Beamforming Flashcards
What does the shape of an ultrasound beam depend on?
- The a/lambda ratio (a is the half aperture)
- The radius of curvature (R or F).
Explain how transmit focusing is achieved electronically.
- Pulses should arrive at the focus at the same time.
- Be delaying the pulse incrementally towards the centre of the aperture, the increased path length at the edges means that the pulses arrive at the focal point at the same time.
- The same delays can be applied to the received pulse aswell.
How is transmit steering achieved electronically?
- Incremental delays across the elements.
- More distal elements have a longer path length, but are fired earlier, meaning the pulses all arrive at the focus at the same time.
- Same delays can be applied for received pulses so that they all reach the signal summer together.
How does the beam width change for a steered beam?
- Beam width is proportional to F*lambda/a
- A steered beam has a smaller effective aperture, and so a wider beam width at the focus.
- a_eff=a*cos(theta)
How is dynamic receive focusing achieved?
- A small aperture will focus at shallow depth, a large focus deep depth.
- By increasing the aperture systematically the focus can be moved in line with the received echo depth.
- This keeps the beamwidth narrow for a much longer distance, improving the lateral resolution.
Explain how multiple-zone transmit focusing is performed.
- Multiple apertures are used to transmit the beam, with the focus at certain depths. (small a = shallow depth)
- Improves the lateral resolution at those depths as the beam is narrower, but reduces the frame rate as multiple transmissions are required for a single image.
What is beam apdoisation?
- Instead of driving each element with the same voltage, a hamming source profile is used (lower voltages to outer elements)
- Hamming profile = a-bcos[2pi*n/(N-1)]
{a = 0.54, b=1-a=0.46}
How does beam apodisation affect the source profile?
- Side lobes are lower, but the width of the main lobe is greater.
- At the focus, the beam profile is the fast-Fourier transform of the source profile.
How does beam apodisation effect the image quality?
- Cystic objects are visible over a greater range of depths.
- Interiors and edges of the objects are clearer.
What is multi-line beam forming?
- Tow or four beams can be received from a single transmission.
- Using multiple apertured for the individual receive beams processed in parallel.
- Gives a high line density and a high frame rate.
How do grating lobes occur?
- To pulses, either received or transmitted, that differ by a path length equal to the wavelength, will constructively interfere causing a signal (received) or a beam (transmitted).
- For this to occur the pitch, p, bust be greater than lambda/2.
- The angles they occur at are given by nlambda = psin(theta)
Why are grating lobes more of a problem for a phased array?
- When the beam is steered, one of the grating lobes can be directly in front of the transducer.
- The signal from this lobe is stronger than normal grating lobes and so the SNR is worse.
At what pitch are grating lobes strongest?
p=lambda/sqrt(2)
How are elements arranges in a 1.5D array?
- Three or five rows of elements in the elevation direction.
- Rows are not completely independent, (i.e. rows 1 &5, 2&4, and 3 can be fired separately).
- Still requires a lens for additional elevation plane focusing.
How are elements arranges in a 1.75D array?
- Three or five rows of elements in the elevation direction.
- Rows are completely independent.
- Still requires a lens for additional elevation plane focusing.
