The Monte Carlo Command Line application (MCCL) is an open-source software package that provides Monte Carlo simulations of radiative transport through heterogeneous turbid media. MCCL is available on GitHub through our virtualphotonics.org website, is actively supported, and carries extensive documentation. Here, we describe the main technical capabilities, the overall software architecture, and the operational details of MCCL.
Researchers have been using single element transducers for photoacoustic microscopy (PAM), but such systems
have limited depth of field due to a single focus. The aim for this project was to develop a high-frequency annular
array transducer for improved depth-of-field PAM. We have designed a concave 40 MHz ultrasound transducer
which has 8 annular array elements with equal area. The outer ring is 12 mm in diameter, the geometric focus is 12
mm, and the space between each annulus is 100 μm. The array was fabricated by lithographically patterning
metalized polyimide film to define back electrodes and signal leads. 9-micron-PVDF film was then press-fit into the
array pattern with epoxy as a backing material and a single drop of epoxy as a bonding layer. The array exhibits high
sensitivity to high-frequency photoacoustic signals. Dynamic focusing of amplified and digitized signals permits
extended depth-of-field imaging compared to the single-element transducer case. Dark-field light-delivery and
3-axis motorized scanning permits 3-D photoacoustic microscopy. Imaging performance in phantoms is discussed.
Optical-resolution photoacoustic microscopy (OR-PAM) is a novel imaging technology for visualizing optically absorbing superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection. Since scanning of the illumination spot is required, OR-PAM imaging speed is limited by both scanning speed and laser pulse repetition rate. Unfortunately, lasers with high repetition rates and suitable pulse durations and energies are not widely available and can be cost-prohibitive and bulky. We are developing compact, passively Q-switched fiber and microchip laser sources for this application. The properties of these lasers are discussed, and pulse repetition rates up to 100 kHz are demonstrated. OR-PAM imaging was conducted using a previously developed photoacoustic probe, which enabled flexible scanning of the focused output of the lasers. Phantom studies demonstrate the ability to image with lateral spatial resolution of 7±2 µm with the microchip laser system and 15±5 µm with the fiber laser system. We believe that the high pulse repetition rates and the potentially compact and fiber-coupled nature of these lasers will prove important for clinical imaging applications where real-time imaging performance is essential.
Both photoacoustic imaging and power Doppler ultrasound are capable of producing images of the vasculature of living subjects, however, the contrast mechanisms of the two modalities are very different. We present a quantitative and objective comparison of the two methods using phantom data, highlighting relative merits and shortcomings. An imaging system for combined photoacoustic and high-frequency power Doppler ultrasound microscopy is presented. This system uses a swept-scan 25-MHz ultrasound transducer with confocal dark-field laser illumination optics. A pulse-sequencer enables ultrasonic and laser pulses to be interlaced so that photoacoustic and power Doppler ultrasound images can be coregistered. Experiments are performed on flow phantoms with various combinations of vessel size, flow velocity, and optical wavelength. For the task of blood volume detection, power Doppler is seen to be advantageous for large vessels and high flow speeds. For small vessels with low flow speeds, photoacoustic imaging is seen to be more effective than power Doppler at the detection of blood as quantified by receiver operating characteristic analysis. A combination of the two modes could provide improved estimates of fractional blood volume in comparison with either mode used alone.
While photoacoustic imaging is capable of producing high-resolution biomedical images with optical absorption contrast, optical property quantification has thus far remained challenging. One reason for this is that laser-induced photoacoustic signal amplitudes are proportional to not only the local optical absorption coefficient, but also the local laser fluence in the tissue. Unfortunately, local laser fluence is often unknown, but could possibly be estimated if local bulk tissue optical properties were known. One method to estimate tissue optical properties is a technique known as oblique incidence diffuse reflectance (OIR). We report on an integrated OIR and photoacoustic imaging system and demonstrate, using phantom experiments, improved ability to quantitatively estimate optical properties of a turbid medium.
