1
Q
what are the three waves we can describe the strength of a wave
A
- amplitude
- power
- intensity
2
Q
other than strength what else can amplitude, power, and intensity express
A
- loudness/ volume of sound
3
Q
what are the 4 definitions of amplitude
A
- particle displacement
- particle velocity
- acoustic pressure of a sound wave
- maximum variation of an acoustic variable
4
Q
what does amplitude indicate about an echo
A
- indicates the strength of the echo or voltage induced in a crystal by a pressure wave
5
Q
what are the 4 acoustic variables
A
- pressure
- density
- particle motion
- temperature
6
Q
what 4 questions should you ask yourself about amplitude
A
- what determines the initial amplitude of a pulse
- as the pulse travels through the medium what is the reduction in amplitude called
- what are the 5 mechanisms that cause attenuation
- does the amplitude of the echo decrease as the echo returns to the transducer
7
Q
what determines the initial amplitude of a pulse
A
- the pulser (output power)
8
Q
as the pulse travels through the medium what is the reduction in amplitude called
A
- attenuation
9
Q
what are the 5 mechanisms that cause attenuation
A
- ABSORPTION
- reflection
- refraction
- wave front divergence
- scatter
10
Q
does the amplitude of the echo decrease as the echo returns to the transducer
A
- yes
11
Q
what is the definition of power
A
- measure of the total energy transmitted summed over the cross sectional area fo the beam per unit time
12
Q
power equation
A
power = intensity x area
13
Q
what is the absolute unit of power
A
- the watt (joules)
14
Q
what is the relative unit of power and intensity
A
- the decibel (db)
15
Q
what is a decibel in relation to a bel
A
- it is 1/10 of a bel
16
Q
what does 0dB equate too
A
- 100% power
17
Q
what does a 3dB loss equate to
A
- 1/2 the original power
18
Q
what is power determined by
A
- the pulser
19
Q
what is the definition of intensity
A
- measure of the strength of a sound wave
- equal to the power per unit area
20
Q
what is the intensity equation
A
intensity = power / area
21
Q
what is the absolute unit of intensity
A
mW/cm^2 or W/cm^2
22
Q
how would you describe the relationship between intensity and power
A
- it is proportional
23
Q
if there is an increase in power then what happens to the intensity
A
the intensity increases
24
Q
how would you describe the relationship between area and intensity
A
- it is inversely proportional
25
if the area decreases what happens to the intensity
- it increases
26
how is intensity related to amplitude
- it is proportional to the square of the pressure amplitude
27
a small change in amplitude will do what to the intensity
- result in a very very large change in intensity due to the square exponent
28
if the amplitude is doubled what happens to the intensity
- it is quadrupled
29
is intensity constant in space and time
- no
30
what are the four different expression of intensity
- spatial average (SA)
- spatial peak (SP)
- temporal average (TA)
- temporal peak (TP)
31
where is the spatial average found
- at the transducer face
32
where is the spatial peak found
- found at a the focal point where the beam area is the smallest
33
how are spatial peak and spatial average related
- by the Beam Uniformity Ratio BUR
34
what is the Beam Uniformity Ratio equation
- BUR = SP / SA
35
what does the BUR tell us about the beam
- tells us the amount of focusing there is
36
highly focused beams have a high or low BUR
- high
37
weakly focused beams have a high or low BUR
- low
38
what number will the BUR always be over and why
- the BUR will always be over 1
| - the spatial peak will always be higher than the averages
39
what would the BUR be in a perfectly uniform beam
- 1
40
what are the two factors that effect the spatial intensities
- spatial peak will increase with an increase in power or focusing
- spatial average will increase with an increase in power
41
what is the temporal average
- over the time the pulse is generated it includes both the ringing and listens phase of the pulse
42
what is the temporal peak
- the highest amplitude in the pulse at any given time
| - instantaneous peak
43
how are the temporal average and temporal peak related
- related by the Duty Factor DF
44
what is the Duty Factor DF equation for TP and TA
- DF = TA / TP
45
what is the Duty Factor
- it is the percent of time the sound is on / the probe is ringing
46
Duty Factor Equation
DF = PD/PRP x 100
47
what should the temporal peak be replaced by and why
- the pulse average PA
| - the pulse is so short that the TP almost equals the PA
48
what is the duty factor equation for TA and PA
- DF = TA / PA
49
what is the pulse average
- pulse average intensity is Thea average of all intensities found within a single pulse
50
what are the two factors that effect temporal intensities
- increase in power
| - increase in duty factor
51
when is the duty factor increase
- if the PRF or PD is increased
52
what is the duty factor for continuous wave CW
- 100% because the crystal is always ringing
| - usually has a higher SPTA than PW
53
combined intensities from highest to lowest
- SPTP
- SPPA
- SPTA
- SATP
- SAPA
- SATA
54
why don't we use SPTP for bioconsiderations
- not fair to use this because it is such a short part of the pulse its very fast
55
what combined intensity do we use for bioconsiderations
- SPTA
56
what is the SPTP intensity also known as
- instantaneous peak
57
what are the three sources that we use for information on effects of ultrasound
