Search

OBJECTIVES Intravascular photoacoustic (IVPA) imaging is being developed to image atherosclerotic plaques, a leading cause of morbidity and mortality in the United States. However, the safety of this imaging modality, which requires repeated irradiation with short laser pulses, has not yet been investigated. This study has two objectives. First, determine in vitro the limit of cumulative fluence that can be applied to cells before death at IVPA relevant wavelengths. Second, evaluate if high single pulse fluences are a potential cause of cell death during IVPA imaging. MATERIALS AND METHODS Experiments were conducted using endothelial cells, macrophages, and smooth muscle cells. The cumulative fluence experiments were conducted at 1064 and 1197 nm, using a high pulse repetition frequency laser. Cells were irradiated with a wide range of cumulative fluences and evaluated for cell death. The thresholds for death were compared to the maximum expected clinical cumulative fluence. To evaluate the effect of single pulse fluences, cells were irradiated at 1064, 1210, and 1720 nm. Light was delivered at a range of pulse energies to emulate the fluences that cells would be exposed to during clinical IVPA imaging. RESULTS At 1064 nm, all three cell types remained viable at cumulative fluences above the maximum expected clinical cumulative fluence, which is calculated based on common IVPA imaging protocols. At 1197 nm, cells were viable near or just below the maximum expected clinical cumulative fluence, with some cell type to cell type variation. All three cell types remained viable after irradiation with high single pulse fluences at all three wavelengths. CONCLUSION The cumulative fluence experiments indicate that safety considerations are likely to put constraints on the amount of irradiation that can be used in IVPA imaging protocols. However, this study also indicates that it will be possible to use IVPA imaging safely, since cumulative fluences could be reduced by as much as two orders of magnitude below the maximum expected clinical cumulative fluence by varying the imaging protocol, albeit at the expense of image quality. The single pulse fluence experiments indicate that cell death from single pulse fluence is not likely during IVPA imaging. Thus, future studies should focus on heat accumulation as the likely mechanism of tissue damage. Lasers Surg. Med. © 2018 Wiley Periodicals, Inc.

In this study the dynamic behavior of a layered viscoelastic medium in response to the harmonic and impulsive acoustic radiation force applied to its surface was investigated both theoretically and experimentally. An analytical solution for a layered viscoelastic compressible medium in frequency and time domains was obtained using the Hankel transform. A special incompressible case was considered to model soft biological tissues. To verify our theoretical model, experiments were performed using tissue-like gel-based phantoms with varying mechanical properties. A 3.5 MHz single-element focused ultrasound transducer was used to apply the radiation force at the surface of the phantoms. A phase-sensitive optical coherence tomography (OCT) system was used to track the displacements of the phantom surface. Theoretically predicted displacements were compared with experimental measurements. The role of the depth dependence of the elastic properties of a medium in its response to an acoustic pulse at the surface was studied. It was shown that the low-frequency vibrations at the surface are more sensitive to the deep layers than high-frequency ones. Therefore, the proposed model in combination with spectral analysis can be used to evaluate depth-dependent distribution of the mechanical properties based on the measurements of the surface deformation.

Using a microbubble-based acoustic radiation force approach, spatial variations of Young's modulus and shear viscosity of the porcine vitreous humors in two groups-young pigs (6 months old) and mature pigs (2 to 3 years old)-were measured in situ. The measurements in these groups (4 specimens in each group) were performed in several positions along an anterior-to-posterior direction. At each position, microbubbles were generated by focusing a nanosecond pulsed laser beam and the displacement of each microbubble in response to an impulsive acoustic radiation force was measured every 10 μs using a custom-made high-pulse-repetition-frequency ultrasound system. Based on measured dynamics of the microbubble, Young's modulus and shear viscosity at various locations of the vitreous were reconstructed. Young's moduli of the young and mature porcine vitreous at anterior region were the highest, whereas the central region had the lowest values, indicating the clear spatial variations in the vitreous humor elasticity in both groups.

We present a preliminary study demonstrating the capability of ultrasound-guided intravascular photoacoustic (IVPA) imaging to visualize the depth-resolved distribution of lipid deposits in atherosclerotic plaques in vivo. Based on the characteristic optical absorption of lipid in the near infrared wavelength range, IVPA imaging at a single, 1720 nm, wavelength was used to provide a spatially resolved direct measurement of lipid content in atherosclerotic arteries. By overlaying an IVPA image with a spatially co-registered intravascular ultrasound (IVUS) image, the combined IVPA/IVUS image was used to visualize the distribution of lipid within the vessel wall. Ultrasound-guided IVPA imaging was performed in vivo in the abdominal aorta of a Watanabe heritable hyperlipidemic (WHHL) rabbit. Subsequently, the excised rabbit aorta filled with a solution of red blood cells (RBC) was then imaged ex vivo, and the histology was obtained in the section adjacent to the imaged cross-section. To demonstrate the potential of future clinical application of IVPA/IVUS imaging, a sample of diseased human right coronary artery (RCA) was also imaged. Both in vivo and ex vivo IVPA images clearly showed the distribution of lipid in the atherosclerotic vessels. In vivo IVPA imaging was able to identify diffuse, lipid-rich plaques in the WHHL rabbit model of atherosclerosis. Furthermore, IVPA imaging at a single wavelength was able to identify the lipid core within the human RCA ex vivo. Our results demonstrate that ultrasound-guided IVPA imaging can identify lipid in atherosclerotic plaques in vivo. Importantly, the IVPA/IVUS images were obtained in presence of luminal blood and no saline flush or balloon occlusion was required. Overall, our studies suggest that ultrasound-guided IVPA imaging can potentially be used for depth-resolved visualization of lipid deposits within the anatomical context of the vessel wall and lumen. Therefore, IVUS/ IVPA imaging may become an important tool for the detection of rupture-prone plaques.

