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