Dual-element needle transducer for intravascular ultrasound imaging

Multiplexed Ultrasound Imaging Using Spectral Analysis on Gas Vesicles

Current advances in ultrasound imaging techniques combined with the next generation contrast agents such as gas vesicles (GV) revolutionize the visualization of biological tissues with spatiotemporal precision. In optics, fluorescent proteins enable understanding of molecular and cellular functions in biological systems due to their multiplexed imaging capability. Here, a panel of GVs is investigated using mid-band fit (MBF) spectral imaging to realize multiplexed ultrasound imaging to uniquely visualize locations of different types of stationary GVs. The MBF spectral imaging technique demonstrates that stationary clustered GVs are efficiently localized and distinguished from unclustered GVs in agarose gel phantom and 3D vessel structures are visualized in ex vivo mouse liver specimens. Mouse macrophages serve as carriers of clustered and unclustered GVs and multiplexing beacons to report cells' spatial locations by emitting distinct spectral signals. 2D MBF spectral images are reconstructed, and pixels in these images are classified depending on MBF values by comparing predetermined filters that predict the existence of cells with clustered and unclustered GVs. This pseudo-coloring scheme clearly distinguishes the locations of two classes of cells like pseudo-color images in fluorescence microscopy.

Design and Micro-Fabrication of Focused High-Frequency Needle Transducers for Medical Imaging

In this study, we report an advanced fabrication technique to develop a miniature focused needle transducer. Two different types of high-frequency (100 MHz) transducers were fabricated using the lead magnesium niobate-lead titanate (PMN-0.3PT) and lithium niobate (LiNbO3) single crystals. In order to enhance the transducer's performance, a unique mass-spring matching layer technique was adopted, in which gold and parylene play the roles of the mass layer and spring layer, respectively. The PMN-0.3PT transducer had a 103 MHz center frequency with a -6 dB bandwidth of 52%, and a signal-to-noise ratio (SNR) of 42 dB. The center frequency, -6 dB bandwidth, and SNR of the LiNbO3 transducer were 105 MHz, 66%, and 44 dB, respectively. In order to compare and evaluate the transducers' performances, an ultrasonic biomicroscopy (UBM) imaging on the fish eye was performed. The results showed that the LiNbO3 transducer had a better contrast resolution compared to the PMN-0.3PT transducer. The fabricated transducer showed an excellent performance with high-resolution corneal epithelium imaging of the experimental fish eye. These interesting findings are useful for the future biomedical implementation of the fabricated transducers in the field of high-resolution ultrasound imaging and diagnosis purpose.

Mechanically Rotating Intravascular Ultrasound (IVUS) Transducer: A Review

Intravascular ultrasound (IVUS) is a valuable imaging modality for the diagnosis of atherosclerosis. It provides useful clinical information, such as lumen size, vessel wall thickness, and plaque composition, by providing a cross-sectional vascular image. For several decades, IVUS has made remarkable progress in improving the accuracy of diagnosing cardiovascular disease that remains the leading cause of death globally. As the quality of IVUS images mainly depends on the performance of the IVUS transducer, various IVUS transducers have been developed. Therefore, in this review, recently developed mechanically rotating IVUS transducers, especially ones exploiting piezoelectric ceramics or single crystals, are discussed. In addition, this review addresses the history and technical challenges in the development of IVUS transducers and the prospects of next-generation IVUS transducers.

Recent Advances in Transducers for Intravascular Ultrasound (IVUS) Imaging

As a well-known medical imaging methodology, intravascular ultrasound (IVUS) imaging plays a critical role in diagnosis, treatment guidance and post-treatment assessment of coronary artery diseases. By cannulating a miniature ultrasound transducer mounted catheter into an artery, the vessel lumen opening, vessel wall morphology and other associated blood and vessel properties can be precisely assessed in IVUS imaging. Ultrasound transducer, as the key component of an IVUS system, is critical in determining the IVUS imaging performance. In recent years, a wide range of achievements in ultrasound transducers have been reported for IVUS imaging applications. Herein, a comprehensive review is given on recent advances in ultrasound transducers for IVUS imaging. Firstly, a fundamental understanding of IVUS imaging principle, evaluation parameters and IVUS catheter are summarized. Secondly, three different types of ultrasound transducers (piezoelectric ultrasound transducer, piezoelectric micromachined ultrasound transducer and capacitive micromachined ultrasound transducer) for IVUS imaging are presented. Particularly, the recent advances in piezoelectric ultrasound transducer for IVUS imaging are extensively examined according to their different working mechanisms, configurations and materials adopted. Thirdly, IVUS-based multimodality intravascular imaging of atherosclerotic plaque is discussed. Finally, summary and perspectives on the future studies are highlighted for IVUS imaging applications.

