Nick Pfeiffer

Angular domain imaging of objects within highly scattering media using silicon micromachined collimating arrays

Optical imaging of objects within highly scattering media, such as tissue, requires the detection of ballistic/quasi-ballistic 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 51/spl times/1000 /spl mu/m etched channels with 102-/spl mu/m spacing over a 10-mm silicon width. With precise array alignment to a laser source, unscattered light passes directly through the channels to the charge coupled device detector and the channel walls absorb the scattered light at angles >0.29/spl deg/. Objects within a scattering medium were scanned quickly with a computer-controlled Z axis table. High-resolution images of 100-/spl mu/m-wide lines and spaces were detected at scattered-to-ballistic ratios of 5/spl times/10/sup 5/: 1, with objects located near the middle of the sample seen at even higher levels. At >5/spl times/10/sup 6/: 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.

Successive order, multiple scattering of two-term Henyey-Greenstein phase functions

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.

Angular Domain Optical Imaging of Structures Within Highly Scattering Material Using Silicon Micromachined Collimating Arrays

Optically Tomography within highly scattering material has focused on Coherence Domain and Time Domain methods: both detecting the shorting 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 μm wide by 10 mm long etched channels with 102 μm spacing very high aspect ration (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 μm lines/spaces at the medium front were observed at scattered to ballistic photon ratios >5×105:1 with a 10 mm beam. Narrowing the beam to 130 μm width produces detectable images >3×108:1. Objects closer to the detector were more visible, and mid point objects were detectable >109:1. Smaller channels and longer arrays should enhance detection by factors of >100.

Optical Imaging of Structures Within Highly Scattering Material Using a Lens and Aperture to Form a Spatiofrequency Filter

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.

Angular Domain Imaging Of Phantom Objects Within Highly Scattering Mediums

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 phantoms 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.

Enhanced angular domain imaging in turbid media using Gaussian line illumination

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 beams 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.

Multispectral Angular Domain Optical Tomography in Scattering Media with Argon and Diode Laser Sources

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 &mgr;m 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 &mgr;m 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 structures. 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.

Enhanced angular domain optical imaging by background scattered light subtraction from a deviated laser source

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 10mm long with a 0.29 degree acceptance angle. At heavy scattering ratios of >1E7 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 scattering 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 5cm thick medium with a scattering ratio of > 1E6, 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 have about images at least 40 times more of our previous scattering limits. Depending on test phantom object location, the contrast level can be increased from 4% of the total dynamic range to over 50% which results in higher definition and visibility of our micro-scale test structures in the turbid medium.

Spatiofrequency filters for imaging fluorescence in scattering media

Researchers have been using simple optics to image optically induced fluorescence in tissues. We now apply the Angular Domain Imaging technique using a Spatiofrequency filter which accepts only photons within a small deviation angle from its original trajectory to image a fluorescing medium beneath a scattering layer. A Rhodamine 6 G dye fluorescing layer or fluorescence slides, under an Intralipid scattering medium was used. By applying ADI with acceptance angle of 0.17°, the structures are distinguishable at low scattering depth depending of the emission wavelength of the fluorescence source. It was established previously that as the acceptance angle increases, the amount of scattered light/noise in the images increases, however, the resolution also deteriorates. Simulations using a Monte-Carlo program are done for both angular filters, Spatiofrequency filter and Linear Collimating Array. Due to the additional positional filtration on top of the angular filtration with Linear Collimating Array, collimators with aspect ratio as low as 10:1 can improve the quality of the fluorescence images significantly in both contrast ratio and resolution.

Fluorescence angular domain imaging of skin tissue phantoms using intralipid-infused solids

Optical imaging through biological tissue has the significant problems of scattering which degrades the image resolution and quality. Research has shown that Angular Domain Imaging (ADI) improves image quality by filtering out the scattered light in the biological tissue images based on the angular direction of photons. The advantage of this technique is that it is independent of the wavelength, coherent, pulse, or duration compared to OCT or time domain. This allows us to couple ADI with conventional fluorescence imaging technique. Previous work was creating test media by varying Intralipid/water concentration to produce different scattering levels. This showed difficulties in producing a consistent scattering medium in liquid states. Hence, ideally we want a reusable solid medium which has a stable scattering characteristic. Our target is to investigate fluorescence ADI on skin with cancerous collagen tissue where healthy collagen fluoresces while the cancerous collagen tissue does not. To mimic the characteristic of skin, a solid scattering medium over a patterned fluorescence material with non-emitting structures is created. We used a solid agar medium, or a transparent polymer, infused with Intralipid at different concentrations, as the scattering medium. The solid media with similar scattering characteristic of skin (μs = 20cm-1, g = 0.85) is placed on top of a fluorescence plastic (415nm excitation, ≈ 530nm emission) which is patterned by strips of non-emitting structures (200-400μm). Using small apertures with acceptance angles of 0.171° a distance away from the solid scattering medium, these non-emitting structures are detectable at shallow scattering tissue depth (1-2mm).