A study of the adult zebrafish ventricular function by retrospective Doppler-gated ultrahigh-frame-rate echocardiography

Superficial Bifurcated Microflow Phantom for High-Frequency Ultrasound Applications

OBJECTIVE

To evaluate and optimize high-frequency ultrasound (HFUS) imaging techniques that visualize the morphology of microscale vasculatures, many studies have used flow phantoms with straight channels. However, the previous phantoms lack the complexity of microvessels to simulate a realistic vascular environment in a shallow depth. This study was aimed at devising a new protocol for fabrication of a microflow phantom with bifurcated geometry at a superficial region.

METHODS

The proposed protocol involved the following features: (i) a bifurcated flow tract model 300 µm in diameter was debossed on the surface of a tissue slab made of polyvinyl alcohol cryogel, and (ii) a wall-less lumen was created via bonding tissue slabs to put a lid on the debossed flow tract. The structure of the created microflow phantom was evaluated using 2-D and 3-D power Doppler imaging with a 30 MHz HFUS modality.

RESULTS

Ultrasound imaging revealed that the desired flow tract with bifurcation was successfully created in the phantom at a depth of 2-5 mm from the ultrasound probe. The diameters of the flow tract measured in the axial direction were 307 ± 3.7 µm in the parent branch and 232 ± 18.2 and 256 ± 23.3 µm in the two daughter branches, respectively.

CONCLUSION

The experiments revealed that the proposed protocol for creating a microscale intricate flow tract with desired dimensions and depth is valid. This new phantom will facilitate further improvement in the ultrasound technologies for the precise visualization of superficial complex vasculatures such as those in skin layers.

Visualization of Human Hand Tendon Mechanical Anisotropy in 3-D Using High- Frequency Dual-Direction Shear Wave Imaging

High-resolution ultrasound shear wave elastography has been used to determine the mechanical properties of hand tendons. However, because of fiber orientation, tendons have anisotropic properties; this results in differences in shear wave velocity (SWV) between ultrasound scanning cross sections. Rotating transducers can be used to achieve full-angle scanning. However, this technique is inconvenient to implement in clinical settings. Therefore, in this study, high-frequency ultrasound (HFUS) dual-direction shear wave imaging (DDSWI) based on two external vibrators was used to create both transverse and longitudinal shear waves in the human flexor carpi radialis tendon. SWV maps from two directions were obtained using 40-MHz ultrafast imaging at the same scanning cross section. The anisotropic map was calculated pixel by pixel, and 3-D information was obtained using mechanical scanning. A standard phantom experiment was then conducted to verify the performance of the proposed HFUS DDSWI technique. Human studies were also conducted where volunteers assumed three hand postures: relaxed (Rel), full fist (FF), and tabletop (TT). The experimental results indicated that both the transverse and longitudinal SWVs increased due to tendon flexion. The transverse SWV surpassed the longitudinal SWV in all cases. The average anisotropic ratios for the Rel, FF, and TT hand postures were 1.78, 2.01, and 2.21, respectively. Both the transverse and the longitudinal SWVs were higher at the central region of the tendon than at the surrounding region. In conclusion, the proposed HFUS DDSWI technique is a high-resolution imaging technique capable of characterizing the anisotropic properties of tendons in clinical applications.

High-frequency ultrasound imaging for monitoring the function of meningeal lymphatic system in mice

The meningeal lymphatic system drains the cerebrospinal fluid from the subarachnoid space to the cervical lymphatic system, primarily to the deep cervical lymph nodes. Perturbations of the meningeal lymphatic system have been linked to various neurologic disorders. A method to specifically monitor the flow of meningeal lymphatic system in real time is unavailable. In the present study, we adopted the high-frequency ultrasound (HFUS) with 1,1'diocatadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI)-loaded microbubble and FePt@PLGA nanoparticle contrast agents to evaluate the flow of the meningeal lymphatic system in 2-month-old mice. Statistical analysis was performed to identify changes of HFUS signals among the microbubbles, FePt@PLGA nanoparticles, and saline control groups. Approximately 15 min from the start of intracerebroventricular injection of contrast agents, their signals were evident at the deep cervical lymph nodes and lasted for at least 60 min. These signals were validated on the basis of the presence of DiI and Fe signals in the deep cervical lymph nodes. Ligation of afferent lymphatic vessels to the deep cervical lymph nodes eliminated the HFUS signals. Moreover, ablation of lymphatic vessels near the confluence of sinuses decreased the HFUS signals in the deep cervical lymph nodes. Glioma-bearing mice that exhibited reduced lymphatic vessel immunostaining signals near the confluence of sinuses had lowered HFUS signals in the deep cervical lymph nodes within 60 min. The proposed method provides a minimally invasive approach to monitor the qualities of the meningeal lymphatic system in real time as well as the progression of the meningeal lymphatic system in various brain disease animal models.

