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D T Delpy

Publications and source records attributed to D T Delpy.

At least 55 records · Page 3Linked to original sources
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↗
PubMed1996

Fetal heart rate changes and cerebral oxygenation measured by near-infrared spectroscopy during the first stage of labour.

OBJECTIVE: To determine the relationship between contraction related changes in fetal heart rate and cerebral oxygenation measured by near-infrared spectroscopy during labour. STUDY DESIGN: A specially designed optical probe was inserted through the dilated cervix and placed against the fetal head in 30 women during labour. Alterations in fetal heart rate during the final hour of the first stage of labour were compared with changes in the cerebral haemoglobin oxygenation index (delta oxyhaemoglobin concentration - delta deoxyhaemoglobin concentration) measured before, during and after uterine contractions. RESULTS: Uterine contractions which were associated with either no alteration, accelerations or early decelerations of the fetal heart rate showed no significant changes in the haemoglobin oxygenation index. Variable, late and prolonged decelerations all showed significant decreases in the haemoglobin oxygenation index (P < 0.01) either during (variable) or after (variable, late and prolonged) the uterine contraction. CONCLUSION: The association between variable, late and prolonged FHR decelerations and significant falls in cerebral oxygenation during late labour suggests that these fetal heart rate patterns are associated with an increased risk of fetal cerebral hypoxia.

Adult↗
PubMed1996

Near-infrared spectroscopy: theory and applications.

In conclusion, NIRS appears to offer both a new monitoring modality and new information about cerebral oxygenation. Technical problems in the application of this technology persist, most notably determination of pathlength and the volume of tissue interrogated. Those familiar with the history of pulse oximetry will recall that although Millikan developed an ear oximeter in 1947, it was not until Aoyagi combined recognition of the pulse signal with spectroscopy in the 1970s that oximetry was transformed into a clinically applicable monitor. In much the same way, NIRS may find the same tremendous usefulness as a noninvasive monitor of cerebral oxygen utilization, pending resolution of the remaining technical problems.

Animals↗
PubMed1996

An investigation of light transport through scattering bodies with non-scattering regions.

Near-infra-red (NIR) spectroscopy is increasingly being used for monitoring cerebral oxygenation and haemodynamics. One current concern is the effect of the clear cerebrospinal fluid upon the distribution of light in the head. There are difficulties in modelling clear layers in scattering systems. The Monte Carlo model should handle clear regions accurately, but is too slow to be used for realistic geometries. The diffusion equation can be solved quickly for realistic geometries, but is only valid in scattering regions. In this paper we describe experiments carried out on a solid slab phantom to investigate the effect of clear regions. The experimental results were compared with the different models of light propagation. We found that the presence of a clear layer had a significant effect upon the light distribution, which was modelled correctly by Monte Carlo techniques, but not by diffusion theory. A novel approach to calculating the light transport was developed, using diffusion theory to analyze the scattering regions combined with a radiosity approach to analyze the propagation through the clear region. Results from this approach were found to agree with both the Monte Carlo and experimental data.

Brain↗
PubMed1996

Measurement of tissue temporal point spread function (TPSF) by use of a gain-modulated avalanche photodiode detector.

This paper describes an opto-electronic cross-correlator designed for measurement of the temporal point spread function (TPSF) of light at the bedside. Ultra-short (< 2 ps) pulses of light from a mode-locked laser were used to illuminate a tissue phantom. The light exiting from the tissue phantom was coupled by an optical fibre to a small-area (200 microns diameter, Hamamatsu S2381) avalanche photodiode (APD). The gain of the photodiode was modulated at the repetition rate of the pulsed laser (82 MHz). Usually the gain was approximately 100, but for a period of approximately 130 ps (FWHM) the gain was increased to approximately 105. A lock-in amplifier, which sampled the integrated APD current, gave an output proportional to the difference between the current in the low- and high-gain states. Hence a small section of the TPSF was selectively sampled. An overall temporal resolution of 275 ps FWHM was achieved. As the timing of the gain modulation was controlled by an all-electronic variable-time-delay system, the whole of the TPSF could be sampled without requiring any moving prism or mirror which is typical of many cross-correlators. Hence the system is mechanically very rugged, which enhances its durability in a portable instrument.

Biophysical Phenomena↗
PubMed1996

Optical properties of multicellular tumour spheroids.