A high pulse repetition frequency ultrasound system for an ex vivo measurement of mechanical properties of an animal crystalline lens was developed and validated. We measured the bulk displacement of laser-induced microbubbles created at different positions within the lens using nanosecond laser pulses. An impulsive acoustic radiation force was applied to the microbubble, and spatio-temporal measurements of the microbubble displacement were assessed using a custom-made high pulse repetition frequency ultrasound system consisting of two 25 MHz focused ultrasound transducers. One of these transducers was used to emit a train of ultrasound pulses and another transducer was used to receive the ultrasound echoes reflected from the microbubble. The developed system was operating at 1 MHz pulse repetition frequency. Based on the measured motion of the microbubble, Young's moduli of surrounding tissue were reconstructed and the values were compared with those measured using the indentation test. Measured values of Young's moduli of four bovine lenses ranged from 2.6 ± 0.1 to 26 ± 1.4 kPa, and there was good agreement between the two methods. Therefore, our studies, utilizing the high pulse repetition frequency ultrasound system, suggest that the developed approach can be used to assess the mechanical properties of ex vivo crystalline lenses. Furthermore, the potential of the presented approach for in vivo measurements is discussed.

An approach to assess the mechanical properties of a viscoelastic medium using laser-induced microbubbles is presented. To measure mechanical properties of the medium, dynamics of a laser-induced cavitation microbubble in viscoelastic medium under acoustic radiation force was investigated. An objective lens with a 1.13 numerical aperture and an 8.0 mm working distance was designed to focus a 532 nm wavelength nanosecond pulsed laser beam and to create a microbubble at the desired location. A 3.5 MHz ultrasound transducer was used to generate acoustic radiation force to excite a laser-induced microbubble. Motion of the microbubble was tracked using a 25 MHz imaging transducer. Agreement between a theoretical model of bubble motion in a viscoelastic medium and experimental measurements was demonstrated. Young’s modulii reconstructed using the laser-induced microbubble approach were compared with those measured using a direct uniaxial method over the range from 0.8 to 13 kPa. The results indicate good agreement between methods. Thus, the proposed approach can be used to assess the mechanical properties of a viscoelastic medium.

Intravascular photoacoustic (IVPA) imaging is a catheter-based, minimally invasive, imaging modality capable of providing high-resolution optical absorption map of the arterial wall. Integrated with intravascular ultrasound (IVUS) imaging, combined IVPA and IVUS imaging can be used to detect and characterize atherosclerotic plaques building up in the inner lining of an artery. In this paper, we present and discuss various representative applications of combined IVPA/IVUS imaging of atherosclerosis, including assessment of the composition of atherosclerotic plaques, imaging of macrophages within the plaques, and molecular imaging of biomarkers associated with formation and development of plaques. In addition, imaging of coronary artery stents using IVPA and IVUS imaging is demonstrated. Furthermore, the design of an integrated IVUS/IVPA imaging catheter needed for in vivo clinical applications is discussed.

Combined intravascular ultrasound and photoacoustic imaging (IVUS/IVPA) is an emerging modality for the improved characterization of atherosclerotic plaques including anatomical features of the imaged vessel and tissue composition. Encouraging initial demonstrations of the technique, however, have been limited by slow catheter rotation and data acquisition rates on the order of several seconds per frame, with off-line image processing. Herein, we present both a system and an integrated catheter capable of real-time IVUS/IVPA imaging with online image processing and display of both IVUS and IVPA images. The integrated IVUS/IVPA catheter is fully contained within a 1 mm outer diameter torque cable with the electrical signals transmitted at the proximal end using a slip ring rotary joint. Additional components include a pulsed laser operating at 10 kHz pulse repetition frequency, motor and servo drive, an ultrasound pulser/receiver, and a 200 MHz digitizer coupled to a system capable of real-time IVUS/IVPA imaging. The system performance was validated on a vessel-mimicking phantom with an embedded coronary stent intended to provide IVPA contrast within content of an IVUS image.

Noninvasively probing the biomechanical properties of crystalline lens has been challenging due to its unique features such as location inside the eye and being optically and ultrasonically transparent. Here we introduce a method of relying on the spectral analysis of the lens surface response to a mechanical stimulation for the depthdependent assessment of lens biomechanical properties. In this method, acoustic radiation force (ARF) is used to remotely induce the deformation on the surface of the crystalline lens, and a phase-sensitive optical coherence tomography (PhS-OCT) system, co-focused with ARF, utilized to monitor the localized temporal response of ARFinduced deformations on the lens surface. The dominant frequency from the amplitude spectra of the surface response is obtained as the indicator of the depthwise elasticity distribution. Pilot experiments were performed on tissue-mimicking layered phantoms and ex vivo porcine crystalline lens. Results indicate that the frequency response of the sample surface is contributed by the mechanical properties of layers located at different depths and the depthdependent elastic properties can be revealed from the amplitude spectrum. Further study will be focused on combining the experimental measurements with theoretical model and inverse numerical method for depth-resolved elastography of the crystalline lens.

The progressive loss of accommodation of the eye, called presbyopia, affects people with age and can result in a complete loss of accommodation by about age 55 years. It is generally accepted that presbyopia is due to an increase in stiffness of the lens. With increasing age, the stiffness of the crystalline lens nucleus increases faster than that of the cortex. During accommodation, the deformation of different parts of the crystalline lens is different and likely changes with age. However, a direct observation of crystalline lens deformation and strain distribution is difficult because although imaging methods such as OCT or Scheimpflug imaging can distinguish cortex and nucleus, they cannot determine their regional deformation. Here, patterns of laser-induced microbubbles were created in gelatin phantoms and different parts of excised animal crystalline lenses and their displacements in response to external deformation were tracked by ultrasound imaging. In the animal lenses, the deformation of the lens cortex was greater than that of nucleus and this regional difference is greater for a 27-month-old bovine lens than for a 6-month-old porcine lens. This approach enables visualization of localized, regional deformation of crystalline lenses and, if applied to lenses from animal species that undergo accommodation, may help to understand the mechanisms of accommodation and presbyopia, improve diagnostics, and, potentially, aid in the development of new methods of lens modifying presbyopia treatments.

