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Non-invasive Tissue oximetry using reflection mode Near
Infrared Spectroscopy system
R Periyasamy*1, Ashutosh Mishra1, Sneh Anand1
1
Center for Biomedical Engineering, IIT Delhi, New Delhi- 110016
ABSTRACT
Oxygen is the basis of human survival. Oxygen diffuses through the arterioles in to the lungs and
complexed with the hemoglobin molecules in the red blood corpuscles (RBCs). Hemoglobin contains
four protein chains and each connected to a hemo molecule. Hence, one hemoglobin structure can
attach to four oxygen atoms. This is how oxygen is circulated in the blood and diffuses through cell
membranes when it reaches the destination (cells and organs). Therefore the level of this oxygenation
that a particular organ receives is very importance as it determines proper functioning of the body
parts (organs). Also, patients suffering from diabetes mellitus(DM) have been suffer from some form
of lower extremity problem: neuropathy (motor, autonomic and sensory), foot deformities and vascular
disease (micro and macro vascular complication) are the chief causes for foot at risk in DM patients
due to oxygen level changes and decreased perfusion in the lower extremities. These cause micro
and macro angiopathy due to reduced oxygen content in the blood (Hypoxia). Also the evaluation of a
patient's oxygenation status in the upper and lower extremities should not be limited to these
parameters (partial oxygen pressure (PO2) and arterial oxygen saturation (SpO2)). However near
infrared (NIR) oximetry is best suited for tissue oxygenation measurement compared to
transcutaneous oximetry, pulse oximetry and laser doppler flowmetry. Also multi regional near infrared
spectroscopy (NIRS) information has proved beneficial for a better understanding of the development
of cerebral injuries and the reduction of neuro development in the newborns. Therefore for
management, detection and diagnosis of such chronic condition like risk status of upper and lower
extremities at early stage, we have developed a NIR based tissue oximetry system which is operated
in the reflection mode. It relies on the optical absorption characteristics of blood and living tissues, thus
requiring an optical source and detector. The optical sources (laser diodes) gives an output power of
20mW (maximum), which is safe as the maximum allowed output power on the body to avoid damage
of any kind (ANSI standards) is 0.2W/cm2. However NIR oximetry probe which can directly be used
on any parts of the human body (like fore finger, toe, ear lobe etc). The process involves an NIR light
from two laser diodes passing through the body tissue and change in the intensity of light detected by
the silicon PIN photodiode after getting reflected back from the body parts( like fore finger). We carry
out the two modes of operation ‘with’ & ‘without’ occlusion. The output values obtained from the photo
diode is in the form of current. A trans-impedance amplifier is used for converting the current in to
voltage and amplifies the signal. This amplified voltage signal is continuously monitored with the CRO
and then evaluated using the Modified Beer-Lambert’s law to calculate relative change in
concentration of oxy hemoglobin Δ [OHb], deoxy hemoglobin Δ [DHb] and total hemoglobin Δ [THb].
Hence our successfully developed non-invasive reflection mode NIRS system can be used to measure
the Tissue Oxygenation value or Tissue Oxygenation Index at any parts of human body.
