A compact photoacoustic transrectal probe is constructed for improved imaging in brachytherapy treatment. A 192 element 5 MHz linear transducer array is mounted inside a small 3D printed casing along with an array of optical fibers. The device is fed by a pump laser and tunable NIR-optical parametric oscillator with data collected by a Verasonics ultrasound platform. This assembly demonstrates improved imaging of brachytherapy seeds in phantoms with depths up to 5 cm. The tuneable excitation in combination with standard US integration provides adjustable contrast between the brachytherapy seeds, blood filled tubes and background tissue.
KEYWORDS: Signal to noise ratio, Monte Carlo methods, Transducers, Tissues, Photoacoustic tomography, Ultrasonics, Scattering, Acquisition tracking and pointing, Photoacoustic spectroscopy, Laser scattering
We compare scanned-mosaicking and blanket illumination schemes for wide-field photoacoustic tomography with potential applications to breast imaging. For each illumination, a locally high-SNR image patch is reconstructed then mosaicked with image patches from other illuminations. Because the beam is not diffused over the entire area, the fluence of the beam can be maximized, therefore maximizing the signal generated. Moreover, the imaging can potentially still be done fast enough within a breath-hold. A Monte Carlo simulation as a function of beam-spot size and depth is performed to quantify this signal gain. We experimentally test both schemes using a 256-element Imasonic ring array on a tissue-mimicking phantom. We were able to verify the simulated signal gain of 2.9x under 0.5 cm of tissue with the experimental data, and measured the signal gain decrease expected when imaging deeper into the tissue. We also measured the effectiveness of averaging the diffused beam versus the scanned-mosaicking approach, and observed that for the same scan times and limited laser power output, scanned-mosaicking was able to produce a higher SNR than the blanket illumination approach. We have shown that this technique will allow wide-area PAT to utilize the maximum SNR available from any system while minimizing the number of acquisitions to reach this SNR.
We propose a theoretical framework for consecutively reconstructing absorption and scattering distributions in turbid soft tissue in an iterative manner. This approach takes advantage of the stability of a recently reported least-squares fixed-point iterative method for reconstructing an optical absorption coefficient map to iteratively update estimates of absorption and scattering for each iteration. Simulations demonstrate that this method converges to an accurate estimate of the optical properties within only a small number of iterations and is robust to noise at realistic signal-to-noise levels.
Quantitatively reconstructing optical absorption using photoacoustic imaging is nontrivial. Theoretical hurdles, such as nonuniqueness and numerical instability, can be mitigated by using multiple illuminations. However, even with multiple illuminations, using ANSI-safety-limited fluence for practical imaging may result in poor performance owing to limited signal-to-noise ratio (SNR). We demonstrate the use of S-sequence coded patterned illumination to boost SNR while preserving the enhanced stability of multiple-illumination iterative techniques.
Quantitative photoacoustic microscopy (qPAT) is challenging. We present an algorithm which consecutively reconstructs absorption and scattering coefficient distributions with an iterative scheme in multiple-illumination photoacoustic tomography (MIPAT). In each iteration, the absorption distribution is estimated with the least-squares fixed-point iteration method. Then the diffusion coefficient is estimated with an updated version of optical fluence based on the previously modified absorption information. This procedure is repeated till an acceptable results is achieved. Simulation examples demonstrate the capability of this method in faithfully recovering the absorption and diffusion coeffient distributions, and fast convergence.
Photoacoustic imaging has great potential for identifying vascular regions for clinical imaging. In addition to assessing angiogenesis in cancers, there are many other disease processes that result in increased vascularity that present novel targets for photoacoustic imaging. Doppler imaging can provide good localization of large vessels, but poor imaging of small or low flow speed vessels and is susceptible to motion artifacts. Photoacoustic imaging can provide visualization of small vessels, but due to the filtering effects of ultrasound transducers, only shows the edges of large vessels. Thus, we have combined photoacoustic imaging with ultrasound power Doppler to provide contrast agent- free vascular imaging. We use a research-oriented ultrasound array system to provide interlaced ultrasound, Doppler, and photoacoustic imaging. This system features realtime display of all three modalities with adjustable persistence, rejection, and compression. For ease of use in a clinical setting, display of each mode can be disabled. We verify the ability of this system to identify vessels with varying flow speeds using receiver operating characteristic curves, and find that as flow speed falls, photoacoustic imaging becomes a much better method for identifying blood vessels. We also present several in vivo images of the thyroid and several synovial joints to assess the practicality of this imaging for clinical applications.
