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SPI Chapter 6
Interaction of Sound and Media
| Question | Answer |
|---|---|
| What is attenuation? | sound waves weaken |
| When a sound wave reaches the transducer, what is the sound wave converted into? | electrical energy |
| What is amplified? | strengthen |
| What is the standard measurement tool for signals? | decibel notation |
| What are decibels based on? | logarithms |
| What is a logarithm (log)? | a number that represents the number of 10s that are multiplied to create the original number |
| What is the log of 1,000,000 | 6 (count the zeros) |
| What is an everyday example of log? | the Richer scale (used for earthquakes) |
| What dos decibel notation report? | relative changes (for ex. doubled or one-tenth larger) |
| What two intensities do decibels require? | actual level at the time of measurement over reference (starting level) and |
| Decibel notation is... | a relative measurement, a comparison, a ratio, a log |
| What are positive decibels? | signals that are increasing in strength, or getting larger |
| What is the decibel notation when a wave's intensity doubles? | the relative change is +3dB |
| What is the decibel notation when intensity increases ten-fold? | the relative change is +10dB |
| What are negative decibels? | signals decreasing in strength, or getting smaller |
| What is the decibel notation when intensity is reduced to 1/2 its original value? | the relative change is -3dB |
| What is the decibel notation when intensity is reduced to 1/10 its original value? | the relative change s -10dB |
| What is the decibel notation when intensity is reduced to one-quarter its original value? | the relative change is -6dB |
| Do we need more than one intensity to calculate decibels? | Yes |
| What two factors attenuation is determined by? | path length and frequency of sound |
| What two factors are directly related to attenuation? | distance and frequency |
| What are the 3 processes that contribute to attenuation? | reflection, scattering, absorption |
| What are the 2 types of reflection? | specular and diffuse |
| What is specular reflection? | smooth boundary reflected in one direction in organized manner at an angle and does not return to the transducer |
| What is diffuse reflection (or back scatter)? | un-smooth boundary and reflects in more than one direction |
| What is an advantage of diffuse reflection? | sound beam can produce reflections that will return to the transducer |
| What is a disadvantage of diffuse reflection? | backscattered signals have a lower strength than specular reflection |
| What is a disadvantage of specular reflection? | the reflection does not return back to the transducer |
| What is scattering? | sound in many directions |
| What type of tissue in the body would scattering take place? | lung tissue because it is filled with air |
| What is scattering directly related to? | frequency |
| What is Rayleigh Scattering? | scattering that occurs the structure's dimensions are much smaller than the beam's wavelength and equally in all directions or "omnidirectional (ex. red blood cells) |
| What id Rayleigh scattering proportional to? | frequency^4 |
| When frequency doubles, what does Rayleigh scattering equals? | 16 |
| What is absorption? | energy that is converted into another energy form, such as heat |
| What is absorption directly related to? | frequency |
| Does high or low frequency waves attenuate more? | High |
| Which body part undergoes extensive absorption of sound travel? | bone |
| What is attenuation coefficient? | the number of decibels of attenuation that occurs when sound travels 1 cm |
| What are the units of attenuation coefficient? | dB/cm |
| What is the advantage of attenuation coefficient? | the value always remains constant no matter the distance |
| What is the formula for total attenuation? | total attenuation (dB) = attenuation coefficient (dB/cm) x distance (cm) |
| In soft tissue, what is attenuation coefficient (dB/cm) directly related to? | frequency (MHz) |
| What is the formula for attenuation coefficient in soft tissue? | atten. coef. (dB/cm) = frequency(MHz)/2 |
| What is the attenuation rate in water? | extremely low |
| What is the attenuation rate in blood, urine, and fluids? | low |
| What is the attenuation rate in fat? | low |
| What is the attenuation rate in soft tissue? | intermediate |
| What is the attenuation rate in muscle? | higher |
| What is the attenuation rate in bone and lung? | even higher |
| What is the attenuation rate in air? | extremely high |
| What is half-value layer thickness? | the distance sound travels in a tissue that reduces the intensity of sound to one-half its original value |
| What are the units for half-value layer thickness? | cm |
| What are the typical values of half-value thickness? | 0.25 to 1.0cm |
| What are some half-value layer thickness synonyms? | penetration depth, depth of penetration, & half-layer boundary layer |
| What are the two factors that half-thickness depends on? | the medium and the frequency of sound |
| What tissue is half-value layer thin in? | lung or bone |
| What tissue is half-value layer thick in? | fluids |
| What is transmission? | the ultrasound's ability to image structure located deep in the body |