Keywords – Near Infrared spectroscopy, Reflection mode, NIRS system, Tissue oxygenation
parameters
INTRODUCTION
In 1895 Wilhelm Conrad Roentgen made the first radiogram of a palm, starting the development of
non-invasive optical diagnostics methods. Over last ten years, optical techniques are improved and
used for clinical monitoring and diagnosis in medicine. The optical technology utilizes light to interact
with tissue. The three photo physical processes widely used in biological studies are: reflection,
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scattering and absorption [1]. Hence for selective optical properties of tissue cells, optical
spectroscopy methods can be used for determination of essential features of tissue. In 1876 Karl van
Vierordt already observed changes in the solar spectrum transmitted by the finger tissues of his own
hand. He discovered that after pressure causing inadequate blood supply, a change occurred in the
spectrum composition obtained, which he related to the changing participation of oxy and deoxy
hemoglobin concentration in the tissues. Non-invasive monitoring of SpO2 based upon skin reflectance
spectrophotometry was first described by Brinkman and Zijlstra in 1949. In 1977, Minolta built the first
oximeter based on the transillumination of an earlobe. The basic application difficulty in an effective
transillumination of thick tissue layers is the low power of radiation to detection [2]. It is necessary to
force the optical power of the source and to apply sensitive photo detectors. Among the above applied
methods (pulse oximetry) of tissue oxygenation parameters (like oxy and deoxy hemoglobin)
measurement, a tendency to develop methods based on detection and analysis of bio optical
phenomena is significant. Transmission and reflectance pulse oximetry, based on the differential
absorption of light in the red to near infrared region, is used routinely for measuring blood oxygenation
[3] and arterial oxygen saturation. Under the noninvasive “illumination from underneath”, reflection
mode NIRS system (both the sources and the detector are placed on the same side of the patient’s
body) is far better than transmittance mode to diagnose and monitor the parameters of tissue. Also
recent literature [4] shows that many critically ill patients (those with DM, peripheral vascular disease
(PVD)) have decreased tissue perfusion and oxygenations are likely to suffer of multiple organ failures
with increased mortality and morbidity. Therefore the aim of our work is to develop simple reflection
mode NIRS system which will be able to measure the tissue oxygenation index at any body parts.
Principle of Near Infrared spectroscopy
A study that deals with interaction of EM radiation (UV, Vis, IR and NIR) is often referred as optical
spectroscopy. NIRS is defined as measurement of spectroscopic changes in optical properties of
tissue in NIR wavelength (650nm to 1100nm) region. NIR light wavelength region is known as
therapeutic optical window. A particular tissue composition depends on blood and water content which
has different absorption values of optical parameters as shown in figure 1. One of the most useful
properties of using NIR wavelengths is that oxygenated hemoglobin and deoxygenated hemoglobin
both absorb light differently in this region.
Figure 1 Absorption spectra of tissue (courtesy:[5])
The effects of radiation influence on the tissue may concern its area and volume. Light tissue
interaction largely depends on the properties of the beam of radiation. Photons in turbid media, such
as most human tissues, are absorbed as well as scattered many times before being transmitted.
Optical radiations that play the role of an effective information carrier should be sufficiently coherent.
High optical density of the tissue should also have possibly high intensity.
Sensor Probe
The sensor consists of a light sources and photo diode. The sources and photodiode can be mounted
side by side to look at changes in reflected light as in figure 2. Traditionally, NIR Oximetry makes use
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of NIR wavelength (780nm, 830nm) to measure oxygen levels in blood. These two wavelengths are
chosen because, at 780 nm, deoxygenated blood has a higher absorption, whereas at 830 nm,
oxygenated blood has a higher absorption.
Figure 2 NIR sensor Probe design
Figure 3 NIR sensor Probe
A detected light intensity level is used to determine the relative change in concentration of tissue
oxygenation parameters. The particular arrangement here uses a rubber pad to hold NIR Laser diodes
and a photo detector as shown in figure 3.
NIRS System Instrumentation
Trans-illumination of hand and foot fingers is possible using efficient reflection mode NIRS system as
shown in figure 4. The NIRS system mainly consists of optical components, electronic circuits and data
processing system. In this system, the optical components are laser diodes as light source for
irradiating the tissue with optical radiation via optical fiber and collect the reflected signal from the
tissue by using PIN photodiode as detector. To achieve a good signal-to-noise ratio when the range of
skin thickness was 2–7 mm, the source-detector separation was set to 1.5cm based on a Monte Carlo
simulation and experiments [6]. Hence source and detector are placed on same side in the probe with
minimum distance of 1.5cm. Although the measurement depth with contact NIRS is about 10 mm and
the probe-tissue separation was 5 mm. The electronic circuit consists of major electronic components
like power supply circuit, pulse generator and timing circuits to drive laser diode, photo detector with
amplifiers as shown in figures 5, 6(a) and 6(b). To reduce the effects of changes in background light
intensity, the incident light was modulated sinusoidally at 300Hz by using pulse generator. The direct
current component of the signal was removed by a capacitor connected to the photodiode in series.