Multifunctional Hydrogel Dressing That Carries Three Antibiotics Simultaneously and Enables Real-Time Ultrasound Bacterial Colony Detection

We have developed a multifunctional hydrogel that can carry three synergistic antibiotics commonly used in clinical practice. This hydrogel was discovered to have drug encapsulation efficiencies of 94% for neomycin, 97% for bacitracin, and 88% for polymyxin B. Drug release data indicated that the release profiles of these three antibiotics were different. A swelling test demonstrated that the hydrogel absorbed liquid after the release of its antibiotics until it became saturated, which occurred within 48 h. Moreover, this hydrogel exhibited excellent antibacterial effects against Escherichia coli and Pseudomonas aeruginosa and biocompatibility; it can thus protect a wound from microbial invasion. When the alginate hydrogel is used to cover a wound, the wound can be checked for colonization at any time using ultrasound imaging; this can thus enable the prevention of wound biofilm formation in the early stages of infection. We evaluated the hydrogel against commercially available wound dressings and discovered that these wound dressings did not have the aforementioned desirable features. In conclusion, our multifunctional hydrogel can carry three types of antibiotics simultaneously and is a suitable medium through which an ultrasound can be performed to detect the growth of colonies in wounds. The hydrogel is expected to make a valuable contribution to the prevention of wound infections in the future.

All-digital transmit beamformer for portable high-frequency ultrasound imaging systems

To meet the requirements of high-frequency ultrasound imaging systems, a transmit-beamforming integrated circuit with higher delay resolution than conventional transmit-beamforming circuits, which are typically implemented using field-programmable gate array chips, is presented. It also requires smaller volumes, allowing for portable applications. Its proposed design includes two all-digital delay-locked loops providing a specified digital control code for a counter-based beamforming delay chain (CBDC) to generate stable and suitable delays for exciting the array transducer elements without variations in process, voltage, and temperature. Moreover, to maintain the duty cycle of long propagation signals, this novel CBDC requires only a few delay cells, significantly reducing hardware costs and power consumption. Simulations were conducted, revealing a maximum time delay of 451.9 ns with a time resolution of 652 ps and a maximum lateral resolution error of 0.04 mm at 6.8 mm.

High-frequency noncontact low-intensity pulsed ultrasound modulates Ca2+-dependent transcription factors contributing to cell migration

Chronic wounds have negative physical and psychological effects on patients and increase the health care burden. Consequently, chronic wound in the elderly population is an important issue. Ultrasound can be a great modality for treating chronic wounds because of its noninvasive and safety characteristics; it can accelerate in vitro and in vivo wound healing. In this study, we developed a novel noncontact ultrasound for wound treatment. We stimulated human epidermal keratinocyte migration using low-intensity pulsed ultrasound (LIPUS) with a noncontact transducer to avoid direct contact with the wound. We also compared the effects of 15-min contact and noncontact transducer stimulation, where a 1-MHz contact transducer (intensity = 40 or 200 mW/cm²) and a 0.45-MHz noncontact transducer (intensity = 30 mW/cm²) were used. Both contact and noncontact LIPUS considerably increased cell migration and activated the calcium (Ca²⁺)-dependent transcription factors cAMP-responsive element-binding protein (CREB) and nuclear factor of activated T cells (NFAT). Furthermore, noncontact transducer stimulation did not cause cell death or affect cell proliferation but significantly increased the Ca²⁺ influx-mediated intracellular Ca²⁺ levels. Ca²⁺-free medium and Ca²⁺ channel blockers effectively inhibited LIPUS-induced Ca²⁺-dependent transcription factor activation and cell migration.

Single-shot Mueller-matrix imaging of zebrafish tissues: In vivo analysis of developmental and pathological features

Zebrafish is a well-established animal model for developmental and disease studies. Its optical transparency at early developmental stages allows in vivo tissues visualization. Interaction of polarized light with these tissues provides information on their structure and properties. This approach is effective for muscle tissue analysis due to its birefringence. To enable real-time Mueller-matrix characterization of unanesthetized fish, we assembled a microscope for single-shot Mueller-matrix imaging. First, we performed a continuous observation of 48 species within the period of 2 to 96 hpf and measured temporal dependencies of the polarization features in different tissues. These measurements show that hatching was accompanied by a sharp change in the angle and degree of linearly polarized light after interaction with muscles. Second, we analyzed nine species with skeletal disorders and demonstrated that the spatial distribution of light depolarization features clearly indicated them. Obtained results demonstrated that real-time Mueller-matrix imaging is a powerful tool for label-free monitoring zebrafish embryos.

