High-frequency Array Transducers Research Articles

High-Frequency Ultrasound Imaging With Sub-Nyquist Sampling.

Implementation of a high-frequency ultrasound (HFUS) beamformer is computationally challenging because of its high sampling rate. This article introduces an efficient beamformer with sub-Nyquist sampling (or bandpass sampling) that is suitable for HFUS imaging. Our approach used channel radio frequency data sampled at bandpass sampling rate (i.e., 4/ 3fc ) and postfiltering-based interpolation to reduce the computational complexity. A polyphase structure for interpolation was used to further reduce the computational burden while maintaining an adequate delay resolution ( δ ). The performance of the proposed beamformer (i.e., 4/ 3fc sampling with sixfold interpolation, δ = 8fc ) was compared with that of the conventional method (i.e., 4fc sampling with fourfold interpolation, δ = 16fc ). Ultrafast coherent compounding imaging was used in simulation, in vitro and in vivo imaging experiments. Axial/lateral resolution and contrast-to-noise ratio (CNR) values were measured for quantitative evaluation. The number of transmit pulse cycles was varied from 1 to 3 using two transducers with different fractional bandwidths (67% and 98%). In the simulation, the proposed and conventional methods showed the similar -6-dB axial beam widths (63.5 and 61.5 μm , respectively) from the two-cycle transmit pulse using the transducer with a bandwidth of 67%. In vitro and in vivo imaging experiments were performed using a Verasonics ultrasound research platform equipped with a high-frequency array transducer (20-46 MHz). The in vitro imaging results using a wire target showed consistent results with the simulation study (i.e., disparity at -6-dB axial resolution). The in vivo feasibility study with a murine mouse model with breast cancer was also performed, and the proposed method yielded a similar image quality compared with the conventional method. From these studies, it was demonstrated that the proposed HFUS beamformer based on the bandpass sampling can substantially reduce the computational complexity while minimizing the loss of spatial resolution for HFUS imaging.

Tumor Contrast Imaging with Gas Vesicles by Circumventing the Reticuloendothelial System

Gas vesicles (GVs) are nanosized structures (45–800 nm) and have been reported to produce non-linear contrast signals, making them an attractive agent for molecular targeting of tumors. One barrier to their use for pre-clinical oncology studies is rapid uptake into the reticuloendothelial system (RES) and consequent rapid decrease in contrast signal after infusion ends and low signal on reperfusion after a bubble burst sequence. The purpose of this study was to examine suppression of the RES and surface modification of GVs to prolong contrast circulation in tumors for ultrasound imaging. Ultrasound imaging to measure dynamics of contrast signal intensity in tumor models was carried out using a 21-MHz high-frequency array transducer with the Vevo 2100 ultrasound system. The non-linear contrast signal from intravenously injected GVs compared with peak enhancement was measured during contrast wash-out and on reperfusion after a contrast burst sequence. Disrupting the RES by saturating the macrophage population or chemically inhibiting the Kupffer cell population with gadolinium or Intralipid preserves 62%–88% of GVs’ contrast enhancement relative to peak during the wash-out phase and 32%–56% on reperfusion compared with 38% and 14%, respectively, for no disruption of the RES, indicating longer circulation of GVs in the tumor. Additionally, coating the GVs with 2-, 5- or 10-kDa polyethylene glycol (PEG) chains resulted in >70% contrast signal retention in the tumors during wash-out and, for 5- or 10-kDa PEG chains, a return to >45% of peak contrast signal on reperfusion. These findings indicate that GVs can be used as a contrast agent for tumor imaging and that disruption of the RES improved recirculation and maintained contrast enhancement caused by GVs in the tumors.

Development of a KNN Ceramic-Based Lead-Free Linear Array Ultrasonic Transducer.

