Digital biomarkers from gaze tests for classification of central and peripheral lesions in acute vestibular syndrome

Acute vestibular syndrome (AVS) is characterised by a sudden vertigo, gait instability, nausea and nystagmus. Accurate and rapid triage of patients with AVS to differentiate central (potentially sinister) from peripheral (usually benign) root causes is a challenge faced across emergency medicine settings. While there exist bedside exams which can reliably differentiate serious cases, they are underused due to clinicians' general unfamiliarity and low confidence interpreting results. Nystagmus is a fundamental part of AVS and can facilitate triaging, but identification of relevant characteristics requires expertise. This work presents two quantitative digital biomarkers from nystagmus analysis, which capture diagnostically-relevant information. The directionality biomarker evaluates changes in direction to differentiate spontaneous and gaze-evoked (direction-changing) nystagmus, while the intensity differential biomarker describes changes in intensity across eccentric gaze tests. In order to evaluate biomarkers, 24 sets of three gaze tests (left, right, and primary) are analysed. Both novel biomarkers were found to perform well, particularly directionality which was a perfect classifier. Generally, the biomarkers matched or eclipsed the performance of quantitative nystagmus features found in the literature. They also surpassed the performance of a support vector machine classifier trained on the same dataset, which achieved an accuracy of 75%. In conclusion, these biomarkers simplify the diagnostic process for non-specialist clinicians, bridging the gap between emergency care and specialist evaluation, ultimately benefiting patients with AVS.

Assessment of bilirubin levels in patients with cirrhosis via forehead, sclera and lower eyelid smartphone images

One of the key biomarkers evaluating liver disease progression is an elevated bilirubin level. Here we apply smartphone imaging to non-invasive assessment of bilirubin in patients with cirrhosis. Image data was processed using two different approaches to remove variation introduced by ambient conditions and different imaging devices-a per-image calibration using a color chart in each image, and a two-step process using pairs of flash/ no-flash images to account for ambient light in combination with a one-time calibration. For the first time, results from the forehead, sclera (white of the eye) and lower eyelid were compared. The correlation coefficients between the total serum bilirubin and the predicted bilirubin via the forehead, sclera and lower eyelid were 0.79, 0.89 and 0.86 (all with p<0.001, n = 66), respectively. Given the simpler image capture for the sclera, the recommended imaging site for this patient cohort is the sclera.

Development of a thermochromic lateral flow assay to improve sensitivity for dengue virus serotype 2 NS1 detection

Dengue disease is a viral infection that has been widespread in tropical regions, such as Southeast Asia, South Asia and South America. A worldwide effort has been made over a few decades to halt the spread of the disease and reduce fatalities. Lateral flow assay (LFA), a paper-based technology, is used for dengue virus detection and identification because of its simplicity, low cost and fast response. However, the sensitivity of LFA is relatively low and is usually insufficient to meet the minimum requirement for early detection. In this study, we developed a colorimetric thermal sensing LFA format for the detection of dengue virus NS1 using recombinant dengue virus serotype 2 NS1 protein (DENV2-NS1) as a model antigen. Plasmonic gold nanoparticles, including gold nanospheres (AuNSPs) and gold nanorods (AuNRs), and magnetic nanoparticles (MNPs), namely iron oxide nanoparticles (IONPs) and zinc ferrite nanoparticles (ZFNPs), were studied for their thermal properties for sensing assays. AuNSPs with 12 nm diameter were chosen due to their great photothermal effect against light-emitting diodes (LEDs). In the thermal sensing assay, a thermochromic sheet is used as a temperature sensor transforming heat into a visible colour. In the typical LFA, the test line is visible at 6.25 ng mL-1 while our thermal sensing LFA offers a visual signal that can be observed at as low as 1.56 ng mL-1. The colorimetric thermal sensing LFA is capable of reducing the limit of detection (LOD) of DENV2-NS1 by 4 times compared to the typical visual readout. The colorimetric thermal sensing LFA can enhance the sensitivity of detection and deliver visuality to the user to translate without the need for an infrared (IR) camera. It has the potential to expand the utilities of LFA and satisfy early diagnostic applications.

Feasibility of smartphone colorimetry of the face as an anaemia screening tool for infants and young children in Ghana

BACKGROUND

Anaemia affects approximately a quarter of the global population. When anaemia occurs during childhood, it can increase susceptibility to infectious diseases and impair cognitive development. This research uses smartphone-based colorimetry to develop a non-invasive technique for screening for anaemia in a previously understudied population of infants and young children in Ghana.

METHODS

We propose a colorimetric algorithm for screening for anaemia which uses a novel combination of three regions of interest: the lower eyelid (palpebral conjunctiva), the sclera, and the mucosal membrane adjacent to the lower lip. These regions are chosen to have minimal skin pigmentation occluding the blood chromaticity. As part of the algorithm development, different methods were compared for (1) accounting for varying ambient lighting, and (2) choosing a chromaticity metric for each region of interest. In comparison to some prior work, no specialist hardware (such as a colour reference card) is required for image acquisition.

RESULTS

Sixty-two patients under 4 years of age were recruited as a convenience clinical sample in Korle Bu Teaching Hospital, Ghana. Forty-three of these had quality images for all regions of interest. Using a naïve Bayes classifier, this method was capable of screening for anaemia (<11.0g/dL haemoglobin concentration) vs healthy blood haemoglobin concentration (≥11.0g/dL) with a sensitivity of 92.9% (95% CI 66.1% to 99.8%), a specificity of 89.7% (72.7% to 97.8%) when acting on unseen data, using only an affordable smartphone and no additional hardware.

CONCLUSION

These results add to the body of evidence suggesting that smartphone colorimetry is likely to be a useful tool for making anaemia screening more widely available. However, there remains no consensus on the optimal method for image preprocessing or feature extraction, especially across diverse patient populations.

A novel smartphone scleral-image based tool for assessing jaundice in decompensated cirrhosis patients

BACKGROUND AND AIM

Serum bilirubin is an established marker of liver disease. Reliable tools for non-invasive assessment of jaundice in cirrhosis patients, at risk of clinical decompensation, are highly desirable. While smartphone-based imaging has been described in neonatal jaundice, it has not been investigated in advanced cirrhosis patients.

METHODS

We included 46 hospitalized patients with acute cirrhosis decompensation and jaundice. Scleral images using an Android smartphone were taken to derive "Scleral Color Values (SCV)," which were matched with same day serum bilirubin measurements. In 29 patients, repeat SCV and bilirubin measurements were performed over time. We analyzed the relationship of SCV and its dynamics with serum bilirubin, clinical scores, and patient outcomes.

