Acquisition principle of biological information

 The acquisition principle of biological information in this system is as follows.

1)   Acquisition principle of biological information

 Based on the Beer-Lambert law, supposing that the incident light to solution in a certain density is Iin, and the light that penetrates the solution is Iout, it is known that the following expression is true.

 That is, if an extinction coefficient ε of solution in a specific wavelength was obtained beforehand, the concentration C of the solution can be obtained by measuring Iin, Iout, and D.

 It is the following modified Beer-Lambert Law that is applied to a medium with scattering by extending the Beer-Lambert Law.

 Here, ΔIout means variation of amount of transmissive light, ΔC for variation of concentration, D for average length of light path, and ΔS for effect variation by scattering.

 Supposing that the incident light to live body in a specific wavelength is Iin (λ), variation of amount of the light that returned ex vivo by being effected by in-vivo absorption and scattering is ΔIout (λ), the extinction coefficient of oxyhemoglobin (OxyHb) is ε oxy (λ), deoxyhemoglobin (DeoxyHb) is ε deoxy (λ), the variation of concentration of OxyHb is ΔCoxy, and the variation of concentration of DeoxyHb is ΔCdeoxy, this equipment is designed presuming that the following expression is true.

 From this expression, the variation of concentration of OxyHb ΔCoxy and the variation of concentration of DeoxyHb ΔCdeoxy are obtained
Here, there are two variables to be obtained such as ΔCoxy, ΔCdeoxy, therefore they are obtained by using the near-infrared extinction coefficient of two wavelengths such as 770 nm and 840 nm shown below for this equipment.

 As for the unit of ΔCoxy, ΔCdeoxy, and ΔCtotalHb, it uses mM·cm (millimolar centimeter) or mM·mm (millimolar millimeter) that still contains the light path length because it cannot be specified. For the case, it has been suggested that the measurement amount should be called variation amount of concentration length of hemoglobin hereafter because the expression of variation amount of concentration of hemoglobin is unreasonable (Professor Okada of Keio University proposed it at the 10th seminar of Japan Optical Functional Brain Imaging Society on December 13, 2008.). Our company also will follow the proposal in the future.

2)   Light modulation

 When trying to obtain biological information on multiple points at the same time, it must be able to recognize where the light that reached a sensor came from. Then, the light modulation technology should be used. The equipment uses the spread spectrum multi-modulation method as light modulation, that is one of the latest digital technologies.

 As for NIRS (Near Infrared Reflectance Spectroscopy), the near-infrared ray in continuous wave (CW) is generally used as a source of light including this equipment. TDMA (Time Division Multiple Access) or FDMA (Frequency Division Multiple Access) has been known as optical multi-modulation method in CW. This equipment has adopted CDMA (Code Division Multiple Access) that is quite different from the technique. CDMA is generally called spread spectrum modulation method from the principle.

 TDMA is a method that controls each light source at the micro level, and only the light source in a specific place injects a light at a certain moment. The sensor side where a light was received can separate the light that came from a specific position by controlling the time. It is extremely advantageous in the easy-to-use as a modulation method, however it has the disadvantages that it is easily influenced by turbulence light, and the biological signal bandwidth is limited to the multiple channels.

 FDMA is a method that light-modulates the light of each source by separate frequency to inject it. The sensor side where a light was received can separate the light that came from a specific position by changing the optical composite signal from each position into an electric signal, then separating it with a frequency-dependent filter. It is less influenced by the turbulence light compared with TDMA, and is advantageous in its simultaneous measurement. However, it has the disadvantage that a design of precise frequency-dependent filter is necessary, the circuit scale becomes complex, and it becomes extremely difficult for supporting multiple channels when it exceeded a certain scale.

 On the other hand, CDMA, that is, the spread spectrum modulation is the most advanced modulation technology, that is used for recent cellular phone and GPS of car navigation system. It is a method that modulates it by using random numbers, and it is difficult to understand the essence of the theory. However, it has the advantages that it is less influenced by turbulence light, it performs simultaneous measurement, it has no big problem to support multiple channels, and the actual circuit scale doesn't become too large. It is a light modulation technology with good prospects.

Overview of TDMA method

 In the figure below, only the light source 1 lights at the time 1, and the light is received at each receiving point. It is the method to light sequentially with time such as the light source 2 at the time 2, the light source 3 at the time 3, and so on in the same way.

Overview of FDMA method

 In the figure below, the light sources 1~5 repeat lighting at the same time at different modulation frequency. It is the method to specify the position and the optical signal strength at each receiving point by utilizing the difference in modulation frequency of the light signal from each light source.

Overview of CDMA (Spread spectrum modulation) method

 In the figure below, the light sources 1~5 repeat lighting at the same time at different modulation frequency. It is the method to specify the position and the optical signal strength at each receiving point by utilizing the difference in random number code of the optical signal from each light source though each light source lights at the same time.

Feature of each method (Use for light measurement)

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