High-frequency array transducers can provide higher imaging resolution than traditional transducers, thus resolving smaller features and producing finer images. Commercially available ultrasonic transducers are mostly made with lead-based piezoelectric materials, which are harmful to the environment and public health. This paper presents the development of the 64-elements high-frequency (18.3 MHz) lead-free linear array ultrasonic transducer based on (K0.44Na0.52Li0.04)(Nb0.86Ta0.1Sb0.04)O3 (KNLNTS) piezoceramic. Array elements were spaced at a 75- pitch, and interconnected via a custom flexible circuit. The two matching layers and a light backing material were used to improve the performance of the array. The developed KNLNTS ceramic-based lead-free linear array exhibited a center frequency of 18.3 MHz, an average -6-dB bandwidth of 42%, an average two-way insertion loss of 41.8 dB, and a crosstalk between the adjacent elements of less than -53 dB near the center frequency. An image of a tungsten wire phantom was acquired using a Verasonics Vantage research ultrasound system. Results from imaging tests demonstrated a good imaging capability with a spatial resolution of axially and laterally, indicating that the lead-free linear array ultrasonic transducer based on KNLNTS ceramics is a promising alternative to lead-based transducers for ultrasound medical imaging.

Spectrum analysis for assessing red blood cell aggregation using high-frequency ultrasound array transducer.

The purpose of this study is to investigate a spectrum analysis technique for detecting and monitoring red blood cell (RBC) aggregation using a high-frequency array transducer. To assess the feasibility of this approach, the backscattered radio-frequency signal from non-aggregated and aggregated RBC samples with two hematocrit levels were acquired by using a 30-MHz linear array transducer and analyzed in frequency domain. Three parameters such as spectral slope, midband fit and Y intercept were extracted in a static condition. Fresh porcine blood was used and degrees of aggregation were changed by diluting plasma concentration. From the experiments, it was demonstrated that the spectral slope related to a size of scatterer progressively declined as the level of aggregation increased; its mean values at hematocrit of 40% were 1.10 and -0.22dB/MHz for RBCs suspended in isotonic phosphate buffered saline and solution with 70% plasma concentrations, respectively. For the midband fit and Y intercept, the mean values were increased by 9.1 and 46.4dB, respectively. These results indicated that the spectrum analysis technique is useful for monitoring RBC aggregation and can be potentially developed for assessing aggregation in clinical applications.

Design and fabrication of relaxor-ferroelectric single crystal PIN–PMN–PT/epoxy 2–2 composite based array transducer

In order to improve the applicability of Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) single crystal and make full use of relaxor ferroelectric single crystal based array transducer on high frequency, PIN–PMN–PT/epoxy 2–2 composite with the width-to-height ratio G of 0.5 in single crystal beam was prepared by a dice-and-fill method. Excellent properties for the fabrication of ultrasonic transducer has been achieved, where ultrahigh thickness electromechanical coupling coefficient (kt), ultrahigh effective electromechanical coupling coefficient (keff), relatively high piezoelectric constant (d33), and low acoustic impedance (Z) are 86%, 88.4%, 1200pC/N and 21 MRayls, respectively. Based on the improved flexibility of as-prepared PIN–PMN–PT/epoxy 2–2 composite, a 12-element array transducer with the center frequency of 10MHz has been designed, fabricated and characterized. The high frequency array transducer has achieved an ultra-broad bandwidth (@-6dB) of 127.7%. It is worth mentioning that all 12 elements in the array work very well. The array transducer based on PIN–PMN–PT/epoxy composite exhibits excellent uniformity, confirming that the relaxor ferroelectric single crystal based 2–2 composite can be used to develop high-performance array transducer in the field of high frequency application.

Multi-particle trapping and manipulation by a high-frequency array transducer.

We report the multiple micro-particle trapping and manipulation by a single-beam acoustic tweezer using a high-frequency array transducer. A single acoustic beam generated by a 30 MHz ultrasonic linear array transducer can entrap and transport multiple micro-particles located at the main lobe and the grating lobes. The distance between trapped particles can be adjusted by changing the transmit arrangement of array-based acoustic tweezers and subsequently the location of grating lobes. The experiment results showed that the proposed method can trap and manipulate multiple particles within a range of hundreds of micrometers. Due to its simplicity and low acoustic power, which is critical to protect cells from any thermal and mechanical damages, the technique may be used for transportation of cells in cell biology, biosensors, and tissue engineering.

