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David T Delpy

Publications and source records attributed to David T Delpy.

At least 19 recordsLinked to original sources
PubMed2006

Measurement of the absolute optical properties and cerebral blood volume of the adult human head with hybrid differential and spatially resolved spectroscopy.

A hybrid differential and spatially resolved spectroscopy (SRS) technique has been developed to measure absolute absorption coefficient (mu(a)), reduced scattering coefficient (mu'(s)) and cerebral blood volume (CBV) in the adult human head. A spectrometer with both differential and SRS capabilities has been used to carry out measurements in 12 subjects. Two versions of the calculation have been considered using the hybrid technique, with one considering water as a chromophore as well as oxy- and deoxy-haemoglobin, and one ignoring water. The CBV has also been measured using a previously described technique based on changing the arterial saturation (SaO(2)) measured separately by a pulse oximeter, resulting in mean +/- SD CBV(a) (intra-individual coefficient of variation) = 2.22 +/- 1.06 ml/100 g (29.9%). (The superscript on CBV indicates the different calculation basis.) Using the hybrid technique with water ignored, CBV(0) = 3.18 +/- 0.73 ml/100 g (10.0%), mu(0)(a)(813 nm) = 0.010 +/- 0.003 mm(-1) and mu'(0)(s)(813 nm) = 1.19 +/- 0.55 mm(-1) (data quoted at 813 nm). With water considered, CBV(w) = 3.05 +/- 0.77 ml/100 g (10.5%), mu(w)(a)(813 nm) = 0.010 +/- 0.003 mm(-1) and mu'(w)(s)(813 nm) = 1.28 +/- 0.56 mm(-1). The mean biases between CBV(0)/CBV(w), CBV(0)/CBV(a) and CBV(w)/CBV(a) are 0.14 +/- 0.09, 0.79 +/- 1.22 and 0.65 +/- 1.24 ml/100 g. The mean biases between mu(0)(a)(813 nm)/mu(w)(a)(813 nm) and mu'(0)(s)(813 nm)/mu'(w)(s)(813 nm) are (5.9 +/- 10.0) x 10(-4) mm(-1) and -0.084 +/- 0.266 mm(-1), respectively. The method we describe extends the functionality of the current SRS instrumentation.

Adult↗
PubMed2006

Blood volume and haemoglobin oxygen content changes in human bone marrow during orthostatic stress.

The interest in, and the need for effective measures to be used in screening, diagnosis, and the follow-up of skeletal pathologies is growing markedly. This paper proposes a completely new and non-invasive technique allowing the study of the human tibia bone marrow (BM) haemodynamics with a time resolution of 1 s. The technique, based on near infrared spectroscopy, is sensitive enough to allow the detection of BM blood volume and/or oxygen saturation changes during orthostatic variations imposed by a tilt bed. An increase in the slope of the bed of 15 degrees is sufficient to detect this phenomenon. The ability to study the possible presence of a neural control of BM haemodynamics is also discussed. No other existing technique currently allows one to obtain the proposed results and this approach might open up a new field of study related to human BM physiology.

Adult↗
PubMed2005

Estimation of cerebral oxy- and deoxy-haemoglobin concentration changes in a layered adult head model using near-infrared spectroscopy and multivariate statistical analysis.

The non-invasive measurement of cerebral oxy- (DeltaHbO(br)2) and deoxy-haemoglobin (DeltaHHb(br)) changes using near-infrared spectroscopy instruments is often affected by the absorption in the extracerebral layer. We have exploited the multivariate calibration (partial least squares, PLS) method to minimize the errors for a range of blood volume, oxygen saturation and extracerebral layer thicknesses. The changes in the mean time of flight of photons (Delta tau) and attenuation (DeltaA) on the surface of a 3D adult head model were simulated using a finite-element method based on the diffusion equation. The PLS was then performed to identify the optimal number of detectors, their positions and weightings, to optimize the estimation of DeltaHbO(br)2 and DeltaHHb(br). We define the 'nominal accuracy' as the accuracy of estimating DeltaHbO(br)2 and DeltaHHb(br) over a nominal range of extracerebral layer thicknesses and 'robustness' as the accuracy beyond the nominal range. The results showed that for one or two detectors, Delta tau performed better than DeltaA while using them together gave the best performance. When more detectors were used, the performances of using Delta tau, DeltaA or both together became comparable, showing that a larger number of detectors can compensate for the performance of a simple DeltaA measurement despite this measurement having a relatively lower sensitivity to intracerebral absorption changes.

