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.
Photoacoustic imaging (PAI) has been proposed as a non-invasive technique for imaging neonatal brain injury. Since
PAI combines many of the merits of both optical and ultrasound imaging, images with high contrast, high resolution, and
a greater penetration depth can be obtained when compared to more traditional optical methods. However, due to the
strong attenuation and reflection of photoacoustic pressure waves at the skull bone, PAI of the brain is much more
challenging than traditional methods (e.g. near infrared spectroscopy) for optical interrogation of the neonatal brain. To
evaluate the potential limits the skull places on 3D PAI of the neonatal brain, we constructed a neonatal skull phantom
(1.4-mm thick) with a mixture of epoxy and titanium dioxide powder that provided acoustic insertion loss (1-5MHz)
similar to human infant skull bone. The phantom was molded into a realistic infant skull shape by means of a CNCmachined
mold that was based upon a 3D CAD model. To evaluate the effect of the skull bone on PAI, a photoacoustic
point source was raster scanned within the phantom brain cavity to capture the imaging operator of the 3D PAI system
(128 ultrasound transducers in a hemispherical arrangement) with and without the intervening skull phantom. The
resultant imaging operators were compared to determine the effect of the skull layer on the PA signals in terms of
amplitude loss and time delay.
Photoacoustic imaging (PAI) has been proposed as a non-invasive technique for the diagnosis and monitoring of
disorders in the neonatal brain. However, PAI of the brain through the intact skull is challenging due to reflection and
attenuation of photoacoustic pressure waves by the skull bone. The objective of this work was to develop a phantom for
testing the potential limits the skull bone places on PAI of the neonatal brain. Our approach was to make acoustic
measurements on materials designed to mimic the neonatal skull bone and construct a semi-realistic phantom. A water
tank and two ultrasound transducers were utilized to measure the ultrasound insertion loss (100 kHz to 5MHz) of several
materials. Cured mixtures of epoxy and titanium dioxide powder provided the closest acoustic match to neonatal skull
bone. Specifically, a 1.4-mm thick sample composed of 50% (by mass) titanium dioxide powder and 50% epoxy was
closest to neonatal skull bone in terms of acoustic insertion loss. A hemispherical skull phantom (1.4 mm skull
thickness) was made by curing the epoxy/titanium dioxide powder mixture inside a mold. The mold was constructed
using 3D prototyping techniques and was based on the hairless head of a realistic infant doll. The head was scanned to
generate a 3D model, which in turn was used to build a 3D CAD version of the mold. The mold was CNC machined
from two solid blocks of Teflon®. The neonatal skull phantom will enable the study of the propagation of photoacoustic
pressure waves under a variety of experimental conditions.
The Radial Angular Filter Array (RAFA) is a novel optical filter consisting of a radially-distributed series of micromachined
channels with a focal length of a few millimeters. The RAFA filters photons passing through the focal point
according to the propagation direction and has proven to be capable of collecting the angular distribution and the spectral
information of photons simultaneously and non-invasively, which allows angle-resolved spectroscopic measurement of a
turbid medium. To explore the feasibility of using this device to characterize the optical abnormalities in human tissues,
we tested the performance of an angle-resolved RAFA-based spectroscopy system to detect absorption targets embedded
within a tissue-mimicking phantom. The body of the phantom was made of 0.1% IntralipidTM/agarose gel (7 mm in
thickness) and the targets were spherical (1.5 mm in radius) and contained 10 μM Indocyanine Green (ICG). The
illumination source was a broadband near infrared (NIR) collimated beam. Photons were angularly filtered by the RAFA
and spectrally resolved by a pushbroom spectrometer. The experimental results confirmed that the RAFA preferentially
filtered photons that carried absorption and scattering information of the embedded targets.
