System Architecture - Frontend
In this problem we look at the paper written by Shafer, The Influence of Front-End Hardware on Digital Ultrasonic Imaging and get to know a bit more on the system Front-End.
Related materials:
by Lasse Løvstakken (lasse.lovstakken@ntnu.no) 12.11.2013
Contents
Front-end overview
- Draw a block diagram of the typical front-end of a modern digital ultrasound system and explain shortly the main function of each block
Transmit chain
- What kind of pulse generators are used in ultrasound imaging systems?
% Bipolar (+1,-1) or tripolar (+1,0,-1), arbitrary waveform generators % (state of the art)
%* *What benefit could you have from transmitting an accurately defined %pulse?* % * Only emit in the transducer band, minimum waste of energy to heat % * No emission of second harmonic (for harmonic imaging) % * Advanced: multi-pulse transmission (MLT), pulse inversion (cancellation % techniques)
- How does the transducer influence the shape of the acoustic pulse?
% It acts as a band pass filter, smoothing out sharp edges and turning the % input into a tapered sinusoidal shape
- In what aspects does the transmit chain influence the ultrasound image quality?
% * Ringing in the pulse means that you degrade the axial resolution % * Insufficient amplitude means that you degrade penetration and also reduce % the ability to do harmonic imaging % * Transmitting outside the transducer band increases heating at the probe % surface, this is very often a limiting factor which needs to be minimized
Receive chain
- Why do we need preamplifiers in an ultrasound system?
% Weak echoes, low voltage (micro-Volt). Need to bring the signal up to % standard levels
- Why do we need depth-dependent amplification?
% Pulse is attenuated as it propagates into the tissue
- What determines the dynamic range in ultrasound imaging?
% * The dynamic range is the relative difference between the weakest and % strongest echos. Due to depth dependent attenuation the dynamic range can % be ~100dB. % * The final dynamic range is determined by the resolution of the % A/D-converter, e.g. 12 bit => 12*6dB = 72dB theoretical dynamic range. % * Some extra dynamic range can also be gained through oversampling and in % the coherent summation (lowers the noise floor).
What are the typical requirements of the analog-to-digital converter in an ultrasound system?
% * Low-end: 12 bit 40MHz % % High-end: 14 bit, 62.5MHz
- What is electrical matching towards the transducer, and why is this important?
% * There is a mismatch in electrical impedance between the cable and % transducer elements, leading to suboptimal transfer of signal energy % * By tuning out this mismatch (typically capacitive) through the addition % of a coil (inductance), a more “effective” system is achieved, e.g. % improved signal-to-noise. % * Most important on receive side to maximize SNR
- In what aspects does the receive chain influence the ultrasound image quality?
% * Noise introduced by amplifiers and A/D-converters limits SNR and % detectability % * The limited dynamic range of A/D converters used infers that signal % compression is needed (through depth-dependent preamplification) % * The number of channels sets a lower limit on the F-number / aperture % that can be achieved, i.e. influences lateral resolution.
Transucer
- Sketch the general modern transducer stack (the mechanical part).
- What kind of material is typically used in modern ultrasound transducers?
% Piezo-composites, e.g. PZT
- What acoustical impedance?
% The acoustical impedance is the (complex) relationship between pressure and (particle) velocity
- Why is the impedance of piezo-composites an issue?
% Because it is very high (stiff) compared to the tissue, causing a % mismatch that results in reflected waves at the interface, i.e. an % inefficient transfer of energy.
- What determines the acoustical impedance?
% * The properties of the tissue, namely: density and speed of sound % * The transducer load is also determined by the area of the transducer % surface, which determines the effective mass that needs to be moved
- How is acoustic matching typically done?
% By introducing (quarter-wave) matching layers in between the % piezo-element and tissue, which has an impedance in between the two, Zm = % sqrt(Zpiezo*Ztissue)
Phased-array systems
- What is particular for a phased-array ultrasound system?
% * That both focusing and (continuous) steering is done electronically, % resulting in a sector image format % * Fine steering delays are needed
- What transducer design characteristics are important for phased-array operation?
% The pitch needs to be sufficiently small to avoid grating lobes, i.e. % less than lambda/2 for a steering angle of +-45deg.
- What front-end characteristics are important to consider for a phased-array system?
% * Smaller elements require more channels % * Transmit delay quantization must be sufficiently small, i.e. the clock % rate of transmit beamformer must be sufficiently high % * Smaller elements means higher electrical impedance for each piezo-element, % electrical impedance matching may be more crucial % * Element directivity is given by the element width. The directivity % pattern needs to be sufficiently wide to allow for large steering angles, % typically +-45 deg for lambda/2 pitch probes.
- What are delay quantization artifacts?
% * The limited discretization of delay values can lead to the same delay % value applied to multiple elements % * This means that these elements effectively appear as one large element, % leading to what appears as grating lobes, often also called quantization % sidelobes due to their origin. % * Mostly a problem for fine steering such as in phased-array operation