How to improve ultrasound system image quality using ultra-low-noise supplies
tissue, it is possible to calculate the distance from the transducer to the organ or interface reflecting the wave. The amplitude of the returning waves determines the brightness of the pixels assigned to the reflection in the ultrasound image, after considerable digital post-processing. Understanding system requirements Despite the conceptual simplicity of the underlying principle, a complete, high-end ultrasound imaging system is a complicated device (Figure 1). The ultimate performance of the system is largely determined by the transducer and analog front-end (AFE), while post-processing of the digitized reflected signal allows algorithms to enhance the situation.
Not surprisingly, system noise of various types is one of the limiting factors in image quality and performance, again analogous to the consideration of bit error rate (BER) versus SNR in digital communication systems. There is a transmit/receive (T/R) switch between the piezoelectric transducer array and the active electronics. The role of this switch is to prevent the high- voltage transmit signals driving the transducer from reaching and damaging the low-voltage receive-side AFE. After the reflection received is amplified and conditioned, it is passed to the analog-to-digital converter (ADC) of the AFE, where it is digitized and undergoes software-based image processing and enhancement. Each of the different imaging modes of an ultrasound system
has different requirements for the dynamic range – and thus SNR – or noise requirements: ■ For black-and-white image mode, a dynamic range of 70 decibels (dB) is required; the noise floor is important as it impacts the maximum depth at which the smallest ultrasound echo can be seen in the far field. This is called penetration, one of the key features of black-and-white mode ■ For pulse wave doppler (PWD) mode, a 130 dB dynamic range is required ■ For continuous wave doppler (CWD) mode, 160 dB is needed. Note that the 1/f noise is particularly important for the PWD and CWD modes, as both of those images include the low-frequency spectrum element below 1kHz, and the phase noise impacts the Doppler frequency spectrum higher than 1kHz These requirements are not easy to meet. As the ultrasound
Figure 2: The highly efficient LT8620 step-down switching regulator includes a SYNC pin so its clocking can be synchronized with other system clocks, minimizing clock intermodulation effects. Image source: Analog Devices
quality (clarity, dynamic range, lack of image speckling, and other figures of merit), it’s important to look at sources which cause loss of signal quality and degradation of SNR. The first one is obvious: due to attenuation, the returns from tissues and organs deeper in the body (such as kidneys) are far weaker than those from those close to the transducer. Therefore, the reflected signal is “gained up” by the AFE so that it occupies as much of the AFE’s input range as possible. For this, an automatic gain control (AGC) function is used. This AGC function is similar to the one used in wireless systems where the AGC assesses wireless RF received signal strength (RSS) and dynamically compensates for its random, unpredictable changes over a span of tens of decibels. However, the situation is different in the ultrasound application than
it is for a wireless link. Instead, the path attenuation is known approximately, as is the velocity of acoustic energy propagation velocity – 1540 meters per second (m/s) in soft tissue, or about five times faster than propagation in air at about 330m/s – and so the attenuation rate is also known. Based on this knowledge, the AFE uses a variable-gain amplifier (VGA) which is arranged as a time- gain compensation (TGC) amplifier. The gain of this VGA is linear-in- dB and is configured such that a linear-versus-time ramping control voltage increases the gain-versus- time to compensate to a large extent for the attenuation. This maximizes SNR and the use of the dynamic range of the AFE.
induced signal noise is beyond the control of the ultrasound system designer, internal system noise must be managed and controlled. For this, it’s important to understand the noise types, their impact, and what can be done to reduce them. The primary areas of concern are switching regulator noise; white noise due to the signal chain, clock, and power; and layout related noise. ■ Switching regulator noise: most switching regulators use a simple resistor to set the switching frequency. The unavoidable tolerance of the nominal value of this resistor introduces different switching frequencies and harmonics as the frequencies of different independent regulators mix and cross-modulate each other. Consider that even a tight- tolerance resistor with a 1% inaccuracy results in a 4kHz harmonic frequency in a 400kHz
transducer frequency is typically from 1MHz to 15MHz, it will be affected by any switching frequency noise within this range. If there are intermodulation frequencies within the PWD and CWD spectrums (from 100Hz to 200kHz), the obvious noise spectrums will appear in the Doppler images, which is unacceptable in the
Noise types and how to address them
Figure 1: A complete ultrasound imaging system is a complex combination of a significant amount of analog, digital, power, and processing functionality; the AFE defines the bounds of system performance. Image source: Analog Devices
ultrasound system. For maximum system performance and image
Although in-body and patient-
we get technical
52
53
Powered by FlippingBook