Ruibin Liu

20 MHz/40 MHz dual element transducers for high frequency harmonic imaging

Concentric annular type dual element transducers for second harmonic imaging at 20 MHz / 40 MHz were designed and fabricated to improve spatial resolution and depth of penetration for ophthalmic imaging applications. The outer ring element was designed to transmit the 20 MHz signal and the inner circular element was designed to receive the 40 MHz second harmonic signal. Lithium niobate (LiNbO3), with its low dielectric constant, was used as the piezoelectric material to achieve good electrical impedance matching. Double matching layers and conductive backing were used and optimized by KLM modeling to achieve high sensitivity and wide bandwidth for harmonic imaging and superior time-domain characteristics. Prototype transducers were fabricated and evaluated quantitatively and clinically. The average measured center frequency for the transmit ring element was 21 MHz and the one-way -3 dB bandwidth was greater than 50%. The 40 MHz receive element functioned at 31 MHz center frequency with acceptable bandwidth to receive attenuated and frequency downshifted harmonic signal. The lateral beam profile for the 20 MHz ring elements at the focus matched the Field II simulated results well, and the effect of outer ring diameter was also examined. Images of a posterior segment of an excised pig eye and a choroidal nevus of human eye were obtained both for single element and dual element transducers and compared to demonstrate the advantages of dual element harmonic imaging.

The acoustic lens design and in vivo use of a multifunctional catheter combining intracardiac ultrasound imaging and electrophysiology sensing

A multifunctional 9F intracardiac imaging and electrophysiology mapping catheter was developed and tested to help guide diagnostic and therapeutic intracardiac electrophysiology (EP) procedures. The catheter tip includes a 7.25-MHz, 64-element, side-looking phased array for high resolution sector scanning. Multiple electrophysiology mapping sensors were mounted as ring electrodes near the array for electrocardiographic synchronization of ultrasound images. The catheter array elevation beam performance in particular was investigated. An acoustic lens for the distal tip array designed with a round cross section can produce an acceptable elevation beam shape; however, the velocity of sound in the lens material should be approximately 155 m/s slower than in tissue for the best beam shape and wide bandwidth performance. To help establish the catheters unique ability for integration with electrophysiology interventional procedures, it was used in vivo in a porcine animal model, and demonstrated both useful intracardiac echocardiographic visualization and simultaneous 3- D positional information using integrated electroanatomical mapping techniques. The catheter also performed well in high frame rate imaging, color flow imaging, and strain rate imaging of atrial and ventricular structures.

Design, Fabrication, and Characterization of a Bifrequency Colinear Array

Ultrasound imaging with high resolution and large penetration depth has been increasingly adopted in medical diagnosis, surgery guidance, and treatment assessment. Conventional ultrasound works at a particular frequency, with a - 6-dB fractional bandwidth of ~ 70% , limiting the imaging resolution or depth of field. In this paper, a bifrequency colinear array with resonant frequencies of 8 and 20 MHz was investigated to meet the requirements of resolution and penetration depth for a broad range of ultrasound imaging applications. Specifically, a 32-element bifrequency colinear array was designed and fabricated, followed by element characterization and real-time sectorial scan (S-scan) phantom imaging using a Verasonics system. The bifrequency colinear array was tested in four different modes by switching between low and high frequencies on transmit and receive. The four modes included the following: 1) transmit low, receive low; 2) transmit low, receive high; 3) transmit high, receive low; and 4) transmit high, receive high. After testing, the axial and lateral resolutions of all modes were calculated and compared. The results of this study suggest that bifrequency colinear arrays are potential aids for wideband fundamental imaging and harmonic/subharmonic imaging.

Characterization of a 40-MHz focused transducer with a fiber grating laser hydrophone

A novel fiber-optic hydrophone based on a dual-polarization, short-cavity fiber grating laser as the sensing element is described. Wet chemical etching was used to fabricate a thinned fiber sensor to extend its frequency response as well as spatial resolution. The lateral beam profile at the focal plane of a 40-MHz lens-focused lithium niobate (LiNbO3) transducer was measured with the fiber sensor, and a tomographic technique was used to compute the transducer profile, which is compared with that obtained by a PVDF hydrophone. The fiber hydrophone has a sensitivity of approximately -259 dB re 1 V/muPa up to 40 MHz, which is higher than that of a commercial PVDF hydrophone. Moreover, it is capable of accurately characterizing the beam generated by high-frequency transducer.

10F-4 Self-Focused 1-3 Composite LiNbO3 Single Element Transducers for High Frequency HIFU Applications

In this paper it is shown that LiNbO3 single crystal may be used as a piezoelectric material for high frequency high intensity focused ultrasound (HIFU) applications for its high Curie temperature and low dielectric constant and superior mechanical properties. Simulation results show that LiNbO3 with a 1-3 composite structure is suitable to make large aperture (diameter 22-24 mm) and high frequency (> 10 MHz) single element transducer with desired impedance and required sustainable driving voltage for the expected acoustic intensity in the focal zone. Prototype transducers with the diameter of 23 mm and a surface curvature designed for f#/l were designed and fabricated. The results are in good agreement with KLM model calculation. The measurement results show that center frequency is 10.5 MHz with the fractional bandwidth larger than 60%. The -6 dB lateral and the axial beam widths were measured by a needle hydrophone and they are 160 mum and 98 mum respectively, which are also in good agreement with theory (147 mum and 83 mum).

