Key terms Test2 Flashcards
Intensity
a beam’s power divided by its area
5 key words related to intensity
spatial
peak
average
temporal
pulsed
intensity measurement methods
SPTP
(spatial peak temporal peak)
SATP
(spatial average temporal peak)
SPTA
(spatial peak temporal average)
SPPA
(spatial peak pulse average)
SAPA
(spatial average pulse average)
SATA
(spatial average temporal average)
commandments of intensity
- intensities may be reported in various ways with respect to time and space
- the different measurements of intensities are important in the study of bioeffects
- because peak measurements are larger than average measurements, SPTP intensity has the highest value, and SATA has the lowest value.
decibel notation
standard measurement tool used to report changes in the degree of attenuation
logarithms
novel method of rating numbers
log-the number of 10s that are multiplied to create a number
(log of 100 is 2)
logrithmic scale is a relative scale
positive and negative decibels
positive decibels report signals that are increasing in strength (getting larger)
negative decibels describe signals that are decreasing in strength (getting smaller)
decibels and intensities
decibels require two intensities:
reference(starting level) and actual level at time of measurement
attenuation
weakening of the soundwave as it moves through a medium
decrease in intensity, power, and amplitude as sound travels
determined by two factors:
path length and frequency of sound
distance and attenuation
distance and attenuation are DIRECTLY related
(the longer the distance the more the sound attenuates)
frequency and attenuation
Frequency and attenuation are DIRECTLY related
(the higher the frequency, the more the sound attenuates)
reflection
as a soundwave hits a boundary, part of it’s energy is redirected (reflected) back to the sound source
specular and diffuse
scattering
random redirection of sound in many directions
absorption
occurs when ultrasonic energy is converted into another energy form, such as heat
absorption is DIRECTLY related to frequency
(higher frequency waves attenuate more than lower frequency waves)
specular reflection
when the boundary is smooth (mirror), the sound is reflected in only one direction in an organized manner
diffuse reflection
when a wave reflects off an irregular surface, it radiates in more than one direction
rayleigh scattering
special form of scattering that occurs when the structure’s dimensions are much smaller than the beam’s wavelength
attenuation coefficient
the number of decibels of attenuation that occurs when sound travels one centimeter
units-dB/cm
attenuation coefficient in soft tissue
0.5 dB/cm/MHz
attenuation in media
air-extremely high attenuation
bone&lung-very high
biologic fluids-low
water-extremely low attenuation
half value layer
the distance sound travels in a tissue that reduces the intensity of sound to 1/2 its original value
half value layer thickness depends on two factors:
the medium, the frequency of sound
impedance
the acoustic resistance to sound traveling in a medium
calculated by multiplying the density of a medium by the speed at which the sound travels in a medium
range equation
time of flight (go-return time)
elapsed time from pulse creation to pulse reception
time of flight is DIRECTLY related to how deep a sound pulse travels
(greater distances prolong the time of flight, while lesser distances shorten the time of flight)
PRP imaging depth
pulse repetition period is DIRECTLY related to the maximum imaging depth
- when the depth of view is shallow, PRP is short
- when the depth of view is deep, PRP is long
PRF imaging depth
period repetition frequency is INVERSELY related to the maximum imaging depth
- when the depth of view is shallow, PRF is high
- when the depth of view is deep, PRF is low
reflection and transmission
when a sound beam strikes a tissue boundary at a 90o angle (normal incidence), reflection occurs only if the media on either side of the boundary have different impedances
- no reflection will occur if the two media have identical impedances
- a small reflection will occur if the impedances are slightly different
- a large reflection will occur if the impedances are substantially different
