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Biomedical subjects

J C Hebden

Publications and source records attributed to J C Hebden.

18 recordsLinked to original sources
PubMed2006

Three dimensional optical imaging of blood volume and oxygenation in the neonatal brain.

Optical methods provide a means of monitoring cerebral oxygenation in newborn infants at risk of brain injury. A 32-channel optical imaging system has been developed with the aim of reconstructing three-dimensional images of regional blood volume and oxygenation. Full image data sets were acquired from 14 out of 24 infants studied; successful images have been reconstructed in 8 of these infants. Regional variations in cerebral blood volume and tissue oxygen saturation are present in healthy preterm infants. In an infant with a large unilateral intraventricular haemorrhage, a corresponding region of low oxygen saturation was detected. These results suggest that optical tomography may provide an appropriate technique for investigating regional cerebral haemodynamics and oxygenation at the cotside.

Blood Volume↗
PubMed2006

Anisotropic photon migration in human skeletal muscle.

It is demonstrated in the short head of the human biceps brachii of 16 healthy subjects (12 males and 4 females) that near infrared photon migration is anisotropic. The probability for a photon to travel along the direction of the muscle fibres is higher (approximately 0.4) than that of travelling along a perpendicular axis (approximately 0.3) while in the adipose tissue the probability is the same (approximately 0.33) in all directions. Considering that the muscle fibre orientation is different depending on the type of muscle considered, and that inside a given skeletal muscle the orientation may change, the present findings in part might explain the intrasubject variability observed in the physiological parameters measured by near infrared spectroscopy techniques. In other words, the observed regional differences might not only be physiological differences but also optical artefacts.

Adult↗
PubMed2005

Three-dimensional whole-head optical tomography of passive motor evoked responses in the neonate.

Optical tomography has been used to reconstruct three-dimensional images of the entire neonatal head during motor evoked responses. Data were successfully acquired during passive movement of each arm on four out of six infants examined, from which eight sets of bilateral images of hemodynamic parameters were reconstructed. Six out of the eight images showed the largest change in total hemoglobin in the region of the contralateral motor cortex. The mean distance between the peak response in the image and the estimated position of the contralateral motor cortex was 10.8 mm. These results suggest that optical tomography may provide an appropriate technique for non-invasive cot-side imaging of brain function.

Algorithms↗
PubMed2005

Recent advances in diffuse optical imaging.

We review the current state-of-the-art of diffuse optical imaging, which is an emerging technique for functional imaging of biological tissue. It involves generating images using measurements of visible or near-infrared light scattered across large (greater than several centimetres) thicknesses of tissue. We discuss recent advances in experimental methods and instrumentation, and examine new theoretical techniques applied to modelling and image reconstruction. We review recent work on in vivo applications including imaging the breast and brain, and examine future challenges.

Diffusion↗
PubMed2003

A method for generating patient-specific finite element meshes for head modelling.

Finite element modelling of fields within the body, whether electrical or optical, requires knowledge of the geometry of the object being examined. It can be clinically impractical to obtain accurate surface information for individual patients, although a limited set of measurements such as the locations of sensors attached to the body, can be acquired more readily. In this paper, we describe how a generic surface taken from an adult head is warped to fit points measured on a neonatal head surface to provide a new, individual surface from which a finite element mesh was generated. Simulations show that data generated from this mesh and from the original neonatal head surface are similar to within experimental errors. However, data generated from a mesh of the best fit sphere were significantly different from data generated from the original neonatal head surface.

Adult↗
PubMed2001

Three-dimensional time-resolved optical tomography of a conical breast phantom.

A 32-channel time-resolved imaging device for medical optical tomography has been employed to evaluate a scheme for imaging the human female breast. The fully automated instrument and the reconstruction procedure have been tested on a conical phantom with tissue-equivalent optical properties. The imaging protocol has been designed to obviate compression of the breast and the need for coupling fluids. Images are generated from experimental data with an iterative reconstruction algorithm that employs a three-dimensional (3D) finite-element diffusion-based forward model. Embedded regions with twice the background optical properties are revealed in separate 3D absorption and scattering images of the phantom. The implications for 3D time-resolved optical tomography of the breast are discussed.

