Design of an Ultrasound Transceiver ASIC with a Switching-Artifact Reduction Technique for 3D Carotid Artery Imaging.

This paper presents an ultrasound transceiver application-specific integrated circuit (ASIC) directly integrated with an array of 12 × 80 piezoelectric transducer elements to enable next-generation ultrasound probes for 3D carotid artery imaging. The ASIC, implemented in a 0.18 µm high-voltage Bipolar-CMOS-DMOS (HV BCD) process, adopted a programmable switch matrix that allowed selected transducer elements in each row to be connected to a transmit and receive channel of an imaging system. This made the probe operate like an electronically translatable linear array, allowing large-aperture matrix arrays to be interfaced with a manageable number of system channels. This paper presents a second-generation ASIC that employed an improved switch design to minimize clock feedthrough and charge-injection effects of high-voltage metal-oxide-semiconductor field-effect transistors (HV MOSFETs), which in the first-generation ASIC caused parasitic transmissions and associated imaging artifacts. The proposed switch controller, implemented with cascaded non-overlapping clock generators, generated control signals with improved timing to mitigate the effects of these non-idealities. Both simulation results and electrical measurements showed a 20 dB reduction of the switching artifacts. In addition, an acoustic pulse-echo measurement successfully demonstrated a 20 dB reduction of imaging artifacts.

A 2-D Ultrasound Transducer With Front-End ASIC and Low Cable Count for 3-D Forward-Looking Intravascular Imaging: Performance and Characterization.

Intravascular ultrasound (IVUS) 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 IVUS (FL-IVUS), instead of the conventional side-looking geometry. Volumetric imaging can be achieved with 2-D array transducers, which present major challenges in reducing cable count and device integration. In this work, we present an 80-element lead zirconium titanate matrix ultrasound transducer for FL-IVUS imaging with a front-end application-specific integrated circuit (ASIC) requiring only four 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%, 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 /Pa. We successfully acquired a 3-D 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 3-D FL-IVUS imaging.

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 micro-beamformer 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.

Front-end receiver electronics for a matrix transducer for 3-D transesophageal echocardiography.

There is a clear clinical need for creating 3-D images of the heart. One promising technique is the use of transesophageal echocardiography (TEE). To enable 3-D TEE, we are developing a miniature ultrasound probe containing a matrix piezoelectric transducer with more than 2000 elements. Because a gastroscopic tube cannot accommodate the cables needed to connect all transducer elements directly to an imaging system, a major challenge is to locally reduce the number of channels, while maintaining a sufficient signal-to-noise ratio. This can be achieved by using front-end receiver electronics bonded to the transducers to provide appropriate signal conditioning in the tip of the probe. This paper presents the design of such electronics, realizing time-gain compensation (TGC) and micro-beamforming using simple, low-power circuits. Prototypes of TGC amplifiers and micro-beamforming cells have been fabricated in 0.35-µm CMOS technology. These prototype chips have been combined on a printed circuit board (PCB) to form an ultrasound-receiver system capable of reading and combining the signals of three transducer elements. Experimental results show that this design is a suitable candidate for 3-D TEE.

A comparison of two- and four-electrode techniques to characterize blood impedance for the frequency range of 100 Hz to 100 MHz.

Measurement setups that characterize the impedance of suspensions of blood over the wide frequency range of 100 Hz to 100 MHz are presented in this paper. The performance of the two- and four-electrode techniques have been compared and evaluated. By applying a combination of the two measurement techniques the best result is achieved when taking into account the main nonidealities, such as electrode polarization impedance and parasitic capacitances. It has been found that the conventional three-element model for the impedance of blood can be used for frequencies up to 1 MHz. For frequencies exceeding 1 MHz, an extended model is introduced where a constant phase angle element is used for modeling the cell membrane and a capacitor C(liq) is added for modeling the electrical capacitance of water in blood.

Catheter-based impedance measurements in the right atrium for continuously monitoring hematocrit and estimating blood viscosity changes; an in vivo feasibility study in swine.

Hematocrit is the most important determinant of whole blood viscosity and it affects thrombosis. As hematocrit can be measured accurately in vitro by using an electrical impedance technique, aim of the present study is to investigate the diagnostic potential of using this technique in vivo to continuously monitor hematocrit. Characteristics of a special catheter for in vivo measurement of electrical resistivity in blood in the right atrium are described. In five anesthetized swine hematocrit is monitored continuously with this catheter while different levels of hemoconcentration are induced. In addition, blood viscosity is increased by inducing 'acute phase' reaction the day before surgery, resulting in variable degree of elevated fibrinogen levels in the five swine. Good reproducibility of the resistivity measurements (S.D < 0.01) and excellent correlation between resistivity data in vivo and hematocrit levels in each swine are found (r2 = 0.95-0.99). Furthermore, stepwise regression analysis of data from all swine shows a highly significant contribution also of other important parameters of blood viscosity, such as fibrinogen, total protein and temperature (cumulative r2 = 0.97). Determining hematocrit continuously in vivo by electrical resistivity measurements with a catheter in the right atrium is feasible and these measurements correlate significantly also with other important parameters of blood viscosity.