Wo-Hsing Chen

Design of focused single element (50-100 MHz) transducers using lithium niobate

This paper discusses two fabrication procedures used to build LiNbO/sub 3/ single element ultrasonic transducers with center frequencies in the 50-100 MHz range. Transducers of varying dimensions were built according to an f-number range of 2.5-3.0. A quarter wavelength silver epoxy matching layer (Z/sub a/=7.3 MRayls), and a silver epoxy backing (Z/sub a/=5.9 MRayls), were used in all designs. The desired focal depths were achieved by either casting an acoustic lens on the transducer face or press-focusing the piezoelectric into a spherical curvature. The lens material EPO-TEK 301 (Z/sub a/=3.1 MRayls) was modeled as a second matching layer. Parylene (Z/sub a/=2.6 MRayls) was used as the second matching layer in the press-focused transducer design. For designs that required electrical impedance matching, a low impedance transmission line coaxial cable was used. All transducers were tested in a pulse-echo insertion loss arrangement, whereby the center frequency, bandwidth, insertion loss, and focal depth were measured. Numerous transducers were fabricated with center frequencies in the 50 to 100 MHz range. The measured -6 dB bandwidths and two-way insertion loss values ranged from 50% to 70% and 12.5 dB to 23.0 dB, respectively. Both fabrication procedures proved successful in producing very sensitive, high frequency single element ultrasonic imaging transducers. The press-focused devices displayed significantly lower insertion loss levels than lensed devices. The Parylene matching layer provided a better acoustic match to the load medium water (Z/sub a/=1.5 MRayls) and a reduced attenuation compared to the lens, which may account for the discrepancy insertion in loss levels. Finally, transmission line tuning demonstrated a 3 dB reduction in insertion loss and a 6% increase in device bandwidth.

The detection and exclusion of the prostate Neuro-Vascular Bundle (NVB) in automated HIFU treatment planning using a pulsed-wave doppler ultrasound system

Men with prostate cancer are likely to develop impotence after prostate cancer therapy if the treatment damages the neuro‐vascular bundles (NVB). The NVB are generally located at the periphery of the prostate gland. To preserve the NVB, a Doppler system is used to detect and localize the associated blood vessels. This information is used during the therapy planning procedure to avoid treatment surrounding the blood vessel areas. The Sonablate®500 (Focus Surgery, Inc.) image‐guided HIFU device is enhanced with a pulse‐wave multi‐gate Doppler system that uses the current imaging transducer and mechanical scanner to acquire Doppler data. Doppler detection is executed after the regular B‐mode images are acquired from the base to the apex of the prostate using parallel sector scans. The results are stored and rendered in 3‐D display, registered with additional models generated for the capsule, urethra, and rectal wall, and the B‐mode data and treatment plan itself. The display of the blood flow can be in 2‐D c...

Fabrication of high frequency (25-75 MHz) single element ultrasonic transducers

The design and fabrication of high frequency single element ultrasonic transducers present a multitude of challenges for the transducer engineer, from size constraints to electrical impedance matching. This paper discusses the trade-offs involved in procedures used to fabricate lithium niobate (LiNbO/sub 3/) and lead titanate (PbTiO/sub 3/) transducers in the 25 MHz to 75 MHz range. Transducers of varying dimensions were built according to an f-number range of 2-3.5. Desired focal depths were achieved with use of either an acoustic lens or a spherically focused piezoceramic. Silver epoxy backing with an acoustic impedance of approximately 5.9 MRayls was used in all designs. All transducers were designed around a 50/spl Omega/ send and receive circuit. Electrical tuning of the transducer to the receive circuitry was achieved by using an RF transformer and/or a length of coaxial cable. All transducers were tested in a pulse-echo arrangement using a Panametrics 5900PR pulser, a Wavetek function generator and a LeCroy digital oscilloscope. The bandwidth, insertion loss, and depth of focus were measured. Numerous transducers were fabricated with -6 dB bandwidths ranging from 40% to 74%, and two-way insertion loss values ranging from -14 dB to -28 dB.

