The advancement of angular domain imaging in mesoscopic reflectance multispectral imaging is reported. The key component is an angular filter array that performs the angular filtration of the back-scattered photons and generates image contrast due to the variances in tissue optical properties. The proposed modality enables multispectral imaging of subsurface features for samples too thick for transillumination angular domain spectroscopic imaging (ADSI) approaches. The validation was carried out with tissue-mimicking phantoms with multiple absorptive features embedded below the surface. Multispectral images in the range of 666 to 888 nm clearly revealed the location of the features with the background scattering levels up to 20 cm−1. The shape of the features was recoverable at depths of up to three to four times the transport mean free path. The spatial resolution was <1 mm and the field-of-view was larger than 2.5 cm×3.0 cm. Furthermore, the attenuation spectra of measured absorptive features were successfully extracted. Target detectability and imaging quality with different background scattering levels, target depths, and illumination focal depths were discussed, as well as the capability of ADSI in reflectance optical mesoscopic imaging and its potential applications.
Angular Domain Imaging (ADI) employs an angular filter to distinguish between quasi-ballistic and scattered photons
based on trajectory. A 2D angular filter array was constructed using 3D printing technology to generate an array of
micro-channels 500 μm x 500 μm with a length of 12 cm. The main barrier to 2D imaging with the 2D angular filter
array was the shadows cast on the image by the 500 μm walls of the angular filter. The objective of this work was to
perform a resolution analysis of the 2D angular filter array. The approach was to position the AFA with a two
dimensional positioning stage to obtain images of areas normally obstructed by the walls of the AFA. A digital light
processor was also incorporated to generate various light patterns to improve the contrast of the images. A resolution
analysis was completed by imaging a knife edge submerged in various uniform scattering media (Intralipid® dilutions
with water). The edge response functions obtained were then used to compute the line spread function and the theoretical
resolution of the imaging system. The theoretical system resolution was measured to be between 110 μm - 180 μm when the scattering level was at or below 0.7% Intralipid®. The theoretical resolution was in agreement with a previous
resolution analysis of a silicon-based angular filter with a similar aspect ratio. The measured resolution was also found to
be smaller than the size of an individual channel, suggesting that the resolution of an AFA based ADI system is not
dependent on the size of the micro-channel.
Angular Domain Imaging (ADI) is a technique that is capable of generating two dimensional shadowgrams of
attenuating targets embedded in a scattering medium. In ADI, an angular filter array (AFA) is positioned between the
sample and the detector to distinguish between quasi-ballistic photons and scattered photons. An AFA is a series of
micro-channels with a high aspect ratio. Previous AFAs from our group were constructed by micro-machining the
micro-channels into a silicon wafer, limiting the imaging area to a one dimensional line. Two dimensional images were
acquired via scanning. The objective of this work was to extend the AFA design to two dimensions to allow for two
dimensional imaging with minimal scanning. The second objective of this work was to perform an initial
characterization of the imaging capabilities of the 2D AFA. Our approach was to use rapid 3D prototyping techniques to
generate an array of micro-channels. The imaging capabilities were then evaluated by imaging a 0.9 mm graphite rod
submerged in a scattering media. Contrast was observed to improve when a second angular filter array was placed in
front of the sample to mask the incoming light.
Angular Domain Imaging (ADI) is an imaging technique that is capable of generating three dimensional images of
attenuating targets embedded in a scattering medium. In ADI, an angular filter is positioned between the sample and the
detector to discriminate between quasi-ballistic photons and scattered photons. Quasi-ballistic photons have undergone
relatively few forward directed scattering events, and can be used to generate a projection image representative of the
imaging target. Scattered photons have undergone many scattering events and contain little information regarding the
imaging target, thereby leading to decreased image contrast. Our implementation of ADI utilizes a silicon micro-channel
array to reject scattered photons based on the angle at which they exit the sample. The objective of this work was to
collect ADI images with a tunable pulsed laser within the visible range. Samples were illuminated at 13 wavelengths
between 460 nm and 700 nm. An angular filter array of 80 μm × 80 μm tunnels 2-cm long was used to select the quasiballistic
photons. Images were detected with a linear 16-bit linear CCD. The phantom consisted of a 0.7 mm attenuating
target submerged in one of four IntralipidR dilutions (0.15%-0.3%) contained within a 1 cm path length cuvette. Image
contrast ranged from 0.02 at 460 nm and 0.3% IntralipidR to 1 at 680 nm at 0.15% IntralipidR. For a given scattering
level, contrast increased at longer wavelengths. Resolution varied minimally with wavelength. The results suggested that
multispectral ADI with a tunable pulsed laser is feasible and may find utility in imaging thin tissue samples in the future.
