SignificanceIn the photoacoustic (PA) technique, the laser irradiation in the time domain (i.e., laser pulse duration) governs the characteristics of PA imaging—it plays a crucial role in the optical-acoustic interaction, the generation of PA signals, and the PA imaging performance.AimWe aim to provide a comprehensive analysis of the impact of laser pulse duration on various aspects of PA imaging, encompassing the signal-to-noise ratio, the spatial resolution of PA imaging, the acoustic frequency spectrum of the acoustic wave, the initiation of specific physical phenomena, and the photothermal–PA (PT-PA) interaction/conversion.ApproachBy surveying and reviewing the state-of-the-art investigations, we discuss the effects of laser pulse duration on the generation of PA signals in the context of biomedical PA imaging with respect to the aforementioned aspects.ResultsFirst, we discuss the impact of laser pulse duration on the PA signal amplitude and its correlation with the lateral resolution of PA imaging. Subsequently, the relationship between the axial resolution of PA imaging and the laser pulse duration is analyzed with consideration of the acoustic frequency spectrum. Furthermore, we examine the manipulation of the pulse duration to trigger physical phenomena and its relevant applications. In addition, we elaborate on the tuning of the pulse duration to manipulate the conversion process and ratio from the PT to PA effect.ConclusionsWe contribute to the understanding of the physical mechanisms governing pulse-width-dependent PA techniques. By gaining insight into the mechanism behind the influence of the laser pulse, we can trigger the pulse-with-dependent physical phenomena for specific PA applications, enhance PA imaging performance in biomedical imaging scenarios, and modulate PT-PA conversion by tuning the pulse duration precisely.
Carotid arteries are important channels delivering blood and oxygen to brain. Atherosclerosis plaque in carotid arteries hinders blood delivery, and plaque rupture causes stroke, leading to high morbidity and motility. Extensive preclinical and clinical studies showed that atherosclerosis inflammation activities are highly related to plaque vulnerability. Thus, visualizing the inflammation of atherosclerosis plaques is important in atherosclerosis vulnerability assessment. In this study, photoacoustic imaging modality was applied for carotid atherosclerosis inflammation identification of mouse in vivo. Deficient apolipoprotein E (ApoE-/-) mice with high-fat diet and normal diet for 16 weeks were employed as atherosclerosis models and control models, respectively. Photoacoustic molecular probes with optical absorption at nearinfrared wavelength and specifically target cluster of differentiation 36 (CD36) were employed to mark inflammation cells in carotid atherosclerosis plaques of mouse in vivo. Noninvasive imaging of atherosclerosis inflammation cells marked by molecular probes was performed by point-to-point scanning with a custom-built acoustic-resolution photoacoustic imaging system. Considering low scattering of near-infrared light in tissues and mature commercialization of laser, excitation wavelength in this research is chosen at 1064 nm. Carotid arteries with and without atherosclerosis plaques have been noninvasively imaged and distinguished. Furthermore, carotid atherosclerosis with different inflammation severity has been analyzed by photoacoustic imaging and immunohistochemistry staining. Photoacoustic signal from atherosclerosis arteries showed high relativity with inflammation severity defined by immunohistochemistry staining, evidencing the reliability of the novel imaging technology in atherosclerosis inflammation identification. This study paves the way for photoacoustic imaging technology to atherosclerosis inflammation identification, severity quantification and even further atherosclerosis therapy.
Intravascular photoacoustic imaging (IVPA) can obtain specific inflammation information and lipid composition in vivo, which is a new method for the diagnosis of atherosclerotic plaques. Numerous IVPA systems have been proposed and pushed towards clinical application. But imaging speed hinders their final clinical translation, considering necessary blood flush or balloon blood blockage operations during the intravascular intervention. In this study, we developed a high-speed IVPA system based on a 1064 nm pulsed laser, with the imaging speed of 60 round/second, about twice speed of the fastest IVPA system. In this system, a 0.9 mm outer diameter catheter was used for simultaneous IVPA and IVUS imaging. A plastic tube with an outer diameter of 1.3 mm was wrapped on the outside of the imaging catheter for protection and blood flushing channels. Firstly, the capability of high-speed imaging was verified by the imaging of a manually moving metal needle. Photoacoustic and ultrasonic images of the needle were obtained. No artifacts were found during the real-time imaging of the needle, which was unavoidable in the low-speed imaging system. Then, an artery excised from abdominal aorta of a New Zealand rabbit was sealed and a certain frequency of flowing water was injected from the one end to simulate the pulsation of blood vessels with a frequency of about 4.5 Hz, which was a typical heart rate of a rabbit. The high-speed IVPA-US imaging of pulsed blood vessels was successfully performed, which proved the feasibility of the system in vivo and even further clinical application.