RESULTS

Of 46 patients, 26 (57%) had alcoholic hepatitis as the decompensation precipitant. Seven patients died during admission; a further 12 following hospital discharge. SCV had an excellent linear correlation with serum bilirubin (rho = 0.90, P < 0.001); changes in SCV and serum bilirubin across different time points, were also closely associated (rho = 0.77, P < 0.001). SCV correlated significantly with CLIF Consortium Acute Decompensation score (rho = 0.38, P < 0.001) and grade of Acute-on-Chronic Liver Failure (rho = 0.42, P = 0.039). SCV was higher in patients who died, however, not significantly (86.1 [IQR 83.0-89.7] vs 82.3 [IQR 78.5-83.3], P = 0.22). The associations of SCV with clinical parameters mirrored those of serum bilirubin.

CONCLUSION

Smartphone-based assessment of jaundice shows excellent concordance with serum bilirubin and is associated with clinical parameters in acute cirrhosis decompensation. This approach offers promise for remote assessment of cirrhosis patients at-risk of decompensation, post hospital discharge.

Regional cerebral and splanchnic tissue oxygen saturation in preterm infants - Longitudinal normative measurements

BACKGROUND

To investigate regional splanchnic and cerebral tissue oxygen saturation in preterm infants <30 weeks gestation.

METHODS

Cerebral (cTOI) and splanchnic (sTOI) Tissue Oxygenation Index were measured weekly in 5 min epochs for a total period of 60 min using NIRS (NIRO-300) for the first 8 weeks of life, in 48 appropriately grown preterm infants born at <30 weeks gestation. Infants who developed HPI and/or NEC (n = 12) and those that died (n = 1) were excluded from our main outcome measure of regional gut and cerebral tissue oxygenation in healthy preterm infants <30 weeks gestation.

RESULTS

Median birthweight 789 g (460-1486), gestational age 25⁺⁶ weeks (23⁺⁰-29⁺¹) and 51.4% female. 217 NIRS measurements were completed across the first 8 weeks of life. Mean weekly cTOI ranged from 56.8-65.4% and sTOI ranged from 36.7-46.0%. Mean cTOI was significantly higher than mean sTOI (p < 0.001) throughout the first 8 weeks of life. Mean cTOI decreased significantly with increasing postnatal age [-0.59% each week (-1.26% to -0.07%) p = 0.04]. None of the examined confounding factors had a significant effect.

CONCLUSIONS

This is the first report of regional cerebral and splanchnic tissue oxygen saturation ranges during the first 8 weeks of life for preterm infants born at <30 weeks gestation.

The Nasal Obstruction Balance Index: A Novel Approach to Improving Correlation Between Unilateral Nasal Airway Measurements and Evaluating Nasal Airway Asymmetry

OBJECTIVES/HYPOTHESIS

Demonstrate that the Nasal Obstruction Balance Index (NOBI) model fulfils the unmet need of improving unilateral correlation between subjective and objective nasal obstruction outcome measures and identifying the more obstructed side. Improve correlation between unilateral objective nasal airway measurements (nasal inspiratory peak flow [NIPF] and acoustic rhinometry [AR]) and subjective Visual Analogue Scale for nasal obstruction (VAS-NO) scores. Improve assessment of nasal airway asymmetry by evaluating unilateral measurements both before and after the application of nasal decongestant; which the patient could better understand. NOBI represents a ratio calculated by taking the difference between left and right nasal airway measurements and divided by the maximum unilateral measurement. It is based on Poiseuille's law and aims to reduce the confounding variables which challenge nasal airway measurement.

STUDY DESIGN

Prospective cohort study.

METHODS

Forty-three controls and 34 patients with nasal obstruction underwent both unilateral and bilateral NIPF, AR and VAS-NO measurements; these were repeated after the application of nasal decongestant. The NOBI values for unilateral NIPF, AR, and VAS-NO were calculated both before and after decongestant.

RESULTS

The correlation between unilateral NIPF and AR measurements was enhanced considerably (r = 0.57, P < .01) when NOBI was applied. The NOBI metric significantly increased the correlation between unilateral NIPF, AR, and VAS-NO scores. Postdecongestant NOBI for NIPF and AR measurements correctly identified the more obstructed side in 82.4% and 94.1% of the deviated nasal septum (DNS) cases, respectively.

CONCLUSION

The NOBI model provides a better correlation between unilateral subjective and objective measurements and identifies the more obstructed side.

LEVEL OF EVIDENCE

Prospective cohort study (level III) Laryngoscope, 2021.

Diagnosing nasal obstruction and its common causes using the nasal acoustic device: A pilot study

Objectives

There is a need to develop a medical device which can accurately measure normal and abnormal nasal breathing which the patient can better understand in addition to being able to diagnose the cause for their nasal obstruction.

The aim is to evaluate the accuracy of the nasal acoustic device (NAD) in diagnosing the common causes for nasal obstruction and diagnosing normal and abnormal (nasal obstruction) nasal breathing.

Methods

This pilot study recruited 27 patients with allergic rhinitis (AR), chronic rhinosinusitis (CRS), and a deviated nasal septum (DNS) which represents the common causes for NO and 26 controls (with normal nasal breathing). Nasal breathing sounds were recorded by the NAD akin to two small stethoscopes placed over the left and right nasal ala. The novel outcome metrics for the NAD include inspiratory nasal acoustic score (INA) score, expiratory nasal acoustic (ENA) score and the inspiratory nasal obstruction balance index (NOBI). The change in acoustic score following decongestant is key in this diagnostic process.

Results

Pre‐decongestant ENA score was used to detect the presence of nasal obstruction in patients compared to controls, with a sensitivity of 0.81 (95% CI: 0.66‐0.96) and a specificity of 0.77 (0.54‐1.00). Post‐decongestant percentage change in INA score was used to identify the presence of AR or CRS, with a sensitivity of 0.87 (0.69‐1.00) and specificity of 0.72 (0.55‐0.89) for AR; and a sensitivity of 0.92 (0.75‐1.00) and specificity of 0.69 (0.52‐0.86) for CRS. Post‐decongestant inspiratory NOBI was used to identify DNS, with a sensitivity of 0.77 (0.59‐0.95) and specificity of 0.94 (0.82‐1.00).

Conclusion

We have demonstrated that the NAD can help distinguish between normal and abnormal nasal breathing and help diagnose AR, CRS, and DNS. Such a device has not been invented and could revolutionize COVID‐19 recovery telemedicine.

Level of Evidence

Diagnostic accuracy study—Level III.

