20 Medical Applications of NIR Spectroscopy
467
20.4.3 Non-invasive Pulsatile NIR Spectroscopy
In most presented applications of short-wave near-IR biospectroscopy so far, the
focus was on integral tissue probing apart from glucose assays using photoplethismography, allowing the probing of the intravascular space through dynamic monitoring of tissue absorption based on subsecond measurements. This technique can
be employed for a spectroscopic measurement of the cardio-vascular pulse wave
that is correlated to periodic changes in blood volume, since the blood is maximally
diffused through the vascular system during the heart systole, whereas at the diastole
blood pressure is minimal.
The major clinical application with exploitation of the cardiac blood volume
modulation is in pulse oximetry providing values of the arterial hemoglobin oxygen
saturation. Meanwhile, it is incorporated into the generally accepted standard of
care and fundamental in the support of critical-care medicine, applied for adult and
neonatal monitoring. An early review on theory and applications of this technique
including practical limitations was published years ago by Mendelson [66]. Two
different wavelengths had been used to measure the actual difference in the absorption spectra of oxygenated and deoxygenated hemoglobins, i.e., for example, at
660 nm (red light) and 940 nm in the SW-NIR, using the ratio of the pulsatile (AC)
to the non-pulsatile (DC) signals at each wavelength for further calculations. This
effective scaling process results in a normalized red/near-infrared ratio that is largely
independent from the incident intensities. Problems at that time were concerned with
low peripheral vascular perfusion, motion artifacts and systematic errors induced by
different hemoglobin variants and derivatives (e.g., HbCO). Over the intervening
years, many patents have been published on advanced pulse oximetry sensors with
the consequence that problems attributed to motion artifacts or straylight have been
diminished.
The commonly used fingertip-type pulse oximeter is taking measurements in
transmission, thus limiting its application to the fingertip or earlobe to provide
physiological parameters. However, their inconvenience for long-term monitoring
in daily life has some shortcomings so that other types of wearable pulse oximeters,
measuring in reflection mode, had been envisaged. For the purpose of developing
reflection pulse oximetry, the light propagation in tissue was simulated to estimate
the measured intensities of reflected light using analytical and numerical solutions
of the diffusion approximation equation for photon migration in the visible and nearinfrared region [67]. A comparison between reflection and transmission modes was
investigated with experimental data and the research results showed that it is possible
to model a reflectance pulse oximeter by simulating the random walk of photons by
diffusion theory.
467
20.4.3 Non-invasive Pulsatile NIR Spectroscopy
In most presented applications of short-wave near-IR biospectroscopy so far, the
focus was on integral tissue probing apart from glucose assays using photoplethismography, allowing the probing of the intravascular space through dynamic monitoring of tissue absorption based on subsecond measurements. This technique can
be employed for a spectroscopic measurement of the cardio-vascular pulse wave
that is correlated to periodic changes in blood volume, since the blood is maximally
diffused through the vascular system during the heart systole, whereas at the diastole
blood pressure is minimal.
The major clinical application with exploitation of the cardiac blood volume
modulation is in pulse oximetry providing values of the arterial hemoglobin oxygen
saturation. Meanwhile, it is incorporated into the generally accepted standard of
care and fundamental in the support of critical-care medicine, applied for adult and
neonatal monitoring. An early review on theory and applications of this technique
including practical limitations was published years ago by Mendelson [66]. Two
different wavelengths had been used to measure the actual difference in the absorption spectra of oxygenated and deoxygenated hemoglobins, i.e., for example, at
660 nm (red light) and 940 nm in the SW-NIR, using the ratio of the pulsatile (AC)
to the non-pulsatile (DC) signals at each wavelength for further calculations. This
effective scaling process results in a normalized red/near-infrared ratio that is largely
independent from the incident intensities. Problems at that time were concerned with
low peripheral vascular perfusion, motion artifacts and systematic errors induced by
different hemoglobin variants and derivatives (e.g., HbCO). Over the intervening
years, many patents have been published on advanced pulse oximetry sensors with
the consequence that problems attributed to motion artifacts or straylight have been
diminished.
The commonly used fingertip-type pulse oximeter is taking measurements in
transmission, thus limiting its application to the fingertip or earlobe to provide
physiological parameters. However, their inconvenience for long-term monitoring
in daily life has some shortcomings so that other types of wearable pulse oximeters,
measuring in reflection mode, had been envisaged. For the purpose of developing
reflection pulse oximetry, the light propagation in tissue was simulated to estimate
the measured intensities of reflected light using analytical and numerical solutions
of the diffusion approximation equation for photon migration in the visible and nearinfrared region [67]. A comparison between reflection and transmission modes was
investigated with experimental data and the research results showed that it is possible
to model a reflectance pulse oximeter by simulating the random walk of photons by
diffusion theory.