Angular domain spectroscopic imaging (ADSI) is a novel technique for the detection and characterization of optical contrast in turbid media based on spectral characteristics. The imaging system employs a silicon micromachined angular filter array to reject scattered light traversing a specimen and an imaging spectrometer to capture and discriminate the largely remaining quasiballistic light based on spatial position and wavelength. The imaging modality results in hyperspectral shadowgrams containing two-dimensional (2D) spatial maps of spectral information. An ADSI system was constructed and its performance was evaluated in the near-infrared region on tissue-mimicking phantoms. Image-based spectral correlation analysis using transmission spectra and first order derivatives revealed that embedded optical targets could be resolved. The hyperspectral images obtained with ADSI were observed to depend on target concentration, target depth, and scattering level of the background medium. A similar analysis on a muscle and tumor sample dissected from a mouse resulted in spatially dependent optical transmission spectra that were distinct, which suggested that ADSI may find utility in classifying tissues in biomedical applications.
Angular Domain Spectroscopic Imaging employs an array of micro-channels to perform angular filtering of light that
traverses a turbid sample to reject moderately to highly scattered light. In this work, we experimentally characterized an
ADSI system by measuring transmission spectra and the first and second derivatives obtained from absorbing and
scattering targets. The derivative analysis was used to estimate the concentration of indocyanine green mixed in a
scattering liquid. The experimental results provided support for ADSI as a potential method for quantitative
spectroscopic imaging of ex vivo tissue samples.
Angular Domain Imaging (ADI) has been previously demonstrated to generate projection images of attenuating targets
embedded within a turbid medium. The imaging system employs a silicon micro-tunnel array positioned between the
sample and the detection system to reject scattered photons that have deviated from the initial propagation direction and
to select for ballistic and quasi-ballistic photons that have retained their forward trajectory. Two dimensional
tomographic images can be reconstructed from ADI projections collected at a multitude of angles. The objective of this
work was to extend the system to three dimensions by collecting several tomographic images and stacking the
reconstructed slices to generate a three dimensional volume representative of the imaging target. A diode laser (808nm,
CW) with a beam expander was used to illuminate the sample cuvette. An Angular Filter Array (AFA) of 80 μm × 80
μm square-shaped tunnels 2 cm in length was used to select for image forming quasi-ballistic photons. Images were
detected with a linear CCD. Our approach was to use a SCARA robot to rotate and translate the sample to collect
sufficient projections to reconstruct a three dimensional volume. A custom designed 3D target consisting of 4 truncated
cones was imaged and reconstructed with filtered backprojection and iterative methods. A 0.5 mm graphite rod was used
to collect the forward model, while a truncated pseudoinverse was used to approximate the backward model for the
iterative algorithm.
The angular filter array (AFA) is a silicon micro-machined optical collimator, which only accepts photons propagating
within a narrow solid angle. It can be used to select photons exiting an imaging sample along a specific direction. This
paper describes a novel Angular Domain Spectroscopic Imaging (ADSI) technique that utilizes deep illumination from
the front surface of the sample and a camera with an AFA to image features embedded inside a turbid medium. This
approach permitted spectroscopic imaging of turbid samples too thick to be imaged in a trans-illumination setup. The
tissue-mimicking test phantom contained three groups of Indocyanine Green doped inclusions at depths from 1 to 3 mm
embedded within an IntralipidTM/agarose gel. The sample was scanned across the AFA and the intensity of the back
scattered light along the direction normal to the surface was acquired as a function of location and wavelength. The
resultant spectral images were captured and analyzed. The experiments demonstrated that ADSI could detect subsurface
features that differed in wavelength-dependent absorption and/or scattering properties from the surrounding medium
with the deep illumination configuration. Deep illumination ADSI may be useful as a non-invasive tissue imaging tool.
By employing high-aspect-ratio parallel microchannels as an angular filter, quasiballistic photons sensitive to internal structures in a turbid medium can be captured. Scattered photons exiting the turbid medium typically exhibit trajectories with random angles compared to the initial trajectory and are mostly rejected by the filter. However, angular filter arrays cannot differentiate between quasiballistic photons (early arriving) and photons that happen to attain a scattered trajectory that is within the acceptance angle (late arriving). Therefore, we have two objectives: (1) to experimentally characterize the angular distribution and proportion of minimally deviated quasiballistic photons and multiply scattered photons in a turbid medium and (2) to combine time and angular gating principles so that early and late arriving photons can be distinguished. From the angular distribution data, the angular filter with angular acceptance about 0.4 deg yields the highest image contrast for transillumination images. The use of angular domain imaging(ADI) with time-gating enables visualization of submillimeter absorbing objects with approximately seven times higher image contrast compared to ADI in a turbid medium with a scattering level of six times the reduced mean free path.
