Photon migration through fetal head in utero using continuous wave, near infrared spectroscopy: clinical and experimental model studies

Review of optical methods for fetal monitoring in utero

The current technology for monitoring fetal wellbeing during child birth is cardiotocography. However, CTG has high false positive rates that lead to unnecessary emergency Cesarean deliveries and false negatives that result in birth injuries. To curtail these issues, fetal pulse oximetery has been a topic of interest for many decades. Fetal pulse oximetry would yield the oxygen saturation of the fetus in utero and provide a more robust marker for clinicians to make decisions about performing emergency Cesarean deliveries. Here, we present a review of biomedical optical developments related to transabdominal fetal pulse oximetery in the biophotonics field and the challenges that must be overcome to make transabdominal pulse oximetry a clinical reality.

Effect of the presence of amniotic fluid for optical transabdominal fetal monitoring using Monte Carlo simulations

About a third of babies are delivered by Cesarean section. There has been an increase in maternal deaths during labor due to complications with subsequent births after a C-section. Therefore, there is a clinical motivation to reduce the C-section rate. Current techniques are, however, inefficient at determining fetal distress leading to a high false positive rate for complications and ultimately a C-section. For the current study, Monte Carlo simulations were used to calculate the amount of signal received on a model of a pregnant mother, as well as, the percent of the signal that comes from the fetal layer. Models with and without a 1 mm amniotic fluid were compared and showed differing trends.

Design and In Vivo Evaluation of a Non-Invasive Transabdominal Fetal Pulse Oximeter

OBJECTIVE

Current intrapartum fetal monitoring technology is unable to provide physicians with an objective metric of fetal well-being, leading to degraded patient outcomes and increased litigation costs. Fetal oxygen saturation (SpO2) is a more suitable measure of fetal distress, but the inaccessibility of the fetus prior to birth makes this impossible to capture through current means. In this paper, we present a fully non-invasive, transabdominal fetal oximetry (TFO) system that provides in utero measures of fetal SpO2.

METHODS

TFO is performed by placing a reflectance-mode optode on the maternal abdomen and sending photons into the body to investigate the underlying fetal tissue. The proposed TFO system design consists of a multi-detector optode, an embedded optode control system, and custom user-interface software. To evaluate the developed TFO system, we utilized an in utero hypoxic fetal lamb model and performed controlled desaturation experiments while capturing gold standard arterial blood gases (SaO2).

RESULTS

Various degrees of fetal hypoxia were induced with true SaO2 values ranging between 10.5% and 66%. The non-invasive TFO system was able to accurately measure these fetal SpO2 values, supported by a root mean-squared error of 6.37% and strong measures of agreement with the gold standard.

CONCLUSION

The results support the efficacy of the presented TFO system to non-invasively measure a wide-range of fetal SpO2 values and identify critical levels of fetal hypoxia.

SIGNIFICANCE

TFO has the potential to improve fetal outcomes by providing obstetricians with a non-invasive measure of fetal oxygen saturation prior to delivery.

Extraction of the Fetal Pulse Curve for Transabdominal Pulse Oximetry using Adaptive and Comb Filters

The fetal pulse curve can be captured by placing light sources and detectors on the belly of a pregnant woman. Following the principle of reflection pulse oximetry, the light emitted into the abdomen is modulated by pulsing maternal and fetal arteries. The acquired signal is a mixture of a weak fetal and a dominating maternal photoplethysmogram (PPG). A first step towards estimation of the fetal oxygen level is the reconstruction of the purely fetal signal in time domain. As already shown in a former work, comb filters are well suited for the task, in case the fetal heart rate is known. In this work we extend the method by utilizing an adaptive noise canceller (ANC) to estimate the fetal pulse rate for comb filter design. Synthetic test signals with constant and time variable pulse rates are generated in order to achieve reproducible conditions. The ANC is fed by the mixed PPG and the maternal reference signal to reduce the dominant maternal components. The fetal pulse rate is computed by evaluating peaks in the resulting signal in time and frequency domain. The findings are used for comb filter design. It is shown that the extraction of the fetal pulse curve from the synthetic mixed PPGs by using the proposed strategy is promising. Clinical test measurements are the next step for evaluation.

