Optics and Photonics Journal, 2012, 2, 352-357
http://dx.doi.org/10.4236/opj.2012.24044 Published Online December 2012 (http://www.SciRP.org/journal/opj)
Method for Improving the Lateral Resolution of
Near-Infrared (NIR) Single Optods: Application to
Subcutaneous Vein Detection and Localization
Yasser S. Fawzy1,2
1Inovia Technologies Ltd., Vancouver, Canada
2National Laser Institute, Cairo University, Cairo, Egypt
Received September 11, 2012; revised October 13, 2012; accepted October 24, 2012
ABSTRACT
NIR backscattering measurements using single source-detector optical probe (optods) can detect absorption areas within
deep tissue layer. However, such optods, are characterized by large separation distance between the source and detec-
tors (>2 cm) and poor lateral resolution (>1 cm), which limits its usage for the localization of small absorption volumes
embedded deep within the tissue such as subcutaneous veins. In this work a method to improve the accuracy of locating
such absorption volumes (areas) using backscattered NIR measurements is suggested and investigated with the aim of
developing an optical sensor for detecting and localizing large subcutaneous veins. The method is based on measuring
the differential signal from three overlapping source-detector pairs arranged within the prob e such that the total photon
sensitivity profile of the probe is maximized along a narrow width area (within the central of the probe) an d minimized
along its sides. The location of th e absorption areas is th en determined when a peak maximum of the measured signal is
detected. Monte Carlo simulation and light transport modeling was used to determine the optimum arrangement of each
source-detector pair within the probe to create the required spatial sensitivity p rofile and demonstrate the validity of the
method. The results showed that the differential optode has more than two times improvement in the lateral resolution
compared to the standard optode. The result also showed that the differential probe can locate subcutaneous veins with
diameter ~5 mm and embedded at ~1.5 cm depth. The method could have a potential for designing and developing an
optical backscattering sensors for detecting and localizing large subcutaneous veins embedded <2 cm depths.
Keywords: Reflectance Measurements; Photon Migration; Optical Diagnosis; Fiber Probe Design
1. Introduction
NIR diffuse reflectance measurements and spectroscopy
is simple and portable optical diagnostic technique that
can be used to detect and to monitor the alteration in tis-
sue physiology and/or pathology, by measuring the
change in absorption coefficient related to such alteratio n.
Several different types of NIR instruments in the CW
domain [1], frequency-domain [2], and time-domain [3]
have been developed over the past 20 years to measure
changes in tissue oxygenation, perfusion, and brain ac-
tivity [4-7]. Depending on the source-detector sep aration,
among other parameters, NIR reflectance measurements
can detect absorption changes from areas or volumes that
are 2 - 3 centimeters deep within the tissues [8,9]. How-
ever the lateral spatial resolution for such single source-
detector arrangement is typically larger than 1 cm for
absorption areas lo cated within such depths [10,11]. This
would limit the application of such probes in the ap-
plications that require detecting as well as localizing the
lateral position of the measured absorption areas within
deep tissue layers.
The aim of this work is to develop a method of reflec-
tance probe design and measurement that could improve
the lateral spatial resolution to the detected NIR diffuse
reflectance signal obtained from large depths within the
tissue. For that respect we have investigated th e possibil-
ity of using multiple source-detector pairs arranged in
certain design such that the spatial sensitivity profile of
the measured signal is maximized along certain localized
area (volume) within the tissue. We have also presented a
method of measurement for using such probe, referred
here as differential probe, to localize the position within
which the absorption changes have occurred. Monte
Carlo simulation was used to determine the optimum
distances to produce the required spatial sensitivity pro-
file and to verify the improvement in the spatial resolu-
tion using simul at e d dat a.
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Y. S. FAWZY 353
2. Method
2.1. Probe Design Method
Our design method is based on measuring the differential
signal obtained using multiple overlapping source-de-
tector pairs interrogate common area (or volume) within
the tissue. The multiple source-detector pairs were ar-
ranged such that the probe photon sensitivity profile is
maximized within a narrow area (volume) along the cen-
ter of the probe. In particular, we have used in this design,
three source-detector pairs arranged such that one pair in
the probe center and the other two arranged at symmetri-
cal distance from the probe center as shown in Figure
1(b). The two detectors at the side of probe are assumed
to have a similar distance (d) from the center of the probe.
The differential probe signal Idiff is equal to


