Biomedical Engineering: Angular Domain Imaging: Imaging through Tissue Papers
Prof. Glenn H. Chapman

This page contains the paper abstracts and full PDF versions of publications on the Biomedical Engineering, Angular Domain Imaging and Imaging through Tissue area. Click on the link to download pdf version.  Papers here are listed in order of publication
 
Note: These papers are for academic use only and are not available for distribution or publication for other purposes.

Refereed Journal Papers

F. Vasefi, M. Belton, B. Kaminska, G.H. Chapman,, and J.L.L. Carson, “Angular Domain Fluorescence Imaging for Small Animal Research”. accepted Journal of Biomedical Optics, 15(01), 016023-1 to 016023-5 2010
F. Vasefi, E. Ng, B. Kaminska, G.H.Chapman, K.Jordan, and J.J. L. Carson, "Transmission and fluorescence angular domain optical projection tomography of turbid media," Appl. Opt. 48, 6448-6457 (2009)
T. Schneider, H. Zhao, J.K. Jackson, G.H. Chapman, J. Dykes, U.O. Häfeli “Generation of biodegradable camptothecin-loaded polymer microspheres using hydrodynamic flow focusing”, J. Pharmaceutical Sciences, Sci 97, 4943-4954 2008. jpharmaceutical_Schneider2008.pdf  285KB
The present study was conducted to investigate the use of hydrodynamic flow focusing for the generation of biodegradable polymer microspheres encapsulating the anticancer drug camptothecin. Poly(D,L-lactide-co-glycolide) (PLGA) and poly (L-lactide) (PLA) were used as the matrix materials. Camptothecin was dissolved in the disperse phase and microspheres with a mean size between 2 and 3 mm generated using hydrodynamic flow focusing. When up to 1 wt.% of the drug was added to PLA, thedrug encapsulation efficiency was 64%. For PLGA, the drug encapsulation efficiency was
between 39 and 46%. Drug release from PLA particles was rapid and complete within 6 h, while drug release from PLGA particles showed no burst effect and followed a first order release profile. The encapsulated camptothecin stayed in its active lactone form, as shown by HPLC, and was able to exert cell toxic effects as shown by a cell viability assay. Hydrodynamic flow focusing is a promising tool for the preparation of drugreleasing biodegradable microspheres typically made by solvent evaporation and/or solvent extraction, as indicated by the successful encapsulation of the anticancer drug camptothecin.
F. Vasefi, B. Kaminska, G.H. Chapman, and J.J. Carson “Image contrast enhancement in angular domain optical imaging of turbid media, Optics Express, Vol. 16 Issue 26, pp. 21492-21504 (2008) optics_express08_fartash-2.pdf 476KB
Imaging structures within a turbid medium using Angular Domain Imaging (ADI) employs an angular filter array to separate weakly scattered photons from those that are highly scattered. At high scattering coefficients, ADI contrast declines due to the large fraction of non-uniform background scattered light still within the acceptance angle. This paper demonstrates various methods to enhance the image contrast in ADI. Experiments where a wedge prism was used to deviate the laser source so that scattered photons could be imaged and subtracted from the image obtained by standard ADI provided the greatest improvement in image contrast.
F. Vasefi, B. Kaminska, P.K.Y. Chan, and G.H. Chapman, “Multi-spectral Angular Domain Optical Imaging in Biological Tissues Using Diode Laser Sources”, Optics Express, Vol. 16 Issue 19, pp.14456-14468 (2008) optics_express08_fartash.pdf 1,274KB
Angular Domain Imaging (ADI) employs micromachined angular filter to detect non-scattered photons that pass through the micro-scale tunnels unattenuated while scattered photons are rejected. This paper describes the construction of an ADI system utilizing diode lasers at three different wavelengths in the range of the red and near infrared spectrum. Experiments are performed to verify the feasibility of ADI at multi-wavelengths. ADI results of chicken breast as a biological scattering medium are presented for different thicknesses. A spatial resolution of <0.5 mm is achieved with 5 mm thick chicken breast using a 975 nm diode laser source.
N. Pfeiffer and G.H. Chapman, “Successive order, multiple scattering of two-term Henyey-Greenstein phase functions”, Optics Express, v16 Issue 18, pp.13637-13642 Sept. 2008. optics_express_pfeiffer08.pdf  240KB
An analytic solution to the problem of determining photon direction after successive scatterings in an infinite, homogeneous, isotropic medium, where each scattering event is in accordance with a two-term Henyey-Greenstein phase function, is presented and compared against Monte Carlo simulation results. The photon direction is described by a probability density function of the dot product of the initial direction and the direction after multiple scattering events, and it is found that such a probability density function can be represented as a weighted series of one-term Henyey-Greenstein phase functions
F. Vasefi, P.K.Y. Chan, B. Kaminska, G.H. Chapman, N. Pfeiffer, “An Optical Imaging Technique Using Deep Illumination in the Angular Domain”, IEEE J. Selected Topics in Quantum Electronics, v 13, pg 1610 – 1620, 2007 jstqe07.pdf 1,027KB
This paper describes a novel optical imaging method, deep illumination Angular Domain Imaging (ADI), for detecting micron-scale objects within highly scattering media. The new optical imaging is a much simpler and less expensive solution compared to other available optical imaging techniques. In principle, deep illumination ADI uses collimation detection capabilities of small acceptance angle devices to extract photons emitted from the scattered light created by a laser source aimed deep beneath the turbid medium surface. The laser source forms an illumination-ball within the medium that emits scattered light in all directions and illuminates objects near the surface from behind. Consequently, when photons from this illumination-ball pass an object and reach the angular filter, light that is not subsequently scattered passes through to a camera detector, whereas scattered photons are rejected by the filter.  Image results obtained are recorded for different phantom locations, phantom sizes, and medium scattering levels. Our images clearly display sub-204 µm phantoms when placed three millimeters deep within a scattering medium with total effective attenuation coefficient up to 5.8 /cm. Preliminary digital image processing shows the image contrast enhancement and the definition improvement.
G. H. Chapman, Member,  M. Trinh, N. Pfeiffer, G. Chu and D. Lee, “Angular Domain Imaging of Objects Within Highly Scattering Media Using Silicon Micromachined Collimating Arrays”, IEEE J. Special Topics on Quantum Electronics, v9, no. 2, 257-266 (2003) jstqe03.pdf 1,105KB
Optical imaging of objects within highly scattering media, such as tissue, requires the detection of ballistic/quasiballistic photons through these media. Recent works have used Phase/Coherence Domain or Time Domain Tomography (femtosecond laser pulses) to detect the shortest path photons through scattering media.  This work explores an alternative, Angular Domain Imaging, which uses collimation detection capabilities of small acceptance angle devices to extract photons emitted aligned closely to a laser source. It employs a high aspect ratio, micromachined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200:1) containing 51x1000 micron etched channels with 102 micron spacing over a 10 mm silicon width.  With precise array alignment to a laser source, unscattered light passes directly through the channels to the CCD detector and the channel walls absorb the scattered light at angles >0.29°. Objects within a scattering medium were scanned quickly with a computer-controlled Z-axis table. High-resolution images of 100 mm wide lines and spaces were detected at scattered-to-ballistic ratios of 5E5:1, with objects located near the middle of the sample seen at even higher levels.  At >5E6:1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction.  Monte-Carlo simulation programs designed to test the Angular Domain Imaging concept showed that the collimator detects the shortest path length photons, as in other Optical Tomography methods. Furthermore the collimator acts as an optical filter to remove scattered light while preserving the image resolution.  Simulations suggest smaller channels and longer arrays could enhance detection by >100.
M. Tank and G.H. Chapman, "Micromachined Silicon Collimating Detector Array to View Objects in Hightly Scattering Medium", Can Jour Elec. & Comp. Eng, .25, no. 1, 13-18 (Jan. 2000), cjece00g.pdf  8069KB
Materials such as turbid liquids and tissues absorb light very weakly, but scatter it very heavily. Detection of objects within such mediums is difficult as the scattered light obscures their presence.  However using laser light and a very high aspect ratio Silicon Micromachined Collimator Array (SMCA) aligned to a CCD array, objects can be detected through a centimeter of a highly scattering medium.
When a collimated laser beam enters a random turbid, high-scattering medium, the photons spread into ballistic (unscattered), scattered and absorbed components. Consider light (ballistic photons) entering a very high aspect ratio (200:1) micromachined collimating array consisting of 50 micron wide etched channels with 100 micron spacing over a 10 mm width of silicon. If the array is carefully aligned to the laser source, the unscattered laser light passes directly through the channels to the CCD, and the channel walls absorb the scattered light that has an angle greater than 0.29 degrees. Images of objects before or within the scattering medium produce a contrast that can be detected even when the scattered light is more than 5,000,000 times greater than the unscattered light. This detection array offers a simple, feasible and direct technique to detect hidden objects in turbid and high scattering media.


