KEYWORDS: 3D image processing, Imaging systems, Ultrasonography, Data acquisition, In vivo imaging, 3D acquisition, Photoacoustic spectroscopy, Lymphatic system, Photoacoustic imaging, 3D modeling
We present an integrated photoacoustic and ultrasonic three-dimensional (3-D) volumetric imaging system based on a two-dimensional (2-D) matrix array ultrasound probe. A wavelength-tunable dye laser pumped by a Q-switched Nd:YAG laser serves as the light source and a modified commercial ultrasound imaging system (iU22, Philips Healthcare) with a 2-D array transducer (X7-2, Philips Healthcare) detects both the pulse-echo ultrasound and photoacoustic signals. A multichannel data acquisition system acquires the RF channel data. The imaging system enables rendering of co-registered 3-D ultrasound and photoacoustic images without mechanical scanning. The resolution along the azimuth, elevation, and axial direction are measured to be 0.69, 0.90 and 0.84 mm for photoacoustic imaging. In vivo 3-D photoacoustic mapping of the sentinel lymph node was demonstrated in a rat model using methylene blue dye. These results highlight the clinical potential of 3-D PA imaging for identification of sentinel lymph nodes for cancer staging in humans.
We developed a novel trimodality system for human breast imaging by integrating photoacoustic (PA) and thermoacoustic (TA) imaging techniques into a modified commercial ultrasound scanner. Because light was delivered with an optical assembly placed within the microwave antenna, no mechanical switching between the microwave and laser sources was needed. Laser and microwave excitation pulses were interleaved to enable PA and TA data acquisition in parallel at a rate of 10 frames per second. A tube (7 mm inner diameter) filled with oxygenated bovine blood or 30 mM methylene blue dye was successfully detected in PA images in chicken breast tissue at depths of 6.6 and 8.4 cm, respectively, for the first time. The SNRs at these depths reached ∼ 24 and ∼ 15 dB, respectively, by averaging 200 signal acquisitions. Similarly, a tube (13 mm inner diameter) filled with saline solution (0.9%) at a depth of 4.4 cm in porcine fat tissue was successfully detected in TA images. The PA axial, lateral, and elevational resolutions were 640 μm, 720 μm, and 3.5 mm, respectively, suitable for breast cancer imaging. A PA noise-equivalent sensitivity to methylene blue solution of 260 nM was achieved in chicken tissue at a depth of 3.4 cm.
Photoacoustic (PA) and thermoacoustic (TA) effects are based on the generation of acoustic waves after tissues absorb
electromagnetic energy. The amplitude of the acoustic signal is related to the temperature of the absorbing target tissue.
A combined photoacoustic and thermoacoustic imaging system built around a modified commercial ultrasound scanner
was used to obtain an image of the target's temperature, using reconstructed photoacoustic or thermoacoustic images. To
demonstrate these techniques, we used photoacoustic imaging to monitor the temperature changes of methylene blue
solution buried at a depth of 1.5 cm in chicken breast tissue from 12 to 42 °C. We also used thermoacoustic imaging to
monitor the temperature changes of porcine muscle embedded in 2 cm porcine fat from 14 to 28 °C. The results
demonstrate that these techniques can provide noninvasive real-time temperature monitoring of embedded objects and
tissue.
We developed a tri-modal system combining photoacoustic (PA) tomography, thermoacoustic (TA) tomography, and
ultrasound (US) imaging. Acquired images of an excised dog prostate were compared to histology results. All three
modalities can image distinct features. Features like the urethra were shown in both TA and US images, but TA gave a
higher contrast-to-noise ratio. Fibrous tissue was more clearly imaged by TA, while the duct structure was better shown
in PA images. These experimental results demonstrate the potential advantages of our tri-modal imaging system.
KEYWORDS: Lymphatic system, Signal to noise ratio, Breast, Tissues, Imaging systems, Biopsy, Tissue optics, Ultrasonics, Photoacoustic imaging, In vivo imaging
A combined ultrasonic and photoacoustic imaging system is presented that is capable of deep tissue imaging. The
system consists of a modified clinical ultrasound array system and tunable dye laser pumped by a Nd:YAG laser. The
system is designed for noninvasive detection of sentinel lymph nodes and guidance of needle biopsies for axillary
lymph node staging in breast cancer patients. Using a fraction of the American National Standards Institute (ANSI)
safety limit, photoacoustic imaging of methylene blue achieved penetration depths of greater than 5 cm in chicken
breast tissue. Photoacoustic imaging sensitivity was measured by varying the concentration of methylene blue dye
placed at a depth of 3 cm within surrounding chicken breast tissue. Signal-to-noise ratio, noise equivalent sensitivity,
and axial spatial resolution were quantified versus depth based on in vivo and chicken breast tissue experiments. The
system has been demonstrated in vivo for detecting sentinel lymph nodes in rats following intradermal injection of
methylene blue. These results highlight the clinical potential of photoacoustic image-guided identification and needle
biopsy of sentinel lymph nodes for axillary staging in breast cancer patients.
