Dr. Stephanie Miller Profile

Dr. Stephanie Miller

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Proceedings Article | 5 March 2021 Presentation + Paper

Near-infrared spectroscopy-based system for assessment of cerebrovascular activity during stress

Stephanie Miller, Kunal Mitra

Proceedings Volume 11638, 116380B (2021) https://doi.org/10.1117/12.2590851

KEYWORDS: Near infrared spectroscopy, Carbon dioxide, Vital signs, Tissues, Oxygen, Heart, Cerebral blood flow

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Cerebral blood flow (CBF) dysregulation has been implicated in a wide variety of conditions. Cerebrovascular reactivity (CVR) to CO₂ may be a biomarker of CBF dysregulation. NIRS-measured regional cerebral tissue oxygen saturation (rSO₂) is a non-invasive measurement made with portable and relatively inexpensive devices. We previously reported that monitoring rSO2 can identify changes in cerebrovascular dynamics in response to hypercapnic breathing challenges. The present study builds on this work, employing rSO₂ to characterize changes in CBF correlates during hypercapnic breathing challenges, with a new focus on correlations with NIRS-measured vital signs. A custom breathing circuit was used to deliver a series of hypercapnic breathing challenges and recovery periods to healthy young adult subjects grouped into three exercise factors. Change in rSO₂ from intra- to pre-challenge (ΔrSO₂) correlated positively with change in heart rate (ΔHRhyp). Athletes showed higher ∆HRhyp than casual and non-exercisers. We previously established that athletes showed higher ΔrSO₂ responses to hypercapnic breathing challenges. While the relationships between ΔrSO₂ and ΔHRhyp and change in end tidal CO₂ (ΔPETCO₂) in response to hypercapnic breathing challenges appears to be in tact in the healthy young adults studied, and more pronounced in athletes, these relationships may not be preserved in cases of compromised CVR. Change in respiration rate correlated negatively with ΔPETCO₂. Establishing baseline values of rSO₂, PETCO₂, HR, and RR may be useful in identifying changes in an individual’s CVR. Combined monitoring of rSO₂, PETCO₂, HR, and RR presents a portable, inexpensive, noninvasive NIRS-based modality for detecting changes in cerebrovascular health.

SPIE Journal Paper | 12 September 2017 Open Access

NIRS-based cerebrovascular regulation assessment: exercise and cerebrovascular reactivity

Stephanie Miller, Kunal Mitra

NPh, Vol. 4, Issue 04, 041503, (September 2017) https://doi.org/10.1117/12.10.1117/1.NPh.4.4.041503

KEYWORDS: Sensors, Near infrared spectroscopy, Oxygen, Magnetic resonance imaging, Tissue optics, Carbon monoxide, Vital signs, Blood pressure, Brain, Tissues

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Alterations to cerebral blood flow (CBF) have been implicated in diverse neurological conditions. Near-infrared spectroscopy (NIRS)-measured regional cerebral tissue oxygen saturation (rSO2) provides an estimate of oxygenation of interrogated cerebral volume useful in identifying variations in oxygen supply to cerebral tissue and in monitoring cerebrovascular function. CO2-inhalation-based hypercapnic breathing challenges were used to simulate CBF dysregulation, utilizing NIRS to monitor the CBF autoregulatory response. A breathing circuit was designed to administer CO2-compressed air mixtures and assess CBF regulatory responses to hypercapnia in 26 healthy young adults. One to three hypercapnic challenges of 5 or 10 min duration were delivered to each subject while continuously monitoring rSO2, partial pressure of end tidal CO2 (PETCO2), and vital signs. Change in rSO2 (ΔrSO2) during CO2 inhalation positively correlated to ΔPETCO2 (R2=0.40). Grouping subjects into three exercise factor levels (h/week), (1) 0, (2) >0 and <10, and (3) >10 showed significantly greater ΔrSO2 responses to CO2 challenges for level 3 subjects but similar PETCO2 responses for the three groups. Exercising greater than 10 h/week may produce a higher resting cerebrovascular reactivity (CVR) to CO2 inhalation. Establishing baseline values of rSO2 and CVR to CO2 may aid in early detection of CBF changes.

