Unit 4 - DMU 3313 Physics

high, echoes, interfaces

When pulses of ___ frequency sound (PW) are transmitted into tissues, ___ are created when the sound waves reach ___ between different types of tissue.

location, quality

When echoes return to the source of the sound, they give information about the ___ and ___ of internal structures.

Reflection, Transmission

When sound waves move from one medium to another, there may be: ___ and/or ___.

Two

At least how many mediums are required for something to happen?

Hyperechoic

Describes relatively strong echoes

Hypoechoic

Describes relatively weak echoes

Isoechoic

Means echoes have similar strengths

Anechoic

Means the absence of echoes

Homogeneous

• Echoes are very similar (brightness)

• Describes tissue or structures that appear uniform in their echo patterns. This means the tissue reflects the same amount of sound waves across its entire area, resulting in a consistent brightness or echo intensity on the ultrasound image.

Heterogeneous

• Very different echoes (different brightnesses)

• Refers to a tissue or structure that has a varied, non-uniform appearance or texture, with differing echogenicity (brightness) across different areas.

Simple

• A structure containing only fluid

• Uniform appearance

Complex

• A structure containing solid or debris-like components

• Refers to a structure that contains both fluid-filled (cystic) and solid components, or that exhibits irregularities or internal structures within the fluid or one that has internal septations (walls) or solid intracystic masses (solids within the fluid)

Interface

• The boundary between two different media

• Usually shown as a horizontal line

Angle of Incidence

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

incidence

In wave physics, ___ refers to the event of a wave encountering a boundary or interface between two different media.

Perpendicular Incidence

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

90 degree incidence, normal incidence, orthogonal incidence

What are some synonymous terms for Perpendicular Incidence?

Reflected (R), Transmitted (T)

If there is perpendicular incidence:

• ___ sound travels in a direction opposite to the incident (I), which is back to the source

• ___ sound does not change direction but continues in the same path as the incident sound

vertical

The line perpendicular to the interface is usually shown as a ___ line.

Oblique Incidence

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

Reflected, equal, incidence, perpendicular

• If there is oblique incidence:

• ___ sound does NOT return to the transducer but travels off at an angle ___ to the angle of ___

• The angle is measured from an imaginary line ___ to the interface

incident angle

θ_i

reflection angle

θ_r

transmission angle

θ_t

incident, reflection

The ___ angle is EQUAL TO the ___ angle!

propagation speed

The transmission angle depends on the ___ ___ in each medium.

impedance, two

Z₁ and Z₂ = ___ of ___ different mediums

Stiffness

Impedance = ___

Specular reflectors

___ ___ cause strong reflections off large, smooth surfaces (ONE direction, so stronger); high amplitude

Non-specular reflectors

___ ___ cause weak scattering off large, rough surfaces or small reflectors (MULTIPLE directions, so weaker); low amplitude

Reflected, echoes

___ sound waves (___) from tissues are used to produce images in DMU.

attenuation, gain, constructive, attenuation

Ultrasound waves interact with internal media (tissues) of the human body in various ways. From these interactions, the ultrasound waves may be either weakened through a process known as ___ or may be strengthened through a process known as ___ (___ interference). The most common/frequent interaction is ___.

Attenuation

What do all of these processes/interactions result in?

1. Geometric Redirection:

• Specular reflection

• Scattering

• Refraction

• Diffraction

2. Divergence

3. Destructive Interference

4. Absorption

strong, specular reflectors

Specular Reflection is ___ reflection from ___ ___.

larger, wavelength, smooth, mirror, Impedance (stiffness)

• Specular Reflector (must be all three):

1. An interface that is ___ than the ultrasound ___

2. The interface is also ___ and ___-like

3. ___ (___) Mismatch

   • Z₁ ≠ Z₂

True

True or False: These is no reflection if sound travels through the same medium throughout.

stronger

The whiter the structure, the ___ the reflection.

Stiffness, propagation speed

___ (Z) affects ___ ___ (c)

dimensions

When referring to reflection and scattering, d = ___.

interface

The dimensions (d) refer to the ___.

angle dependent, incidence, 90 degrees, non-frequency dependent

Specular Reflection is ___ ___ because the angle of ___ must be ___ ___ for our transducer to capture the echoes. It is ___-___ ___.

Specular Reflection participates in the ___ of the ___. It diminishes the ___ of the transmitted ultrasound ___.