Zebrafish Models of Cardiac Disease: From Fortuitous Mutants to Precision Medicine

Heart disease is the leading cause of death worldwide. Despite decades of research, most heart pathologies have limited treatments, and often the only curative approach is heart transplantation. Thus, there is an urgent need to develop new therapeutic approaches for treating cardiac diseases. Animal models that reproduce the human pathophysiology are essential to uncovering the biology of diseases and discovering therapies. Traditionally, mammals have been used as models of cardiac disease, but the cost of generating and maintaining new models is exorbitant, and the studies have very low throughput. In the last decade, the zebrafish has emerged as a tractable model for cardiac diseases, owing to several characteristics that made this animal popular among developmental biologists. Zebrafish fertilization and development are external; embryos can be obtained in high numbers, are cheap and easy to maintain, and can be manipulated to create new genetic models. Moreover, zebrafish exhibit an exceptional ability to regenerate their heart after injury. This review summarizes 25 years of research using the zebrafish to study the heart, from the classical forward screenings to the contemporary methods to model mutations found in patients with cardiac disease. We discuss the advantages and limitations of this model organism and introduce the experimental approaches exploited in zebrafish, including forward and reverse genetics and chemical screenings. Last, we review the models used to induce cardiac injury and essential ideas derived from studying natural regeneration. Studies using zebrafish have the potential to accelerate the discovery of new strategies to treat cardiac diseases.

High-Frequency Ultrasound Elastography for Assessing Elastic Properties of Skin and Scars

Scars are a type of fibrous tissue that typically forms during the wound healing process to replace damaged skin. Because studies have indicated a high correlation between scar stiffness and clinical symptoms, assessing the mechanical properties of scar is crucial for determining an appropriate treatment strategy and evaluating the treatment's efficacy. Shear wave elastography (SWE) is a common technique for measuring tissue elasticity. Because scars are typically a few millimeters thick, they are thin-layer tissues, and therefore, the dispersion effect must be considered to accurately estimate their elasticity. In this study, high-frequency ultrasound (HFUS) elastography was proposed for estimating the elastic properties of scars by using the Lamb wave model (LWM). An external vibrator was used to generate elastic waves in scar tissue and skin, and the propagation of the elastic waves was tracked through 40-MHz ultrafast ultrasound imaging. The elasticity was estimated through shear wave models (SWMs) and LWMs. The effectiveness of using HFUS elastography was verified through phantom and human studies. The phantom experiments involved bulk phantoms with gelatin concentrations of 7% and 15% and 2-4-mm-thick thin-layer 15% gelatin phantoms. The studies of three patients with eight cases of scarring were also conducted. The phantom experimental results demonstrated that the elasticity estimation biases for the thin-layer mediums were approximately -36% to -50% and 3% to -9% in the SWMs and LWMs, respectively, and the estimated shear moduli were 12.8 ± 5.4 kPa and 74.8 ± 26.8 kPa for healthy skin and scar tissue, respectively. All the results demonstrated that the proposed HFUS elastography has a great potential for improving the accuracy of elasticity estimations in clinical dermatological diagnoses.

High-Spatiotemporal-Resolution Visualization of Myocardial Strains Through Vector Doppler Estimation: A Small-Animal Study

High-frequency ultrasound (HFUS) imaging is extensively used for cardiac diseases in small animals due to its high spatial resolution. However, there is a lack of a system that can provide a 2-D high-spatiotemporal dynamic visualization of mouse myocardial strains. In this article, a dynamic HFUS (40 MHz) high-resolution strain imaging was developed through the vector Doppler imaging. Following in vitro tests using a rubber balloon phantom, in vivo experiments were performed on wild-type (WT) and myocardial infarction (MI) mice. High-resolution dynamic images of myocardial strains were obtained in the longitudinal, radial, and circumferential directions at a frame rate of 1 kHz. Global peak strain values for WT mice were -19.3% ± 1.3% (longitudinal), 31.4% ± 1.7% (radial in the long axis), -19.9% ±.8% (circumferential), and 34.4% ± 1.9% (radial in the short axis); those for the MI mice were -16.1% ±.9% (longitudinal), 26.8% ± 2.9% (radial in the long axis), -15.2% ± 2.7% (circumferential), and 21.6% ± 4.8% (radial in the short axis). These results indicate that the strains for MI mice are significantly lower than those for WT mice. Regional longitudinal strain curves in the epicardial, midcardial, and endocardial layers were measured and the peak strain values for WT mice were -22.% and -16.8% in the endocardial and epicardial layers, respectively. However, no significant difference in the layer-based values was noted for the MI mice. Regional analysis results revealed obvious myocardial strain variation in the apical anterior region in the MI mice. The experimental results demonstrate that the proposed dynamic cardiac strain imaging can be useful in high-performance imaging of small-animal cardiac diseases.