The optimal photoacoustic probe design is the key to obtain highest imaging sensitivity in photoacoustic computed tomography. Two commonly used probe design types are dark- and bright-field designs. We proposed a design for photoacoustic probe called quasibright-field illumination and compared the performance of all three kinds of probes theoretically and experimentally. Our conclusion is that the proposed quasibright-field illumination photoacoustic probe is superior compared to the existing probe designs as demonstrated. However, each type of illumination should still have its own advantages under certain circumstances. The dark-field illumination is capable of minimizing surface interference signals and reducing their contributions to the background of deeper signals. Hence, it should perform better when imaging samples with high optical absorbance at the surface layer. The bright field may perform better under circumstance when phase distortion is less. We also designed and fabricated three kinds of probes using a single multimode optical fiber for laser energy delivery instead of fiber bundle. Single fiber probes are low cost, transmit laser energy efficiently, and are compact for easy handling. Thus, our study not only provides a method for probe design but also a guidance for cost-effective transducer array-based photoacoustic probe design and manufacturing in the future.
KEYWORDS: Transducers, Photoacoustic tomography, Acoustics, Signal detection, Imaging systems, Signal to noise ratio, Data acquisition, Quartz, Ultrasonics, Sensors
A dual-foci transducer with coplanar light illumination and acoustic detection was applied for the first time. It overcame the small directivity angle, low-sensitivity, and large datasets in conventional circular scanning or array-based photoacoustic computed tomography (PACT). The custom-designed transducer is focused on both the scanning plane with virtual-point detection and the elevation direction for large field of view (FOV) cross-sectional imaging. Moreover, a coplanar light illumination and acoustic detection configuration can provide ring-shaped light irradiation with highly efficient acoustic detection, which in principle has a better adaptability when imaging samples of irregular surfaces. Phantom experiments showed that our PACT system can achieve high resolution (∼0.5 mm), enhanced signal-to-noise ratio (16-dB improvement), and a more complete structure in a greater FOV with an equal number of sampling points compared with the results from a flat aperture transducer. This study provides the proof of concept for the fabrication of a sparse array with the dual-foci property and large aperture size for high-quality, low-cost, and high-speed photoacoustic imaging.
For the diagnosis and evaluation of ophthalmic diseases, imaging and quantitative characterization of vasculature in the iris are very important. The recently developed photoacoustic imaging, which is ultrasensitive in imaging endogenous hemoglobin molecules, provides a highly efficient label-free method for imaging blood vasculature in the iris. However, the development of advanced vascular quantification algorithms is still needed to enable accurate characterization of the underlying vasculature. We have developed a vascular information quantification algorithm by adopting a three-dimensional (3-D) Hessian matrix and applied for processing iris vasculature images obtained with a custom-built optical-resolution photoacoustic imaging system (OR-PAM). For the first time, we demonstrate in vivo 3-D vascular structures of a rat iris with a the label-free imaging method and also accurately extract quantitative vascular information, such as vessel diameter, vascular density, and vascular tortuosity. Our results indicate that the developed algorithm is capable of quantifying the vasculature in the 3-D photoacoustic images of the iris in-vivo, thus enhancing the diagnostic capability of the OR-PAM system for vascular-related ophthalmic diseases in vivo.
We developed a miniaturized, simple and full field-of-view photoacoustic/ultrasonic endoscopy system, and used a flexible coil to transmit the rotational torque from the rotary stage, which enables a 360o field-of-view imaging in vivo. The developed imaging catheter was fully encapsulated by a single-use protective polyamide tube. A B-scan rate up to 5 Hz (200 A-lines/B-scan) was achieved. Three-dimensional photoacoustic and ultrasound images of the rectum from a SD rat were acquired in vivo. It suggests that this PAE system can be of great interest for clinical translation for a variety of endoscopic applications.
