A method to locate an absorber embedded in a semi-infinite turbid medium by spatially-resolved continuous-wave (SRCW) diffuse reflectance measurements is introduced. The depth of the absorber is assessed by single wavelength SRCW diffuse reflectance measurements by two detectors in a radial row. The ratio of perturbations introduced by the defect at two detectors is used to be matched with the ratio-versus-depth curve, which are generated by approximate formulas of continuous wave diffuse reflectance. The error due to approximation and the error in depth assessment are studied for different cases revealing favorable source-detector placements with respect to planar position of the defect. The effect of lateral displacement of the source with respect to defect is studied. A strategy to overcome errors introduced by erroneous prediction of background medium optical properties is suggested. Theoretical results indicate that the depth of the absorber can be obtained with 0.1 mm precision independent of its absorption coefficient and its size for the values chosen in the study. The approach is tested experimentally and it is observed that theoretical results fit with experimental data.
Accurate estimation of concentration changes in muscles by continuous wave near-IR spectroscopy for muscle measurements suffers from underestimation and crosstalk problems due to the presence of superficial skin and fat layers. Underestimation error is basically caused by a homogeneous medium assumption in the calculations leading to the partial volume effect. The homogeneous medium assumption and wavelength dependence of mean partial path length in the muscle layer cause the crosstalk. We investigate underestimation errors and crosstalk by Monte Carlo simulations with a three layered (skin-fat-muscle) tissue model for a two-wavelength system where the choice of first wavelength is in the 675- to 775-nm range and the second wavelength is in the 825- to 900-nm range. Means of absolute underestimation errors and crosstalk over the considered wavelength pairs are found to be higher for greater fat thicknesses. Estimation errors of concentration changes for Hb and HbO2 are calculated to be close for an ischemia type protocol where both Hb and HbO2 are assumed to have equal magnitude but opposite concentration changes. The minimum estimation errors are found for the 700/825- and 725/825-nm pairs for this protocol.
Potential of depth resolution of continuous-wave (CW) illumination in diffuse optical imaging is explained. It is known
both experimentally and numerically that in CW measurements photons traversing a homogenous, semi-infinite, highly
scattering medium between a source and a detector located on the surface of the medium follow paths that the volume
interrogated resembles a banana-shape. Also is known that, sensitivity profile of photon propagation in CW
measurements is non-uniformly distributed in depth, reaching a maximum at a certain value depending on geometry,
source-detector separation, and optical properties of the medium. The presence of an inclusion with a higher absorption
coefficient with respect to that of the background in a homogeneous medium can be estimated by increasing time-rate-ofphoton-
injection into the medium. The inclusion is assumed to be at a depth between the optode pair such that distances
to optodes are the same. An increment in the time-rate-of-photon-injection will give different detection slopes depending
on the depth of the inclusion, because the number of photons which is blocked by the inclusion is high if it resides at a
depth where the sensitivity profile has a higher value. In this work, preliminary results of Monte-Simulation of light
propagation show that measuring slopes of increase in detected light intensity for different interoptode distances are
different for extreme case of screen between optodes blocking all photons below a certain depth. Specification of
inclusion with this method may enable us to make predictions about the depth and optical properties of the inclusion to
be used as a priori information to be used image reconstruction in diffuse optical tomography that may be integrated
imaging systems.
In this work, preliminary results of a method to calculate anisotropy constant of scattering phase function (g) of a homogeneous, semi-infinite slab are shown. The method relies on measuring steady-state diffuse reflectance from the medium. In order to test the method, Monte-Carlo simulation of photon propagation in turbid media is utilized. Simulation depends on capturing diffuse-reflected photon packets which are injected perpendicularly by a point source in a continuous-wave manner. Photon packets are captured by a hemi-sphere like detector residing on the upper plane of the slab a few centimeters away from the source. The surface of the detector is divided into 120 sub-regions with equal area. Exit angles are converted into spherical coordinates. Therefore, the intersection of a particular photon packet with the detector surface is evaluated and the weight of photon packet is added to its corresponding sub-area. This process is repeated for all packets leaving the medium through the base of the hemisphere in each run. To evaluate the effect of anisotropy constant of the phase function on hemispheric weight accumulation the following simulations are performed. Optical properties of the medium are chosen as 10 cm-1 for scattering coefficient, 0.1 cm-1 for absorption coefficient, and 1.0 for refractive index. Detector is a non-scattering and non-absorbing medium whose refractive index is 1.0. Twenty millions of packets are used in the simulations. All optical properties except g are kept constant, while g is chosen to be 0.30, 0.50, 0.70, 0.75 and 0.80 for different runs.
In this work, 2D and 3D distribution of exit angles of diffuse-reflected photons from a semi-infinite, homogeneous slab are obtained by Monte-Carlo simulations. During simulations, a single point source emits incident photons normally onto the slab. For the calculation of exit angles, we consider the exit angles from a hemisphere with a varying radius. Hemispheres are distributed on the upper side of the slab for reflectance geometry. In both 2D and 3D cases, only photons exiting the slab through bases of hemispheres are considered. As a photon intersects the hemisphere surface, exit angles of the photon with respect to a spherical coordinate system whose origin is on the center of base of the hemisphere are recorded. In the 2D case, projection of the direction of exit of a photon onto upper surface of the slab is tallied hence only a single exit angle (polar angle) with respect to a polar coordinate system is considered. In the 3D case however, two exit angles, namely polar and azimuth angles, are recorded. Our results show that, in both 2D and 3D distribution of exit angles of diffuse-reflected photons have similar patterns for all hemispheres. The distributions of exit angles are observed to aggregate on angles away from the source. This property is preserved independent of the hemisphere position with respect to the point source.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.