Acoustic Radiation Force (ARF) stimulation is actively used in ultrasound elastography to estimate mechanical properties of tissue. Compared with ultrasound imaging, OCT provides advantage in both spatial resolution and signal-to-noise ratio. Therefore, a combination of ARF and OCT technologies can provide a unique opportunity to measure viscoelastic properties of tissue, especially when the use of high intensity radiation pressure is limited for safety reasons. In this presentation we discuss a newly developed theoretical model of the deformation of a layered viscoelastic medium in response to an acoustic radiation force of short duration. An acoustic impulse was considered as an axisymmetric force generated on the upper surface of the medium. An analytical solution of this problem was obtained using the Hankel transform in frequency domain. It was demonstrated that layers at different depths introduce different frequency responses. To verify the developed model, experiments were performed using tissue-simulating, inhomogeneous phantoms of varying mechanical properties. The Young’s modulus of the phantoms was varied from 5 to 50 kPa. A single-element focused ultrasound transducer (3.5 MHz) was used to apply the radiation force with various durations on the surface of phantoms. Displacements on the phantom surface were measured using a phase-sensitive OCT at 25 kHz repetition frequency. The experimental results were in good agreement with the modeling results. Therefore, the proposed theoretical model can be used to reconstruct the mechanical properties of tissue based on ARF/OCT measurements.

We demonstrate the feasibility of using the dominant frequency of the sample surface response to a mechanical stimulation as an effective indicator for sensing the depthwise distribution of elastic properties in transparent layered phantom samples simulating the cortex and nucleus of the crystalline lens. Focused ultrasound waves are used to noninvasively interrogate the sample surface. A phase-sensitive optical coherence tomography system is utilized to capture the surface dynamics over time with nanometer scale sensitivity. Spectral analysis is performed on the sample surface response to ultrasound stimulation and the dominant frequency is calculated under particular loading parameters. Pilot experiments were conducted on homogeneous and layered tissue-mimicking phantoms. Results indicate that the mechanical layers located at different depths introduce different frequencies to the sample surface response, which are correlated with the depth-dependent elasticity of the sample. The duration and the frequency of the ultrasound excitation are also investigated for their influences on this spectrum-based detection. This noninvasive method may be potentially applied for localized and rapid assessment of the depth dependence of the mechanical properties of the crystalline lens.

In this study the dynamic behavior of a layered viscoelastic medium in response to the impulsive acoustic radiation force applied to its surface was investigated. To verify our theoretical model, experiments were performed using tissue-like gel-based phantoms of varying mechanical properties. A 3.5 MHz single-element focused transducer was used to apply the radiation force at the surface of the phantoms and a phase-sensitive OCT system was used to track the displacements on the phantom surface. The results of this study demonstrate good agreement between theoretical predictions and experimental measurements. It was demonstrated that layers at different depths introduce responses at different frequencies. Therefore, the proposed model in combination with spectral analysis can be used to evaluate depth dependant distribution of the mechanical properties based the measurements on the tissue surface.

Combined intravascular ultrasound and intravascular photoacoustic (IVUS/IVPA) imaging has been previously established as a viable means for assessing atherosclerotic plaque morphological and compositional characteristics using both endogenous and exogenous contrast. In this study, IVUS/IVPA imaging of atherosclerotic rabbit aortas following systemic injection of gold nanorods (AUNRs) with peak absorbance within the tissue optical window is performed. Ex vivo imaging results reveal a high photoacoustic signal from localized AUNRs in regions with atherosclerotic plaques. Corresponding histological staining further confirms the preferential extravasation of AUNRs in atherosclerotic regions with compromised luminal endothelium and acute inflammation. The ability to detect AUNRs using combined IVUS and photoacoustic imaging in the presence of luminal saline and luminal blood is evaluated using both spectroscopic and single wavelength IVPA imaging techniques. Results demonstrate that AUNR detection within the arterial wall can be achieved using both methods, even in the case of imaging through luminal blood.

Pilot studies of in vivo combined intravascular ultrasound (IVUS) and intravascular photoacoustic (IVPA) imaging are reported. A recently introduced prototype of an integrated IVUS/IVPA imaging catheter consisting of a single-element ultrasound transducer and a light delivery system based on a single optical fiber was adapted and used for in vivo imaging of a coronary stent deployed in a rabbit's thoracic aorta in the presence of luminal blood. The results suggest that in vivo IVUS/IVPA imaging is feasible using the integrated IVUS/IVPA imaging catheter. The challenges of in vivo combined IVUS/IVPA imaging are discussed, and further improvements on the design of the catheter and the clinical imaging system are proposed.

The mechanical properties of ex vivo animal lenses from three groups were evaluated: old bovine (25–30 mo old, n = 4), young bovine (6 mo old, n = 4) and young porcine (6 mo old, n = 4) eye globes. We measured the dynamics of laser-induced microbubbles created at different locations within the crystalline lenses. An impulsive acoustic radiation force was applied to the microbubble, and the microbubble displacements were measured using a custom-built high pulse repetition frequency ultrasound system. Based on the measured dynamics of the microbubbles, Young's moduli of bovine and porcine lens tissue in the vicinity of the microbubbles were reconstructed. Age-related changes and location-dependent variations in the Young's modulus of the lenses were observed. Near the center, the old bovine lenses had a Young's modulus approximately fivefold higher than that of young bovine and porcine lenses. The gradient of Young's modulus with respect to radial distance was observed in the lenses from three groups.