Signal Separation for Transabdominal Non-invasive Fetal Pulse Oximetry using Comb Filters

Non-invasive fetal pulse oximetry is the application of reflection pulse oximetry to the abdomen of a pregnant woman. Light sources and detectors areplaced on the belly. Emitted photons travel through maternal and fetal tissue and back to the detectors. The captured photoplethysmogram (PPG) is a complex mixture of the maternal and fetal pulse curve. A purely fetal PPG in time domain is needed to estimate the oxygen level of the unborn child. In this work we describe the application of comb filters to separate the fetalfrom the maternal signal. Finite element simulations and phantom measurements are utilized to generate and measure synthetic signals at different heart rates and noise levels. Comb filters with peak frequencies matched to the fetal heart rate are applied to the mixed PPGs. The filtered signals prove that the extraction of the fetal signal is sufficient even at a distance between the maternal and the fetal signal magnitudes of around 80 dB. The resulting signal quality is sufficient for beat to beat analysis and feature extraction in the time domain. We conclude that comb filtering is a suitable signal separation method for non-invasive fetal pulse oximetry.

Simulation study on the effect of tissue geometries to fluence composition for non-invasive fetal pulse oximetry

Transabdominal fetal pulse oximetry is a method to estimate the state of oxygenation of a fetus in-utero, utilizing the principle of reflection pulse oximetry. The extraction of fetal related information from a mixed fetal-maternal signal is elementary. Minimizing the ratio of purely maternal components of the signal at the detector side obviously facilitates signal separation. In this paper we analyze the influence of tissue geometries to the fluence composition at the surface of the abdomen. Monte-Carlo method is used to compute photon propagation in spherical layered tissue models. Spatial fluence distributions at the surface of the models are visualized and discussed. Our results show the characteristic effects of the distance between the fetus and the surface and the radius of the abdomen to the fluence composition at the detector. Further, the simulations indicate suitable source-detector configurations considering various anatomical conditions. We conclude that an adoption of the source-detector configuration to the individual tissue geometry at hand is necessary to achieve a proper signal composition and quality. Utilizing simulations for sensor design enhances the understanding of photon distributions in complex tissue geometries and supports a successful implementation of transabdominal fetal pulse oximetry.

New frontiers in time-domain diffuse optics, a review

The recent developments in time-domain diffuse optics that rely on physical concepts (e.g., time-gating and null distance) and advanced photonic components (e.g., vertical cavity source-emitting laser as light sources, single photon avalanche diode, and silicon photomultipliers as detectors, fast-gating circuits, and time-to-digital converters for acquisition) are focused. This study shows how these tools could lead on one hand to compact and wearable time-domain devices for point-of-care diagnostics down to the consumer level and on the other hand to powerful systems with exceptional depth penetration and sensitivity.

Utility of near infrared spectroscopy for the screening of the growth restricted fetus

A myriad of factors have been linked to increased risk for intrauterine growth restriction and the associated complications; the majority of which are based on observational statistics of demographics, socioeconomics and patient history. Unfortunately, there is a paucity of factors available that can appropriately address the underlying anatomy and physiology responsible for intrauterine growth restriction. To this point, it becomes necessary to use data acquisition modalities capable of addressing both the etiology and pathology in an effort to improve clinical management strategies. Near-infrared spectroscopy, although not traditionally used in standard, clinical screening has proven valuable for risk assessment in a number of recent investigational studies. Simulations based on the current literature are presented to assess near infrared spectroscopy utility regarding the ability to distinguish between the normal fetus and the growth restricted fetus. Findings are presented for all simulated data as well as the equipment-specific data derived from the NIRO-100 system (Hamamatsu Photonics, Hamamatsu, Japan). Results suggest an overall sensitivity and specificity on the order of 62% and 58%, respectively, and NIRO-100 sensitivity and specificity on the order of 85% and 92%, respectively.