Smartphone screening for neonatal jaundice via ambient-subtracted sclera chromaticity

Jaundice is a major cause of mortality and morbidity in the newborn. Globally, early identification and home monitoring are significant challenges in reducing the incidence of jaundice-related neurological damage. Smartphone cameras are promising as colour-based screening tools as they are low-cost, objective and ubiquitous. We propose a novel smartphone method to screen for neonatal jaundice by imaging the sclera. It does not rely on colour calibration cards or accessories, which may facilitate its adoption at scale and in less economically developed regions. Our approach is to explicitly address three confounding factors in relating colour to jaundice: (1) skin pigmentation, (2) ambient light, and (3) camera spectral response. (1) The variation in skin pigmentation is avoided by imaging the sclera. (2) With the smartphone screen acting as an illuminating flash, a flash/ no-flash image pair is captured using the front-facing camera. The contribution of ambient light is subtracted. (3) In principle, this permits a device- and ambient-independent measure of sclera chromaticity following a one-time calibration. We introduce the concept of Scleral-Conjunctival Bilirubin (SCB), in analogy with Transcutaneous Bilirubin (TcB). The scleral chromaticity is mapped to an SCB value. A pilot study was conducted in the UCL Hospital Neonatal Care Unit (n = 37). Neonates were imaged using a specially developed app concurrently with having a blood test for total serum bilirubin (TSB). The better of two models for SCB based on ambient-subtracted sclera chromaticity achieved r = 0.75 (p<0.01) correlation with TSB. Ambient subtraction improved chromaticity estimates in proof-of-principle laboratory tests and screening performance within our study sample. Using an SCB decision threshold of 190μmol/L, the sensitivity was 100% (specificity 61%) in identifying newborns with TSB>250μmol/L (area under receiver operating characteristic curve, AUROC, 0.86), and 92% (specificity 67%) in identifying newborns with TSB>205μmol/L (AUROC 0.85). These results are comparable to modern transcutaneous bilirubinometers.

Pilot Study to Detect Changes in Blood Flow in the External Auditory Meatus During Hemodialysis

Blood flow to internal organs is reported to fall during hemodialysis (HD). As such, noninvasive monitoring devices are required to detect changes in perfusion, which could then be used for therapeutic interventions. We report on a pilot study monitoring blood flow in the outer auditory meatus. We measured the maximum pulse wave amplitude and indicators of blood flow by analyzing red and green color changes in the outer auditory meatus from video recordings made using an otoscope fitted with a digital camera during HD treatments. We studied 61 patients, 43 (71.5%) male, mean age 64.9 ± 12.7 years. Weight fell from 72.8 ± 22.5 kg predialysis to 71.5 ± 22.1 kg postdialysis (P < 0.001). BP did not significantly change (predialysis 142 ± 29/67 ± 18 to 143 ± 25/68 ± 17 mm Hg postdialysis). The maximum pulse wave amplitude in the external auditory meatus fell from 0.21 (0.1-0.55) to 0.14 (0.04-0.4) after 90 min, P < 0.001, and remained low thereafter, and the change at the end of the dialysis session was associated with percentage weight loss (r = -0.37, P = 0.003). Green and red pixel values did not change (predialysis 0.339 [0.333-0.345] to 0.302 [0.291-0.33] post, and 0.301 [0.293-0.328] predialysis to 0.339 [0.334-0.347] post, respectively). This pilot study showed that the maximum pulse wave amplitude measured in the external auditory meatus fell during the dialysis session, and that the fall was associated with fluid removal. This could potentially lead to the development of a monitoring device, which could fit in the ear and record during the dialysis session.

Investigating the nasal cycle using unilateral peak nasal inspiratory flow and acoustic rhinometry minimal cross-sectional area measurements

OBJECTIVE

To plot the nasal cycle using unilateral peak nasal inspiratory flow (UPNIF) and unilateral minimal cross-sectional area (UMCA) readings demonstrating a linear relationship in normal nasal function. Additionally, to determine how this changes in abnormal nasal function.

DESIGN

A cross-sectional study measuring UPNIF and UMCA in controls demonstrating normal nasal function and in patients with nasal obstruction.

SETTING

Royal National Throat Nose and Ear Hospital, London.

PARTICIPANTS

A total of 39 participants, 26 controls and 13 patients, were recruited. Controls exhibited normal nasal function with SNOT-22 <5. Patients nasal obstruction symptoms secondary to inflammation or structural abnormality with SNOT-22 >9.

MAIN OUTCOME MEASURES AND RESULTS

Airflow rates and resistance values were derived from UPNIF and UMCA measurements respectively based on Poiseuille's laws. Ratios between right and left UPNIF and UMCA values were taken to adjust for confounding factors. The relationship of 1/Resistance Ratio and Airflow Rate Ratio demonstrated a linear of direct proportionality of strong correlation and statistical significance (correlation coefficient = 0.76, P « 0.01). This suggests that data points from controls with a normal nasal cycle lie closely along the regressed line, whilst those lying significantly away were shown to belong to patients with nasal dysfunction. Olfactory dysfunction appears to be a sensitive discriminator in predicting this.

CONCLUSION

This study demonstrates the directly proportional relationship of 1/Resistance Ratio and Airflow Rate Ratio in normal nasal function. Furthermore, nasal pathology can be predicted if data points lie significantly outside these normal limits. Further studies are needed to validate exact normal and abnormal thresholds.

Jaundice Eye Color Index (JECI): quantifying the yellowness of the sclera in jaundiced neonates with digital photography

The sclera is arguably a better site than the skin to measure jaundice-especially in dark-skinned patients-since it is free of skin pigment (melanin), a major confounding factor. This work aims to show how the yellowness of the sclera can be quantified by digital photography in color spaces including the native RGB and CIE XYZ. We also introduce a new color metric we call "Jaundice Eye Color Index" (JECI) which allows the yellowness of jaundiced sclerae to be predicted for a specific total serum bilirubin level in the neonatal population.

Blood transfusion in preterm infants improves intestinal tissue oxygenation without alteration in blood flow

BACKGROUND AND OBJECTIVE

The objective of the study was to investigate the splanchnic blood flow velocity and oximetry response to blood transfusion in preterm infants according to postnatal age.

MATERIALS AND METHODS

Preterm infants receiving blood transfusion were recruited to three groups: 1-7 (group 1; n = 20), 8-28 (group 2; n = 21) and ≥29 days of life (group 3; n = 18). Superior mesenteric artery (SMA) peak systolic (PSV) and diastolic velocities were measured 30-60 min pre- and post-transfusion using Doppler ultrasound scan. Splanchnic tissue haemoglobin index (sTHI), tissue oxygenation index (sTOI) and fractional tissue oxygen extraction (sFTOE) were measured from 15-20 min before to post-transfusion using near-infrared spectroscopy.

RESULTS

The mean pretransfusion Hb in group 1, 2 and 3 was 11, 10 and 9 g/dl, respectively. The mean (SD) pretransfusion SMA PSV in group 1, 2 and 3 was 0·63 (0·32), 0·81 (0·33) and 0·97 (0·40) m/s, respectively, and this did not change significantly following transfusion. The mean (SD) pretransfusion sTOI in group 1, 2 and 3 was 36·7 (19·3), 44·6 (10·4) and 41·3 (10·4)%, respectively. The sTHI and sTOI increased (P < 0·01), and sFTOE decreased (P < 0·01) following transfusion in all groups. On multivariate analysis, changes in SMA PSV and sTOI following blood transfusion were not associated with PDA, feeding, pretransfusion Hb and mean blood pressure.