Development of sector scanning ultrasonic backscatter microscope

The Ultrasound Backscatter Microscope (UBM) is a noninvasive high frequency imaging tool used frequently for imaging the eye, skin and blood vessels. Currently, most UBM systems employ a linear motor control to obtain a transverse scan. This paper reports the implementation of a UBM that performs sector scan with a servo-controlled motor to manipulate a single element transducer. The advantage of applying a sector scan versus transverse linear scan is that the transducer needs to travel less distance to acquire an image of the same area. The transducer sector movement is achieved by a brief sweep that needs only a small open area for scanning. The servomotors sweep angle has a small are of 5 degrees and provides enough width for an image. Because of the small angle sweeping, the image can be displayed in a linear format as the image in transverse scan without further calculation. The UBM system can be operated within the 50-100 MHz frequency range. Images have been acquired with this approach on excised human eye specimen. The quality of the image compares favorably with that obtained with the conventional UBM. The results indicate that the sector scan is an alternative method for UBM scanning. Future work includes the development of a hand held probe that houses a small transducer and servomotor capable of sector scanning.

High-frequency transducers for medical ultrasonic imaging

A wide variety of fabrication techniques and materials produce ultrasound transducers with very different performance characteristics. High frequency (50 MHz), focused single element transducers using lead zirconate titanate (PZT) fiber composite, lead titanate (PbTiO3) ceramic, polyvinylidene fluoride (PVDF) polymer and lithium niobate (LiNbO3) single crystal are compared in design and performance. The transducers were all constructed with a 3 mm aperture and f- number of 2 - 3. Design considerations discussed include optimization of designs using different lens, backing and matching materials for acoustic matching and the use of several electrical tuning techniques to match the transducers to the 50(Omega) circuitry. Transducers were tested for insertion loss and -6dB bandwidth using a quartz flat- plate target. Insertion loss measurements between transducers were -20dB to -50dB with bandwidths in the range of 50 - 120%. Through the use of an ultrasound backscatter microscope (UBM), the transducer were compared using in vitro images of the human eye. Images of a wire phantom were also made for comparison of lateral and axial resolution of each device.

Real‐Time Tissue Change Monitoring on the Sonablate® 500 during High Intensity Focused Ultrasound (HIFU) Treatment of Prostate Cancer

Sonablate® 500 (SB‐500) HIFU devices have been successfully used to treat prostate cancer non‐invasively. In addition, Visually Directed HIFU with the SB‐500 has demonstrated higher efficacy. Visually Directed HIFU works by displaying hyperechoic changes on the B‐mode ultrasound images. However, small changes in the grey‐scale images are not detectable by Visually Directed HIFU. To detect all tissue changes reliably, the SB‐500 was enhanced with quantitative, real‐time Tissue Change Monitoring (TCM) software. TCM uses pulse‐echo ultrasound backscattered RF signals in 2D to estimate changes in the tissue properties caused by HIFU. The RF signal energy difference is calculated in selected frequency bands (pre and post HIFU) for each treatment site. The results are overlaid on the real‐time ultrasound image in green, yellow and orange to represent low, medium and high degree of change in backscattered energy levels. The color mapping scheme was derived on measured temperature and backscattered RF signals fro...

Design of a real time digital beamformer for a 50MHz annular array ultrasound transducer

A Field Programmable Gate Array (FPGA) based real time beamformer was developed for a six-ring annular array ultrasound transducer. Six analog to digital converters (AD9054, Analog Devices Inc.) were used to digitized the echoes at 200MHz. A Xilinx Virtex E FPGA chip which works at a 200MHz clock was used to delay the digitized echoes for beamforming. The delay for each channel was accomplished in two steps. A programmable FIFO was used for the delays of integer multiples of the clock period, a 4-tap Fractional Delay (FD) FIR filter was used for the delays less than one clock period. A high speed Cypress FIFO was used to transfer the summed beam to a DSP microprocessor (ADSP21065L). The DSP microprocessor completes envelope detection, imaging processing and transfers the image data to a computer for display through a PCI bus I/O card (PCI6534, National Instruments). The source codes for FPGA were written in VHDL language and schematic capture. A lookup table method based multiplier was designed to improve the speed of algorithm. The whole beamformer was designed in a pipeline structure; it is capable of working at 240MHz clock frequency after implemented in ISE Foundation 4.2i (Xilinx Inc). Using a Gaussian modulated sinusoidal pulse, with a 50MHz center frequency and a 50% bandwidth, the Matlab simulation study shows that the FD filter gave a maximal error of 11.2% in amplitude from the ideal waveform, and a 0.3% maximum mean square error when the required delay was 0.2 of the clock period.

