Near-infrared photons: a non-invasive probe for studying bone blood flow regulation in humans

Table 1 Optical instruments utilized to investigate bone blood flow in humans

Instrument name	Possible source types	Possible number of light wavelengths	Light source intensity	Detected parameters for light (possible detector types)	Measured physiological parameters
PPG [32]	LED	1	Constant	Intensity (PMT, APD)	ΔBF (a.u.)
NIRS [33]	LED, CW laser, white light	1, 2, 3, …, to continuous	Constant	Intensity (PMT, APD)	Hb, HbO₂, ΔHb, ΔHbO₂, (a.u.) and %SO₂
DCS [34]	CW laser	1	Constant	Temporal correlation (SPD)	BF (cm² s ⁻¹)
IMS [35]	IM laser	2, 3, …	Sinusoidal modulation	Intensity and intensity phase shifts (PMT, APD)	Hb, HbO₂, (μM) and %SO₂
TRS [36]	Pulsed laser	2, 3, …	Pulsed	Photon DTOF (SPD)	Hb, HbO₂, (μM) and %SO₂
LDF [37]	CW laser	1	Constant	Doppler frequency shift (PMT, APD, SPD)	BF, BS, #rbc (a.u.)

Note that concentrations are relative to the volume of the investigated region of interest. For the present purposes the source-detector geometrical configuration is the same for all the instruments and is schematically represented in Fig. 1. The bibliographic references appearing here are technical references.
PPG photoplethysmography, NIRS near infrared spectroscopy, DCS diffuse correlation spectroscopy, IMS intensity modulated spectroscopy, TRS time resolved spectroscopy, LDF laser-Doppler flowmetry, LED light emitting diode, CW continuous wave, IM intensity modulated, DTOF distribution of time-of-flight, BF mean blood flow, Δ BF change in BF, Hb deoxy-hemoglobin concentration, H b O ₂ oxy-hemoglobin concentration, Δ[H b change in Hb, Δ]H b O ₂, change in H b O ₂, % S O ₂ blood oxygen saturation, BS mean blood speed, #rbc number moving red blood cells, PMT photo-multiplier tube, APD avalanche photodetector, SPD single-photon detector, a.u. arbitrary units

ISSN: 1880-6805