Fluence mapping inside the highly scattering medium using ...

Fluence mapping inside the highly scattering medium using
reflection mode acousto-optics
Altaf Hussain
Biomedical Photonic Imaging group, MIRA Institute for Biomedical Technology and Technical
Medicine, University of Twente, PO Box 217, 7500 AE Enschede, The Netherlands
1. Introduction
In recent years, optical imaging modalities received considerable attention of researchers and are moving toward clinical
applications. Optical properties of soft biological tissue such as scattering and absorption are related to its morphology
and biochemical composition. Optical imaging modalities, such as photoacoustic (PA) and acousto optics (AO) provides
very useful structural and functional information about tissue physiology [1-4]. However, these imaging modalities suffer
from quantification problem. The quantification problem is a result of unknown local light fluence in scattering medium.
Since, images of soft biological tissue obtained with optical imaging technique are weighted with unknown local light
fluence. It is difficult to get quantitative information about µa without knowing the local fluence. Several attempts have
been made to compensate for fluence variation in PA imaging [5-7], as well in acousto-optics [4, 8]. Main challenge
remains to predict the local fluence for a medium with unknown and inhomogeneous optical properties. Previously we
presented a purely experimental strategy to do fluence independent photoacoustic imaging [9]. The strategy proposed
combination of two photoacoustic experiments with illumination at two points and one transmission mode AO
experiment.
We propose a theoretical concept for experimentally measuring the local light fluence in absolute terms in scattering
media. The proposed methodology is based on an experimental technique that allows local labeling of light with known
efficiency and detection of labeled light in pure reflection mode. We validate our theoretical concept using Monte Carlo
simulations, results are presented in numerical experiment section. Experimentally we exploit the feasibility of using
reflection mode AO as techniques to locally tag the light. Since tagging efficiency of AO is unknown, therefor, the
proposed expression for local fluence is simplified, with few justifiable assumptions. Our experimental results on
measuring the fluence in scattering medium using reflection mode AO are presented in experiment section.
2. Theory
In this section, we describe the mathematical model of our proposed methodology. This model is based on two
principles, one photon path reversibly in scattering medium [9, 10] and local labeling of photons inside the scattering
medium. Consider a scattering medium depicted in Fig. 1, where point 1 is on the surface and point 2 is inside the
medium. The fluence rate ϕ1,2 at point 2 due to injection of light at point 1 with power P1 can be written as,
( ). Where Pr(1,2) is the probability per unit area and per unit solid angle that a photon injected at point 1 will
cross the aperture placed at point 2, traversing any possible trajectory.
Pl,1
(a) (b)
Ω1
P 1
A 1
A2
Fig. 1. a) Schematic of medium and photon trajectories from injection point 1 to detection point 1 through internal point “2” (labeling
volume). b) MC simulation results, estimated light fluence using model (squares) and actual simulated fluence (circles).

