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In the early 1990s, “portable” phones roughly the size of modern laptop computers (sometimes called bag phones) were common. Within two decades, pocket-sized mobile phones could send and receive email and text messages, take photos, check stocks, schedule appointments, and, of course, make calls to anywhere in the world. Similarly, early so-called portable ultrasound systems were cart-based, power-hungry, and expensive. Recent advances in chip integration and power-optimization techniques have enabled much more compact and efficient ultrasound systems.
These advances have produced more portable, power-efficient ultrasound devices with improved imaging performance and expanded functionality. Higher dynamic range, lower power consumption, and more compact system-level ICs enable high-quality images suitable for diagnostic use. Future ultrasound devices may become truly handheld tools for clinicians.
Ultrasound signal chain
All ultrasound systems use a transducer at the end of a relatively long cable, typically around two meters. That cable contains at least eight—and up to 256—miniature coaxial lines and is one of the most expensive components in the system. In almost all systems, transducer elements directly drive the cable. The cable capacitance loads the transducer elements and causes significant signal attenuation, which requires a highly sensitive receiver to preserve dynamic range and achieve optimal system performance.
On the transmit side (Tx path), the beamformer sets the pulse-sequence delay pattern for the desired focus. The beamformer output is then amplified by high-voltage transmit amplifiers to drive the transducer. These amplifiers are controlled by digital-to-analog converters (DACs) or high-voltage FET switch arrays to shape the transmit pulses and improve energy transfer to the transducer elements. On the receive side, the transmit/receive (T/R) switch (commonly a diode bridge) blocks high-voltage Tx pulses. Some arrays use high-voltage multiplexers/demultiplexers to reduce the amount of transmit and receive hardware at the expense of flexibility.
The time-gain-compensation (TGC) receive path consists of a low-noise amplifier (LNA), a variable-gain amplifier (VGA), and an analog-to-digital converter (ADC). The VGA typically provides linear dB gain control that matches the attenuation of reflected ultrasound signals. Under operator control, the TGC path maintains image uniformity during scanning. A low-noise LNA is critical to minimize the noise contribution of the subsequent VGA. Where input impedance matching is required, active impedance control yields the best noise performance.
The VGA compresses the input signal's wide dynamic range to match the ADC input range. The LNA's input-referred noise limits the smallest discernible input signal, while output-referred noise is dominated by the VGA and limits the maximum instantaneous dynamic range at a given gain-control voltage. That limit is set by the quantization noise floor, which is determined by the ADC resolution. Early ultrasound systems used 10-bit ADCs; most modern systems use 12- or 14-bit ADCs.
Anti-aliasing filters (AAF) limit the signal bandwidth and suppress unwanted noise in the TGC path prior to sampling by the ADC.
Beamforming in medical ultrasound refers to phase alignment and summation of signals that originate from a common source but are received by different transducer elements at different times. In continuous-wave Doppler (CWD) paths, receiver channels are phase-shifted and summed to extract coherent information. Beamforming performs two main functions: steering the transducer array to increase effective gain and defining a focal point within the body where echoes are localized.
Beamforming can be implemented in two different ways: analog beamforming (ABF) and digital beamforming (DBF). The main difference between ABF and DBF systems is how beamforming is performed, and both approaches require good channel-to-channel matching. ABF uses analog delay lines and summation and therefore needs only a single precision, high-resolution, high-speed ADC. DBF systems are currently more common; they employ many high-speed, high-resolution ADCs. In DBF architectures, signals are sampled as close to the transducer elements as possible, then delayed and digitally summed.