Refereed Conference Proceedings

F. Vasefi, E. Ng, M. Najiminaini, G. Albert, B. Kaminska, G.H. Chapman, and J.J.L. Carson, “Angular Domain Spectroscopic Imaging of turbid media using silicon micro-machined micro-channel arrays”, accepted in Photonics West, BIOS10, Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues VIII, San Francisco, Jan. 2010
F. Vasefi, E. Ng, M. Najiminaini, B. Kaminska, G.H. Chapman, H. Zeng, and J.J.L. Carson, “Angle-resolved diffused scattered light spectroscopy using radial angular filter arrays” accepted for Photonics West, BIOS10, Optical Interactions with Tissue and Cells XXI, San Francisco, Jan. 2010
P.B. Tsui, G. Chiang, G.H. Chapman, N. Pfeiffer, B. Kaminska, “Spatiofrequency filters for imaging fluorescence in scattering media”, accepted for Photonics West, BIOS10, Optical Interactions with Tissue and Cells XXI, San Francisco, Jan. 2010
N. Pfeiffer, G.H. Chapman, and B. Kaminska, “Optical Imaging of Structures Within Highly Scattering Material Using an Incoherent Beam and a Spatial Filter”, accepted for Photonics West, BIOS10, Optical Interactions with Tissue and Cells XXI, San Francisco, Jan. 2010
M. Najiminaini, F. Vasefi, B. Kaminska., G.H. Chapman, and J.J.L Carson,. Macroscopic fluorescent lifetime imaging in turbid media using angular filter arrays. The 30th Annual International Conference of IEEE EMBS 2009, pp 5364-5368. Minneapoli, Sept. 2009.
F. Vasefi, B.S.L. Hung, B. Kaminska1, G.H. Chapman, and J.J.L. Carson, “Angular domain optical imaging of turbid media using enhanced micro-tunnel filter arrays”, Diffuse Optical Imaging II, v 7369, 73691N1-1N6, Munich, Germany, July 2009 diffuse_optical_imaging09.pdf  1,939KB
We experimentally characterized the angular distribution and proportion of minimally deviated quasi-ballistic (snake) photons versus multiply scattered photons in a homogenous turbid medium. The study examined the angular distribution of photons propagating through and exiting the highly scattering medium over a narrow range about the axis of a collimated light source in trans-illumination mode. The measurements were made using an angular  domain imaging system that employed one of five silicon micro-machined arrays of micro-tunnels each with a range of different acceptance angles and micro-tunnel structures. The balance between quasi-ballistic photons and unwanted multiply scattered photons accepted by the micro-machined angular filters was measured in order to determine the optimum range of acceptance angles for the system. The experiments were performed in tissue mimicking phantoms using a 2-cm thick optical cell with 0.25% IntralipidTM and a near infrared laser. This paper also  presents experimental results of the angular domain imaging system employing novel micro-tunnel arrays with minimal internal reflection which can accept the non-scattered light exiting from the turbid medium within its small acceptance angle more efficiently. Our experiments reveal that image contrast was improved from 20% to 30% by employing an angular filter array with minimal internal reflection compared to conventional square-shaped filter arrays of identical size.
E. Ng, F. Vasefi, B. Kaminska, G.H. Chapman, and J.J. L. Carson, “Image contrast enhancement during time-angular domain imaging through turbid media by estimation of background scattered light”, Photonics West, BIOS09, Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues VII, v7182, 71821C1 – 1C12, San Jose, Jan 2009. photonw09bios_Ng_angular_distribution.pdf 8,112KB
Time-angular domain imaging (TADI) employs an angular filter array, which functions to accept quasi-ballistic photons with trajectories near the axis of a collimated light source. At high scattering coefficients, image contrast declines due to background signals from scattered photons that have trajectories compatible with the angular filter array. We attempted to correct for the background signal using a temporal discrimination technique  and image subtraction. During TADI through turbid media, photons at early arrival times represent a mixture of quasi-ballistic and scattered photons, while late arriving photons represent scattered photons. We captured two TADI images of a resolution target suspended midway through a 2 cm thick cuvette filled with 0.30% Intralipid. A 780 nm, 100 ps pulsed laser (PicoTA, PicoQuant) was used to trans-illuminate the cuvette. Detection was performed after the angular  filter array (500 elements with 60 µm × 60 µm square-shaped cross section and 1 cm length) with a gated camera (Picostar HR, LaVision). The first TADI image was collected at a short gate delay with respect to the minimum transit time, and resulted in a projection of the target. A long gate delay was used to collect the second TADI image and the projection of the target was not apparent. A corrected image (two - one) was digitally computed. Analysis of the first image compared to the corrected image revealed a 2.1-fold increase in contrast-to-noise ratio for the corrected image. Therefore, images collected with TADI were improved by processing successive images at different gate delays.
F. Vasefi, B. Kaminska, J.J.L. Carson, and G.H. Chapman, “Angular domain florescent lifetime imaging (ADFLIM) in turbid media”, BIOS09, Photonics West, Multiphoton Microscopy in the Biomedical Sciences IX, v71830I1-0I9, San Jose, Jan 2009  photonw09bios_fartash_florescent.pdf 9,041KB
We describe a novel florescent lifetime imaging methodology applicable to fluorophores embedded in turbid media. The method exploits the collimation detection capabilities of an angular filter device to extract photons emitted by a fluorophore embedded at depth within the medium. A laser source is used to excite the fluorophore within the medium. Photons emitted by the fluorophore that are not scattered to a high degree pass through the angular filter array and are detected by the intensified CCD camera (200 ps minimum gate width). Scattered photons are rejected by the filter and do not pass through to the camera. We fabricated angular filter arrays using silicon bulk micromachining and found that an array of 80 µm square aperture micro-tunnels, 1.5 cm in length accepted photons with trajectories within 0.4° of the axes of the micro-tunnels. The small acceptance angle rejected most of the scattered light exiting the turbid medium.
F. Vasefi, B. Kaminska, J.J.L. Carson, and G.H. Chapman, “Effect of time gating and polarization discrimination of propagating light in turbid media in Angular Domain Imaging (ADI)”, Photonics West, BIOS09, Imaging, Manipulation, and Analysis of Biomolecules, Cells, and Tissues VII, v7182, 718217-1 – 17-10, San Jose, Jan 2009.  photonw09bios_fartash_time_gating.pdf  1,741KB
