We describe a 3-D multiline parallel beamforming scheme for real-time volumetric ultrasound imaging using a prototype matrix transesophageal echocardiography probe with diagonally diced elements and separated transmit and receive arrays. The elements in the smaller rectangular transmit array are directly wired to the ultrasound system. The elements of the larger square receive aperture are grouped in 4 × 4-element sub-arrays by micro-beamforming in an application-specific integrated circuit. We propose a beamforming sequence with 85 transmit–receive events that exhibits good performance for a volume sector of 60° × 60°. The beamforming is validated using Field II simulations, phantom measurements and in vivo imaging. The proposed parallel beamforming achieves volume rates up to 59Hz and produces good-quality images by angle-weighted combination of overlapping sub-volumes. Point spread function, contrast ratio and contrast-to-noise ratio in the phantom experiment closely match those of the simulation. In vivo 3-D imaging at 22-Hz volume rate in a healthy adult pig clearly visualized the cardiac structures, including valve motion.@en

This paper presents the design, fabrication and characterization of a miniature PZT-on-CMOS matrix transducer for real-time pediatric 3-dimensional (3D) transesophageal echocardiography (TEE). This 3D TEE probe consists of a 32 × 32 array of PZT elements integrated on top of an Application Specific Integrated Circuit (ASIC). We propose a partitioned transmit/receive array architecture wherein the 8 × 8 transmitter elements, located at the centre of the array, are directly wired out and the remaining receive elements are grouped into 96 sub-arrays of 3 × 3 elements. The echoes received by these sub-groups are locally processed by micro-beamformer circuits in the ASIC that allow pre-steering up to ±37°. The PZT-on-CMOS matrix transducer has been characterized acoustically and has a centre frequency of 5.8 MHz, -6 dB bandwidth of 67%, a transmit efficiency of 6 kPa/V at 30 mm, and a receive dynamic range of 85 dB with minimum and maximum detectable pressures of 5 Pa and 84 kPa respectively. The properties are very suitable for a miniature pediatric real-time 3D TEE probe.

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Until now, no matrix transducer has been realized for 3D transesophageal echocardiography (TEE) in pediatric patients. In 3D TEE with a matrix transducer, the biggest challenges are to connect a large number of elements to a standard ultrasound system, and to achieve a high volume rate (>200 Hz). To address these issues, we have recently developed a prototype miniaturized matrix transducer for pediatric patients with micro-beamforming and a small central transmitter. In this paper we propose two multiline parallel 3D beamforming techniques (μBF25 and μBF169) using the micro-beamformed datasets from 25 and 169 transmit events to achieve volume rates of 300 Hz and 44 Hz, respectively. Both the realizations use angle-weighted combination of the neighboring overlapping sub-volumes to avoid artifacts due to sharp intensity changes introduced by parallel beamforming. In simulation, the image quality in terms of the width of the point spread function (PSF), lateral shift invariance and mean clutter level for volumes produced by μBF25 and μBF169 are similar to the idealized beamforming using a conventional single-line acquisition with a fully-sampled matrix transducer (FS4k, 4225 transmit events). For completeness, we also investigated a 9 transmit-scheme (3 × 3) that allows even higher frame rates but found worse B-mode image quality with our probe. The simulations were experimentally verified by acquiring the μBF datasets from the prototype using a Verasonics V1 research ultrasound system. For both μBF169 and μBF25, the experimental PSFs were similar to the simulated PSFs, but in the experimental PSFs, the clutter level was ∼10 dB higher. Results indicate that the proposed multiline 3D beamforming techniques with the prototype matrix transducer are promising candidates for real-time pediatric 3D TEE.

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Data acquisition from 2D transducer arrays is one of the main challenges for the development of emerging miniature 3D ultrasound imaging devices, such as 3D trans-esophageal (TEE) and intra-cardiac echocardiography (ICE) probes (Fig. 10.5.1). The main obstacle lies in the mismatch between the large number of transducer elements (103 to 104) and the limited cable count (<;200). Recent advances in transducer-on-CMOS integration have enabled the use of in-probe subarray beamforming based on delay-and-sum (DAS) circuits [1] to reduce the channel count by an order of magnitude. Further reduction calls for in-probe digitization to enable more advanced data processing and compression in the digital domain. However, prior designs [2-4] compromise on transducer pitch (> half wavelength) to accommodate the ADC and consume >9mW/element, which translates into unacceptable self-heating in miniature 3D probes.@en

The huge amount of data that needs to be transferred between probe and imaging system becomes a major issue when the data transfer capacity is limited, e.g. in handheld systems, wireless probes and miniaturized probes. The amount of data can be significantly reduced by using two-stage beamforming. The first stage consists of a fixed focus algorithm that compresses channel data to scanline data. This can be done by integrated electronics in the handle. In the second stage the scanline data is further beamformed in the imaging system to obtain images that are synthetically focused at all depths. Here we present a wave equation two-stage beamforming method for phased array imaging that is computationally efficient and outperforms PSASB, a time-of-flight alternative, in terms of lateral resolution and contrast-to-noise ratio.@en