Estimating the neovascularity of human finger tendon through high frequency ultrasound micro-Doppler imaging

OBJECTIVE

Neovascularization of injured tendons prolongs the proliferative phase of healing, but prolonged neovascularization may cause improper healing and pain. Currently, ultrasound Doppler imaging is used for measuring the neovascularization of injured tendons (e.g., Achilles tendon). However, the resolution of state-of-the-art clinical ultrasound machines is insufficient for visualizing the neovascularization in finger tendons. In this study, a high-resolution micro-Doppler imaging (HFDI) based on 40-MHz ultrafast ultrasound imaging was proposed for visualizing the neovascularization in injured finger tendons during multiple rehabilitation phases.

METHOD

The vessel visibility was enhanced through a block-wise singular value decomposition filter and several curvilinear structure enhancement strategies, including the bowler-hat transform and Hessian-based vessel enhancement filtering. HFDI was verified through small animal kidney and spleen imaging because the related vessel structure patterns of mice are well studied. Five patients with finger tendon injuries underwent HFDI examination at various rehabilitation phases after surgery (weeks 1156), and finger function evaluations were performed for comparisons.

RESULTS

The results of small animal experiments revealed that the proposed HFDI provides excellent microvasculature imaging performance; the contrast-to-noise ratio of HFDI was approximately 15 dB higher than that of the conventional singular value decomposition filter, and the minimum detectable vessel size for mouse kidney was 35 m without the use of contrast agent. In the human study, neovascularization was clearly observed in injured finger tendons during the early phase of healing (weeks 1121), but it regressed from week 52 to 56. Finger rehabilitation appears to help reduce neovascularization; neovascular density decreased by approximately 1.8%8.0% in participants after 4 weeks of rehabilitation.

CONCLUSION

The experimental results verified the performance of HFDI for microvasculature imaging and its potential for injured finger tendon evaluations.

Ultrasonic technologies in imaging and drug delivery

Ultrasonic technologies show great promise for diagnostic imaging and drug delivery in theranostic applications. The development of functional and molecular ultrasound imaging is based on the technical breakthrough of high frame-rate ultrasound. The evolution of shear wave elastography, high-frequency ultrasound imaging, ultrasound contrast imaging, and super-resolution blood flow imaging are described in this review. Recently, the therapeutic potential of the interaction of ultrasound with microbubble cavitation or droplet vaporization has become recognized. Microbubbles and phase-change droplets not only provide effective contrast media, but also show great therapeutic potential. Interaction with ultrasound induces unique and distinguishable biophysical features in microbubbles and droplets that promote drug loading and delivery. In particular, this approach demonstrates potential for central nervous system applications. Here, we systemically review the technological developments of theranostic ultrasound including novel ultrasound imaging techniques, the synergetic use of ultrasound with microbubbles and droplets, and microbubble/droplet drug-loading strategies for anticancer applications and disease modulation. These advancements have transformed ultrasound from a purely diagnostic utility into a promising theranostic tool.

32-Channel transmit beamformer with high timing resolution for high-frequency ultrasound imaging systems

This paper presents a 32-channel high timing resolution transmit-beamforming circuit for use in high-frequency ultrasound imaging systems. Conventional transmit-beamforming circuits are typically implemented using field-programmable gate array (FPGA) chips. However, it is difficult for FPGAs to provide high timing resolution to meet the beamforming requirements of high-frequency ultrasound imaging systems. The proposed transmit-beamforming design can generate stable and suitable delays to excite 32-channel array transducer elements without variations in the process, voltage, and temperature. In addition, the proposed low-complexity architecture can maintain the duty cycle of long prorogation signals with low hardware cost to meet the timing requirements of a large channel number array transducer. The proposed designed transmit beamformer uses 0.18-µm CMOS technology for a 30-MHz high-frequency linear array, and the simulation results show that the proposed transmit-beamforming application-specific integrated circuit can achieve a maximum time delay of 619.5 ns with a time resolution of 617 ps.

