# Wavelengths for Optimal NIRS Device Functionality

As described in previous blog posts about Near Infrared Spectroscopy (NIRS), NIRS is a valuable tool to measure oxygen concentration in the body. It can be used on various parts of the body including the muscles and brain. These applications of NIRS to measure oxygen concentration is useful in metabolic kinetics research, diagnoses of disease conditions [1], and as an athletic performance measure. Recent advancements in NIRS technology have allowed for the development of portable NIRS devices that can be worn while exercising. In this post, I will be focusing on the use of NIRS technology for the measurement of muscle oxygenation.

The main benefit of NIRS is that it can measure oxygen concentration, as discussed above. In order to do this the use of spectroscopy methodology is used. Spectroscopy is when light of a specific wavelength is transmitted through a substance and the amount of light the substance absorbs is measured and called the absorbance. From the absorbance, the concentration of a solute can be determined using Beer-Lambert’s Law. The Beer-lambert’s Law relates the light intensity to the product of the molar absorptivity (ε, L/mol*m), the substance concentration (c, mol/L), and the path length  (L, m). Due to the path length not being perfectly defined, a modified version of the Beer-Lambert’s Law is used (Equation 1) that accounts for any scattering (g) of the light beam as it travels through the tissue. The initial intensity of the light (I) then is the sum of the light that is reflected, the light that is transmitted to the detector and the light that is scattered throughout the tissue.

log10(I0/I) = εcL+g     (1)

As I have outlined, the main function of NIRS for measuring muscle oxygenation is to be able to measure the absorbance of the hemoglobin in the blood. To do this the right wavelengths must be able to penetrate the body and get picked up by the detector where the absorbance can be measured. By solving the problem of what wavelength of light to use, the device can effectively function for its purpose and be reliable. The wavelength of light appropriate depends on what is being looked at. In the case of NIRS, the absorbance of hemoglobin in the blood is the target substance. When oxygen is bound to hemoglobin (oxyhemoglobin), the hemoglobin has a different absorbance value (Figure 1) than if there is no oxygen bound to it (deoxyhemoglobin). The question is: what is the appropriate wavelength to use?

Figure 1. Absorbance curves for oxyhemoglobin (red) and deoxyhemoglobin (blue).

https://commons.wikimedia.org/wiki/File:Oxy_and_Deoxy_Hemoglobin_Near-Infrared_absorption_spectra.png

In order to work through and solve the problem of what wavelength to use, we must first consider what is known and what is unknown. Referring to Equation 1, the modified Beer-Lambert Law, the absorptivity is a known variable dependent on the solute looking at. Another known variable is the path length that can be determined by measuring the length between the light source or probe and the detector surface. That leaves the solute concentration and the scattering term as the unknowns. In order to solve the equation for concentration and determine the optimal wavelengths for NIRS device functionality, the scattering term needs to be eliminated from the equation. In order to do this, we need to realize that the scattering term is very variable; it depends on the subject, the location, and muscle the NIRS device is being used on. For the device to be used by a larger population the scattering term must be normalized and this is accomplished by finding the change in concentration. This is why concentrations are reported not in absolute concentrations but in relative measures, percentages. By subtracting two absorbances (change in absorbance), the scattering terms will cancel out assuming the scattering term is the same over one location. Now the equation for change in concentration is:

log10(I0/I) = εΔcL     (2)

Now when we account for the fact that NIRS measures not only oxyhemoglobin concentrations but also deoxyhemoglobin concentrations we need to expand our equation to account for both. That means that the change in concentration is the change in concentration of oxyhemoglobin and the change of concentration in deoxyhemoglobin.

log10(I0/I) = (εO2ΔcO2 + εHbΔcHb)L     (3)

When we add another unknown thought we need to generate another equation to be able to solve for the two unknown concentrations leaving us with the following equations, Equation 4.

https://cdn.iopscience.com/images/0031-9155/51/5/N02/Full/pmb211409eqn04.gif

Now that we have equations where the subscripts λ1 and λ2 are the two wavelengths [2] and the subscripts O2 is for oxyhemoglobin and Hb is for deoxyhemoglobin. There is one unknown in each so we can now determine a wavelength that will be optimal for both oxyhemoglobin and deoxyhemoglobin so the NIRS device will be able to measure oxygen concentration in the muscles. Solving the equations so that one is sensitive to oxyhemoglobin, and one is more sensitive to deoxyhemoglobin, we come up with the optimal range of 0.7 um to 2.5 um. The near infrared region of light. This is reasonable and coincides with what research says that near infrared light is the optimal waveforms to have light waves penetrate the muscles and measure oxygen concentrations [3].

