Maysam Shabanimotlagh

A Prototype PZT Matrix Transducer With Low-Power Integrated Receive ASIC for 3-D Transesophageal Echocardiography

This paper presents the design, fabrication, and experimental evaluation of a prototype lead zirconium titanate (PZT) matrix transducer with an integrated receive ASIC, as a proof of concept for a miniature three-dimensional (3-D) transesophageal echocardiography (TEE) probe. It consists of an array of 9 × 12 piezoelectric elements mounted on the ASIC via an integration scheme that involves direct electrical connections between a bond-pad array on the ASIC and the transducer elements. The ASIC addresses the critical challenge of reducing cable count, and includes front-end amplifiers with adjustable gains and microbeamformer circuits that locally process and combine echo signals received by the elements of each 3 × 3 subarray. Thus, an order-of-magnitude reduction in the number of receive channels is achieved. Dedicated circuit techniques are employed to meet the strict space and power constraints of TEE probes. The ASIC has been fabricated in a standard 0.18-μm CMOS process and consumes only 0.44 mW/channel. The prototype has been acoustically characterized in a water tank. The ASIC allows the array to be presteered across ±37° while achieving an overall dynamic range of 77 dB. Both the measured characteristics of the individual transducer elements and the performance of the ASIC are in good agreement with expectations, demonstrating the effectiveness of the proposed techniques.

Low-power receive electronics for a miniature real-time 3D ultrasound probe

Transesophageal echocardiography (TEE) involves the use of an ultrasound transducer mounted at the tip of an endoscope to make ultrasonic images of the human heart from the esophagus. In conventional TEE probes, each transducer element is wired-out by a micro-coaxial cable to an external imaging system. To obtain real-time three-dimensional images, however, a two-dimensional transducer array with more than 1000 elements is required. Direct wiring of so many elements through an endoscope is not feasible, so channel reduction should be performed locally. In this paper, we present an application-specific integrated circuit (ASIC) that includes low-noise amplifiers, programmable-gain amplifiers and micro-beamformer circuits that locally process and combine the signals received by sub-groups of the transducer array. Thus, an order-of-magnitude channel reduction is achieved. The acoustic characterization of the prototype ASIC with a co-integrated transducer array will be presented.

A front-end ASIC with receive sub-array beamforming integrated with a 32 × 32 PZT matrix transducer for 3-D transesophageal echocardiography

This paper presents a power- and area-efficient front-end ASIC that is directly integrated with an array of 32 × 32 piezoelectric transducer elements to enable the next-generation miniature ultrasound probes for real-time 3-D transesophageal echocardiography. The 6.1 × 6.1 mm2 ASIC, implemented in a low-voltage 0.18 μm CMOS process, effectively reduces the number of cables required in the probes narrow shaft by means of 96 sub-array beamformers, which have a compact element-matched layout and employ mismatch-scrambling to enhance the dynamic range. The ASIC consumes less than 230 mW while receiving and its functionality has been successfully demonstrated in a 3-D imaging experiment.

Improving the Performance of a 1-D Ultrasound Transducer Array by Subdicing

In medical ultrasound transducer design, the geometry of the individual elements is crucial since it affects the vibration mode of each element and its radiation impedance. For a fixed frequency, optimal vibration (i.e., uniform surface motion) can be achieved by designing elements with very small width-to-thickness ratios. However, for optimal radiation impedance (i.e., highest radiated power), the width should be as large as possible. This leads to a contradiction that can be solved by subdicing wide elements. To systematically examine the effect of subdicing on the performance of a 1-D ultrasound transducer array, we applied finite-element simulations. We investigated the influence of subdicing on the radiation impedance, on the time and frequency response, and on the directivity of linear arrays with variable element widths. We also studied the effect of varying the depth of the subdicing cut. The results show that, for elements having a width greater than 0.6 times the wavelength, subdicing improves the performance compared with that of nonsubdiced elements: the emitted pressure may be increased up to a factor of three, the ringing time may be reduced by up to 50%, the bandwidth increased by up to 77%, and the sidelobes reduced by up to 13 dB. Moreover, this simulation study shows that all these improvements can already be achieved by subdicing the elements to a depth of 70% of the total element thickness. Thus, subdicing can improve important transducer parameters and, therefore, help in achieving images with improved signal-to-noise ratio and improved resolution.

