This thesis describes low-power application-specific integrated circuit (ASIC) designs to mitigate the constraint of cable count in miniature 3-D TEE probes. Receive cable count reduction techniques including subarray beamforming and digital time-division multiplexing (TDM) have been explored and the effectiveness of these techniques has been demonstrated by experimental prototypes. Digital TDM is a reliable technique to reduce cable count but it requires an in-probe datalink for high-speed data communication. A quantitative study on the impact of the datalink performance on B-mode ultrasound image quality has been introduced in this thesis for data communications in future digitized ultrasound probes. Finally, a high-voltage transmitter prototype has been presented for effective cable count reduction in transmission while achieving good power efficiency. @en

This paper presents an ultrasound receiver ASIC in 180nm CMOS that enables element-level digitization of echo signals in miniature 3D ultrasound probes. It is the first to integrate an analog front-end and a 10-b Nyquist ADC within the 150 μ m element pitch of a 5-MHz 2D transducer array. To achieve this, a hybrid SAR/shared-single-slope architecture is proposed in which the ramp generator is shared within each 2 × 2 subarray. The ASIC consumes 1.54mW/element and has been successfully demonstrated in an acoustic imaging experiment.

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This paper presents a power- and area-efficient approach to digitizing the echo signals received by piezoelectric transducer elements, commonly used for ultrasound imaging. This technique utilizes such elements not only as sensors but also as the loop filter of an element-level δσ analog to digital converter (ADC). The receiver chain is thus greatly simplified, yielding savings in area and power. Every ADC becomes small enough to fit underneath a 150 μm × 150 μm transducer element, enabling simultaneous acquisition and digitization from all the elements in a 2-D array. This is especially valuable for miniature 3-D probes. Experimental results are reported for a prototype receiver chip with an array of 5×4 element-matched ADCs and a transducer array fabricated on top of the chip. Each ADC consumes 800 μW from a 1.8 V supply and achieves a SNR of 47 dB in a 75% bandwidth around a center frequency of 5 MHz.@en

This paper presents an area- and power-efficient application-specified integrated circuit (ASIC) for 3-D forward-looking intravascular ultrasound imaging. The ASIC is intended to be mounted at the tip of a catheter, and has a circular active area with a diameter of 1.5 mm on the top of which a 2-D array of piezoelectric transducer elements is integrated. It requires only four micro-coaxial cables to interface 64 receive (RX) elements and 16 transmit (TX) elements with an imaging system. To do so, it routes high-voltage (HV) pulses generated by the system to selected TX elements using compact HV switch circuits, digitizes the resulting echo signal received by a selected RX element locally, and employs an energy-efficient load-modulation datalink to return the digitized echo signal to the system in a robust manner. A multi-functional command line provides the required sampling clock, configuration data, and supply voltage for the HV switches. The ASIC has been realized in a 0.18-μm HV CMOS technology and consumes only 9.1 mW. Electrical measurements show 28-V HV switching and RX digitization with a 16-MHz bandwidth and 53-dB dynamic range. Acoustical measurements demonstrate successful pulse transmission and reception. Finally, a 3-D ultrasound image of a three-needle phantom is generated to demonstrate the imaging capability.

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This paper presents a front-end application-specified integrated circuit (ASIC) integrated with a 2-D PZT matrix transducer that enables in-probe digitization with acceptable power dissipation for the next-generation endoscopic and catheter-based 3-D ultrasound imaging systems. To achieve power-efficient massively parallel analog-to-digital conversion (ADC) in a 2-D array, a 10-bit 30 MS/s beamforming ADC that merges the subarray beamforming and digitization functions in the charge domain is proposed. It eliminates the need for costly intermediate buffers, thus significantly reducing both power consumption and silicon area. Self-calibrated charge references are implemented in each subarray to further optimize the system-level power efficiency. High-speed datalinks are employed in combination with the subarray beamforming scheme to realize a 36-fold channel-count reduction and an aggregate output data rate of 6 Gb/s for a prototype receive array of 24 x 6 elements. The ASIC achieves a record power efficiency of 0.91 mW/element during receive. Its functionality has been demonstrated in both electrical and acoustic imaging experiments.

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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

Intravascular ultrasound is an imaging modality used to visualize atherosclerosis from within the inner lumen of human arteries. Complex lesions like chronic total occlusions require forward-looking intravascular ultrasound (FL-IVUS), instead of the conventional side-looking geometry. Volumetric imaging can be achieved with 2D array transducers, which present major challenges in reducing cable count and device integration. In this work we present an 80-element lead zirconium titanate (PZT) matrix ultrasound transducer for FL-IVUS imaging with a front-end application-specific integrated circuit (ASIC) requiring only 4 cables. After investigating optimal transducer designs we fabricated the matrix transducer consisting of 16 transmit (TX) and 64 receive (RX) elements arranged on top of an ASIC having an outer diameter of 1.5 mm and a central hole of 0.5 mm for a guidewire. We modeled the transducer using finite element analysis and compared the simulation results to the values obtained through acoustic measurements. The TX elements showed uniform behavior with a center frequency of 14 MHz, a -3 dB bandwidth of 44 &#x0025; and a transmit sensitivity of 0.4 kPa/V at 6 mm. The RX elements showed center frequency and bandwidth similar to the TX elements, with an estimated receive sensitivity of 3.7 &#x03BC;V/Pa. We successfully acquired a 3D FL image of three spherical reflectors in water using delay-and-sum beamforming and the coherence factor method. Full synthetic aperture acquisition can be achieved with frame rates on the order of 100 Hz. The acoustic characterization and the initial imaging results show the potential of the proposed transducer to achieve 3D FL-IVUS imaging.