CONCLUSION

Pretransfusion baseline splanchnic tissue oximetry and blood flow velocity varied with postnatal age. Blood transfusion improved intestinal tissue oxygenation without altering mesenteric blood flow velocity irrespective of postnatal ages.

Cerebral blood flow and oximetry response to blood transfusion in relation to chronological age in preterm infants

OBJECTIVE

Preterm infants frequently receive blood transfusion (BT) and the aim of this study was to measure the effect of BT on cerebral blood flow and oxygenation in preterm infants in relation to chronological age.

PATIENTS

Preterm infants undergoing intensive care recruited to three chronological age groups: 1 to 7 (Group 1; n=20), 8 to 28 (Group 2; n=21) & ≥29days of life (Group 3; n=18).

METHODS

Pre and post-BT anterior cerebral artery (ACA) time averaged mean velocity (TAMV) and superior vena cava (SVC) flow were measured. Cerebral Tissue Haemoglobin Index (cTHI) and Oxygenation Index (cTOI) were measured from 15-20min before to 15-20min post-BT using NIRS. Vital parameters and blood pressure were measured continuously.

RESULTS

Mean BP increased significantly, and there was no significant change in vital parameters following BT. Pre-BT ACA TAMV was higher in Group 2 and 3 compared to Group 1 (p<0.001). Pre-BT ACA TAMV decreased significantly (p≤0.04) in all 3 groups; pre-BT SVC flow decreased significantly in Group 1 (p=0.03) and Group 3 (p<0.001) following BT. Pre-BT cTOI was significantly lower in Group 3 compared to Group 1 (p=0.02). cTHI (p<0.001) and cTOI (p<0.05) increased significantly post-BT in all three groups. PDA had no effect on these measurements.

CONCLUSION

Baseline cTOI decreases and ACA TAMV increases with increasing chronological age. Blood transfusion increased cTOI and cTHI and decreased ACA TAMV in all groups. PDA had no impact on the baseline cerebral oximetry and blood flow as well as changes following blood transfusion.

Smart biosensors for multiplexed and fully integrated point-of-care diagnostics

Point-of-care diagnostics (PoC) and personalised medicine are highly valuable for the improvement of world health. Smartphone PoC platforms which precisely diagnose diseases and track their development through the detection of several bioanalytes represent one of the newest and most exciting advancements towards mass-screening applications. Here we focus on recent advances in both multiplexed and smartphone integrated PoC sensors.

Gradient-Based Quantitative Image Reconstruction in Ultrasound-Modulated Optical Tomography: First Harmonic Measurement Type in a Linearised Diffusion Formulation

Ultrasound-modulated optical tomography is an emerging biomedical imaging modality which uses the spatially localised acoustically-driven modulation of coherent light as a probe of the structure and optical properties of biological tissues. In this work we begin by providing an overview of forward modelling methods, before deriving a linearised diffusion-style model which calculates the first-harmonic modulated flux measured on the boundary of a given domain. We derive and examine the correlation measurement density functions of the model which describe the sensitivity of the modality to perturbations in the optical parameters of interest. Finally, we employ said functions in the development of an adjoint-assisted gradient based image reconstruction method, which ameliorates the computational burden and memory requirements of a traditional Newton-based optimisation approach. We validate our work by performing reconstructions of optical absorption and scattering in two- and three-dimensions using simulated measurements with 1% proportional Gaussian noise, and demonstrate the successful recovery of the parameters to within ±5% of their true values when the resolution of the ultrasound raster probing the domain is sufficient to delineate perturbing inclusions.

Effect of blood transfusion on intestinal blood flow and oxygenation in extremely preterm infants during first week of life

BACKGROUND

Extremely preterm infants receive frequent blood transfusions in the first week of life. The aim of this study was to measure the effect of blood transfusion on intestinal blood flow and oxygenation during the first week of life in extremely preterm infants.

STUDY DESIGN AND METHODS

Superior mesenteric artery (SMA) peak systolic velocity (PSV) and diastolic velocities were measured 30 to 60 minutes before and after transfusion. Splanchnic tissue hemoglobin index (sTHI), splanchnic tissue oxygenation index (sTOI), and splanchnic fractional tissue oxygen extraction (sFTOE) were measured continuously from 15 to 20 minutes before to after transfusion along with vital variables.

RESULTS

Twenty infants were studied (median gestational age, 26 weeks). Ten infants were partially fed (15-68 mL/kg/day). Heart rate and SaO2 remained unaltered; blood pressure increased significantly (p < 0.01) after transfusion. Mean SMA PSV (p = 0.63) and diastolic velocity (p = 0.65) remained unaltered. Mean pretransfusion SMA PSV was similar in partially fed (0.78 m/sec) compared to unfed infants (0.52 m/sec; p = 0.06) and the response to transfusion was not dissimilar. There was a significant increase in sTHI (mean difference, 32.3%; p < 0.01) and sTOI (14.6%; p = 0.03) and decrease in sFTOE (22.1%; p < 0.01) after transfusion. There was no significant difference in sTHI or sTOI between fed and unfed infants and their response to transfusion.

CONCLUSIONS

Blood transfusion increased blood pressure and intestinal tissue oxygenation but did not alter blood flow velocities. Partial feeding had no impact on intestinal blood flow and tissue oxygenation changes.

Screening neonatal jaundice based on the sclera color of the eye using digital photography

A new screening technique for neonatal jaundice is proposed exploiting the yellow discoloration in the sclera. It involves taking digital photographs of newborn infants' eyes (n = 110) and processing the pixel colour values of the sclera to predict the total serum bilirubin (TSB) levels. This technique has linear and rank correlation coefficients of 0.75 and 0.72 (both p<0.01) with the measured TSB. The mean difference ( ± SD) is 0.00 ± 41.60 µmol/l. The receiver operating characteristic curve shows that this technique can identify subjects with TSB above 205 µmol/l with sensitivity of 1.00 and specificity of 0.50, showing its potential as a screening device.

Spatial sensitivity and penetration depth of three cerebral oxygenation monitors

The spatial sensitivities of NIRO-100, ISS Oximeter and TRS-20 cerebral oxygenation monitors are mapped using the local perturbation method to inform on their penetration depths and susceptibilities to superficial contaminations. The results show that TRS-20 has the deepest mean penetration depth and is less sensitive than the other monitors to a localized absorption change in the superficial layer. However, an integration time of more than five seconds is required by the TRS-20 to achieve an acceptable level of signal-to-noise ratio, which is the poorest amongst the monitors. With the exception of NIRO-100 continuous wave method, the monitors are not significantly responsive to layer-wide absorption change that occurs in the superficial layer.