Data processing system include digital CRO to display voltage waveform related to detected light
signal (or) intensity changes due to blood chromophores (oxy and deoxy hemoglobin) present in the
tissue. Therefore our NIRS system can measures any change in oxygenated hemoglobin,
deoxygenated hemoglobin, total hemoglobin, and oxygenation index in the tissue.
Figure 4 Block diagram reflection mode NIRS system
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Figure 5 Photo detector with amplifier circuit
Working principle of NIRS system
Two NIR wavelength laser diode of 780 nm 830nm are driven by 300Hz modulation frequency
generated by timer circuit as shown in figure 6(a) and 6(b). The light emitting from the laser diode was
transmitted through the skin or tissue and change in light intensity is detected by photodiode. The
photodiode is connected to a trans-impedance amplification circuit that converts a current to an
appropriately enhanced voltage signal as in figure 5. The amplifier circuit uses an LM358 dual op-amp
to provide two identical broadly-tune band pass stages with gain of 100. The output intensity change in
terms of voltage output was observed through the use of an oscilloscope. By the acquired voltage we
can calculate the tissue oxygen parameters with the help of modified Beer Lambert law.
Figure 6(a) Pulse generator and timing circuit
Figure 6 (b) Timing diagram to drive laser diode
Comparison between NIRS system and Pulse oximetry
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Different wavelengths are used in both these techniques but NIRS is far more penetrating
effect than Pulse oximeter because of the usage of sources of light in NIR wavelength region.
Pulse oximeter considers only the arterial compartment by time gating the measurements,
where as NIRS provides a global assessment of all the vascular compartments (Arterial,
Venous and capillary).
NIRS uses more specific wavelengths than Pulse oximeter and therefore can characterize
more chromophores than the other.
Near-infrared spectroscopy is able to measure hemo dynamics, metabolic and fast neuronal
responses to brain activation with inexpensive and portable instrumentation. Pulse oximeter is
a reliable and commonly used to monitor systemic oxygen supply only.
Pulse oximeter utilizes the arterial oscillations to extract arterial oxygen saturation SaO 2 and
does not exploit all of the information from the heartbeat oscillations NIRS method measure
relative changes in pulsatile components of the cerebral blood flow and cerebral blood volume
based on the shape of the heartbeat pulse waveform.
NIRS can be used in patients with low perfusion states and peripheral vascular. It gives exact
oxygen level in the blood.
WORK DONE
Protocol for Data Collection
Data was collected on two subjects (mean age 25 years) in the fore finger using our developed NIRS
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system. The subject is first asked to relax for at least five minutes by meditation. Also the patient’s
body part (forefinger) is cleaned to avoid any unnecessary disturbance by dirt etc in the light reflection
process. Now we place the finger ‘without occlusion’ on the NIRS probe as shown in figure 7 and
switch on the power supply and observe the change in light intensity waveform in the digital
Oscilloscope (CRO).
Figure 7 Placement Fore finger over NIRS probe
Then note down the peak (voltage in mV) value from the CRO. After that use a “cuff” to cause
occlusion and increase in the blood flow to the finger (as done by the Blood Pressure (BP) measuring
instrument).Once the occlusion is done for 5-10 seconds, then place the forefinger on the NIRS probe
and note down the peak (voltage in mV) value from the CRO.
PRELIMINARY RESULTS
Then the two peak value obtained from CRO are processed and calculate the change in concentration
of oxy hemoglobin Δ [HbO2], deoxy hemoglobin Δ [DHb] and total hemoglobin Δ [THb] using modified
Beer Lambert law. ‘Δ’ denotes change in chromophores. ‘[ ]‘ denotes concentration.
Modified Beer-Lambert’s Law
OD= -log10 (I/ I0) = ε.C.L + G
G is the factor that accounts for the measurement geometry, C is the concentration, ε is called as
molar extinction coefficient and L is the optical path length value in the scattering medium. This path
length this defined as the source-detector separation “d” multiplied by the differential path length factor
(DPF) “B”. The typical values of DPF have been reported in the literature for muscles of various body
parts as 4 to 6mm.