We have implemented a hand-held photoacoustic and ultrasound probe for image-guided needle biopsy using a
modified clinical ultrasound array system. Pulsed laser light was delivered via bifurcated optical fiber bundles
integrated with the hand-held ultrasound probe. We photoacoustically guided needle insertion into rat sentinel lymph
nodes (SLNs) following accumulation of indocyanine green (ICG). Strong photoacoustic image contrast of the needle
was achieved. After intradermal injection of ICG in the left forepaw, deeply positioned SLNs (beneath 2-cm thick
chicken breast) were easily indentified in vivo and in real time. Further, we confirmed ICG uptake in axillary lymph
nodes with in vivo and ex vivo fluorescence imaging. These results demonstrate the clinical potential of this hand-held
photoacoustic system for facile identification and needle biopsy of SLNs for cancer staging and metastasis detection in
humans.
KEYWORDS: Photoacoustic spectroscopy, 3D image processing, Ultrasonography, Imaging systems, Photoacoustic imaging, Transducers, Lymphatic system, 3D acquisition, In vivo imaging, Data acquisition
Photoacoustic tomography provides both structural and functional imaging in vivo based on optical absorption contrast.
A novel imaging system that incorporates a two-dimensional matrix ultrasound probe for combined photoacoustic and
ultrasonic three-dimensional (3D) volumetric imaging is presented. The system consists of a tunable dye laser pumped
by a Nd:YAG laser, a commercial ultrasound imaging system (Philips iU22) with a two-dimensional matrix transducer
(Philips X7-2, 2500 elements, 2-7 MHz), and a multichannel data acquisition system which allows us to acquire RF
channel data. Compared with alternative 3D techniques, this system is attractive because it can generate co-registered 3D
photoacoustic and ultrasound images without mechanical scanning. Moreover, the lateral resolution along the azimuth
and elevational directions are measured to be 0.77 ± 0.06 mm and 0.96 ± 0.06 mm, respectively, based on reconstructed
photoacoustic images of phantoms containing individual human hairs. Finally, in vivo 3D photoacoustic sentinel lymph
node mapping using methylene blue dye in a rat model is demonstrated.
Optical absorption is closely associated with many physiologically important parameters, such as the
concentration and oxygen saturation of hemoglobin, and it can be used to quantify the concentrations of non-fluorescent
molecules. We introduce a method to quantify the absolute optical absorption based upon the acoustic spectra of
photoacoustic (PA) signals. This method is self-calibrating and thus insensitive to variations in optical fluence. Factors
such as the detection system bandwidth and acoustic attenuation can affect the quantification but can be canceled by
measuring the acoustic spectra at two optical wavelengths. This method has been implemented on various PA systems,
including optical-resolution PA microscopy, acoustic-resolution PA microscopy, and reconstruction based PA array
systems. We quantified the optical absorption coefficients of phantom samples at various wavelengths. We also
quantified the oxygen saturation of hemoglobin in live mice.
By modifying a clinical ultrasound array system, we develop a novel handheld photoacoustic probe for image-guided needle biopsy. The integration of optical fiber bundles for pulsed laser light delivery enables photoacoustic image-guided insertion of a needle into rat axillary lymph nodes with accumulated indocyanine green (ICG). Strong photoacoustic contrast of the needle is achieved. After subcutaneous injection of the dye in the left forepaw, sentinel lymph nodes are easily detected, in vivo and in real time, beneath 2-cm-thick chicken breast overlaying the axillary region. ICG uptake in axillary lymph nodes is confirmed with fluorescence imaging both in vivo and ex vivo. These results demonstrate the clinical potential of this handheld photoacoustic system for facile identification and needle biopsy of sentinel lymph nodes for cancer staging and metastasis detection in humans.