Adult↗
PubMed2005

Linear and nonlinear reconstruction for optical tomography of phantoms with nonscattering regions.

Most research in optical imaging incorrectly assumes that light transport in nonscattering regions in the head may be modeled by use of the diffusion approximation. The effect of this assumption is examined in a series of experiments on tissue-equivalent phantoms. Images from cylindrical and head-shaped phantoms with and without clear regions [simulating the cerebrospinal fluid (CSF) filled ventricles] and a clear layer (simulating the CSF layer surrounding the brain) are reconstructed with linear and nonlinear reconstruction techniques. The results suggest that absorbing and scattering perturbations can be identified reliably with nonlinear reconstruction methods when the clear regions are also present in the reference data but that the quality of the image degrades considerably if the reference data does not contain these features. Linear reconstruction performs similarly to nonlinear reconstruction, provided the clear regions are present in the reference data, but otherwise linear reconstruction fails. This study supports the use of linear reconstruction for dynamic imaging but suggests that, in all cases, image quality is likely to improve if the clear regions are modeled correctly.

Algorithms↗
PubMed2005

Optimal determination of detector placement in cerebral NIR spectroscopy of neonates using chemometric techniques.

This paper investigates the optimal placement of NIRS optodes in order to maximise the detection of haemoglobin changes in cortical grey matter resulting from an evoked response in neonates. The analysis is based upon predictions of optical signal at the surface of the head, using a Finite Element based model of light diffusion in tissue. Using the generated intensity data, the combination of optode positions, which maximise the signal from cortical grey matter whilst minimising that from surface tissue or cerebral white matter, is determined using a Chemometric statistical analysis. The neonatal head is modelled as a 2 dimensional circle with 3 layers corresponding to the skin/scalp, and grey and white matter. A wide range of absorption coefficients for each layer is simulated, based upon physiologically reasonable values for parameters. Surface intensity at 10 different optode positions have been generated for a total of 31,250 combinations of these variables for the 3 layers. It was found that with 3 optodes at 5, 15, and 50 mm apart from the source, the smallest root-mean-square error between the estimated and modelled values can be obtained. Increasing the number of optodes further does not improve the performance.

Cerebral Cortex↗
PubMed2005

Rate of change in cerebral oxygenation and blood pressure in response to passive changes in posture: a comparison between pure autonomic failure patients and controls.

The cardiovascular and cerebrovascular responses to head-up postural change are compromised in pure autonomic failure (PAF) patients because of sympathetic denervation. The aim of this study was to characterize the rate of change of systemic mean blood pressure (MBP) and cerebral haemodynamics in response to passive posture changes. Nine PAF patients and 9 age-matched controls took part in this study. MBP and oxy- (O2Hb), deoxy-haemoglobin (HHb), and tissue oxygenation index (TOI) on the forehead were continuously monitored non-invasively using the Portapres and near-infrared spectroscopy (NIRS), respectively. From visual inspection of the haemoglobin difference signal (Hb(diff) = O2Hb-HHb), seven distinct phases were marked (1: supine, 2: start passive tilt, 3: head up to 60 degrees degrees, 4: end of tilt, 5: tilt reversal, 6: return to supine, 7: rest); the same time points were used for all of the other signals. For each phase, the slope was calculated using a linear regression algorithm. Significant differences were found between PAF patients and controls in the Hb(diff) slope magnitudes for phases 3 (P < .05) and 5 (P = .01), and the duration of phase 2 (P < .05). MBP slope magnitudes showed significant differences for phases 2 (P < .01) and 5 (P < .01). These differences in the rate of change suggest differences in blood vessel resistance related to sympathetic activation.

Aged↗
PubMed2005

Measurement of CMRO2 in neonates undergoing intensive care using near infrared spectroscopy.

Greater understanding of the rate of oxygen delivery and uptake in sick preterm and term infants undergoing intensive care is an important aim of brain-orientated neonatal medicine. Near infrared spectroscopy (NIRS) is a continuous, non-invasive and portable technique which can be used to measure cerebral blood flow (CBF) in infants. It is also possible to use spatially resolved spectroscopy to measure absolute mean cerebral oxygen saturation (SmcO2). The aim of this study was to investigate the derivation of cerebral metabolic rate for oxygen (CMRO2) from these two measurements. Nine preterm infants were studied, of median (range) gestational age 25 (23-37) weeks. A NIRO300 was used to measure CBF and SmcO2 simultaneously over the right and left hemisphere. Median (range) left and right cerebral hemisphere values for CMRO2 were 0.95 (0.79-1.53) ml 100g(-1) x min(-1) and 0.88 (0.69-1.46) ml 100g(-1) x min(-1), respectively. No significant difference was seen between the left- and right-sided values. These values are similar to median (range) values previously reported in infants using positron emission tomography or more invasive NIRS methods. Further work is necessary to define limits on the use of this technique, particularly in the assumption of the venous:arterial compartment volume ratio across different infants.

