Physics Unit 4

What information is given by the echoes return to the source of the sound?

What information is given by the echoes return to the source of the sound?

Location and Quality

What occurs when sound waves move from one medium to another?

Reflection and/or Transmission

Describes relatively strong echoes

Hyperechoic

Describes relatively weak echoes

Hypoechoic

Echoes have similar strengths

Isoechoic

Absence of echoes

Anechoic

Angle between the direction the ultrasound beam is traveling and a line perpendicular to the interface between the two media

Angle of Incidence

The boundary between two different media

Interface

Refers to the direction of travel of ultrasound waves perpendicular to the interface between two media

Perpendicular Incidence

Normal incidence, orthogonal incidence, and 90 degree incidence are all synonyms to…

Perpendicular incidence

For perpendicular incidence what direction does the reflected sound travel in relation to the incident?

Opposite direction (back towards the source)

Does the transmitted sound go the same or opposite direction of the incident sound?

Same direction

the direction of travel of the incident sound that is not perpendicular to the boundary between two media

Oblique incidence

The texture of organs is the result of ____ reflectors

non-specular

An interface that is larger than the ultrasound wavelength

Specular reflectors

Texture of specular reflectors

Smooth

Specular Reflection equation

Transmission = Incident - Reflection

The two types of Non-specular Reflectors

Diffuse Reflector & Rayleigh Scatterer

An interface that is larger than the ultrasound wavelength, but is rough

Diffuse Reflector

It is smaller than the ultrasound wavelength, and either smooth or rough

Rayleigh scatter

What texture does diffuse reflector have?

Rough

What texture does Rayleigh scatter have?

Rough or smooth

Sound waves are weakened

attenuation

Sound waves are strengthened

Gain

The processes that result in attenuation of the beam (7)

Specular Reflection

Refraction

Scattering

Diffraction

Divergence

Destructive interference

Absorption

The change in direction of ultrasound when it crosses an interface

Refraction

What are the two requirements for refraction to occur?

Oblique incidence and Different propagation speeds

This depends on the difference in propagation speed between the tissues

Degree of beam refraction

Refraction degree ____ if the difference in velocity increases

increases

What does frequency do after crossing the interface?

Remains the same

This changes in proportion to the change in velocity

Wavelength

Snell’s Law

When the incidence becomes a critical angle, the angle of transmission is …

90 degrees

Occurs when the beam travels from a low velocity medium into a high velocity medium

Total Reflection

Angle = 0 degrees, Sin is…

0.000

Angle = 30 degrees, Sin is…

0.500

Angle = 45 degrees, Sin is…

0.707

Angle = 60 degrees, Sin is…

0.866

Angle = 90 degrees, Sin is…

1.000

Th redirection of sound in many directions

Scattering

Scatter which travels back in the direction it came from originally

Backscatter

What is parenchyma?

Tissue texture

What is scattering produced by?

Non-specular reflectors

What texture does diffuse reflectors reflect off of?

Rough texture

What texture does Rayleigh scatterers reflect off of?

Smooth or rough

Spreading of the ultrasound beam after passing through a small opening or around an obstacle

Diffraction

The smaller the source, the ____ the diffraction

Larger

If diffraction increases, attenuation will ____

increase

Spreading out of the ultrasound wave as it travels farther from its source

Divergence

The more divergent the wave becomes, the ____ it becomes

weaker

The grainy appearance of the sonographic imaging

Acoustic Speckle

What factors contribute to attenuation?

Absorption, reflection, scattering, refraction

The value of attenuation in dB per centimeter the wave travels for each Megahertz

Attenuation coefficient

What is attenuation coefficient units

dB/cm - MHz

What is the attenuation coefficient in soft tissue?

0.5 dB/cm - MHz

Attenuation Equation

Att. = ac x f x L

Attenuation and frequency are ______ ______

Directly Proportional

Attenuation and Path Length are _____

Directly proportional

Penetration determines the ____

Upper limit of frequency

As frequency increases, penetration _____

decreases

As penetration increases, frequency _____

decreases

What determines the upper limit of frequency

Penetration

What determines the lower limit of frequency?

Resolution

High frequency provides ____ resolution

better

Low frequency transducer are used for _____

Deep structures

The thickness of material that will reduce the intensity to half its original value

HVL

What does HVL stand for?

Half-Value Layer

HVL equation

L = Att. / ac x f

What is the HVL of soft tissue

6 cm/ f

Increased attenuation from strongly attenuating structures

Shadowing

Decreased attenuation from weakly attenuating structures

Enhancement

When the sound travels faster or slower than the machine assumes

Propagation speed error

What happens during a propagation speed error

Objects appear deeper or shallower than they actually are or objects appear a different shape than they actually are