Biomedical subjects

Simon R Arridge

Publications and source records attributed to Simon R Arridge.

6 recordsLinked to original sources

PubMed2003

Time-series estimation of biological factors in optical diffusion tomography.

We apply state space estimation techniques to the time-varying reconstruction problem in optical tomography. We develop a stochastic model for describing the evolution of quasi-sinusoidal medical signals such as the heartbeat, assuming these are represented as a known frequency with randomly varying amplitude and phase. We use the extended Kalman filter in combination with spatial regularization techniques to reconstruct images from highly under-determined time-series data. This system also naturally segments activity belonging to different biological processes. We present reconstructions of simulated data and of real data recorded from the human motor cortex (Franceschini et al 2000 Optics Express 6 49-57). It is argued that the application of these time-series techniques improves both the fidelity and temporal resolution of reconstruction in optical tomography.

Algorithms↗

PubMed2003

Optical tomography of a realistic neonatal head phantom.

We have begun clinical trials of optical tomography of the neonatal brain. To validate this research, we have built and imaged an anatomically realistic, tissue-equivalent neonatal head phantom that is hollow, allowing contrasting objects to be placed inside it. Images were reconstructed by use of two finite-element meshes, one generated from a computed tomography image of the phantom and the other spherical. The phantom was filled with a liquid of the same optical properties as the outer region, and two perturbations were placed inside. These were successfully imaged with good separation between the absorption and scatter coefficients. The phantom was then refilled with a liquid of increased absorption compared with the background to simulate the brain, and the absolute properties of the two regions were found. These were used as a priori information for the complete reconstruction. Both perturbations were visible, superimposed on the increased absorption of the central region. The head-shaped mesh performed slightly better than the spherical mesh, particularly when the absorption of the central region of the phantom was increased.

Head↗

PubMed2003

Optode positional calibration in diffuse optical tomography.

Although diffuse optical tomography is a highly promising technique used to noninvasively image blood volume and oxygenation, the reconstructed data are sensitive to systemic difference between the forward model and the actual experimental conditions. In particular, small changes in optode location or in the optode-tissue coupling coefficient significantly degrade the quality of the reconstruction images. Accurate system calibration therefore is an essential part of any experimental protocol. We present a technique for simultaneously calibrating optode positions and reconstructing images that significantly improves image quality, as we demonstrate with simulations and phantom experiments.

Calibration↗

PubMed2003

Application of a B-spline active surface technique to the measurement of cervical cord volume in multiple sclerosis from three-dimensional MR images.

PURPOSE: To evaluate the ability of a B-spline active surface technique to detect cervical spinal cord atrophy due to multiple sclerosis (MS) compared with intensity-based contouring. MATERIALS AND METHODS: In a previously reported study, the cervical spinal cords of 28 MS patients and 13 age-matched controls were imaged with a volume-acquired inversion-prepared fast spoiled gradient echo sequence at baseline and after one year. The images were reanalyzed using the B-spline technique and the results compared with the results obtained in the original report using intensity-based contouring. RESULTS: The mean cervical spinal cord volume determined by the active surface programme was 6487 mm(3) in 28 patients compared with 7117 mm(3) in controls (P = 0.002, corrected for age and gender). The patients' cervical spinal cord volumes were associated with expanded disability status scale scores (parameter estimate = -1.21 x 10(-3), r(2) = 0.39, P = 0.001). The patients' cervical spinal cord volumes did not decrease significantly over one year, unlike the mean cervical spinal cord areas at C2/3 calculated using intensity-based contouring. CONCLUSION: The active surface technique can detect cervical spinal cord atrophy due to MS, which has functional significance. However, this methodology is less sensitive at detecting small serial changes compared with the previously reported method.

Atrophy↗

PubMed2003

State-estimation approach to the nonstationary optical tomography problem.

We propose a new numerical approach to the nonstationary optical (diffusion) tomography (OT) problem. The assumption in the method is that the absorption and/or diffusion coefficients are nonstationary in the sense that they may exhibit significant changes during the time that is needed to measure data for one traditional image frame. In the proposed method, the OT problem is formulated as a state-estimation problem. Within the state-estimation formulation, the absorption and/or diffusion coefficients are considered a stochastic process. The objective is to estimate a sequence of states for the process when the state evolution model for the process, the observation model for OT experiments, and data on the exterior boundary are given. In the proposed method, the state estimates are computed by using Kalman filtering techniques. The performance of the proposed method is evaluated on the basis of synthetic data. The simulations also illustrate that further improvements to the results in nonstationary applications can be obtained by adjustment of the measurement protocol.

Models, Theoretical↗

PubMed2002

Three-dimensional optical tomography of the premature infant brain.

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.

Brain↗