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.
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 (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.
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.
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.
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.
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