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 slick 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