Effect of blood in the cerebrospinal fluid on the accuracy of cerebral oxygenation measured by near infrared spectroscopy

Near infrared spectroscopy (NIRS) is an optical technique used to examine the oxygenation state of tissues such as the brain in patients, including those with brain injury. We have examined the effect of a cerebrospinal fluid (CSF) contaminant, specifically haemoglobin, on the sensitivity of cerebral NIRS signals through computer simulation. Previous models of light transport in the head have shown that the clear CSF layer has a profound effect on the sensitivity profile of the NIRS signal due to its low absorbing, low scattering qualities. In subarachnoid haemorrhage, which may accompany brain injury, the principal near infrared chromophore, haemoglobin, is released into the CSF. Sensitivity was measured through forward modeling and the presence of haemoglobin within the CSF was modeled by increasing the absorption coefficient of the layer, with sensitivity quantified in terms of the partial pathlength of light within the brain. The model demonstrated that increases in the CSF absorption led to a marked decrease in the sensitivity to changes in the brain layer. This suggests that blood or other contaminants in the CSF may have a significant effect on the utility of NIRS for measurement of cerebral oxygenation, and merits further investigation.

Linear reconstruction of absorption perturbations in coherent ultrasound-modulated optical tomography

Discrete form sensitivity functions are derived for a number of measurement types employed in autocorrelation-based ultrasound-modulated optical tomography. The Jacobian for a particular problem is constructed from the set of such sensitivity functions arising from a raster scan of a focused ultrasound field through a turbid medium. A linear reconstruction of an absorption perturbation is performed in a simulated difference data experiment, and the performance of the measurement types is compared under different degrees of added noise.

Intraoperative assessment of human spinal cord perfusion using near infrared spectroscopy with indocyanine green tracer technique

BACKGROUND CONTEXT

Despite the significant interest in the assessment of human cerebral perfusion, investigations into human spinal cord perfusion (SCP) are scarce. Current intraoperative monitoring of spinal cord relies on the assessment of neural conduction as a surrogate for SCP. However, there are various inherent limitations associated with the use of these techniques. Near infrared spectroscopy (NIRS) has been successfully used for monitoring and assessment of human cerebral perfusion and has shown promising results in intraoperative assessment of SCP in animal models.

PURPOSE

The aim of this study was to investigate whether it is possible to monitor physiological changes in human SCP intraoperatively using NIRS with indocyanine green (ICG) tracer technique. We used this technique to calculate the human spinal cord carbon dioxide (CO₂) reactivity index. In addition, we investigated whether the lamina causes significant attenuation of NIRS signals.

STUDY DESIGN/SETTING

Intraoperative human experimental study.

PATIENT SAMPLE

Eighteen patients undergoing elective posterior cervical spine surgery.

OUTCOME MEASURES

Carbon dioxide reactivity of human SCP.

METHODS

Nine patients underwent transdural assessment of SCP, with an additional nine patients undergoing translaminar measurements. Patients' SCP was continuously monitored using an NIRO-500 NIRS monitor via a set of purpose built optodes. Their arterial ICG concentration was simultaneously assessed using a pulse dye densitometer. Patients' end-tidal CO₂ was gradually increased by 7.5 mm Hg and then returned back to baseline. Three sets of measurements were taken: baseline, hypercapnic, and return to baseline.

RESULTS

After hypercapnia, SCP increased by a mean of 57.2 ± 23.3% in the transdural group and 46.6 ± 36.3% in the translaminar group. Carbon dioxide reactivity index was 7.6 ± 3.2%ΔSCP/mm Hg in the transdural group and 6.4 ± 5.3 %ΔSCP/mm Hg in the translaminar group. There was no significant difference in the increase in SCP (p=.475) or the CO₂ reactivity index (p=.581) observed between the transdural and the translaminar groups.

CONCLUSIONS

Intraoperative NIRS with ICG tracer technique can identify an increase in the SCP in response to hypercapnia. It is possible to use this technique for monitoring SCP over the dura and the lamina. This technique could potentially be used to provide insight in to the pathophysiology and autoregulation of commonly acquired spinal cord conditions. Further research assessing the use of NIRS for monitoring of SCP is required.

Acousto-optic imaging of a color picture hidden behind a scattering layer

An imaging technique has been developed to image a color picture hidden behind a 5 mm thick, highly scattering layer with low transmittance of 0.24%. Small vibrations (< 1 µm) were induced in the hidden picture, causing a time-varying speckle pattern on the scattering layer in the front, which is captured by a CCD camera and quantified as speckle contrast difference (SCD). With two lasers at 543 nm and 633 nm, the imaging system raster-scans the front of the scattering layer and the resulting SCD image reveals the color features of the hidden picture.

Classifying hidden colors behind an opaque layer with the acoustically modulated laser speckle contrast technique

The acoustically modulated laser speckle contrast technique has been employed to quantify and classify 25 colors (made up by different percentages of the two base colors cyan and magenta) hidden behind a 5 mm thick opaque layer with 0.24% transmittance. The main components included two He-Ne lasers (543 and 633 nm), a consumer grade digital camera (Nikon 1 J1), focusing optics and a loudspeaker. The camera captured the laser speckle patterns with the sound on and off, respectively, from which the speckle contrast differences were calculated and used in a nearest neighbor classification algorithm. The classification accuracy was between 55% and 88% depending on the underlying reflectance of all the colors to be classified.

Development of a hybrid microwave-optical tissue oxygenation probe to measure thermal response in the deep tissue

The design of a new non-invasive hybrid microwave-optical tissue oxygenation probe is presented, which consists of a microwave biocompatible antenna and an optical probe. The microwave antenna is capable of inducing localised heat in the deep tissue, causing tissue blood flow and therefore tissue oxygenation to change. These changes or thermal responses are measured by the optical probe using near-infrared spectroscopy. Thermal responses provide important information on thermoregulation in human tissue. The first prototype of the biocompatible antenna was developed and placed on the human calf for in vivo experiments. The measured results include oxy-, deoxy- and total haemoglobin concentration changes (ΔHbO2/ΔHHb/ΔHbT), tissue oxygenation index and the normalised tissue haemoglobin index for two human subjects. Both ΔHbO2 and ΔHbT show an increase during 5 min of microwave exposure. The thermal response, defined as the ratio of the increase in ΔHbT to the time duration, is 7.7 μM/s for subject 1 (fat thickness = 6.8 mm) and 18.9 μM/s for subject 2 (fat thickness = 5.0 mm), which may be influenced by the fat thicknesses. In both subjects, ΔHbO2 and ΔHbT continued to increase for approximately another 70 s after the microwave antenna was switched off.