Fixed-point iteration shows promise for quantitative reconstruction of optical absorption in photoacoustic tomography. However, there are issues that prevent the technique from being practical including: non-uniqueness of scattering and absorption profiles, divergence with over-iteration, and sensitivity to noise. Multiple illumination has been proposed to deal with the first problem, and may help with the second. The issue of noise may be balanced out by increasing the regularization parameter at the expense of the exactness of the reconstruction. In a multiple-illumination setup with a circular geometry where fluence is abundant, using a patterned illumination with a decoding step may provide an alternative which will boost SNR. We present a simple sequence of patterned illuminations based on an S-sequence that serves to improve SNR. While the forward model of the iterative method may be applied directly to the patterned excitations, including the decoding step improves SNR in an individual image by a factor equal to the size of the S-sequence, thus greatly improving convergence for a given value of regularization and SNR. For example, with 15 illuminations, 50-60dB noise levels with S-sequence patterned illuminations gives similar simulated performance to the 70dB case with single-source illuminations. This technique will allow the application of fixed-point iteration techniques in a broader range of SNR conditions without resorting to averaging.
Using a 0.8-mm-diameter image guide fiber bundle consisting of 30,000 single-mode fibers and an external linear array transducer, we demonstrate a dual-mode photoacoustic system capable of ultrasound-guided microendoscope insertion and photoacoustic imaging. The array optical resolution photoacoustic microendoscopy (AOR-PAME) system is designed to visualize the placement of the distal end of an endoscopy probe several centimeters into tissue, transmit scanning focused laser pulses into tissues via the fiber bundle, and acquire the generated photoacoustic signals. A ytterbium-doped fiber laser is tightly focused and is scanned across the proximal tip of the image guide fiber bundle using a two-dimensional galvanometer scanning mirror system. The end of the fiber bundle is used in contact mode with the object. The capabilities of AOR-PAME are demonstrated by imaging carbon fiber networks embedded in tissue-mimicking phantoms and the ears of a 60-g rat. The lateral resolution and signal-to-noise ratio are measured as 9 μm and 40 dB, respectively.
Recently we demonstrated the feasibility of Optical-Resolution Photoacoustic Micro-Endoscopy (OR-PAME) using an image guide fiber. However, the use of an ultrasound transducer for signal collection limited useful applications. We demonstrate detection of OR-PAM signals using an external array transducer in order to make endoscopic imaging practical for clinical use for the first time. The array system is able to visualize the placement of the image-guide fiber using pulse-echo ultrasound then switch to an OR-PAME acquisition mode.
Photoacoustic signals are captured by a Verasonics ultrasound system using an L7-4 linear array transducer. A high-repetition-rate 532-nm fiber laser was used as the excitation source. This light was focused and raster scanned into a 800m-diameter image-guide fiber bundle consisting of 30,000 individual fiber elements. The operator finds the end of the endoscope using a flash ultrasound imaging mode, then captures endoscopic data by clicking a button. This activates the motion of scanning mirrors into the end of the image guide, and engages an endoscopic capture sequence. Endoscopic data are used to form a maximum amplitude image by simply taking the maximum of the absolute value of the signal across the 64 center channel lines used for capture. Using this technique, we have captured images of carbon fibers with a resolution of 6 microns at an SNR of greater than 30dB. Electronic focusing is expected to improve the SNR. The use of an ultrasound array transducer for both endoscope guidance and data collection allows for a much smaller endoscope footprint while opening up clinical possibilities.
The metabolic rate of oxygen consumption, an important indicator of tissue metabolism, can be expressed as the change of net blood oxygen flux into and out of a tissue region per 100 g of tissue. In this work, we propose a photoacoustic and Doppler ultrasound method for imaging local blood oxygen flux of a single vessel. An imaging system for combined photoacoustic and high-frequency 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 Doppler ultrasound images are co-registered. Since the mean flow speed can be measured by color Doppler ultrasound, the vessel cross-sectional area can be measured by power Doppler or structural photoacoustic imaging, and multi-wavelength photoacoustic methods can be used to estimate oxygen saturation (sO2) and total concentration of haemoglobin (CHb), all of the parameters necessary for oxygen flux estimation can be provided. The accuracy of the flow speed and sO2 estimation has been investigated. In vitro sheep blood phantom experiments have been performed at different sO2 levels and mean flow speeds. Blood oxygen flux has been estimated, and the uncertainty of the measurement has been quantified.