Micro-ultrasound for preclinical imaging

Over the past decade, non-invasive preclinical imaging has emerged as an important tool to facilitate biomedical discovery. Not only have the markets for these tools accelerated, but the numbers of peer-reviewed papers in which imaging end points and biomarkers have been used have grown dramatically. High frequency 'micro-ultrasound' has steadily evolved in the post-genomic era as a rapid, comparatively inexpensive imaging tool for studying normal development and models of human disease in small animals. One of the fundamental barriers to this development was the technological hurdle associated with high-frequency array transducers. Recently, new approaches have enabled the upper limits of linear and phased arrays to be pushed from about 20 to over 50 MHz enabling a broad range of new applications. The innovations leading to the new transducer technology and scanner architecture are reviewed. Applications of preclinical micro-ultrasound are explored for developmental biology, cancer, and cardiovascular disease. With respect to the future, the latest developments in high-frequency ultrasound imaging are described.

Acoustic microscopy: latest developments and applications

Scanning acoustic microscopy (SAM) introduced a new form of contrast that is based on the mechanical properties of what is imaged. For bone, cartilage, tendon and cardiovascular tissues, micro-acoustic properties provided important information on biomechanical properties. Biomechanics of these tissues are especially important for assessing the pathophysiology. Cells are considered to consist of visco-elastic materials and SAM is capable of imaging information on viscosity by attenuation and information on elasticity by sound speed. Recent developments such as ultrasound impedance microscopy, 3D ultrasound microscopy and high frequency array transducer may realize a clinically applicable SAM in the near future.

TU-D-210A-04: Development of High Frequency Ultrasound Transducers for Medical Imaging

Ultrasonic imaging is one of the most important and still growing diagnostic tools today. Ultrasonic imaging is more appealing as a clinical imaging modality compared to such imaging modalities as magnetic resonance imaging(MRI), nuclear imaging, and x‐ray computed tomography(CT) in that it is more cost‐effective, non‐invasive, capable of real‐time operation, and portable while providing images of comparable quality and resolution. State‐of‐the‐art ultrasonic scanners offer real‐time gray scale images of anatomical detail with millimeter spatial resolution superimposed on which is a map of Doppler blood flow information displayed in color. Clinical applications of these devices are still expanding and the operating frequencies of these devices seem to inch higher and higher. High frequency (HF) ultrasound (>20 MHz) yields improved spatial resolution at the expense of a shallower depth of penetration. There are a number of clinical problems that may benefit from high frequency ultrasonic imaging. Intravascular imaging with probes mounted on catheter tips at frequencies higher than 20 MHz with the highest frequency being 60 MHz has been used to characterize atherosclerotic plaque and to guide stent placement and angioplastic procedures. The medical efficacy of ultrasonic imaging of anterior segments of the eye at frequencies higher than 50 MHz in diagnosing glaucoma and ocular tumors and in assisting refractive surgery has been demonstrated. Applications in dermatology for defining tumor involvement and for monitoring treatment, in vascular surgery for characterizing atherosclerotic plaques, and in small animal imaging are also being investigated. To further expand the role of HF ultrasound, probe performance must be improved since current HF ultrasound imaging devices, dubbed “ultrasound biomicroscopes” or “UBMs” by a number of investigators use mechanically scanned single element transducers. Compared to electronic scanning with linear arrays, mechanical scanning of a single element transducer has three major drawbacks: poorer resolution, slower frame rate, and motion of the probe. Single element transducers can only produce beams with a fixed focus which means the spatial resolution of the device is best only within the depth of focus, i.e., in a very tight zone and degrades rapidly beyond the focal point. Mechanical motion of the transducer limits the frame rate, is unreliable, and may cause discomfort and, at worst, hazard to the patient. My presentation will introduce several HF ultrasound transducers currently being developed by the NIH Transducer Resource Center at the University of California. Emphasis will be placed on the construction and implementation of high frequency array transducers. Learning Objectives: 1. Familiarize audience with high frequency ultrasound transducers and their applications.