Photoacoustic computed tomography (PACT) has emerged as a unique and promising technology for multiscale biomedical imaging. To fully realize its potential for various preclinical and clinical applications, development of systems with high imaging speed, reasonable cost, and manageable data flow are needed. Sparse-sampling PACT with advanced reconstruction algorithms, such as compressed-sensing reconstruction, has shown potential as a solution to this challenge. However, most such algorithms require iterative reconstruction and thus intense computation, which may lead to excessively long image reconstruction times. Here, we developed a principal component analysis (PCA)-based PACT (PCA-PACT) that can rapidly reconstruct high-quality, three-dimensional (3-D) PACT images with sparsely sampled data without requiring an iterative process. In vivo images of the vasculature of a human hand were obtained, thus validating the PCA-PACT method. The results showed that, compared with the back-projection (BP) method, PCA-PACT required ∼50% fewer measurements and ∼40% less time for image reconstruction, and the imaging quality was almost the same as that for BP with full sampling. In addition, compared with compressed sensing-based PACT, PCA-PACT had approximately sevenfold faster imaging speed with higher imaging accuracy. This work suggests a promising approach for low-cost, 3-D, rapid PACT for various biomedical applications.
Using a water-immersion optical objective in conjunction with a miniature 40-MHz ultrasonic transducer, we developed reflection-mode photoacoustic microscopy with a transverse resolution as high as 320 nm. Here, we further integrated two-photon microscopy capability into the system to enable multimodality in vivo biomedical imaging at submicron resolution. As a result, the system is capable of tri-modality label-free imaging of microvasculature, collagen, and cell morphology, based on the contrast of optical absorption, second-harmonic generation, and autofluorescence, respectively. In addition, we demonstrated simultaneous microscopic imaging of neuron and microvasculature in the brain cortex of a living mouse, which may offer new opportunities for studying the mechanisms of neurovascular coupling.
KEYWORDS: Signal to noise ratio, Photoacoustic microscopy, In vivo imaging, Signal processing, Image resolution, Acoustics, Image processing, Signal detection, Transducers, Carbon
We propose an improved version of a synthetic aperture focusing technique (SAFT) based on a delay-multiply-and-sum algorithm for acoustic-resolution photoacoustic microscopy (AR-PAM). In this method, the photoacoustic (PA) signals from multiple scan-lines are combinatorially coupled, multiplied, and then summed. This process can be considered a correlation operation of the PA signals in each scan-line, so the spatial coherent information between the PA signals can be efficiently extracted. By applying this method in conventional AR-PAM, lateral resolution and signal-to-noise ratio in out-of-focus regions are much improved compared with those estimated from the previously developed SAFT, respectively, thereby achieving the extension of the imaging focal region. Our phantom and in vivo imaging experiments prove the validity of our proposed method.
Photoacoustic-computed microscopy (PACM) differs from conventional photoacoustic microscopy (PAM) imaging techniques in a way that thousands of optical foci are generated simultaneously using a two-dimensional microlens array, and raster-scanning these optical foci provides wide-field images. A major limitation of PACM is the slow imaging speed caused by the high power pulsed lasers and large amount of acoustic detectors. Here, we addressed this problem through compressed sensing and image inpainting. Compressed sensing minimizes the number of transducer elements used to acquire each frame, while inpainting minimizes the scanning steps. Combining these two approaches, we improved the imaging speed by sixteen times.
Intravascular spectroscopic photoacoustic technology can image atherosclerotic plaque composition with high sensitivity and specificity, which is critical for identifying vulnerable plaques. Here, we designed and engineered a catheter of 0.9 mm in diameter for intravascular photoacoustic (IVPA) imaging, smaller than the critical size of 1 mm required for clinical translation. Further, a quasifocusing photoacoustic excitation scheme was developed for the catheter, producing well-detectable IVPA signals from stents and lipids with a laser energy as low as ∼30 μJ/pulse. As a result, this design enabled the use of a low-energy, high-repetition rate, ns-pulsed optical parametric oscillator laser for high-speed spectroscopic IVPA imaging at both the 1.2-μm and 1.7-μm spectral bands for lipid detection. Specifically, for each wavelength, a 1-kHz IVPA A-line rate was achieved, ∼100-fold faster than previously reported IVPA systems offering a similar wavelength tuning range. Using the system, spectroscopic IVPA imaging of peri-adventitial adipose tissue from a porcine aorta segment was demonstrated. The significantly improved imaging speed, together with the reduced catheter size and multiwavelength spectroscopic imaging ability, suggests that the developed high-speed IVPA technology is of great potential to be further translated for in vivo applications.
