Understanding the tumor microenvironment is critical to characterizing how cancers operate and predicting their response to treatment. We describe a novel, high-resolution coregistered photoacoustic (PA) and pulse echo (PE) ultrasound system used to image the tumor microenvironment. Compared to traditional optical systems, the platform provides complementary contrast and important depth information. Three mice are implanted with a dorsal skin flap window chamber and injected with PC-3 prostate tumor cells transfected with green fluorescent protein. The ensuing tumor invasion is mapped during three weeks or more using simultaneous PA and PE imaging at 25 MHz, combined with optical and fluorescent techniques. Pulse echo imaging provides details of tumor structure and the surrounding environment with 100-μm3 resolution. Tumor size increases dramatically with an average volumetric growth rate of 5.35 mm3/day, correlating well with 2-D fluorescent imaging (R = 0.97, p < 0.01). Photoacoustic imaging is able to track the underlying vascular network and identify hemorrhaging, while PA spectroscopy helps classify blood vessels according to their optical absorption spectrum, suggesting variation in blood oxygen saturation. Photoacoustic and PE imaging are safe, translational modalities that provide enhanced depth resolution and complementary contrast to track the tumor microenvironment, evaluate new cancer therapies, and develop molecular contrast agents in vivo.
Most single element hydrophones depend on a piezoelectric material that converts pressure changes to electricity.
These devices, however, can be expensive, susceptible to damage at high pressure, and/or have limited bandwidth and
sensitivity. The acousto-electric (AE) hydrophone is based on the AE effect, an interaction between electrical current and
acoustic pressure generated when acoustic waves travel through a conducting material. As we have demonstrated
previously, an AE hydrophone requires only a conductive material and can be constructed out of common laboratory
supplies to generate images of an ultrasound beam pattern consistent with more expensive hydrophones. The sensitivity
is controlled by the injected bias current, hydrophone shape, thickness and width.
In this report we describe simulations aimed at optimizing the design of the AE hydrophone with experimental
validation using new devices composed of a resistive element of indium tin oxide (ITO). Several shapes (e.g., bowtie and
dumbbell) and resistivities were considered. The AE hydrophone with a dumbbell configuration achieved the best beam
pattern of a 2.25MHz ultrasound transducer with lateral and axial resolutions consistent with images generated from a
commercial hydrophone (Onda Inc.). The sensitivity of this device was ~2 nV/Pa. Our simulations and experimental
results both indicate that designs using a combination of ITO and gold (ratio of resistivities = ~18) produce the best
results. We hope that design optimization will lead to new devices for monitoring ultrasonic fields for biomedical
imaging and therapy, including lithotripsy and focused ultrasound surgery.
Clinical ultrasound (US) imaging and therapy require a precise knowledge of the intensity distribution of the
acoustic field. Although piezoelectric hydrophones are most common, these devices are limited in terms of, for example,
type of materials, cost, and performance at high frequency and pressure. As an alternative to conventional acoustic
detectors, we describe acoustoelectric hydrophones, developed using photolithographic fabrication techniques, where the
induced voltage (phase and amplitude) is proportional to both the US pressure and bias current injected through the
device. In this study a number of different hydrophone designs were created using indium tin oxide (ITO). A constriction
of the current path within the hydrophone created a localized "sensitivity zone" of high current density. The width of this
zone ranged from 30 to 1000 μm, with a thickness of 100 nm. A raster scan of the US transducer produced a map of the
acoustic field. Hydrophones were evaluated by mapping the pressure field of a 2.25 MHz single element transducer, and
their performance was compared to a commercial capsule hydrophone. Focal spot sizes at -6 dB were as low as 1.75 mm,
comparing well with the commercial hydrophone measurement of 1.80 mm. Maximum sensitivity was 2 nV/Pa and up to
the 2nd harmonic was detected. We expect improved performance with future devices as we optimize the design.
Acoustoelectric hydrophones are potentially cheaper and more robust than the piezoelectric models currently in clinical
use, potentially providing more choice of materials and designs for monitoring therapy or producing arrays for imaging.