Incident, Reflection

Transmission = ___ — ___

I, R

T = ___ — ___

Transmission

T =

Incident

I =

Reflection

R =

Scattering

___ is the redirection of sound in many directions by non-specular reflectors

tissue texture

Scattering helps image ___ ___

Parenchyma

The functional tissue of an organ or body part

Backscatter

___ is scatter which travels back in the direction it came from originally

weak, evenly

Scattering produces ___ echoes because the intensity is ___ divided among echoes

Diffuse Reflector, Rayleigh Scatterer

Non-specular Reflectors can be a ___ ___ or a ___ ___

larger, wavelength, rough, reflection, scattering

Diffuse Reflector:

• An interface (d = dimensions) that is ___ than the ultrasound ___

• ___

  • Produces diffuse ___/___

smaller, wavelength, smooth, rough, scattering

Rayleigh Scatterer:

• An interface (d = dimensions) that is ___ than the ultrasound ___

• Either ___ OR ___

  • Produces Rayleigh ___

non-angle dependent, frequency dependent

Scattering is ___-___ ___ because the echoes are reflected in many directions. It is strongly ___ ___.

scatterers, size, impedance, frequency

Scattering intensity is proportional to the total number of ___, ___ of the scatterers, acoustic ___, and ___.

less, incidence

Scattering makes ultrasound (US) imaging ___ dependent on ___ angle.

Refraction

___ refers to the change in direction of ultrasound when it crosses an interface (boundary)

Oblique, angle, propagation speeds, Velocity

There are two requirements for refraction to occur (must be BOTH):

1. ___ incidence

   • Beam approaches at an ___

2. Different ___ ___ on either side of the boundary

   • ___ mismatch

   • c₁ ≠ c₂

lateral, reflectors

***Refraction results in ___ displacement of ___***

reflection, incidence

At oblique incidence, the angle of ___ = the angle of ___

True

True or False: Since the transducer cannot capture the reflected beam from oblique incidence, it is not useful to DMU.

frequency

In refraction, ___ remains the same after crossing the interface.

wavelength

In refraction, ___ changes in proportion to the change in velocity.

normal

The line perpendicular to the interface is also referred to as “the ___”

normal

The angle of transmission can move toward or away from “the ___”

Snell’s Law

The refraction of ultrasound is governed by ___ ___

sinθ_i / sinθ_t = c_i / c_t

Snell’s Law:

sin of the angle of incidence

sinθ_{i =}

sin of the angle of transmission

sinθ_{t =}

velocity of the medium of incidence

c_{i =}

velocity of the medium of transmission

c_{t =}

propagation speed

The degree of beam refraction depends on the difference in ___ ___ between the tissues

increases

Refraction degree increases if the difference in velocity ___

greater

If propagation speed through the medium of transmission is greater than the medium of incidence, the transmission angle is ___ than the incidence angle

smaller

The smaller the difference between the propagation speeds, the ___ the change that will occur

greater

The greater the difference between the propagation speeds, the ___ the change that will occur

critical angle, Total Reflection, low, high

When the angle of incidence becomes a ___ ___, the angle of transmission = 90°. This means that the transmitted beam follows the interface without transmission in the medium, a phenomenon called ___ ___. This phenomenon occurs when the beam travels from a ___ velocity medium into a ___ velocity medium.

c_i / c_t ≅ θ_i / θ_t

Formula for calculating the Refraction Angle:

Snell’s Law

What formula do you use to calculate the Critical Angle?

0

sin 0° =

0.5

sin 30° =

0.707

sin 45° =

0.866

sin 60° =

1

sin 90° =

Huygens’ Principle

When a wave is disturbed, each point where there is a disturbance becomes the source of a spherical wave

disturbance

A ___ is simply when there is something in the way of the wave

Diffraction

___ is the spreading out or bending of sound waves around an obstacle or through a small opening

Lateral

___ resolution is affected by diffraction

increases

If diffraction increases, attenuation ___

size

The degree of diffraction is related to the ___ of the source

larger

The smaller the source, the ___ the diffraction

Divergence

___ refers to the spreading out of the ultrasound wave as it travels farther from its source

Huygens’ Principle

The theoretical explanation of the divergence is offered by ___ ___

weaker

The more divergent the wave becomes (far from the straight path), the ___ it becomes (more attenuated)

False

True or False: A point-like source (very small) emits sound spherically. This is an isotropic emission of sound, which means with equal strength in all directions. This type of emission of sound is extremely efficient in DMU (because it’s very weak).

True

True or False: The larger the source (transducer) becomes, the more directional and less divergent the emission of sound becomes; consequently, the ultrasound wave is less attenuated.

superposition of waves

If two or more ultrasound waves overlap, they will undergo a process known as ___ ___ ___

destructive, constructive, complete destructive

From the superposition of waves, a new wave may result, which can be either:

• Weaker - ___ interference

• Stronger - ___ interference

• Total cancellation - ___ ___ interference