Ultrasound fields: Arrays Flashcards
What type of transducers are used in diagnostic imaging?
Array transducers which have multiple elements (share ground electrode and matching layers)
(made of PZT)
What are array dimensions characterised by?
pitch (from centre of one PZT to another)
element width
kerf width (distance between elements)
elevation height (of elements)
How are arrays pulsed?
In groups large enough to get long near field zone
What is the length of the near field zone?
a^2 / λ
(a = half the length of group)
What does pulsing outer elements cause first?
Beam to converge
Why are time delays introduced?
To electronically focus the transmit beam along the scan line
Reduces beam width to increase lateral resolution for a specific depth range
(same applies for beam steering)
How are rectangular elements different to circular elements?
Fields are similar in focal region but differ in near field
What does azimuth plane define?
The lateral image resolution
What does the elevation plane define
Slice thickness defined by beam dimensions in the orthogonal
What is the slick thickness when there is no focusing in elevation plane?
In near field S ≈ b
In divergent far field, it is defined by divergence angle
(b = element length)
What is the slice thickness when there is focusing in elevation plane using a lens?
S ≈ 1.41λF/b
Length of focal zone: D ≈ 9.7 λ(F/b)^2
What do phased array transducers produce?
Grating lobes due to the finite spacing between the elements
When do grating lobes appear?
Path-length difference between adjacent elements is equal to wavelength
How can effects of grating lobes be reduced?
By reducing element pitch (d)
If d is small enough, the path-length difference between successive elements < λ so no grating lobe
What is the requirement for avoiding grating lobes?
d < λ/2
When does grating lobe angle increase?
As element spacing is reduced
What happens when kerf width is decreased?
Energy in side lobe decreases compared to main lobe
What are linear array and curvilinear array made up of ?
Elements used in groups and beam is electronically focused
Linear used for: vascular imaging
Curvilinear used for: abdominal imaging
What is sector array?
All elements used simultaneously and beam is electronically focused and steered
used for: cardiac imaging
What can endocavity and endoscopic array be?
Either linear, curvilinear or sector transducers
Used for transvaginal imaging
What are intravascular arrays?
Can be multi-element, or single element with rotating transducer or acoustic mirror
What is a 3D imaging probe?
Can be made from 2D sector array, or moving 1D array
