In The Name Of God
CHAPTER 2
Attenuation in tissues
Attenuation includes the effects of
both scattering and absorption in the
loss of pressure amplitude as the
ultrasound wave propagates through
a medium.
Amplitude attenuation coefficient (α)
The amplitude attenuation coefficient (α) is
given by sum of the scattering coefficient
(αs ) and absorption coefficient (αa)
α= αa +αs and we have:
p  pmeax
0
Maximum particle velocity (u0) and
maximum displacement (s0) are related
according to: u0=p0/ρc and s0=p0/(2пfcρ)
Amplitude
Absorption
coefficient
ultrasonic beam follows
function.
of
the
an exponential
p  pmex
0
P0 and Pm are pressure and peak pressure
at a point
Intensity describes the amount of energy
flowing through a unit cross sectional
area per unit of time.
The intensity of the beam also decreases
exponentially with distance from the
z
I

I
e
sound source:
m
  2a
The special unit for these coefficient is
the Neper (NP) per centimeter.
The attenuation coefficient at 1 MHZ
for various tissue types are different.
Level (db)= 10 log
A
( Max )2  ( I max )
A
I
Level (db)= 20 log
A
( Max ) 10log( I max )
A
I
dB=8.868np
0
.
693
Ln
2
HVL 



IMAX
I n
2
nnumbern of HVL
The intensity loss in decibles caused
by attenuation as the ultrasound
beam passes through a medium is:
Intensity loss (dB) = μfz
Attenuation refers to all processes
(except reflection) that act to reduce
ultrasound intensity.
100
loss (dB)  10 log(
)
%R