Journal Article↗
PubMed2001

Time resolved optical tomography of the human forearm.

A 32-channel time-resolved optical imaging instrument has been developed principally to study functional parameters of the new-born infant brain. As a prelude to studies on infants, the device and image reconstruction methodology have been evaluated on the adult human forearm. Cross-sectional images were generated using time-resolved measurements of transmitted light at two wavelengths. All data were acquired using a fully automated computer-controlled protocol. Images representing the internal scattering and absorbing properties of the arm are presented, as well as images that reveal physiological changes during a simple finger flexion exercise. The results presented in this paper represent the first simultaneous tomographic reconstruction of the internal scattering and absorbing properties of a clinical subject using purely temporal data, with additional co-registered difference images showing repeatable absorption changes at two wavelengths in response to exercise.

Adult↗
PubMed1999

Effect of lateral boundaries on contrast functions in time-resolved transillumination measurements.

The method of images is employed to insert the effects of the presence of a single lateral boundary on contrast functions previously derived for an infinite slab using a random walk model of photon transport. The predictions of the model for zero and extrapolated boundary conditions are compared with Monte Carlo (MC) simulations and with experimental results obtained using a homogenous phantom with tissue-like optical properties. As expected, the extrapolated boundary condition yields better agreement between the theoretical predictions and results obtained from MC and experiments. This indicates that the random walk model has potential as a forward model in iterative imaging schemes developed for optical tomography.

Biophysical Phenomena↗
PubMed1997

Optical imaging in medicine: I. Experimental techniques.

The overwhelming scatter which occurs when optical radiation propagates through tissue severely limits the ability to image internal structure using measurements of transmitted intensity. A broad range of methods has been proposed during the past decade or so in order to improve imaging performance. Direct methods involve isolating an unscattered or least-scattered component of transmitted scattered light. Indirect methods generally involve measuring some characteristic of the temporal distribution of transmitted light, or an equivalent in the frequency domain, and obtaining a computational solution to the inverse problem. In this paper, we review the experimental techniques which have been proposed in order to explore both direct and indirect imaging. The relative merits and limitations of the various experimental methods are discussed, and we consider the future directions and likelihood of success of optical imaging in medicine.

Diagnostic Imaging↗
PubMed1997

Optical imaging in medicine: II. Modelling and reconstruction.

The desire for a diagnostic optical imaging modality has motivated the development of image reconstruction procedures involving solution of the inverse problem. This approach is based on the assumption that, given a set of measurements of transmitted light between pairs of points on the surface of an object, there exists a unique three-dimensional distribution of internal scatterers and absorbers which would yield that set. Thus imaging becomes a task of solving an inverse problem using an appropriate model of photon transport. In this paper we examine the models that have been developed for this task, and review current approaches to image reconstruction. Specifically, we consider models based on radiative transfer theory and its derivatives, which are either stochastic in nature (random walk, Monte Carlo, and Markov processes) or deterministic (partial differential equation models and their solutions). Image reconstruction algorithms are discussed which are based on either direct backprojection, perturbation methods, nonlinear optimization, or Jacobian calculation. Finally we discuss some of the fundamental problems that must be addressed before optical tomography can be considered to be an understood problem, and before its full potential can be realized.

Diagnostic Imaging↗
PubMed1997

Evaluation of spatial resolution as a function of thickness for time-resolved optical imaging of highly scattering media.

Previous experimental and theoretical investigations of the utility of time-resolved methods as a means of optical imaging through the human breast have indicated that a spatial resolution of approximately 1 cm is achievable by isolating the shortest path length photons which propagate through the tissue. Studies have also shown that resolution may be improved further by extrapolating the measured distribution using an appropriate model of photon transport. The experiments described here were performed in order to observe the relationship between achievable spatial resolution and the thickness of the medium. For a given time gate, an improvement in the spatial resolution was observed as the object thickness was reduced. Overall, the results indicate that a breast compression of about 1 cm may improve the limiting spatial resolution by as much as 7 mm. Less encouraging is the implication that temporal extrapolation over several orders of magnitude in intensity is required to achieve a comparable improvement in spatial resolution.

Biophysical Phenomena↗
PubMed1997

Diagnostic imaging with light.

