Understanding pigmentation’s effect on pulse oximetry is critical amid evidence that pulse oximetry is less accurate for patients with pigmented skin. Optical phantoms can help validate oximeters, but commercial phantoms do not vary pigmentation. We develop a resin-based 3D printing method that generates mechanically flexible phantoms with tunable optical properties and <100 µm diameter channels. Using a reflectance-mode Maxim 86171 pulse oximeter, we evaluate how photoplethysmogram waveforms change as phantom pigmentation increases, and test an algorithm for estimating pigmentation from waveforms alone. 3D-printed phantoms can provide a platform for testing pulse oximeter performance across the spectrum of human pigmentation.
Pulse oximeters are widely used in healthcare systems to estimate blood oxygen saturation level (SpO2) using red and infrared light. Recent clinical and simulation studies reported that in darkly pigmented subjects oximeter over-estimates SpO2 which could lead to higher rates of occult hypoxemia in highly pigmented subjects. The probable solutions to solve this over-estimation bias could be modification of the current oximeter design, calibration enrollment or modification of oximeter ratio (R). In this study, a modification of the current oximeter ratio (R) was presented by using different combinations of currently estimated oximeter parameters. Simulation results showed that modified oximeter ratio reduces over-estimation bias in highly pigmented subjects compared to the conventional oximeter ratio. In the regions near hypoxemia threshold (90% oxygenation level), the over-estimation bias in the simulated test cohort could reduce from 1.36% to -0.01% if modified oximeter ratio is used. Results show that modification of oximeter ratio could be used in future to improve oximeter accuracy and produce pigmentation independent outcomes.
The ability to perform routine monitoring of bone quality is crucial for patients with bone diseases such as osteoporosis. Current assessments of bone quality are expensive and cannot be used regularly without exposing patients to ionizing radiation. Alternatively, visible-near infrared (Vis-NIR) spectroscopy is a non-invasive, non-ionizing technique that can be used to assess the compositional properties of bone. Recently, studies have reported agreement between transcutaneous Vis-NIR spectroscopic measures of bone quality and conventional radiographic measures collected from the second metacarpal bone of the hand. Computational simulations using Monte-Carlo (MC) modeling offer a valuable tool to better understand the relative contributions from the underlying bone in comparison with the superficial skin, as well as to investigate the relative benefits of specific fiberoptic illumination/collection geometries for transcutaneous measurement of metacarpal bone. To inform the model, skin from above the 2nd metacarpal bone and the bone itself were dissected from human cadaver hands. Reflectance and transmittance measurements of the skin and bone tissues were taken using an integrating sphere setup in the range of 400 nm-1800 nm. Optical properties were estimated using the Inverse Adding Doubling (IAD) technique. MC models of skin-bone tissues were created using these estimated optical properties as well as physical measurements of tissue thickness, and simulations of fiber-optic Vis-NIR measurements were performed. Results indicate up to 30% of the absorbance signal arises from contributions from the bone in specific spectral ranges.
Significance: Fiber-optic extended-wavelength diffuse reflectance spectroscopy (EWDRS) using both visible/near-infrared and shortwave-infrared detectors enables improved detection of spectral absorbances arising from lipids, water, and collagen and has demonstrated promise in a variety of applications, including detection of nerves and neurovascular bundles (NVB). Development of future applications of EWDRS for nerve detection could benefit from the use of model-based analyses including Monte Carlo (MC) simulations and evaluation of agreement between model systems and empirical measurements.Aim: The aim of this work is to characterize agreement between EWDRS measurements and simulations and inform future applications of model-based studies of nerve-detecting applications.Approach: A model-based platform consisting of an ex vivo microsurgical nerve dissection model, unique two-layer optical phantoms, and MC model simulations of fiber-optic EWDRS spectroscopic measurements were used to characterize EWDRS and compare agreement across models. In addition, MC simulations of an EWDRS measurement scenario are performed to provide a representative example of future analyses.Results: EWDRS studies performed in the common chicken thigh femoral nerve microsurgical dissection model indicate similar spectral features for classification of NVB versus adjacent tissues as reported in porcine models and human subjects. A comparison of measurements from unique EWDRS issue mimicking optical phantoms and MC simulations indicates high agreement between the two in homogeneous and two-layer optical phantoms, as well as in dissected tissues. Finally, MC simulations of measurement over a simulated NVB indicate the potential of future applications for measurement of nerve plexus.Conclusions: Characterization of agreement between fiber-optic EWDRS measurements and MC simulations demonstrates strong agreement across a variety of tissues and optical phantoms, offering promise for further use to guide the continued development of EWDRS for translational applications.
