The novel methods and systems receive radiation which exits from the medium. Then, one or more optical properties of the medium are derived using the received radiation and one or more calibration factors, wherein the calibration factors are variables. Subsequently, a distribution of the optical properties in the medium is determined using the derived optical properties.

Diffusion tomography in dense media

Abstract: Publisher Summary This chapter provides an overview of the inverse problem and discusses the strategies that are adopted in order to solve it. One of the models employed to solve the inverse problem is the model of photon transport in dense media. Some insight into light propagation in diffusive media can be gained by examining infinite media. In particular, verification of optical scattering and absorption parameters is frequently made with source and detector fibers immersed in a large container, and far from the container walls. In a finite domain, however, boundary conditions are used. The Robin-to-Neumann map—a linear operator mapping boundary sources to boundary data—can also be employed to solve the inverse problem. An important tool in scattering problems, in general, is the approximation of the change in the field due to a change in state, developed in a series based on known functions for the reference state.

Optical Tomography of the Breast using a 32-Channel Time-Resolved Imager

Abstract: Studies on volunteers are being performed to assess three-dimensional time-resolved optical tomography as a means of detecting and specifying breast disease. The dual-wavelength instrument is employed to generate images of the tissue optical properties.

Anisotropic effect in light scattering and some implications in optical tomography

Abstract: In this paper a possible model for anisotropic light scattering is proposed, and applied on the equations used to model light propagation in optical tomography. A simultaneous reconstruction of anisotropic parameters and the absorption coefficient is presented.

Time resolved optical imaging of the newborn infant brain: initial clinical results

Abstract: A 32-channel time-resolved imaging system has been used to acquire three-dimensional (3D) optical tomography data on newborn infants for the first time. Multiple sources and detectors were coupled to the infant head using a custom-designed helmet. Temporal distributions of transmitted light were recorded during illumination by picosecond pulses of light at 780 nm and 815 nm in order to generate 3D images of the brain.

Simultaneous estimation of optical anisotropy and absorption in medical optical tomography

Abstract: In this paper we propose a possible model for anisotropic light propagation, and present simultaneous reconstruction of anisotropy parameters and optical absorption for optical tomography. The anisotropic model is introduced into the Radiative Transfer Equation (RTE). The most commonly used approximation to the RTE in optical tomography is the Diffusion Equation (DE). In anisotropic case, the diffusion coefficient in the DE assumes a tensor form. We present the diffusion tensor as an eigenvalue decomposition corresponding to the directions and the strength parameters of anisotropy. The numerical approximation is done in two dimensions using the finite element (FE) method. We then consider the inverse problem of reconstructing the optical absorption when the directions of anisotropy are assumed to be known, but the strength may vary. For this estimation to be successful, the strength parameters are reconstructed simultaneously with the absorption. We present numerical examples of cases in which the location of anisotropy is truly known, and in which there is an error in the assumed location.

Oxygen saturation and blood-volume derivation from multi-wavelength time-resolved optical tomography data

Abstract: We present an analysis of the accuracy of oxygen saturation and blood volume maps derived from temporal optical tomographic data. Simulations and then clinical data are used to demonstrate different techniques for extracting the parameters.

Optical Tomography of a Realistic Head-Shaped Phantom

Abstract: Optical tomography images have been reconstructed from data generated using a head-shaped finite element model. The images were significantly improved when a priori information was included. Further results will be presented from a realistic phantom.