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Ultrasound Physics
ultrasound physics
| Question | Answer |
|---|---|
| As sound travels through a medium, what term describes the effects of the medium on the sound wave? | acoustic propagation properties |
| Describe the characteristics of a sound wave | mechanical and longitudinal waves composed of a series of compressions and rarefactions |
| Acoustic variables identify certain waves as sound waves, these quantities are... | -pressure -density -particle motion |
| What is the most commonly used unit to measure acoustic pressure? | pascals; Pa; 1 Pa = 1 N/m^2 |
| Define pressure | force/area; 1 Pa = 1 N/m^2 |
| What is the unit of force? | newton; N |
| Define interference | when sound waves exist at the same location and time |
| Define constructive interference | an interaction of echoes that leads to reinforcement rather than destruction of waves; in-phase |
| Define destructive interference | an interaction of echoes that leads to partial or complete destruction of the sound wave; out-of-phase |
| Describe the mechanics of a sound wave as it travels through a vacuum | IT'S A TRICK! sound cannot travel through a vacuum |
| What is the relationship between frequency and period? | inverse, reciprocal |
| What is the range of periods commonly found in US waves? | 0.1-0.5 microsecons |
| Define frequency, in relation to US | NUMBER of cycles per second |
| Define period, in relation to US | TIME it takes for one cycle to occur |
| Define wavelength | LENGTH of space of one cycle |
| With standard ultrasonic imaging, what happens to the period of a wave as it propagates? | TRICK! it stays the same |
| What determines the period of an US wave? | (frequency of) the sound source; transducer |
| What is the unit of frequency? | hertz; Hz; cycles/sec |
| What range of frequencies are typically emitted by US transducers? | approximately 2-10 megahertz; 2-10 million cycles/sec |
| Define ultrasound | acoustic wave with a frequency of at least 20,000 Hz |
| Define infrasound | acoustic wave with a frequency less than 20 Hz |
| What is the relationship between frequency and attenuation? | direct relationship; as frequency increases, attenuation increases |
| What is the unit for intensity? | power/area; I = P/A > I = watt/cm^2 |
| What is the equation for a wave's amplitude? | (max value-min value) / 2 |
| What is the unit for power? | watts |
| What is the relationship between initial power and amplitude? | direct relationship; as power increases, amplitude increases |
| What is the relationship between frequency and depth of penetration? | inverse; the higher the frequency, the lower the imaging depth |
| What is the relationship between amplitude and final power? | direct relationship; as amplitude decreases, final power decreases SQUARED(if only one half of the original amplitude remains, then only one-forth of the power remains) |
| What is the relationship between beam area and beam intensity? | inverse; as beam area increases, beam intensity decreases; I = P/A |
| Without changing the transducer, the US tech can adjust which of the following: frequency, period, amplitude, power, intensity | amplitude, power, intensity |
| What is the relationship between intensity and amplitude? | direct relationship; as intensity increases, amplitude increases SQUARED |
| What is the relationship between intensity and power? | direct; as power of the beam increases, intensity increases |
| Define wavelength | the distance from the beginning to the end of one cycle |
| What is the typical range of wavelength in diagnostic US? | 0.15-0.75 mm |
| What determines the frequency of an ultrasonic wave? | the sound source; the transducer |
| What determines the wavelength of an ultrasonic wave? | the source AND the medium- determined in part by frequency, which is determined by the transducer, and partly by the speed of sound of the wave, which is established by the medium |
| What is the speed of sound in soft tissue? | 1540 meters per second, which is approximately one mile per second |
| What is the equation for wavelength? | wavelength = speed of sound/frequency mm = 1.54/MHz |
| What is the relationship between frequency and wavelength? | inverse; as frequency increases, wavelength decreases |
| What is the relationship between frequency and image quality? | direct; higher frequency usually yields greater detail and therefore higher quality images |
| What is the relationship between wavelength and image quality? | inverse; larger wavelengths yield less detail in an image and therefore lower quality images |
| What is the relationship between (speed of) medium and wavelength? | direct; as a wave travels through a faster medium, wavelength increases |
| What is the unit of propagation speed? | speed = distance/time |
| Rank the following highest to lowest in regards to propagation speed: air, bone, soft tissue, tendon, fat | bone, tendon, soft tissue, fat, air |
| What is the relationship between frequency and speed of sound | TRICK! frequency has no effect on sound speed; for example, the speed of sound in soft tissue is always 1540m/s regardless of frequency |
| What determines the speed at which a wave travels through a medium? | medium ONLY; all sound waves of any frequency, period, intensity, and power travel at the same speed in a given medium |
| What properties of the medium determine sound speed? | density and stiffness |
| What is the relationship between density/stiffness of a medium and propagation speed? | direct; as density/stiffness increases, propagation speed increases |
| What is the relationship between elasticity of a medium and propagation speed? | inverse; as elasticity increases, propagation speed decreases |
| Two sound waves with frequencies of 5 and 3 MHz travel to a depth of 8 cm in the same medium and then reflect back to the surface of the body. Which acoustic wave arrives first at the surface of the body? | TRICK! both sound waves arrive at the same time because all sound waves travel at the same speed in a given medium, regardless of frequency or any other features |