13 2
2
I
II where I1, I
2, I 3 are the intensity meas-
ured by the first, second (central) and third source-de-
-tector pairs respectively. Monte Carlo simulation, de-
scribed below, was used to calculate the spatial sensitiv-
ity profile [12,13] of this differential detection sequence.
The photon sensitivity profile was calculated for different
distances (d) to find the optimum distance (d) that produce
the smallest (width) area with maximum photon density
along the center of the probe. The width of this high pho-
ton density area obtained from such arrangement should
determine the lateral resolution of the designed probe.
2.2. Measured Method
The method of measurement is based on scanning the
tissue with the probe, and measur es the change in attenu-
ated light over the scanned area. The position of the ab-
sorber is localized when it falls within the central of the
probe which has the maximum photon density (maximum
probe sensitivity). In terms of reflectance measurements,
this will corresponds to a peak maximum during the
measurement scan. We have verified the measurement
method using a Monte Carlo simulation by modeling the
total diffuse reflectance measured by the differential
probe from a scattering media contains two subcutaneous
veins (diameter ~5 mm each) separated by ~5 mm dis-
tance as shown in Figure 2. The absorption coefficient of
the subcutaneous veins was chosen to simulate realistic
conditions of the subcutaneous veins embed de d wi thi n f at
d
3 3 2 1 3 2 1
Figure 1. (a) Standard probe; (b) Differential probe. Detec-
tor (framed circle), source (solid black).
x
y
Probe Scan Measurement
Turbid Media
Absorption rods
Figure 2. Schematic diagram of the measurement method.
and muscle layers (~8 folds increase). To model the
scanning measurements of the probe we have acquired
different measurements at interval distances in the radial
direction


0,4 cm,0.5 mm.rr dr  The radial dis-
tance is referenced to the position of the maximum sensi-
tivity point in each probe (probe center).
2.3. Monte Carlo Simulation
Monte Carlo simulations were performed using custom-
ized software based on the algorithm s and technique s used
by Phral et al. [14]. In particular, the Monte Carlo simu-
lation has been customized to estimate the spatial sensi-
tivity profile (photon density distribution) and the total
diffuse reflectance measured from the differential probe.
The meas ured diffuse reflectance signal wa s calculated by
recording the photons that hit the detectors when it escape
the sample and propagate to any of the three detectors
plane with an angle, to the surface normal, less than the
NA cone of the fiber detector. A detector fiber with 400
µm diameter and NA ~ 0.4 was used for the simulated
reflectance probes. The spatial sensitiv ity profile calcula-
tions required to keep track of the escaped photons in
three dimensions. To simulate differential probe detection,
the photon is reco rded to hit the probe when the weighted
intensity of the photon detected by the second (central)
detector is larger than the half su m of the weighted in ten-
sities detected by the two other detectors in the side. This
would be corresponding to detecting the signal only when
(Idiff > 0).
3. Results
Figure 3 shows the spatial sensitivity (photon density)
profile of the single standard probe and the triple differ-
ential probe. As shown in the figure the photon density of
the differential probe has an overall increase within the
central of the probe while it was reduced along the side
of the probe. Also, the width of the area with increased
photon density was significantly reduced compared to

Y. S. FAWZY 354
-3
-2
-1
02015 -10 -505101520
-Max
-Min
(a)
-3
-2
-1
02015-10-50510 15 20
-Max
-Min
(b)
Figure 3. Photon Sensitivity Profile for (a) Single source-detector probe and (b) Differential triple source-detector probe
(units in 0.5*cm).
Figure 5 shows the radial sensitivity profile (at depth
~10 mm) of the differential probe in comparison to the
single probe. The radial sen sitivity profile was calculated
by recording all the radial position of the photons de-
tected by the probe detectors from depths between 10 -
12 mm. As shown in Figure 5, the width of the radial
sensitivity profile was reduced more than two times
compared to the standard probe. This would result in
improving the localization accuracy of an absorption
structure by two times.
that of the single source-detector probe.
The distance between the source-detectors pairs de-
termine the resultant sensitivity pr ofile of the probe. Dis-
tance (d) between the center of the detector on the side
(detector 1 or 3) and the probe center was varied and the
width of the resultant sensitivity profile (wi), defined
here as the area with photon sensitivity >75% of the
maximum, was calculated. Figure 4 shows the width of
the sensitivity profile calculated for a range of distances
(d) between –8 mm to 8 mm. As shown in the figure, the
width of the sensitivity p rofile (wi) is minimized to 4mm
along distances (d) between (0 - 3 mm). The figure also
shows that the optimum distance range for minimizing
the sensitivity profile width depends on the background
optical properties of the tissu e being investigated.
The depth sensitivity profile was also calculated for
each probe by recording all the photons that hit the probe
detectors with intensity larger than 1/e of the maximum
detected photon intensity. Figure 6, shows that both the
ifferential probe and the standard probe interrogate d

