L3: Beamforming, Focusing, Steering, Ultrasound Modes Flashcards
A single point of an ultrasound source will radiate
isotropically - in three dimensions e.g. speakers
How can we modify the beam?
- Beam stepping (scanning across the field)
- Beam focussing (for lateral resolution)
- Beam steering (to move beam without moving probe)
Why do we not want a curved shape
As the wave spreads, it will be longer for the time to come back - different times the wave will hit the boundary of surface - hitting at the same time
Where waves have the same phase, they will ___
interfere constructively
When they are 180 out of phase, they
_____
interfere destructively
As we increase the number of transducers,
Interference causes the wavefront to become flatter
In the centre of the beam, the intensity
becomes
stronger (more directional) with
each emitter
Past a certain point,
more transducers just broaden the beam without any benefit
To produce a parallel sided US beam, the active transducer width (apenture) must be
at least 10 times the ultrasound wavelength
* Around 10 crystal elements wide
To create an ultrasound (B-mode) image across an entire field,
the beam must be stepped.
When focussing, where is the superior lateral resolution
Ultrasound beams are focused to give superior
lateral resolution at a desired point
Transducer focussing can either be
- Fixed
- Adjustable (we want to choose where focal point is at)
What is affected when we adjust the transducer focussing?
Lateral resolution and slice thickness are affected.
Fixed Focussing (lens)
a) An acoustic lens in front of the element
(b) A concave piezoelectric element
(c) An acoustic mirror behind the crystal
Narrow the beam into that region
Dynamic Aperture
Focussing is where
we can adjust electronically the focal point
How do we change Dynamic Aperture
Focussing?
By changing the number of crystal eleents activated at a single time, which changes the size of the beam at the source and increases the NEar Zone Length
We increase the transducer which
causes the NZL length to increase and the focal point further away, beam width to also increase,
Shorter delay gives longer
the focal depth (less focused)
Greater delay gives
shorter the focal depth
(more focused)
Why do we have a time delay in phase focussing?
Introduced between electrical pulses to each crystal across the array.
* The outer crystals will be pulsed first.
* Other elements are then pulsed in sequence, working
toward the centre of the array
* The centremost element will receive the last pulse.
Weak focusing →
Weak focusing → longer focal zone → greater focal depth
Strong focusing →
Strong focusing → shorter focal zone → shorter
focal depth
Array type transducers (only) can be adjusted to _____
focus to a specific depth for each transmitted pulse
Beam steering
By changing the phase of the waves relative to each other, we can change where the waves interfere the most