Echocardiography is a portable, safe, and low-cost imaging technique for accurate assessment of the heart. In transesophageal echocardiography (TEE) the esophagus is utilized as the imaging window to examine the cardiac anatomy and function. In conventional TEE probes, a one-dimensional (1D) ultrasound array is employed to obtain two-dimensional (2D) cross-sectional images of the heart. Since cardiac morphology, leakage of valves and function of the outflow tracts are all three-dimensional (3D) phenomena, it is beneficial to interpret them from 3D images. Therefore, there is high clinical demand for matrix TEE probes that are capable of providing real-time volumetric images [1]. Several matrix arrays (Philips X7-2t, Siemens V5M TEE, General Electric 6VTD) have been developed for this purpose, however all of them are large in size (~10 cm3) and uncomfortable to use on non-anesthetized patients [2]. We aim to develop a matrix TEE probe with a small head volume (<1 cm3), which is suitable for long term monitoring of cardiac system on adults and in babies. We have developed a prototype of a small matrix TEE probe, which consists of a piezoelectric matrix transducer directly mounted on an Application Specific Integrated Circuit (ASIC). The ASIC performs the task of micro-beamforming, signal amplification and efficient data reduction. The piezoelectric matrix array consist of a 32×32 PZT elements with a pitch of 150 μm × 150 μm. The transmit aperture consists of 8×8 elements at the centre of the array, which are directly wired out to the ultrasound system. The remaining 864 elements are used in receive and are organized in 96 sub-arrays of 3×3 elements to reduce the cable count with a factor of 9. The signals from the individual elements in a sub-array are combined to a single output signal using a micro-beamformer on the ASIC. The micro-beamformer allows pre-steering of 0◦, ±17◦, and ±37◦ angles in both lateral and elevation directions. By recording datasets for different pre-steering angles, and by processing and combining them, a large volume image can be constructed. Acoustic performance of the prototype is evaluated in a water tank. The transmit transfer function of a single element is measured by applying a 20 cycle sinusoidal voltage, sweeping from 3 to 8 MHz with steps of 50 kHz. The output pressure is recorded by a calibrated hydrophone. It is found that the transducer has a central frequency of 5 MHz, a bandwidth of 40% and a transmit efficiency of 6.4 kPa/V (at 51 mm). To characterize the micro-beamforming function, three delay angles of 0◦, 17◦ and 37◦ were programmed. While transmitting with a well-defined external source, the output voltage from a sub-group was recorded from -50◦ to +50◦ degrees. We observe that the theoretical values of the beam profile agree well with the measurement results, especially with regard to the position of the grating lobes and side lobes. @en

Data acquisition from 2-D transducer arrays has become one of the main challenges for the development of endoscopic and catheter-based 3-D ultrasound imaging devices. Front-end ASICs with sub-array pre-beamforming have been reported that reduce the cable number by an order of magnitude. Further channel reduction requires digitization in the front-end ASIC to facilitate more in-probe data processing functions. Prior solutions, however, are too large and power-hungry to be integrated in a miniature ultrasound probe. In this work, we present a front-end ASIC with an element-pitch-matched layout that combines sub-array beamforming, digitization and high-speed data transmission. It achieves a 36-fold channel-count reduction and a record power-efficiency with less than 1 mW/element power dissipation in receive.@en

This paper presents a front-end application-specific integrated circuit (ASIC) that demonstrates the feasibility of inprobe digitization for next-generation miniature 3-D ultrasound probes with acceptable power- and area-efficiency. The proposed design employs a low-power charge-domain ADC that is directly merged with the sample-and-hold delay lines in each subarray, and high-speed datalinks at the ASIC periphery to realize an additional channel-count reduction compared to prior work based on analog subarray beamforming. The 4.8 × 2 mm² ASIC, which has a compact layout element-matched to a 5-MHz 150-μm-pitch PZT matrix transducer, achieves an overall 36-fold channel-count reduction and a state-of-the-art power-efficiency with less than 1 mW/element power dissipation while receiving, which is acceptable even when scaled up to a 1000-element probe. The prototype ASIC has been fabricated in a 0.18 μm CMOS process. Its functionality has been successfully evaluated with both electrical and acoustical measurements.

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Ultrafast 3D transesophageal echocardiographic (TEE) imaging, combined with 3D echo particle image velocimetry (ePIV), would be ideal for tracking the complex blood flow patterns in the heart. We are developing a miniature pediatric matrix TEE transducer that employs micro-beamforming (μBF) and allows high framerate in 3D. In this paper, we assess the feasibility of 3D ePIV with a high frame rate, small aperture transducer and the influence of the micro-beamforming technique. We compare the results of 3D ePIV on simulated images using the μBF transducer and an idealized, fully sampled (FS) matrix transducer. For the two transducers, we have simulated high-framerate imaging of an 8.4mm diameter artery having a known 4D velocity field. The simulations were performed in FieldII. 1000 3D volumes, at a rate of 1000 volumes/sec, were created using a single diverging transmission per volume. The error in the 3D velocity estimation was measured by comparing the ePIV results of both transducers to the ground truth. The results on the simulated volumes show that ePIV can estimate the 4D velocity field of the arterial phantom using these small-aperture transducers suitable for pediatric 3D TEE. The μBF transducer (RMSE 44.0%) achieved comparable ePIV accuracy to that of the FS transducer (RMSE 42.6%).

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The design of 3D TEE transducers poses severe technical challenges: channel count, electronics integration with high and low voltages, heat dissipation, etc. We present an adult matrix TEE probe with separate transmit (Tx) and receive (Rx) arrays allowing optimization in both Tx and Rx [1]. Tx elements are directly wired out, Rx employs integrated micro-beamformers in low-voltage (1.8/5.0V) chip technology. The prototype is fully integrated into a gastroscopic tube.

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