Angular Domain Imaging (ADI) employs an angular filter array to accept photons within a small acceptance angle along the axis of an aligned laser light source and preferentially reject scattered light. Simulations show that the accepted photons travel the shortest paths between source and detector and are therefore the earliest to arrive. We fabricated angular filter arrays using silicon bulk micromachining and found that an array of 60 µm square shape microtunnels 1 cm in length accepted photons within 0.48 degree of axis of the micro-tunnels. This small acceptance angle rejected most of the scattered light and sub-millimeter resolution targets could be resolved in a few centimeters of turbid medium with at least six times reduced mean free path. ADI through media with higher scattering coefficients was not achievable due to unwanted acceptance of late arriving scattered photons. To reject the late arriving photons, we added time-domain filtration and linear polarization to ADI. The implementation of a time-gated camera, a 780 nm femtosecond pulsed laser, and linear polarization to our ADI system resulted in improved image contrast. The use of ADI with time-gating (gate width 250 ps) and linear polarization enabled visualization of sub-millimeter absorbing objects with approximately eight times higher  image contrast compared to ADI in a scattering medium equivalent  to six times reduced mean free path.
F. Vasefi, B. Kaminska, J.J.L. Carson, K. Jordan and G.H. Chapman, “Angular domain optical projection in turbid media”, Photonics West, BIOS09, Optical Tomography and Spectroscopy of Tissues VIII, v 7174, 71740D1-0D10, San Jose, Jan 2009. photonw09bios_fartash_optical_projection.pdf 785KB
Most high-resolution optical tomography techniques employ coherence domain or time domain methodologies to capture non-scattered photons in turbid media. Angular Domain Optical Projection Tomography (ADOPT) uses an angular filter array (AFA) to observe photons that propagate through a specimen with small angular deviation. We constructed an ADOPT system consisting of an AFA micro-machined silicon micro-tunnel array with each micro-tunnel 60 µm  wide, 60 µm high, 10 mm long, and separated by 5 µm thick walls. The range of acceptance angles was 0° to 0.5°. The system also included an 808 nm CW diode laser, beam shaping optics, a sample cuvette, a Keplerian lens system, and a CMOS camera. Testing was performed with a target consisting of two graphite rods (0.9 mm diameter) suspended in the cuvette by a rotation stage. The target was placed in a manner that the line of laser light was perpendicular to the long axis of the rods. A multitude of projections were collected at increments of 1.8° and compiled into a sinogram. A transverse image was reconstructed from the sinogram using filtered backprojection. The submillimeter targets embedded in the 2 cm thick scattering medium (reduced scattering coefficient <= 2.4/cm) were discernable in both the sinograms and the reconstructed images. The results suggest that ADOPT may be a useful technique for tomographic imaging of thick biological specimens (i.e. up to 8 mm across).
F. Vasefi, B. Kaminska, J.J.L. Carson, and G.H. Chapman, “Angular distribution of quasi-ballistic light measured through turbid media using angular domain optical imaging” Photonics West, BIOS09, Optical Interactions with Tissue and Cells XX, v7175, 717509-1 – 09, San Jose, Jan 2009. photonw09bios_fartash_image_contrast.pdf 1,585KB
We experimentally characterized the angular distribution and proportion of minimally deviated quasi-ballistic photons versus multiply scattered photons in a turbid medium. The study examined the angular distribution of photons propagating through  and exiting the highly scattering medium over a narrow range  about the axis of a collimated light source in trans-illumination  mode. The measurements were made using an angular domain imaging  system that employed one of three silicon micro-machined arrays of micro-tunnels each with a different range of acceptance angles. The balance between quasi-ballistic photons and unwanted multiply scattered photons accepted by the micro-machined angular filters was measured in order to determine the optimum range of acceptance angles for the system. The experiments were performed in tissue mimicking phantoms using a 1-cm thick optical cell with 0.7% Intralipid and an 808 nm diode laser. A maximum spatial resolution and contrast of 150 µm and 6% were observed by selecting the proper acceptance angle, respectively. Image contrast was improved further (about 10 times) by subtraction of the background signal representative of the multiply scattered photons by inserting an optical wedge into the light path. The measurements indicated that the highest spatial resolution  and contrast for trans-illumination images through the sample were obtained for the angular filter with an angular acceptance range of 0.4° to 0.5°. It was concluded that this range of acceptance angles was small enough to attenuate the majority of multiply scattered photons, but still accept a significant proportion of quasi-ballistic photons, thereby optimizing both image resolution and contrast.
P. Tsui, G.H. Chapman, R.L.K. Cheng, N. Pfeiffer, B. Kaminska, and F. Vasefi, “Spatiofrequency Filter in Turbid Medium Enhanced by Background Scattered Light Subtraction from a Deviated Laser Source”, Photonics West, BIOS09, Optical Interactions with Tissue and Cells XX, v. 7175, 71750A1-0A12, San Jose, Jan 2009 photonw09bios_tsui.pdf  5,894KB
Angular Domain Imaging is an optical tomography technique that filters out scattered light by accepting only photons with small deviation angles from their original trajectories. Previously, angular filters of linear collimating array (0.29° acceptance) or spatiofrequency filter of a +50mm lens with a 214um aperture (0.25° acceptance) were used. In the linear collimating array system, using a wedge prism to deviate the light source by 2-3x the acceptance angle creates a second image of only the scattered components which can then be subtracted from the filtered image to enhance detectability. We now apply this technique to the spatiofrequency filter system at an angle 2x the acceptance. Utilizing several wavelengths of laser sources with different beam symmetries, test phantoms are placed in a 5cm thick sample of diluted intralipid solution, with a maximum SR of 1.64×106:1 (µs' = 1.8/cm). By digitally subtracting the background scattered light, test phantoms previously unobservable are now distinguishable. Using background subtraction, the SR limitation of the SFF system improves 3x under full illumination and ~40x under line of light illumination. The improvement under partial illumination is similar to the result using the collimator array, but with resolution limited by the optics used in the system.
N. Pfeiffer, P.K.Y. Chan,, G.H. Chapman, F. Vasefi and B. Kaminska, “Optical Imaging of Structures Within Highly Scattering Material Using a Lens and Aperture to Form a Spatiofrequency Filter”, Photonics West, Optical Interactions with Tissue and Cells XIX, v6854, pp 68541D1-1D12, San Jose, Jan 2008. photonw08bios-aperture.pdf 1,720KB