Measurements of acoustic radiation force of ultrahigh frequency ultrasonic transducers using model-based approach

Even though ultrahigh frequency ultrasonic transducers over 60 MHz have been used for single-cell-level manipulation such as intracellular delivery, acoustic tweezers, and stimulation to investigate cell phenotype and cell mechanics, no techniques have been available to measure the actual acoustic radiation force (ARF) applied to target cells. Therefore, we have developed an approach to measure the ARF of ultrahigh frequency ultrasonic transducers using a theoretical model of the dynamics of a solid sphere in a gelatin phantom. To estimate ARF at the focus of a 130 MHz transducer, we matched measured maximum displacements of a solid sphere with theoretical calculations. We selected appropriate ranges of input voltages and pulse durations for single-cell applications, and the estimated ARF was in the range of tens of μN. To gauge the influence of pulse duration, an impulse of different pulse durations was estimated. Fluorescence resonance energy transfer live cell imaging was demonstrated to visualize calcium transport between cells after a target single cell was stimulated by the developed ultrasonic transducer.

FRET-Based Ca2+ Biosensor Single Cell Imaging Interrogated by High-Frequency Ultrasound

Fluorescence resonance energy transfer (FRET)-based biosensors have advanced live cell imaging by dynamically visualizing molecular events with high temporal resolution. FRET-based biosensors with spectrally distinct fluorophore pairs provide clear contrast between cells during dual FRET live cell imaging. Here, we have developed a new FRET-based Ca²⁺ biosensor using EGFP and FusionRed fluorophores (FRET-GFPRed). Using different filter settings, the developed biosensor can be differentiated from a typical FRET-based Ca²⁺ biosensor with ECFP and YPet (YC3.6 FRET Ca²⁺ biosensor, FRET-CFPYPet). A high-frequency ultrasound (HFU) with a carrier frequency of 150 MHz can target a subcellular region due to its tight focus smaller than 10 µm. Therefore, HFU offers a new single cell stimulations approach for FRET live cell imaging with precise spatial resolution and repeated stimulation for longitudinal studies. Furthermore, the single cell level intracellular delivery of a desired FRET-based biosensor into target cells using HFU enables us to perform dual FRET imaging of a cell pair. We show that a cell pair is defined by sequential intracellular delivery of the developed FRET-GFPRed and FRET-CFPYPet into two target cells using HFU. We demonstrate that a FRET-GFPRed exhibits consistent 10–15% FRET response under typical ionomycin stimulation as well as under a new stimulation strategy with HFU.

Dual-Element Intravascular Ultrasound Transducer for Tissue Harmonic Imaging and Frequency Compounding: Development and Imaging Performance Assessment

OBJECTIVE

For accurate diagnosis of atherosclerosis, the high spatial and contrast resolutions of intravascular ultrasound (IVUS) images are a key requirement. Increasing the center frequency of IVUS is a simple solution to meet this requirement. However, this leads to a reduction in imaging depth due to the frequency-dependent attenuation of ultrasound. Here, we report a recently developed dual-element IVUS transducer for tissue harmonic imaging (THI) and frequency compounding to increase the spatial and contrast resolutions of IVUS images, while maintaining the imaging depth to assess the overall morphological change of blood vessels.

METHODS

One 35-MHz element is used for producing general IVUS images and the other 70-MHz element is for receiving the second harmonic signals induced by the 35-MHz ultrasound. The fundamental and second harmonic signals can also be used for frequency compound imaging to further improve contrast resolution. The spatial and contrast resolutions achieved by the developed transducer were evaluated through wire and tissue-mimicking phantom imaging tests. Additionally, the images of a stent deployed in a tissue-mimicking phantom and an excised pig artery were acquired to assess clinical usefulness of the transducer.

RESULTS

The results demonstrated that the developed IVUS transducer enables us to simultaneously examine the overall morphological change of blood vessels by the 35-MHz ultrasound images and the near vessel layers such as the intima, the media, and the adventitia by either THI or compound images with high spatial and contrast resolutions. In addition, the developed transducer facilitates the simultaneous acquisition of 35- and 70-MHz fundamental images when needed.

CONCLUSION

The developed dual-element IVUS transducer makes it possible to fully realize the potential benefits of IVUS in the diagnosis of atherosclerosis.