Fetal transabdominal pulse oximeter studies using a hypoxic sheep model

OBJECTIVE

This study investigates the validity of transabdominal pulse oximetry using a sheep fetal hypoxia model with fetal arterial hemoglobin saturation.

METHODS

Four pregnant ewes were anaesthetized and cannulated through the brachial artery to measure direct arterial blood saturation, SaO(2). Next, the transabdominal pulse oximeter was used to measure indirect measurement of the arterial saturation of the fetus, SpO(2), from the maternal abdomen. Hypoxia was induced by a balloon placed in the maternal aorta.

RESULTS

There is a linear relationship between SaO(2), arterial blood saturation values of the fetus, and SpO(2), the values measured by the transabdominal pulse oximetry with a slope of 0.75 (r(2)=0.76).

CONCLUSION

This information can be used to calibrate the transabdominal pulse oximeter as a measurement of fetal arterial saturation. With these results, we can advance the accurate, no-risk, noninvasive transabdominal fetal pulse oximeter for human use. This research may contribute to the more accurate diagnosis of the diseases of the fetus including Hypoxic Ischemic Encephalopathy.

Effect of errors in baseline optical properties on accuracy of transabdominal near-infrared spectroscopy in fetal sheep brain during hypoxic stress

A continuous-wave (cw) near-infrared spectroscopy (NIRS) instrument has been developed to noninvasively quantify fetal cerebral blood oxygen saturation (StO2). A linear Green's function formulism was used to analytically solve the photon diffusion equation and extract the time-varying fetal tissue oxy- and deoxy-hemoglobin concentrations from the NIR measurements. Here we explored the accuracy with which this instrument can be expected to perform over a range of fetal hypoxic states. We investigated the dependence of this accuracy on the accuracy of the reference optical properties chosen based on the literature. The fetal oxygenation of a pregnant ewe model was altered via maternal aortic occlusion. The NIR cw instrument was placed on the maternal abdomen directly above the fetal head, continuously acquiring diffuse optical measurements. Blood was sampled periodically from the fetus to obtain fetal arterial saturation (SaO2) measurements from blood gas analysis. The NIR StO2 values were compared with the fetal SaO2 measurements. Variations in the NIR results due to uncertainty in the reference optical properties were relatively small within the fetal SaO2 range of 30 to 80%. Under hypoxic conditions, however, the variability of the NIR StO2 calculations with changes in the assumed reference properties became more significant.

Transabdominal fetal pulse oximetry with near-infrared spectroscopy

OBJECTIVE

The purpose of this study was to determine the feasibility of noninvasive fetal pulse oximetry in the human fetus with transabdominal continuous-wave near-infrared spectroscopy.

STUDY DESIGN

The instrument has 3 wavelength light-emitting diodes (735, 805, and 850 nm) as light sources and a photomultiplier tube as a detector. This instrument was used in 6 pregnant women (>36 weeks of gestation). First, a fetal heart rate was obtained with a fetal heart rate monitor. Then, the depth of fetal tissue (head) from the maternal abdomen was determined by ultrasound examination; the distance between the optodes (light source and the detector) was set to be approximately twice the depth of the fetus (7-11 cm). The data analysis was based on the modified Beer-Lambert law and the use of optical densities at 735 and 850 nm to obtain the concentration changes of the oxyhemoglobin and deoxyhemoglobin. The saturation was expressed as the percent of oxygen saturation equal to 100 x oxyhemoglobin/(oxyhemoglobin + deoxyhemoglobin). We recorded the spectroscopy data and the fetal heart rate for approximately 3 to 10 minutes in each patient.

RESULTS

The mean oxygen saturation values of each of the 6 individual fetuses ranged from 50% to 74% (overall mean saturation, 61% +/- 14.8% [SD]).

CONCLUSION

This preliminary data indicate that transabdominal fetal pulse oximetry is feasible for human patient application. The measured values were similar to those that are obtained with transvaginal pulse oximetry. Future studies should correlate transabdominally obtained measurements with those measurements that are obtained by transvaginal fetal pulse oximetry.

