Chapter 6 Flashcards
Positive decibels
indicate that the signal strengthened (final intensity > initial intensity)
Negative Decibels
indicate that the signal has weakened (final intensity < initial intensity)
3 dB intensity ratio 2 intensity doubled
6 dB intensity ratio 4 intensity increase 4 fold
9 dB intensity ratio 8 intensity increase 8 fold
10 dB intensity ratio 10 intensity increase 10 fold
20 dB intensity ratio 100 intensity increase 100 fold
30 dB intensity ratio 1000 intensity increase 1000 fold
40 dB intensity ratio 10000 intensity increase 10,000 fold
Intensity ratios
Attenuation
The decrease in intensity, power and amplitude as sound travels
expressed in dB
Attenuation is determined by two factors
- Path length or distance traveled by the sound wave
- Frequency of sound
Path length and attenuation are directly related
Path Length ∝ Attenuation
Frequency and attenuation are directly related
Frequency ∝ attenuation
Three processes contribute to attenuation
Reflection
Scattering
Absorption
Reflection
a portion of the sound wave is redirected back towards the sound source when it strikes a boundary between 2 medium
There are two forms of reflection
- Specular
- Diffuse
Specular reflection
is when the sound is reflected off the boundary back towards the probe in only one direction in an organized manner
One limitation of specular reflectors
is that once the wave is slightly off axis (non perpendicular to the boundary) the reflection does not return to the transducer
An advantage of diffuse reflections
is that the transducer receives reflections from interfaces that it is not perpendicular to
a second advantage to backscatter
is that heterogeneous tissue acts as a back scatter and allows us to see tissue texture
A disadvantage to backscatter
is that backscattered signals have lower strength than specular reflections
Scattering
is the random redirection of sound in many different directions
Disadvantage to Scattering
Little to none of the redirected sound wave makes it back to the probe
Rayleigh scattering
is a special type of scattering that occurs when the interface/structure diameter is much smaller than the wavelength
Sound is redirected equally in all directions (little towards probe)
Disadvantage to Rayleigh Scattering
Little to none of the redirected sound wave makes it back to the probe
Rayleigh scattering ∞ frequency4
rayleigh scattering is directly related to frequency quadrupled
Absorption
occurs when ultrasonic energy is converted into another energy form such as heat
absorption is directly related to frequency
absorption ∝ frequency
Attenuation Coefficient
is the number of decibels of attenuation that occurs when sound travels one centimeter; units = dB/cm
Attenuation Coefficient is directly related to frequency
attenuation coefficient ∝ frequency
Half Value Layer Thickness
is the distance that sound travels in a tissue that reduces the intensity of sound to one-half its original value
Acoustic Impedance
it is the resistance offered to sound traveling through the medium
AKA: characteristic impedance
Acoustic Impedance
is a characteristic of the tissue, specifically it is the resistance offered to sound traveling through the medium
AKA characteristic impedance
units of rayls
typical values are 1.25-1.75 Mrayls
Incidence
the angle at which the wave strikes the boundary
Normal incidence
is when the incident sound beam strikes the boundary at exactly 90°
Also called perpendicular, orthogonal, right angle, or 90 degree incidence
Oblique incidence
is when the incident sound beam strikes the boundary at an angle other than 90°
Also called nonperpendicular incidence
Incident intensity
is the sound wave’s intensity immediately before it strikes a boundary
Reflected Intensity
is the intensity of the reflected portion of the sound wave, immediately after it reflects off the boundary
Transmitted Intensity
is the intensity of the transmitted portion of the sound wave immediately after it crosses the boundary
Intensity Reflection Coefficient
The percentage of the intensity that is reflected back when a sound beam strikes the boundary between two media
Intensity Transmission Coefficient
The percentage of the intensity that propagates forward or is transmitted when the beam strikes the interface between two media
In oblique incidence if we have reflection the
incident angle = reflection angle
Refraction
the soundwave may bend or change direction as it crosses the boundary
Refraction occurs only if two conditions are satisfied; you must have
- Oblique incidence
- Different propagation speeds between the 2 media
the critical angle
The incident angle at which no transmission into the second medium takes place
(>t = 90 degrees)
