Evaluation of the robustness of cerebral oximetry to variations in skin pigmentation using a tissue-simulating phantom

Ali Afshari; Rolf B. Saager; David Burgos; William C. Vogt; Jianting Wang; Gonzalo Mendoza; Sandy Weininger; Kung-Bin Sung; Anthony J. Durkin; Anthony J. Durkin; T. Joshua Pfefer

doi:10.1364/BOE.454020

Biomedical Optics Express
Vol. 13,
Issue 5,
pp. 2909-2928
(2022)
•https://doi.org/10.1364/BOE.454020

Evaluation of the robustness of cerebral oximetry to variations in skin pigmentation using a tissue-simulating phantom

Ali Afshari, Rolf B. Saager, David Burgos, William C. Vogt, Jianting Wang, Gonzalo Mendoza, Sandy Weininger, Kung-Bin Sung, Anthony J. Durkin, and T. Joshua Pfefer

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Ali Afshari, Rolf B. Saager, David Burgos, William C. Vogt, Jianting Wang, Gonzalo Mendoza, Sandy Weininger, Kung-Bin Sung, Anthony J. Durkin, and T. Joshua Pfefer, "Evaluation of the robustness of cerebral oximetry to variations in skin pigmentation using a tissue-simulating phantom," Biomed. Opt. Express 13, 2909-2928 (2022)

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Abstract

Clinical studies have demonstrated that epidermal pigmentation level can affect cerebral oximetry measurements. To evaluate the robustness of these devices, we have developed a phantom-based test method that includes an epidermis-simulating layer with several melanin concentrations and a 3D-printed cerebrovascular module. Measurements were performed with neonatal, pediatric and adult sensors from two commercial oximeters, where neonatal probes had shorter source-detector separation distances. Referenced blood oxygenation levels ranged from 30 to 90%. Cerebral oximeter outputs exhibited a consistent decrease in saturation level with simulated melanin content; this effect was greatest at low saturation levels, producing a change of up to 15%. Dependence on pigmentation was strongest in a neonatal sensor, possibly due to its high reflectivity. Overall, our findings indicate that a modular channel-array phantom approach can provide a practical tool for assessing the impact of skin pigmentation on cerebral oximeter performance and that modifications to algorithms and/or instrumentation may be needed to mitigate pigmentation bias.

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Data availability

Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.

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Fig. 1. Estimated absorption coefficient spectra for highly pigmented epidermis (M_f =43%) compared to oxygenated and deoxygenated blood (150 g/liter Hb) [19,25].

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Fig. 2. Schematic of modular cerebral oximetry phantom with layer thicknesses noted (left, not to scale) and illustration of CVM design along with a photo of the phantom (right).

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Fig. 3. Optical properties of the neonatal and adult CVMs in comparison to literature data [38,48,49]. Absorption due to blood was added analytically to the measured µ_a of the proprietary customized resins.

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Fig. 4. Measured absorption spectra of epidermal TMM samples (normalized at 700 nm), as compared to human melanin (from Eq. (4)). Images of TMM samples are shown.

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Fig. 5. Absorption (left) and reduced scattering coefficients (right) of epidermis layer TMMs for the range of M_f values studied, compared with target data from the literature (dashed lines) [19,52].

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Fig. 6. Pigmented epidermal phantom layers used for oximeter testing, in position on the CVM.

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Fig. 7. Neonatal phantom case results for Oximeter A neonatal sensor and Oximeter B neonatal-pediatric sensor, at two ctHb values and three M_f levels.

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Fig. 8. Pediatric phantom case results for Oximeter A pediatric sensor and Oximeter B neonatal-pediatric sensor, at two ctHb values and three M_f levels.

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Fig. 9. Adult phantom case results for Oximeter A and Oximeter B adult sensors, at two ctHb values and three M_f levels. *For Oximeter A measurements at ctHb = 46 µM, Layer 2 thickness of 5 mm was used as the device did not display results for 7.5 mm.

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Fig. 10. Change in StO₂ relative to M_f = 2% case for Oximeter A neonatal sensor and oximeter B neonatal-pediatric sensor, using neonatal CVMs with two ctHb levels.

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Fig. 11. Change in StO₂ relative to M_f = 2% case for Oximeter A pediatric sensor and Oximeter B neonatal-pediatric sensor using neonatal CVMs with two ctHb levels.

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Fig. 12. Change in StO₂ relative to M_f = 2% case for Oximeter A and B adult sensors using adult CVMs with two ctHb levels. Scalp/skull thickness for Oximeter A with ctHb of 46 µM is 5 mm.

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Fig. 13. Surface reflectance of sensors, (a) as measured with a spectrophotometer, along with photos of sensors including Oximeter A (b) neonate (c) pediatric and (d) adult, as well as Oximeter B (e) neonate/pediatric and (f) adult.

