Physics 2 - Module 1 & 3 Flashcards
Most common material used to for the transducer crystal
PZT
Occurs when a mechanical pressure deforms the crystal, changing the orientation of the dipoles, and produces a small electrical voltage
Piezoelectric effect (direct)
Occurs when an electrical voltage changes the orientation of the dipoles, causing the crystal to expand and contract
Reverse piezoelectric effect (indirect)
Which mode of vibration is most desirable?
Thickness
What is the Currie temperature for PZT?
350
The frequency at which the crystal likes to ring at. It is determined by the crystal material and thickness. AKA fundamental frequency
Resonant frequency
Frequency determined by the AC voltage sent to the crystal. If the voltage is altered, the crystal is forced to ring at a frequency different then the fundamental
Driving frequency
The frequency you are using to scan
Operating frequency
Two times the resonant frequency
2nd harmonic
How to determine the thickness of the crystal
=1/2 wavelength
How does crystal thickness affect frequency?
Thicker crystal = lower frequency
What is the purpose of the backing/damping material?
Reduce the SPL which will improve axial resolution. Absorb sound so that reflections don’t occur from behind the crystal.
What else does the damping material do that is not a good thing?
Reduces the amplitude therefore reducing sensitivity
As we increase the amount of damping material, what happens to the length of the pulse?
It becomes shorter
What is the Z value of the damping material?
Similar to the crystal for absorption to occur
How to have a probe that is capable of 2D and doppler? (2-3 cycles per pulse vs. minimum of 4)
Dynamic damping
What does the matching layer do?
Has a Z value between that of the skin and the crystal to help sound get into the patient.
What is the thickness of the matching layer?
1/4 wavelength
How is bandwidth related to SPL and resolution?
The wider the bandwidth, the shorter the SPL and the better the resolution.
A wider bandwidth also means more frequencies to choose from
Is low Q factor good or bad?
It is good.
Using nanosecond time delays to change when the voltage excites the crystal.
phasing
What is the purpose of phasing?
Controls the beam and allow for focusing and steering
What is segmental sequencing?
Firing groups of crystals at a time so that information is not missed.
How to correct for grating lobes?
Apodization
The voltages to the crystals are gradually weakened from the inside out
Apodization
Near fields have __________ beams due to interference between the wavelets and far fields have ________ beams.
Non-uniform, uniform
Grating lobes are a result of what kind of vibration?
Length and width vibration resulting in crosstalk
Equal to 2 NZL’s
Usable beam shape
What happens to the NZL as the frequency or diameter of the crystal increases?
It also increases
What happens to the angle of divergence in the far field if the frequency or diameter of the crystal increases?
It decreases
Increasing the delays in transmit focusing does what to the focus?
Increases it
Bringing received echoes into phase so that they don’t cancel each other out.
Dynamic receive focus
What determines the width of the beam?
Aperture, frequency, distance from transducer face