Accurate three dimensional (3D) mapping of bioelectric sources in the body with high spatial resolution is important
for the diagnosis and treatment of a variety of cardiac and neurological disorders. Ultrasound current source density
imaging (UCSDI) is a new technique that maps electrical current flow in tissue. UCSDI is based on the acousto-electric
(AE) effect, an interaction between electrical current and acoustic pressure waves propagating through a conducting
material and has distinct advantages over conventional electrophysiology (i.e., without ultrasound). In this study, UCSDI
was used to simultaneously image current flow induced in two tissue phantoms positioned at different depths. Software
to simulate AE signal was developed in Matlab™ to complement the experimental model and further characterize the
relationship between the ultrasound beam and electrical properties of the tissue. Both experimental and simulated images
depended on the magnitude and direction of the current, as well as the geometry (shape and thickness) and location of
the current sources in the ultrasound field (2.25MHz transducer). The AE signal was proportional to pressure and current
with detection levels on the order of 1 mA/cm<sup>2</sup> at 258kPa. We have imaged simultaneously two separate current sources in tissue slabs using UCSDI and two bridge circuits to accurately monitor current flow through each source. These results are consistent with UCSDI simulations of multiple current sources. Real-time 3D UCSD images of current flow automatically co-registered with pulse echo ultrasound potentially facilitates corrective procedures for cardiac and neural abnormalities.
Understanding the tumor microenvironment is critical to characterizing how cancers operate and predicting how they
will eventually respond to treatment. The mouse window chamber model is an excellent tool for cancer research,
because it enables high resolution tumor imaging and cross-validation using multiple modalities. We describe a novel
multimodality imaging system that incorporates three dimensional (3D) photoacoustics with pulse echo ultrasound for
imaging the tumor microenvironment and tracking tissue growth in mice. Three mice were implanted with a dorsal skin
flap window chamber. PC-3 prostate tumor cells, expressing green fluorescent protein (GFP), were injected into the skin.
The ensuing tumor invasion was mapped using photoacoustic and pulse echo imaging, as well as optical and fluorescent
imaging for comparison and cross validation. The photoacoustic imaging and spectroscopy system, consisting of a
tunable (680-1000nm) pulsed laser and 25 MHz ultrasound transducer, revealed near infrared absorbing regions,
primarily blood vessels. Pulse echo images, obtained simultaneously, provided details of the tumor microstructure and
growth with 100-μm3 resolution. The tumor size in all three mice increased between three and five fold during 3+ weeks
of imaging. Results were consistent with the optical and fluorescent images. Photoacoustic imaging revealed detailed
maps of the tumor vasculature, whereas photoacoustic spectroscopy identified regions of oxygenated and deoxygenated
blood vessels. The 3D photoacoustic and pulse echo imaging system provided complementary information to track the
tumor microenvironment, evaluate new cancer therapies, and develop molecular imaging agents in vivo. Finally, these
safe and noninvasive techniques are potentially applicable for human cancer imaging.
Recent clinical studies have demonstrated that photoacoustic (PA) imaging, in conjunction with pulse echo (PE)
ultrasound is a promising modality for diagnosing breast cancer. However, existing devices are unwieldy and are hard to
integrate into the clinical environment. In addition, it is difficult to illuminate thick samples because light must be
directed around the transducer. Conventional PA imaging designs involve off-axis illumination or transillumination
through the object. Whereas transillumination works best with thin objects, off-axis illumination may not uniformly
illuminate the region of interest. To overcome these problems we have developed an attachment to an existing clinical
linear array that can efficiently deliver light in line with the image plane. This photoacoustic enabling device (PED)
exploits an optically transparent acoustic reflector to co-align the illumination with the acoustic waves, enabling realtime
PA and PE imaging. Based on this concept, we describe results from three types of PEDs in phantoms and rat
tissue. The most recent version is fabricated by rapid prototyping, and attached to a 10 MHz linear array. Real-time PA
and PE images of a 127-μm diameter wire were consistent with our expectations based on the properties of the
ultrasound transducer. Comparisons with and without the PED of another test phantom printed on transparency
demonstrated that the PED does not appreciably degrade or distort image quality. The PED offers a simple and
inexpensive solution towards a real-time dual-modality imaging system for breast cancer detection. It could also be
adapted for virtually any kind of ultrasound transducer array and integrated into routine ultrasound exams for detection
of cancerous lesions within 1-2 cm from the probe surface.