This paper presents a novel optical filter called the Radial Angular Filter Array (RAFA) for real-time measurement of
the angular and spectral distribution of diffuse light exiting a turbid medium. The RAFA consists of a radiallydistributed
series of 48 micro-channels micro-machined into a silicon substrate. To test the device, we constructed an
angle-resolved spectroscopy system by integrating a wideband light source, the RAFA, and an imaging spectrometer.
The collimated broadband light source was configured to trans-illuminate a turbid sample over a wide range of
wavelengths in the near infrared spectral region. The RAFA was used to collect the angular distribution of light exiting
the turbid sample. The imaging spectrometer decomposed the output of the RAFA into hyperspectral images
representative of scatter angle and wavelength. By scanning the RAFA and imaging spectrometer over the sample, the
intensity of the scattered light was acquired as a function of location on the sample surface, wavelength, and angle
relative to the surface normal. With angle resolved spectroscopy it will be possible to characterize the optical properties
of turbid samples in great detail.
We experimentally characterized a novel Angular Domain Spectroscopic Imaging (ADSI) technique for the detection
and characterization of optical contrast abnormalities in turbid media. The new imaging system employs silicon micromachined
angular filtering methodology, which has high angular selectivity for photons exiting the turbid medium. The
angular filter method offers efficient scattered light suppression at moderate levels of scattering (i.e. up to 6 reduced
mean free paths). An ADSI system was constructed from a broadband light source, an Angular Filter Array (AFA), and
an imaging spectrometer. The free-space collimated broadband light source was used to trans-illuminate a turbid sample
over a wide range of wavelengths in the near infrared region of the spectrum. The imaging spectrometer decomposed the
output of the AFA into hyperspectral images representative of spatial location and wavelength. It collected and angularly
filtered a line image from the object onto the CCD camera with the spatial information displayed along one axis and
wavelength information along the other. The ADSI system performance was evaluated on tissue-mimicking phantoms as
well as fresh chicken breast tissue. Collected images with the ADSI displayed differences in image contrast between
different tissue types.
Photoacoustic imaging is a hybrid imaging modality capable of producing contrast similar to optical
imaging techniques but with increased penetration depth and resolution in turbid media by encoding the information
as acoustic waves. In general, it is important to characterize system performance by parameters such as sensitivity,
resolution, and contrast. However, system characterization can extend beyond these metrics by implementing
advanced analysis via singular value decomposition. A method was developed to experimentally measure a matrix
that represented the imaging operator for the system. Analysis of the imaging operator was done via singular value
decomposition so that the capability of the system to reconstruct objects and the inherent system sensitivity to those
objects could be understood. The results provided by singular value decomposition were compared to simulations
performed on an ideal system with matching transducer arrangement and defined object space.
Angular domain imaging (ADI) generates a projection image of an attenuating target within a turbid medium by
employing a silicon micro-tunnel array to reject photons that have deviated from the initial propagation direction. In this
imaging method, image contrast and resolution are position dependent. The objective of this work was to first
characterize the contrast and resolution of the ADI system at a multitude of locations within the imaging plane. The
second objective was to compare the reconstructions of different targets using filtered back projection and iterative
reconstruction algorithms. The ADI system consisted of a diode laser laser (808nm, CW, ThorLabs) with a beam
expander for illumination of the sample cuvette. At the opposite side of the cuvette, an Angular Filter Array (AFA) of
80 μm x 80 μm square-shaped tunnels 1 cm in length was used to reject the transmitted scattered light. Image-forming
light exiting the AFA was detected by a linear CCD (16-bit, Mightex). Our approach was to translate two point
attenuators (0.5 mm graphite rod, 0.368 mm drill bit) submerged in a 0.6% IntralipidTM dilution using a SCARA robot
(Epson E2S351S) to cover a 37x37 and 45x45 matrix of grid points in the imaging plane within the 1 cm path length
sample cuvette. At each grid point, a one-dimensional point-spread distribution was collected and system contrast and
resolution were measured. Then, the robot was used to rotate the target to collect projection images at several projection
angles of various objects, and reconstructed with a filtered back projection and an iterative reconstruction algorithm.
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.
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% IntralipidTM. 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.
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.
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