Evaluation of cardiac dysfunction in adult zebrafish using high frequency echocardiography

AIMS

Recently, the zebrafish has gained attention as an innovative experimental model to decipher molecular and cellular mechanisms involved in cardiovascular development and diseases. Nevertheless, the use of zebrafish models has been challenged because the transparency of these fish, which allows for accurate cardiac evaluation, disappears in adulthood. In this study, the epicardial outline method was performed to investigate the feasibility of echocardiography in assessing cardiac function in pathological adult zebrafish.

MATERIALS AND METHODS

We attempted to estimate heart failure in adult zebrafish treated with three distinct regulators of cardiac function: phenylhydrazine hydrochloride (PHZ), doxorubicin (DOX), and ethanol. B-mode and Doppler images were evaluated at frequencies of up to 50 MHz and 40 MHz, respectively. The correlation between alterations in cardiac function, haemoglobin concentration, and myocardial histopathology were assessed.

KEY FINDINGS

Cardiac output (CO) in PHZ-treated zebrafish was significantly higher than that in control zebrafish (151 ± 67 vs. 84 ± 37 μl/min, P = 0.004), whereas ejection fraction (EF) was lower (36.3 ± 10.9 vs. 50.9 ± 8.7%, P < 0.001), indicating typical high output heart failure derived from anaemia. Additionally, ventricular dysfunction in DOX-treated zebrafish was characterised by low CO (57 ± 38 μl/min) and EF (28.8 ± 10.4%), accompanied by an enlarged ventricle in diastole and systole, representing low output heart failure. For ethanol-treated zebrafish, EF was markedly reduced (39.6 ± 7.2%) indicating a dilated heart, while CO remained unchanged (90 ± 40 μl/min).

SIGNIFICANCE

The epicardial outline method is an effective way of using echocardiography to assess cardiac dysfunction in pathological adult zebrafish, unlocking a major bottleneck in this research field with limited cardiac functional assays.

Characterization of Hand Tendons Through High-Frequency Ultrasound Elastography

Tendon stiffness plays an important role in the tendon healing process, and many studies have indicated that measuring the shear wave velocity (SWV) on tendons relates to their stiffness. Because the thickness of hand tendons is a few millimeters, high-resolution imaging is required for visualizing hand tissues. However, the resolution of current ultrasound elastography systems is insufficient. In this study, a high-frequency (HF) ultrasound elastography system is proposed for measuring the SWVs of hand tendons. The HF ultrasound elastography system uses an external vibrator to create shear waves on hand tendons. Then, it uses a 40-MHz HF ultrasound array transducer with ultrafast ultrasound imaging technology to measure the SWV for characterizing hand tendons. A handheld device that combines a transducer and a vibrator allows the user to scan hand tissues. The biases of HF ultrasound elastography were measured in gelatin phantom experiments and were less than 6% compared to standard mechanical testing approach. Human experiments showed the ability to use HF ultrasound elastography to distinguish different SWVs of hand tendons. The SWVs were 0.73 ± 0.65 m/s and 1 ± 0.54 m/s for flexor digitorum superficialis (FDS) and flexor digitorum profundus (FDP), respectively, and 0.52 ± 0.14 m/s and 4.02 ± 0.77 m/s for extensor tendon under stretch and contraction conditions, respectively. The simplicity and convenience of the HF ultrasound elastography system for measuring hand tendon stiffness make it a promising tool for evaluating the severity of hand injuries and the performance of rehabilitation after hand injuries.

High-frequency ultrasound deformation imaging for adult zebrafish during heart regeneration

Background

The adult human heart cannot efficiently generate new cardiac muscle cells in response to injury, and, therefore, cardiac injury results in irreversible damage to cardiac functions. The zebrafish (Danio rerio) is a crucial animal model in cardiac research because of its remarkable capacity for tissue regeneration. An adult zebrafish can completely regenerate cardiac tissue without a scar being formed, even after 20% of its ventricular myocardium has been resected. Zebrafish have been utilized in developmental biology and genetics research; however, the details of myocardium motions during their cardiac cycle in different regeneration phases are still not fully understood.

Methods

In this study, we used a 70-MHz high-resolution ultrasound deformation imaging system to observe the functional recovery of zebrafish hearts after amputation of the ventricular apex.

Results

The myocardial deformation and cardiac output (CO) were measured in different regeneration phases relative to the day of amputation. In response to the damage to the heart, the peak systolic strain (ε_max) and strain during ejection time (ε_ej) were lower than normal at 3 days after the myocardium amputation. The CO had normalized to the baseline values at 7 days after surgery.