Quantifying the effect of subdicing on element vibration in ultrasound transducers

In ultrasound imaging, one of the challenges of transducer design is to maximize the acoustic energy that is radiated into the medium. The geometry of the transducer elements is one of the major aspects that affects the transmit efficiency. In this study we apply simulations to investigate the effect of subdicing on the transmit performance of an ultrasound linear array transducer. We focus on the influence of subdicing on the average emitted pressure, surface velocity and radiation impedance of linear arrays with variable element widths. The simulations are performed using the Finite Element Analysis (FEA) software PZFlex (Weidlinger Associates, Los Altos, CA) and the results obtained show the improvement in transmit performance when subdicing elements having a width above 0.6λ.

Optimizing the directivity of piezoelectric matrix transducer elements mounted on an ASIC

Over the last decade, clinical studies show a strong interest in real-time 3D imaging. This calls for ultrasound probes with high-element-count 2D matrix transducer arrays. These may be interfaced to an imaging system using an in-probe Application Specific Integrated Circuit (ASIC) that takes care of signal amplification, element switching, sub-array beamforming, etc. Since the ASIC is made from silicon and is mounted directly behind the transducer elements, it can acoustically be regarded as a rigid plate that can sustain traveling lateral waves. These waves lead to acoustical cross-talk between the elements, and results in extra peaks in the directivity pattern. We propose two solutions to this problem, based on numerical simulations. One approach is to decrease the phase velocity in the silicon by reducing the silicon thickness and absorbing the energy using a proper backing material. Another solution is to disturb the waves inside the silicon plate by sub-dicing the back-side of the ASIC. We conclude that both solutions can be used to improve the directivity pattern.

The directivity of piezoelectric matrix transducer elements mounted on an ASIC

Over the last decade, clinical studies show a strong interest for real-time 3D imaging. This calls for ultrasound probes with high-element-count 2D matrix transducer array, interfaced to an imaging system using an in-probe Application Specific Integrated Circuit (ASIC) that takes care of element selection, signal amplification, sub-array beamforming, etc. Since the ASIC is based on silicon and is mounted directly behind the transducer elements, it can be regarded as a rigid plate that can sustain travelling waves, which effectively lead to acoustical cross-talk between the elements. We hypothesize that the cross-talk can be diminished by reducing the thickness of the ASIC and using a proper backing, and we investigate this by simulation.

The role of sub-dicing in the acoustical design of an ultrasound matrix transducer for carotid arteries imaging

Accurate diagnostics of stenosis and blood flow distribution in carotid arteries requires transducers capable of producing 3D volume images with high frame rate for real time imaging. In the process of designing a matrix probe, an important goal is to realize the acoustic stack with high sensitivity and bandwidth. In this study, we employ a finite element analysis to evaluate the effect of sub-dicing on the performance of an acoustic stack in a piezoelectric matrix array. The array is integrated with an Application Specific Integrated Circuit (ASIC), which performs the task of signal amplification and efficient data reduction. The results show that two sub-dicing cuts can improve the sensitivity by 40%, bandwidth by 20%, and reduce the ringing time by 43%, which are all desired for improving the image quality.

The acoustical performance of an ultrasound matrix transducer directly stacked on a silicon chip

Clinical studies have shown a high demand for 3D real time ultrasound imaging for accurate diagnosis. One way to scan a volume with high frame rate is to design a 2D matrix transducer allowing beamforming in both lateral and elevation directions. Recent developments in manufacturing technologies allow the integration of a matrix of piezoelectric elements on Application Specific Integrated Circuit (ASIC). The ASIC is responsible for beamforming, amplification, switching, and significant channel count reduction. Traditionally, the backing material for a linear/phased array is composed of a mixture of epoxy with powder of heavy materials, which causes strong attenuation. This avoids energy being reflected back into to the elements and results in short pulses, high bandwidth, and consequently high axial resolution of the image. However, the ASIC acoustically behaves like a hard and non-absorbing backing material. Therefore, the acoustical energy of the piezoelectric pillar propagates into the chip and affects...