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Forward-looking intravascular ultrasound (FL-IVUS) transducers are needed to image complex lesions in the coronary arteries, such as chronic total occlusions (CTOs). To achieve 2D and 3D FL-IVUS imaging, transducer arrays can be integrated at the tip of the catheter. However, connecting the elements is challenging due to the limited space available. In this work, we present a FL-IVUS matrix transducer consisting of 16 transmit and 64 receive elements, which are interfaced with an ASIC that requires only 4 micro-coaxial cables. The transducer performance was characterized by hydrophone measurements and FL imaging of three spherical reflectors @en

This work presents a compact ADC architecture capable of digitizing the signals received by every individual element of a 2D ultrasound transducer array. An element-matched layout of 150 μm × 150 μm is realized by exploiting each piezo-electric transducer element not only as the signal source, but also as the electro-mechanical loop-filter of a continuoustime band-pass ΔΣ ADC, thus minimizing the required circuit blocks. The transducer's frequency response, which is inherently matched with the signal bandwidth of interest, provides noise shaping to the ADC. A prototype chip has been fabricated in a 0.18 μm CMOS technology, featuring 20 ADCs located directly underneath a 150 μm-pitch piezo-electric transducer array fabricated on top of the chip. Each ADC, clocked at 200 MHz, consumes 800 μψ from a 1.8 V supply, and achieves an SNR of 47 dB in a 75% bandwidth around a center frequency of 5 MHz. Acoustic measurements show that the ADC successfully digitizes incoming echo signals.@en

This paper presents a front-end ASIC for forward-looking intravascular ultrasound (IVUS) imaging. The ASIC is intended to be mounted at the tip of a catheter and can interface a total of 80 piezo-electric transducer elements with an imaging systems using only 4 cables, thus significantly reducing the system complexity compared to the prior art. It is capable of switching high-voltage transmit pulses to 16 transmit elements, and capturing the resulting echo signals using 64 multiplexed receive elements. The ASIC digitizes the received signals locally, providing more robust communication than prior analog approaches. Measurements show that the ASIC effectively switches transmit pulses up to 30 V, and digitizes echo signals with a bandwidth of 16 MHz, while consuming only 10 mW. Acoustic measurements in combination with a prototype transducer array demonstrate pulse transmission and reception. Finally, a B-mode image of a needle phantom demonstrates the imaging capability.

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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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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 power-and area-efficient front-end application-specific integrated circuit (ASIC) that is directly integrated with an array of 32 × 32 piezoelectric transducer elements to enable 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 receive (RX) cables required in the probe's narrow shaft by ninefold with the aid of 96 delay-and-sum beamformers, each of which locally combines the signals received by a sub-array of 3 × 3 elements. These beamformers are based on pipeline-operated analog sample-and-hold stages and employ a mismatch-scrambling technique to prevent the ripple signal associated with the mismatch between these stages from limiting the dynamic range. In addition, an ultralow-power low-noise amplifier architecture is proposed to increase the power efficiency of the RX circuitry. The ASIC has a compact element matched layout and consumes only 0.27 mW/channel while receiving, which is lower than the state-of-the-art circuit. Its functionality has been successfully demonstrated in 3-D imaging experiments.

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A general challenge in 3D volumetric imaging is the large channel count. One solution uses integrated microbeamformers. The reconstruction of the entire volume from these micro-beamformed datasets can be performed in many ways. In this paper we propose two 3D multiline beamforming techniques, suitable for producing volumes at high frame rate and compare the image qualities to a fully-sampled matrix. The performance of the proposed beamforming techniques was evaluated with simulations in FieldII. Results show that the proposed simple volume reconstruction technique (using 25 transmissions) produces volumes at very high frame rate, but with sharp intensity changes within the volume. The proposed advanced technique (using 169 transmissions) produces volumes very similar to a fully-sampled matrix transducer despite the micro-beamforming.@en

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 probe's 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.@en

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.@en

This paper presents a power- and area-efficient 9-channel LNA array for piezoelectric ultrasound transducers to enable real-time 3D imaging with miniature endoscopic and catheter-based probes. In view of the relatively low impedance of piezoelectric transducers, the LNA is implemented as a capacitive feedback voltage amplifier, rather than a trans-impedance amplifier, to achieve a better noise-power trade-off. The use of a current-efficient inverter-based OTA with optimized bias scheme and dual-rail regulation further improves the power efficiency of the LNA while keeping the area compact: 0.01 mm2 per channel. Electrical and acoustic measurement results show that the proposed LNA achieves a 0.6 mPa/√Hz input-referred noise at 4 MHz while consuming only 0.135 mW, which represents a noise efficiency 2.5 × better than the state-of-the-art.@en