The combination of ultrasonic and photoacoustic imaging modalities has yet to be realized in the high-frequency
regime (>20MHz) where spatial resolution may permit visualization of the microvasculature. In this work,
we characterize the in-vivo performance of a custom ultrasound-photoacoustic B-scanning imaging system. This
system utilizes a combined ultrasound/photoacoustic probe attached to a voice-coil capable of approximately 1cm
lateral translation at a rate of up to 15Hz. The probe is comprised of a 25MHz ultrasound transducer, configured
confocally with a conical mirror-based dark-field laser delivery system. The fast-scanning mode permits realtime
ultrasound imaging. The imaging speed of the photoacoustic mode is limited by the repetition rate of the
532nm laser (up to 20Hz). Signals from the transducer are amplified by a 39dB preamp with an additional
time-gain compensation stage of up to 24dB. Control of the system is through a digital input-output PCI card,
which acts as a pulse-sequencer and permits software control of time-gain compensation. This setup permits
interlaced pulse sequences for excellent registration of ultrasonic and photoacoustic data, as well as separate timegain
compensation curves for photoacoustic and ultrasound modalities. We have managed to achieve a lateral
resolution of 155 μm and an axial resolution of 40 μm. The system is used to visualize the finger and palm of a
hand to almost 1cm ultrasound depths and multiple millimeter-scale photoacoustic depths. Photoacoustic images
are overlaid on the ultrasound images for simultaneous visualization of the microvasculature and surrounding
tissue.
The photoacoustic signal of an optical absorber in a turbid medium is proportional to the local laser fluence, the optical
absorption coefficient and the Gruneisen parameter. The local fluence at a subsurface absorber is determined by the
initial incident fluence and optical properties of the media. Knowledge of laser fluence at subcutaneous tissue locations
will improve our ability to estimate local chromophore concentrations and will lead to more quantitative estimates of
blood oxygen saturation with photoacoustics. By integrating an oblique incidence reflectance (OIR) system in a
photoacoustic imaging system, we are able to estimate optical properties of the turbid medium. To do this, we use a
unique photoacoustic probe consisting of a 45-degree optical prism in an optical index-matching fluid. An oblique CWlaser
beam interrogates the tissue surface at the same location as a pulsed laser, used for photoacoustic interrogation.
Photoacoustic signals collected from the tissue are deflected by the prism to a focused 10 MHz ultrasound transducer.
Diffuse light from the CW-laser is collected by a CCD camera and analyzed to estimate the bulk absorption and
scattering coefficients. We fixed a tube filled with known concentrations of an absorbing dye below the probe in an
Intralipid bath. We obtained the OIR and photoacoustic measurements for different Intralipid concentrations (providing a
μs' between 1 and 10 cm-1). The OIR measurements were used to estimate the bulk optical parameters. Using these
values, models of light transport were then used to calculate the local laser fluence to normalize the photoacoustic
measurements. The corrected photoacoustic signals show direct proportionality to the tube dye concentrations
irrespective of bulk turbid medium properties.
We developed a unique reflection-mode photoacoustic technique sensitive to optical scattering in turbid media. We
focused a small laser spot on to the surface of a turbid medium and captured the photoacoustic signal by a focused
ultrasound transducer. The amplitude of the photoacoustic signal for different surface illumination spot locations is an
effective estimate of the Green's function of light transport in turbid media. Our results for different concentrations of
Intralipid indicate that this method is capable of distinguishing small changes in the reduced scattering coefficient. In
this work, we present experimental measurements for an Intralipid phantom with reduced scattering coefficients of 3, 4,
and 5 cm-1, and show that Monte Carlo simulations of light transport accurately reproduce experimental curves. This
means that we can estimate transport-regime optical properties of the media given a suitable fitting algorithm.
Optical-resolution photoacoustic microscopy is a novel imaging technology for visualizing optically-absorbing
superficial structures in vivo with lateral spatial resolution determined by optical focusing rather than acoustic detection.