Intravascular photoacoustic (IVPA) imaging can characterize atherosclerotic plaque composition on the basis of the optical absorption contrast between different tissue types. Given the high optical absorption of lipid at 1720 nm wavelength, an atherosclerotic rabbit aorta was imaged at this wavelength ex vivo using an integrated intravascular ultrasound (IVUS) and IVPA imaging catheter in the presence of luminal blood. Strong optical absorption of lipid combined with low background signal from other tissues provides a high-contrast, depth-resolved IVPA image of lipid. The ability to image lipid at a single wavelength without removing luminal blood suggests that in vivo detection of lipid in atherosclerotic plaques using combined IVUS/IVPA imaging is possible.

Combined intravascular photoacoustic (IVPA) and intravascular ultrasound (IVUS) imaging has been previously established as a viable means for imaging atherosclerotic plaques using both endogenous and exogenous contrast. In this study, IVUS/IVPA imaging of an atherosclerotic rabbit aorta following injection of gold nanorods (AuNR) with peak absorbance within the tissue optical window was performed. Ex-vivo imaging results revealed high photoacoustic signal from localized AuNR. Corresponding histological cross-sections and digital photographs of the artery lumen confirmed the presence of AuNR preferentially located at atherosclerotic regions and in agreement with IVPA signal. Furthermore, an integrated IVUS/IVPA imaging catheter was used to image the AuNR in the presence of luminal blood. The results suggest that AuNR allow for IVPA imaging of exogenously labeled atherosclerotic plaques with a comparatively low background signal and without the need for arterial flushing.

Metal needles are commonly used for drug delivery or biopsy collection in clinical settings. Needle deflection and deformation can occur when inserting needles into soft, non-homogeneous tissues which can affect the location accuracy of insertion. Therefore, the ability to visualize both anatomical surrounding structures and the advancing needle is required. Ultrasound is commonly used for image-guidance of needles; however, specular reflections from the metal surface can deflect the acoustic beam away from the transducer when the needle is even slightly angled from the US transducer thereby rendering the needle invisible in the image. Photoacoustic imaging has been proposed for guidance of metal needles and other metal objects in-vivo. The high optical absorption coefficient of stainless steel can provide high photoacoustic imaging contrast. The photoacoustic signal is produced omni-directionally from the metal surface allowing for greater detection of needles at increasing injection angles compared to ultrasound imaging. In the current work, needles were inserted into excised tissue and imaged using an ultrasound array transducer and a pulsed 800 nm laser. The results showed that at a shallow 10° insertion angle, the photoacoustic ratio of needle signal to background was four-times higher compared to ultrasound. Furthermore, the surrounding tissue composition was observed to have an effect on photoacoustic signal enhancement which correlated with the change of the Gruneisen coefficient of the surrounding tissue environment, suggesting that the photoacoustic signal amplitude could be used to ascertain surrounding tissue composition. Photoacoustic imaging provides sufficient depth penetration for this application and offers excellent image contrast.

Successful diagnosis and treatment of atherosclerosis demands imaging modalities that can characterize the composition of atherosclerotic plaques, stage the disease, and guide interventional therapy. In this chapter, combined intravascular photoacoustic (IVPA) and intravascular ultrasound (IVUS) imaging is used to address these issues. Based on the difference in optical absorption spectra, lipid-rich tissues can be differentiated using spectroscopic IVPA imaging. Using gold nanoparticles as contrast agent, events happening at the molecular and cellular levels may be visualized in IVPA images. IVPA imaging can also monitor stent deployment during interventional therapy procedures. Design of integrated IVUS/IVPA catheters is discussed. Based on the structural information provided by IVUS images, IVPA images may add further clinically relevant information, helping the management of atherosclerosis.

An approach to assess the mechanical properties of a viscoelastic medium using laser-induced microbubbles is presented. A 532 nm wavelength nanosecond pulsed laser beam was focused into a phantom to create a microbubble at the desired location. A 3.5 MHz ultrasound transducer was used to generate acoustic radiation force to excite the laser-induced microbubble. Motion of the microbubble was tracked using a 25 MHz imaging transducer. An agreement between a theoretical model of bubble motion in viscoelastic medium and experimental measurements was demonstrated. Young's moduli reconstructed using the laser-induced microbubble approach were compared with those of direct measurements. Thus, the proposed approach can be used to assess the mechanical properties of viscoelastic medium such as soft tissue.

Due to their optical absorption properties, metallic nanoparticles are excellent photoacoustic imaging contrast agents. A silver nanosystem is presented here as a potential contrast agent for photoacoustic imaging and image-guided therapy. Currently, the nanosystem consists of a porous silver layer deposited on the surface of spherical silica cores ranging in diameter from 180 to 520 nm. The porous nature of the silver layer will allow for release of drugs or other therapeutic agents encapsulated in the core in future applications. In their current PEGylated form, the silver nanosystem is shown to be nontoxic in vitro at concentrations of silver up to 2 mg∕ml. Furthermore, the near-infrared absorbance properties of the nanosystem are demonstrated by measuring strong, concentration-dependent photoacoustic signal from the silver nanosystem embedded in an ex vivo tissue sample. Our study suggests that silver nanosystems can be used as multifunctional agents capable of augmenting image-guided therapy techniques.

The vulnerability of atherosclerotic plaques that are formed in the arterial walls due to atherosclerosis depends on both their distribution and composition. The distribution of the plaques can be imaged using an intravascular ultrasound (IVUS) imaging which is a clinically approved minimally-invasive method. The recently introduced intravascular photoacoustic (IVPA) imaging may be used to obtain the necessary information about the composition of the plaques. Previous studies using excised rabbit arteries have demonstrated that the combined IVUS/IVPA imaging may simultaneously provide the morphology and functional information of plaques. However, for in-vivo IVUS/IVPA imaging, an integrated IVUS/IVPA imaging catheter capable both of delivering light into a vessel lumen with consequent detection of photoacoustic transients and of probing the arterial walls in pulse-echo mode is required. In the current study, an advanced prototype of the integrated IVUS/IVPA imaging catheter based on a 40-MHz single-element ultrasound transducer and a 600-μm-core single optical fiber is introduced. Unlike previously reported prototypes, the current integrated IVUS/IVPA imaging catheter is capable of cross-sectional imaging of vessel walls via mechanical rotation of the catheter. The performance of the integrated IVUS/IVPA catheter was evaluated in tissue-mimicking phantoms with and without the presence of blood in a lumen. The results of our study suggest that the approach used to develop integrated IVUS/IVPA imaging catheter can be successfully translated to the clinical environment for in-vivo combined IVUS/IVPA imaging.