Photoacoustic (PA) imaging is a rapidly developing imaging modality that can detect optical contrast agents with high
sensitivity. While detectors in PA imaging have traditionally been single element ultrasound transducers, use of array
systems is desirable because they potentially provide high frame rates to capture dynamic events, such as injection and
distribution of contrast in clinical applications. We present preliminary data consisting of 40 second sequences of coregistered
pulse-echo (PE) and PA images acquired simultaneously in real time using a clinical ultrasonic machine.
Using a 7 MHz linear array, the scanner allowed simultaneous acquisition of inphase-quadrature (IQ) data on 64
elements at a rate limited by the illumination source (Q-switched laser at 20 Hz) with spatial resolution determined to be
0.6 mm (axial) and 0.4 mm (lateral). PA images had a signal-to-noise ratio of approximately 35 dB without averaging.
The sequences captured the injection and distribution of an infrared-absorbing contrast agent into a cadaver rat heart.
From these data, a perfusion time constant of 0.23 s<sup>-1</sup> was estimated. After further refinement, the system will be tested
in live animals. Ultimately, an integrated system in the clinic could facilitate inexpensive molecular screening for
coronary artery disease.
Conventional methods for mapping cardiac current fields have either poor spatial resolution (e.g. ECG) or are time
consuming (e.g., intra-cardiac catheter electrode mapping). We present a method based on the acousto-electric effect
(AEE) and lead field theory for minimally-invasive mapping of 2D current distributions. The AEE is a pressure-induced
conductivity modulation in which focused ultrasound can be used as a spatially-localized pressure source. As a proof of
principle we generated a 2D dipole field in a thin bath of 0.9% NaCl solution by injecting 28 mA through a pair of
electrodes. A 7.5 MHz transducer was focused on the bath from below. A recording electrode was rotated along the
boundary of the bath in 20° steps. For each angle, the transducer was swept over the bath in a raster scan. A pulse-echo
and an AEE voltage trace were acquired at each point. The AEE traces were combined in post-processing as if coming
from a multi-electrode circular array. The direction and magnitude of the current field at each point in the plane was
estimated from the AEE and compared to simulation. The potential field was independently mapped using a roving
monopolar electrode. The correlation coefficient between this map and the simulated field was 0.9957. A current source
density analysis located the current source and sink to within 1±2 mm of their true position. This method can be
extended to 3 dimensions and has potential for use in rapid mapping of current fields in the heart with high spatial
Conventional methods for mapping cardiac current fields lack either spatial resolution (e.g. ECG) or are time consuming (e.g., intra-cardiac catheter electrode mapping). We present a method based on the acousto-electric effect (AEE) with potential for rapid mapping of current fields in the heart with high spatial resolution. The AEE is a pressure-induced conductivity modulation, in which focused ultrasound can be used as a spatially localized pressure source. When an ultrasound beam is focused between a pair of recording electrodes in a homogeneous conductive medium, an induced voltage will be produced due to the pressure-modulated conductivity and local current density. The amplitude of the voltage change should be proportional to fluctuations in current density, such as those generated during the cardiac cycle, in the region of focused ultrasound. Preliminary experiments demonstrate the feasibility of this method. A 540 kHz ultrasound transducer is focused between two tin electrodes lying parallel to the beam axis. These electrodes inject current into a 0.9% saline solution. A pair of insulated stainless steel electrodes exposed at the tip is used to record voltage. To simulate a cardiac current, a low frequency current waveform is injected into the sample such that the peak current density (8 mA/cm<sup>2</sup>) approximates cardiac currents. The transducer is pulsed at different delays after waveform initiation. Delays are chosen such that the low frequency waveform is adequately sampled. Using this approach an emulated ECG waveform has been successfully reconstructed from the ultrasound modulated voltage traces.