Since scanning of the illumination spot is required, the imaging speed is limited by the scanning speed and the laser
pulse repetition rate. Unfortunately, lasers with high-repetition rate and suitable pulse durations and energies are
difficult to find. We are developing compact laser sources for this application. Passively Q-switched fiber and microchip
lasers with pulse repetition rates up to 300 kHz are demonstrated. Using a diode-pumped microchip laser fiber-coupled
to a large mode-area Yb-doped fiber amplifier we obtained 60μJ
1-ns pulses at the frequency-doubled 532-nm
wavelength. The pulse-repetition rate was determined by the power of the microchip laser pump source at 808nm and
may exceed 10 kHz. Additionally, a passively Q-switched fiber laser utilizing a Yb-doped double-cladding fiber and an
external saturable absorber has shown to produce 250ns pulses at repetition rates of 100-300 KHz. A photoacoustic
probe enabling flexible scanning of the focused output of these lasers consisted of a 45-degree glass prism in an optical
index-matching fluid. Photoacoustic signals exiting the sample are deflected by the prism to an ultrasound transducer.
Phantom studies with a 7.5-micron carbon fiber demonstrate the ability to image with optical rather than acoustic
resolution. We believe that the high pulse-repetition rates and the potentially compact and fiber-coupled nature of these
lasers will prove important for clinical imaging applications where realtime imaging performance is essential.
Photoacoustic imaging has emerged as a promising technique for visualizing optically absorbing structures with
ultrasonic spatial resolution. Since it relies on optical absorption of tissues, photoacoustic imaging is particularly
sensitive to vascular structures even at the micro-scale. Power Doppler ultrasound can be used to detect moving blood
irrespective of Doppler angles. However, the sensitivity may be inadequate to detect very small vessels with slow flow
velocities. In this work, we merge these two synergistic modalities and compare power Doppler ultrasound images with
high-contrast photoacoustic images. We would like to understand the advantages and disadvantages of each technique
for assessing microvascular density, an important indicator of disease status. A combined photoacoustic and highfrequency
ultrasound system has been developed. The system uses a swept-scan 25 MHz ultrasound transducer with
confocal dark-field laser illumination optics. A pulse-sequencer enables ultrasonic and laser pulses to be interlaced so
that photoacoustic and Doppler ultrasound images are co-registered. Experiments have been performed on flow
phantoms to test the capability of our system and signal processing methods. Work in progress includes in vivo color
flow mapping. This combined system will be used to perform blood oxygen saturation and flow estimations, which will
provide us with the parameters to estimate the local rate of metabolic oxygen consumption, an important indicator for
many diseases.
Measurement of tissue optical properties impacts both optical diagnostic and theraputic applications. Although existing photoacoustic imaging techniques provide optical absorption contrast, we present a photoacoustic technique that demonstrates sensitivity to the optical scattering coefficient of a turbid medium. By incrementing the distance between a surface illumination spot and a subsurface absorber and measuring the photoacoustic amplitude of the absorber, we can effectively estimate the Green's function of light transport in a turbid medium. Our results for different concentrations of Intralipid indicate that this technique has the ability to distinguish small changes of the reduced scattering coefficient. It has the potential to be used for in vivo studies to obtain reduced scattering coefficients of biological tissues. These findings will potentially improve the calculation of subcutaneous fluence in photoacoustic-based techniques and laser dosimetry studies in live tissues.
High moisture extrusion technology is capable of producing meat analogs which assemble real meat.
Since visual and textural properties are the key factor for consumer acceptance, assessing fiber
formation in extruded products is important for producing quality meat analogs with a great texture.
Recently, we developed a photon migration method to assess fiber formation in meat analogs. In this
paper, we present an implementation of this method in a real time scanning system. Acquired images
were processed to characterize the fiber formation. This system provides a fast, non destructive means
to determine the fiber formation in meat analogs.
Recently a realtime photoacoustic microscopy system has been demonstrated. Unfortunately, however, displayed B-scan
images were sometimes difficult to interpret as there was little structural context. In this work, we provide structural
context for photoacoustic microscopy images by adding ultrasound biomicroscopy as a complementary and synergistic
modality. Our system uses a voice-coil translation stage capable of 1" lateral translation, and can operate in excess of 15
Hz for 1-cm translations, providing up to 30 ultrasound frames per second. The frame-rate of the photoacoustic
acquisitions is limited by the 20-Hz pulse-repetition rate of the laser, but can be increased with a faster-repetition-rate
laser. Data from the system is streamed in real time from a 2GS/s PCI data acquisition card to the host PC at rates as
high as 200 MB/s. The system should prove useful for various in vivo studies, including combined ultrasound Doppler
and photoacoustic imaging.