In this solution, we made the assumption that the scattering term was the same in the same location but that is not necessarily the truth. If the light was angled differently when it was absorbed, there could be different amounts of scatter even in the same location. Without this simplification, the equation would be much more complex because it would have to take into consideration all factors that affect the scattering of light waves in the tissue which would require frequent calibrations and mathematical adjustments. Now that we have answered the question of what wavelengths are optimal for the NIRS to measure oxyhemoglobin and deoxyhemoglobin concentrations we can start measuring concentration!

References:

[1]  Adami A, Rossiter HB. Principles, insights, and potential pitfalls of the noninvasive determination of muscle oxidative capacity by near-infrared spectroscopy. J Appl Physiol (1985) 124: 245–248, 2018. doi:10.1152/japplphysiol.00445.2017. https://journals.physiology.org/doi/full/10.1152/japplphysiol.00445.2017

[2] “About NIRS (Principle of Operation and How It Works),” About NIRS (Principle of Operation and How It Works) | SHIMADZU EUROPA. [Online]. Available: https://www.shimadzu.eu.com/about-nirs-principle-operation-and-how-it-works. [Accessed: 13-May-2020].

[3] L. Kocsis, P. Herman, and A. Eke, “The modified Beer–Lambert law revisited,” Physics in Medicine and Biology, vol. 51, no. 5, 2006.

# Using NIRS to non-invasively monitor muscle oxygenation during exercise

Skeletal muscles are the basis of all movement in the human body, and athletes work years to train their muscles to be powerful yet efficient. Even if a single muscle could allow a person to lift a car, it would not be very useful if the muscle could no longer create forceful contraction again for several hours. The muscle also must be efficient in the use of oxygen, ions, and other substrates that allow for contraction to be able to quickly recover and be prepared for repeated contraction. Muscle oxygenation is particularly important for both endurance and power of a muscle because it is necessary to produce ATP to power muscle cells to contract. Heart rate and blood oxygen delivery are helpful for getting an idea of an athlete’s efficiency, but they do not tell the whole story for the muscle. At the muscle, the balance between delivery and consumption of oxygen explains its efficiency [1]. To measure muscle oxygen saturation, a technique called near-infrared spectroscopy (NIRS) is used to get real time data to inform athletes of the state of their muscles during training. This is a powerful tool for maximizing athletic gains in muscles from training and to see the state of the muscle over time and after rest.

Early NIRS instrumentation was contained to the lab, but recently portable versions have become more common, which is very important for its use in both the medical and research fields. In medicine, NIR has been used for study of septic shock, free tissue transfer, real-time tissue perfusion during surgery, cancer nanotechnology, and peripheral arterial disease.  For this post, the use of NIR in exercise will be highlighted. In exercise, NIRS is a great tool because it is a non-invasive method that can be applied locally to muscles or tissues of interest and provide real time data during exercise. NIRS is highly sensitive to changes in muscle tissue oxygenation [2, 3, 4], and it reflects the balance between oxygen delivery and utilization, unlike measurements of arterial or venous blood samples which have been used previously and are minimally invasive [2]. NIRS works by measuring the percentage of oxygenated hemoglobin to total hemoglobin (oxygenated and deoxygenated hemoglobin) to give muscle oxygenation. Hemoglobin is the main oxygen carrying protein in the blood and can carry 4 oxygen molecules (O2). Oxygenated and deoxygenated hemoglobin scatter NIR light (600-1000 nm) differently, so their relative concentrations can be found from their molecular absorption coefficients. To do this, three to four different wave lengths of light will be used to determine the concentrations of each based on the change in molecular absorption coefficients at different wavelengths (Fig 1). NIR light must be used as it: 1) passes through skin, bone, and most biological tissue, and 2) is the appropriate wavelength where the small amount of absorption that occurs is predominately from hemoglobin (Fig 2) [5].  As the muscle performs work, the muscle oxygenation will decrease as a function of the work and the training of the muscle.