Atherosclerosis is characterized by formation and development of the plaques in the inner layer of the vessel wall. To detect and characterize atherosclerotic plaques, we previously introduced the combined intravascular ultrasound (IVUS) and intravascular photoacoustic (IVPA) imaging capable of assessing plaque morphology and composition. The utility of IVUS/IVPA imaging has been demonstrated by imaging tissue-mimicking phantoms and ex vivo arterial samples using laboratory prototype of the imaging system. However, the clinical realization of a IVUS/IVPA imaging requires an integrated intravascular imaging catheter. In this paper, two designs of IVUS/IVPA imaging catheters--side fire fiber-based and mirror-based catheters--are reported. A commercially available IVUS imaging catheter was utilized for both pulse-echo ultrasound imaging and detection of photoacoustic transients. Laser pulses were delivered by custom-designed fiber-based optical systems. The optical fiber and IVUS imaging catheter were combined into a single device. Both designs were tested and compared using point targets and tissue-mimicking phantoms. The results indicate applicability of the proposed catheters for clinical use.

The ability to visualize and track temporarily or perma- nently implanted metal devices is important in many applications ranging from diagnosis to therapy. Specifically, reliable imaging of metal needles is required in today's clinical settings. Currently, ultra- sound is utilized to image a needle inserted into tissue in real time. However, the diagnostic value and tracking ability of these images depends highly on the orientation of the needle, and also its proximity to regions of interest in the tissue. We examine the use of photoacous- tic imaging combined with current ultrasound imaging methods to obtain high-contrast images of commonly used needles in the body. Experiments were performed using 21 G and 30 G needles inserted into ex vivo porcine tissue and tissue-mimicking phantoms. The needles and surrounding tissue were imaged using an ultrasound im- aging system interfaced with the pulsed laser source necessary for photoacoustic imaging. The results suggest that photoacoustic imag- ing, combined with ultrasound imaging, is capable of real-time, high- contrast, and high-spatial-resolution visualization of metal implants within anatomical landmarks of the background tissue. © 2010 Society of

Nanoparticles are attracting considerable interest as contrast agents for many different imaging modalities. Moreover, imaging the events at the cellular and molecular level is possible by using nanoparticles that have the desired targeting moiety. Unfortunately, ultrasound imaging cannot visualize the nano-structures directly due to its limited spatial resolution and contrast. We present a new technique, pulsed magneto-acoustic imaging, capable of imaging magnetic nanoparticles indirectly. In this method, a high-strength pulsed magnetic field is used to induce motion within the magnetically labeled tissue and ultrasound is used to detect internal tissue motion. Experiments on tissue-mimicking phantoms and ex-vivo animal tissues demonstrated a clear contrast between normal and iron-laden samples labeled with 5 nm magnetic nanoparticles. In addition, the sensitivity of this new imaging technique was investigated for different concentrations of magnetic agents. The results of the study suggest that magnetic nanoparticles can be used as contrast agents in pulsed magneto-acoustic imaging. Furthermore, PMA imaging could become an imaging tool capable of visualizing the cellular and molecular composition of deep-lying structures.

An ultrasound-based method to locally assess the shear modulus of a medium is reported. The proposed approach is based on the application of an impulse acoustic radiation force to an inhomogeneity in the medium and subsequent monitoring of the spatio-temporal response. In our experimental studies, a short pulse produced by a 1.5-MHz highly focused ultrasound transducer was used to initiate the motion of a rigid sphere embedded into an elastic medium. Another 25 MHz focused ultrasound transducer operating in pulse-echo mode was used to track the displacement of the sphere. The experiments were performed in gel phantoms with varying shear modulus to demonstrate the relationship between the displacement of the sphere and shear modulus of the surrounding medium. Because the magnitude of acoustic force applied to sphere depends on the acoustic material properties and, therefore, cannot be used to assess the absolute value of shear modulus, the temporal behavior of the displacement of the sphere was analyzed. The results of this study indicate that there is a strong correlation between the shear modulus of a medium and spatio-temporal characteristics of the motion of the rigid sphere embedded in this medium.

The effectiveness of an imaging technique is often based on the ability to image quantitatively both morphological and physiological functions of the tissue. Here we present several ultrasound-based imaging techniques capable of visualizing both structural and functional properties of living tissue. Each imaging system utilizes custom-made, targeted nanoparticles developed to probe specific molecular events. Therefore, images of these nanoparticles display molecular processes in the body. Furthermore, the developed nanoparticle contrast agents can also be used for image-guided molecular therapy. For each imaging system, the basic physics and principles behind each approach are described. Experimental aspects of each imaging system including fabrication of integrated imaging probes and associated imaging hardware, and design of targeted contrast agents are discussed. Finally, biomedical and clinical applications of the developed imaging approaches ranging from microscopic to macroscopic imaging of cardiovascular diseases, cancer detection, diagnosis, therapy and therapy monitoring are demonstrated and discussed.

Gold nanoparticles targeting epidermal growth factor receptor via antibody conjugation undergo molecular specific aggregation when they bind to receptors on cell surfaces, leading to a red shift in their plasmon resonance frequency. Capitalizing on this effect, we demonstrate the efficacy of the molecular specific photoacoustic imaging technique using subcutaneous tumor-mimicking gelatin implants in ex-vivo mouse tissue. The results of our study suggest that highly selective and sensitive detection of cancer cells is possible using multiwavelength photoacoustic imaging and molecular specific gold nanoparticles.