Light propagation in a turbid medium with insonified microbubbles

Surfactant stabilized microbubbles are widely used clinical contrast agents for ultrasound imaging. In this work, the light propagation through a turbid medium in the presence of microbubbles has been investigated. Through a series of experiments, it has been found that the optical attenuation is increased when the microbubbles in a turbid medium are insonified by ultrasound. Such microbubble enhanced optical attenuation is a function of both applied ultrasound pressure and microbubble concentration. To understand the mechanisms involved, a Monte Carlo (MC) model has been developed. Under ultrasound exposure, the sizes of microbubbles vary in space and time, and their dynamics are modeled by the Rayleigh-Plesset equation. By using Mie theory, the spatially and temporally varying optical scattering and scattering efficiency of microbubbles are determined based on the bubble sizes and internal refractive indices. The MC model is shown to effectively describe a medium with rapidly changing optical scattering, and the results are validated against both computational results using an N-layered diffusion equation model and experimental results using a clinical microbubble contrast agent (SonoVue®).

Superficial heat reduction technique for a hybrid microwave-optical device

Microwave applicator in the form of a circularly polarized microstrip patch antenna is proposed to provide localized deep heating in biological tissue, which causes blood vessels to dilate leading to changes in tissue oxygenation. These changes are monitored by an integrated optical system for studying thermoregulation in different parts of the human body. Using computer simulations, this paper compares circularly and linearly polarized antennas in terms of the efficiency of depositing electromagnetic (EM) energy and the heating patterns. The biological model composes of the skin, fat and muscle layers with appropriate dielectric and thermal properties. The results show that for the same specific absorption rate (SAR) in the muscle, the circularly polarized antenna results in a lower SAR in the skin-fat interface than the linearly polarized antenna. The thermal distribution is also presented based on the biological heat equation. The proposed circularly polarized antenna shows heat reduction in the superficial layers in comparison to the linearly polarized antenna.

Measuring the reflectance of hidden color objects with acoustically modulated laser speckle

We demonstrate a technique to measure the reflectance of colored objects, including blue, green, and yellow, hidden behind an opaque slab based on acoustically modulated laser speckle. One colored paper at a time was placed behind a 1 cm thick opaque slab with an air gap of 5 mm. Small periodic movements (nanometer scale) at 200 Hz were induced in the colored paper. A coherent red He-Ne laser illuminated the front of the slab, producing acoustically modulated speckle patterns, which were captured by a CCD camera. The magnitude of the time-varying speckle intensity is indicative of the hidden colored paper's reflectance.

Highly parallel Monte-Carlo simulations of the acousto-optic effect in heterogeneous turbid media

The development of a highly parallel simulation of the acousto-optic effect is detailed. The simulation supports optically heterogeneous simulation domains under insonification by arbitrary monochromatic ultrasound fields. An adjoint method for acousto-optics is proposed to permit point-source/point-detector simulations. The flexibility and efficiency of this simulation code is demonstrated in the development of spatial absorption sensitivity maps which are in broad agreement with current experimental investigations. The simulation code has the potential to provide guidance in the feasibility and optimization of future studies of the acousto-optic technique, and its speed may permit its use as part of an iterative inversion model.

Spatial sensitivity of acousto-optic and optical near-infrared spectroscopy sensing measurements

Near-infrared spectroscopy (NIRS) is a popular sensing technique to measure tissue oxygenation noninvasively. However, the region of interest (ROI) is often beneath a superficial layer, which affects its accuracy. By applying focused ultrasound in the ROI, acousto-optic (AO) techniques can potentially minimize the effect of physiological changes in the superficial layer. Using absorption perturbation experiments in both transmission and reflection modes, we investigated the spatial sensitivity distributions and mean penetration depths of an AO system based on a digital correlator and two popular NIRS systems based on i. intensity measurements using a single source and detector configuration, and ii. spatially resolved spectroscopy. Our results show that for both transmission and reflection modes, the peak relative sensitivities of the two NIRS systems are near to the superficial regions, whereas those of the AO technique are near to the ROIs. In the reflection mode, when the ROI is deeper than 14 mm, the AO technique has a higher absolute mean sensitivity than the two NIRS techniques. As the focused ultrasound is moved deeper into the turbid medium, the mean penetration depth increases accordingly. The focused ultrasound can shift the peak relative sensitivity of the AO measurement toward its focused region.

Effects of assuming constant optical scattering on haemoglobin concentration measurements using NIRS during a Valsalva manoeuvre

Resolving for changes in concentration of tissue chromophores in the human adult brain with near-infrared spectroscopy has generally been based on the assumption that optical scattering and pathlength remain constant. We have used a novel hybrid optical spectrometer that combines multi-distance frequency and broadband systems to investigate the changes in scattering and pathlength during a Valsalva manoeuvre in 8 adult volunteers. Results show a significant increase in the reduced scattering coefficient of 17% at 790nm and 850nm in 4 volunteers during the peak of the Valsalva. However, these scattering changes do not appear to significantly affect the differential pathlength factor and the tissue haemoglobin concentration measurements.

Fast Monte Carlo simulations of ultrasound-modulated light using a graphics processing unit

Ultrasound-modulated optical tomography (UOT) is based on "tagging" light in turbid media with focused ultrasound. In comparison to diffuse optical imaging, UOT can potentially offer a better spatial resolution. The existing Monte Carlo (MC) model for simulating ultrasound-modulated light is central processing unit (CPU) based and has been employed in several UOT related studies. We reimplemented the MC model with a graphics processing unit [(GPU), Nvidia GeForce 9800] that can execute the algorithm up to 125 times faster than its CPU (Intel Core Quad) counterpart for a particular set of optical and acoustic parameters. We also show that the incorporation of ultrasound propagation in photon migration modeling increases the computational time considerably, by a factor of at least 6, in one case, even with a GPU. With slight adjustment to the code, MC simulations were also performed to demonstrate the effect of ultrasonic modulation on the speckle pattern generated by the light model (available as animation). This was computed in 4 s with our GPU implementation as compared to 290 s using the CPU.

Precision of coherence analysis to detect cerebral autoregulation by near-infrared spectroscopy in preterm infants

Coherence between spontaneous fluctuations in arterial blood pressure (ABP) and the cerebral near-infrared spectroscopy signal can detect cerebral autoregulation. Because reliable measurement depends on signals with high signal-to-noise ratio, we hypothesized that coherence is more precisely determined when fluctuations in ABP are large rather than small. Therefore, we investigated whether adjusting for variability in ABP (variability(ABP)) improves precision. We examined the impact of variability(ABP) within the power spectrum in each measurement and between repeated measurements in preterm infants. We also examined total monitoring time required to discriminate among infants with a simulation study. We studied 22 preterm infants (GA<30) yielding 215 10-min measurements. Surprisingly, adjusting for variability(ABP) within the power spectrum did not improve the precision. However, adjusting for the variability(ABP) among repeated measurements (i.e., weighting measurements with high variability(ABP) in favor of those with low) improved the precision. The evidence of drift in individual infants was weak. Minimum monitoring time needed to discriminate among infants was 1.3-3.7 h. Coherence analysis in low frequencies (0.04-0.1 Hz) had higher precision and statistically more power than in very low frequencies (0.003-0.04 Hz). In conclusion, a reliable detection of cerebral autoregulation takes hours and the precision is improved by adjusting for variability(ABP) between repeated measurements.