This paper reviews the evolution of optical imaging in diagnostic radiology and examines recent progress. Although the idea has been around for many decades, interest in the development of an effective method has never been so great. Optical imaging presents several potential advantages over existing radiological techniques. First, the radiation is non-ionizing and therefore reasonable doses can be repeatedly employed without harm to the patient. Second, optical methods offer the potential to differentiate between soft tissues with different optical absorption or scatter, but which are indistinguishable using other modalities. And third, specific absorption by natural chromophores (such as haemoglobin) allows functional information to be obtained. Principal clinical applications include a means of detecting breast disease and a cerebral imaging modality for mapping oxygenation and haemodynamics in the brain of newborn infants or cortical functional activity in adults. Past attempts to image tissues with light have been severely restricted by the overwhelming scatter which occurs when optical radiation spreads through tissue: however, recent innovations in technology have suggested once again that it may be a practical possibility.

Adult↗
PubMed1995

The spatial resolution performance of a time-resolved optical imaging system using temporal extrapolation.

Optical imaging methods are being explored as a potential means of screening for breast cancer. Previous investigations of time-resolved imaging techniques have suggested that due to the lack of photons with sufficiently small pathlengths, the spatial resolution achievable through a human breast would be unlikely to be better than a centimeter. Experimental results presented here indicate, however, that higher resolution may be achieved by extrapolating the measured temporal distribution of transmitted photons. This is performed using a least-squares fit between data and an analytic model of photon transport. The spatial resolution of a time-resolved imaging system was evaluated by measuring the edge response produced by an opaque mask embedded in the center of a 51-mm-thick, very highly scattering medium. The limiting spatial resolution was improved from about 13 mm to about 5 mm.

Breast Neoplasms↗
PubMedDate not supplied

Tissue phantom for optical diagnostics based on a suspension of microspheres with a fractal size distribution.

We demonstrate experimentally the possibility of reproducing the phase function, absorption, and scattering coefficients of a real biological tissue (adult brain white matter and liver) using a suspension of polystyrene microspheres with a fractal size distribution. The design of a light scattering goniometer with a cylindrical cell in air is discussed, and phase function measurements using the device are described. The scattering coefficient is measured using transmission spectrophotometry and the absorption and reduced scattering coefficients are measured using a time-resolved method. A good match between real tissue and phantom parameters is demonstrated.

Animals↗
PubMedDate not supplied

Transillumination imaging performance: a time-of-flight imaging system.

A series of Monte Carlo simulations have been performed in order to investigate the feasibility of constructing a time-of-flight transillumination imaging system that could be used as an effective tool for screening for breast disease. The conceptual design of such a system is described, and simulated images are presented that demonstrate its likely performance. It is found that, whereas the spatial resolution achievable with such a system is only dependent upon its temporal resolution, the scattering characteristics of the tissue being imaged will strongly affect the ultimate imaging performance of such a system.

Breast Neoplasms↗
PubMedDate not supplied

Transillumination imaging performance: spatial resolution simulation studies.

Monte Carlo calculations were used to simulate the propagation of visible and near-infrared light through homogenous tissue in order to quantitate the potential spatial resolution performance for transillumination imaging. Specifically, the relative effectiveness of coaxial collimation and time of flight (TOF) detection for improving spatial resolution was investigated. The results demonstrate that significant improvements in spatial resolution can be achieved through these techniques, with TOF methods offering superior performance for a given level of detected signal intensity.

Computer Simulation↗
PubMedDate not supplied

Evaluating the spatial resolution performance of a time-resolved optical imaging system.

An imaging system is being developed as a possible means of screening for breast cancer using harmless doses of visible or near-infrared radiation. This system produces transmission images of highly scattering objects by recording and discriminating between the times-of-flight of transmitted photons. Experiments have been performed to evaluate the likely spatial resolution performance of such a system. This involved measuring the edge profile produced by an opaque mask embedded in a highly scattering medium, and evaluating the spatial resolution as a function of the period of time over which transmitted light was integrated. The results suggest that a resolution of a few millimeters is achievable with a system with a temporal resolution of about 10 ps.

Breast Neoplasms↗