The long-term goal of our research is to develop photoacoustic and Doppler ultrasound imaging methods for noninvasive
estimation of the oxygen consumption rate (MRO2) in vivo. Previously, we have demonstrated a combined
photoacoustic and high-frequency Doppler ultrasound system and shown the feasibility of flow velocity and oxygen
saturation (sO2) estimation using double-ink flow phantoms. In this work, the results of in vitro sheep blood experiments
are presented. Blood oxygen flux has been estimated at different sO2 levels and mean flow speeds, and the uncertainty of
the measurement has been quantified. In vivo experiments have been performed on Swiss Webster mice to provide coregistered
photoacoustic and Doppler flow images with imaging depths of ~2mm. Doppler bandwidth broadening
technique has been used to obtain transverse flow velocity. The diameter of the blood vessel is ~500μm and the mean
flow speed is 15cm/s. We are working towards sO2 estimation in vivo and 3D oxygen consumption imaging of tumors at
depths beyond OR-PAM.
KEYWORDS: Tumors, Ultrasonography, Imaging systems, In vivo imaging, Photoacoustic spectroscopy, Photoacoustic imaging, Blood, Tissues, Signal to noise ratio, Breast
Tyrosinase, a key enzyme in the production of melanin, has shown promise as a reporter of genetic activity.
While green fluorescent protein has been used extensively in this capacity, it is limited in its ability to provide
information deep in tissue at a reasonable resolution. As melanin is a strong absorber of light, it is possible
to image gene expression using tyrosinase with photoacoustic imaging technologies, resulting in excellent resolutions
at multiple-centimeter depths. While our previous work has focused on creating and imaging MCF-7
cells with doxycycline-controlled tyrosinase expression, we have now established the viability of these cells in a
murine model. Using an array-based photoacoustic imaging system with 5 MHz center frequency, we capture
interleaved ultrasound and photoacoustic images of
tyrosinase-expressing MCF-7 tumors both in a tissue mimicking
phantom, and in vivo. Images of both the tyrosinase-expressing tumor and a control tumor are presented
as both coregistered ultrasound-photoacoustic B-scan images and
3-dimensional photoacoustic volumes created
by mechanically scanning the transducer. We find that the
tyrosinase-expressing tumor is visible with a signal
level 12dB greater than that of the control tumor in vivo. Phantom studies with excised tumors show that the
tyrosinase-expressing tumor is visible at depths in excess of 2cm, and have suggested that our imaging system is
sensitive to a transfection rate of less than 1%.
Brachytherapy is a form of radiation therapy commonly used in the treatment of prostate cancer wherein sustained radiation doses can be precisely targeted to the tumor area by the implantation of small radioactive seeds around the treatment area. Ultrasound is a popular imaging mode for seed implantation, but the seeds are difficult to distinguish from the tissue structure. In this work, we demonstrate the feasibility of photoacoustic imaging for identifying brachytherapy seeds in a tissue phantom, comparing the received intensity to endogenous contrast. We have found that photoacoustic imaging at 1064 nm can identify brachytherapy seeds uniquely at laser penetration depths of 5 cm in biological tissue at the ANSI limit for human exposure with a contrast-to-noise ratio of 26.5 dB. Our realtime combined photoacoustic-ultrasound imaging approach may be suitable for brachytherapy seed placement and post-placement verification, potentially allowing for realtime dosimetry assessment during implantation.
Recently, we have developed a combined photoacoustic and high-frequency Doppler ultrasound system with a single
element transducer to estimate the metabolic rate of oxygen consumption in small animal models. However, the long
scanning time due to mechanical motion may be a limitation of our swept-scan system. In this work, the single element
transducer was replaced by a clinical array transducer which may provide more accurate flow velocity estimations,
higher frame rates, improved penetration depth, and improved depth-of-field due to dynamic focusing capabilities. We
used an array system from Verasonics Inc. which enables flexible pulse-sequence programming and parallel channel data
acquisition, along with a pulsed laser and optical parametric oscillator. For flow estimation, we implemented a flash-
Doppler sequence which transmits ensembles of plane-wave excitations. Echo signals are beamformed and subjected to
wall-filtering and Kasai flow estimation algorithms. High frame rates over a wide region can be achieved. Combined
interlaced photoacoustic and Doppler imaging on flow phantoms has been performed on this system. We demonstrate the
ability to image animal blood to depths of 1.5-cm with high signal-to-noise with both modalities. The light penetration is
2-cm. We discuss the performance of Doppler flow estimation and photoacoustic oxygen saturation estimation and their
role in future work of estimating oxygen consumption.
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.