A hybrid multi-distance phase and broadband spatially resolved spectrometer and algorithm for resolving absolute concentrations of chromophores in the near-infrared light spectrum

For resolving absolute concentration of tissue chromophores in the human adult brain with near-infrared spectroscopy it is necessary to calculate the light scattering and absorption, at multiple wavelengths with some depth resolution. To achieve this we propose an instrumentation configuration that combines multi-distance frequency and broadband spectrometers to quantify chromophores in turbid media by using a hybrid spatially resolved algorithm. Preliminary results in solid phantoms as well as liquid dynamic homogeneous and inhomogeneous phantoms and in-vivo muscle measurements showed encouraging results.

The effect on cerebral tissue oxygenation index of changes in the concentrations of inspired oxygen and end-tidal carbon dioxide in healthy adult volunteers

BACKGROUND

A variety of near-infrared spectroscopy devices can be used to make noninvasive measurements of cerebral tissue oxygen saturation (ScO2). The ScO2 measured by the NIRO 300 spectrometer (Hamamatsu Photonics, Japan) is called the cerebral tissue oxygenation index (TOI) and is an assessment of the balance between cerebral oxygen delivery and utilization. We designed this study to investigate the effect of systemic and intracranial physiological changes on TOI.

METHODS

Fifteen healthy volunteers were studied during isocapneic hyperoxia and hypoxemia, and normoxic hypercapnea and hypocapnea. Absolute cerebral TOI and changes in oxy- and deoxyhemoglobin concentrations were measured using a NIRO 300 spectrometer. Changes in arterial oxygen saturation (SaO2), ETCO2, heart rate, mean arterial blood pressure (MBP), and middle cerebral artery blood flow velocity (Vmca) were also measured during these physiological challenges. Changes in cerebral blood volume (CBV) were subsequently calculated from changes in total cerebral hemoglobin concentration.

RESULTS

Baseline TOI was 67.3% with an interquartile range (IQR) of 65.2%-71.9%. Hypoxemia was associated with a median decrease in TOI of 7.1% (IQR -9.1% to -5.4%) from baseline (P < 0.0001) and hyperoxia with a median increase of 2.3% (IQR 2.0%-2.5%) (P < 0.0001). Hypocapnea caused a reduction in TOI of 2.1% (IQR -3.3% to -1.3%) from baseline (P < 0.0001) and hypercapnea an increase of 2.6% (IQR 1.4%-3.7%) (P < 0.0001). Changes in SaO2 (P < 0.0001), ETCO2 (P < 0.0001), CBV (P = 0.0003), and MBP (P = 0.03) were significant variables affecting TOI. Changes in Vmca (P = 0.7) and heart rate (P = 0.2) were not significant factors.

CONCLUSION

TOI is an easy-to-monitor variable that provides real-time, multisite, and noninvasive assessment of the balance between cerebral oxygen delivery and utilization. However, TOI is a complex variable that is affected by SaO2 and ETCO2, and, to a lesser extent, by MBP and CBV. Clinicians need to be aware of the systemic and cerebral physiological changes that can affect TOI to interpret changes in this variable during clinical monitoring.

Estimating a modified Grubb's exponent in healthy human brains with near infrared spectroscopy and transcranial Doppler

The relationship between cerebral blood volume (CBV) and flow (CBF) has been widely studied. One of the most significant early studies was by Grubb et al (1974 Stroke 5 630-9), who conducted hypercapnia studies in primates with positron emission tomography (PET) and empirically found CBV = 0.8 CBF(0.38). The exponent used here has since been known as the Grubb's exponent. In this paper, we define a similar exponent known as the modified Grubb's exponent, G', which is based on CBV and cerebral blood flow velocity (CBFV) estimated by near infrared spectroscopy (NIRS) and transcranial Doppler respectively, i.e. G' = log(CBV/CBV(0))/log(CBFV/CBFV(0)), where CBV(0) and CBFV(0) are baseline values. The aim of this study was to estimate the nominal value of the modified Grubb's exponent in healthy human brains. We conducted hypercapnia and hypocapnia studies on 14 healthy adult subjects. The correlation coefficient between log(CBV/CBV(0)) and log(CBFV/CBFV(0)) is 0.71 (p < 0.0001). We found a modified Grubb's exponent of 0.13 (the 95% confidence bounds are 0.10 and 0.17) which is expectedly lower than the conventional Grubb's exponents estimated by other techniques. The modified Grubb's exponent is a simple measure to quantify the hemodynamics between local CBV and global CBFV in the brain and as such may provide insight on brain physiology. Both NIRS and transcranial Doppler techniques are non-invasive and portable, facilitating future studies in other population groups such as brain-injured patients.

Comparison of cerebral oxygen saturation in premature infants by near-infrared spatially resolved spectroscopy: observations on probe-dependent bias

Spatially resolved spectroscopy (SRS) allows the estimation of absolute tissue oxygen saturation, the ratio of oxygenated to total hemoglobin concentration, which may facilitate the comparison of results among patients. Eighty-two premature infants were included over two years. The cerebral tissue oxygenation index (c-TOI) was measured using NIRO 300 (Hamamatsu Photonics KK). c-TOI was measured at several positions in each infant. c-TOI varied over time, increasing in the first third and decreasing in the last third of the study period (p<10(-6)). Two probes were used in the study, and a highly significant difference was found between these (p<10(-6)). The mean difference was 8.5% (95%CI 5.4 to 11.6%). After correction for this difference, there was no variation over time. A conclusive explanation for the bias could not be identified. Since the study groups were well distributed, the bias had no influence on the results of our clinical study. We investigated an unexpected but highly significant probe-dependent bias in c-TOI with no conclusive explanation. Hence, comparisons of absolute TOI between groups of patients and among studies should be regarded with caution. A better strategy to detect potential instrumental problems will be useful in preventing biased c-TOI from occurring.

Increase in cerebral aerobic metabolism by normobaric hyperoxia after traumatic brain injury

OBJECT

Traumatic brain injury (TBI) is associated with depressed aerobic metabolism and mitochondrial dysfunction. Normobaric hyperoxia (NBH) has been suggested as a treatment for TBI, but studies in humans have produced equivocal results. In this study the authors used brain tissue O(2) tension measurement, cerebral microdialysis, and near-infrared spectroscopy to study the effects of NBH after TBI. They investigated the effects on cellular and mitochondrial redox states measured by the brain tissue lactate/pyruvate ratio (LPR) and the change in oxidized cytochrome c oxidase (CCO) concentration, respectively.