Conclusions

Our results confirm that the imaging system constructed for this study is suitable for examining zebrafish cardiac functions during heart regeneration.

Brain aromatase modulates cardiac functions in embryonic zebrafish

Oestradiol (E₂) is known as a female reproductive hormone with pleiotropic effects on the cardiovascular system. Local E₂ biosynthesis such as in the brain and myocardial cells have important physiological and pathophysiological roles. E₂ production is catalysed by aromatase (Aro) enzyme. In teleost, two Aro isoforms are distinctly expressed in the ovary and brain. In this study, the role of brain Aro (AroB) in modulating cardiovascular system is investigated. AroB MO-mediated knockdown decreased ventricular functions. Moreover, embryos injected with AroB MO displays a sign in developing heart failure. All the effects caused by AroB MO were partially reversed by exposure to E₂. Taken together, this study demonstrates the role of AroB in modulating normal cardiovascular function in zebrafish embryos.

Monitoring of Adult Zebrafish Heart Regeneration Using High-Frequency Ultrasound Spectral Doppler and Nakagami Imaging

This paper reports the feasibility of Nakagami imaging in monitoring the regeneration process of zebrafish hearts in a noninvasive manner. In addition, spectral Doppler waveforms that are typically used to access the diastolic function were measured to validate the performance of Nakagami imaging. A 30-MHz high-frequency ultrasound array transducer was used to acquire backscattered echo signal for spectral Doppler and Nakagami imaging. The performances of both methods were validated with flow and tissue-mimicking phantom experiments. For in vivo experiments, both spectral Doppler and Nakagami imaging were simultaneously obtained from adult zebrafish with amputated hearts. Longitudinal measurements were performed for five zebrafish. From the experiments, the E/A ratio measured using spectral Doppler imaging increased at 3 days post-amputation (3 dpa) and then decreased to the value before amputation, which were consistent with previous studies. Similar results were obtained from the Nakagami imaging where the Nakagami parameter value increased at 3 dpa and decreased to its original value. These results suggested that the Nakagami and spectral Doppler imaging would be useful techniques in monitoring the regeneration of heart or tissues.

Analysis and Design of High-Frequency 1-D CMUT Imaging Arrays in Noncollapsed Mode

High-frequency ultrasound imaging arrays are important for a broad range of applications, from small animal imaging to photoacoustics. Capacitive micromachined ultrasonic transducer (CMUT) arrays are particularly attractive for these applications as low noise receiver electronics can be integrated for an overall improved performance. In this paper, we present a comprehensive analysis of high-frequency CMUT arrays based on an experimentally verified CMUT array simulation tool. The results obtained on an example, a 40-MHz 1-D CMUT array for intravascular imaging, are used to obtain key design insights and tradeoffs for receive only and pulse-echo imaging. For the receiver side, thermal mechanical current noise, plane wave pressure sensitivity, and pressure noise spectrum are extracted from simulations. Using these parameters, we find that the receiver performance of CMUT arrays can be close to an ideal piston, independent of gap thickness, and applied dc bias, when coupled to low noise electronics with arrays utilizing smaller membranes performing better. For pulse-echo imaging, thermal mechanical current noise limited signal-to-noise ratio is observed to be dependent on the maximum available voltage and gap thickness. In terms of bandwidth, we find that the Bragg resonance of the array, related to the fill factor, is a significant determinant of the high frequency limit and the fluid loaded single membrane resonance determines the lower limit. Based on these results, we present design guidelines requiring only fluid loaded single membrane simulations and membrane pitch to achieve a desired pulse-echo response. We also provide a design example and discuss limitations of the approach.

In Vivo Visualization of Vasculature in Adult Zebrafish by Using High-Frequency Ultrafast Ultrasound Imaging

OBJECTIVE

Zebrafish has been recently considered an ideal vertebrate for studying developmental biology, genetics, particularly for modeling tumorigenesis, angiogenesis, and regeneration in vivo. However, when a zebrafish matures completely, its body loses transparency, thus making conventional optical imaging techniques difficult for imaging internal anatomy and vasculature. Acoustic wave penetration outperforms optical methods, high-frequency (>30 MHz) ultrasound (HFUS) was consequently an alternative imaging modality for adult zebrafish imaging, particularly for echocardiography However, visualizing peripheral vessels in a zebrafish by using conventional HFUS is still difficult.

METHODS

In the present study, high-frequency micro-Doppler imaging (HFμDI) based on ultrafast ultrasound imaging was proposed for zebrafish dorsal vascular mapping in vivo. HFμDI uses a 40-MHz ultrasound transducer, which is an ultrafast ultrasound imaging technology with the highest frequency available currently. Blood flow signals were extracted using an eigen-based clutter filter with different settings. Experiments were performed on an 8-month-old wild-type AB-line adult zebrafish.

