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100 Physical Prin of Medicine 1 Fall 2013 > CVT 100 #9 Ultrasound > Flashcards

CVT 100 #9 Ultrasound Flashcards

1
Q

Sound is

A

particles in a medium vibrating around a rest point.

It is energy traveling through a medium.

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2
Q

How is sound propagated?

A

Particles are disturbed by the sound source. This disturbance is transmitted to adjacent particles, and so forth. In this way the sound is propagated.

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3
Q

What does sound require to be a sound?

A

Sound requires a source of disturbance and a medium to travel through (no medium—no sound, e.g., a vacuum)

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4
Q

Production of sound:

A

Source vibrates, disturbs particles, creates ACOUSTIC PRESSURE

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5
Q

There is no net movement of the particles in the medium. What moves through the medium?

A

It is the energy that moves through the medium, leaving the particles where they were

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6
Q

Does the water move?

A

Nope—just the energy.

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7
Q

The energy is propagated by?

A

The region of acoustic pressure is transmitted to adjacent particles in the medium— the energy is propagated.

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8
Q

This creates areas in the medium of COMPRESSION AND RAREFACTION

A

|||||| | | | | | |||||||| | | | | | |||||||| | | | | |||||||||

These are regions of higher and lower acoustic pressure, due to higher and lower concentrations of particles.

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9
Q

Wave:

A

A propagating (i.e., traveling) variation in quantity.

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10
Q

Sound is a

A

propagating variation in acoustic pressure.

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11
Q

Two types of waves:

A

Longitudinal and Transverse

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12
Q

Transverse wave:

A

moves across (90° to) direction of propagation

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13
Q

Longitudinal wave:

A

moves in the direction of (axial to) propagation

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14
Q

What type of wave is sound?

A

Sound is a longitudinal wave. The acoustic variations occur along the direction of travel.

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15
Q

The changes in acoustic pressure can be graphed as?

A

The changes in acoustic pressure can be graphed as a sine wave.

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16
Q

Areas of higher pressure that go above the baseline are:

A

compression

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17
Q

Areas that go below the baseline are:

A

rarefaction

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18
Q

What does zero represent?

A

the rest point of the particles in the medium

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19
Q

In that sine-wave graphical depiction of sound, the positive peak represents _______________, the negative peak represents ___________, and the zero represents the rest point.

A

Compression

Rarefaction

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20
Q

Waves move in cycles:

A

Repetitions of the physical changes in the medium. One cycle of sound would be one compression/rarefaction in the medium.

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21
Q

Frequency =

A

number of cycles in one second

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22
Q

Hertz (Hz) =

A

Cycles per second

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23
Q

****test
Sound that humans can hear:
Below this:
Above this:

A

20 to 20,000 Hz
infrasound (think of elephants)
ultrasound (think of bats)

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24
Q

Medical diagnostic ultrasound uses very high frequencies:
vascular:
echo:
Therapeutic ultrasound:

A

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

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25
Q

Commonly used units: How many Hz and how to write it in scientific notation?
KHz =
MHz =

A 
KHz = 1,000 Hz = 1 x 3 10 Hz 
MHz = 1,000,000 Hz = 1 x 106 Hz
26
Q

Period:

A

Time it takes to complete one cycle

27
Q

Period and frequency are reciprocal:

A

f= 1 = cycles
—
P second

P = 1 = seconds
—
f cycle

28
Q

A shorter period means a

A longer period means a

A

higher frequency— you can fit more cycles into a second.

lower frequency— you can’t fit as many cycles into a second.

29
Q 
Example:
5 cycles/sec or 5 Hz =        
4 Hz (cycle/sec)= 
400 cycle/sec= 
4 MHz =
A

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

30
Q

If the period is 0.5 msec, what is the frequency?

A

f=1/P
f=1/0.5ms=2MHz OR
f=1/0.5x10^-6s = 2,000,000MHz

31
Q

If the frequency is 3.5 MHz, what is the period?

A

P=1/f
P=1/3.5MHz = 1/3.5x10^6Hz = .0000002857 s/c
or .286x10^-6s
or .286ms/c

32
Q

Source of ultrasound: crystals

A

lead zirconate titanate (PZT)

33
Q

****test

The piezolelectric effect:

A

Voltage in, sound out and sound in, voltage out More on this later.

34
Q

**test

The piezoelectric effect was discovered by

A

Pierre Curie (with brother Jacques Curie): electric potential created by distorting crystal.

35
Q

**test

Gabriel Lipmann predicted

A

the reverse effect mathematically and the brothers confirmed it experimentally.

36
Q 
*********test
Paul Langevin (France): developed?
A

sonar (he studied under Pierre Curie and had an affair with Marie Curie, the cad)

37
Q 
********test
Ian Donald (Scotland):
A

first efforts at medical use of ultrasound

38
Q

**test
The resonant frequency of a piezoelectric crystal is dependent on ___________.

What frequency is Thin:
What frequency is Thick:

A

its thickness.

Thin: higher frequency
Thick: lower frequency

39
Q

The source of sound determines:

A
  1. Frequency
  2. Period
  3. Amplitude
  4. Intensity
40
Q

The medium determines:

A

Propagation speed (how fast sound travels through the medium)

Usually given as c (constant) for ultrasound calculations

41
Q

Propagation speed depends on:

A

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

42
Q

Density is the concentration of matter:

A

mass per unit volume

g/cm3
kg/m3

43
Q

What does and does not carry sound very well?

A

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…

44
Q

Aluminum carries sound better than lead. Why?

Which one has a high propagation speed?

A

Both are pretty stiff, but lead is more dense; this slows down the travel of sound.

aluminum

45
Q

Propagation speed
Increases with:
Decreases with:

A

Increases with greater stiffness of the medium.
Decreases with greater density of the medium.

(Density and stiffness aren’t necessarily related.)

46
Q

Greater stiffness means

Greater density means

A

less particle motion, so faster travel.

more particles to move, so slower travel.

47
Q

****test
Average speed in soft tissue:

What is range gating?

A

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.

48
Q

Speed of sound in soft tissue:

A

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

49
Q

**test

Wavelength is:

A

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.

50
Q

Note the inverse relationship between wavelength and frequency:

A

Longer wavelength means lower frequency, shorter wavelength means

51
Q

Trick question: What happens to frequency if you double the propagation speed?

A

NOTHING.

The SOURCE governs frequency, not the medium.
The MEDIUM governs propagation speed, not the source.

52
Q

**TEST
The source determines

The medium determines

A

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.

53
Q

What happens to wavelength if frequency is doubled?

A

HALFED

54
Q

What happens to wavelength if propagation speed is doubled?

A

DOUBLED

55
Q

Next concept: Acoustic impedance

A

Opposition of the medium to sound travel
Greater acoustic impedance means stronger echo

SYMBOL: Z

56
Q

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?

A

the differences in acoustic impedance

It creates a huge change in acoustic impedance. All the ultrasound is bounced back, none gets through.

57
Q

Echoes are created by

A

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.

58
Q

Units of acoustic impedance:

A

Rayls (remember that)

59
Q

Interface: Two media create a boundary that reflects some of the ultrasound. The bigger the ∆Z, __________________.

A

the more sound is reflected (the stronger the echo).

60
Q

What is ∆Z?

A

acoustic impedance

100 Physical Prin of Medicine 1 Fall 2013 (9 decks)

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