Coronary atherosclerosis is a complex disease accompanied by the development of plaques in the arterial wall. Since the vulnerability of the plaques depends on their composition, the appropriate treatment of the arteriosclerosis requires a reliable characterization of the plaques' geometry and content. The intravascular ultrasound (IVUS) imaging is capable of providing structural details of the plaques as well as some functional information. In turn, more functional information about the same plaques can be obtained from intravascular photoacoustic (IVPA) images since the optical properties of the plaque's components differ from that of their environment. The combined IVUS/IVPA imaging is capable of simultaneously detecting and differentiating the plaques, thus determining their vulnerability. The potential of combined IVUS/IVPA imaging has already been demonstrated in phantoms and ex-vivo experiments. However, for in-vivo or clinical imaging, an integrated IVUS/IVPA catheter is required. In this paper, we introduce two prototypes of integrated IVUS/IVPA catheters for in-vivo imaging based on a commercially available single-element IVUS imaging catheter. The light delivery systems are developed using multimode optical fibers with custom-designed distal tips. Both prototypes were tested and compared using an arterial mimicking phantom. The advantages and limitations of both designs are discussed. Overall, the results of our studies suggest that both designs of integrated IVUS/IVPA catheter have a potential for in-vivo IVPA/IVUS imaging of atherosclerotic plaques.

A new metallodielectric nanoparticle consisting of a silica core and silver outer cage was developed for the purpose of enhancing photoacoustic imaging contrast in pancreatic tissue. These nanocages were injected into an ex vivo porcine pancreas and imaged using a combined photoacoustic and ultrasound (PAUS) assembly. This custom-designed PAUS assembly delivered 800 nm light through a fiber optical light delivery system integrated with 128 element linear array transducer operating at 7.5 MHz center frequency. Imaging results prove that the nanocage contrast agents have the ability to enhance photoacoustic imaging contrast. Furthermore, the value of the combined PAUS imaging modality was demonstrated as the location of nanocages against background native tissue was evident. Future applications of these nanocage contrast agents could include targeting them to pancreatic cancer for enhancement of photoacoustic imaging for diagnosis and therapy.

An ultrasound-based technique capable of detection and spatio-temporal characterization of microbubbles induced in water by femtosecond laser is reported. A highly focused single-element ultrasound transducer was used both to detect passive acoustic emission of the microbubbles and to probe the microbubbles at different stage of their evolution. The location of origin and wall of the microbubble was assessed from temporal characteristics of the passive acoustic emissions and of the pulse-echo signals. The radius of the microbubble was estimated as the distance between the origin of the bubble and its wall. The ultrasound characterization of microbubbles induced by femtosecond pulses agreed well with theoretical predictions based on the well-known Rayleigh-based model of bubble behavior in liquid. The results of this study demonstrate that femtosecond laser-induced microbubbles with typical sizes of several tens of micrometers can be characterized by the developed ultrasound technique.

There is a need for an imaging technique that can reliably identify and characterize the vulnerability of atherosclerotic plaques. Catheter-based intravascular ultrasound (IVUS) is one of the imaging tools of the clinical evaluation of atherosclerosis. However, histopathological information obtained with IVUS imaging is limited. We present and discuss the applicability of a combined intravascular photoacoustic (IVPA) and intravascular ultrasound (IVUS) imaging approach to assess both vessel structure and tissue composition thus identifying rupture-prone atherosclerotic plaques. Photoacoustic (or optoacoustic and, generally, thermoacoustic) imaging relies on the absorption of electromagnetic energy, such as light, and the subsequent emission of an acoustic wave. Therefore, the amplitude and temporal characteristics of the photoacoustic signal is primarily determined by optical absorption properties of different types of tissues and can be used to differentiate the lipid, fibrous and fibro-cellular components of an inflammatory lesion. Simultaneous IVUS and IVPA imaging studies were conducted using 40 MHz clinical IVUS imaging catheter interfaced with a pulsed laser system. The performance of the IVPA/IVUS imaging was assessed using phantoms with point targets and vessel-mimicking phantoms. To detect the lipids in the plaque, ex-vivo IVPA imaging studies of a normal and an atherosclerotic rabbit aorta were performed at a 532 nm wavelength. To assess plaque composition, multi-wavelength (680–950 nm) spectroscopic IVPA imaging studies were carried out. Finally, molecular and cellular IVPA imaging was demonstrated using plasmonic nanoparticles. Overall, our studies suggest that plaque detection and characterization can be improved using the combined IVPA/IVUS imaging.

Highly proliferative cancer cells over express molecular markers such as epidermal growth factor receptor (EGFR). When specifically targeted gold nanoparticles bind to EGFR they tend to cluster on the cell membrane leading to an optical red-shift in the plasmon resonance frequency and an increase in absorption in the red region. These changes in optical properties provide the opportunity for photoacoustic imaging to differentiate cancer cells from surrounding benign cells - the contrast in photoacoustic imaging is based on the optical absorption properties. The imaging experiments were performed using ex-vivo tumor models. The results of our study indicate that photoacoustic imaging is capable of highly sensitive and selective detection of cancer cells by utilizing the plasmon resonance coupling effect of EGFR targeted gold nanoparticles.