METHODS

The authors studied 8 adults with TBI within the first 48 hours postinjury. Inspired oxygen percentage at normobaric pressure was increased from baseline to 60% for 60 minutes and then to 100% for 60 minutes before being returned to baseline for 30 minutes.

RESULTS

The results are presented as the median with the interquartile range in parentheses. During the 100% inspired oxygen percentage phase, brain tissue O2 tension increased by 7.2 kPa (range 4.5-9.6 kPa) (p < 0.0001), microdialysate lactate concentration decreased by 0.26 mmol/L (range 0.0-0.45 mmol/L) (p = 0.01), microdialysate LPR decreased by 1.6 (range 1.0-2.3) (p = 0.02), and change in oxidized CCO concentration increased by 0.21 mumol/L (0.13-0.38 micromol/L) (p = 0.0003). There were no significant changes in intracranial pressure or arterial or microdialysate glucose concentration. The change in oxidized CCO concentration correlated with changes in brain tissue O(2) tension (r(s)= 0.57, p = 0.005) and in LPR (r(s)= -0.53, p = 0.006).

CONCLUSIONS

The authors have demonstrated oxidation in cerebral cellular and mitochondrial redox states during NBH in adults with TBI. These findings are consistent with increased aerobic metabolism and suggest that NBH has the potential to improve outcome after TBI. Further studies are warranted.

Impaired autoregulation in preterm infants identified by using spatially resolved spectroscopy

OBJECTIVE

The absence of cerebral autoregulation in preterm infants has been associated with adverse outcome, but its bedside assessment in the immature brain is problematic. We used spatially resolved spectroscopy to continuously measure cerebral oxygen saturation (expressed as a tissue-oxygenation index) and used the correlation of tissue-oxygenation index with spontaneous fluctuations in mean arterial blood pressure to assess cerebral autoregulation.

PATIENTS AND METHODS

The tissue-oxygenation index and mean arterial blood pressure were continuously measured in very premature infants (n = 24) of mean (+/-SD) gestational age of 26 (+/-2.3) weeks at a mean postnatal age of 28 (+/-22) hours. The correlation between mean arterial blood pressure and tissue-oxygenation index in the frequency domain was assessed by using cross-spectral analysis techniques (coherence and transfer-function gain). Values of coherence reflect the strength of linear correlation, whereas transfer-function gain reflects the amplitude of tissue-oxygenation index changes relative to mean arterial blood pressure changes.

RESULTS

High coherence (coherence > or = 0.5) values were found in 9 infants who were of lower gestational age, lower birth weight, and lower mean arterial blood pressure than infants with coherence of < 0.5; high-coherence infants also had higher median Clinical Risk Index for Babies scores and a higher rate of neonatal deaths. Coherence of > or = 0.5 predicted mortality with a positive predictive value of 67% and negative predictive value of 100%. In multifactorial analysis, coherence alone was the best predictor of mortality and Clinical Risk Index for Babies score alone was the best predictor of coherence.

CONCLUSIONS

High coherence between mean arterial blood pressure and tissue-oxygenation index indicates impaired cerebral autoregulation in clinically sick preterm infants and is strongly associated with subsequent mortality. Cross-spectral analysis of mean arterial blood pressure and tissue-oxygenation index has the potential to provide continuous bedside assessment of cerebral autoregulation and to guide therapeutic interventions.

Light transport in tissue by 3D Monte Carlo: influence of boundary voxelization

Monte Carlo (MC) based simulations of photon transport in living tissues have become the "gold standard" technique in biomedical optics. Three-dimensional (3D) voxel-based images are the natural way to represent human (and animal) tissues. It is generally believed that the combination of 3D images and MC based algorithms allows one to produce the most realistic models of photon propagation. In the present work, it is shown that this approach may lead to large errors in the MC data due to the "roughness" of the geometrical boundaries generated by the presence of the voxels. In particular, the computed intensity of the light detected on the tissue surface of a simple cubic tissue phantom may display errors from -80% to 120%. It is also shown that these errors depend in a complex manner on optical and geometrical parameters such as the interoptode distance, scattering coefficient, refractive index, etc. and on the degree of voxelization ("roughness") of the boundaries. It is concluded that if one wants to perform reliable 3D Monte Carlo simulations on complex geometries, such as human brain, skin or trabecular bone, it is necessary to introduce boundary meshing techniques or other equivalent procedures in the MC code to eliminate the deleterious effect of voxelization.

Theoretical investigation of measuring cerebral blood flow in the adult human head using bolus Indocyanine Green injection and near-infrared spectroscopy

To investigate the accuracy of measuring cerebral blood flow (CBF) using a bolus injection of Indocyanine Green (ICG) detected by near-infrared spectroscopy in adult human heads, simulations were performed using a two-layered model representing the extracerebral and intracerebral layers. Modeled optical data were converted into tissue ICG concentration using either the one-detector modified Beer-Lambert law (MBLL) method, or the two-detector partial path-length (PPL) method. The CBFs were estimated using deconvolution and blood flow index techniques. Using the MBLL method, the CBFs were significantly underestimated but the PPL method improved their accuracy and robustness, especially when used as relative measures. The dispersion of the arterial input function also affected the CBF estimates.

Near-infrared spectroscopic quantification of changes in the concentration of oxidized cytochrome c oxidase in the healthy human brain during hypoxemia

The near-IR cytochrome c oxidase (CCO) signal has potential as a clinical marker of changes in mitochondrial oxygen utilization. We examine the CCO signal response to reduced oxygen delivery in the healthy human brain. We induced a reduction in arterial oxygen saturation from baseline levels to 80% in eight healthy adult humans, while minimizing changes in end tidal carbon dioxide tension. We measured changes in the cerebral concentrations of oxidized CCO (Delta[oxCCO]), oxyhemoglobin (Delta[HbO(2)]), and deoxyhemoglobin (Delta[HHb]) using broadband near-IR spectroscopy (NIRS), and estimated changes in cerebral oxygen delivery (ecDO(2)) using pulse oximetry and transcranial Doppler ultrasonography. Results are presented as median (interquartile range). At the nadir of hypoxemia ecDO(2) decreased by 9.2 (5.4 to 12.1)% (p<0.0001), Delta[oxCCO] decreased by 0.24 (0.06 to 0.28) micromoles/l (p<0.01), total hemoglobin concentration increased by 2.83 (2.27 to 4.46) micromoles/l (p<0.0001), and change in hemoglobin difference concentration (Delta[Hbdiff]=Delta[HbO(2)]-Delta[HHb]) decreased by 12.72 (11.32 to 16.34) micromoles/l (p<0.0001). Change in ecDO(2) correlated with Delta[oxCCO] (r=0.78, p<0.001), but not with either change in total hemoglobin concentration or Delta[Hbdiff]. This is the first description of cerebral Delta[oxCCO] during hypoxemia in healthy adults. Studies are ongoing to investigate the clinical relevance of this signal in patients with traumatic brain injury.