Measurement of the optical properties of the adult human head with spatially resolved spectroscopy and changes of posture.
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Adult↗
Biomedical subjects
David T Delpy
Publications and source records attributed to David T Delpy.
Spectral characteristics of spontaneous oscillations in cerebral haemodynamics are posture dependent.
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Adult↗
Assessment of an in situ temporal calibration method for time-resolved optical tomography.
A 32-channel time-resolved optical imaging device is developed at University College London to produce functional images of the neonatal brain and the female breast. Reconstruction of images using time-resolved measurements of transmitted light requires careful calibration of the temporal characteristics of the measurement system. Since they can often vary over a period of time, it is desirable to evaluate these characteristics immediately after, or prior to, the acquisition of image data. A calibration technique is investigated that is based on the measurement of light back-reflected from the surface of the object being imaged. This is facilitated by coupling each detector channel with an individual source fiber. A Monte Carlo model is employed to investigate the influence of the optical properties of the object on the back-reflected signal. The results of simulations indicate that their influence may be small enough to be ignored in some cases, or could be largely accounted for by a small adjustment to the calibrated data. The effectiveness of the method is briefly demonstrated by imaging a solid object with tissue-equivalent optical properties.
Brain↗
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↗
Nitric oxide does not inhibit cerebral cytochrome oxidase in vivo or in the reactive hyperemic phase after brief anoxia in the adult rat.
In this study, near-infrared spectroscopy was applied to examine whether cytochrome oxidase in the rat brain is inhibited by nitric oxide in vivo. During normoxia, intravenous N(G)-nitro-L-arginine methyl ester (L-NAME) administration significantly decreased the cerebral saturation of hemoglobin with oxygen but did not alter the cytochrome oxidase redox state. Anoxia significantly reduced the cytochrome oxidase. The time course of the recovery of the redox state during reoxygenation was not altered by L-NAME. The results suggest that in adult rats, cytochrome oxidase is not inhibited by nitric oxide, either in physiologic conditions or during reoxygenation after a brief anoxic period.
Animals↗
Noninvasive measurement of cerebral blood flow in adults using near-infrared spectroscopy and indocyanine green: a pilot study.
This pilot study was designed to determine the feasibility of measuring cerebral blood flow noninvasively after an intravenous bolus of indocyanine green using near-infrared spectroscopy and pulse dye-densitometry. Feasibility aside, this study did not attempt to validate the measured values of cerebral blood flow against an established method of measurement. Twelve healthy volunteers were investigated after peripheral intravenous injection of indocyanine green. Arterial and cerebral changes in indocyanine green concentration were measured using pulse dye-densitometry and near-infrared spectroscopy, respectively. Two methods of calculating cerebral blood flow were used, and a blood flow index was also estimated. Absolute cerebral blood flow was calculated using a modification of the Fick principle and a deconvolution algorithm to derive the impulse residue function. Mean (range) estimated cerebral blood flow for the Fick method was 8.2 mL/100 g/min (4.2-16.2 mL/100 g/min) and 8.3 mL/100 g/min (4.7-15.3 mL/100 g/min) for the impulse residue function method. The impulse residue function method provided a more precise intrasubject estimation of cerebral blood flow compared with the modified Fick principle, with a coefficient of variation of 10.1% versus 25.5%. The blood flow index was 8.6 mg/sec (range: 5.6-17.3 mg/sec) with an intrasubject coefficient of variation of 12.0%. Estimation of cerebral blood flow using near-infrared spectroscopy and pulse dye-densitometry can be made at the bedside after intravenous injection of indocyanine green, and the precision can be improved using a deconvolution algorithm. Notwithstanding the low values obtained for absolute cerebral blood flow, further investigation and validation of this bedside technique is warranted.
Adult↗
Quantitative near infrared spectroscopy measurement of cerebral hemodynamics in newborn piglets.
