Abstract
We present and validate a multi-wavelength time-domain near-infrared spectroscopy (TD-NIRS) system that avoids switching wavelengths and instead exploits the full capability of a supercontinuum light source by emitting and acquiring signals for the whole chosen range of wavelengths. The system was designed for muscle and brain oxygenation monitoring in a clinical environment. A pulsed supercontinuum laser emits broadband light and each of two detection modules acquires the distributions of times of flight of photons (DTOFs) for 16 spectral channels (used width 12.5 nm / channel), providing a total of 32 DTOFs at up to 3 Hz. Two emitting fibers and two detection fiber bundles allow simultaneous measurements at two positions on the tissue or at two source-detector separations. Three established protocols (BIP, MEDPHOT, and nEUROPt) were used to quantitatively assess the system’s performance, including linearity, coupling, accuracy, and depth sensitivity. Measurements were performed on 32 homogeneous phantoms and two inhomogeneous phantoms (solid and liquid). Furthermore, measurements on two blood-lipid phantoms with a varied amount of blood and Intralipid provide the strongest validation for accurate tissue oximetry. The retrieved hemoglobin concentrations and oxygen saturation match well with the reference values that were obtained using a commercially available NIRS system (OxiplexTS) and a blood gas analyzer (ABL90 FLEX), except a discrepancy occurs for the lowest amount of Intralipid. In-vivo measurements on the forearm of three healthy volunteers during arterial (250 mmHg) and venous (60 mmHg) cuff occlusions provide an example of tissue monitoring during the expected hemodynamic changes that follow previously well-described physiologies. All results, including quantitative parameters, can be compared to other systems that report similar tests. Overall, the presented TD-NIRS system has an exemplary performance evaluated with state-of-the-art performance assessment methods.
Highlights
Time-domain near-infrared spectroscopy (TD-NIRS) uses short pulses of light to non-invasively and in real-time monitor tissue optical properties, i.e. absorption and reduced scattering (μ′s) coefficients, which are related to the concentrations of chromophores in tissue, e.g. oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb)
We report the most relevant performance tests from three established protocols for photon migration instruments (MEDPHOT [29], basic instrument protocol (BIP) [30], and nEUROPt [31]), which were intended for quantitatively assessing the performance of timedomain instruments that use pulsed laser sources, single-photon detectors, and time-correlated single-photon counting electronics
The two instrument response function (IRF) can be used in data analysis to remove drift in N, m1, and V, by assuming that the drift was constant over time
Summary
Time-domain near-infrared spectroscopy (TD-NIRS) uses short pulses of light to non-invasively and in real-time monitor tissue optical properties, i.e. absorption (μa) and reduced scattering (μ′s) coefficients, which are related to the concentrations of chromophores in tissue, e.g. oxyhemoglobin (HbO2) and deoxyhemoglobin (Hb). The advancements have enabled TD-NIRS measurements at larger source-detector separations, e.g. 9 cm [14], at longer wavelengths, e.g. in the range from 600 to 1350 nm [15], with more sources and detectors, e.g. 16 sources and 8 detectors with fibers [16], or 1032 detectors [17], or 36288 detectors [18], with fast acquisitions, e.g. wavelength switching at 160 Hz [19], more compact, e.g. 200 × 160 × 50 mm3 [20], wearable [22,21] and wireless [23], or with an improved light harvesting capability that allows collecting more photons than restricted by the pile-up limit [24]. Another direction of advancements involves integrating various imaging techniques, e.g. TD-NIRS and diffuse correlation spectroscopy [25,26]
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