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bjlewis44
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Ultrasound
High frequency "sounds" inaudible to the human hear. >20kHz
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Sound
- traveling variation of acoustic variables
- ; audible 20Hz - 20kHz
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Frequency
number of cycles in a wave that occur in one second.
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Acoustic variables
- Pressure
- Density
- Particle Motion
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Wave
traveling variation of some quantity or quantities
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Hertz
one cycles per second (Hz)
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Kilohertz
- 1000 cycles per second
- kHz
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Megahertz
- in million cycles per second
- MHz
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Infra
"below" audible sounds and is a frequency too low for human hearing
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ultrasound
"above" a frequency too high for human hearing.
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Period
time it takes for one cycle
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Frequency and period relationship?
- Inverse.
- As frequency increases period decreases.
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Wavelength
the length of a cycles in space
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Propagation Speed
speed at which a wave moves through a medium
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Frequency and Wavelength relationship?
- inversely
- as frequency increases, wavelength decreases
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Propagation speeds in solids and gases
- highest in solids
- lower in gases
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Propagation speed in tissue
1.54 mm/microsec.
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Harmonics
even and odd multiples of fundamental frequency
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What is PRF?
- Pulsed Repetition Frequency
- # of pules per second
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What is PRP
time from beginning of one pulse to the next
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PRF and PRP relationship?
inversely
- PRF increases
- PRP decreases
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Pulse duration
TIME for a pulse to occur
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Duty Factor
fraction of TIME that pulsed ultrasound is on
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PRP increases, PRP decreases and Duty Factor...
increases
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Spatial Pulse Length
DISTANCE in space that a pulse takes up
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# of Cycles in a pulse increases, SPL increaces. If frequency increases, wavelength and SPL....
decrease
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What do shorter pulses do to image resolution
improves resolution
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Bandwidth
range of frequencies contained in a pulse
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Intensity (I)
rate at which energy passes through a unit area
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How is Intesity related to power?
- proportional
- Intensity = power (mW)/area (cm^2)
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If beam power increases, intensity increases. If beam area ____, intensity _____.
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If frequency increases penetration ______
decreases
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Attenuation
weakening of sound as it propagates
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If the attenuation coefficient ______, attenuation ______
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If path length _____ attenuation ______
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As frequency ______ increases, attenuation ______.
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Perpendicular Incidence
denotes a directoin of travel of US wave perpendicular to the boundary between two media.
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Impedance
determines how much of an incident sound wave is reflected back into the first medium and how much is transmitted into the second medium.
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If density _____, impedance ______.
If propogation speed _______, impedance _____.
- increases
- increases
- increases
- increases
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Oblique incidence
denotes a direction of travel of the incident US that is NOT perpendicular to the boundary between two medium.
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Refraction
"turn aside" - change of direction of sound when it crosses a boundary
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Scattering
redirection of sound in many directions by rough surfaces, like tissue and blood.
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Backscatter
sound scattered in the direction from which it came.
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Speckle
grainy dots or "noise" in the display representing interference between constructive and destructive.
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