Transabdominal near infrared oximetry of hypoxic stress in fetal sheep brain in utero

The feasibility of transabdominal near-infrared (NIR) spectroscopy for detecting and quantifying fetal hypoxia in utero is demonstrated in a pregnant ewe model. A frequency domain NIR spectroscopy probe, consisting of two detectors and six sources operating at three wavelengths (675, 786, and 830 nm), was placed on the maternal abdomen directly above the fetal head. Fetal hypoxia was indirectly induced through occlusion of uterine blood flow for approximately 3 min. NIR photon diffusion measurements were made during a baseline period, during hypoxia of the fetus, and during recovery. Fetal blood samples were drawn from the fetal brachial artery and jugular veins at several time points during the cycle. Seven hypoxic cycles were induced in a total of five pregnant ewes. The NIR measurements were analyzed by using a two-layer diffusion model to deconvolve the fetal blood saturation from that of the pregnant ewe. Fetal hypoxia was detected. Good agreement was found between fetal blood saturation determined by the transabdominal NIR method and arterial and venous fetal blood saturation quantified from fetal blood samples by using a hemoximeter.

Feasibility of in vivo multichannel optical imaging of gene expression: experimental study in mice

PURPOSE

To develop and test a multichannel reflectance imaging system for small animals on the basis of a previously developed single-channel setup.

MATERIALS AND METHODS

The imaging system was composed of modular parts, including a light source, excitation filters, emission filters, and a charged-coupled device for recording images. On the basis of generated excitation and absorption spectra of green fluorescent protein (GFP), tricarbocyanine 5.5 (Cy5.5), and indocyanine green (ICG), filters were selected to allow spectral separation and optimize resultant recorded signal. The system was tested by using a combination of the fluorochromes to confirm spectral separation. In vivo tests were performed in nude mice with tumors that expressed cathepsin B, which could be evaluated by using a Cy5.5-based activatable probe and GFP. For each in vivo tumor type and channel, statistical analysis was performed on the basis of signal intensity in the region of interest.

RESULTS

The different fluorochromes were readily distinguished with the system; characteristics such as power were determined for all wavelengths. The system demonstrated a linear response for GFP, a monotonic response for Cy5.5 over a range of more than three orders of magnitude of concentration, and a more complex response for ICG. In vivo analysis demonstrated the ability to image GFP expression and cathepsin B expression separately in tumors: As expected, marked differences were observed in GFP-expression imaging between tumor types (1,363 arbitrary units [AU] +/- 236 [SD] vs 110 AU +/- 11 for GFP-positive and GFP-negative tumors, respectively; P <.001), whereas similar cathepsin B expression (1,070 AU +/- 285 vs 1,168 AU +/- 367; P >.5) was observed. Histologic analysis confirmed in vivo findings.

CONCLUSION

Imaging multiple gene expressions simultaneously in vivo by using optical imaging is feasible.

Modeling photon transport in transabdominal fetal oximetry

The possibility of optical oximetry of the blood in the fetal brain measured across the maternal abdomen just prior to birth is under investigation. Such measurements could detect fetal distress prior to birth and aid in the clinical decision regarding Cesarean section. This paper uses a perturbation method to model photon transport through an 8-cm-diam fetal brain located at a constant 2.5 cm below a curved maternal abdominal surface with an air/tissue boundary. In the simulation, a near-infrared light source delivers light to the abdomen and a detector is positioned up to 10 cm from the source along the arc of the abdominal surface. The light transport [W/cm2 fluence rate per W incident power] collected at the 10 cm position is Tm = 2.2 x 10(-6) cm(-2) if the fetal brain has the same optical properties as the mother and Tf = 1.0 x 10(-6) cm(-2) for an optically perturbing fetal brain with typical brain optical properties. The perturbation P=(Tf - Tm)/Tm is -53% due to the fetal brain. The model illustrates the challenge and feasibility of transabdominal oximetry of the fetal brain.