Brain↗
PubMed2004

Imaging changes in blood volume and oxygenation in the newborn infant brain using three-dimensional optical tomography.

Induced haemodynamic and blood oxygenation changes occurring within the brain of a ventilated newborn infant have been imaged in three dimensions using optical tomography. Noninvasive measurements of the flight times of transmitted light were acquired during illumination of the brain by laser pulses at wavelengths of 780 nm and 815 nm. The oxygen and carbon dioxide partial pressures were adjusted through alterations to the ventilator settings, resulting in changes to the cerebral blood volume and oxygenation. Three-dimensional images were generated using the physiologically associated differences in the measured data, obviating the need for data calibration using a separate reference measurement. The results exhibit large changes in absorption coefficient at both wavelengths. Images corresponding to differences in concentrations of oxy- and deoxyhaemoglobin are in qualitative agreement with known physiological data.

Blood Flow Velocity↗
PubMed2004

Investigation of cerebral haemodynamics by near-infrared spectroscopy in young healthy volunteers reveals posture-dependent spontaneous oscillations.

Autonomic reflexes enable the cardiovascular system to respond to gravitational displacement of blood during changes in posture. Spontaneous oscillations present in the cerebral and systemic circulation of healthy subjects have demonstrated a regulatory role. This study assessed the dynamic responses of the cerebral and systemic circulation upon standing up and the posture dependence of spontaneous oscillations. In ten young healthy volunteers, blood pressure and cerebral haemodynamics were continuously monitored non-invasively using the Portapres and near-infrared spectroscopy (NIRS), respectively. Oscillatory changes in the cerebral NIRS signals and the diastolic blood pressure (DBP) signal have been identified by the fast Fourier analysis. Blood pressure increased during standing and returned to basal level when volunteers sat on a chair. The mean value of cerebral tissue oxygen index (TOI) as measured by NIRS did not demonstrate any significant changes. Oscillatory changes in DBP, oxyhaemoglobin concentration [O2Hb] and TOI showed a significant increase when subjects were standing. Investigation of the low frequency component (approximately 0.1 Hz) of these fluctuations revealed posture dependence associated with activation of autonomic reflexes. Systemic and cerebral changes appeared to preserve adequate blood flow and cerebral perfusion during standing in healthy volunteers. Oscillatory changes in [O2Hb] and TOI, which may be related to the degree of cerebral sympathetic stimulation, are posture dependent in healthy subjects.

Adolescent↗
PubMed2003

Monitoring cytochrome redox changes in the mitochondria of intact cells using multi-wavelength visible light spectroscopy.

We have developed an optical system based on visible light spectroscopy for the continuous study of changes in the redox states of mitochondrial cytochromes in intact mammalian cells. Cells are suspended in a closed incubation chamber in which oxygen and nitric oxide (NO) concentrations can be monitored during respiration. Simultaneously the cells are illuminated with a broad-band tungsten-halogen light source. Emergent light in the visible region (from 490-650 nm) is detected using a spectrophotometer and charge-coupled device camera system. Intensity spectra are then converted into changes in optical attenuation from a 'steady-state' baseline. The oxidised-minus-reduced absorption spectra of the mitochondrial cytochromes are fitted to the attenuation spectra using a multi-wavelength least-squares algorithm. Thus, the system can measure changes in the redox states of the cytochromes during cellular respiration. Here we describe this novel methodology and demonstrate its validity by monitoring the action of known respiratory chain inhibitors, including the endogenous signalling molecule NO, on cytochrome redox states in human leukocytes.

Cytochromes↗
PubMed2003

A new method for the measurement of cerebral blood volume and total circulating blood volume using near infrared spatially resolved spectroscopy and indocyanine green: application and validation in neonates.