- epidemiology
- in vitro cell studies
- animal studies (invivo)
58
what is an epidemiological study and what is the time line of it
- people are monitored over the course of several years that were exposed to ultrasound in utero
- conducted over a long term
59
what is the purpose of an epidemiological study
- look for cause and effect
60
what is an in vitro study
- exposes macromolecules, membran transport systems, cells, or clumps of cells suspended in liquid to ultrasound
- it is difficult to say that an effect on cells will have a direct clinical significance
- the results of in vitro studies can give valuable information to set thresholds for in vivo studies
61
what have in vivo studies shown
- potential for bio effects exists
62
what 6 adverse effects have in vivo studies shown
- fetal weight reduction
- postpartum mortality
- fetal abnormalities
- tissue lesions
- hind limb paralysis
- blood flow stasis
63
what two more positive effects have been reported from in vivo studies
- wound repair enhancement
| - tumor regression
64
what does the official AIUM statement say about bio effects
- there are no adverse effects observed if the intensities are kept below a certain threshold
65
what is the highest safe intensity level for an unfocused probe
- 100 mW/cm^2
66
what is the highest safe intensity level for a focused probe
- 1000 mW/cm^2 (1W/cm^2)
67
what are the two main categories of bio effects
- thermal
| - non thermal (mechanical)
68
what is an important thing to remember about thermal bioeffects
- attenuation is primarily due to absorption which is the conversion of sound to heat
69
do adult of fetal tissues handle temperature increases better
- adult tissues
70
no adverse effects occur when the temp increases is less than what number
- 2ºC
71
what other temperature range is considered safe as long as exposure time is reasonable
- between 2-6ºC
72
the higher the exposure time leads to what
- more chance of adverse effects at higher temperatures
| - 6ºC increase in temp will not produce effects if exposure time is under 16 minutes
73
what two categories are included in mechanical effects
- radiation force
| - cavitation
74
what is radiation force
- force exerted by the sound on the medium which can deform and disrupt structures
75
what can radiation force cause
- can cause flow in absorbing fluids resulting in shear forces
76
what is cavitation
- production and behaviour of bubbles in a liquid medium
77
what are the to divisions of cavitation
- stable and transient
78
what is stable cavitation
- simple oscillation of the bubbles that can result int eh streaming of liquid resulting in shear stresses
79
what is transient cavitation
- when the bubble actually collapses producing shock waves
| - the shock waves can result in localized extremely high temperatures and has even emitted light in clear fluids
80
what is the standard set in 1992 for real time scanning called
- the Output Display Standard (ODS)
81
what 2 quantities are displayed by the ODS
- thermal index TI
| - mechanical index MI
82
what is the thermal index
- ratio of acoustic power produced by the transducer to the power required to raise the temperature in tissue 1ºC
- assumed value
83
what are the three other TI categories
- TIS
- TIB
- TIC
84
TIS
- TI for soft tissue and is the MOST COMMON
85
TIB
- TI for bone near the focus and is used for OB
86
TIC
- TI for bone near the surface as in the case of a transcranial study
87
what does the mechanical index represent
- likelihood that the energy used will lead to cavitation
88
what is the MI proportional to
- peak rarefaction pressure so if the pressure doubles the MI doubles as well
89
what is the MI inversely proportional to
- proportional to the square root of the frequency so the frequency would have to quadruple for the MI to halve
90
is there a chance for MI and TI to be underestimated and if yes then in what circumstance
- yes it can be
| - in presence of a large fluid collection but these situations usually allow for less power
91
what is the max TI
6ºC
92
what is the max MI
1.9
93
cardiac SPTA limit
430 mW/cm^2
94
peripheral vascular SPTA limit
20 mW/cm^2
95
ophthalmic SPTA limit
17 mW/cm ^2
96
fetal and other (abd, peds, SS, neonatal, brain) SPTA limit
94 mW/cm^2
97
typical output intensity SPTA for b scan
18.7 mW/cm^2
98
typical output intensity SPTA for m mode
73 mW/cm^2
99
typical output intensity for SPTA for PW
1140 mW/cm^2
100
typical output intensity SPTA for color
234 mW/cm^2
101
what are the two different applications of ultrasound
- scanned
| - non scanned
102
what are the 3 non scanned applications of ultrasound
- CW
- PW
- m mode
103
what do the non scanned applications of ultrasound pose risk of
- highest risk of thermal effects with W having the most potential due to the fact they transmit repeatedly in a straight line
104
what other non thermal adverse effects can occur
- when gas bodies are present in the circulation contrast agents in the blood stream pose a concern since these findings were confirmed at intensities below the accepted threshold
- when MI is below 0.4 there is little apparent risk
105
what 4 adverse effects have been found from scanning the gas bodies
- induction of premature ventricular contractions
- microvascular leakage with petechiae (micro bleeds)
- glomerular capillary hemorrhage
- local cell killing
106
ALARA
- balancing between benefit and risk
| - As Low As Reasonable Achievable