Angular Domain Imaging (ADI) is a high resolution, ballistic imaging method that utilizes the angular spectrum of photons to filter multiply-scattered photons which have a wide distribution of angles from ballistic and quasi-ballistic photons which exit a scattering medium with a small distribution of angles around their original trajectory.  Such spatial gating has been previously accomplished using a scanning array of collimating holes micromachined into a silicon wafer section.  We now extend that work to include using a wide-beam, full-field, converging lens and pinhole aperture system to capture images in a single exposure.  We have developed an analysis of resolution and sensitivity trade-offs of such a system using Fourier optics theory to show that the system resolution is primarily governed by collimation ability at larger aperture sizes and by spatiofrequency (Fourier space-gated) filtering at smaller aperture sizes.  It is found that maximum sensitivity is achieved when spatiofrequency resolution and collimation resolution are equal. Planar, high contrast, phantom test objects are observed in 5 cm thick media with effective scattered to ballistic photon ratios >1.25×107:1 using a wide-beam, full-field lens and aperture system.  Comparisons are made between ballistic imaging with the lens and aperture system and with the scanning silicon micromachined collimating array.  Monte-Carlo simulations with angular tracking validate the experimental results.
F. Vasefi, G.H. Chapman, P.K.Y. Chan, B. Kaminska and N. Pfeiffer, “Enhanced Angular Domain Optical Imaging by Background Scattered Light Subtraction from a Deviated Laser Source”, Photonics West, Optical Interactions with Tissue and Cells XIX, v6854,  pp 68541E1-1E12, San Jose, Jan 2008. photonw08bios-wedge.pdf 605KB
Imaging structures within a turbid medium using Angular Domain Imaging (ADI) employs angular filter array aligned to a laser source to separate ballistic and quasi-ballistic photons from the highly scattered light by means of angular filtration. The angular filter consists of a high aspect ratio linear array of silicon micromachined tunnels, 51 micron wide by 10 mm long with a 0.29 degree acceptance angle.  At heavy scattering ratios of >1E7:1 image detectability declines due to the non-uniform scattered background light fraction still within the acceptance angle.  This scattered signal can be separated out by introducing a wedge prism to deviate the laser source where it enters the medium by an angle slightly larger than the acceptance angle. This creates a second image consisting of pure scattered photons with the filtration characteristics of the angular filter, and a pixel by pixel correspondence to the fully scattered illumination emitted from the medium. Experiments used an 808 nm laser diode, collimated to an 8×1 mm line of light, entering a 5 cm thick medium with a scattering ratio of >1E6:1, with a wedge prism creating a 0.44 degree deviation.  Digitally subtracting the deviated scattered signal from the original image significantly reduced the scattered background and enhanced image contrast. We can obtain images with scattering ratios at least 40 times more than our previous scattering limits. The contrast level can be increased from 0.04 of the total dynamic range to over 0.50, depending on test phantom object location, which results in higher definition and visibility of our micro-scale test structures in the turbid medium. 
P.K.Y. Chan, F. Vasefi, G.H. Chapman, B. Kaminska, and N. Pfeiffer, “Multispectral Angular Domain Optical Tomography in Scattering Media with Argon and Diode Laser Source”, Proc. Photonics West, Optical Interactions with Tissue and Cells XVIII, v6435,  64350M1-0M12, San Jose, Jan 2007. photonw07bios.pdf 3,303KB
Angular Domain Imaging (ADI) within highly scattering media employs micromachined angular filter tunnels to detect nonscattered photons which pass through the tunnels unattenuated while scattered photons collide with the tunnel walls. Each tunnel is micromachined approximately 51 µm wide by 10 mm long in silicon, giving a maximum acceptance angle of 0.29 degrees. The ADI technique is inherently independent of wavelength, and thus multispectral laser sources can be incorporated. Previous ADI experiments employed a 488-514 nm Argon ion laser source. This paper describes the construction of a new imaging system utilizing a high-power (up to 0.5 W) laser diode at the 670 nm wavelength, along with an aspheric and cylindrical lens system for shaping the beam into a collimated line of light. ADI results of biological samples (i.e. chicken breast tissue) are also presented. Image resolution is 204 µm or better in compressed chicken breast tissue approximately 3.8 mm in thickness. Digital image processing techniques are employed to improve image contrast, definition, and detectability of test phantoms. Because silicon is 40% reflective, scattered light at up to three times the acceptance angle is not sufficiently absorbed by the angular filter tunnels and contributes significant background noise, thus decreasing image contrast and detectability. Roughening of the tunnel surface using a NH4OH etchant solution scatters light hitting the walls, thus allowing it to be absorbed. Images after roughening show dramatic reductions in background scattered light levels between tunnels, suggesting that further experiments will make progress towards improved contrast and detectability of structures.
F. Vasefi, P. K.Y. Chan, B. Kaminska, and G. H. Chapman, "Deep Illumination Angular Domain Imaging Within Highly Scattering Media Enhanced by Image Processing", Proc. SPIE Optics East: Smart Medical and Biomedical Sensor Technology IV, v 6380 63800Q1-Q12 Boston, MA Oct. 2006 optics_east06.pdf  1,318KB
Deep Illumination Angular Domain Imaging employs a micromachined angular filter array to detect photons emitted from the scattered light created by a laser source aimed deep beneath the turbid medium surface. As this source light is scattered, a ball of illumination is formed within the medium. This deep illumination source emits scattered light in all directions and illuminates objects near the surface from behind. When photons from this illumination ball pass an object and reach the angular filter, light that was not subsequently scattered, passes through to a camera detector whereas scattered photons are rejected by the filter. The angular filter consists of an array of high-aspect ratio channels fabricated via silicon bulk micromachining. Under illumination by an argon ion (488-514 nm) laser, two-dimensional phantom test objects were observed in high scattering media up to 3 mm deep in the medium at effective scattering coefficients, µseff up to 5.8 cm-1. Scan results are reconstructed and enhanced using various image processing techniques to enhance the spatial image resolution and image contrast and to reduce noise.
F. Vasefi, P.K.Y. Chan, B. Kaminska, and G. H. Chapman, “Subsurface Bioimaging Using Angular Domain Optical Backscattering Illumination” Proc. 28th Inter. Conf. IEEE-Engineering in Medicine and Biology Society 2006. EMBS '06, 1932 - 1936, New York, Aug 2006. IEEE_embc_nyc06.pdf  911KB
While Coherence or Time Domain Optical tomography within highly scattering media observes the shortest path photons over the dominant randomly scattered background light, Backscattering Angular Domain Imaging employs micromachined collimators to detect photons within small angles of laser light reflected from the high scattering medium. These angular filters are composed of micromachined semicircular silicon collimator channels. By illuminating from the front side phantom test objects were observed in scattering media up to 3 mm deep in the medium at effective scattered to ballistic ratios from 1:1 to greater than 3E12:1. Results from carbon coating the collimators using a sputtering system to decrease internal reflectivity are shown.