Cable shared dual-frequency catheter for intravascular ultrasound

This study proposes a catheter consisting of dual-frequency transducer for intravascular ultrasound. Both ultrasonic elements with different frequencies were connected to one coaxial cable to make the connection simple. The aperture size of the ultrasound elements were 0.4×0.6 mm2 and 0.3×0.4 mm2 for the low frequency element and high frequency element, respectively. The center frequency and bandwidth of the fabricated low frequency transducer were 33.8 MHz and 49.3%, respectively. Meanwhile, the center frequency and bandwidth of the high frequency transducer were 80.6 MHz and 50.3%, respectively. Imaging evaluations of wire phantom, tissue phantom and vessel tissue demonstrated good imaging capability of the dual-frequency catheter. The spatial resolution are 19 μm axially and 128 μm laterally for the high frequency transducer, and 37 μm axially and 199 μm laterally for the low frequency transducer. Band-pass filters were designed to separate the mixed echo signals. After filtering, the images from different ultrasound elements can be successfully identified, indicating the feasibility of the proposed cable shared dual-frequency imaging strategy. The proposed method has simple structure, good imaging resolution, and large penetration depth, showing good application prospect for intravascular ultrasound.

Acoustic mechanogenetics

From basic studies in understanding the role of signaling pathways to therapeutic applications in engineering new cellular functions, efficient and safe techniques to monitor and modulate molecular targets from cells to organs have been extensively developed. The developmental advancement of engineering devices such as microscope and ultrasonic transducers allows us to investigate biological processes at different scales. Synthetic biology has further emerged recently as a powerful platform for the development of new diagnostic and therapeutic molecular tools. The synergetic amalgamation between engineering tools and synthetic biology has rapidly become a new front in the field of bioengineering and biotechnology. In this review, ultrasound and its generated mechanical perturbation are introduced to serve as a non-invasive engineering approach and, integrated with synthetic biology, to remotely control signaling and genetic activities for the guidance of cellular functions deep inside tissue with high spatiotemporal resolutions. This ultrasound-based approach together with synthetic biology has been applied in immunotherapy, neuroscience, and gene delivery, paving the way for the development of next-generation therapeutic tools.

Novel Configurations of Ultrahigh Frequency (≤600 MHz) Analog Frontend for High Resolution Ultrasound Measurement

In this article, an approach to designing and developing an ultrahigh frequency (≤600 MHz) ultrasound analog frontend with Golay coded excitation sequence for high resolution imaging applications is presented. For the purpose of visualizing specific structures or measuring functional responses of micron-sized biological samples, a higher frequency ultrasound is needed to obtain a decent spatial resolution while it lowers the signal-to-noise ratio, the difference in decibels between the signal level and the background noise level, due to the higher attenuation coefficient. In order to enhance the signal-to-noise ratio, conventional approach was to increase the transmit voltage level. However, it may cause damaging the extremely thin piezoelectric material in the ultrahigh frequency range. In this paper, we present a novel design of ultrahigh frequency (≤600 MHz) frontend system capable of performing pseudo Golay coded excitation by configuring four independently operating pulse generators in parallel and the consecutive delayed transmission from each channel. Compared with the conventional monocycle pulse approach, the signal-to-noise ratio of the proposed approach was improved by 7⁻9 dB without compromising the spatial resolution. The measured axial and lateral resolutions of wire targets were 16.4 µm and 10.6 µm by using 156 MHz 4 bit pseudo Golay coded excitation, respectively and 4.5 µm and 7.7 µm by using 312 MHz 4 bit pseudo Golay coded excitation, respectively.

A 35 MHz/105 MHz Dual-Element Focused Transducer for Intravascular Ultrasound Tissue Imaging Using the Third Harmonic

The superharmonic imaging of tissue has the potential for high spatial and contrast resolutions, compared to the fundamental and second harmonic imaging. For this technique, the spectral bandwidth of an ultrasound transducer is divided for transmission of ultrasound and reception of its superharmonics (i.e., higher than the second harmonic). Due to the spectral division for the transmission and reception, transmitted ultrasound energy is not sufficient to induce superharmonics in media without using contrast agents, and it is difficult that a transducer has a −6 dB fractional bandwidth of higher than 100%. For the superharmonic imaging of tissue, thus, multi-frequency array transducers are the best choice if available; transmit and receive elements are separate and have different center frequencies. However, the construction of a multi-frequency transducer for intravascular ultrasound (IVUS) imaging is particularly demanding because of its small size of less than 1 mm. Here, we report a recently developed dual-element focused IVUS transducer for the third harmonic imaging of tissue, which consists of a 35-MHz element for ultrasound transmission and a 105-MHz element for third harmonic reception. For high quality third harmonic imaging, both elements were fabricated to have the same focus at 2.5 mm. The results of tissue mimicking phantom tests demonstrated that the third harmonic images produced by the developed transducer had higher spatial resolution and deeper imaging depth than the fundamental images.

