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 (<inline-formula><tex-math notation="LaTeX">${\text{5.42 nV}}/\sqrt{\text{Hz}}$</tex-math></inline-formula> input-referred noise). Overall, the AFE consumes <inline-formula><tex-math notation="LaTeX">${\text{7.8 mW}}$ </tex-math></inline-formula> (static power) and occupies <inline-formula><tex-math notation="LaTeX"> ${\text{1.5}}\,\text{mm}\times {\text{1.5}}\,\text{mm}$</tex-math></inline-formula> in AMS <inline-formula> <tex-math notation="LaTeX">${\text{0.35}}\,\mu \text{m}$</tex-math></inline-formula> 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 <inline-formula><tex-math notation="LaTeX">${\text{7}}\,\text{Hz}$ </tex-math></inline-formula>, the power consumption is <inline-formula><tex-math notation="LaTeX"> ${\text{3.4}}\,\text{mW}/\text{channel}$</tex-math></inline-formula> across an aperture of 64 elements. B-mode images were obtained using a database of ultrasound signals (<inline-formula><tex-math notation="LaTeX"> ${\text{2.5}}\,\text{MHz}$</tex-math></inline-formula> 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 <inline-formula> <tex-math notation="LaTeX">${\text{13}}\%$</tex-math></inline-formula>. Image quality/frame rate may be tuned by varying the degree of spatial compounding.

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