Define:

Attenuation

What are the units?

The decrease in the three bigness parameters, intensity, power, and amplitude of a sound wave as it travels.

The further US travels, the more attenuation occurs.

Units are in decibels, dB (must be negative, since the attenuation causes intensity to decrease)

Attenuation is _______________ to frequency and distance.

Proportional

What are the factors that affect attenuation?

Frequency and Path Length (Distance)

What happens to attenuation with a shorter distance?

Less attenuation

What happens to attenuation with a longer path length?

More attenuation

What happens to attenuation with lower frequency?

Less attenuation

What happens to attenuation with higher frequency?

More attenuation

What are the three components of attenuation?

absorption

scattering

reflection

Which component of attenuation is particular to bioeffects and why?

Absorption--sound converted into heat

Attenuation ultimately limits the __________  __________ from which meaningful reflections are obtained.

maximum depth

What is the purpose of ultrasound gel?

Gel is used to remove air from the path of the ultrasound beam to reduce attenuation.

What occurs when propagating sound energy strikes a boundary between two media and some returns to the transducer?

Reflection

What is specular reflection?

Reflections from a smooth reflector (mirror) are specular and return in one direction.

Specular reflections also occur when the ____________ is much ____________ than the ____________ in the boundary.

wavelength

smaller

irregularities

What are some examples of specular reflectors and how does it appear on the monitor?

 Diaphragm

Bladder

Tendons

Bright reflection

What is Backscatter or Diffuse Reflection?

The redirection of sound in all directions.

When a boundary is rough, reflected sound is ____________ and ____________

disorganized

random

Back scatter also occurs when the boundary has irregularities that are approximately ____________ ____________ ____________ as the sound's wavelength. 

the same size

What are some good examples of diffuse reflectors and how do they appear on the monitor?

Kidneys

Liver

Thyroid

Heart chambers

Blood vessels

Appear homogeneous (speckled)

What happens when this occurs?

d < λ

Particle diameter is less than the wavelength, sound is is uniformly distributed in all directions. This is known as Rayleigh scattering.

What is Rayleigh Scattering related to?

frequency⁴

What is and organized reflection called?

Specular

What is a disorganized reflection called?

Diffuse or Back Scatter

What is organized scattering called?

Rayleigh Scattering

What is disorganized scattering called?

Scatter

Define:

Attenuation Coefficient

What are the units?

The amount (toll) of attenuation per centimeter (pays). A way to report attenuation without dealing with distance.

dB/cm/Mhz

What is attenuation coefficient related to?

Frequency

True or False?

As frequency of sound increases, the attenuation coefficient increases

True

True or False?

Attenuation coefficient changes when the path length changes.

False

Attenuation coefficient does not change when distance changes.

What is the attenuation coefficient in soft tissue and what is the total attenuation equation?

0.5 db/cm/Mhz

equation:

tot attenuation (dB) = distance (cm) x attenuation coefficient (dB/cm) x frequency (MHz)

Define:

Impedance

What are the units and how are they represented?

What are the typical values in sonography?

It is the resistance to the propagation of sound through a medium.

Rayls, often represented by the letter "Z"

Typical values between 1.25 - 1.75 MZ

What is the Impedance equation?

Z = ρc or Z = ρv

impedance (rayls) = density (kg/m³) x propagation speed (m/s)

True of False?

The less dense a material is, the more it will reflect the sonographic waves.

False

The more dense the material is, the more it reflects the sonographic waves.

What is it called when portions of an image are brighter than the surrounding tissues, or tissues that appear brighter than normal?

Hyperechoic

What term describes structures with equal echo brightness?

Isoechoic

What is it called when portions of an image are not as bright as surrounding tissues, or tissues that appear less bright than normal?

Hypoechoic

An extreme form of hypechoic, meaning entirely without echos (echofree)

Anechoic

A portion of tissue or an image that has similar echo characteristics throughout

Homogeneous

A portion of tissue or an image that has differing echo characteristics throughout

Heterogeneous

Define:

Normal Incidence

Name the 4 synonyms

Normal incidence means that the incident sound beam strikes the boundary at exactly 90 degrees

Remember: PORN

perpendicular

orthogonal

right angle

ninety degrees

Define:

Oblique Incidence

Oblique incidence occurs when the incident sound beam strikes the boundary at any angle other than 90°

What is an oblique incidence less than 90°?

Acute

What is an oblique incidence greater than 90°?

obtuse

Define:

Incident Intensity

The intensity of the sound wave at the instant prior to striking a boundary.

Define:

Reflected Intensity

The portion of the incident intensity that, after striking a boundary, changes direction and returns back from where is came.

Define:

Transmitted Intensity

The portion of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling.

What is the Reflection and Transmission equation?

What are the units?

Define:

Conservation of energy

Incident Intensity (P_i) = Reflected Intensity (P_r) + Transmitted Intensity (P_t)

W/cm²

the law of conservation of energy states that the total energy of an isolated system remains constant; it is said to be conserved over time.

Define:

Intensity Reflection Coefficient (IRC)

The percentage of the US intensity that bounces back when the sound strikes a boundary

Define:

Intensity Transmission Coefficient (ITC)

The percentage of the incident intensity that, after striking a boundary, continues on in the same general direction that it was originally traveling.

IRC Formula?

IRC (%) = (Z₂-Z_1/Z₂+Z₁)² x 100

Define:

Refraction

Refraction is transmission with a bend. Refraction is a change in direction as sound transmits from one medium to another.

What are the two things required for refraction to occur?

oblique incidence and different speeds

Define:

Snell's Law

What is the equation?

Snell's law relates the directions of the wave before and after it crosses the boundary between the two media

θ_t/θ_i = c₂/c₁

sin (transmission angle)/sin (incident angle) = propagation speed 2/propagation speed 1

What do you call the time needed for a pulse to travel to and from the transducer and the reflector?

go-return time

or

time-of-flight

or

round trip time

What is the range equation?

to solve for Time

t = 2d/v

to solve for Velocity

v = 2d/t

to solve for Distance (depth)

d = vt/2

What is the average speed in soft tissue?

1.54 mm/μs

What is the 13 Microsecond Rule?

In soft tissue, every 13 μs of go-return time means the reflector is 1 cm deeper in the body.