Mechanogenetics for the remote and noninvasive control of cancer immunotherapy

While cell-based immunotherapy, especially chimeric antigen receptor (CAR)-expressing T cells, is becoming a paradigm-shifting therapeutic approach for cancer treatment, there is a lack of general methods to remotely and noninvasively regulate genetics in live mammalian cells and animals for cancer immunotherapy within confined local tissue space. To address this limitation, we have identified a mechanically sensitive Piezo1 ion channel (mechanosensor) that is activatable by ultrasound stimulation and integrated it with engineered genetic circuits (genetic transducer) in live HEK293T cells to convert the ultrasound-activated Piezo1 into transcriptional activities. We have further engineered the Jurkat T-cell line and primary T cells (peripheral blood mononuclear cells) to remotely sense the ultrasound wave and transduce it into transcriptional activation for the CAR expression to recognize and eradicate target tumor cells. This approach is modular and can be extended for remote-controlled activation of different cell types with high spatiotemporal precision for therapeutic applications.

Development of a 3 French Dual-Frequency Intravascular Ultrasound Catheter

Coronary plaque morphology, including plaque size and fibrous cap thickness, is thought to contribute to the risk of plaque rupture and future cardiac events. Dual-frequency intravascular ultrasound has been proposed as a possible technique to visualize both large-scale features and superficial detail of coronary plaque; however, it has not been found to be feasible within the constraints of a clinically functional intravascular ultrasound catheter. In this study, we describe the design and fabrication of a dual-frequency catheter using a bidirectional transducer stack with center frequencies of approximately 30 and 80 MHz. We describe how the high-frequency transducer achieves significantly improved axial and lateral resolution (16 and 120 µm, respectively, vs. 50 and 220 µm) at the expense of penetration depth. Finally, imaging of ex vivo human coronary artery segments reveals that the catheter can provide complementary images of the deeper arterial wall and superficial plaque features.

Direct and sustained intracellular delivery of exogenous molecules using acoustic-transfection with high frequency ultrasound

Controlling cell functions for research and therapeutic purposes may open new strategies for the treatment of many diseases. An efficient and safe introduction of membrane impermeable molecules into target cells will provide versatile means to modulate cell fate. We introduce a new transfection technique that utilizes high frequency ultrasound without any contrast agents such as microbubbles, bringing a single-cell level targeting and size-dependent intracellular delivery of macromolecules. The transfection apparatus consists of an ultrasonic transducer with the center frequency of over 150 MHz and an epi-fluorescence microscope, entitled acoustic-transfection system. Acoustic pulses, emitted from an ultrasonic transducer, perturb the lipid bilayer of the cell membrane of a targeted single-cell to induce intracellular delivery of exogenous molecules. Simultaneous live cell imaging using HeLa cells to investigate the intracellular concentration of Ca²⁺ and propidium iodide (PI) and the delivery of 3 kDa dextran labeled with Alexa 488 were demonstrated. Cytosolic delivery of 3 kDa dextran induced via acoustic-transfection was manifested by diffused fluorescence throughout whole cells. Short-term (6 hr) cell viability test and long-term (40 hr) cell tracking confirmed that the proposed approach has low cell cytotoxicity.

Impedance matching network for high frequency ultrasonic transducer for cellular applications

An approach for the design of an impedance matching network (IMN) for high frequency ultrasonic transducers with large apertures based on impedance analysis for cellular applications is presented in this paper. The main objectives were to maximize energy transmission from the excitation source to the ultrasonic transducers for cell manipulation and to achieve low input parameters for the safe operation of an ultrasonic transducer because the piezoelectric material in high frequency ultrasonic transducers is prone to breakage due to its being extremely thin. Two ultrasonic transducers, which were made of lithium niobate single crystal with the thickness of 15 μm, having apertures of 4.3 mm (fnumber=1.23) and 2.6mm (fnumber=0.75) were tested. L-type IMN was selected for high sensitivity and compact design of the ultrasonic transducers. The target center frequency was chosen as the frequency where the electrical admittance (|Y|) and phase angle (θz) from impedance analysis was maximal and zero, respectively. The reference center frequency and reference echo magnitude were selected as the center frequency and echo magnitude, measured by pulse-echo testing, of the ultrasonic transducer without IMN. Initial component values and topology of IMN were determined using the Smith chart, and pulse-echo testing was analyzed to verify the performance of the ultrasonic transducers with and without IMN. After several iterations between changing component values and topology of IMN, and pulse-echo measurement of the ultrasonic transducer with IMN, optimized component values and topology of IMN were chosen when the measured center frequency from pulse-echo testing was comparable to the target frequency, and the measured echo magnitude was at least 30% larger than the reference echo magnitude. Performance of an ultrasonic transducer with and without IMN was tested by observing a tangible dent on the surface of a plastic petridish and single cell response after an acoustic pulse was applied on a target cell.