Photoacoustic microscopy (PAM) is becoming a vital tool for various biomedical studies, including functional and molecular imaging of cancer. However, due to the use of a focused ultrasonic transducer for photoacoustic detection, the image quality of conventional PAM degrades rapidly away from the ultrasonic focal zone. To improve the image quality of PAM for out-of-focus regions, we have developed compressed sensing based virtual-detector photoacoustic microscopy (CS-PAM). Through phantom and in vivo experiments, it has been demonstrated that CS-PAM can effectively extend the depth of focus of PAM, and thus may greatly expand its potential biomedical applications.
Intravascular ultrasound (IVUS) plays a vital role in assessing the severity of atherosclerosis and has greatly enriched our knowledge on atherosclerotic plaques. However, it mainly reveals the structural information of plaques. In contrast, spectroscopic and molecular photoacoustic imaging can potentially improve plaque composition identification, inflammation detection, and ultimately the stratification of plaque vulnerability and risk. In this work, we developed an integrated intravascular ultrasound and optical-resolution photoacoustic microscopy (IVUS-PAM) system with a single catheter as small as 1 mm in diameter, comparable to that of existing clinical IVUS catheters. In addition, by using a GRIN lens to focus the excitation laser pulse, the system provides an optical-diffraction limited photoacoustic lateral resolution as fine as 19.6 micrometers, ~10-fold finer than that of conventional intravascular photoacoustic imaging and existing IVUS technology. The system employs a custom-made miniaturized single-element ultrasonic transducer with a dimension of ~0.5 mm, a centre frequency of ~40 MHz, and a fractional bandwidth of ~60%. The IVUS-PAM can simultaneously acquire co-registered IVUS images with an axial resolution of ~40 micrometers and a lateral resolution of ~200 micrometers. In the future, IVUS-PAM may open up new opportunities for improved high-resolution vulnerable plaque imaging and image-guided stent deployment.
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.
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.
KEYWORDS: Imaging systems, Photoacoustic spectroscopy, In vivo imaging, Skin, Arteries, Photoacoustic microscopy, Real time imaging, Photoacoustic imaging, Ultrasonography, Tissue optics
With a refined ultrasound-array-based real-time photoacoustic microscopy (UA-PAM) system, we demonstrate the feasibility of noninvasive in vivo imaging of human pulsatile dynamics. The system, capable of real-time B-scan imaging at 50 Hz and high-speed 3-D imaging, is validated by imaging the subcutaneous microvasculature in rats and humans. After the validation, a human artery around the palm-wrist area is imaged, and its pulsatile dynamics, including the arterial pulsatile motion and changes in hemoglobin concentration, is monitored with 20-ms B-scan imaging temporal resolution. To our knowledge, this is the first demonstration of real-time photoacoustic imaging of human physiological dynamics. Our results show that UA-PAM can potentially enable many new possibilities for studying functional and physiological dynamics in both preclinical and clinical imaging settings.
KEYWORDS: Acquisition tracking and pointing, Photoacoustic tomography, In vivo imaging, Data acquisition, Compressed sensing, Sensors, Reconstruction algorithms, Ultrasonics, Tomography, Wavelets
The data acquisition speed in photoacoustic computed tomography (PACT) is limited by the laser repetition rate and the number of parallel ultrasound detecting channels. Reconstructing an image with fewer measurements can effectively accelerate the data acquisition and reduce the system cost. We adapt compressed sensing (CS) for the reconstruction in PACT. CS-based PACT is implemented as a nonlinear conjugate gradient descent algorithm and tested with both phantom and in vivo experiments.
Using realtime ultrasound array photoacoustic microscopy (UA-PAM), we demonstrated the feasibility of
noninvasive in vivo imaging of human pulsatile dynamics, as well as
3-D dynamic imaging of sentinel
lymph nodes (SLNs) in a murine model. The system, capable of realtime B-scan imaging at 50 Hz and
high-speed 3-D imaging, was validated by imaging the subcutaneous microvasculature in rats and humans.