Fig. 1: Molecular Absorption Coefficient Profiles for Oxygenated and Deoxygenated Hemoglobin [5]

Fig 2: Light Absorption by Wavelength [5]

A patent on google patent claims to leverage this technology in a wearable article of clothing for athletes to be able to measure muscle oxygenation real-time (Fig 3) [6]. The patent claims to be a method and apparatus for assessing tissue oxygenation saturation through two main claims that summarize to: a portable apparatus that is a wearable article capable of measuring oxygenation saturation of at least one of a skin dermis layer, adipose layer, or muscular fascial layer of a user during physical activity using at least one near-infrared spectroscopy probe including at least one near-infrared light source and at least one photodetector. In short, the patent is a claim on a portable, wearable NIRS device for tissue oxygenation levels. NIRS has been a research method for decades, so the novel part of this patent lies in the incorporation of this technology into a wearable article of clothing.

Fig 3: Figure from patent illustrating wearable shirt, shorts, and socks using NIRS

Fig4: Figures from patent showing example data of muscle oxygenation average during constant rate running at different grades (top) and real time data from medial gastrocnemius muscle during weighted exercise and unweighted control (bottom)

This patent pertains primarily to the measurement of tissue during exercise (Fig 4). This could be of use for athletes during training to be able to compare what levels of exercise cause certain levels of muscle oxygen saturation loss. For example, highly trained athletes often train at high altitude to reduce oxygen in the air so that their body adapts to becoming more efficient with oxygen usage. This prompts higher performance when returning to normal oxygen levels. Using NIRS could allow them to find a training regime that caused the same hypoxia in muscle without traveling to higher altitude (they will still miss out on some of the pulmonary and cardio vascular advantages that training at altitude can produce). This may also be helpful in rehabilitation as the change in muscle oxygenation is an indicator that the muscle is being used and can inform physical therapists if the patient is engaging the correct muscles during rehab. Additionally, the device may also have merit in the medical realm for monitor muscle oxygenation in patients with chronic heart failure, peripheral vascular disease, chronic obstructive pulmonary disease, and varying muscle diseases [3, 4].

1. Patent title: Method and apparatus for assessing tissue oxygenation saturation
2. Patent number: US20170273609A1
3. Patent filing date: 2017-03-22
4. Patent issue date: Patent Pending
5. How long it took for this patent to issue: TBD
6. Inventor(s): Luke G. Gutwein, Clinton D. Bahler, Anthony S. Kaleth
7. Assignee (if applicable): Indiana University Research and Technology Corp
8. U.S. classification: A61B5/0075
9. How many claims: 20

References and Further Reading

[2] Bhambhani, Y. N. (2004). Muscle Oxygenation Trends During Dynamic Exercise Measured by Near Infrared Spectroscopy. Can. J. Appl. Physiol., 29(4), 504–523.

[3] Hamaoka, T., Mccully, K. K., Quarisma, V., Yamamoto, K., & Chance, B. (2007). Near-infrared spectroscopy / imaging for monitoring muscle oxygenation and oxidative metabolism. Jounal of Biomedical Optics, 12(6), 1–16. http://doi.org/10.1117/1.2805437

[4] Boushel, R., & Piantadosi, C. A. (2000). Near-infrared spectroscopy for monitoring muscle oxygenation. Acta Physiol Scand, 168, 615–622. http://doi.org/10.1046/j.1365-201x.2000.00713.x

[5] Shimadzu Commercial Website https://www.ssi.shimadzu.com/products/imaging/labnirs-principle-of-operation.html

[7] Ferrari, M., Muthalib, Makii, & Quarisma, V. (2011). The use of near-infrared spectroscopy in understanding skeletal muscle physiology : Phil. Trans. R. Soc. A, 369, 4577–4590. http://doi.org/10.1098/rsta.2011.0230

[8] Artinis Commercial Site https://www.artinis.com/portamon#portamon-software