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Wavelengths for Optimal NIRS Device Functionality

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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.

What’s the Scoop on Cupping?

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My first exposure to cupping was seeing the perfectly circular bruises on Michel Phelp’s during the 2016 Summer Olympics. Since then, I have come across it many times in the University of Delaware athletic training room seeing athletes performing exercises with cups suctioned to their back. I have even tried it myself a couple of times to see what the hype was about and if I felt a difference using this type of recovery method. 

Figure 1. Michael Phelps swimming with visible cupping markers (bruises) on his shoulders. 

Now if you haven’t heard about cupping you may be wondering: what is cupping? Cupping is the application of plastic, glass, bamboo, or ceramics cups [1] to the skin via suction. The suction can either be created naturally by heating up the inside of the cup using a flame and allowing it to cool on the skin creating negative pressure and lifting/stretching the skin up. The other way to get this pressure is to use a suction device.[1] There are also two types of cupping, similar to needling; there are both wet and dry methods. Dry cupping is exactly the procedure I described above while wet cupping is when small cuts are made on the skin before the cup is applied and blood is drawn out. [1] The original idea behind this technique was that it was regulating Qi in the body. More recently, people claim that it promotes blood flow and therefore has a positive effect on the healing process, reducing soreness and pain. There are still many who find cupping bizarre and disgusting due to the often dark bruising and the odd look of the skin suctioned into cups. In particular, a Forbes article by Steven Salzberg goes as describes it as “someone giving you a massive hickey, and then doing another dozen or so all over your back, or legs, or wherever ” [2]. So by now, you should have a pretty clear image that while there are many advocates and cupping has been gaining interest (especially if professional athletes on the world stage have used it), there are still many skeptics and people who say it is harmful. Let’s see exactly what the research says about cupping. Is it beneficial? Harmful?

 

An article published in The Journal of Alternative and Complementary Medicine by a group of Australian and German researchers performed a systematic meta-analysis of clinical trials evaluating the effects of cupping on athletes. [3] They found 11 valid (according to their criteria) trials with a combined total of 498 participants from China, the United States, Greece, Iran, and the United Arab Emirates. Participants received cupping 1 to 20 times in daily or weekly intervals either alone or in combination with another procedure, like acupuncture.[3] The study found no conclusive results however. Even though there were improvements to the participant’s perception of pain, an increased range of motion, and lower levels of creatine kinase, there were large variations between symptom intensity and recovery measures, and other metrics.[3] There are also some limitations to this study. One of the main concerns is the reliability of the data. The researchers report an unclear or high risk of bias in many of the trials and they also mention that none of the trials reported safety. 

 

Another study published in 2016 in the Journal of Novel Physiotherapies evaluated the effects of various soft tissue mobilization techniques, including cupping, on active myofascial trigger-points in 20 amateur soccer players.[4] Athletes received cupping once a week for three weeks. They found that all techniques used, including cupping, improved pain pressure threshold and pain sensitivity significantly. [4] The researchers concluded that more research must be done to fully be able to draw a conclusion. Some limitations of the study were the small sample size (n = 20) and that the study was limited to only amateur soccer players. Other studies, including the previously mentioned study viewed multiple different sports instead of one. This also provided a much larger sample size compared to this study.  

 

Overall, there appears to be no definite answer, at least at this time, on if cupping helps promote healing and reduce pain and muscle soreness. For some, it appears to be beneficial in relieving pain but due to a limited number of studies and the questionable accuracy of others, there is no conclusive data for or against cupping. As the first-mentioned study by Bridgett et. al stated, “ No explicit recommendation for or against the use of cupping for athletes can be made. More studies are necessary for conclusive judgment on the efficacy and safety of cupping in athletes.” [3].

 

 

References:

[1] NCCIH. “Cupping.” November 2018. Retrieved from: https://www.nccih.nih.gov/health/cupping

[2] Steven Salzberg. “ The Ridiculous and Possibly Harmful Practice of Cupping”.  May 2019. Retrieved from: https://www.forbes.com/sites/stevensalzberg/2019/05/13/the-ridiculous-and-possibly-harmful-practice-of-cupping/#57ce2d2331f3

[3] Rhianna Bridgett, Petra Klose, Rob Duffield, Suni Mydock, and Romy Lauche.The Journal of Alternative and Complementary Medicine.Mar 2018. 208-219.http://doi.org/10.1089/acm.2017.0191

[4] Fousekis, Konstantinos et al. “The Effectiveness of Instrument-assisted Soft Tissue Mobilization Technique(Ergoné Technique), Cupping and Ischaemic Pressure Techniques in the Treatment of Amateur AthletesàMyofascial Trigger Points.” (2016).

 

Questions to Consider:

  1. Have you ever gotten cupping done? If yes, what are your thoughts? Did you find it beneficial? If no, was there a reason why?
  2. How do you think studies looking at cupping should compare its effects for the most accurate evaluation? Should they compare across different sports because the benefits should not be sport dependent or within one sport to get a better comparison?

