Attenuation and Absorption

Question 1

What is absorption?

Answer 1

Dissipation of acoustic energy (into heat) which leads to a reduction in signal amplitude

Question 2

What is attenuation?

Answer 2

Any amplitude reduction (geometric spreading, scattering, absorption..)

Question 3

What is α?

Answer 3

The absorption coefficient which in the plane wave equation p(x,t) gives the decay in amplitude
Units: 1/cm = Nepers/cm
or dB/cm

Question 4

How can do we convert from dB/cm to Np/cm?

Answer 4

α(dB/cm) ≈ 8.7α (Np/cm)

Question 5

What happens to amplitude of wave after travelling a distance of x = 1/α?

Answer 5

The wave will decay and amplitude will fall by 1/e ≈ 0.37

Question 6

In decibels what is equal?

Answer 6

The pressure and intensity absorption coefficient

Question 7

What are the absorption mechanism for monoatomic gases?

Answer 7

Viscosity: Different parts of the medium moving with different velocities exert a
shear stress that resists relative motion (friction within fluid)

Thermal conductive

Question 8

What are the absorption mechanisms in more complex fluids?

Answer 8

Viscous

Relaxation: After disturbance of external degrees of freedom by an acoustic wave,
there is a characteristic relaxation time to re-establish equilibrium

Question 9

What is the frequency dependence for viscous absorption?

Answer 9

α = α_0 f^2 so significant at higher frequencies

Question 10

What is the frequency dependence for relaxation absorption?

Answer 10

α = α_0 f^2 for ωt &laquo_space;1
α = α_0 f for ωt&raquo_space; 1

Question 11

How do cellular-level absorption mechanisms work in the blood?

Answer 11

Viscous losses due to relative motion between cells and plasma which have different densities (significant < 10 MHz)

Question 12

How do molecular-level absorption mechanisms work in the blood?

Answer 12

Viscous losses due to internal friction (grows with f^2)

Large number of molecular relaxation processes associated with different blood proteins (significant at least from 0.1-100MHz)

Question 13

When does diffusive scattering from blood cells become significant?

Answer 13

> 10 MHz

Question 14

How does the attenuation grow in blood?

Answer 14

with ~ f^1.6

Question 15

What is the power law for attenuation in soft tissues?

Answer 15

α = α_0 f^y (Np/cm)
α = 8.7 α_0 f^y (dB/cm)

a_0 (dB/cm/MHz^y)
y is frequency dependence

Question 16

What is the rough rule of thumb of the attenuation of tissue?

Answer 16

~ 1 dB per MHz per cm

Question 17

What frequencies are better for deep tissue imaging?

Answer 17

lower frequencies

Question 18

What happens to the ultrasound as the wave travels further and does it lead to?

Answer 18

The further it travels the more they get absorbed

This means reflections from deeper features will have smaller amplitudes

Question 19

What is the time gain compensation?

Answer 19

Detected signal is multiplied by exponentially increasing factor using amplifier with adjustable gain

Gain is set proportional to attenuation coefficient

Question 20

How is compensation applied?

Answer 20

It is applied automatically but sliders can be used to make further adjustments

Question 21

Where does the heat go from the mechanisms?

Answer 21

It ends up as heat as thermal molecular motion

Question 22

Why is knowledge of absorption coefficient important?

Answer 22

Thermal molecular motion is the basis of ultrasonic thermal ablation, treatment planning and assessing safety

Question 23

Why is time gain compensation used?

Answer 23

To correct for attenuation effects

Question 24

How does a plane wave decay in pressure?

Answer 24

by e^ - αx

Question 25

How does a plane wave decay in intensity?

Answer 25

e^ - 2αx