Ultrasound technology, a widely used non-invasive tool in medical diagnostics and other applications, has shifted from static to dynamic images, and from black-and-white presentation to color Doppler images. These important enhancements are largely due to the introduction of digital ultrasound technology. While these advances have increased the effectiveness and versatility of ultrasound imaging, it is equally important for these systems to offer improved image quality via advances in the head- end ultrasound probe, and the analog front-end (AFE) that drives the probe and captures the return signals. One of the impediments to achieving this improved image quality is noise, so the design goal is to increase the signal-to-noise ratio (SNR) of the system. This can be achieved in part by addressing noise due to the various power supply rails in the system. Note that such noise is not a single, simple entity. Instead, it has various characteristics and attributes which determine how it ultimately impacts system performance. This article will look at the basic principle of ultrasound imaging, and then focus on different factors that affect image quality, primarily noise from the power supplies. It will use DC-DC regulator devices from Analog Devices as examples of power supply components that can greatly improve SNR and other
aspects of ultrasound system performance.
Basics of ultrasound imaging
The concept is simple: generate a sharp acoustic pulse, then “listen” for its echo reflection as it encounters obstacles or various interfaces between organs and their differing acoustic impedances. By doing these impulse-return sequences repeatedly, the reflections can be used to create an image of the reflecting surfaces. For most modes of ultrasound, the array of piezoelectric transducers sends a limited number of wave cycles (typically two to four) as a pulse. The frequency of these waves in each cycle is usually in the range of 2.5 to 14 megahertz (MHz). The array is controlled via beamforming techniques analogous to a phased-array RF antenna, so the overall ultrasound pulse can be focused and steered to create a scan. The transducer then switches to receive mode to sense the return of the reflected waves from within the body. Note that the transmit/receive timing ratio is typically about 1%/99%, with a pulse repetition frequency usually between 1 and 10 kilohertz (kHz). By timing the pulse from its transmission to received echoes and knowing the speed at which the ultrasound energy propagates through body
How to improve ultrasound system image quality using ultra-low-noise supplies Written by Bill Schweber
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