Real-time temperature monitoring with high spatial resolution (~1 mm) and high temperature sensitivity (1 °C or better)
is needed for the safe deposition of heat energy in surrounding healthy tissue and efficient destruction of tumor and
abnormal cells during thermotherapy. A temperature sensing technique using thermoacoustic and photoacoustic
measurements combined with a clinical Philips ultrasound imaging system (iU22) has been explored in this study. Using
a tissue phantom, this noninvasive method has been demonstrated to have high temporal resolution and temperature
sensitivity. Because both photoacoustic and thermoacoustic signal amplitudes depend on the temperature of the source
object, the signal amplitudes can be used to monitor the temperature. The signal is proportional to the dimensionless
Grueneisen parameter of the object, which in turn varies with the temperature of the object. A temperature sensitivity of
0.5 °C was obtained at a temporal resolution as short as 3.6 s with 50 signal averages.
Sentinel lymph node biopsy (SLNB) has become the standard method for axillary staging in breast cancer patients,
relying on invasive identification of sentinel lymph nodes (SLNs) following injection of blue dye and radioactive tracers.
While SLNB achieves a low false negative rate (5-10%), it is an invasive procedure requiring ionizing radiation. As an
alternative to SLNB, ultrasound-guided fine needle aspiration biopsy has been tested clinically. However, ultrasound
alone is unable to accurately identify which lymph nodes are sentinel. Therefore, a non-ionizing and noninvasive
detection method for accurate SLN mapping is needed.
In this study, we successfully imaged methylene blue dye accumulation in vivo in rat axillary lymph nodes using a
Phillips iU22 ultrasound imaging system adapted for photoacoustic imaging with an Nd:YAG pumped, tunable dye
laser. Photoacoustic images of rat SLNs clearly identify methylene blue dye accumulation within minutes following
intradermal dye injection and co-registered photoacoustic/ultrasound images illustrate lymph node position relative to
surrounding anatomy. To investigate clinical translation, the imaging depth was extended up to 2.5 cm by adding
chicken breast tissue on top of the rat skin surface. These results raise confidence that photoacoustic imaging can be used
clinically for accurate, noninvasive SLN mapping.
Dye-labeled protein microspheres, submicron in size and capable of producing thermoelastically generated ultrasound in response to laser stimulation, are presented as contrast agents for photoacoustic imaging. Incident laser energy absorbed by fluorescein isothiocyanate (FITC)-labeled elastin submicrospheres results in thermoelastically generated sound production. Plotted A-line graphs reveal a distinctive morphology and a greater than two orders of magnitude increase in signal amplitude subsequent to converting FITC elastin into submicrospheres (despite a four orders of magnitude decrease in concentration). Evidence of nonlinearity and enhancement of ultrasound backscatter indicate a potential use in contrast-enhanced harmonic imaging. Photoacoustic and ultrasound imaging of FITC-elastin submicrospheres in a water-filled phantom vessel shows enhanced contrast at low concentration and clear delineation of the phantom vessel wall.
In-vivo photoacoustic/ultrasound (PA/US) imaging of nude mice was investigated using a photoacoustic imaging system
based on a commercial ultrasound scanner HDI-5000. Raw per-channel data was captured and beamformed to generate
each individual photoacoustic image with a single laser shot. An ultra-broadband CL15-7 linear array with a center
frequency of 8 MHz, combined with a Schott Glass fiber bundle, was used as a compact high resolution imaging probe,
with lateral and axial PA resolutions of about 300µm and 200µm, respectively. The imaging system worked in a dual
PA-US mode, with the ultrasound outlining the tissue structure and the photoacoustic image showing the blood vessels.
PA signals were generated by exposing mice to ultra-short optical pulses from a Nd:YAG-pumped OPO laser operating
in a wavelength range of 700-950nm. The corresponding ultrasound images were generated in the regular B-mode with
standard delay-and-sum beamforming algorithm. The system resolution was sufficiently high to identify and clearly
distinguish the dorsal artery and the two lateral veins in the mouse tail. Both the saphena artery and the ischiatic vein on
the cross-section of the mouse leg were clearly outlined in the PA images and correctly overlaid on the ultrasound image
of the tissue structure. Similarly, cross-section PA images of the mouse abdomen revealed mesenteric vasculatures
located below the abdominal wall. Finally, a successful PA imaging of the mouse thoracic cavity unveiled the ascending
and descending aorta. These initial results demonstrate a great potential for a dual photoacoustic/ultrasound imaging
modality implemented on a commercial ultrasound imaging scanner.
