We demonstrated the potential of carbon nanoparticles (CNPs) as exogenous contrast agents for both thermoacoustic
(TA) tomography (TAT) and photoacoustic (PA) tomography (PAT). In comparison to deionized water, the CNPs
provided a four times stronger signal in TAT at 3 GHz. In comparison to blood, The CNPs provided a much stronger
signal in PAT over a broad wavelength range of 450-850 nm. Specifically, the maximum signal enhancement in PAT
was 9.4 times stronger in the near-infrared window of 635-670 nm. In vivo blood-vessel PA imaging was performed
non-invasively on a mouse femoral area. The images, captured after the tail vein injection of CNPs, show a gradual
enhancement of the optical absorption in the vessels by up to 230%. The results indicate that CNPs can be potentially
used as contrast agents for TAT and PAT to monitor the intravascular or extravascular pathways in clinical applications.
Photoacoustic tomography (PAT) of the human brain is challenging due to the fact that the skull strongly absorbs and scatters light, and attenuates and distorts ultrasound as well. For the first time, we demonstrated the feasibility of PAT through a whole adult human skull. A photon recycler (PR) was built to increase light transmittance through the skull. Both a graphite target and a canine brain were imaged through the skull. Use of the PR was found to improve the photoacoustic signal-to-noise ratio by a factor of 2.4. In addition, subtraction of photoacoustic signals that arise from light absorption within the skull significantly improved the contrast of the target. Our results indicate that PAT can potentially be applied to in vivo human brain imaging.
A challenge in photoacoustic tomography (PAT) brain imaging is to compensate for aberrations in the measured photoacoustic data due to their propagation through the skull. By use of information regarding the skull morphology and composition obtained from adjunct x-ray computed tomography image data, we developed a subject-specific imaging model that accounts for such aberrations. A time-reversal-based reconstruction algorithm was employed with this model for image reconstruction. The image reconstruction methodology was evaluated in experimental studies involving phantoms and monkey heads. The results establish that our reconstruction methodology can effectively compensate for skull-induced acoustic aberrations and improve image fidelity in transcranial PAT.
We report an investigation of image reconstruction in photoacoustic tomography for objects that possess heterogeneous material and acoustic attenuation distributions. When the object contains materials, such as bone and soft-tissue, that are modeled using power law attenuation models with distinct exponents, we demonstrate that the effects of acoustic attenuation due to the most strongly attenuating material can be compensated for if the attenuation of the other less attenuating material(s) are neglected. Experiments with phantom objects are presented to validated our findings.
A photoacoustic tomography (PAT) system using a virtual point ultrasonic transducer was developed for transcranial
imaging of monkey brains. The virtual point transducer provided a 10 times greater field-of-view (FOV) than finiteaperture
unfocused transducers, which enables large primate imaging. The cerebral cortex of a monkey brain was
accurately mapped transcranially, through up to two skulls ranging from 4 to 8 mm in thickness. The mass density and
speed of sound distributions of the skull were estimated from adjunct X-ray CT image data and utilized with a timereversal
algorithm to mitigate artifacts in the reconstructed image due to acoustic aberration. The oxygenation saturation
(sO2) in blood phantoms through a monkey skull was also imaged and quantified, with results consistent with
measurements by a gas analyzer. The oxygenation saturation (sO2) in blood phantoms through a monkey skull was also
imaged and quantified, with results consistent with measurements by a gas analyzer. Our experimental results
demonstrate that PAT can overcome the optical and ultrasound attenuation of a relatively thick skull, and the imaging
aberration caused by skull can be corrected to a great extent.
A photoacoustic tomography system (PAT) using virtual point ultrasonic transducers was developed and applied to image a monkey brain. The custom-built transducers provide a 10-fold greater field-of-view (FOV) than finite-aperture unfocused transducers as well as an improved signal-to-noise ratio (SNR) and reduced artifacts rather than negative-lens transducers. Their tangential resolution, radial resolution, and (SNR) improvements were quantified using tissue phantoms. Our PAT system can achieve high uniformity in both resolution (<1 mm) and SNR (>8) within a large FOV of 6 cm in diameter, even when the imaging objects are enclosed by a monkey skull. The cerebral cortex of a monkey brain was accurately mapped transcranially, through a skull ranging from 2 to 4 mm in thickness. This study demonstrates that PAT can overcome the optical and ultrasound attenuation of a relatively thick skull and can potentially be applied to human neonatal brain imaging.
