Acoustic Variables
Pressure (Pascals (Pa)): |
concentration of force in an area |
Density (km/cm^3): |
concentration of mass in a volume |
Distance (cm or mm): |
refers to the distance a particle moves. |
Used to differentiate between the different types of waves (heat, light, sound etc). If one of the 3 variables have rhythmic oscillations then it is a sound waves.
Basic Review
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All waves carry what? Energy
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Sound must travel in a ______ line Straight
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ALL sound waves are ______ and ______ Longitudinal and mechanical
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Sound (mechanical waves) need a ______ to travel through Medium
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Molecules in a sound wave are ________ and _______. Compressed and rarefied
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Acoustic Propagation Properties The effect medium has on sound waves.
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Bioeffects The effects sound waves have on body tissue.
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What are 2 types of mechanical waves? Transverse and longitudinal.
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What is the average speed of sound in soft tissue? 1540 m/s OR 1.54 mm
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Another word for stiffness is? Bulk modulus
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Stiffness and speed are _____ related. Directly; increased speed = increased stiffness.
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Speed and density are _____ related. Inversely; increased speed = decreased density.
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Sound travels the same speed no matter what _____. Frequency; 5Mhz probe and 15 Mhz probe will travel at the same speed.
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Acoustic Parameters
Period (microseconds): |
Time it take to complete one cycle. |
Source, NO |
Frequency (MHz): |
# of cycles per second. |
Source, NO |
Power (Watts): |
Rate of energy |
Source, YES by adjusting output power. |
Intensity (W/cm^2) |
Concentration of energy. |
Source, YES by adjusting output power. |
Amplitude (Pa): |
The difference between the baseline and peak of a wave (bigness). |
Source, YES by adjusting output power. |
Wavelength (mm): |
Distance to complete one cycle. (1.54mm/frequency) |
Source AND medium, NO |
Propagation speed (m/s) |
How fast a sound wave travels through a medium. |
Medium, NO |
Used to define characteristics of a continuous wave (wave that is unable to produce an image).
- Period and frequency are reciprocals.
- Frequency is inversely related to period and wavelength.
- Wavelength and period are directly related.
- Propagation speed is determined by stiffness and density.
Intensity
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Used to evaluate tissue exposure to sound energy Determines the effects sound has on tissue.
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Intensity is important when studying what? Bioeffects
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The strongest intensity is at they ______ of the beam. Center/Focus (smallest area).
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What intensity is the most important when studying bioeffects? SPTA
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SPTP is the _______ intensity Highest
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SATA is the _______ intensity. Lowest
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ALL intensities have units of? W/cm^2 (power/area)
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5 Parameters of Pulsed Sound
Pulse duration (microseconds): |
Time is takes to complete one pulse. |
Source, NO |
Spatial Pulse Length (mm): |
Distance it takes to complete one pulse. |
Source, NO |
Pulse Repetition Period (PRP): |
Time from the start of one pulse to the start of the next (includes transmit and receiving time). |
Source, YES by adjusting depth (Directly related). |
Pulse Repetition Frequency (PRF) (kHz): |
# of pulses per second. |
Source, Yes by adjusting depth (inversely related). |
Duty Factor (%): |
Percentage of time that the pulse is on. |
Source, YES by adjusting depth (inversely related). |
Characteristics used to define a pulse wave (wave that is able to produce an image).
- A pulse is made up of multiple cycles.
- 3 out the 5 parameter can be adjusted using depth.
How Sound Travels Through Media
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Attenuation Decrease in power, intensity, and amplitude due to sound waves decreasing as they propagate through media.
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Distance and attenuation are _____ related. Directly; increased distance = increased attenuation.
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Frequency and attenuation are ______ related. Directly; increased frequency = increased attenuation.
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3 processes that contribute to attenuation: 1. Reflection 2. Scattering 3. Absorption
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2 types of reflection (energy reflected back): 1. Specular: smooth boundary, one direction 2. Diffuse: irregular border, multiple directions.
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Scattering Waves redirected in many directions due to small tissue interface; when tissue is < wavelength. Directly related to frequency.
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Rayleigh scattering When structures are MUCH smaller than the beams wavelength. EX: RBC. Rayleigh scattering = frequency^4. They are directly proportional; increased frequency = increased Rayleigh scattering.
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Absorption Ultrasonic energy is converted into heat. Directly related to frequency.
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Attenuation Coefficient (dB/cm) Used to compare the amount of attenuation in certain circumstances. Measured in decibels for when sound travels 1 cm.
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Total attenuation = Attenuation coefficient x Distance (cm) EX: depth = 5cm AC = 2 dB/cm then total attenuation = 10 dB.
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Attenuation Coefficient in soft tissue = frequency/2 |
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Half layer thickness (penetration depth or half-boundary layer) The distance sound travels in tissue that reduces intensity in half; Thin half layer = attenuates more
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Impedance (rayls) Resistance to sound traveling in a medium. Impedance = density x speed.
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Normal incidence Sound beam strikes boundary at 90 degrees (orthogonal, perpendicular, right). Reflection occurs if the boundaries have different impedances.
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Oblique incidence Sound beam strikes at any other angle other than 90 degrees; angle of incidence = angle of reflection.
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Incident intensity (%) Intensity before striking boundary; incident intensity = reflected intensity + transmitted intensity.
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Reflected intensity Intensity of sound wave after striking boundary and retuning.
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Transmitted intensity intensity of sound wave after striking boundary.
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Refraction Transmission with a bend. Can only occur IF 1. oblique incidence and 2. different propagation speeds of 2 media.
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