Photoacoustic imaging of the microvasculature with a high-frequency ultrasound array transducer

Visualization of microvascular networks could provide new information about function and disease. We demonstrate the capabilities of a 30-MHz ultrasound array system for photoacoustic microscopy of small (< or = 300 microm) vessels in a rat. 3D images obtained by translating the array in the elevation direction are compared with photographs of excised skin. The system is shown to have 100-microm lateral resolution, 25-microm axial resolution, and 3-mm imaging depth. To our knowledge this is the first report on photoacoustic microscopy of the microvasculature with a high-frequency array transducer. It is anticipated that the system can be used for studying and diagnosing a number of diseases including cancer, atherosclerosis, dermatological disorders, and peripheral microvascular complications in diabetes.

Standardisierte sonographische Diagnostik (13 MHz) der Sehne des M. tibialis posterior - Normalbefunde an Fußgesunden

Tibialis posterior dysfunction is often diagnosed at a very late stage. However, the early diagnosis of tibialis posterior dysfunction is crucial for therapeutic aspects and especially for the operative prognosis. The morphological correlate of the tibialis posterior dysfunction consists of degenerative changes and thickening of the posterior tibial tendon [2, 9, 11]. By means of a high frequency linear array transducer, a standardized technique of examination as well as reference values of cross-sections of posterior tibial tendon are introduced. Investigating 51 healthy subjects (102 feet) without any foot deformities, standardized planes were defined by use of a 13 MHz linear array transducer in order to delineate the posterior tibial tendon. At exact anatomic landmarks, tendon diameters were measured using two longitudinal sections proximal and distal to the medial malleolus. Likewise, two diameters and the resulting roughly calculated cross-section of the tendon were determined, using two transverse sections at the level of the subtalar joint facet and the medial malleolus. The findings obtained by ultrasound in four female patients with the diagnosis of a tibialis posterior dysfunction confirmed during surgery are compared to the aforementioned 51 healthy subjects. A healthy tendon appears homogeneous and echo-rich in orthogonal ultrasound and displays average areas of 18.4 sq.mm. (SD 5.9 sq.mm.) at the subtalar joint facet level and 19.2 sq.mm (SD 4.8 sq.mm.) at medial malleolus level in transverse sections. The corresponding areas obtained in patients with tibialis posterior dysfunction were clearly enlarged than in healthy subjects. In 85% of all feet examined at the level of the medial malleolus, the tendon is surrounded by a hypoechoic halo which has a size smaller than two times the cross-section of the flexor digitorum longus tendon. Reference values of tendon thickness and of intratendinous echo-structures at reproducible landmarks facilitate delimitations from pathological tendon alterations. The exact delineation of intratendineal echoes by high frequency array transducers and standardized examination techniques that measure tendons size is a prerequisite to enable an early assessment and registration of degenerative alterations and thickening of the posterior tibial tendon.

Electromechanical properties of (Pb, Ln) (Ti, Mn) O3 ceramics (Ln = rare earths)

Electromechanical properties of PbTiO3 ceramics modified by the partial substitution of rare earths for Pb, particularly, (Pb, Ln) (Ti, Mn) O3 (Ln = La, Pr, Nd, Sm, or Gd) ceramics, are examined. It is found that the Sm substitution for Pb dramatically increases the ratio of electromechanical coupling factor of the thickness dilatational mode to that of the planar extensional mode. Extensive studies of the resonant properties of rectangular strip resonators make it clear that the Sm-substituted PbTiO3 ceramics are very useful for high-frequency array transducer applications. It is also shown that the pulse responses of array transducers can be improved using Sm substituted PbTiO3 ceramics instead of the other PbTiO3 ceramics studied here.

Design of ultrasound transducers using new piezoelectric ceramic materials

Three parameters α, DR and D n ( A), i.e. the efficiency, the dynamic range and the normalized axial resolution are used to evaluate the performance of ultrasound transducers for diagnostic imaging equipment. These parameters determine optimum conditions for use in the design of high frequency array transducers. A newly-developed piezoelectric ceramic material composed of (Pb,Sr)(Ti,Zr)O 3 or Pb[(Mg 1 3 , Nb 2 3 ), Ti,Zr]O 3 which satisfies the above-mentioned conditions was used to fabricate a co-phase array ultrasound transducer. The transducer showed good electrical and acoustic performance in accordance with theoretical considerations.