| What is the relationship between power and frequency? | TRICK! there is none |
| Define pulse duration | the TIME a transducer is creating a pulse; does not include listening time |
| What are the units of pulse duration? | units of time; microseconds |
| What is the typical value of pulse duration in diagnostic US? | 0.3-2 microseconds |
| What determines the pulse duration? | the sound source; the transducer |
| What is the equation for pulse duration? | pulse duration = # of cycles x period |
| What is the relationship between frequency and pulse duration? | inverse; as frequency increases, pulse duration decreases (when the # of cycles is the same) |
| What is the relationship between pulse duration and imaging quality? | inverse; as pulse duration increases, axial resolution decreases, and therefore image quality decreases |
| Define spatial pulse length? | LENGTH of one pulse from beginning to end |
| What determines a waves spatial pulse length? | sound source AND medium; SPL depends partially on wavelength, which is determined by the sound source and the medium |
| What is the equation for spatial pulse length? | SPL = # of cycles x wavelength |
| What is the relationship between frequency and spatial pulse length (given same number of cycles)? | inverse; as frequency increases, spatial pulse length decreases (when same # of cycles) |
| Define pulse repetition period | TIME from the start of one pulse to the start of the next pulse; pulse duration plus listening time |
| What is the relationship between imaging depth and pulse duration? | TRICK! there is none; the time the transducer is "pulsing" doesn't change, only the listening time changes |
| What is the relationship between imaging depth and pulse repetition period? | direct; as imaging depth increases, pulse repetition period increases |
| Define pulse repetition frequency | NUMBER of pulses per second |
| What determines the pulse repetition frequency? | bad question.. officially-the source only; but also the medium, indirectly |
| What is the relationship between frequency and pulse repetition frequency? | none. frequency is determined by transducer, PRF is determined by imaging depth |
| What is the relationship between pulse repetition frequency and pulse repetition period? | reciprocal |
| What is the relationship between imaging depth and pulse repetition frequency? | inverse; as imaging depth increases, pulse repetition frequency decreases |
| Pulse repetition frequency and pulse repetition period only change when what is altered? | imaging depth |
| What is a typical value for the duty factor of pulsed sound wave in diagnostic imaging? | 0.001 or 0.1% US - transducers spend the vast majority of time receiving signals and only a small fraction of time transmitting |
| What is the value of the duty cycle for a continuous wave ultrasound? | 1.0 or 100% - continuous wave transducers are always transmitting a signal; no picture is produced |
| What is the relationship between depth of view and duty factor? | inverse; as imaging depth increases, duty factor decreases |
| What is the equation for duty factor? | duty factor = pulse duration/pulse repetition period |
| Define duty factor | the percentage of time the system is transmitting a signal |
| Define Im | maximum intensity (Im) observed in an US wave when averaged over the largest one-half cycle |
| What does the beam uniformity coefficient measure and what is the equation for it? | it's an index of how evenly an US beam is distributed throughout space; BUC = spatial peak / spatial average (SP/SA) |
| From highest to lowest, order the measurements of US beam intensity. | -SPTP/SPPA spatial peak temporal peak/spatial peak pulse average (pulse duration only) -SATP/SAPA spatial ave. temp. peak/spatial ave. pulse ave. (includes listening time) -SPTA spatial peak temp average -SATA spatial ave. temp. ave. (TP aka PA) |
| Which intensity is most closely correlated to tissue heating? | SPTA spatial peak temporal average |
| Define decibel | unit that quantifies the intensity of a sound beam by COMPARING it to a given level on a logarithmic scale; ratio; relative |
| Describe the 3 decibel rule | a change of 3 decibels is equal to doubling (+3) or halving (-3) the intensity of the original sound beam |
| Describe the 6 decibel rule | a change of 6 decibels is equal to 4-fold (+6) or 1/4 (-6) of the intensity of the original sound beam |
| Describe the 10 decibel rule | a change of 10 decibels is equal to 10-fold (+10) or 1/10 (-10) of the intensity of the original sound beam |
| Describe the 20 decibel rule | a change of 20 decibels is equal to 100-fold (+20) or 1/100 (-20) of the intensity of the original sound beam Each 10 dB indicates a tenfold increase or decrease |
| What are the units of attenuation? | decibels; dB |
| Define attenuation | loss of strength of sound as sound travels through a medium |
| What are the contributors to attenuation? | -absorption- conversion of sound to heat -reflection- increases path length -scattering- dispersion of sound in different directions rather than back to the transducer |
| What determines attenuation? | -frequency -path length |
| Define Rayleigh scattering | equal redirection of sound waves that occurs when the tissue interface is much smaller than the wavelength of the sound beam; related to frequency^4 |
| What kind of reflection is caused by a rough or irregular border between two media? | backscatter aka diffuse reflection |
| What kind of reflection is caused by a large, smooth boundary? | specular reflection |
| Define attenuation coefficient | amount of attenuation PER CENTIMETER |
| What is the equation for attenuation coefficient? | atten. coeff. = frequency / 2 |
| What is the equation for total attenuation? | total atten. = atten. coeff. x path length |
| Define half value layer | the thickness of tissue where sound intensity is reduced to 1/2 (-3 dB) of its original value |
| What determines the half value layer? | frequency and medium |
| Rank the following highest to lowest in regards to attenuation: water, muscle, fat, bone, air, blood | air, bone, muscle, fat, blood, water |
Created by:
geminijester617