Multicellular tumour spheroids grown in vitro have been widely used in cancer research as an experimental preparation with many of the characteristics of tumours. They provide a model system for understanding the optical behaviour of tumour tissue, which is of interest in novel diagnostic and therapeutic procedures. Optical measurements on fresh spheroids in the wavelength range of 600-1000 nm yielded scattering coefficients, absorption coefficients and g values (mean cosine of scatter) of 160-90 mm-1, 0.1-0.4 mm-1 and 0.99, respectively. Following fixation, considerably higher values of scattering and absorption coefficients were seen. The values are compared with those reported elsewhere for excised tumour tissue and interpretations of the optical behaviour are suggested.

Biophysical Phenomena↗
PubMed1996

The effect of scalp ischaemia on measurement of cerebral blood volume by near-infrared spectroscopy.

Near-infrared spectroscopy (NIRS) is a noninvasive method of quantifying changes in cerebral haemodynamics from changes in the absorption of near-infrared light by oxyhaemoglobin and deoxyhaemoglobin. Measurement of neonatal cerebral blood volume (CBV) by NIRS was described in 1990 but it has been suggested that, in adults, scalp and skull blood content contribute a significant amount to the cerebral haemodynamic variables quantifiable by NIRS. To investigate this, CBV was measured in nine adult subjects, in the frontal region of the head, before and after inflating a pneumatic tourniquet proximal to the measurement site. Because a change in scalp blood content could potentially alter the pathlength of light passing through the head and hence affect the measured CBV, the optical pathlength factor was therefore also measured before and after tourniquet inflation. Blood flow occlusion was confirmed by laser Doppler velocimetry. The results showed that tourniquet inflation had no effect on the estimated value of CBV or the differential pathlength factor. We conclude that, provided the distance between light entry and exit on the surface of the scalp is sufficiently large, changes in scalp blood flow have no effect on NIRS measurement of cerebral haemodynamics.

Adult↗
PubMed1996

Use of near infrared spectroscopy to estimate cerebral blood flow in conscious and anaesthetized adult subjects.

Near infrared spectroscopy (NIRS) can be used to quantify cerebral haemodynamic states non-invasively and to estimate cerebral blood flow (CBF). In the first part of this study we have compared CBF measurements in conscious and anaesthetized subjects. In the second part we have compared paired measurements made during anaesthesia, first on the scalp and then the dura after craniotomy. Mean CBF was 17 (SD 7) ml 100 g-1 min-1 in the conscious subjects compared with 21 (8) ml 100 g-1 min-1 on the scalp during anaesthesia (P > 0.1). Mean CBF on the dura was 68 (21) ml 100 g-1 min-1 (P < 0.0001). Computer modelling suggests that the difference in magnitude between scalp and dura measurements of CBF is likely to be caused by the optical effect of extracerebral tissue which powerfully scatters light passing through it but does not contribute significantly to the measured CBF because it has only a small blood content itself. The results lend support to this method of estimating CBF although formal validation by comparison with an established technique is needed.

Adult↗
PubMed1996

Influence of respiration and changes in expiratory pressure on cerebral haemoglobin concentration measured by near infrared spectroscopy.

Near infrared spectroscopy (NIRS) was used to measure the changes in concentration of cerebral oxy- and deoxygenated haemoglobin ([HbO2] and [Hb]) in six healthy adult volunteers spontaneously breathing against increased expiratory pressures (IEPs) between 0 and 20 cm H2O. During expiration, an increase in [HbO2] was recorded, accompanied by a smaller decrease in [Hb], producing a small increase in total cerebral haemoglobin concentration ([Hbsum]). The mean plus/minus SD change in [Hbsum] at the maximum 1EP of 20 cm H2O was 1.2 +/- 0.7 micromol L-1 (equivalent to 1.4%). Changes in [Hbsum] correlated with IEP level (r = 0.95) and changes in MABP (r = 0.96). The results suggest that homeostatic mechanism do not maintain cerebral blood volume or flow constant over the period of a single breath in normal adults.

Adult↗
PubMed1996

The noninvasive measurement of absolute cerebral deoxyhemoglobin concentration and mean optical path length in the neonatal brain by second derivative near infrared spectroscopy.