Photoacoustic imaging offers a new and complementary contrast mechanism to the traditional structural contrast
of ultrasound. While the combination of these two modes has been demonstrated in the past with single-element
transducers, array transducers offer clear advantages in both modes by eliminating mechanical scanning and
allowing image formation from a single excitation. Given the abundance of commercially available ultrasound
systems, it is desirable to use them as much as possible. However, these systems often only allow access to
beamformed RF data. We discuss the applicability of ultrasound beamformers for photoacoustic imaging, and
find that with only software-defined control over the speed of sound, walking aperture reconstruction is optimally
performed using a speed correction factor of 1.414. When sector-scanning is used, a different strategy is required.
We also demonstrate a new photoacoustic-ultrasound imaging system based on a Verasonics ultrasound array
system. The system streams raw channel data to a 6-core PC at up to 1.4GB/s via PCI-Express, allowing
interlaced ultrasound and photoacoustic data to be acquired and reconstructed at realtime rates. Using an L7-4
linear array transducer, we demonstrate the performance of this system and discuss potential applications. The
system should provide new opportunities for clinical and pre-clinical imaging.
The metabolic rate of oxygen consumption (MRO2) quantifies tissue metabolism, which is important for diagnosis of
many diseases. For a single vessel model, the MRO2 can be estimated in terms of the mean flow velocity, vessel crosssectional
area, total concentration of hemoglobin (CHB), and the difference between the oxygen saturation (sO2) of blood
flowing into and out of the tissue region. In this work, we would like to show the feasibility to estimate MRO2 with our
combined photoacoustic and high-frequency ultrasound imaging system. 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 Doppler ultrasound images are co-registered. Since the mean flow
velocity can be measured by color Doppler ultrasound, the vessel cross-sectional area can be measured by power
Doppler or photoacoustic imaging, and multi-wavelength photoacoustic methods can be used to estimate sO2 and CHB,
all of these parameters necessary for MRO2 estimation can be provided by our system. Experiments have been performed
on flow phantoms to generate co-registered color Doppler and photoacoustic images. To verify the sO2 estimation, two
ink samples (red and blue) were mixed in various concentration ratios to mimic different levels of sO2, and the result
shows a good match between the calculated concentration ratios and actual values.
Brachytherapy is a technique commonly used in the treatment of prostate cancer that relies on the precise
placement of small radioactive seeds near the tumor location. The advantage of this technique over traditional
radiation therapies is that treatment can be continuous and uniform, resulting in fewer clinic visits and a shorter
treatment duration. Two important phases of this treatment are needle guidance for implantation, and post-placement
verification for dosimetry. Ultrasound is a common imaging modality used for these purposes, but it
can be difficult to distinguish the seeds from surrounding tissues, often requiring other imaging techniques such as
MRI or CT. Photoacoustic imaging may offer a viable alternative. Using a photoacoustic system based on an L7-
4 array transducer and a realtime ultrasound array system capable of parallel channel data acquisition streamed
to a multi-core computer via PCI-express, we have demonstrated imaging of these seeds at an ultrasound depth
of 16 mm and laser penetration depths ranging up to 50 mm in chicken tissue with multiple optical wavelengths.
Ultrasound and photoacoustic images are coregistered via an interlaced pulse sequence. Two laser pulses are used
to form a photoacoustic image, and at these depths, the brachytherapy seeds are detected with a signal-to-noise
ratio of over 26dB. To obtain this result, 1064nm light was used with a fluence of 100mJ/cm2, the ANSI limit
for human skin exposure at this wavelength. This study demonstrates the potential for photoacoustic imaging
as a candidate technology for brachytherapy seed placement guidance and verification.
Optical reporter genes, such as green fluorescence protein, are powerful research tools that allow visualization of gene
expression. We have successfully used tyrosinase as a reporter gene for photoacoustic imaging. Tyrosinase is the key
regulatory enzyme in the production of melanin which has a broad optical absorption spectrum. MCF-7 cells were stably
transfected with tyrosinase under the control of an inducible promoter. For photoacoustic experiments, MCF-7 cells were
resuspended at 108 cells/mL and injected in 700 μm (inner diameter) plastic tubing. Photoacoustic signal of MCF-7 cells
expressing tyrosinase were >20-fold greater than those of untransfected MCF-7 cells. Photoacoustic signal of tyrosinaseexpressing
MCF-7 cells were approximately 2-fold lesser and greater than those of blood at 576 and 650 nm,
respectively, suggesting that photoacoustic signal from blood and tyrosinase-expressing cells can be separated by dualwavelength
analysis. Photoacoustic signal from tyrosinase-expressing MCF-7 cells covered by chicken tissue could even
be detected at a laser penetration depth of 4 cm, suggesting that tyrosinase can be used to image gene expression in
relatively deep tissues. The current data suggests that tyrosinase is a strong reporter gene for photoacoustic imaging.
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.
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.
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.
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