Photoacoustic microscopy and tomography are hybrid biomedical imaging technologies that provide optical absorption
contrast with ultrasonic spatial resolution. To date, photoacoustic methods have provided little information about the
optical scattering properties of tissues. Yet scattering is a key tissue parameter with diagnostic potential for a number of
diseases. Moreover, quantitative knowledge of the optical scattering coefficient may prove valuable for improving
quantitative estimates of blood oxygen saturation and other functional parameters. We present a new photoacoustic
method that shows promise for sensing the local reduced scattering coefficient of tissues.
We developed a polarization sensitive imaging system that can acquire reflectance images in turbid
samples using incident light of different polarization states. Using this system, we studied polarization
imaging on bovine sternomandibularis muscle strips using light of two orthogonal linearly polarized states.
We found the obtained polarization sensitive reflectance images had interesting patterns depending on
the polarization states. In addition, we computed four elements of the Mueller matrix from the acquired
images. As a comparison, we also obtained polarization images of a 20% Intralipid"R" solution and
compared the results with those from muscle samples. We found that the polarization imaging patterns
from Intralipid solution can be described with a model based on single-scattering approximation. However,
the polarization images in muscle had distinct patterns and can not be explained by this simple model.
These results implied that the unique structural properties of skeletal muscle play important roles in
modulating the propagation of polarized light.
A clear understanding of how light propagation in muscle is important for developing optical methods for muscle
characterization. We investigated photon migration in muscle by imaging the optical reflectance from fresh prerigor
skeletal muscles. We found the acquired reflectance patterns can not be described using existing theories. In order to
quantify the equi-intensity contours of acquired reflectance images, we developed a numerical fitting function. Using
this model, we studied the changes of reflectance profile during stretching and rigor process. The observed unique
anisotropic features diminished after rigor completion. These results suggested that muscle sarcomere structures played
important roles in modulating light propagation in whole muscle. To explain the observed patterns, we incorporated the
sarcomere diffraction in a Monte Carlo model and we showed that the resulting reflectance profiles quantitatively
resembled the experimental observation.
Animal meat products may not be the best choice for many people in the world due to various reasons such
as cost, health problems, or religious restrictions. High moisture (40-80%) extrusion technology shows a great
promise for texturizing vegetable proteins into fibrous meat alternatives. Soy protein which is healthy, highly
nutritious, low in both fat and carbohydrate has been used in high moisture extrusion process to produce
meat analogs with well formed fiber that resemble chicken or turkey breast meat. Assessing fiber formation in
extruded products is important for controlling extrusion quality in manufacturing process. Although several
methods have been studied for quantifying fiber formation in extrudates, their applications for real time quality
control in manufacturing process have been challenging. We explored the possibility of applying a
nondestructive method based on backscattered reflectance to measure the fiber formation of extruded soy
proteins. An image processing method was developed to extract the light reflectance profile at the extrudates'
surface. We applied the anisotropic continuous time random walk (CTRW) theory to quantitatively describe
the fiber formation in extrudates based on extracted surface reflectance profiles. This method has a potential
to be used as a non-destructive, fast, real time quality control tool for products with fibrous structures.
High moisture extrusion using twin-screw extruders shows great promise of producing meat analog products with vegetable proteins. The resulting products have well defined fiber formations; resemble real meat in both visual appearance and taste sensation. Developing reliable non-destructive techniques to quantify the textural properties of extrudates is important for quality control in the manufacturing process. In this study, we developed an image processing technique to automatically characterize sample fiber formation using digital imaging. The algorithm is based on statistical analysis of Hough transform. This objective method can be used as a standard method for evaluating other non-invasive methods. We have compared the fiber formation indices measured using this technique and a non-invasive fluorescence polarization method and obtained a high correlation.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.