Quadrature Synthetic Aperture Beamforming Front-End for Miniaturized Ultrasound Imaging.

A quadrature synthetic aperture front-end receiver for B-mode ultrasound imaging is presented. The receiver targets small-scale imaging applications such as capsule endoscopy and low-cost portable devices. System complexity, area, power consumption, and cost are minimized using synthetic aperture beamforming (SAB), whereby signals are processed in a sequential manner using only a single channel. SAB is combined with quadrature (I/Q) sampling, which further reduces the bandwidth and computational load. I/Q demodulation is carried out using a full custom analog front-end (AFE), which comprises a low-noise, variable gain preamplifier, followed by a passive mixer, programmable gain amplifier (PGA) and active lowpass filter. A novel preamplifier design is proposed, with quasi-exponential time-gain control and low noise (${\text{5.42 nV}}/\sqrt{\text{Hz}}$ input-referred noise). Overall, the AFE consumes ${\text{7.8 mW}}$ (static power) and occupies ${\text{1.5}}\,\text{mm}\times {\text{1.5}}\,\text{mm}$ in AMS ${\text{0.35}}\,\mu \text{m}$ CMOS. Real-time SAB is carried out using a Spartan-6 FPGA, which dynamically apodises and focuses the data by interpolating and applying complex phase rotations to the I/Q samples. For a frame rate of ${\text{7}}\,\text{Hz}$ , the power consumption is ${\text{3.4}}\,\text{mW}/\text{channel}$ across an aperture of 64 elements. B-mode images were obtained using a database of ultrasound signals ( ${\text{2.5}}\,\text{MHz}$ center frequency) derived from a commercial ultrasound machine. The normalized root mean squared error between the quadrature SAB image and the RF reference image was ${\text{13}}\%$. Image quality/frame rate may be tuned by varying the degree of spatial compounding.

Comparison of synthetic aperture architectures for miniaturised ultrasound imaging front-ends.

BACKGROUND: Point of care ultrasonography has been the focus of extensive research over the past few decades. Miniaturised, wireless systems have been envisaged for new application areas, such as capsule endoscopy, implantable ultrasound and wearable ultrasound. The hardware constraints of such small-scale systems are severe, and tradeoffs between power consumption, size, data bandwidth and cost must be carefully balanced. METHODS: In this work, two receiver architectures are proposed and compared to address these challenges. Both architectures uniquely combine low-rate sampling with synthetic aperture beamforming to reduce the data bandwidth and system complexity. The first architecture involves the use of quadrature sampling to minimise the signal bandwidth and computational load. Synthetic aperture beamforming (SAB) is carried out using a single-channel, pipelined protocol suitable for implementation on an FPGA/ASIC. The second architecture employs compressive sensing within the finite rate of innovation framework to further reduce the bandwidth. Low-rate signals are transmitted to a computational back-end (computer), which sequentially reconstructs each signal and carries out beamforming. RESULTS: Both architectures were tested using a custom hardware front-end and synthetic aperture database to yield B-mode images. The normalised root-mean-squared-error between the quadrature SAB image and the RF reference image was [Formula: see text] while the compressive SAB error was [Formula: see text] for the same degree of spatial compounding. The sampling rate is reduced by a factor of 2 (quadrature SAB) and 4.7 (compressive SAB), compared to the RF sampling rate. The quadrature method is implemented on FPGA, with a total power consumption of [Formula: see text] mW, which is comparable to state-of-the-art hardware topologies, but with significantly reduced circuit area. CONCLUSIONS: Through a novel combination of SAB and low-rate sampling techniques, the proposed architectures achieve a significant reduction in data transmission rate, system complexity and digital/analogue circuit area. This allows for aggressive miniaturisation of the imaging front-end in portable imaging applications.