Y. S. FAWZY 355
Figure 4. Design optimization for improving the lateral resolution for turbid media with different optical scattering proper-
ties (µa = 0.01 mm–1).
Sensitivity
Figure 5. Radial sensitivity profile at 10 mm depth for the total diffuse reflectance collected by the standard probe and the
differential probe.
similar depths (up to 2 cm) when measuring the diffuse
reflectance signal from the same tissue optical properties.
The result of the virtual scanning measurements is
shown in Figure 7. For such measurement the back-
ground optical properties was (µs’ = 1.5 mm–1, µa = 0.01
mm–1) and the two absorption rods was assumed to have
a diameter of 5 mm and µa = 0. 08 mm–1. As shown in
figure the measured signal detected using the differential
probe measurement has two remarkable peak minimum
corresponding to the position of the absorption rods. The
figure also shows the diffuse reflectance measured during
the virtual scanning (along different distances) using the
standard probe measurement. The differential signal
plotted in the figure is equal to (Idiff + 0.4). As shown in
the figure the position of the two absorption rods are not
differentiated using the standard probe measurements.
4. Discussion
Improving the lateral resolution of NIR reflectance
measurement could be of great benefit for a number of
applications that require a method fo r detecting as well as
localizing the position of absorption volume within deep
tissue layers. A potential application is the detection and
localization of subcutaneous veins (for example internal
jugular vein) for accurate puncturing positioning [15,16].
Also such probes could be sed to monitor tissue oxy- u

Y. S. FAWZY 356
Sensitivity
Figure 6. Depth sensitivity profile for the total diffuse reflectance measured by the standard probe and the diffe re ntial probe .
Figure 7. Results of the virtual scanning measurements, using Monte Carlo simulation, showing the total diffuse reflectance
measured using the differential and the standard reflectance probes. The two rectangles on the bottom of the graph indicate
the actual position of the two absorption rods.
genation from deep veins by localizing the area or the
volume from where the signal is measured [17].
Our results showed that the differential probe can de-
tect and localize the attenuation changes from area
(volume) with diameter ~5 mm at depths ~1.5 cm. Com-
pared to the standard probe with the same source-detec-
tor distance, the designed probe has at least two folds
improvement in the lateral reso lution and consequently in
the localization accuracy of the small absorption volume
embedded deep within the tissue. This improvement in
the spatial resolution is the resultant of the differential
measurements of the signals detected by multiple sour ce-
detector pairs interrogating common area or volume.
The design analysis for the optimum source-detector
arrangement and spacing within the differential probe
showed that the optical properties of the investigated
tissue should be taken into consideration when calculat-
ing the optimum source-detector distances that would
improve the spatial sensitivity of the probe. Thus, a pri-
ori information about the range of the tissue optical
properties being investigated would be needed to make
use of this method in practice. It should be noted also
that finer lateral resolution could be achieved using the
above approach but with more complex fiber arrange-

Y. S. FAWZY 357
ments and with a priori knowledge about the optical
properties range of the tissue being investigated. Never-
theless, the objective of this work is mainly to illustrate
the validity of the proposed method and the concept for
designing diffuse reflectance probes that have improved
lateral resolution (<5 mm) and yet interrogate d eep tissue
layers within ~1.5 cm depth.
In addition, the proposed method is not li mited to CW
measurements and could be used with other types of NIR
instruments including time-resolved and frequency mo-
dulated reflectance measurements. However, other de-
sign parameters such as detectors SNR, dynamic range,
source-detector pairs switching, and tissue background
optical properties, need to be investigated before devel-
oping an experimental differential probe that could lo-
calize deep small absorption changes from different
modes of reflectance measurements.
5. Conclusion
We have proposed and investigated a new method for
improving the lateral resolution of the optical back-scat-
tering (reflectance) measurements. The method could
have potential for improving the accuracy of the locating
absorption structure (such as vein) embedded deep within
the biological tissue. Future work will involve develop-
ing an optical probe based the suggested method and
verifying the validity of this approach using experimental
measurements on tissue simulating phantoms.
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