Pulsed Ultrasound Flashcards
What happens in a pulsed system?
An image is produced by the transducer by pausing or “listening” after transmitting.
How many ways can a pulse be generated?
Two
What are the two ways a pulse can be generated?
1) The electrical output from a generator is gated to apply a rapidly alternating voltage to the crystal
2) A charged capacitor is discharged through the crystal which is simulated to vibrate at a resonance frequency
What does a capacitor do?
It stores a charge
How does pulsed ultrasound differ from continuous wave ultrasound.
In continuous wave ultrasound the crystal never stops vibrating, the transducer never pauses to “listen” for the returning reflectors and therefore cannot produce an image.
What does PD mean?
Pulse Duration
What is pulse duration?
Pulse duration (PD) is the time it takes for one pulse to occur.
What is the formula for PD?
PD = T x Cycles in a pulse
What is the unit of PD?
μs
What is RD?
Ringdown
What is Ringdown?
Cycles in a pulse
What is SPL?
Spatial Pulse Length
What is Spatial Pulse Length?
The length of the pulse (distance).
What is the formula for SPL?
SPL = Wavelength x Cycles in a pulse
SPL = λ x RD
What is the unit of SPL?
mm
How does frequency effect wavelength?
They are inversely related. Increasing the frequency decreases the wavelength, decreasing the frequency increases the wavelength.
How does frequency effect SPL?
Increasing frequency decreases wavelength, which therefore decreases the SPL (SPL = wavelength x RD)
How does decreasing the cycles in a pulse effect SPL?
Decreasing the cycles in a pulse (RD) would also decrease SPL
(SPL = λ x RD)
How does SPL effect an image?
SPL effects resolution.
Decreasing the SPL will improve an image.
How would a shorter pulse effect the image?
A shorter pulse will give better resolution.
To get a shorter pulse would you increase or decrease SPL?
Decrease
To get a shorter pulse would you increase or decrease RD?
Decrease
What is PRF?
Pulse Repetition Frequency
What is Pulse Repetition Frequency?
The number of pulses emitted in one second.
How many pulses to ultrasound probes emit per second? (Roughly)
A few thousand
What is the unit of PRF?
Hz or kHz
What is the formula for PRF?
PRF = 1/PRP
What is the formula for frequency?
f = 1/T
How is the frequency of a probe and the pulse repetition frequency (PRF) related?
They aren’t.
What is PRP?
Pulse Repetition Period
What is Pulse Repetition Period?
The time from the beginning of one pulse to beginning of next.
How is PRP and “go return” related?
They are the same.
What is the formula for PRP?
PRP = D/C
Same as Range equation
What is the unit of PRP?
s or μs
How are PRF and PRP related to frequency and period….and therefore eachother?
PRP is a period, PRF is a frequency and therefore they are inverses of eachother just like period and frequency.
What is the formula for PRF?
PRF = 1/PRP
Or
PRF = 1/(D/C) therefore PRF = C/D
What does D mean?
Depth
What is DF?
Duty Factor?
What is Duty Factor?
The fraction of time that pulsed ultrasound is “on”.
What is the unit of DF?
No unit.
It’s a percentage. %
What is the DF of continuous wave?
100% (on all the time)
How does a higher PRF effect DF? Why?
It increases the DF because there is less “listening time” between pulses.
How does a higher PD effect DF? Why?
It increases the DF because there is less “listening time” between pulses
What is the formula for DF?
DF = PD/PRP x 100
What happen to the units of PD and PRP in the Duty Factor equation?
Since they are both μs they just cancel out.
Does Pulse Duration include dead time?
No
What properties determine velocity?
The density and compressibility of the medium
What is density?
Density is the mass of a medium per unit volume (or the concentration of matter).
If density is the ONLY factor that changes and it increases, what happens to the speed of sound?
It decreases.
Why does the speed of sound decrease when density increases? (If density is the only factor that changes).
An increase in density is an increase in mass, more mass requires more energy for particle motion.
What is compressibility?
The fractional decrease in volume when pressure is applied to a material.
(OR how easy it is to squish)
Is the velocity slower or faster in an easy to compress material?
Slower
When a material is stiff the sound (energy) transfers how?
Very well, quickly.
Do dense or squishy materials transfer sound better?
What’s an example?
Dense materials do.
Ex. Bone, has low compressibility and therefore increased sound velocity.
What is the more important (dominant) factor in sound velocity, compressibility or density?
Compressibility is dominant.
What is Bulk Modulus?
It is the opposite (reciprocal) of compressibility.
OR “stiffness”
What is the negative ratio of stress and strain?
Bulk modulus
As bulk modulus increases, the compressibility…?
Decreases.
As bulk modulus increases, velocity….?
Increases.
What is elasticity?
The ability of an object to return to it’s original shape and volume after a force is no longer acting on it.
How does ultrasound effect elasticity?
Ultrasound waves cause elastic deformation by separation and compression of molecules.
Summarize the relationship between density and stiffness?
Give an example
Typically an increase in density is accompanied by an increase in stiffness with stiffness being the dominant factor.
Ex. Bone is more dense than soft tissue but it is also more stiff.
What is the sound velocity in air?
330 m/s
What is the sound velocity in bone?
4080 m/s
Compare air and bone to determine which would have the higher sound velocity and explain why.
Bone is much denser than air, it is also much more stiff therefore the sound velocity in bone is much higher than in air.
Compare the sound velocity between water and mercury, explain.
Mercury is much more dense than water.
We are told that mercury is much more dense than water but in order to determine whether is has a higher sound velocity than water we have to think about stiffness. Do water and mercury have different compressibility? No, they are both liquidy and similar so since stiffness is the dominant factor and it is the same, the only thing changing is the density. When ONLY the density increases, the sound velocity decreases, therefore the sound velocity in mercury decreases so water has a higher velocity.
How does temperature effect the speed of sound?
As temperature increases the velocity of sound also increases.
Why does temperature not effect the speed of sound when we are scanning the human body?
The human body has little temp variation so it’s not an issue when scanning patients.
What is important regarding temp and scanning phantoms?
Phantoms must be scanned at a consistent temp since a one degree change will alter the velocity of the sound.
