Ch 7 Range Equation & Ch 8 Transducer Basics Flashcards
What is time of flight also called?
go-return-time
What is time of flight?
Elapsed time from pulse creation to pulse reception
How is time of flight related to depth?
Directly, shallow imaging = shorter ToF, deep imaging = longer ToF
How do you calculate depth from go-return time?
1.54 (mm/us) x go-return time (us) / 2
When does the 13 microsecond rule apply?
Whenever sound is traveling through soft tissue
What is the 13 microsecond rule?
For every 13 us of go-return-time, the object creating the reflection is 1 cm deeper into the body
If GRT is 26 us, how deep is the structure creating the reflection?
2 cm deep
If GRT is 26 us, what is the total distance traveled by the pulse?
4 cm
How is PRP related to go-return time?
PRP is the go-return time of a sound pulse between the transducer and the bottom of the image
How is PRP related to depth?
Directly related
If depth is 10 cm, how long is the PRP?
130 us
If depth is 5 cm what is PRP?
5 x 13 = 65 us
How far can sound travel in 1 second?
To & from a depth of 77,000 cm, or to & from a depth of 7.7 cm 10,000 times
How is PRF related to depth?
Inversely
When depth is shallow, PRF is
High (can travel to & from depth more times/sec)
When depth is deep, PRF is
Low (to & from depth less time/sec)
How is PRF calculated using depth?
PRF = 77,000 (cm/s) / depth (cm)
What is a transducer?
Any device that converts one form of energy to another
How does an US machine convert energy?
Electrical to acoustic
What are the two functions of transducers?
Transmission and reception
What is transmission?
Electrical energy from the system is converted to sound
What is reception?
Reflected sound pulse is converted into electricity
What is the piezoelectric effect?
The property of certain materials to create voltage when they are mechanically deformed or when pressure is applied to them
What is reverse piezoelectric effect?
Materials change shape when voltage is applied
What are piezoelectric materials AKA?
Ferroelectric materials
What are piezoelectric materials?
Materials which convert sound into electricity and vice versa
How are piezoelectric materials sourced?
Some are natural ex. Quartz, tourmaline, in US mostly synthetic
What is a common synthetic piezoelectric material used in US?
Lead zirconate titanate (PZT)
What are the 7 components of a basic transducer?
- case
- electrical shield
- acoustic insulator
- PZT/active element
- wire
- matching layer
- backing layer
What does the transducer case do?
- protects the internal components of the transducer from damage
- insulates the patient from electrical shock
What is the electrical shield?
Thin metallic barrier lining inside of case
- prevents electrical signals in the air from contaminating clinically important signals
What is the acoustic insulator?
Cork or rubber layer, isolates internal components from case and prevent vibrations in the case from inducing electrical voltage in PZT
What results in reflections at boundaries?
Differences in impedance
The impedance of PZT compared to impedance of skin is
20 times greater
If sound traveled directly from the PZT to skin what would happen?
Majority of sound would reflect back into PZT, only a small fraction would transmit, resulting in extremely limited clinical imaging
What does the matching layer do?
It provides an impedance in between the active element and skin
How does the matching layer affect reflections?
decreases reflection at the boundary of PZT and skin, increases transmission
What does gel do?
Provides an impedance in between the matching layer and skin, “coupling” transducer to patient
Order in decreasing order of impedance:
Gel, matching layer, PZT, skin
PZT > matching layer > gel > skin
How thick is the matching layer?
1/4 wavelength thick
How thick is the active element?
1/2 wavelength thick
What is backing material AKA? What does it do?
Damping material, keeps pulses short
What would happen without backing material?
PZT will vibrate longer than it should
If the PZT vibrates for too long how does it affect the image?
Pulse has increased length and duration, image has lower resolution and accuracy
What are the characteristics of damping material?
- high degree of sound absorption
- acoustic impedance similar to PZT
Why does the backing material need to have similar impedance to PZT?
Sound pulse moves from PZT into backing material where it is absorbed
What are three consequences of using backing material?
- Decreased sensitivity
- Wide bandwidth
- Low quality factor
Why does backing material cause decreased sensitivity?
- reduces PZT vibration during transmission and reception
- makes transducers less responsive to small sound reflections returning from body
What is bandwidth?
Range of frequencies in the pulse
How does backing material create wide bandwidth?
PZT is restricted from vibrating freely, short duration sound click is released containing sound at many different frequencies
What kind of bandwidth do continuous wave transducers produce?
Narrow bandwidth due to lack of backing material
If a transducer with a 3 MHz frequency produces a pulse ranging from 1-5 MHz, what is the bandwidth?
5-1 = 4 MHz
Long duration events are ________ bandwidth, short duration events are _______ bandwidth.
Narrow, wide
What is quality factor?
A unitless number inversely related to bandwidth
How is quality factor calculated?
QF = main frequency / bandwidth
Why does backing material cause low quality factor?
Backing material causes wide bandwidth which results in low QF
Which probes have a low Q factor? Which probes have high Q factor?
Wide bandwidth, narrow bandwidth