Design, fabrication and characterization of a bi-frequency co-linear array (7.5MHz/15MHz)

Ultrasound imaging with high resolution and large field of depth is important in disease diagnosis, surgery guidance and post-surgery assessment. Conventional ultrasound imaging arrays work at a particular frequency, with -6dB fractional bandwidth of <; 100%, limiting the resolution or field of depth in many ultrasound imaging cases. This paper presented design of a 7.5 MHz / 15 MHz bi-frequency co-linear array prototype with a wide bandwidth of 5MHz-20 MHz, which can be significant in a broad range of biomedical ultrasound imaging applications. To demonstrate the concept, a 32-element 1-D linear sub-array was fabricated, followed by element characterization and beamforming tests using a Verasonics system. Beam steering at +/- 40 degree was achieved without obvious side lobes. The initial results suggest great potential of this bi-frequency co-linear array for medical imaging with high resolution and large field of depth.

Development of a C-Scan phased array ultrasonic imaging system using a 64-element 35MHz transducer

Phased array imaging systems provide the features of electronic beam steering and dynamic depth focusing that cannot be obtained with conventional linear array systems. This paper presents a system design of a digital ultrasonic imaging system, which is capable of handling a 64-element 35MHz center frequency phased array transducer. The system consists of 5 parts: an analog front-end, a data digitizer, a DSP based beamformer, a computer controlled motorized linear stage, and a computer for post image processing and visualization. Using a motorized linear stage, C-scan images, parallel to the surface of scanned objects may be generated. This digital ultrasonic imaging system in combination a 35 MHz phased array appears to be a promising tool for NDT applications with high spatial resolution. It may also serve as an excellent research platform for high frequency phased array design and testing as well as ultrasonic array signal algorithm developing using systems raw RF data acquisition function.

DESIGN AND FABRICATION OF HIGH-INTENSITY FOCUSED ULTRASOUND PHASED ARRAY FOR LIVER TUMOR THERAPY

Noninvasive surgery of the liver tumors has been carried out by using the high-intensity focused ultrasound (HIFU). However, the liver tumor can be moved by the human respirations and heartbeats, which may cause the ablation and damage of normal tissues during the sonications of HIFU. The purpose of this study was to design and fabricate a cylindrical HIFU phased array transducer for treating the moving liver tumor efficiently. The total number of the element was 512 but only 256 channels were required since the elements along the elevation direction were connected in pairs with respect to the central line of the array. Field II software was used to simulate the acoustic field, and a formula for predicting the spatial averaged intensity at focus was developed based on the practical factors. The results of the simulations showed that the cylindrical HIFU phased array in water had a dynamic focusing range from 145 to 175 mm in the depth direction and a steering range from -15 to 15 mm in azimuthal direction with respect to the center of the array. After the dissipation of cables and the attenuation of various media, the designed array could still generate the intensity at focus up to 1095 W/cm2 when the input electrical power was approximately 410 W. The prototype of the array was fabricated and the preliminary test was completed. The testing results showed that each element of the array prototype can work well.

A Bi-Frequency Co-Linear Array Transducer for Biomedical Ultrasound Imaging

Ultrasound imaging with high resolution and large field of depth has been increasingly adopted in medical diagnosis, surgery guidance and treatment assessment because of its relatively low cost, non-invasive and capability of real-time imaging. There is always a tradeoff between the resolution and depth of field in ultrasound imaging. Conventional ultrasound works at a particular frequency, with −6 dB fractional bandwidth of < 100%, limiting the resolution or field of depth in many ultrasound imaging cases.In this paper, a bi-frequency co-linear array covering a frequency range of 5 MHz-20 MHz was investigated to meet the requirements of resolution and depth of field for a broad range of ultrasound imaging applications. As a demonstration, a 31-element bi-frequency co-linear array was designed and fabricated, followed by element characterization and real time sectorial scan (S-scan) phantom imaging using a Verasonics system.Copyright

Fabrication and Characterization of High Frequency Phased Arrays for NDE Imaging

PMN-PT single crystal 1-3 composite high frequency phased arrays with center frequency of 35 MHz were fabricated and characterized for silicon carbide (SiC) NDE imaging applications. The 35 MHz 64-element array was successfully prototyped using PMN-PT single crystal and PC-MUT technology. The broad bandwidth > 90% and high sensitivity (echo amplitude > 500 mV from the impulse response with 0 gain) was observed with reasonably high uniformity. These high frequency phased arrays are promising for ceramic NDE imaging.