1What is a pulse?
A collection of cycles that travel together as a single unit
2What are the two components of pulsed ultrasound?
Transmit (talking, "time on") and receive (listening, "time off")
3What does the pulser do?
Produces the electrical waveforms that drive the transducer
4What does the master synchronizer do?
Maintains and organizes the proper timing and interaction of the system's components
5What is time of flight (go-return time)?
The elapsed time from pulse creation to pulse reception; 13 µs per cm of depth
6What is wavelength?
The distance one cycle occupies in space; λ = c ÷ f; mm
7What is period?
The time of one cycle; P = 1 ÷ f; µs
8What is pulse duration?
The time from the start of a pulse to the end of that pulse (talking time); PD = P × # cycles; µs
9What is pulse repetition period?
The time from the start of one pulse to the start of the next (PD + listening time); PRP = 1 ÷ PRF; ms
10What is pulse repetition frequency?
The number of pulses occurring in one second; PRF = c ÷ 2D; Hz or kHz
11What is spatial pulse length?
The distance a pulse occupies in space from start to end; SPL = λ × # cycles; mm
12What is duty factor?
The fraction of time the system is transmitting a pulse; DF = PD ÷ PRP; unitless; ideal 0.1–1%
13What is the duty factor of continuous wave?
100%, because the system is always transmitting
14How many cycles are in a typical imaging pulse?
2–4 cycles
15Which parameters change when imaging depth changes?
PRP, PRF, and duty factor (only listening time changes)
16What determines SPL?
Both the source and the medium
17What does a transducer do?
Converts one form of energy into another
18What happens during transmission?
Electrical energy is converted into sound
19What happens during reception?
The reflected sound pulse is converted into electricity
20What is the piezoelectric effect?
Mechanical stress (compression) applied to PZT generates electricity
21What is the reverse piezoelectric effect?
A voltage applied to PZT makes it change shape (electrical energy to mechanical energy)
22What is PZT?
Lead zirconate titanate; the most common piezoelectric material
23What is polarization (poling)?
During manufacturing, PZT is heated to the Curie point in a magnetic field so its dipoles align
24What is the Curie point?
The temperature (about 328–365°C) that PZT is heated to during poling
25What is depolarization?
Heating PZT to the Curie point again after manufacturing, which makes it lose its piezoelectric properties forever
26Why should a transducer never be autoclaved?
Heat causes it to lose its polarity (depolarize)
27List the 7 components of a transducer
Case, electrical shield, acoustic insulator, PZT (active element), wire, matching layer, backing material
28What is the case?
A plastic, hermetically sealed unit that protects the operator from accidental shock
29What is the electrical shield?
A thin metallic lining inside the case that keeps outside electrical signals out; prevents noise
30What is the acoustic insulator?
Rubber or cork that prevents vibrations in the case
31What does the wire do?
Provides an electrical connection between the PZT and the US system
32What is resonant (natural) frequency?
The natural frequency at which a crystal vibrates at its highest amplitude; determined mainly by thickness
33What is the ideal crystal thickness?
½ wavelength
34How are PZT thickness and frequency related?
Inversely; thinner crystal = higher frequency
35How are speed of sound in PZT and frequency related?
Directly; faster speed = higher frequency
36What two characteristics determine the frequency of a pulsed wave transducer?
Speed of sound in the PZT and thickness of the PZT
37What is the matching layer?
Layer in front of the PZT that increases the efficiency of sound transfer into the body and protects the PZT; ¼ wavelength thick
38What is the purpose of the matching layer?
To reduce reverberations and provide efficient transmission of sound from the PZT to soft tissue
39Why is the matching layer needed?
PZT impedance is about 20× greater than skin; without it, most sound would reflect back into the PZT
40What is the backing (damping) material?
Material immediately behind the PZT that reduces ringing and shortens the pulse