We have used second differential near infrared spectroscopy of water to determine the mean optical path length of the neonatal brain. By obtaining the ratio of the second differential features of deoxyhemoglobin to those of water, the absolute cerebral concentration of deoxyhemoglobin can be monitored continuously and noninvasively. Nineteen neonates were studied; the gestational age at birth varied from 23 to 38 wk, and the postconceptual age, when the spectra were recorded, ranged from 35 to 48 wk. The calculated mean deoxyhemoglobin concentration was 14.6 +/- 4.0 microM; the differential path length factor (mean optical path length/optode separation) calculated from the water peak at 730 nm was 4.66 +/- 1.01, and that calculated at the 830-nm peak was 3.91 +/- 0.75. These values are consistent with path length measurements using laser time-of-flight spectroscopy on postmortem neonates and phase-resolved spectroscopy on live neonates. Induced arterial oxygen saturation decreases from 98 to 93% showed no significant change in the mean optical path length, despite significant cerebral desaturation. Changes in the deoxyhemoglobin concentration after this procedure were identical, whether measured by second differential analysis at 760 nm or by multilinear regression over the wavelength range 740-900 nm. When combined with existing methods of measuring total cerebral hemoglobin concentration, second differential near infrared spectroscopy can be used to derive the mean cerebral oxygen saturation. A preliminary experiment outlined the feasibility of this approach and yielded a saturation value of 63%, consistent with near infrared sampling of a predominantly venous pool in the brain.

Brain↗
PubMed1996

Measurement of cranial optical path length as a function of age using phase resolved near infrared spectroscopy.

Near infrared spectroscopy (NIRS) has been used to measure concentration changes of cerebral hemoglobin and cytochrome in neonates, children, and adults, to study cerebral oxygenation and hemodynamics. To derive quantitative concentration changes from measurements of light attenuation, the optical path length must be known. This is obtained by multiplying the source/ detector separation by a laboratory measured differential path length factor (DPF) which accounts for the increased distance traveled by light due to scattering. DPF has been measured by time of flight techniques on small populations of adults and postmortem infants. The values for adults are greater than those for newborns, and it is not clear how to interpolate the present data for studies on children. Recent developments in instrumentation using phase resolved spectroscopy techniques have produced a bedside unit which can measure optical path length on any subject. We have developed an intensity modulated optical spectrometer which measures path length at four wavelengths. Two hundred and eighty three subjects from 1 d of age to 50 y were studied. Measurements were made at a fixed frequency of 200 MHz and a source detector separation of 4.5 cm. Results suggest a slowly varying age dependence of DPF, following the relation DPF690 = 5.38 + 0.049A0.877, DPF744 = 5.11 + 0.106A0.723, DPF807 = 4.99 + 0.067A0.814, and DPF832 = 4.67 + 0.062A0.819, where DPF690 is the DPF measured at 690 nm and A is age is expressed in years from full term. There was a wide scatter of values, however, implying that ideally DPF should be measured at the time of each study.

Adolescent↗
PubMed1995

Regional changes in cerebral haemodynamics as a result of a visual stimulus measured by near infrared spectroscopy.

Near infrared spectroscopy (NIRS) is used to measure global changes in cerebral haemodynamics. We have adapted the technique to measure regional changes in response to a visual stimulus. Ten volunteers were exposed to a computer generated visual stimulus designed to activate a large area of the visual cortex, including V1, V2, V3, V4 and V5. The stimulus was on for 30 s and off for 30 s. Changes in the concentrations of oxyhaemoglobin ([HbO2]) and deoxyhaemoglobin ([Hb]) were measured using a commercial spectrometer (NIRO500), over the occipital cortex. The data were summed over ten cycles. As a control, the experiment was repeated over the frontal cortex. For each subject [HbO2] increased during stimulation, and decreased when the stimulus was off. The mean (+/- s.e.m.) change in [HbO2] was 0.54 +/0 0.14 micromol 1(-1). The change in total haemoglobin concentration, given by [HbO2] + [Hb] was 0.61 +/- 0.21 micromol 1(-1), equivalent to a rise in cerebral blood volume of 0.04 +/- 0.01 ml 100 g(-1) which is about 2% of the total cerebral blood volume. There was no significant change in [HbO2] over the frontal cortex, implying that the changes in blood volume originated in the occipital lobe. This demonstrates that NIRS provides a non-invasive method of measuring regional changes in cerebral haemodynamics as a result of visual stimulation.

Adult↗
PubMed1995

Performance comparison of several published tissue near-infrared spectroscopy algorithms.