Severely premature infants are often at increased risk of cerebral hemorrhage and/or ischemic injury caused by immature autoregulatory control of blood flow to the brain. If blood flow is too high, the infant is at risk of hemorrhage, whereas too little blood flow can result in ischemic injury. The development of a noninvasive, bedside means of measuring cerebral hemodynamics would greatly facilitate both diagnosis and monitoring of afflicted individuals. It is to this end that we have developed a near infrared spectroscopy (NIRS) system that allows for quantitative, bedside measurement of cerebral blood flow (CBF), cerebral blood volume (CBV), and mean transit time (MTT). The technique requires an i.v. injection of the near infrared chromophore indocyanine green. Six newborn piglets, median age of 18 h (range 6-54 h), median weight of 1.75 kg (range 1.5-2.1 kg), were studied. Measurements of CBF, CBV, and MTT were made at normocapnia, hypocapnia, and hypercapnia to test the technique over a range of hemodynamic conditions. The accuracy of our new approach has been determined by direct comparison with measurements made using a previously validated computed tomography technique. Paired t tests showed no significant difference between computed tomography and NIRS measurements of CBF, CBV, and MTT, and mean biases between the two methods were -2.05 mL x min(-1) x 100 g(-1), -0.18 mL x 100 g(-1), and 0.43 s, respectively. The precision of NIRS CBF, CBV, and MTT measurements, as determined by repeated-measures ANOVA, was 9.71%, 13.05%, and 7.57%, respectively.
Animals↗
Effects of hepatic ischaemia/reperfusion injury in a rabbit model of Indocyanine Green clearance.
Hepatic ischaemia/reperfusion (I/R) injury is a major cause of primary non-function of the graft after liver transplantation. The ability to assess the severity of ischaemic injury would be of prognostic value and allow the possibility of therapeutic interventions. Currently there is no reliable clinical method for assessing the severity of hepatic ischaemic injury. The hepatic handling of Indocyanine Green as a technique for monitoring the severity of I/R injury has been investigated in the present study. A rabbit model of lobar ischaemia was used. At laparotomy, left lobe hepatic ischaemia was produced for 30, 45 or 60 min, followed by 60 min of reperfusion. Liver function tests, bile excretion and flow in the hepatic microcirculation were measured in animals subjected to I/R injury and in controls. Indocyanine Green was given after reperfusion and its concentration was measured directly in the liver using near-infrared spectroscopy. Indocyanine Green hepatic uptake and excretion rates were calculated. I/R injury produced significant increases in hepatic serum enzymes and decreases in bile excretion and hepatic microcirculation in all I/R groups in comparison with controls. There was a significant reduction in Indocyanine Green uptake and excretion in the I/R groups in comparison with controls, which was correlated with the duration of ischaemia. Indocyanine Green uptake was correlated significantly with flow in the hepatic microcirculation, and its excretion was correlated significantly with the severity of liver damage, as reflected by the changes in serum enzymes and bile excretion. In conclusion, I/R injury affects the hepatic handling of Indocyanine Green, and direct quantification of the uptake and excretion of this dye by near-infrared spectroscopy may be used to objectively assess the degree of I/R injury.
Alanine Transaminase↗
Time-resolved optical mammography using a liquid coupled interface.
A method has been devised for generating three-dimensional optical images of the breast using a 32-channel time-resolved system and a liquid-coupled interface. The breast is placed in a hemispherical cup surrounded by sources and detectors, and the remaining space is filled with a fluid with tissue-like optical properties. This approach has three significant benefits. First, cups can accommodate a large range of breast sizes, enabling the entire volume of the breast to be sampled. Second, the coupling of the source and detector optics at the surface is constant and independent of the subject, enabling intensity measurements to be employed in the image reconstruction. Third, the external geometry of the reconstructed volume is known exactly. Images of isolated targets with contrasting absorbing and scattering properties have been acquired, and the performance of the system has been evaluated in terms of the contrast, spatial resolution, and localization accuracy. These parameters were strongly dependent on the location of the targets within the imaged volume. Preliminary images of a healthy human subject are also presented, which reveal subtle heterogeneity, particularly in the distribution of scatter. The ability to detect an absorbing target adjacent to the breast is also demonstrated.
Breast↗