Results analysis and Calculation
Using our designed NIRS probe on a subject’s right forefinger, we can note down the peak value (mV)
“With” and “Without occlusion” as shown in the table 1. ΔOD values at 780nm and 830nm are
mentioned in the table 2 for each subject was found by using the formula
ΔOD = -log 10 (V with occlusion / V without occlusion)
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Table 1: Peak value (mV) “with” and “without occlusion”
Laser diode
wavelength
780nm
830nm
Subject
1
without
Occlusi
on
Subject
1 with
Occlusi
on
Subject
2
without
Occlusi
on
Subject 2
with
Occlusio
n
18mV
42mV
20mV
40mV
23mV
38mV
21mV
41mV
Table 2: Change in optical density
Optical path length L =0.6
cm
No of Subjects
ΔOD780nm
ΔOD830nm
Subject1
-0.367
-0.218
Subject2
-0.301
-0.290
Calculation
Finally, we use the following formula to obtain the value of Δ [OHb], Δ [DHb] and Δ [THb]
Δ [HbO2] = [(εDHb830X ΔOD780) - (ε DHb 780 X ΔOD830)] / [L ( ε HbO2780 X εDHb830 - ε HbO2830 X εDHb780)]
Δ [HbO2] = 83.6 µM/L
Δ [DHb] = [(εHbO2780X ΔOD830) - (ε HbO2 830X ΔOD780)] / [L ( ε HbO2780 X εDHb830 - ε HbO2830 X εDHb780)]
Δ [DHb] =10.5 µM/L
Δ [THb] = Δ [HbO2] + Δ [DHb]
Δ [THb] =83.6+10.5=94.1µM/L
Tissue oxygenation TO = (Δ [HbO2] / Δ [THb]) x100
T.O = 88.8%
Limitation of NIR based tissue oximetry
There are factors like fluorescent or direct sunlight will cause the tissue oximetry to estimate false
reading of tissue oxygenation value. Nail polish might also be problematic to obtaining a true reading,
especially for patients wearing black, green, and blue nail polish. In these cases, the clinician can
either remove the nail polish. There has been no supporting evidence that other nail polish colors or
that acrylic nails affect the reading. In case of ICU patients, it has been noted that the NIR tissue
oximetry probes are not opaque and external light source will particularly be a problem. In addition,
motion (i.e. movement of probes), such as encountered in the pediatric ages, will alter the signal-tonoise ratio, thus stabilization of the hand or the foot.
SUMMARY OF PRELIMINARY RESULT
In the recent years, the research has been focused on the development of non invasive methods to
monitor physiological parameters. Techniques like pulse oximetry, laser doppler instruments for
imaging and analysis for instance have scaled new heights. But researchers have now developed
instruments that readily give the values of SpO2, pulse waveform and heart rate variability (HRV) using
photoplethysmograph (PPG). In this study, we present preliminary result obtained by in house
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designed NIRS probe show oxygen consumption level in the forefinger in terms of change in optical
density (Δ OD). During occlusion, the deoxy hemoglobin will go high and the tissue oxygenation in the
particular area will be less and vice versa in the form of change in concentration of oxy hemoglobin,
deoxy hemoglobin and total hemoglobin. In this system we have used 300Hz modulated light source
to correct the external noise. To suppress the low-frequency noise effectively; it is necessary to
increase the modulation frequency and the cutoff frequency of the high-pass filter that is connected to
the photodiode in series. Preliminary results, suggest that this system can be used for assessing
tissue oxygenation level in the upper and lower extremity of diabetic patients as well as cerebral
oxygenation in quantitative and non-invasive manner.
CONCLUSION AND FUTURE WORK
Hence, we have successfully developed a simple NIRS instrument which makes use of reflection
mode spectroscopy of NIR light wavelength on the tissue level of the human body and gives the
desired output in the form of the waveform which is connected to the digital CRO. This work might be
taken further to develop multi wavelength NIRS system and real time algorithm to map the NIRS signal
for displaying the tissue oxygenation level in the upper and lower extremities of human body using Mat
lab 7 or Lab View.
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