L4: Ultrasound Interaction and Attenuation

Lec 4

Attenuation

Decrease/weakening of the sound as it propagates through a medium

Attenuation

Reduction in amplitude and intensity as sound travels

Decibel (dB)

Unit of Attenuation is ___________

Total attenuation (dB) = attenuation coefficient (dB/cm) x path length (cm) x frequency

What is the formula for Total Attenuation?

Low-frequency ultrasound

Worse resolution

Less attenuation

Deeper penetration

High-frequency ultrasound

Better resolution

Greater attenuation

Less penetration

Frequency of the Sound

Type of Tissue

Path Length

The amount of attenuation that occurs will depend on the: (3)

Attenuation Coefficient

The relationship of attenuation to distance and depends on the tissues traversed and the frequency of the ultrasound wave

Decibel per centimeter (dB/cm)

Unit for Attenuation Coefficient is _________

0.18

Attenuation Coefficient
Material: Blood

0.6

Attenuation Coefficient
Material: Fat

0.5 - 0.8

Attenuation Coefficient
Material: Soft Tissue

20

Attenuation Coefficient
Material: Skull

40

Attenuation Coefficient
Material: Lung

0.0022

Attenuation Coefficient
Material: Water

Directly Proportional

What is the relationship of Attenuation to Attenuation Coefficient?

Ultrasonic Half Value Layer Thickness

Refers to the thickness of a particular tissue medium required to attenuate the ultrasound beam intensity by 50%, which results in a 3 dB reduction in intensity

50%, 3 dB

Refers to the thickness of a particular tissue medium required to attenuate the ultrasound beam intensity by ___, which results in a ___ reduction in intensity

cm or any unit of distance

Unit for HVT/HVL ___

Inversely Proportional

What is the relationship of HVL to Frequency and Attenuation Coefficient?

Reflection

Refraction

Scattering

Absorption

Divergence

Main Processes causing Attenuation: (5)

Incident Energy

Sound hits the acoustic interface

Echo

Reflected sound beam

Transmitted Sound

Sound that penetrates the acoustic interface

Reflection

The major interaction of interest for diagnostic ultrasound

Angle of Incidence

Refers to the angle of deviation from a perpendicular line to the surface of the tissue

Width of Tissue Boundary/Reflector

If the tissue boundary width is less than the wavelength of the ultrasound wave, the ultrasound wave will not be reflected

Specular or Non-specular/Diffuse

Reflectors can either be ____________

Acoustic Impedance Mismatch

The difference in acoustic impedance between two tissues accounts for the amount of reflection that will occur at the tissue border.

increase, increase

An ____ in acoustic impedance mismatch will ____ the rate of Reflection.

Absorption

It is the dominant factor contributing to attenuation of ultrasound in soft tissues

Viscosity

A measure of the frictional forces between particles of the medium as they move past one another. The greater these frictional forces the more heat generated by the vibrating particles.

Relaxation Time

The length of time taken by medium particles to revert to their original positions within the medium following displacement by an ultrasound pulse.

Frequency of Sound

The _____________ affects the amount of heat generated through both the viscous drag and the relaxation process.

Scattering

It is responsible for providing the internal texture of organs in the image

Speckle

The random image pattern created by backscatter is termed ______

Rayleight Scattering

When the scatter is equal in all direction

Refraction

The deviation of ultrasonic energy into new directions contributes to the loss of beam intensity.

Snell’s Law

Describes the physics of refraction

Snell’s Law

Measure relationship between angle of incidence and angle of refraction when a beam of sound passes through an interface between two tissues where the speed of sound is different

Divergence

Spreading out of beam energy as it moves away from the source

Acoustic Boundaries

Positions within tissue where the acoustic impedance change. These boundaries are very important in ultrasound imaging.

Tissue Interface

Acoustic Boundaries is also known as _____

Acoustic Window

Compartments which allow ultrasound to pass readily through them

Acoustic Barriers

Impedes transmission of ultrasonic energy

REFRACTION

Required an oblique incident sound wave and a different propagation speed of sound wave on the side of boundary.