Beam forming Flashcards
What does a beam former determine?
- Shape, size & position of beam (controlling signals to & from the probe)
In beam forming, what happens in transmission? and reception?
T = signals generated to drive each element R = combines echo sequence from ALL elements to 1 sequence
What 2 things are required to be processed before a beam former is used?
1) amplification
2) digitisation
How many elements does a linear array typically have? (common number)
128
Why is it typically 128 or 256 and not ‘rounded’?
due to the binary processing, easier for digital control and processing
All transducer dimensions are proportional to the wavelength, what does this mean for high f transducers?
- high f transducers are smaller than low f
Why do the rear electrode have separate electrical signals compared to front?
- allows signals to be made to and from each element individually
What is this image showing?
IMAGE HERE - linear array transducer active group of elements
Theres only 1 active group of elements at one time. 1 element at the end is dropped and another is added at the front.
In active group of elements, what is less vs transmission?
few elements used for reception (R) vs transmission (T)
When would the reception element number increase?
if R echoes are from a deeper target and eventually exceed # for T.
When 1 element is dropped and another added, a new scan line is interrogated with new R & T, centred on the line. When is this process repeated until?
all scan lines across FOV are interrogated & a whole sweep along the array is performed
What is the beam width/shape? and what will reduced BW do?
1) size of the beam in T & R
2) increase lateral res
What is FOV?
physical region interrogated by consecutive ultrasound beams which corresponds to the image
What governs the FOV?
arrangement of elements
Describe element excitation?
ADD iMAGE?
- elements are individually excited via the separate/ individual electrode (rear)
- element groups are used to form the beam
- elements maybe excited at different times
What does interference of the waves play a role in?
different points of the transducer surface plays a major role in shaping the resultant beam
What is ‘Constructive interference’?
waves are in phase, amplitude at a point is the sum of 2 waves
What is ‘Destructive interference’?
out of phase, and either partially or completely cancel each other out.
In transmission, the operate sets the focus (transmission focus) at depth optimum for lateral resolution. Why?
Ensure transmission beam is narrow as possible
What is needed to form a large amplitude pulse?
all pulses arriving at the same time in order to concentrate power into narrow ‘focal zone’.
What area of elements need to be transmitted earlier and why?
outside ones, to ensure all pulses arrive simultaneously
What happens to points outside the focal zone? (manufacturing)
different elements arrive at different times producing a weak acoustic noise
What is this picture showing (effects of transmit delay slide. Pic. A)?
outer fired before the inner elements, creating a focused US beam. All US waves overlap
What is this picture showing (effects of transmit delay slide. Pic. B)?
- dynamic focusing (phased array)
- flexibility for making beam shapes and sending sound waves to the direction desired.
How is BW made narrower?
increase # of transmit focal zones
What does the increasing # of transmit focal zones effect?
effecting FR = > focal zone = reduced FR
What does variable delay lines allow?
focusing at different depths
How can lateral res be improved?
increasing the focus (beam)
what improves lateral res over a large area of an image?
multiple focal zones
having multiple focal zones can do what?
New pulse is required to create a new focal zone
- decrease FR
- decrease temporal res
What is coprocessing allow?
data from 3 focal zones (collected & processed in sync) for 3 nonadjacent lines of sight
Why is coprocessing used?
- data (from all 3) can be displayed in 1
- high FR maintained
Describe this image
Delay and sum method
1) electrical signals delayed by all element apart from outside = imposed signals > elements closer to centre
2) outside: > delay for elements closer to centre of active group (signals align in phase at summing point w. large signal obtained from desired receive focal zone)
3) weak summed signal (acoustic noise) produced from echoes elsewhere
with R focusing is this usually an automatic approach?
Yes w/ no reception receive focus control
(DYNAMIC FOCUSING & APERTURE)
–> ideal depth for R focus = depth of original ( at echoes arriving)
the effective BW is a product of what?
T focal zone and R BW
IMAGE
in dynamic focusing and aperture, making the scanner sensitive at specific depth (R focus) results in what?
- R beam narrowed
- improved lateral res
What is required for the R focus sensitivity to remain high?
elements in active group remain simultaneously for result electronic echo signal.
How are the outside elements focus and R vs centre elements controlled?
Figure 3.18
electronically delaying echo signals (centre)
DELAY and SUM METHOD
when the delay and sum method (at desired focus) is used, when would weak signals appear?
from echoes elsewhere
What does the US machine automatically change with using dynamic focusing & aperture?
IMAGE 3.19
delays & aperture = R focus advances
Why is there no receive function available to the operator?
because ideal depth (R focus) is DEPTH ORIGIN
In dynamic focusing & aperture, if aperture expands what happens to BW and foci?
BW & all foci expand as well and remained constant
What happens if it stops expanding?
deeper foci become greater.
why is there no point using large group of R echoes at superficial targets?
elements far from the centre of the group wouldn’t receive them
IMAFE 3.20 (Used for deeper targets)
What is multiple-zone focusing?
IMAGE 3.22
- sub-divides each scan line into 2 or more depth zones.
- interrogates each zone with a separate T pulse
- stays focused at the centre
Whats the advantage of using multiple-zone focusing?
increased lateral res
When is a narrow effective transmission beam seen in multiple-zone focusing? What can this also do?
- the greater number of transmission focal zones w/ greater depths
- lower FR - longer duration
Why is parallel beam forming used?
- dynamic focusing used in R - effect R beam is narrower than transmit/T beam
- Transmit beam - accommodates 2 R beams
- sharing the same pulse (targets on 2 adjacent lines) - processed together
Where would parallel beam forming be used?
Cardiac imaging - high FR required.
What is Plane wave imaging?
IMAGE 3.38
plane waves pulses transmitted as wide as FOV to interrogate many/all scan lines simultaneously.
What is the key benefit to Plane wave imaging?
- high FR - FOV interrogated within 1 transmit receive cycle
With high FR what disadvantage does it come with?
1) poor lateral res because width of transmit beam
2) high acoustic noise from scatters from sides of R beam axis
High acoustic noise leads to what?
1) poor contrast res
2) penetration & sensitivity - further reduced w/ amplitude
What properties does Phased array have?
- produced ‘sector’ scan format (point at centre)
- small footprint
- narrow elements - don’t need to be sub-diced like wide lineary array elements
- ALL elements caused for T and R
