CVT 100 #9 Ultrasound Flashcards
Sound is
particles in a medium vibrating around a rest point.
It is energy traveling through a medium.
How is sound propagated?
Particles are disturbed by the sound source. This disturbance is transmitted to adjacent particles, and so forth. In this way the sound is propagated.
What does sound require to be a sound?
Sound requires a source of disturbance and a medium to travel through (no medium—no sound, e.g., a vacuum)
Production of sound:
Source vibrates, disturbs particles, creates ACOUSTIC PRESSURE
There is no net movement of the particles in the medium. What moves through the medium?
It is the energy that moves through the medium, leaving the particles where they were
Does the water move?
Nope—just the energy.
The energy is propagated by?
The region of acoustic pressure is transmitted to adjacent particles in the medium— the energy is propagated.
This creates areas in the medium of COMPRESSION AND RAREFACTION
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These are regions of higher and lower acoustic pressure, due to higher and lower concentrations of particles.
Wave:
A propagating (i.e., traveling) variation in quantity.
Sound is a
propagating variation in acoustic pressure.
Two types of waves:
Longitudinal and Transverse
Transverse wave:
moves across (90° to) direction of propagation
Longitudinal wave:
moves in the direction of (axial to) propagation
What type of wave is sound?
Sound is a longitudinal wave. The acoustic variations occur along the direction of travel.
The changes in acoustic pressure can be graphed as?
The changes in acoustic pressure can be graphed as a sine wave.
Areas of higher pressure that go above the baseline are:
compression
Areas that go below the baseline are:
rarefaction
What does zero represent?
the rest point of the particles in the medium
In that sine-wave graphical depiction of sound, the positive peak represents _______________, the negative peak represents ___________, and the zero represents the rest point.
Compression
Rarefaction
Waves move in cycles:
Repetitions of the physical changes in the medium. One cycle of sound would be one compression/rarefaction in the medium.
Frequency =
number of cycles in one second
Hertz (Hz) =
Cycles per second
****test
Sound that humans can hear:
Below this:
Above this:
20 to 20,000 Hz
infrasound (think of elephants)
ultrasound (think of bats)
Medical diagnostic ultrasound uses very high frequencies:
vascular:
echo:
Therapeutic ultrasound:
3.5–10 MHz for vascular
2.5–5 MHz for echo (possibly higher for pediatric echo)
Therapeutic ultrasound is in the hundreds of thousands of Hz
Commonly used units: How many Hz and how to write it in scientific notation?
KHz =
MHz =
KHz = 1,000 Hz = 1 x 3 10 Hz MHz = 1,000,000 Hz = 1 x 106 Hz
Period:
Time it takes to complete one cycle
Period and frequency are reciprocal:
f= 1 = cycles
—
P second
P = 1 = seconds
—
f cycle
A shorter period means a
A longer period means a
higher frequency— you can fit more cycles into a second.
lower frequency— you can’t fit as many cycles into a second.
Example: 5 cycles/sec or 5 Hz = 4 Hz (cycle/sec)= 400 cycle/sec= 4 MHz =
1/5 seconds/cycle (0.2 sec) OR 1/5 Hz
1/4 sec/cycle
1/400 sec/cycle
4 cycles/msec OR .25 msec/cycle
If the period is 0.5 msec, what is the frequency?
f=1/P
f=1/0.5ms=2MHz OR
f=1/0.5x10^-6s = 2,000,000MHz
If the frequency is 3.5 MHz, what is the period?
P=1/f
P=1/3.5MHz = 1/3.5x10^6Hz = .0000002857 s/c
or .286x10^-6s
or .286ms/c
Source of ultrasound: crystals
lead zirconate titanate (PZT)
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The piezolelectric effect:
Voltage in, sound out and sound in, voltage out More on this later.
**test
The piezoelectric effect was discovered by
Pierre Curie (with brother Jacques Curie): electric potential created by distorting crystal.
**test
Gabriel Lipmann predicted
the reverse effect mathematically and the brothers confirmed it experimentally.
*********test Paul Langevin (France): developed?
sonar (he studied under Pierre Curie and had an affair with Marie Curie, the cad)
********test Ian Donald (Scotland):
first efforts at medical use of ultrasound
**test
The resonant frequency of a piezoelectric crystal is dependent on ___________.
What frequency is Thin:
What frequency is Thick:
its thickness.
Thin: higher frequency
Thick: lower frequency
The source of sound determines:
- Frequency
- Period
- Amplitude
- Intensity
The medium determines:
Propagation speed (how fast sound travels through the medium)
Usually given as c (constant) for ultrasound calculations
Propagation speed depends on:
the medium. c = the square root of B/p
(if B/p goes down, c goes down)
B=bulk modulus: stiffness
p = density of medium
Density is the concentration of matter:
mass per unit volume
g/cm3
kg/m3
What does and does not carry sound very well?
Gases don’t carry sound very well: not dense, but too elastic
Liquids do much better (whales use it)
Solids do very much better: listen to train tracks…
Aluminum carries sound better than lead. Why?
Which one has a high propagation speed?
Both are pretty stiff, but lead is more dense; this slows down the travel of sound.
aluminum
Propagation speed
Increases with:
Decreases with:
Increases with greater stiffness of the medium.
Decreases with greater density of the medium.
(Density and stiffness aren’t necessarily related.)
Greater stiffness means
Greater density means
less particle motion, so faster travel.
more particles to move, so slower travel.
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Average speed in soft tissue:
What is range gating?
c = 1540 m/sec
The scanner knows this, and knows time of travel, so it knows distance (depth) d=r x t
This is range-gating.
Speed of sound in soft tissue:
1540 m/sec or 1.54 mm/μsec
Transit time for 1 cm: ~ 6.5 μsec
Round trip: 13 μsec
4 cm: 52 μsec
10 cm: 130 μsec
15 cm: 195 μsec
**test
Wavelength is:
Distance of one cycle.
**propagation speed frequency(They are not directly related) The formula is c=y*f, y=c/f, f=c/y
(y is upside down in the formula)
(Don’t confuse it with period—time of one cycle)
So wavelength depends on both the source of sound AND the medium.
Note the inverse relationship between wavelength and frequency:
Longer wavelength means lower frequency, shorter wavelength means
Trick question: What happens to frequency if you double the propagation speed?
NOTHING.
The SOURCE governs frequency, not the medium.
The MEDIUM governs propagation speed, not the source.
**TEST
The source determines
The medium determines
how many cycles per second get generated.
how fast the sound can travel.
The medium offers some degree of acoustic impedance— we’ll get to that shortly.
What happens to wavelength if frequency is doubled?
HALFED
What happens to wavelength if propagation speed is doubled?
DOUBLED
Next concept: Acoustic impedance
Opposition of the medium to sound travel
Greater acoustic impedance means stronger echo
SYMBOL: Z
Propagation speed of bone: 4080 m/sec
Propagation speed of tissue: 1540 m/sec
If bone is so fast, why does it make such crummy-looking echoes?
the differences in acoustic impedance
It creates a huge change in acoustic impedance. All the ultrasound is bounced back, none gets through.
Echoes are created by
changes of acoustic impedance
When the sound reaches an interface between tissues having different acoustic impedances, some of the sound is reflected, and the rest travels on.
Units of acoustic impedance:
Rayls (remember that)
Interface: Two media create a boundary that reflects some of the ultrasound. The bigger the ∆Z, __________________.
the more sound is reflected (the stronger the echo).
What is ∆Z?
acoustic impedance