Photoacoustic signals, detected by a transducer array, need to be beamformed for subsequent use in a limited view angle tomography such as B-scan imaging. In the presence of the light scattering or phase aberration, the spatial resolution and contrast in the photoacoustic images are degraded. Phase aberration due to tissues with inhomogeneous acoustic speeds is a major source for image degradation. However, a constant speed of sound (e.g., 1540 m/s) is typically assumed in photoacoustic imaging. Such an assumption can affect the quality of photoacoustic image since changes in sound velocity cause significant phase errors in beamforming. An adaptive weighting method such as coherence factor (CF) technique can improve the ultrasound and photoacoustic image quality significantly. In addition, photoacoustic images can be further improved by applying adaptive beamforming techniques developed for ultrasound imaging. In this study, an adaptive photoacoustic image reconstruction technique that combines an adaptive weighting factor (CF) and an adaptive apodization called minimum variance method (MV) is introduced. Although MV method calculates the optimal apodization weighting factors which minimize the variance of the beamformed signal, it can lead to unexpected weighting factors since it is data dependant. In this case, CF weighting can help to avoid this problem by weighting the output from the MV method based on signal coherence. Simulations were performed to analyze the spatial resolution using a point targets and to demonstrate improvement in phase aberration correction. Numerical studies demonstrated the superior performance of MV adaptive method combined with CF weighting.

An ultrasound technique to measure the spatial and tempo- ral behavior of the laser-induced cavitation bubble is introduced. The cavitation bubbles were formed in water and in gels using a nanosec- ond pulsed Nd:YAG laser operating at 532 nm. A focused, single- element, 25-MHz ultrasound transducer was employed both to detect the acoustic emission generated by plasma expansion and to acousti- cally probe the bubble at different stages of its evolution. The arrival time of the passive acoustic emission was used to estimate the loca- tion of the cavitation bubble's origin and the time of flight of the ultrasound pulse-echo signal was used to define its spatial extent. The results of ultrasound estimations of the bubble size were compared and found to be in agreement with both the direct optical measure- ments of the stationary bubble and the theoretical estimates of bubble dynamics derived from the well-known Rayleigh model of a cavity collapse. The results of this study indicate that the proposed quantita- tive ultrasound technique, capable of detecting and accurately mea- suring laser-induced cavitation bubbles in water and in a tissue-like medium, could be used in various biomedical and clinical applications. © 2008 Society of Photo-Optical Instrumentation Engineers. DOI: 10.1117/1.2937478

A recently developed ultrasound technique is evaluated by measuring the behavior of a cavitation bubble that is induced in water by a femtosecond laser pulse. The passive acoustic emission during optical breakdown is used to estimate the location of the cavitation bubble's origin. In turn, the position of the bubble wall is defined based on the active ultrasonic pulse-echo signal. The results suggest that the developed ultrasound technique can be used for quantitative measurements of femtosecond laser-induced microbubbles.

The motion of a rigid sphere in a viscoelastic medium in response to an acoustic radiation force of short duration was investigated. Theoretical and numerical studies were carried out first. To verify the developed model, experiments were performed using rigid spheres of various diameters and densities embedded into tissue-like, gel-based phantoms of varying mechanical properties. A 1.5 MHz, single-element, focused transducer was used to apply the desired radiation force. Another single-element, focused transducer operating at 25 MHz was used to track the displacements of the sphere. The results of this study demonstrate good agreement between theoretical predictions and experimental measurements. The developed theoretical model accurately describes the displacement of the solid spheres in a viscoelastic medium in response to the acoustic radiation force.

An ultrasound-based method to detect and characterize the laser-induced microbubbles was developed. This method is based on temporal measurement of passive acoustic emission from cavity during laser-tissue interaction and simultaneous active pulse-echo ultrasound probing of the cavitation bubble. These measurements were used to estimate the location of the nanosecond laser induced cavity and to monitor the spatial and temporal behavior of the microbubble. The measurements agreed with estimates derived from a well-known Rayleigh model of the cavity collapse. Overall, the studies indicate that the developed ultrasound technique can be used to detect and accurately measure laser-induced microbubbles in tissue.

To perform ultrasound imaging using an array transducer, a focused ultrasound beam is transmitted in a particular direction within the tissue and the received backscattered ultrasound wave is then dynamically focused at every position along the beam. The ultrasound beam is scanned over the desired region to form an image. The photoacoustic imaging, however, is distinct from conventional ultrasound imaging. In photoacoustic imaging the acoustic transients are generated simultaneously in the entire volume of the irradiated tissue – no transmit focusing is possible due to light scattering in the tissue. The photoacoustic waves are then r ecorded on every element of th e ultrasound tran sducer array at once and processed to form an image. Therefore, compared to ultrasound imaging, photoacoustic imaging can utilize dynamic receive focusing only. In this paper, we describe the image formation algorithms of the array-based photoacoustic and ultrasound imaging system and present me thods to improve the quality of photoacoustic images. To evaluate the performance of photoacoustic imaging using an array transducer, numerical simulations and phantom experiments were performed. First, to evaluate spatial resolution, a point source was imaged using a combined ultrasound and photoacoustic imaging system. Next, image quality was assessed by imaging tissue imaging phantoms containing a circular inclusion. Finally, the photoacoustic and ultrasound images from the combined imaging system were analyzed. Keywords: Photoacoustic imaging, ultrasound imaging, array transducer, beamforming, image formation, image reconstruction, delay and sum

In many clinical and research applications including cancer diagnosis, tumor response to therapy, reconstructive surgery, monitoring of transplanted tissues and organs, and quantitative evaluation of angiogenesis, sequential and quantitative assessment of microcirculation in tissue is required. In this paper we present an imaging technique capable of spatial and temporal measurements of blood perfusion through microcirculation. To demonstrate the developed imaging technique, studies were conducted using phantoms with modeled small blood vessels of various diameters positioned at different depths. A change in the magnitude of the photoacoustic signal was observed during vessel constriction and subsequent displacement of optically absorbing liquid present in the vessels. The results of the study suggest that photoacoustic, ultrasound and strain imaging could be used to sequentially monitor and qualitatively assess blood perfusion through microcirculation.

