|Optical Coherence Tomography (OCT), Blood oxygen saturation, FDOCT, Retina
|Hemoglobin is a protein presents in the blood, which is responsible for carrying oxygen to all organs in the human
body. When Hemoglobin charged with oxygen in lungs it forms as Oxy-Hemoglobin (HbO2) or oxygenated blood.
After delivering oxygen to cells it becomes as de-oxygenated blood (Hb). The amount of oxygen actually present in the
blood compared to maximum amount it can hold is called blood oxygen saturation (sO2). In clinics, local, non invasive
and non contact measurement of sO2 has great importance. The eye deceases like Diabetic Retinopathy (DR),
neovasularisation are directly related to the blood flow and retinal hypoxia this would leads to low vision or vision
loss[1, 2]. It can be used for diagnosing deceases like Peripheral Vascular Diseases (PVD), Compartment syndrome,
perfusion . Furthermore, some studies are suggesting that by measuring tumours hypoxia one can predict the
carcinogenic nature of cells, thus it would be useful for determining metastases state of carcinoma tumour . Blood
has different absorption properties at different wavelengths of the light. This is the main key feature for measuring sO2
levels in blood. Pulse oximetry is also works on the same principle but with it, it is not possible to measure local or
depth dependent sO2 levels in blood.
|Optical Coherence Tomography (OCT) works on low coherence interferometry and useful for studying high resolution
cross sectional images of biological objects . In OCT, another modality called Spectroscopic Optical Coherence
Tomography (SOCT) which is used for extracting local spectroscopic properties of biological samples . Measuring
sO2 with OCT has several advantages over conventional sO2 measurement techniques. It’s high spatial resolution would
enable us to resolve different layers of the sample, extract it’s local spectroscopic properties, due to its non invasive,
high speed it can be used to measure bloods flow velocity and generates enface images, cross sectional images . For
studying spectroscopic properties of an eye with OCT, the wavelength band selection is the most important since it
contains 90% water molecules. Hence it is important to choose central wavelength whose absorption coefficients and
scattering effects are lowest. In OCT for ophthalmology purpose, 800 nm, 1060 nm, 1300 nm central wavelengths are
most popular. In case of spectroscopic study of retina and choroid, 1060 nm is the optimum central wavelength. It has
been experimentally proved that, at 1060 nm water molecules absorption spectrum shows local minima and scattering
effects of blood above 800 nm decreases approximately with -1.7 . Also, the depth dependent broadening of the axial point spread function of OCT has suppressed and water shows very less percent of dispersion effects [9,10]. The water
molecules absorption is very high at 1300 nm wavelength, whereas 1060 nm central wavelength shows less absorption
by Retinal Pigment Epithelium (RPE), hence deeper penetration depth can be achieved by it and also, this enables us
clear visualization of choroidal vessels below the choroid capillary layer [9-11] .
|Few people has attempted to study spectroscopic properties of blood with the OCT. SOCT was used to study the
absorption properties of HbO2 and Hb samples . Also 780 nm, 810 nm central wavelength bands has been used in
OCT for measuring absorption coefficients of diluted 100% oxygenated or deoxygenated samples . In vivo 575 nm
as central wavelength band was also implemented for extracting sO2 levels in normal mouse dorsal skin , 840 nm as
central wavelength band used for measuring the sO2 levels in human eye . But, these central wavelength bands have
several constraints. Our study will be the first attempt to measure sO2 in retina with OCT using 1060 nm as central
|The main objective of this work is to explore measurement of sO2 levels in human retinal blood by using FDOCT with
1060 nm as central wavelength and having a 120 nm spectral bandwidth which is optimized wavelength band for
extracting spectroscopic properties of deep retina and choroid[16-19].
|FDOCT spectral data from normal human eye (Asian, 24 yrs) was collected and used in this study. The FDOCT system
has penetration depth about 2.5 mm. Then data was analysed by custom made codes written using Labview (National
Instruments, Inc., Austin, Texas). In order to estimate the levels of sO2 in retinal capillaries, capillary portions in retina was identified from B-scan image which comprises of 512 x 1500 pixels. A volume (512x1500x128) projection image
was presented Fig.1.
|Fig.1. SDOCT volume projection image. Red color line showed in the above figure is 76th (slice) extracted for B-scan image as shown in Fig.2.
Green circles represent vein and artery portions.
|Fig.2. Extracted B-scan image of human retina from Fig 1 marked as red line. The portions (a) and (b) is the vein and artery vessel respectively, from
where the data was extracted.
|The vein and an artery portions have been identified by visual inspection of the SDOCT fundus image [12, 20] it can be
seen in fig.1 and fig.2. In present study, four fringe patterns from the edges of the vein, artery and its adjacent tissues,
as shown in Fig. 2(a) and (b) was sampled. These fringes were averaged in order to avoid specular reflections . In
blood, concentration of HbO2 and Hb can be calculated by assuming light absorption depends only on concentration
state of HbO2 and Hb. Intensity of SDOCT A-line can be expressed as 
IV.RESULTS AND DISCUSSION
|In this work, HbO Hb C ,C
values for vein and artery were estimated at 0.5 mm depth. The values are 15.4 g/L-mm, 75.5
g/L-mm for vein and 43.8 g/L-mm, 14.4 g/L-mm for artery respectively. According to Robles et al., [18, 20] , the
lowest feasible CHb value was 1.2 g/L-mm, which is supporting our calculated values. Within this method the maximum
depth limitation was up to 1 mm, however in our work we used 0.5 mm depth range. The measured sO2 level in the
vein was 16.8% and for an artery it was 75.3%. Accuracy of our measurement of sO2 level may be influenced by
wavelength band due to some portion of radiation also absorbed by water molecules .
|In summary, the sO2 level in retinal layer was measured by using FDOCT data with 1060 nm wavelength band and
least squares method. The obtained results are well comparable with existing results. So, we can conclude that 1060 nm
wavelength band is optimal for measuring sO2 levels in retino-choroidal complex. We successfully implemented least
squares method for measuring concentrations of Hb and HbO2. We also observed sO2 levels of the vein are less than an
|The authors are gratefully acknowledged the financial support of the DST (IDP/MED/10/2010), Govt. of India. We are
also thankful to computational optics group, university of Tsukuba, Japan for providing FDOCT data from human eye.
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