Photoacoustic (PA) experiments were performed using a modified commercial ultrasound scanner equipped with an
array transducer and a Nd:YAG pumped OPO laser. The contrast agent SIDAG (Bayer Schering Pharma AG, Germany),
used to enhance the optical absorption, demonstrated an expected pharmacokinetic behavior of the dye in the tumor and
in the bladder of the nude mice. A typical behavior in the tumor consisted of an initial linear increase in PA signal
followed by an exponential decay. PA signal approached the pre-injection level after about one hour following the dye
injection, which was consistent with the behavior for such contrast agents when used in other imaging modalities, such
as fluorescence imaging. The in-vivo spectral PA data from the mouse bladder, conducted 1.5 hours after the dye injection, clearly demonstrated presence of the dye. The multi-spectral PA data was obtained at 760nm, 784nm and 850nm laser excitations. The PA
intensities obtained at these three wavelengths accurately matched the dye absorption spectrum.
In addition, in the kidney, a clearance organ for this contrast agent, both in-vivo and ex-vivo results demonstrated a
significant increase (~ 40%) in the ratio of PA signal at 760nm (the peak of the dye absorption) relative to the signal at
850nm (<1% absorption), indicating significant amounts of the dye in this organ.
Our initial results confirm the desired photoacoustic properties of the contrast agent, indicating its great potential to be
used for imaging with a commercial array-based ultrasound scanner.
We studied the nature of photoacoustic signals that were generated under a variety of conditions from vessel-mimicking
polyethylene tubes. The vessels, filled with a range of contrast agents, were buried in tissue-like phantoms that possessed
low to high optical absorption and scattering properties. In a photoacoustic image, we observed that either a single spot
or two distinct spots could represent a single vessel depending on the strength of the infused contrast agent and on the
size of the vessels. We typically found linear increase of the photoacoustic intensity with laser excitation power as well
as with absorption coefficient of the contrast agent. However, we found that there is an optimum excitation power for
achieving the best photoacoustic signal. If a vessel is buried in a highly absorbing background, increasing the laser
power beyond a certain limit reversibly reduces the photoacoustic signal from the vessel, eventually decreasing it to
zero. We also studied the blood-to-tissue absorption contrast requirement for observing the photoacoustic signal from a
vessel buried in an absorbing and scattering tissue. We find that, in order to distinguish the photoacoustic signal from its
background, the absorption coefficient of contrast agent in the vessel must be at least 2.5 times larger than that of the
surroundings.
A one of a kind photoacoustic system has been built around a Philips iU22 ultrasound scanner. The modified channel
board architecture allows access to the raw per-channel photoacoustic data, while keeping all of the imaging capabilities
of an actual commercial ultrasound scanner. A captured photoacoustic data frame is Fourier beamformed to generate a
single laser shot photoacoustic image. In addition to the photoacoustic data, the system supplies the beamformed
ultrasound data, providing a truly dual-modality imaging capability. A tunable OPO laser system (700-900nm), pumped
by an Nd:YAG solid state laser, is used as an illumination source with 5ns long pulses. An FPGA-based electronic
board synchronizes the iU22 start of frame with the laser firing, currently permitting photoacoustic imaging at a rate of
10 Hz (laser repetition rate limit). At that imaging frame rate the photoacoustic system, consisting of a PC modified
with 32 Gbytes of acquisition memory and an FPGA array, is able to store several minutes of continuously captured
data, enabling monitoring and off-line analysis of dynamic photoacoustic events and/or fast scanning for performing
pseudo-3D imaging. The system can use all of the standard iU22 array transducers both for photoacoustic imaging, and
in all of the ultrasound imaging modes.
Protein nanospheres capable of frequency controlled oscillation in response to laser stimulation are presented as contrast
agents for photoacoustic imaging. Incident laser energy absorbed by dye-labeled protein nanospheres causes
thermoelastically generated sound production. Plotted A-line graphs reveal a distinctive morphology and greater than 2
orders of magnitude increase in signal amplitude subsequent to converting labeled proteins into nanospheres. Evidence
of nonlinearity and enhancement of ultrasound backscatter indicate a potential use in contrast-enhanced harmonic
imaging. Photoacoustic and ultrasound imaging of protein nanospheres in phantom vessels show enhanced contrast at
low concentration and clear delineation of the phantom vessel wall.
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