F. Vasefi, B. Kaminska, G. H. Chapman, and P.K.Y. Chan, “Angular Domain Imaging for Tissue Mapping” Proc. IEEE-EMBS Conf. on Bio-Micro and Nanosystems, pg. 39 – 45, San Franscisco, Jan. 2006. embs06.pdf 3,066KB
The paper reviews the optical imaging techniques emphasizing the imaging approaches appropriate for tissue mapping. Angular Domain Imaging is introduced as a successful implementation of the optical imaging for tissue mapping that utilizes micromachined collimators detecting photons within small angles of aligned laser light sources. The experiments were undertaken with transillumination of the scattering medium and with backscattering of photons. The results are presented and discussed.
G.H. Chapman, J. Rao, T. Lui and N. Pfeiffer, “Enhanced Angular Domain Image in Turbid Medium using Gaussian Line Illumination”, Proc. SPIE Photonics West: Laser Interaction with Tissue and Cells XVII, v 6084, 60841D1-1D11,  San Jose, CA, Jan. 2006.
photonw06bios.pdf 4,648KB
Coherence or Time Domain Optical tomography within highly scattering media observes the shortest path photons over the dominant randomly scattered background light.  Angular Domain Imaging employs micromachined collimators detecting photons within small angles of aligned laser light sources.  These angular filters are micromachined silicon collimator channels 51 microns wide by 10 mm long on 102 micron spacing, giving an acceptance angle of 0.29 degrees at a CMOS detector array.  Phantom test objects were observed in scattering media 5 cm thick at effective scattered to ballistic ratios from 1:1 to greater than 1E8:1.  Line and space test objects detection limits are set by detector pixel size (5.5 microns) not collimator hole spacing.  To maximize the ballistic/quasi-ballistic photons observed, a line of light aligned with the collimator holes increases detectability by reducing the amount of scattered background light.  A Cylindrical Spherical Cylindrical beam expander/shrinker creates a 16 mm by 0.35 mm line of light.  Best results occur when the scattering medium, collimator and detector are within 3X the Rayleigh Range of the beam’s narrow vertical axis, allowing imaging of 51 micron lines/spaces at 3E8:1 scattering ratios.  Restricting the light to a 1 mm line extends this to 8E9:1.  Carbon coating the SMCA to reduce reflectivity shows that at high scattering levels absorbing walls will reduce background light, improving contrast.  ADI has also been shown to work when the illumination is unaligned with the detector.  This allows for side illumination with detection of structures at depths of 3mm with a scattering ratio of 1E6:1.
G.H. Chapman, P.K.Y. Chan, J. Dudas, J. Rao and N. Pfeiffer, “Angular Domain Image Detectability with Changing Turbid Medium Scattering Coefficients”, Proc. SPIE Photonics West: Laser Interaction with Tissue and Cells XVI, v, 5695, pg 160-171, San Jose, CA, Jan. 2005. photonw05bios-c.pdf 2,358KB
Optical tomography within highly scattering media has usually employed coherence domain and time domain imaging, which observe the shortest path photons over the dominant randomly scattered background light. An alternative, Angular Domain Imaging, employs micromachined collimators which detect photons within a small angle of the aligned laser light source.  These angular filters consist of micromachined silicon collimator channels 51 micron wide by 10 mm long on 102 micron spacing giving an acceptance angle of 0.29 degrees at a CCD detector.  Phantom test objects were observed in turbid mediums ranging from 1 to 5 cm thick at effective scattered to ballistic ratios from 1:1 to greater than  100,000,000:1.  Simple line and space test objects detection limits are set by detector pixel size not collimator hole spacing. Restricting the light emission to only the collimating array hole area provides increased detectability by reducing the amount of scattered light background.  This is best done using cylindrical spherical cylindrical lens beam expanders/shrinkers to create a wide line of light of small thickness aligned to the collimator array.  As object locations within the medium are moved closer to the detector/collimator, image detectability appears to depend on the scattering ratio after the test object rather than the total medium scattering.  Hence, objects located closer to the detector than the middle of the medium are observed at a much higher scattering levels than those nearer the light source
N. Pfeiffer, and G.H. Chapman, “Monte Carlo Simulations of the Growth and Decay of Quasi-Ballistic Photon Fractions with Depth in an Isotropic Medium” ”, Proc. SPIE Photonics West: Laser Interaction with Tissue and Cells XVI, v, 5695,   pg 136-147, San Jose, CA, Jan. 2005. photonw05bios-p.pdf  165KB
Quasi-ballistic or “snake” photons carry useable information on the internal structure of scattering mediums such as tissues.  By defining quasi-ballistic photons to be those photons that have been scattered but have not exceeded a specified radial distance threshold from their initial trajectory (equivalent to the resolving limit of the quasi-ballistic photons) and by using the Henyey-Greenstein phase function, Monte Carlo modeling has shown that the number of quasi-ballistic photons increases with depth in an isotropic scattering medium until a maximum is reached and then the quantity decreases.  The quantity of quasi-ballistic photons at a specified depth can be shown to be governed by two competing processes: the decay of ballistic photons into quasi-ballistic photons and the decay of quasi-ballistic photons into scattered photons.  These well-defined behaviors allow one to write a rate equation governing the growth and decay in the quantity of quasi-ballistic photons with depth.  It is found that as the anisotropy factor increases with forward scattering and as the resolution limit is widened, the quantity of quasi-ballistic photons begins to exceed the quantity of ballistic photons at a specified depth and the rate of decay of quasi-ballistic photon quantity decreases.  The development of a rate equation for the formation of quasi-ballistic photons allows one to analyze how efficient various detection methods are in extracting these quasi-ballistic photons, and it can be seen that there is a compromise between desired resolution and the effective scattering ratio at a detector.
N. Pfeiffer,  B. Wai, and G.H. Chapman,” Angular Domain Imaging of phantom objects within highly scattering mediums”  Proc. SPIE  Photonics West: Laser Interaction with Tissue and Cells XV, v 5319, San Jose, CA pp. 135-145, Jan. 2004 photonw04bios.pdf 1,250KB
