Charles D. Emery

Update on 2-D array transducers for medical ultrasound

l 1/2 -D and 2-D arrays offer a myriad of new imaging modalities and benefits when compared to the linear array. However, with added benefits come many problems and challenges and l 1/2 -D and 2-D arrays are no exception. The authors give possible solutions to a number of these challenges. The increase in transducer channels needed in a 1 1/2 -D and 2-D array can be reduced using a sparse periodic or sparse random array. The complexity of the fabrication is overcome using a multilayer flexible connector designed and fabricated using microelectronic techniques. The low SNR of 1 1/2 -D and 2-D arrays can be circumvented with the application of multi-layer ceramic elements to optimize the SNR given a specific transmit and receive configuration. In addition, optoelectronic transmitters allow for the reduction in size and increase in flexibility of the transducer cable because of the use of fiber optics. With the reduction in the number of channels, improvement in transducer fabrication, and increase in transducer SNR, l 1/2 -D and 2-D arrays will be accepted as viable replacements for the linear arrays of today.

Hybrid multi/single layer array transducers for increased signal-to-noise ratio

In medical ultrasound imaging, two-dimensional (2-D) array transducers are necessary to implement dynamic focusing in two dimensions, phase correction in two dimensions and high speed volumetric imaging. However, the small size of a 2-D array element results in a small clamped capacitance and a large electrical impedance, which decreases the transducer signal-to-noise ratio (SNR). We have previously shown that SNR is improved using transducers made from multi-layer PZT, due to their lower electrical impedance. In this work, we hypothesize that SNR is further increased using a hybrid array configuration: in the transmit mode, a 10 /spl Omega/ electronic transmitter excites a 10 /spl Omega/ multi-layer array element; in the receive mode, a single layer element drives a high impedance preamplifier located in the transducer handle. The preamplifier drives the coaxial cable connected to the ultrasound scanner. For comparison, the following control configuration was used: in the transmit mode, a 50 /spl Omega/ source excites a single layer element, and in the receive mode, a single layer element drives a coaxial cable load. For a 5/spl times/102 hybrid array operating at 7.5 MHz, maximum transmit output power was obtained with 9 PZT layers according to the KLM transmission line model. In this case, the simulated pulse-echo SNR was improved by 23.7 dB for the hybrid configuration compared to the control. With such dramatic improvement in pulse-echo SNR, low voltage transmitters can be used. These can be fabricated on integrated circuits and incorporated into the transducer handle.

Ultrasonic imaging using a 5-MHz multilayer/single-layer hybrid array for increased signal-to-noise ratio

Conventional diagnostic ultrasound scanners are bulky and require significant amounts of electrical power during operation. Reducing the size, weight, and consumption of electrical power is made easier through the use of highly integrated compact transmit and receive electronics that may be incorporated in the transducer handle. This necessitates the use of low voltage transmitters and low power receive preamplifiers. Conventional scanners typically use approximately 100-V pulses during transmit; therefore, decreasing the transmit voltage to 15 V decreases the transmit sensitivity. Conventional receive electronics that are located at the scanner degrade the received signal-to-noise ratio (SNR) because the array element cannot efficiently drive the coaxial cable. Transmit sensitivity and received SNR can be radically improved using a multilayer/single-layer hybrid array making integration of electronics into the transducer handle more feasible. In this paper, we discuss the design, fabrication, and testing of a 5-MHz hybrid linear array. The hybrid array included 16 multilayer transmit elements (10 /spl Omega/ impedance) and 24 single-layer receive elements at a half wavelength element pitch. Low voltage transmitters with an output resistance of 7 /spl Omega/ and high impedance JFET preamplifiers using 15 V for biasing were located adjacent to the hybrid array in the transducer handle. The transmit sensitivity and received SNR of the hybrid array were compared with a conventional array using 50-/spl Omega/ transmitters and receive preamplifiers at the scanner. The transmit sensitivity improved by 12.8 dB, and the received SNR improved by 7.8 dB, yielding an overall improvement of 20.6 dB, which compared well with predictions from the KLM model. Images of phantoms and in vivo images of the kidney obtained with the Siemens Model 1200 phased array system showed the increased SNR using the hybrid array. Estimates of penetration in tissue mimicking phantoms (/spl alpha/=0.5 dB/(cm MHz)) improved by 7 cm compared with the control.