Modern mobile phone imaging sensors wide availability and high quality have enabled development of low-cost imaging and sensing approaches that utilize the camera, including those which detect diffuse optical interactions and produce quantitative transcutaneous measures analogous to clinical techniques for bilirubin and oxygenation sensing. Concurrently, recent clinical studies report overestimation bias from dark skinned patients in transcutaneous bilirubinometery (TcB) and pulse oximetry. Here, Monte Carlo simulations of TcB and oximetery were used to investigate the source of possible racial biases in clinical measurements. Simulations of device calibration studies with dark, mixed, and light skinned cohorts were tested against groups with similar and different racial distributions. Results implicate a combination of tissue optics and biased enrollment in calibration studies for systematic overestimation in both TcB and oximetry. Next, identical Monte Carlo simulations were performed with a 2D image sensor capable of detecting spatially resolved diffuse reflectance. Quantification models were developed from simulated calibration studies where reflectance was extracted from 1 to 5 unique sensor regions of interest (ROI), followed by evaluation against test cohorts with different racial distributions. The results indicated overestimation bias in darkly pigmented subjects could be reduced through incorporation of an increasing number of sensor ROI’s. Models for quantification of bilirubin were then developed using clinical data from our mobile phone based TcB study, and increasing number of sensor ROI’s improved model performance (r2) . These results suggest promise for the development of mobile image-sensor based spatially resolved diffuse reflectance for improving accuracy and reducing racial bias in transcutaneous measurements.
Pulse oximetry is a common tool to perform a non-invasive optical estimate (SpO2) of arterial blood oxygen saturation level (SaO2). Although the principle of pulse oximetry has been established for a long time Recent clinical studies reported oximeter over-estimation bias in black patients. Measurement accuracy is an important factor, as over-estimation could impact clinical decision-making. Prior Monte-Carlo (MC) simulation-based studies showed increased melanin could reduce the oximeter signal intensity. These studies didn’t show the impact of pigmentation on calibration equation development in a population cohort. Extending MC simulations to study the influence of bias in calibration model enrollment, along with the corresponding optical estimation errors would offer insight into the basis of important clinical observations. Here, an MC simulation platform was developed to assess how pigmentation distribution in the racial demographics could impact calibration model development. MC simulations of oximeter measurements from <1200 simulated patient finger models were generated using a stochastic sampling-based technique, where patient optical properties (including pigmentation) were statistically assigned to generate a variation of measurements across different population cohorts. MC simulations of oximeter calibration studies representative of prior FDA 510(k) guidelines e.g.- minimum 20% darkly pigmented population) in comparison with alternative enrollment distributions. Performance of oximeter calibration equations was evaluated with unique population distributions of test subjects. Results showed that even if the calibration equations were developed from a representative population cohort, the predicted SpO2 show overestimation in high pigmentation cohorts. This over-estimation minimizes when the calibration is generated from distributions with an increased pigmented subject enrollment. The sensitivity to detect hypoxia in the highly pigmented cohort (sensitivity=0.95) is lower than the low pigmented cohort(sensitivity=0.98) when the representative population distribution was used to develop the calibration equation.
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