Most optical tomography work within highly scattering media has employed coherence domain and time domain methodologies, both detecting the shortest path photons over the dominant randomly scattered background. Angular domain imaging instead uses micromachined collimators to observe only those photons within a small angle of the aligned laser light source, which simulations show are the shortest path photons, while rejecting heavily scattered light. These angular filters consist of micromachined silicon collimator channels 51 micron wide by 10 mm or 20 mm long on 102 micron spacing giving acceptance angles of 0.29 to 0.15 degrees on a CCD detector. Phantom test objects were observed in mediums ranging from 1 to 5 cm thick at scattered to ballistic ratios of 500,000:1 to 10,000,000:1 depending on the illumination pattern. Object detection was retained at the same scattering levels for either 1 cm or 5cm thick mediums, demonstrating little dependence on medium thickness. Detection was also independent of the object size: phantoms ranging from thin structures of 100 micron wide lines and spaces to 4 mm spheres were detected at approximately the same scattering ratios. Minimum size resolution depends on CCD pixel size, not the collimator characteristics. Furthermore, detection was a function of the scattering ratio produced after the phantom's position, not of the whole medium"s scattering ratio.  This means objects nearer the detector are much more observable. Longer collimators significantly increase the scattered light rejection. Monte-Carlo simulations with angular tracking demonstrate the object size independence and are undertaken to verify the other behaviors.
G.H. Chapman, M. Trinh, G. Chou, N. Pfeiffer and D. Lee, "Optical imaging objects within highly scattering material using Silicon Micromachined Collimating Arrays", SPIE Photonics West, Optical Tomography and Spectrocopy of Tissue V, San Jose, (BIOS) v4955, pg 464-473, Jan. 2003. bios03.pdf 1,495KB
Optically Tomography within highly scattering material has focused on Coherence Domain and Time Domain methods: both detecting the shortest path photons over the dominant randomly scattered background light. Angular Domain Imaging instead uses collimators, small acceptance angle filters, to observe only those photons closely aligned to a laser light source. A linear collimating array was fabricated using silicon surface micromachining consisting of 51 micron wide by 10 mm long etched channels with 102 micron spacing very high aspect ratio (200:1) 20 mm wide array. With careful array alignment to a laser source, restricted to a linear beam, the unscattered laser light passes directly through the channels to a CCD detector, and the channel walls absorb the scattered light at angles >0.29 degrees. With a computer controlled Z axis objects within a 1 cm thick scattering material were scanned quickly. High contrast 150 micron lines/spaces at the medium front were observed at scattered to ballistic photon ratios >5E5:1 with a 10 mm beam. Narrowing the beam to 130 micron width produces detectable images >3E8:1.  Objects closer to the detector were more visible, and mid point objects were detectable >1E9:1. Smaller channels and longer arrays should enhance detection by factors of >100.
G.H. Chapman, M.S. Tank, G. Chou and M. Trinh "Optical imaging objects within highly scattering mediums using Siliocn Micromachined Collimating Arrays", Proceedings SPIE Photonics West, BIOS Optical Fibers and sensors in Biomedical Applications II (BO10), v4616, pg  187-198,  San Josa, CA Jan. 2002. photonw02s12j.pdf 3173 KB
Optical imaging of objects within highly scattering media requires the detection of ballistic/quasiballistic photons through these media. Recent works have used Phase/Coherence Domain or Time Domain Tomography (femtosecond pulses) to detect the shortest path photons through scattering media. Our collimation detection uses Small Acceptance Angle Devices to extract photons emitted within a small source angle. This work employs a high aspect, micromachined collimating detector array fabricated by high-resolution silicon surface micromachining. Consider a linear collimating array of very high aspect ratio (200:1) containing 51x1000 microns etched channels with 102 microns spacing over a 10mm silicon width.  With precise array alignment to a laser source, unscattered light passes directly through the channels to the CCD detector and the channel walls absorb the scattered light at angles >0.29°. Objects within a scattering medium were scanned quickly with a computer-controlled Z-axis table. High-resolution images of 100 micron wide lines and spaces were detected at scattered-to-ballistic ratios of 500,000:1. At >5,000,000:1 ratios, a uniform background of scattered illumination degrades the image contrast unless recovered by background subtraction.  Simulations suggest smaller channels and longer arrays could enhance detection by factors greater than 100. Detection using Silicon Micromachined Collimating Arrays are also nearly wavelength independent.
G.H. Chapman, and D.E. Bergen, "A High Speed Lensless Integrated Proximity Sensor", Proceedings IEEE Int'l Conf. on Innovative Systems in Silicon, pg 126-135, Oct. 1997.  isis97.pdf  575KB
An integrated sensor array has been developed using lensless techniques that produces a high accuracy proximity measurement from contact to 18 mm. An integrated optical detector and processing circuitry chip was developed for this application. It locates the brightest spot within an optical array which will output the proximity position every 5.3 microsec.
G.H. Chapman, D.E. Bergen, T. Singh and T.S. Weeks, "Laser Hole Drilling in Thick Polypropylene sheets for Proximity Sensors", Lasers as Tools for Manufacturing of Durable Goods and Microelectronics, San Jose, SPIE Proceedings v2703, pg 421-429 (1996) photonw96.pdf  583KB
Laser micromachining of polypropylene for transducer applications has the advantage of creating small (<100 micron) structures through very thick materials (>400 micron). Normally translucent polypropylene formed using carbon as a dye is an excellent laser machining material having a high optical absorption, and a low thermal conductivity. For an optical alignment system a matrix of high aspect ratio holes of <130 microns diameter with <300 micron spacing was needed through thick (>400 micron) sheets. This alignment sensor is to be used on the end of a robot arm and will aid in the manipulation of the arm. Using an argon ion laser focused through a 50 mm lens (5.2 micron R(1/e^2) spot, 55.2 micron focal depth), holes as small as 30 microns on 150 micron spacing were achieved in 400-500 micron thick black polypropylene sheets with consistent results. Best results currently are achieved with a laser power of only 0.3 W, using 10-100 pulse stream of 10-100 microsec pulses, and duty cycles of <10%. Shorter duty cycles require more power, as do shorter pulse durations, and both result in larger holes at wider spacings. Minimum repeatable hole separation is controlled by the lip of material formed around the hole. These settings have achieved 41x29 (1189 hole) arrays on a sample, with a computer driven submicron XYZ positioning system. Commercially available opaque white polypropylene required 17 times the power, and achieved holes of only 127 microns, with 500 microns spacing in 500 micron thick material. Thicker (1mm) black polypropylene produces 144 micron holes on 500 micron spacings due to the lip material, and required a 100 mm lens.