A new technique known as tissue dye densitometry (TDD) has been developed to simultaneously measure cerebral blood volume (CBV) and total circulating blood volume (TCV) using near infrared (NIR) spatially resolved spectroscopy (SRS) and the injection of indocyanine green (ICG). Using a medical NIR spectrometer with SRS capability (NIRO-300, Hamamatsu KK), a new parameter is calculated known as the ICG Hb index (IHI), which represents the ratio of ICG concentration to Hb concentration in tissue. Acting as a tracer, ICG is cleared by the liver over 15 min, providing a change of tracer concentration (DeltaCICG,tis), which allows the calculation of the total Hb concentration in tissue (tcHb) using the equation: tcHbtis (micro molar) = DeltaCICG,tis/DeltaIHI. The CBV can subsequently be calculated from tcHbtis given the absolute Hb concentration in blood (g/dL), from which the ICG concentration in blood (DeltaCICG,bl) is obtained. By back-extrapolating the DeltaCICG,bl curve to the peak time, the initial ICG concentration in tissue blood (C0ICG,bl) can be found and TCV can then be calculated. The TCV of 17 neonates were measured using the TDD technique and for comparison using the previously reported fetal Hb dilution technique (FHD). The mean TCV measured by the FHD and TDD techniques were 70.19 +/- 13.73 mL/kg and 70.80 +/- 32.54 mL/kg. The Bland Altman plot showed that the bias was 0.61 +/- 34.34 mL/kg and limits of agreement (2 SD) were -68.07 mL/kg and 69.30 mL/kg. The agreement is limited and the TDD technique needs further validation and development for use in a clinical environment.

Blood Transfusion↗
PubMed2003

Near-infrared light propagation in an adult head model. I. Modeling of low-level scattering in the cerebrospinal fluid layer.

Adequate modeling of light propagation in a human head is important for quantitative near-infrared spectroscopy and optical imaging. The presence of a nonscattering cerebrospinal fluid (CSF) that surrounds the brain has been previously shown to have a strong effect on light propagation in the head. However, in reality, a small amount of scattering is caused by the arachnoid trabeculae in the CSF layer. In this study, light propagation in an adult head model with discrete scatterers distributed within the CSF layer has been predicted by Monte Carlo simulation to investigate the effect of the small amount of scattering caused by the arachnoid trabeculae in the CSF layer. This low scattering in the CSF layer is found to have little effect on the mean optical path length, a parameter that can be directly measured by a time-resolved experiment. However, the partial optical path length in brain tissue that relates the sensitivity of the detected signal to absorption changes in the brain is strongly affected by the presence of scattering within the CSF layer. The sensitivity of the near-infrared signal to hemoglobin changes induced by brain activation is improved by the effect of a low-scattering CSF layer.

Adult↗
PubMed2003

Near-infrared light propagation in an adult head model. II. Effect of superficial tissue thickness on the sensitivity of the near-infrared spectroscopy signal.

It is important for near-infrared spectroscopy (NIRS) and imaging to estimate the sensitivity of the detected signal to the change in hemoglobin that results from brain activation and the volume of tissue interrogated for a specific source-detector fiber spacing. In this study light propagation in adult head models is predicted by Monte Carlo simulation to investigate the effect of the superficial tissue thickness on the partial optical path length in the brain and on the spatial sensitivity profile. In the case of source-detector spacing of 30 mm, the partial optical path length depends mainly on the depth of the inner skull surface whereas the spatial sensitivity profile is significantly affected by the thickness of the cerebrospinal fluid layer. The mean optical path length that can be measured by time-resolved experiments increases when the skull thickness increases whereas the partial mean optical path length in the brain decreases when the skull thickness increases. These results indicate that it is not appropriate to use the mean optical path length as an alternative to the partial optical path length to compensate the NIRS signal for the difference in sensitivity caused by variation of the superficial tissue thickness.

Adult↗
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

The oxygen dependency of cerebral oxidative metabolism in the newborn piglet studied with 31P NMRS and NIRS.

Mean cerebral saturation and changes in the oxidation state of the CuA centre of cytochrome oxidase were measured by near infra-red spectroscopy simultaneously with phosphorous metabolites and intracellular pH measured using 31P NMR spectroscopy during transient anoxia (inspired oxygen fraction = 0.0 for 105 seconds) in the newborn piglet brain. By collecting high quality 31P spectra every 10 seconds, it was possible to resolve the delay between the onset of anoxia and the fall in PCr and to show that the CuA centre of cytochrome oxidase reduced simultaneously with the fall in PCr. From these observations it is concluded that, at normoxia, oxygen tension at the mitochondrial level is substantially above a critical value at which oxidative metabolism becomes oxygen dependent.

Animals↗