Improved Signal-to-Noise Ratio in Hybrid 2-D Arrays: Experimental Confirmation

2-D array transducers have shown significant promise for medical ultrasound over conventional linear arrays, at the cost of increasing the number of channels, difficulty of fabrication and array element impedance. The increase in element impedance reduces the power coupled to a 2-D array element from a conventional 50 Ω source in transmit mode. If the array is sparse, which is typical of 2-D arrays, then the net power coupled into the front acoustic load is reduced when compared to a fully sampled aperture. Furthermore, the received signal-to-noise ratio (SNR), when measured through a nonideal amplifier, is degraded because the high impedance 2-D array transducer element cannot efficiently drive the coaxial cable. The reduction in transmit sensitivity and received SNR can be circumvented with the application of multilayer piezoelectric elements. The improvement in transmit occurs because the transducer impedance is better matched to the impedance of the source. In receive, multilayer elements allow more of the open circuit received voltage to fall across the input of the high impedance preamplifier. In this case, the same number of layers are used in transmit and receive. Recently, it has been suggested that separate optimization of the transmit channel and receive channel (a hybrid array) would further improve the pulse-echo SNR. In this paper, we fabricated and tested a hybrid array operating at 1 MHz using a multilayer transmit element and single layer receive element. A 7 Ω transmitter and high impedance preamplifier were placed adjacent to the transmit and receive elements within the transducer assembly. The hybrid pulse-echo SNR improved by 26.4 dB over the conventional array. The experimental result showed good agreement with the KLM model. Furthermore, KLM simulations showed that as the operating frequency of the array increases, the overall improvement over the conventional array increases. For example, a 1.5-D array operating at 2 MHz had an improvement of 30 dB whereas a 7.5 MHz 1.5-D array showed an increase of approximately 38 dB. The separate optimization of the transmit and receive channel for 2-D arrays showed even greater improvement than for 1.5-D arrays. For example, a 2 MHz 2-D array had an improvement of over 44 dB.

2-D array transducers for medical ultrasound at Duke University: 1996

l 1/2 -D and 2-D arrays offer a myriad of new imaging modalities and benefits when compared to the linear array. However, with added benefits come many problems and challenges. The increase in transducer channels needed in a l 1/2 -D and 2-D array can be reduced using a sparse periodic or sparse random array. The complexity of the fabrication is overcome using a multi-layer flexible connector designed and fabricated using microelectronic techniques. The low SNR of 1 1/2 -D and 2-D arrays can be increased with the application of multi-layer ceramic elements to optimize the SNR given a specific transmit and receive configuration. We are also looking at PZT/polymer composites for SNR improvement as well as better acoustic matching to tissue. In addition, opto-electronic transmitters allow for the reductions in size and increase in flexibility of the transducer cable because of the use of fiber optics.

Ultrasonic Imaging Using Optoelectronic Transmitters

Conventional ultrasound scanners utilize electronic transmitters and receivers at the scanner with a separate coaxial cable connected to each transducer element in the handle. The number of transducer elements determines the size and weight of the transducer cable assembly that connects the imaging array to the scanner. 2-D arrays that allow new imaging modalities to be introduced significantly increase the channel count making the transducer cable assembly more difficult to handle. Therefore, reducing the size and increasing the flexibility of the transducer cable assembly is a concern. Fiber optics can be used to transmit signals optically and has distinct advantages over standard coaxial cable to increase flexibility and decrease the weight of the transducer cable for large channel numbers. The use of fiber optics to connect the array and the scanner entails the use of optoelectronics such as detectors and laser diodes to send and receive signals. In transmit, optoelectronics would have to be designed to produce high-voltage wide-bandwidth pulses across the transducer element. In this paper, we describe a 48 channel ultrasound system having 16 optoelectronic transmitters and 32 conventional electronic receivers. We investigated both silicon avalanche photodiodes (APDs) and GaAs lateral photoconductive semiconductor switches (PCSSs) for producing the transmit pulses. A Siemens SI-1200 scanner and a 2.25 MHz linear array were used to compare the optoelectronic system to a conventional electronic transmit system. Transmit signal results and images in tissue mimicking phantoms of cysts and tumors are provided for comparison.

Ultrasonic imaging using optoelectronic transmitters

Conventional ultrasound scanners utilize electronic transmitters and receivers at the scanner with a separate coaxial cable connected to each transducer element in the handle. 2-D arrays which allow new imaging modalities to be introduced significantly increase the channel count and degrade the handling ability of the transducer cable assembly. Fiber optics which is a medium used to transmit signals optically has distinct advantages over standard coaxial cable to increase flexibility and decrease the weight of the transducer cable for large channel numbers. In transmit, optoelectronics would have to be designed to produce high voltage wide bandwidth pulses across the transducer element. In this paper, we describe a 48 channel ultrasound system having 16 optoelectronic transmitters and 32 conventional electronic receivers. A silicon avalanche photodiode (APD) and GaAs photoconductive switch (PCSS) were investigated for producing the transmit pulse. A Siemens SI-1200 scanner and a 2.25 MHz linear array were used to compare the optoelectronic system to the conventional electronic system. Transmit signal results and images in phantoms of cysts and tumors are provided for comparison.

Signal to noise ratio of hybrid multilayer/single layer 2-D arrays

In our previous work, we examined the improved transmit sensitivity of 2-D array transducers using multilayer PZT. In receive mode, the signal to noise ratio (SNR) requires a more complex examination. Typically, a coaxial cable connects a single layer element to a preamplifier at the scanner. In this configuration, the high impedance 2-D array element is unable to drive the coaxial cable and preamplifier efficiently. In addition to signal losses, the electronic noise from the preamplifier usually dominates the thermal noise of the transducer element which reduces the SNR. In this study, a 1.5-D 7.5 MHz array element was analyzed using the KLM model. We show the receive SNR increases by 8.7 dB compared to the control when a high impedance preamplifier is placed adjacent to the single layer transducer element. The receive SNR improvement combined with the transmit improvement using multilayer PZT yields an overall improvement of 23.7 dB. A 1.5-D 1 MHz hybrid array was fabricated and tested to confirm the simulation results.