The Near-Electromagnetic Spectrum Is Just a Name

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Humans require oxygen for everyday life. It is a key component in many body functions so it logically follows that measuring oxygen and oxygen usage in the body can be extremely beneficial. Healthcare and sports are two main fields that come to mind. On the market today, there are hundreds of devices that measure various  body metrics and oxygen levels and saturation are no exception. One device that measures the the oxygenation saturation in body tissues is a portable near-infrared spectroscopy (NIRS) apparatus created by Gutwein et al.[1]. Gutwein et al. applied for a patent titled “Method and Apparatus for Assessing Tissue Oxygenation Saturation” on March 22, 2017 and the patent  was approved and filed roughly 5 months later on September 28, 2017. Gutwein et al. also had a previous patent “Portable Near-Infrared Spectroscopy Apparatus” filed in October 2016. The basic patent information is outline below[1]:

  1. Patent title: Method and Apparatus for Assessing Tissue Oxygenation Saturation
  2. Patent number: US 2017/0273609 A1
  3. Patent filing date: March 22, 2017
  4. Patent issue date: September 28, 2017
  5. Inventors: Luke G. Gutwein, Clinton D. Bahler, Anthony S. Kaleth
  6. Assignee: Indiana University Research and Technology Corporation
  7. U.S. Classification: CPC – A61B: 5/14552, 5/6807, 5/02055
  8. Claims: 20

Invention & Claims

This invention is an apparatus and method developed for assessing tissue oxygenation saturation during physical activity. The portable near-infrared spectroscopy (NIRS) apparatus comprises of a wearable article of clothing, namely a shirt, pair of shorts or a calf sleeve. It also contains a spectroscopy probe, a near-infrared light source and a photodetector coupled to the article of clothing used. The probe is configured to measure oxygenation saturation in skin dermis, adipose tissue (fat) or muscle fascial layer[1].

Figure 1. Image depicting user wearing device. The wireless probe can be incorporated into different wearable articles.

Why Is This Important?

There is a need for such a device in a multiple fields, namely healthcare and sports. This NIRS apparatus has applications in clinical settings, sports industry and in exercise physiology research. Clinicians, doctors and researchers can benefit from a portable NIRS device in monitoring and diagnosing disease states, including:

  • septic shock
  • real-time tissue perfusion analysis during surgery
  • peripheral arterial disease

The NIRS device is highly sensitive to changes in Muscle tissue oxygenation (StO2) and during exercise, the NIRS signal is considered to reflect  the balance between oxygen delivery and utilization. This can have multiple applications in sports including looking at how efficient oxygen flow is and to show what muscles are specifically being activated and which aren’t; this can indicate whether or not proper form is being used[1].

Tech Talk: How It Works

NIRS is an optical technique that allows for continuous non-invasive monitoring. This technique is founded on the Beer-Lambert Law. The Beer- Lambert Law is a linear relationship between absorbance and concentration of an absorbing species[2].

Where A is the measured absorbance, a(𝜆) is the wavelength-dependent absorptivity coefficient, b is the path length, and c is the analyte concentration.

The NIRS device measures hemoglobin oxygen saturation in microvessels (arterioles, capillaries, venules) by applying this Beer-Lambert Law and the using the differences in light absorption coefficients of oxyhemoglobin and deoxyhemoglobin. The device uses these absorption characteristics to calculate changes in oxygenated & deoxygenated hemoglobin in skin, fat, or muscle tissues[1].

What Sets It Apart?

Early devices that measured oxygenation saturation were mostly limited to research usage. Since then there have been advances in the field leading to the creation of smaller probes and utilizing wireless probes instead. The PortaMon created by Artinis Medical Systems (B.V. Netherlands) is currently the most common portable NIRS device on the market. There are still problems with these devices however. Namely issues with:

  • Device size
  • Motion signal artifacts
  • Adipose (fat) tissue thickness

The size of the probe is based off the required penetration depth. In order to get the 2-6 cm of depth required, the probes tend to be bulkier. Another issue is motion signal artifacts that raise questions concerning the reliability of some devices. These artifacts are created by instabilities (for example, lose of contact) at the skin-probe interface. Finally, the thickness of the adipose tissue can limit the depth of penetration achieved[1].

The main advantage of this device is that it can measure levels in skin, fat or muscle tissue over any sport or activity including running, bicycling, swimming and weightlifting. Some additional features are that this NIRS device and method can measure the user’s heart rate, respiratory rate and body temperature[1].

Citations:

[1] Gutwein et al. (2017). Method and Apparatus for Assessing Tissue Oxygenation Saturation. US 2017/0273609 A1. U.S. Patent and Trademark Office

[2] Beer-Lambert Law. (n.d.). Retrieved March 10, 2020, from http://life.nthu.edu.tw/~labcjw/BioPhyChem/Spectroscopy/beerslaw.htm