Interaction Of Sound And Media Flashcards
What results in increased attenuation?
Higher frequency and longer path length. The further ultrasound travels, the more attenuation occurs.
The decrease in intensity, power and amplitude of a soundwave as it travels.
Attenuation
What results in less attenuation?
Lower frequency and shorter path length. In soft tissue, lower frequency results in less attenuation. Thus, we penetrate further with lower frequency sound.
What are three components of attenuation?
- Absorption – primary, sound converted into heat.
- Scattering
- Reflection
A 3 MHz sound beam travels through 2 media. It attenuates 5 dB in medium a and 6 dB in medium be. What is the total attenuation that the sound beam undergoes as it travels through both media?
11 dB
This occurs when propagating sound energy strikes a boundary between two media and some returns to the transducer.
Reflection
Reflections from a smooth reflector, like a mirror, are called what?
Specular reflections, and they return in one direction.
Specular reflectors are well seen when sound strikes the reflector at how what degrees?
90°
What type of reflection occurs when a boundary is rough, and reflected sound is disorganized and random?
Diffuse reflection or backscatter, (occurs with blood)
When a reflector is much smaller than the wavelength of sound, sound is uniformly distributed in all directions, or omnidirectional. What is this interaction of sound and media called?
Rayleigh scattering
How is Rayleigh scattering related to frequency?
Rayleigh scattering is related to frequency^4.
Name two organized, systematic interactions between sound and media.
Specular reflection
Rayleigh scattering
Name 2 disorganized, chaotic interactions between sound and media.
Diffuse reflection
scatter
What is attenuation coefficient?
The amount of attenuation per centimeter. A way to report attenuation without dealing with distance. Attenuation coefficient is related frequency.
What are the units of measurement for attenuation coefficient?
Decibels per centimeter
dB/cm
What is the attenuation coefficient in soft tissue? How is it expressed mathematically?
It is approximately one half of the frequency (MHz).
0.5 dB/cm/MHz
What is the equation for total attenuation?
Total attenuation (dB) = path length (cm) x attenuation coefficient (dB/cm)
Which of the following is the best estimate for the attenuation coefficient of sound traveling in soft tissues? A. 1.8 dB/cm/MHz B. 0.77 dB/cm/MHz C. 0.45 dB/cm/MHz D. 1.0 dB/cm/MHz
C. .45 dB/cm/MHz
A calculation of the opposition of the propagation of sound through a given medium.
Impedance
What are the units for impedance?
…and it’s abbreviation?
Rayls, often represented by the letter Z.
What are the typical values for impedance?
1,250,000 - 1,750,000 Rayls
What is the equation for impedance?
Impedance (Rayls) = density (kg/m^3) x propagation speed (m/s)
Name the three components of attenuation. Which one is the dominant contributor to attenuation?
Absorption, reflection, and scattering. Absorption attenuates sound the most.
As the path length increases, attenuation of ultrasound in soft tissue…
Increases
Total attenuation in soft tissue is related to ______ & ______?
Path length and frequency of the sound
Is attenuation in lung tissue greater than, less then, or equal to that of soft tissue?
Greater than
Is attenuation in bone greater than, less than, or equal to that of soft tissue?
Greater than
Is attenuation in air greater than, less than, or equal to that of soft tissue?
Much greater than
Is attenuation in water greater than, less then, or equal to that of soft tissue?
Much less than
What are the units of attenuation?
Decibels
What is the relationship between propagation speed and the attenuation coefficient in soft tissues?
Speed and attenuation coefficient are unrelated.
What is the relationship between ultrasound frequency and the attenuation coefficient in soft tissues?
In soft tissue, the attenuation coefficient in dB/cm is about half of the ultrasonic frequency in megahertz.
As the path length increases, the attenuation coefficient of ultrasound in soft tissue ________.
Remains the same
Acoustic impedance = _____x_____
Density (kg/m^3) x propagation speed (m/s)
Impedance is important in ______ at boundaries.
Reflections
Which is better to use while examining a carotid artery, a 7.5 or 3.0 MHz transducer?
7.5 MHz transducer is better because the higher frequency transducer produces the better image. We can use the higher frequency transducer because the structure is superficial and attenuation is not a problem.
In soft tissue, attenuation coefficient is directly related to ______.
Frequency (if frequency doubles, attenuation coefficient will double).
Rayleigh scattering is related to _____.
Frequency^4
If frequency doubles, Rayleigh scattering increases by a factor of 16. (2x2x2x2=16)
What are other names for normal incidence?
Perpendicular, orthogonal, right angle, and 90°.
‘PORNN’
What is oblique incidence?
Anything other than 90°; not at right angles.
What are two types of oblique angles?
Acute angles, which are less than 90°, and obtuse angles, which are greater than 90°, are oblique.
The intensity of the soundwave at the instant prior to striking a boundary.
Incident intensity
The portion of the incident intensity that, after striking a boundary, changes direction and returns back from where it came.
Reflected intensity
The portion of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling.
Transmitted intensity
Due to the principle of the conservation of energy, what is the equation for incident intensity?
Incident intensity = reflected intensity + transmitted intensity
What is the intensity reflection coefficient?
The percentage of the ultrasound intensity that bounces back when the sound strikes a boundary
What is the intensity transmission coefficient?
The percentage of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling.
When intensity reflection coefficient and intensity transmission coefficient are added together, what is the result?
100%
How are coefficients reported?
As percentages
With normal incidence, reflection occurs only if the two media at the boundary have ________________.
Different acoustic impedances
Will transmission or reflection occur with oblique incidence?
I don’t know! Transmission and reflection may or may not occur with oblique incidence. There are no simple rules to predict if so.
With oblique incidence, incident intensity equals what?
Incident intensity = transmitted+ reflected
With oblique incidence reflection angle equals what?
Reflection angle = incident angle
What is refraction?
Refraction is transmission with a bend.
A change in direction as sound transmits from one medium to another
What his refraction, Alex?
What is required for refraction to occur?
We fraction requires both: oblique incidence and different speeds.
What law describes the physics of refraction?
Snell’s Law
Soundwaves strike a boundary at normal incidence. The impedances of the two media are identical. What percentage of the soundwave is refracted?
0%, remember refraction requires oblique incidence and different speeds.
Which of these media has the greatest attenuation and the slowest speed? Bone, tendon, fat, air?
Air
Which of these media has the greatest attenuation and the fastest speed? Bone, tendon, lung, fat, air?
Bone
Which of the following describes the physics of refraction mathematically?
Bernoulli’s principal, Snell’s Law, Pouiseulle’s law, Murphy’s Law, Bernoulli’s law?
Snell’s Law
Which of the following terms does not belong with the others? Oblique, normal, perpendicular, orthogonal,90°?
Oblique
Which of the following terms best describes a reflection arising from a rough boundary? Non-diffuse, absolute, Rayleigh, smooth, non-specular?
Non-specular
A sound wave strikes a boundary between two media with different impedance is. The angle of incidence is non-orthogonal. Which of the following statements is most correct? All of the sound will be reflected. Some of the sound will be reflected. Some of the sound may be reflected. Some of the sound will be refracted. None of the sound will be refracted.
Some of the sound may be reflected.