RESULTS

Blood vessels, including intersegmental vessels, parachordal vessel, dorsal longitudinal anastomotic vessel, and dorsal aorta, from the dorsal side of the zebrafish were clearly observed in two-dimensional (2-D) and 3-D HFμDI.

CONCLUSION

The maximum image depth of HFμDI and the minimal diameter of vessel can be detected were 4 mm and 36 μm, respectively; they were determined without any use of microbubbles. The maximum flow velocity range was approximately 3-4 mm/s on the dorsal vessels of the adult zebrafish.

SIGNIFICANCE

Compared with conventional ultrasound Doppler imaging, HFμDI exhibited superior small vessel imaging.

A Langendorff-like system to quantify cardiac pump function in adult zebrafish

Zebrafish are increasingly used as a vertebrate model to study human cardiovascular disorders. Although heart structure and function are readily visualized in zebrafish embryos because of their optical transparency, the lack of effective tools for evaluating the hearts of older, nontransparent fish has been a major limiting factor. The recent development of high-frequency echocardiography has been an important advance for in vivo cardiac assessment, but it necessitates anesthesia and has limited ability to study acute interventions. We report the development of an alternative experimental ex vivo technique for quantifying heart size and function that resembles the Langendorff heart preparations that have been widely used in mammalian models. Dissected adult zebrafish hearts were perfused with a calcium-containing buffer, and a beat frequency was maintained with electrical stimulation. The impact of pacing frequency, flow rate and perfusate calcium concentration on ventricular performance (including end-diastolic and end-systolic volumes, ejection fraction, radial strain, and maximal velocities of shortening and relaxation) were evaluated and optimal conditions defined. We determined the effects of age on heart function in wild-type male and female zebrafish, and successfully detected hypercontractile and hypocontractile responses after adrenergic stimulation or doxorubicin treatment, respectively. Good correlations were found between indices of cardiac contractility obtained with high-frequency echocardiography and with the ex vivo technique in a subset of fish studied with both methods. The ex vivo beating heart preparation is a valuable addition to the cardiac function tool kit that will expand the use of adult zebrafish for cardiovascular research.

Pharmacological Modulation of Hemodynamics in Adult Zebrafish In Vivo

Introduction

Hemodynamic parameters in zebrafish receive increasing attention because of their important role in cardiovascular processes such as atherosclerosis, hematopoiesis, sprouting and intussusceptive angiogenesis. To study underlying mechanisms, the precise modulation of parameters like blood flow velocity or shear stress is centrally important. Questions related to blood flow have been addressed in the past in either embryonic or ex vivo-zebrafish models but little information is available for adult animals. Here we describe a pharmacological approach to modulate cardiac and hemodynamic parameters in adult zebrafish in vivo.

Materials and Methods

Adult zebrafish were paralyzed and orally perfused with salt water. The drugs isoprenaline and sodium nitroprusside were directly applied with the perfusate, thus closely resembling the preferred method for drug delivery in zebrafish, namely within the water. Drug effects on the heart and on blood flow in the submental vein were studied using electrocardiograms, in vivo-microscopy and mathematical flow simulations.

Results

Under control conditions, heart rate, blood flow velocity and shear stress varied less than ± 5%. Maximal chronotropic effects of isoprenaline were achieved at a concentration of 50 μmol/L, where it increased the heart rate by 22.6 ± 1.3% (n = 4; p < 0.0001). Blood flow velocity and shear stress in the submental vein were not significantly increased. Sodium nitroprusside at 1 mmol/L did not alter the heart rate but increased blood flow velocity by 110.46 ± 19.64% (p = 0.01) and shear stress by 117.96 ± 23.65% (n = 9; p = 0.03).

Discussion

In this study, we demonstrate that cardiac and hemodynamic parameters in adult zebrafish can be efficiently modulated by isoprenaline and sodium nitroprusside. Together with the suitability of the zebrafish for in vivo-microscopy and genetic modifications, the methodology described permits studying biological processes that are dependent on hemodynamic alterations.