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Proceedings Article | 9 March 2016 Paper

NIRS-based noninvasive cerebrovascular regulation assessment

S. Miller, I. Richmond, J. Borgos, K. Mitra

Proceedings Volume 9690, 96900W (2016) https://doi.org/10.1117/12.2213951

KEYWORDS: Carbon dioxide, Oxygen, Near infrared spectroscopy, Sensors, Vital signs, Blood pressure, Tissue optics, Heart, Cerebral blood flow, Traumatic brain injury

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Alterations to cerebral blood flow (CBF) have been implicated in diverse neurological conditions, including mild traumatic brain injury, microgravity induced intracranial pressure (ICP) increases, mild cognitive impairment, and Alzheimer’s disease. Near infrared spectroscopy (NIRS)-measured regional cerebral tissue oxygen saturation (rSO2) provides an estimate of oxygenation of the interrogated cerebral volume that is useful in identifying trends and changes in oxygen supply to cerebral tissue and has been used to monitor cerebrovascular function during surgery and ventilation. In this study, CO₂-inhalation-based hypercapnic breathing challenges were used as a tool to simulate CBF dysregulation, and NIRS was used to monitor the CBF autoregulatory response. A breathing circuit for the selective administration of CO2-compressed air mixtures was designed and used to assess CBF regulatory responses to hypercapnia in 26 healthy young adults using non-invasive methods and real-time sensors. After a 5 or 10 minute baseline period, 1 to 3 hypercapnic challenges of 5 or 10 minutes duration were delivered to each subject while rSO₂, partial pressure of end tidal CO₂ (PETCO₂), and vital signs were continuously monitored. Change in rSO₂ measurements from pre- to intrachallenge (ΔrSO₂) detected periods of hypercapnic challenges. Subjects were grouped into three exercise factor levels (hr/wk), 1: 0, 2:>0 and <10, and 3:>10. Exercise factor level 3 subjects showed significantly greater ΔrSO₂ responses to CO₂ challenges than level 2 and 1 subjects. No significant difference in ΔPETCO2 existed between these factor levels. Establishing baseline values of rSO₂ in clinical practice may be useful in early detection of CBF changes.

Proceedings Article | 7 March 2016 Paper

Simulation of the dependence of spatial fluence profiles on tissue optical properties

S. Miller, K. Mitra

Proceedings Volume 9706, 97061Q (2016) https://doi.org/10.1117/12.2213841

KEYWORDS: Scattering, Tissue optics, Tissues, Laser tissue interaction, Absorption, Anisotropy, Optical properties, Laser therapeutics, Monte Carlo methods, Skin

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Medical laser applications are promoted as safe, effective treatments for a multiplicity of concerns, ranging from hyperthermal skin rejuvenation to subcutaneous tumor ablation. Chromophore and structural protein concentration and distribution within a patient’s tissue vary from patient to patient and dictate the interaction of incident radiative energy of a specific wavelength with the target tissue. Laser parameters must be matched to tissue optical and thermal properties in order to achieve the desired therapeutic results without inducing unnecessary tissue damage, although accurate tissue optical properties are not always measured prior to and during laser therapies. A weighted variable step size Monte Carlo simulation of laser irradiation of skin tissue was used to determine the effects of variations in absorption (μ_a) and scattering coefficients (μs) and the degree of anisotropy (g) on the radiant energy transport per mm² in response to steady-state photon propagation. The three parameters were varied in a factorial experimental design for the ranges of 0.25/mm ≤ μ_a ≤ 2.0/mm, 30.0/mm ≤ μ_s ≤ 140.0/mm, and 0.65 ≤ g ≤ 0.99 in order to isolate their impacts on the overall fluence distribution. Box plots of the resulting fluence profiles were created and compared to identify ranges in which optical property variance could be considered to significantly impact the spatial variance of fluence within the simulation volume. Results indicated that accurate prediction of the fluence profiles that will be achieved by any given medical laser treatment is unlikely without pre-treatment assessment of the tissue optical properties of individual patients.

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