Angular Domain Imaging Thesis

Darren Edward Bergen, "Development of Vision Skin: A Proximity and Imaging Sensors", Masters of Applied Science Thesis, Simon Fraser University, Burnaby, BC, Canada 1994  bergenthesis.pdf 1086KB
A new sensor called "Vision Skin" was built and studied in this thesis. Aimed at robotic applications it will give a high accuracy proximity distance measurement of approaching objects and also generate a low resolution image. The sensor operates in the near range from between 0 to 20 mm, from the surface of contact. The proximity mode operation uses laser triangulation of the object's surface in combination with the imaging capability. The sensor makes use of an array of small holes in thick material, called a mask. The mask was created in 500 micron thick polypropylene using an Argon ion laser. The holes produced were 30 microns in diameter with a hole to hole spacing of 150 microns. These small holes created small acceptance angles (3.4 degrees), thus restricting the amount of light seen by the sensor. With the mask the sensor needs no lenses for the imaging or proximity modes and thus no depth of focus problems occur in the close range. To measure proximity, a laser spot is projected onto the surface of the object to be observed. The hole with the largest intensity is the location over the sensor surface of the laser spot. The restricted light action of the mask means the location of the spot can be accurately determined and with the angle of the laser beam simple trigonometry can be used to calculate height. The tested prototype used modified charged coupled device (CCD) sensors with the masks placed on the photoarray. Experiments have shown the sensors have a proximity ranging from +/- 0.033 mm at 1 mm to +/- 0.067 mm at 2 mm using an interpolation of light intensities. At distances of 0 to 6 mm, the sensor is capable of imaging a simple object. An integrated circuit was designed in Mitel 1.5 micron CMOS to integrate both the proximity extraction and photodetection. The circuit was simulated, and submitted to CMC. Further work in the sensor requires integration of the laser, and faster acquisition hardware and software.
Gary Chu, "Probing Structures in Scattering Medium using a Silicon Micromachined Collimator Array", Bachelor of Applied Science Thesis, Simon Fraser University, Burnaby, BC, Canada 2001 gchuthesis.pdf 1,628KB
Optical Tomography (OT) is a much-researched area whose goal is to determine shapes and structures inside a turbid medium using light.  Using optical wavelengths for in tissue probing has the advantage of not exposing the user to any harmful radiation. The Silicon Micromachined Collimator Array (SMCA) is an array of long tubes with diameter to length ratio of 1:200 that blocks input light with angles greater than 0.29o.  When aligned with a laser source, the SMCA allows objects inside turbid media to be imaged directly.  The aims of the thesis are to develop a system to automate the data collection process, to characterize an existing SMCA, to adopt a light scattering Monte Carlo simulation for the SMCA application, and to use the simulation to analyze the SMCA.  The automation system reduced the time needed to capture one data image from hours to minutes.  Using the automation system, a set of images were captured and showed that the existing SMCA is able to discern structures inside a turbid medium up to a scattering level of 0.99999 (i.e. 105 scattered noise photons for every unscattered signal photon.)  The Monte Carlo simulation produced a number of numerical relationships between the design parameters of the SMCA to the characteristics of the medium and the performance of the SMCA.  These results provide directions for the future development of the SMCA.
Maria Tu Khanh Trinh, "Pushing the limits of Optical Tomography using a Silicon Micromachined Collimator Array". BASc thesis, Simon Fraser University, Burnaby, BC, Canada 2002 mtrinhthesis.pdf 3945K
Detecting objects optically within human tissue is difficult because this medium scatters most of the light that enters it, while absorbing very little. Optical Tomography (OT) is a method of imaging objects within highly scattering media, such as tissue, by extracting both the unscattered (ballistic) photons and slightly scattered (quasi-ballistic) photons from the light traveling through them. These ballistic and quasi-ballistic photons are then collected to reconstruct information about the structures and shapes inside the media, while the scattered photons are filtered out as noise.  Existing techniques use Coherence Domain Tomography (phase matching) or Time Domain Tomography (measured by femtosecond pulses) to separate the photons traveling the shortest path through the media. Both techniques require costly and cumbersome equipment, as well as complicated calibration.  This thesis presents a collimating technique that separates ballistic and quasi-ballistic photons from the scattered ones on the basis of a photon’s angle of deviation from the source laser beam.  This work employs a small-acceptance-angle device called a Silicon Micromachined Collimating Array (SMCA) fabricated by high-resolution surface micromachining.  The SMCA is a linear collimating array of very high aspect ratio (200:1) containing parallel tunnels that are 51 micron in diameter and 10 mm in length.  With precise array alignment to a laser source, unscattered photons pass directly through the tunnels and are detected by a Charge-Coupled Device (CCD), while the tunnel walls absorb the scattered photons traveling at angles greater than 0.29° to the source beam.  Objects within different scattering media are quickly scanned using a computer-controlled Z-axis table, which is capable of vertical, incremental movement.  This thesis presents improved fabrication techniques for the collimating devices and a superior experimental setup that substantially increases the system sensitivity.  The use of a laser beam with a linear cross-section has produced high-resolution images of objects embedded within media up to scattering ratios of 1E8:1 scattered-to-ballistic photons.  Images at even higher scattering levels are recovered by simple contrast enhancement.
Desmond A. Lee, "Imaging Objects at Various Depths in Scattering Mediums Using a Silicon Micromachined Collimating Array”, B.A.Sc Thesis, School of Engineering Sciences, Simon Fraser University, Burnaby, BC Canada, 2002  dleethesis.pdf 30,766K
Optical Tomography (OT) utilizes light as a means of imaging and has the potential to be an alternative to conventional methods of imaging organic tissue for the use in medical scanning.  X-rays have long been used for medical imaging but are inherently dangerous due to their ionizing effects on biological tissue.  Light at near infrared wavelengths can penetrate centimeters into human tissue and is safe at power levels below well known limits.  However, light passing through turbid mediums, such as soft tissue, becomes highly scattered.  The difficulty of extracting an image from light passing through turbid mediums is a direct result of the high probability of light scattering.  The key to OT is the separation of the non scattered light from the scattered light.  Non scattered light consists of photons that travel directly through the medium without deviation (ballistic) and photons that deviate slightly but remain close to their original path (quasi-ballistic).
Paulman Konn Yan Chan, " Angular Filters for Angular Domain Imaging Optical Tomography in Highly Scattering Media", MASc Thesis, School of Engineering Sciences, Simon Fraser University, Burnaby, BC Canada, 2008 Paulman_Chan_MAsc_Thesis.pdf 7,976 KB
Angular Domain Imaging performs Optical Tomography imaging through highly scattering media by rejecting scattered light through micromachined Angular Filter Array (AFA) tunnels or an aperture-based Spatial Filter (SF) while accepting non-scattered light with only small angular deviations from the source. AFA imaging using a laser diode source (λ = 670 nm) resolves 153 µm structures at a Scattering Ratio (SR) of 1E7:1 in a milk-based scattering solution and 204 µm patterns for chicken tissue ≤ 3.8 mm thick. Carbon deposition and NH4OH silicon roughening of the AFA tunnels is shown to reduce background scattered light. Smaller tunnel geometries and scan steps improve image resolution. A lens with aperture ADI Spatial Filtering was studied to verify theoretical predictions of the tradeoff between resolution and scattered light rejection. SF ADI with an Argon laser resolved 102 µm structures in a SR = 1.4×1E7:1 milk solution.
Nicholas Pfeiffer, "Imaging of turbid media using trajectory filter methods", PhD Thesis, Simon Fraser University, Burnaby, BC 2009, pfieffer_phdthesis.pdf 7,014KB
The new proposed solution for separation of ballistic and quasi-ballistic photons from scattered photons is through the use of a Silicon Micromachined Collimating Array (SMCA).  The SMCA is a linear array of 51 x 10000 micron tunnels etched into silicon using surface micromachining methods.  The 200:1 length to width aspect ratio of the SMCA creates an angle of acceptance of 0.29 degrees.  Thus, photons entering the SMCA tunnels at angles > 0.29 degrees will be absorbed and fail to reach the light detector.
This thesis explores the improvement of image visibility as test structures are moved to deeper depths in a scattering medium such that they are closer to the detector.  Test structures were observed in a 10mm thick test container at zero depth as well depths of 2mm to 8mm at 1mm increments.  The CCD detector is placed at the back of the test container (a 10mm depth).  Experimental results have shown that test structures in mediums with scattering ratios near the imaging limits produce images that increase in visibility the deeper in the medium they are.  Test structure observability is clearly higher at depths ranging from the center of the container to the back.  Maximum filterable limits of the SMCA have been experimentally shown to be competitive with competing methods of OT at a scattering ratio of 1E8:1.
Optical imaging through biological tissues and other scattering media is challenging, as the scattered light creates an extremely high background noise level that makes it difficult to detect objects that are embedded within the media. This thesis examines a relatively unexplored method of separating scattered light from unscattered light that has application to optical imaging through turbid media. The method creates an optical filter that blocks photons based upon their exit trajectory direction.  Such a trajectory filter can be used with a collimated beam that transmissively illuminates a scattering medium to create an imaging system in which a shadowgram is formed from those photons that pass through the filter and have a trajectory close to that of the collimated beam.  Experiments have shown that such a system is effective up to measured optical depths of 18 to 21 and scattering ratios of 108 to 109 using both coherent and incoherent sources.A micromachined linear array of 50 µm x 10 mm collimating holes was developed earlier as a photon trajectory filter and was used to successfully image through media in which the ratio of scattered to unscattered light is extremely high (>107).  These results are much better than simple theory would predict.  This thesis provides a theoretical basis for the trajectory filter system to allow its performance to be characterized and compared against other optical imaging methods, such as time-domain imaging.  Using Monte Carlo simulations, it is found that the trajectory filter method is more effective than pathlength-based methods for imaging through turbid media with moderate levels of scattering, up to ~20 optical depths, and that it can be combined with other imaging methods to further improve contrast.  Advantages of the trajectory filter method include coherence and wavelength invariance and the ability to perform either wide beam, full-field or narrow beam, scanned imaging.  Experimental results are presented for laser and incoherent beams using two types of trajectory filters: spatiofrequency and linear collimating hole array.  It is found that the trajectory filter method offers a viable means of transmissively imaging through moderately scattering media at optical and near infrared wavelengths.