We have collected multiwavelength near-infrared (NIR) attenuation spectra on human forearm muscle, the adult rat head, and newborn piglet head to compare the changes in chromophore concentration derived from these data using published algorithms from four groups. We find differences between the results from the algorithms on each data set, particularly in their estimation of cytochrome oxidase (cyt-aa3) redox changes. We also find some differences when applying the same algorithm to the three data sets, suggesting possible difficulties in transferring algorithms between different physiological systems (e.g., Kurth, C. D., Steven, J. M., Benaron, D., and Chance, B. (1993) J. Clin. Monit. 9, 163-170). We have also compared the algorithms using simulated data generated using measured hemoglobin absorption spectra and a diffusion model for light transport in tissue. We find that while the algorithms from three groups are in broad agreement, that published by Piantadosi (Piantadosi, C. A. (1993) Methods Toxicol. 2, 107-126) produces significantly different results for cyt-aa3 and HbO2. Either the hemoglobin spectra used to produce the simulated data are inaccurate or the modeling is incorrect, or this algorithm is erroneous.

Adult↗
PubMed1995

The effect of overlying tissue on the spatial sensitivity profile of near-infrared spectroscopy.

The exact volume of brain tissue interrogated in near-infrared spectroscopy (NIRS) studies of cerebral oxygenation in unknown, the inhomogeneity of the head and resulting variation in optical paths making determination of the interrogated volume difficult. In this study, the spatial sensitivity profiles in simple two-component inhomogeneous cylindrical models are predicted by Monte Carlo simulation in order to reveal the effects of the overlying tissue on the volume of tissue interrogated by NIRS. The predictions are validated by experimental measurements on solid "tissue equivalent' cylindrical phantoms. The phantom diameter is 30 mm and the thickness of the outer layer is 2 mm. The optical properties of the inner cylinder match those of adult brain white matter (mu' s1 = 6.4 mm-1 and mu' a1 = 0.02 mm-1), but those of the outer layer are changed (mu' s0 from 1.7 to 8.3 mm-1 and mu' a0 from 0.002 to 0.1 mm-1). Results show that the spatial sensitivity profile is largely confined to the outer layer at small optode spacing (< 15 degrees) and to the inner layer at large spacing (> 120 degrees). At intermediate angles, the sensitivity profiles are sensitive to the optical properties of the outer layer. A low mu' s0 or mu' a0 moves the profile toward the surface, at high mu' a0 it moves in towards the inner layer and at high mu' a0 it is similar to the homogeneous case.

Biophysical Phenomena↗
PubMed1995

Optical pathlength measurements on adult head, calf and forearm and the head of the newborn infant using phase resolved optical spectroscopy.

We have used an intensity modulated optical spectrometer, which measures the phase shift across tissue experienced by intensity modulated near-infrared light, to determine the absolute optical pathlength through tissue. The instrument is portable and takes only 5 s to record pathlength at four wavelengths (690 nm, 744 nm, 807 nm and 832 nm). The absolute pathlength divided by the known spacing between the light source and detector on the skin is the differential pathlength factor (DPF) which previous studies have shown is approximately constant for spacings greater than 2.5 cm. DPF results are presented for measurements on 100 adults and 35 newborn infants to determine the statistical variation on the DPF. All measurements were made at a frequency of 200 MHz with source-detector spacings of > 4 cm. Results at 807 nm show a DPF of 4.16(+/- 18.8%) for adult arm, 5.51(+/- 18%) for adult leg, 6.26(+/- 14.1%) for adult head and 4.99(+/- 9%) for the head of a newborn infant. A wavelength dependence was obtained for DPF on all tissues and a difference in DPF between male and female was observed for both the adult arm and leg. The results can be used to improve the quantitation of chromophore concentration changes in adults and newborn infants.

Adult↗
PubMed1995

An improved design for a stable and reproducible phantom material for use in near-infrared spectroscopy and imaging.

In this note, we describe an improved phantom material for use in near-infrared spectroscopy and imaging. The material consists of a clear epoxy resin with absorbing dyes and amorphous silica spheres as scattering particles. It is possible to calculate the scattering coefficient and angular scattering distribution of the material from Mie theory, using the known size and refractive index of the silica spheres together with the measured refractive index of the resin (approximately 1.56). We show a good agreement between prediction and experimental measurements. The scattering properties of the material closely match those of tissue in the near-infrared wavelength region, having an anisotropy factor, g, of approximately 0.93. The absorption coefficient of the epoxy is low (approximately 0.001 mm-1), and addition of the dyes produces an absorption coefficient that covers the same range as that of tissue.

Biophysical Phenomena↗