The photons addressing the internal point 2 can be “labeled/modulated” using strongly focused ultrasound and detected at
the surface of the medium (AO). Assuming incoming fluence rate ϕ1,2 in internal point 2 (labeling volume) and that all
the photons entering the labeling volume get labeled (labeling efficiency is unity) and leave without absorption. The
power of detected tagged photons at point 1, can be written as, ( ) By applying photon path reversibility
principle Pr(1,2)= Pr(2,1), and rearranging aforementioned equations for fluence at point 2 and power of tagged photons
measured at 1, we get the expression for local fluence rate at point 2,
√ (1)
If we take into account all the parameters regarding labeling volume and detection system, the expression for the absolute
local fluence rate can be written as.
√
√ ( )
This expression for local fluence contains excitation parameter P1, instrumental parameters (Ω1, A1 and A2) and
externally measureable quantity Pl,1. Hence, Eqn. 2 suggests that local light fluence can be measured in optically
inhomogeneous medium experimentally. Reflection mode AO is a technique which allows local labeling of light using
ultrasonic modulation and detection of ultrasonically modulated light in reflection mode [11].
3. Numerical experiment
To proof our methodology we used Monte Carlo simulation program for light transport in turbid media [12]. We
modified the program to be able to “label” photons addressing a certain region of interest in medium. This region of
interest, we call labeling volume, was considered of spherical shape in our simulations. We simulated a scattering
medium of dimensions 40x40x4 cm 3 , containing a spherical absorber/labeling volume of diameter 1 mm. The optical
properties of background medium µs ’ =7.5/cm and µa=0.01/cm were different from the optical properties of labeling
volume µs ’ =7.5/cm and µa=1/cm.
In simulations we injected 10^7 photons into the medium through a circular aperture A1 of diameter 3 mm. Photons
addressing the labeling volume during their random walk through the scattering medium were “labeled”. Labeled
photons leaving the medium through aperture A1 within the opening angle of 25 degrees were detected to simulate
reflection mode AO. We used Eqn. 2 to estimate the fluence at the position of labeling volume in scattering medium. The
quantities P1 and Pl,1 power of incident photons and detected labeled photons respectively in Eqn. 2, were replaced by the
weight of injected photons and detected labeled photons. Results from Monte Carlo simulations presented in Fig. 1b,
show that estimation of the local fluence inside the scattering medium using our proposed methodology.
4. Experimental validation
Acousto optics is a technique that allows such local labeling of light [13], and detection of ultrasonically labeled in
reflection configuration [11]. However, unlike presented theoretical model (Eqn. 1) the labeling volume of AO is of
complicated shape and its labeling efficiency is unknown. As a result measuring fluence in absolute terms using
reflection mode AO is not possible at this stage. Therefore, we used Eqn. 1 instead and show that we can measure fluence
variation in scattering medium. We used speckle contrast detection method to measure ultrasonically modulated
backscattered light []. The speckle contrast decreases in the presence of ultrasound. Change in speck contrast (ΔC)
between US ON and OFF is can be measured and it is approximately proportional to the intensity of locally
ultrasonically modulated light [14]. This means Eqn. 1 can be written as, √ The experiment was performed on
a soft tissue mimicking cubical phantom of dimensions 3x3x3 mm 3 . The phantom was made of 3% Agar gel and a
dilution of 4% Intralipid (20%), resulting in approximate reduced scattering coefficient 7.5 cm -1 . US transducer was
attached to the xy translation stage. Speckle contrast change (ΔC) in the backscattered light was measured by scanning
the ultrasound focus along the optical axis (x-axis). To validate our results we measured the fluence invasively using an
optical fiber.
Fig. 4 shows the normalized fluence, measured acousto-optically (red) using Eqn. 1 and with the fiber (black). Horizontal
axis represents the depth from the surface along optical axis (x-axis). We measured the fluence acousto optically twice,
before (Fig. 2a) and after (Fig. 2b) measuring the fluence with the fiber, to see if introducing the fiber into the medium
effects the local fluence, results show no significant change.

Fig. 4. Comparison of acousto optically measured (circles) light fluence with invasively measured (squares) light fluence using optical
fiber, in both cases the measured signal has been normalized to maximum value of one. a) measurement before inserting the fiber into
medium, b) measurement after inserting the fiber into medium.
Results presented in Fig. 4, show that acousto optically measured local light fluence, using our proposed methodology, is
in good agreement with invasively measured fluence using optical fiber. Presented results are average of fifteen AO
measurements and error bars are obtained by calculating the error propagation based on standard deviation in acoustic
measurements. The error in estimated fluence increases with depth as it is expected since SNR in reflection mode AO
becomes lower at higher depth.
We presented a theory that proposes a method to measure the local light fluence in scattering medium experimentally,
without the need of any prior knowledge about the optical properties of the medium. Our theoretical and numerical
simulation model assumes a simplistic labeling volume and unit tagging efficiency. However, it is not the case at
experimental level, when using AO as labeling technique. Therefore, we simplified our model (Eqn. 2) to use reflection
mode AO as technique to measure local fluence in scattering medium, which gives the fluence map in relative terms with
an unknown pre-factor. Here in this proceeding, we have shown in real experimental settings that using reflection mode
AO local light fluence can be measured. Our present experiments are done on homogeneous scattering medium and our
goal in future is to investigate the applicability of this method in heterogeneous (scattering and absorption) medium.
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