The weakening of sound waves as they propagate through a medium

Results in a decrease in intensity, power, and amplitude

Reported as a relative change in negative dB

Unrelated to propagation speed

Depends on frequency, travel distance aka path length, and tissue through which the wave travels

Based on logarithms

A ratio comparing two intensities to report relative changes

dB = 10log(intensity₁/intensity₂)

P <> dB

0.001 <> -30

0.01 <> -20

0.1 <> -10

0.0125 <> -9

0.25 <> -6

0.5 <> -3

0.79 <> -1

1 <> 0

1.26 <> 1

2 <> 3

4 <> 6

5 <> 7

8 <> 9

10 <> 10

20 <> 13

40 <> 16

50 <> 17

100 <> 20

1000 <> 30

Reflection

Absorption

Scattering

Redirection of a sound wave that strikes a boundary back to the sound source

Likely to occur when the dimension of the boundary s larger than the wavelength of the sound

Specular vs Diffuse 

Occurs when sound strikes a smooth boundary and is reflected in only one direction in an organized manner

May not actually end up back at the receiver

Diaphragm

Organ boundaries

Heart walls

Vessel walls

Calcifications

Pacemaker

Occurs when a wave reflects off of an irregular surface to radiate in more than one direction

Advantage: Interfaces at suboptimal angles to the sound beam can still produce reflections that will return to the transducer

Disadvantage: Returning signals are lower strength than initial signal

The random redirection of sound in many directions

Occurs when a tissue interface is ≤ wavelength of the sound

In ultrasound, wavelengths are generally between 0.1-0.7mm (based on c/f = λ calculation), while most tissue interfaces within the body are in the μm range

High frequencies scatter sound more than lower frequency beams -> the closer the ratio between the frequency and interface thickness is to 1, the more scattering effect occurs

Occurs when the dimension of the reflector is much smaller than the wavelength

Redirects sound energy in all directions equally

Example: red blood cells

Rayleigh scattering = frequency⁴

Occurs when ultrasonic energy is converted into another energy form (heat)

Largest component of attenuation

Directly related to higher frequencies

The number of decibels of attenuation that occurs when sound travels one centimeter at a constant frequency through an unchanging medium

A way of reporting the amount of energy lost due to attenuation

Units = dB/cm

Directly related to frequency in soft tissue only:

= frequency (MHz) ÷ 2

Total amount of attenuation over the entire path length

Attenuation coefficient (dB/cm) * distance traveled (cm)

The distance sound travels in soft tissue that reduces the sound to one-half of its original intensity

OR the depth of tissue that results in 3dB of attenuation to the intensity

Units = cm

Typical values 0.25-1.0 cm

AKA penetration depth AKA depth of penetration AKA half-boundary layer

The passing of a sound wave through a tissue interface

Necessary to image deeper in the body

The acoustic resistance to sound traveling in a medium

Influences the amount of reflection that occurs

Calculated: density (kg/m³) * propagation speed (m/s)

Units = rayls (Z)

Typical values = 1,250,000-1,750,000 rayls

The angle at which the wave strikes the tissue interface

Determines the behavior of the pulse

Normal (Right) and Oblique (Obtuse and Acute)

The sound beam strikes the boundary at 90°

AKA perpendicular AKA orthogonal AKA right angle

Reflection will only occur if the media on either side of the boundary have different impedances

Occurs when the sound beam strikes the boundary at any angle other than 90°

Includes acute and obtuse angles

The sound wave's intensity before it strikes a boundary

Reflected intensity + transmitted intensity

Percentage of the intensity that is reflected when a beam strikes a boundary

Bone and soft tissue ≈ 50%

Air and soft tissue ≈ 99%

Soft tissue and soft tissue ≈ 1%

The percentage of the intensity that continues forward when a beam strikes a boundary between two media

= 100% - intensity reflection coefficient

% = [(Z₂ - Z₁)/(Z₂ + Z₁)]² * 100

Z₁ = medium 1 = the medium that the sound is currently in

Z₂ = medium 2 = the tissue that the sound is entering

% = (transmitted intensity ÷ incident intensity) * 100

OR 

1 - intensity reflection coefficient

VERY COMPLEX

May occur between two media with same impedance

Incident intensity = transmitted + reflected energy

Reflection angle = incident angle

Transmission with a "bend" or a change in direction of sound

Occurs with oblique incidence between two media with different propagation speeds

Snell's Law: [sin(transmission θ)/sin(incident θ)] = (medium 2 speed)/(medium 1 speed)

Shortcut: if speed₂ < speed₁, then transmission θ < incident angle (same with = and >)