Monte Carlo Photon Transport Simulator Software

Photon Transport Simulator Manuals
N. Pfeiffer, “Development of software tools for simulation of photon transport in scattering and absorbing media: the Photon Transport Simulator. ENSC 892 project report, Simon Fraser University, Burnaby, BC 2002 photon_transport_simulator_report.pdf 951 KB
Photon Transport Simulator User Manual v100.pdf  662KB
Development of NURB Surface and Visualization Extensions to the Photon Transport Simulator (Pfeiffer, 2003).pdf 239KB
Angular domain imaging (ADI) has been suggested as an alternative to time domain imaging (TDI) and coherence domain imaging (CDI) for optical tomography  applications.  Prior experimental results have established the viability of ADI as a method of achieving imaging through highly scattering mediums up to scattering ratios of approximately 1E8.

Prior work by the author and others at Simon Fraser University (SFU) have demonstrated the usefulness of numerical simulation of the photon transport process in understanding experimental data.  Such numerical simulation has been conducted using extensions of known Monte Carlo software.  However, the limitations of that software toolset have restricted the range of models capable of being investigated.

This report describes a new software tool, the Photon Transport Simulator (PTS), that has been developed by the author specifically for investigating angular domain imaging problems.  The new software, written in the ANSI C language, allows a wide range of photon transport problems to be investigated.  It includes the ability to define scattering and absorbing media of arbitrary geometry (rectangles, circles, cubes, cylinders, and semi-spheres), location, and orientation.  Each medium may have different optical properties such as scattering coefficient, absorption coefficient, and index of refraction.  The mediums may intersect, allowing collimating holes to be defined in absorbing media, or diverging lenses to be defined by the superposition of a semi-sphere of air on a cube of glass, for example.

The PTS software toolset is object oriented and has three primary types of objects: mediums, photon sources, and density maps.  Each object can be of arbitrary geometry and multiple objects are allowed.

The software tool also allows a flexible method of working with human-readable input and output files that define the model and a wide variety of user-configurable methods of reporting results.  Results can include multiple photon density maps on arbitrary planes through the model with each density map providing a correlation between user-specified photon parameters.  In addition, photon position history can be recorded for specified photons for post-processing.

Testing of the new software tool with a variety of test cases has shown that the results are comparable to those obtained with other known Monte Carlo software.

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Last updated  Aug 5, 2010