There is a need for a clinical imaging technique to reliably diagnose and adequately age deep vein thrombosis (DVT). Maturation of DVT is associated with the reorganization and possibly significant changes in the optical absorption of the blood clot. In this paper, we explore photoacoustic imaging to age DVT. Photoacoustic imaging is a technique capable of remote measurements of optical absorption of tissue. Theoretical and experimental studies were performed using DVT-mimicking phantoms. The results of our study suggest that combined ultrasound and photoacoustic imaging of thrombi can provide information about the structure and age of deep vein thrombosis.

Tissue engineering is an interdisciplinary field that combines various aspects of engineering and life sciences and aims to develop biological substitutes to restore, repair or maintain tissue function. Currently, the ability to have quantitative functional assays of engineered tissues is limited to existing invasive methods like biopsy. Hence, an imaging tool for non-invasive and simultaneous evaluation of the anatomical and functional properties of the engineered tissue is needed. In this paper we present an advanced in-vivo imaging technology - ultrasound biomicroscopy combined with complementary photoacoustic and elasticity imaging techniques, capable of accurate visualization of both structural and functional changes in engineered tissues, sequential monitoring of tissue adaptation and/or regeneration, and possible assistance of drug delivery and treatment planning. The combined imaging at microscopic resolution was evaluated on tissue mimicking phantoms imaged with 25 MHz single element focused transducer. The results of our study demonstrate that the ultrasonic, photoacoustic and elasticity images synergistically complement each other in detecting features otherwise imperceptible using the individual techniques. Finally, we illustrate the feasibility of the combined ultrasound, photoacoustic and elasticity imaging techniques in accurately assessing the morphological and functional changes occurring in engineered tissue.

A hybrid imaging system is proposed for cancer detection, diagnosis and therapy monitoring by integrating three complementary imaging techniques - ultrasound, photoacoustic and elasticity imaging. Indeed, simultaneous imaging of the anatomy (ultrasound imaging), cancer-induced angiogenesis (photoacoustic imaging) and changes in biomechanical properties (elasticity imaging) of tissue is based on many synergistic features of these modalities and may result in a unique and important imaging tool. To facilitate the design and development of a real-time imaging system for clinical applications, we have investigated the core components of the imaging system using numerical simulations. Differences and similarities between each imaging technique were considered and contrasted. The results of our study suggest that the integration of ultrasound, photoacoustic and elasticity imaging is possible using a custom designed imaging system.

An advanced in-vivo imaging technology; namely, combined ultrasound, elasticity and photoacoustic imaging, capable of visualizing both structural and functional properties of living tissue, is presented. This hybrid imaging technology is based on the fusion of the complementary imaging modalities and takes full advantage of the many synergistic features of these systems. To highlight fundamental differences and similarities between the imaging systems and to appreciate advantages and limitations of each imaging system, the basic physics of each imaging system is described. The experimental aspects of combined imaging including hardware, signal and image processing algorithms, etc. are presented. Noise and primary artifacts associated with each imaging modality and combined imaging system are analyzed, and techniques to increase and optimize contrast-to-noise and signal-to-noise ratios in the images are discussed. Finally, biomedical and clinical applications of the combined ultrasound, elasticity and photoacoustic imaging ranging from macroscopic to microscopic imaging of pathology are demonstrated and discussed

The goal of this study was to investigate the dynamic behavior of a rigid sphere in viscoelastic medium in response to the impulsive acoustic radiation force. Theoretical and numerical studies were carried out first. Then, to verify our theoretical and numerical models, the experiments were performed using rigid spheres of various diameters and densities embedded into tissue-like gel-based phantoms of varying mechanical properties. A 1.5 MHz single-element focused transducer was used to apply the desired radiation force. Another single-element focused transducer operating at 25 MHz was used to track the displacements of the sphere. The results of this study demonstrate good agreement between theoretical predictions and experimental measurements. The developed theoretical model accurately describes the displacement of the solid spheres in viscoelastic medium in response to the acoustic radiation force and thus can be used to evaluate mechanical properties of tissue

Combination of three complementary imaging technologies - ultrasound imaging, elastography, and optoacoustic imaging - is suggested for detection and diagnostics of tissue pathology including cancer. The fusion of these ultrasound-based techniques results in a novel imaging system capable of simultaneous imaging of the anatomy (ultrasound imaging), cancer-induced angiogenesis (optoacoustic imaging) and changes in mechanical properties (elasticity imaging) of tissue to uniquely identify and differentiate pathology at various stages. To evaluate our approach, analytical and numerical studies were performed using heterogeneous phantoms where ultrasonic, optical and viscoelastic properties of the materials were chosen to closely mimic soft tissue. The results of this study suggest that combined ultrasound-based imaging is possible and can provide more accurate, reliable and earlier detection and diagnosis of tissue pathology. In addition, monitoring of cancer treatment and guidance of tissue biopsy are possible with a combined imaging system.

Acoustic radiation force on a scatterer in tissue depends on the compressibility and shear modulus of both the tissue and the scatterer. This force is related to the monopole and dipole scattering coefficients. The finite shear modulus of the tissue decreases the radiation force in comparison with the force exerted on the same scatterer surrounded by liquid. Shear moduli for soft tissue range from several kilopascals (breast, liver) to tens of kilopascals and higher for cornea, cartilage, and cancerous tissue. As reported previously, the radiation force on a bubble in tissue having 100 kPa shear modulus is 50% less than if the bubble is in water. This difference decreases for scatterers with finite shear moduli, examples of which are reported here. Additionally, displacement of a scatterer due to radiation force is inversely proportional to the shear modulus of the tissue, which permits measurement of the latter. Experiments demonstrating this technique are reviewed. In these experiments, the radiation force is applied to a gas microbubble produced by laser-induced optical breakdown, while displacement of the microbubble is measured by high-frequency ultrasound as a function of time. Results are reported for tissue-mimicking phantoms and animal crystalline lenses in vitro.