Accurate corneal thickness measurement using ultrasound

In this study, we developed a faster and more accurate method for the detection of corneal thickness using an ultrasonic pulse-echo method. After a ultrasound pulse was transmitted to the cornea, the echo was received in radio frequency. The corneal thickness was calculated from the time interval between the two echoes reflected at front and rear interfaces of the cornea. The time interval between two echoes was obtained by detecting the peaks of the digitized echoes. A phase-adjusting method, which is superior to the often-used interpolation approach, was used to improve the time resolution. Based on this method, a MCS-8031 microprocessor based pachymeter for measuring corneal thickness was developed. The center frequency of the ultrasonic transducer works was 30MHz and the echoes were digitized at a 180MHz sampling frequency. The device showed an accuracy of 1 micrometers for 10 repeated measurements.

Optimization of pulse transmission in a high-frequency ultrasound imaging system

The effect of cables that connect the transducer and an imaging system can no longer be ignored when the ultrasound frequency is higher than 20MHz. The length of transmission line between transducer and electronic system critically affects the amplitude and bandwidth of pulse transmission. We report an evaluation method for assessing the effects of the cables and results obtained from this method. A time domain PSpice simulation model including high frequency impulse generator and receiver, a transducer simulated with a Mason model and a lossy transmission line has been developed. The model allows the simulation of the echo pulse shape at the input of the receiver. The investigation is carried out with a transducer of center frequency 45MHz and 48% bandwidth. The effect of cable length on the amplitude, space pulse length and bandwidth of received pulse is studied. Moreover, the effect of impedance tuning is also studied between the system and transducer when the cable is not used. Both experimental and simulation results show that the system performance is optimized by a cable length of 60cm to achieve higher sensitivity. If a suitable impedance matching method is applied between the system and the transducer without the cable connected, the system performance in sensitivity would be better than that with a cable connected. However, the results also show that achieving a highest sensitivity would increase pulse length. The results show that optimizing high frequency pulse transmission with high sensitivity and acceptable pulse shape, the pulser and receiver should be placed as close as possible to transducer with suitable impedance matching. The results also suggest the shorted pulse length could occur at a certain cable length but the result is also dependent on the original pulse shape.

Fabrication of high-frequency single-element ultrasonic transducers using lithium niobate

The design and fabrication of high frequency single element ultrasonic transducers present a multitude of challenges for the transducer engineer, from size constraints to electrical impedance matching. This paper discusses the trade-offs involved in a procedure used to fabricate transducers with center frequencies in the 25 MHz - 100 MHz range using 36 degree rotated Y-cut lithium niobate (LiNbO3) as the active element. Transducers of varying dimensions were built according to an f-number range of 2 - 3.5. A (lambda) /4 silver epoxy matching layer with an acoustic impedance of 7.3 Mrayls was used. Desired focal depths were achieved with use of an acoustic lens. Silver epoxy backing with an acoustic impedance of approximately 5.9 Mrayls was also used in all designs. All transducers were designed around a 50(Omega) send and receive circuit. Electrical tuning of the transducer to the receive circuitry was achieved by using an RF transformer. All transducers were tested in a pulse-echo arrangement using a Panametrics 5900PR pulser, a Wavetek function generator and a LeCroy digital oscilloscope. The bandwidth, insertion loss, and depth of focus were measured. Several transducers were fabricated with -6dB bandwidths ranging from 62% to 74%, and two-way insertion loss values ranging from -14dB to -22dB.