Physics Review Ch. 5, 6, 7 Flashcards
How do you calculate intensity?
a beam’s power divided by its area.
units: W/cm^2
What are other names for normal incidence?
perpendicular
orthogonal
right angle
90 degrees
Difference between normal and oblique incidence
normal incidence strikes the boundary at exactly 90 degrees
oblique incidence strikes the boundary at any angle other than 90 degrees (acute and obtuse)
Define incidence intensity
The sound waves intensity immediately before it strikes a boundary
Define reflected intensity
Is the intensity of the portion of the incident sound beam that, after striking a boundary, returns back from where it came
Define transmission intensity
Is the intensity of the portion of the incident beam that, after striking a boundary, continues forward in the same general direction that it was traveling
IRC
Intensity Reflection Coefficient; the percentage of intensity that bounces back, after it strikes the boundary between 2 media. (1%)
ITC
Intensity Transmission Coefficient; the percentage of intensity that passes in the forward direction between 2 media. (99%)
Define Impedance
The acoustic resistance to sound traveling in a medium. Calculated by multiplying the density of a medium by the speed at which sound travels in the medium.
Units: rayls; Z
Impedance-what is takes to achieve reflection
Reflection of an ultrasound wave depends upon the difference in acoustic impedances of the two media at a boundary.
Define specular reflectors
When the boundary is smooth, the sound is reflected in only on direction in an organized manner
Conservation of energy
Applies at the boundary between two media.
IRC + ITC = 100%
Reflection and transmission with oblique incidence
We cannot predict; it’s too complex
Define Refraction
Is a change in direction of wave propagation when traveling from one medium to another
Refraction occurs only when there is what?
- Oblique Incidence. (not normal)
2. Different Propagation speeds between the media.
What is refraction associated with?
Transmission
Attenuation
The decrease in intensity, power, and amplitude as sound travels.
Units: dB
Attenuation is determined by?
- Path length
2. Frequency of sound
What three processes contribute to attenuation?
- Reflection
- Scattering
- Absorption
What is impedance determined by?
Medium
Define scattering
Is the random redirection of sound in many directions. (Disorganized and chaotic)
Define Rayleigh scattering
Is a special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength. (Organized and omnidirectional)
*omnidirectional-in all directions
Relationship between scattering and frequency
directly related; Rayleigh scattering is proportional to frequency^4
Formula for Impedance
Impedance (rayls) = density (kg/m^3) * prop. speed (m/s)
Formula for IRC and ITC
100% = IRC + ITC
What are other names for decibel notation?
relative measurement
comparison
ratio
logarithmic
Half-Value Layer
Is the depth of tissue that results in 3 dB of attenuation to the intensity.
May be called: penetration depth, depth of penetration, and half-boundary layer
Units: cm
Half-value layer thickness depends on what two factors?
- The medium
2. The frequency of sound
Half-Value Layer conditions
Thin Half Value: -High frequency sound -Media with high attenuation rate Thick Half Value: -Low frequency sound -Media with low attenuation rate
Five key words that are related to intensity
- Spatial
- Peak
- Average
- Temporal
- Pulsed
Six methods for measuring intensity
- SPTP
- SATP
- SPTA
- SATA
- SPPA
- SAPA
What is the most relevant intensity with respect to tissue heating?
SPTA
Importance of the study of bioeffects
The different methods evaluate tissue exposure to sound energy.
T/F Spatial peak intensity is always higher than the spatial average intensity
True
For continuous wave ultrasound, which intensities are the same?
Since the beam is always “on” the pulse average and temporal average intensities are the same. SPTA=SPPA and SATA=SAPA
Rank of intensities from largest to smallest
SPTP -> Im -> SPPA -> SPTA -> SATA
When a wave’s intensity doubles, the relative change is
+3 dB
When intensity increases ten-fold, the relative change is
+10 dB
When the intensity is reduced to one-half its original value, the relative change is
-3 dB
When the intensity is reduced to one-tenth its original value, the relative change is
-10 dB
Attenuation conditions
More attenuation: -Longer distances -Higher frequencies Less attenuation: -Shorter distances -lower frequencies
A 15 MHz transducer is used for what?
Small parts- breast, thyroid, scrotum
What frequency is used for vascular?
7 MHz-12 MHz
When examining AB/OB the frequency should be between
3 MHz-6MHz
Attenuation is not related to?
propagation speed
Define Attenuation coefficient
Is the number of decibels of attenuation that occurs when sound travels one centimeter.
Units: dB/cm
When sound travels to a depth of 5 cm, the attenuation coefficient remains 2 dB/cm. What is the total attenuation?
10 dB (5 cm * 2 dB/cm = 10 dB)
Total attenuation (dB) = atten. coef. (dB/cm) * distance (cm)
Attenuation coefficient in soft tissue
Attenuation Coefficient = 0.5 dB/cm/MHz
Attenuation in media
Water- extremely low Blood, urine- low Fat- low Soft tissue- intermediate Muscle- higher Bone and lung- even higher Air- extremely high
At the boundary, the reflected and transmitted intensities are added together. The result must equal?
The incident intensity
If the intensity reflection coefficient is 45%, what percent of sound is transmitted into the body?
55%
45% + ? = 100%
Refraction does not occur when
The speeds of the two media are identical..
The angles of incidence and transmission will be equal.
When will the transmission angle be greater than the incident angle?
When the speed of medium 2 is greater than the speed of medium 1
When will the transmission angle be less than the incident angle?
When the speed of medium 2 is less than the speed of medium 1
Define reflection
As sound strike’s a boundary, a portion of the wave’s energy may be redirected back to the sound source
Two types of reflection
- specular
2. diffuse or backscatter
Diffuse reflection
When a wave reflects off an irregular surface, it radiates in more than one direction
A sound wave with an intensity of 30 W/cm^2 strikes a boundary and is totally reflected. What is the intensity reflection coefficient?
100%, since the wave is totally reflected
Conditions for impedance
Media have identical impedance-no reflection
Slightly different impedance-small reflection
Substantially different impedance-large reflection
What is Time of Flight?
elapsed time from pulse creation to pulse reception. Also called go-return time
What is the 13-microsecond rule?
for every 13 microseconds of go return time, the object creating the reflector is 1cm deeper in soft tissue.
Relationship between time-of-flight and how deep a sound pulse travels
They are directly related. Greater distances prolong the time-of-flight. Lesser distances shorten the time-of-flight
Snell’s Law
It quantifies the physics of refraction.
sin (transmission angle)/sin (incident angle) = speed of Medium 2/speed of Medium 1