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Tables (2)

Table 1. Summary of optical properties for phantom layers 2, 3 and CVM.

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Table 2. Summary of phantom cases implemented

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Equations (6)

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neonatal CVM : {μ_{s}}^{'} = (2.33 x 10^{4}) λ^{- 1.58}

adult CVM : {μ_{s}}^{'} = (2.29 x 10^{4}) λ^{- 1.51}

μ_{a, e p i} = M_{f} μ_{a, m e l} + (1 - M_{f}) μ_{a, 0}

μ_{a, m e l} = (519 {c m}^{- 1}) {(\frac{λ}{500 n m})}^{- 3.53}

{μ_{s}}^{^{'}} = (6.0 x 10^{3}) λ^{- 1.22}

A_{r m s} = \sqrt{\frac{\sum_{i = 1}^{n} {(S t O_{2 i} - S_{R} O_{2 i})}^{2}}{n}} .

Phantom component	Neonatal case					Pediatric case
	Wavelength (nm)					Wavelength (nm)
	690	730	770	810	870	690	730	770	810	870
L2 µ_s’ (mm^-1)	1.104	1.024	0.959	0.891	0.745	1.104	1.024	0.959	0.891	0.745
L2 µ_a (mm^-1)	0.012	0.011	0.011	0.011	0.011	0.012	0.011	0.011	0.011	0.011
L3 µ_s’ (mm^-1)	0.384	0.345	0.308	0.274	0.228	0.384	0.345	0.308	0.274	0.228
L3 µ_a (mm^-1)	0.007	0.007	0.006	0.005	0.004	0.007	0.007	0.006	0.005	0.004
CVM µ_s’ (mm^-1)	0.736	0.684	0.638	0.585	0.519	0.736	0.684	0.638	0.585	0.519
CVM µ_a (mm^-1)	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017

Phantom component	Adult case
	Wavelength (nm)
	690	730	770	810	870
L2 µ_s’ (mm^-1)	1.104	1.024	0.959	0.891	0.745
L2 µ_a (mm^-1)	0.012	0.011	0.011	0.011	0.011
L3 µ_s’ (mm^-1)	0.384	0.345	0.308	0.274	0.228
L3 µ_a (mm^-1)	0.007	0.007	0.006	0.005	0.004
CVM µ_s’ (mm^-1)	1.179	1.113	1.046	0.978	0.855
CVM µ_a (mm^-1)	0.015	0.014	0.014	0.015	0.016

Phantom component	Neonatal case	Pediatric case	Adult case
CVM (optical properties)	Neonate	Neonate	Adult
CVM (ctHb)	33, 75 µM	33, 75 µM	46, 75 µM
Epidermal layer thickness (L1)	0.1 mm	0.1 mm	0.1 mm
Scalp/skull thickness (L2)	1 mm	5 mm	7.5 mm
CSF thickness (L3)	1 mm	1 mm	2.5 mm
Total extracerebral thickness	3.1 mm	7.1 mm	11.1 mm

Phantom component	Neonatal case					Pediatric case
	Wavelength (nm)					Wavelength (nm)
	690	730	770	810	870	690	730	770	810	870
L2 µ_s’ (mm^-1)	1.104	1.024	0.959	0.891	0.745	1.104	1.024	0.959	0.891	0.745
L2 µ_a (mm^-1)	0.012	0.011	0.011	0.011	0.011	0.012	0.011	0.011	0.011	0.011
L3 µ_s’ (mm^-1)	0.384	0.345	0.308	0.274	0.228	0.384	0.345	0.308	0.274	0.228
L3 µ_a (mm^-1)	0.007	0.007	0.006	0.005	0.004	0.007	0.007	0.006	0.005	0.004
CVM µ_s’ (mm^-1)	0.736	0.684	0.638	0.585	0.519	0.736	0.684	0.638	0.585	0.519
CVM µ_a (mm^-1)	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017	0.017

Phantom component	Adult case
	Wavelength (nm)
	690	730	770	810	870
L2 µ_s’ (mm^-1)	1.104	1.024	0.959	0.891	0.745
L2 µ_a (mm^-1)	0.012	0.011	0.011	0.011	0.011
L3 µ_s’ (mm^-1)	0.384	0.345	0.308	0.274	0.228
L3 µ_a (mm^-1)	0.007	0.007	0.006	0.005	0.004
CVM µ_s’ (mm^-1)	1.179	1.113	1.046	0.978	0.855
CVM µ_a (mm^-1)	0.015	0.014	0.014	0.015	0.016

Abstract

Data availability

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Figures (13)

Tables (2)

Equations (6)

Biomedical Optics Express