Ultrasound Physics Ch 6

sound waves ___ as they travel in the body

weaken, or attenuate

logarithm

represents the number of 10s that are multiplied to create the original number

decibel (dB)

common unit for measuring electrical signal strength and brightness of images

report relative changes

decibel notation

relative measurement

a comparison

a ratio

logarithmic

positive decibels

report signals that are increasing in strength, or getting larger

negative decibels

describe signals that are decreasing in strength, or getting smaller

3dB

double

10dB

ten times larger

-3dB

half

-10dB

one-tenth

attenuation

decrease in intensity, power, and amplitude as sound travels

attenuation is determined by:

path length

frequency of sound

more attenuation

longer distances

higher frequencies

less attenuation

shorter distances

lower frequencies

three processes that contribute to attenuation

reflection

scattering

absorption

reflection

weakens the portion of the sound wave that continues in the forward direction

two forms of reflection

specular

diffuse

specular reflection

when the boundary is smooth, the sound is reflected in only one direction in an organized manner

diffuse reflection (backscatter)

when a wave reflects off an irregular surface, it radiates in more than one directions

scattering

random redirection of sound in many directions

rayleigh scattering

occurs when a structure’s dimensions are much smaller than the beam’s wavelength

relationship between rayleigh scattering and frequency

rayleigh scattering is proportional to frequency^4

absorption

occurs when ultrasonic energy is converted into another energy form

attenuation coefficient

number of decibels of attenuation that occurs when sound travels one centimeter

Units: dB/cm

attenuation equation

total attenuation (dB) = atten. coefficient (dB/cm) x distance (cm)

attenuation coefficient equation

atten. coef. (dB/cm) = frequency (MHz)/2

atten. coef. = 0.5 dB/cm/MHz

half-value layer thickness

distance sound travels in a tissue that reduces the intensity of sound to ½ its original value

Units: cm

impedance

acoustic resistance to sound traveling in a medium

impedance equation

impedance = density x speed

normal incidence

the incident sound beam strikes the boundary at exactly 90 degrees

oblique incidence

incident sound beam strikes the boundary at any angle other than 90 degrees

incident intensity

the sound wave’s intensity immediately before it strikes a boundary

reflected intensity

intensity of the portion of the incident sound beam that after striking a boundary, returns back from where it came

transmitted intensity

intensity of the portion of the incident beam that after striking a boundary, continues forward in the same general direction that it was traveling