Angular Domain Spectroscopic Imaging (ADSI) is a novel technique for the detection and characterization of optical
contrast abnormalities in ex-vivo breast tissue samples based on spectral characteristics. The imaging system employs a
spatial filter called an angular filter array to reject scattered photons traversing a sample. The system employs an
imaging spectrometer to capture and discriminate the largely remaining quasi-ballistic photons based on spatial position
and wavelength. Spectral data were obtained from samples obtained from two patients, one sample contained invasive
mammary carcinoma, and the other one contained normal fat and fibrous tissue. Principal component analysis using
transmission absorption spectra obtained with ADSI was able to differentiate tumor versus normal tissue regions.
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.
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.
Quantum dots have been used in a wide variety of biomedical applications. A key advantage of these particles is that
their optical properties depend predictably on size, which enables tuning of the emission wavelength. Recently, it was
found that CdSe/ZnS quantum dots lose their ability to photoluminescence after exposure to gamma radiation (J. Phys.
Chem. C., 113: 2580-2585 (2009). A method for readout of the loss of quantum dot photoluminescence during exposure
to radiation could enable a multitude of real-time dosimetry applications. Here, we report on a method to image
photoluminescence from quantum dots from a distance and under ambient lighting conditions. The approach was to
construct and test a time-gated imaging system that incorporated pulsed illumination. The system was constructed from a
pulsed green laser (Nd:YAG, 20 pulses/s, 5 ns pulse duration, ~5 mJ/pulse), a time-gated camera (LaVision Picostar, 2
ns gate width), and optical components to enable coaxial illumination and imaging. Using the system to image samples
of equivalent concentration to the previous end-point work, quantum dot photoluminescence was measureable under
ambient room lighting at a distance of 25 cm from the sample with a signal to background of 7.5:1. Continuous exposure
of samples to pulsed laser produced no measureable loss of photoluminescence over a time period of one hour. With
improvements to the light collection optics the range of the system is expected to increase to several metres, which will
enable imaging of samples during exposure to a gamma radiation source.
Angular Domain Spectroscopic Imaging (ADSI) is a novel technique for the detection and characterization of optical
contrast abnormalities in a turbid medium. The imaging system employs silicon micro-machined angular filtering
methodology, which has high angular selectivity for photons exiting the turbid medium. The ADSI system performance
was evaluated on tissue samples from a dissected mouse. Images collected with the ADSI displayed differences in image
contrast between different tissue types.
Gold nanorods (AuNRs) are of interest for many biomedical applications due to their tunable optical properties. AuNRs
efficiently absorb light in the near-infrared (NIR) region, which induces effects such as hyperthermia and/or cell killing
by localized microbubble formation through photothermal conversion. Our objective was to study the potential of
AuNRs to elicit photothermal conversion effects due to pulsed laser exposure at depth within tissue-like phantoms. The
approach was to measure photothermal conversion in inclusion-containing phantoms representative of breast cancer.
Tissue-like phantoms were prepared with hemoglobin at 10 μM in a homogeneous mixture of 1% agarose and 1%
Intralipid to mimic the optical properties of human breast tissue. Polyethylene glycol AuNR-loaded gel spheres (at an
equivalent optical density of 0.67 at 800 nm) were prepared with hemoglobin at 20 μM in a homogeneous mixture of 1%
agarose and 1% Intralipid. The spherical gel inclusions were cast into the phantom material at a depth of 0, 5, 10, or 15
mm. Phantoms were then exposed to nanosecond pulsed-NIR light (800 nm; 5 ns pulse duration; 17-100 mJ/cm2; 10-
1000 pulse count). Each phantom was then cut longitudinally and imaged with a NIR camera. The images were
examined with image analysis software. Preliminary results indicated that the greatest extent of photothermal conversion
occurred in spherical AuNR-loaded gels next to the phantom surface. Based on these results, we concluded that within
ANSI limits of laser exposure photothermal therapy with AuNR-based agents will be limited to surface lesions and/or
lesions accessible with needle-based light delivery.