A uniform energy field of microwave illumination on sample is assumed for microwave-induced thermoacoustic
tomography. However, microwave transmitting out of the waveguide surface is nonuniform due to microwave diffraction,
which would lead to uneven excitation of thermoacoustic pressure. Hence, the thermoacoustic images may be distorted
especially at the edge of microwave radiation. A fast thermoacoustic tomography system at 6 GHz was developed for
phantom study and in vivo animals imaging. The effects of microwave distribution inhomogeneity on nonuniform
excitation of acoustic pressure were theoretically studied and a corresponding calibration algorithm for image distortion
was also provided and experimentally verified. The distribution formulas of microwave field were derived using a
Huygens diffraction principle model. Then a point microwave absorber moved under the microwave waveguide to
measure the microwave field distribution. The measure data is in good agreement with the deduced result.
Once the calibration map was obtained via the theoretical calculation, the TAT (thermoacoustic tomography)
images could be calibrated by dividing the reconstructed image by the calibration map. Thermoacoustic images without
and with calibration were reconstructed for comparison. According to the statistical results, after calibration the
thermoacoustic contrast can be enhanced 2 times or more. Also it can be supposed that the farther the distance away
from the illumination centre, the greater signal-noise-ratio (SNR) could be enhanced by the calibration. The results of
experiment showed that this method could achieve even distribution of SNR and improve the reconstructed image
quality. Therefore, this calibration method has potential application in solving the problem of imaging distortion
especially at the edge of microwave illumination.
Based on the measurement of ultrasonic waves induced by electromagnetic pulses, photoacoustic imaging and
thermoacoustic imaging can reveal optical or dielectric properties of tissues that are closely related to the physiological
and pathological status of tissues and they have became the promising clinical imaging modalities. In this paper, a high
frame rate tomography instrument with 64-channel parallel data-acquisition system was designed and developed for
photoacoustic biomedical imaging. In the system, the pulse-laser-induced ultrasonic signals are converted to voltage
signals by a 128-element linear ultrasound transducer array. The 128-channel signals are acquired by the 64-channel
parallel data-acquisition system twice through the 2:1 multiplexer, and the OPO (optical parametric oscillator) laser
provides laser with a pulse repetition rate of 15 Hz. Therefore, the acquisition rate can reach about 7 frames per second
and photoacoustic images can be displayed dynamically. Based on the above signal acquisition system, biomedical
application of photoacoustic imaging was explored and successfully performed. Fast photoacoustic tomography for
flow-field visualization was demonstrated by flowing object. Thermoacoustic tomography was developed to detection of
low-density foreign targets in small animals which was indistinguishable with X-ray. All the experimental results show
that photoacoustic and thermoacoustic imaging with fast parallel data-acquisition system is a highly-efficient approach
for functional imaging of biomedical tissues.
Microwave-induced thermoacoustic or photoacoustic imaging is a nonionizing imaging modality based on the difference
in microwave or light absorption of various biological tissues. The advantage of this imaging over traditional optical
imaging is that it retains intrinsic microwave or optical contrast characteristics while benefiting from the
diffraction-limited high spatial resolution of ultrasound. Various tissues present particular characteristics in their
absorption spectra. Photoacoustic imaging can map optical absorption distribution whereas microwave-induced
thermoacoustic imaging is related to the electrical properties in the objects. Muscles which have a rich blood supply
provide excellent optical contrast. In contrast high water or ion content tissues, such as muscle tissues or malignant
tissues, demonstrate high contrast to fatty tissues employing microwave radiation. Besides different contrast mechanisms,
microwave-induced thermoacoustic imaging may find unique applications because microwave radiation provides a
deeper penetration depth in biological tissues than optical radiation. The frequency spectrum of thermoacoustic images is
much lower and the spatial resolution is poorer due to the pulse width of microwave radiation.
Therefore, thermoacoustic and photoacoustic imaging are associated with different characteristics based on
information from tissue properties over an electromagnetic spectrum from microwave to optical bands. The
reconstructed thermoacoustic and photoacoustic images with their image blending may be more accurate and
comprehensive than was previously available. Also the experimental results show that breast cancer detection is a
promising, and specific application field by microwave-induced thermoacoustic imaging. We are able to provide
multimodality, complementary, and low cost images which can be potentially used for early cancer detection and
imaging.
The customary methods for localizing the foreign body in human body present several disadvantages, radioscopy has
significant radiation exposure, MRI can not work with the metal, and also they are expensive and neither is portable.
Microwave-induced thermoacoustic imaging which shares similar mechanism with photoacoustic imaging can achieve
high ultrasound resolution and good microwave absorption contrast. Besides, microwave can reach deeply hidden sample
in biological tissue because of its long wavelength, hence, this new imaging modality can be potentially applied to detect
the foreign body hidden in human body. Our detecting methods were validated by imaging simulated foreign body
embedded in biological tissue at different depths, the unknown samples embedded deeply were discernible and the signal
to noise ratio was good. By employing the multi-element linear transducer array and phase-controlled focus algorithm, a
circular scan of thermoacoustic signals at 20 angles needs no more than 2 minutes. Our established thermoacoustic
imaging system has potential to supply a novel and fast method for surgical localization of an unknown foreign body.
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