Modeling GATAD1-Associated Dilated Cardiomyopathy in Adult Zebrafish

Animal models have played a critical role in validating human dilated cardiomyopathy (DCM) genes, particularly those that implicate novel mechanisms for heart failure. However, the disease phenotype may be delayed due to age-dependent penetrance. For this reason, we generated an adult zebrafish model, which is a simpler vertebrate model with higher throughput than rodents. Specifically, we studied the zebrafish homologue of GATAD1, a recently identified gene for adult-onset autosomal recessive DCM. We showed cardiac expression of gatad1 transcripts, by whole mount in situ hybridization in zebrafish embryos, and demonstrated nuclear and sarcomeric I-band subcellular localization of Gatad1 protein in cardiomyocytes, by injecting a Tol2 plasmid encoding fluorescently-tagged Gatad1. We next generated gatad1 knock-out fish lines by TALEN technology and a transgenic fish line that expresses the human DCM GATAD1-S102P mutation in cardiomyocytes. Under stress conditions, longitudinal studies uncovered heart failure (HF)-like phenotypes in stable KO mutants and a tendency toward HF phenotypes in transgenic lines. Based on these efforts of studying a gene-based inherited cardiomyopathy model, we discuss the strengths and bottlenecks of adult zebrafish as a new vertebrate model for assessing candidate cardiomyopathy genes.

The Zebrafish Heart as a Model of Mammalian Cardiac Function

Zebrafish (Danio rerio) are widely used as vertebrate model in developmental genetics and functional genomics as well as in cardiac structure-function studies. The zebrafish heart has been increasingly used as a model of human cardiac function, in part, due to the similarities in heart rate and action potential duration and morphology with respect to humans. The teleostian zebrafish is in many ways a compelling model of human cardiac function due to the clarity afforded by its ease of genetic manipulation, the wealth of developmental biological information, and inherent suitability to a variety of experimental techniques. However, in addition to the numerous advantages of the zebrafish system are also caveats related to gene duplication (resulting in paralogs not present in human or other mammals) and fundamental differences in how zebrafish hearts function. In this review, we discuss the use of zebrafish as a cardiac function model through the use of techniques such as echocardiography, optical mapping, electrocardiography, molecular investigations of excitation-contraction coupling, and their physiological implications relative to that of the human heart. While some of these techniques (e.g., echocardiography) are particularly challenging in the zebrafish because of diminutive size of the heart (~1.5 mm in diameter) critical information can be derived from these approaches and are discussed in detail in this article.

Use of echocardiography reveals reestablishment of ventricular pumping efficiency and partial ventricular wall motion recovery upon ventricular cryoinjury in the zebrafish

Aims

While zebrafish embryos are amenable to in vivo imaging, allowing the study of morphogenetic processes during development, intravital imaging of adults is hampered by their small size and loss of transparency. The use of adult zebrafish as a vertebrate model of cardiac disease and regeneration is increasing at high speed. It is therefore of great importance to establish appropriate and robust methods to measure cardiac function parameters.

Methods and Results

Here we describe the use of 2D-echocardiography to study the fractional volume shortening and segmental wall motion of the ventricle. Our data show that 2D-echocardiography can be used to evaluate cardiac injury and also to study recovery of cardiac function. Interestingly, our results show that while global systolic function recovered following cardiac cryoinjury, ventricular wall motion was only partially restored.

Conclusion

Cryoinjury leads to long-lasting impairment of cardiac contraction, partially mimicking the consequences of myocardial infarction in humans. Functional assessment of heart regeneration by echocardiography allows a deeper understanding of the mechanisms of cardiac regeneration and has the advantage of being easily transferable to other cardiovascular zebrafish disease models.

High-resolution tissue Doppler imaging of the zebrafish heart during its regeneration

The human heart cannot regenerate after injury, whereas the adult zebrafish can fully regenerate its heart even after 20% of the ventricle is amputated. Many studies have begun to reveal the cellular and molecular mechanisms underlying this regenerative process, which have exciting implications for human cardiac diseases. However, the dynamic functions of the zebrafish heart during regeneration are not yet understood. This study established a high-resolution echocardiography for tissue Doppler imaging (TDI) of the zebrafish heart to explore the cardiac functions during different regeneration phases. Experiments were performed on AB-line adult zebrafish (n=40) in which 15% of the ventricle was surgically removed. An 80-MHz ultrasound TDI based on color M-mode imaging technology was employed. The cardiac flow velocities and patterns from both the ventricular chamber and myocardium were measured at different regeneration phases relative to the day of amputation. The peak velocities of early diastolic inflow, early diastolic myocardial motion, late diastolic myocardial motion, early diastolic deceleration slope, and heart rate were increased at 3 days after the myocardium amputation, but these parameters gradually returned to close to their baseline values for the normal heart at 7 days after amputation. The peak velocities of late diastolic inflow, ventricular systolic outflow, and systolic myocardial motion did not significantly differ during the heart regeneration.