After the validation, a human superficial palmar was imaged, and its pulsatile dynamics monitored, with
20-ms B-scan imaging temporal resolution. In addition, noninvasive photoacoustic sentinel lymph node
(SLN) mapping with high spatial resolution has the potential to reduce the false negative rate and eliminate
the use of radioactive tracers. Upon intra-dermal injection of Evans blue, the system maps SLNs accurately
in mice and rats. Furthermore, the ~6 s 3-D imaging temporal resolution offers the capability to
quantitatively and noninvasively monitor the dye dynamics in SLNs in vivo through sequential 3-D
imaging. The demonstrated capability suggests that high-speed 3-D photoacoustic imaging should facilitate
the understanding of the dynamics of various dyes in SLNs, and potentially help identify SLNs with high
accuracy. With the results shown in this study, we believe that
UA-PAM can potentially enable many new
possibilities for studying functional and physiological dynamics in both preclinical and clinical imaging
settings.
The data acquisition speed in photoacoustic computed tomography (PACT) is limited by the laser repetition
rate and the number of parallel ultrasound detecting channels. Reconstructing PACT image with a less number of
measurements can effectively accelerate the data acquisition and reduce the system cost. Recently emerged
Compressed Sensing (CS) theory enables us to reconstruct a compressible image with a small number of projections.
This paper adopts the CS theory for reconstruction in PACT. The idea is implemented as a non-linear conjugate
gradient descent algorithm and tested with phantom and in vivo experiments.
We present an in vivo reflection-mode photoacoustic microscopy system that performs B-scan imaging at
50 Hz with realtime beamforming and 3-D imaging of 166 B-scan frames at 1 Hz with post-beamforming.
To our knowledge, this speed is currently the fastest in high frequency photoacoustic imaging. In addition,
with a custom fiber based light delivery system, the imaging device is capable of performing handheld
operation. Software for image processing and display with clinically user-friendly graphic user interface
(GUI) is developed. The system has axial, lateral, and elevational resolutions of 25, 70, and 200 μm,
respectively, and can image 3 mm deep in scattering biological tissue. Volumetric images of subcutaneous
vasculature in murine are demonstrated in vivo. The system is anticipated to have potential clinical
applications in skin melanoma detection due to its unique ability to image in realtime and to image
anatomical sites inaccessible to other imaging systems.
We present an in vivo dark-field reflection-mode photoacoustic microscopy system that performs cross-sectional (B-scan) imaging at 50 Hz with real-time beamforming and 3-D imaging consisting of 166 B-scan frames at 1 Hz with postbeamforming. To our knowledge, this speed is currently the fastest in photoacoustic imaging. A custom-designed light delivery system is integrated with a 30-MHz ultrasound linear array to realize dark-field reflection-mode imaging. Linear mechanical scanning of the array produces 3-D images. The system has axial, lateral, and elevational resolutions of 25, 70, and 200 µm, respectively, and can image 3 mm deep in scattering biological tissues. Volumetric images of subcutaneous vasculature in rats are demonstrated in vivo. Fast 3-D photoacoustic microscopy is anticipated to facilitate applications of photoacoustic imaging in biomedical studies that involve dynamics and clinical procedures that demand immediate diagnosis.
While photoacoustic imaging has emerged as a promising modality in recent years, a key drawback of practical and
widespread use of the technique has been slow imaging rates. We present a 30-MHz array-based photoacoustic imaging
system that can acquire and display photoacoustic images in realtime. Realtime display is very helpful and provides the
system operator the ability to better navigate and position the probe for selecting a desired anatomical field of view. The
system is capable of imaging at 50 frames per second to depths of a few mm in tissue. We used this system to
successfully image the beating hearts of young athymic nude mice in vivo. Also of interest was the ability to visualize
microvascular changes during respiration.
The optical properties diffuse reflection and transmission of normal porcine aorta and
thermally coagulated aorta were measured using low power laser irradiation at 1060 nm, 633 nm,
514 nm, and 325 nrn (reflection measurements only). The results from three of the wavelengths
(1060 nm, 633 nm, and 514 nm) were used with a Delta-Eddington diffusion approximation
model12 to calculate absorption coefficients, j.ta, and effective scattering coefficients, j.ts, of the
normal and thermally coagulated aorta at these wavelengths. The results show a strong wavelength
dependency of irreversible changes in optical properties of thennally damaged aorta, with a trend of
increasing changes in reflection approaching smaller wavelengths.
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