| What is acoustic impedance? | acoustic resistance to sound traveling in a medium |
| What is the formula for acoustic impedance? | acoustic impedance (rayls) = density (kg/m^3) x speed (m/s) |
| Is a tissue's impedance measured? | No, its calculated |
| What is the unit for impedance? | rayls (Z) |
| What are the typical values for impedance? | 1,250, 000 to 1,750,000 rayls |
| What is impedance determined by? | medium |
| What is a synonym for acoustic impedance? | characteristic impedance |
| What is incidence? | sound pulse striking many tissues as is travels in the body |
| What are the 3 types of angles? | acute, right, obtuse |
| What are oblique angles? | acute and obtuse angles |
| What is normal incidence? | sound beam strikes the boundary at exactly 90 degrees |
| What are the typical values of normal incidence? | 90 degrees |
| What are synonyms for normal incidence? | perpendicular, orthogonal, right angle, 90 degrees |
| What is oblique incidence? | sound beam strikes the boundary at at any angle other than 90 degrees |
| What are the typical values of oblique incidence? | must not equal 80 degrees |
| What are synonyms for oblique incidence? | not a right angle or non-perpendicular |
| What is incident intensity? | the sound wave's intensity immediately before it strikes a boundary |
| What is reflected intensity? | after striking a boundary, returns back from where it came |
| What is transmitted intensity? | after striking the boundary it continues forward |
| What is the formula for incident intensity? | incident (starting) intensity = reflected intensity + transmitted intensity |
| What is intensity reflected coefficient (IRC)? | the percentage of intensity that bounces back when a sound beam strikes the boundary between 2 media |
| In clinical imaging, how much of sound waves's intensity is reflected at a boundary between 2 soft tissues? | 1% or less |
| What amount of a wave is reflected when sound strikes a boundary such as between soft tissue and bone, or between soft tissue and air? | a greater percentage |
| What is intensity transmission coefficient (ITC)? | the percentage of intensity that passes in a forward direction when the beam strikes an interference between two media |
| In clinical imaging, how much of sound waves's intensity is transmitted at a boundary between 2 soft tissues? | 99% or more |
| What amount of a wave is transmitted when sound strikes a boundary such as between soft tissue and bone, or between soft tissue and air? | a smaller percentage |
| What are the coefficients IRC and ITC both reported as? | percentages and are dimensionless |
| When a sound beam strikes a boundary and energy is conserved, how much intensity is accounted for? | 100% |
| What formula applies to energy of conservation? | 100% = IRC (%) + ITC (%) |
| If IRC and ITC are added, the result must equal? | 100% |
| In normal incidence, reflection occurs only if the media on either side of the boundary have what? | different impedances |
| Will reflection occur if the two media have identical impedances? | No |
| Will a small reflection occur if the impedances are slightly different? | Yes |
| Will a large reflection occur if the impedances are substantially different? | Yes |
| What is the formula for IRC? | IRC (%) = [Z2-Z1/Z2+Z1]^2 x 100 |
| What is the formula for ITC? | ITC (%) = transmitted intensity/incident intensity + 100 = 1 - IRC |
| With normal incidence, when sound strikes an interference it will be? | straightforward |
| With oblique incidence, when sound strikes an interference it will be? | complicated to predict if sound reflects or transmits |
| With oblique incidence, will reflection occur even when the impedances of 2 media are identical? | May occur |
| With oblique incidence, will reflection be absent even when the impedances are identical? | Maybe |
| What are the 2 physical principles that always apply to reflection with oblique incidence? | conservation of energy and reflection angle = incident angle |
| The angle between the incident sound beam and the imaginary line is what to the "angle of incidence"? | perpendicular |
| What is refraction? | bending, change in direction |
| What 2 conditions satisfy refraction? | oblique incidence and different speeds of 2 media |
| What tissue exaggerates refraction (bending) | bone - soft tissue |
| What is Snell's Law? | sin (transmission angle)/sin (incident angle) = speed of medium 2/speed of medium 1 |
| Will refraction occur when the speed of 2 media are identical? | Yes |
| Speed 2 = Speed 1 | no refraction; transmission angle = incident angle |
| Speed 2 greater than Speed 1 | transmission angle greater than incident angle |
| Speed 2 less than Speed 1 | transmission angle is less than incident angle |
| What requirement is needed for reflection with normal incidence? | different impedances required |
| What requirement is needed for reflection with oblique incidence? | we cannot predict, it's too complex |
| What requirement is needed for transmission? | derived from reflection info; law of conservation of energy |
| What requirement is need for refraction? | oblique incidence and different speeds |
Created by:
Lazacha
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