Sono Physics Ch.6-7

Interactions of Sound and Media

What is Attenuation?

The gradual weakening of a sound wave as it travels through tissue.

Decibel Notation ($dB$)

◦ Measure a change in signal strength

◦ Based on a logarithmic scale

◦ Compare a starting value to a final value

Why dB uses Logs?

Because ultrasound Intensity changes over extremely large ranges.

What are Decibels - dB?

Measures how much a beam’s strength changes as it travels through the body.

Compares 2 Intensity Levels:

◦ Original intensity (before)

◦ New intensity (after)

Common dB Values:

+3dB = Intensity x 2

-3dB = Intensity/ 2

+10dB = I x 10

-10dB = I / 10

Decibels Measured as a Ratio

Ratio = New Intensity / Original Intensity

Ex: 100 → 10

Intensity decreased by a factor of 10

So, Change = -10dB Decibels measures a difference, not an actual Value

Components of Attenuation:

Reflection, Scattering and Absorption

What are two factors that Increase Attenuation?

-Distance (Path Length)

-Frequency

Higher Frequency = Greater Attenuation

Frequency

Higher Frequency = Greater Attenuation

Lower frequency = less attenuation and deeper penetration

Reflection

Occurs when sound encounters a boundary between two tissues

◦ Part of the sound wave returns to the transducer

◦ Reflection is the most important component of Attenuation because it creates the echoes used to form an image

Specular Reflection

Occurs at smooth, flat boundaries

◦ Vessel walls

◦ Diaphragm

◦ Organ capsules

Echoes return in a single organized direction

Limitations of Specular Reflection

◦ Requires the beam to strike surface nearly perpendicular

◦ If beam is off-axis, echo may miss transducer

Diffuse Reflection (Back Scatter)

Occurs when sound strikes an irregular surface

◦ Echoes are scattered in many directions

◦ Most Images are created from Backscatter

Advantage:

◦ Produces echoes even when the beam is not perpendicular

Limitation of Diffuse Reflection (Backscatter):

Weaker echoes than specular reflection

Scattering

Occurs when the reflecting surface is very small

◦Sound waves are redirected in many directions

◦About the same size or smaller than the sound's wavelength

◦Lung tissue scatters sound strongly because of the many tiny air-filled alveoli.

◦Higher frequency sound scatters more than lower frequency sound

Scattering Relationship with Frequency is what?

Directly Related

Rayleigh Scattering

Occurs when scatterer is much smaller than wavelength

◦ Sound is scattered equally in all directions

◦ Omnidirectional scattering

Ex: Red Blood Cells

Rayleigh Scattering Relationship with Frequency is what?

Inversely Proportional

Rayleigh Scattering ∝ Frequency⁴

◦ If frequency doubles:

2⁴ = 16

◦ Scattering increases 16-fold

Absorption

Occurs when sound energy is converted into another form of energy

◦ Most commonly converted into heat

Important Facts:

◦ Bone absorbs more sound than any other tissue

◦ Absorption is directly related to frequency

◦ Absorption is the largest contributor to attenuation

Absorption Relationship with Frequency is what?

Directly Related

Higher Frequency → More Absorption

Lower Frequency → Less Absorption

Attenuation Coefficient

The amount of sound energy lost per centimeter traveled

Units: dB/cm

In Soft Tissue:

Attenuation Coefficient Formula:

≈ Frequency ÷ 2

or 0.5 dB/cm/MHz

Attenuation Coefficient Relationship with Frequency is what?

Directly Related

Higher Frequency → Higher Attenuation per cm

Attenuation Coefficient Example

12 Mhz ÷ 2 = 6 dB/cm

Total Attenuation Formula

Total Attenuation (dB) = Attenuation Coefficient (dB/cm) x distance (cm

Half-Value Layer Thickness (HVL)

The distance sound travels before its intensity is reduced by 50%

◦ 50% intensity loss = 3 dB decrease

Units:

◦Cm

◦ Any unit of length

Also Known As:

◦ Penetration Depth

◦ Depth of Penetration

◦ Half-Boundary Layer

Thin Half-Value Layer:

More Attenuation

◦High-frequency sound

◦Highly attenuating media

◦Bone

◦Lung

Thick Half-Value Layer:

Less Attenuation

◦Low-frequency sound

◦Low-attenuating media

◦Water

◦Fluids

Acoustic Impedance

Resistance a medium offers to the transmission of sound.

Key Facts:

Symbol = Z

Measured in Rayls (MRayls)

◦ Calculated, not directly measured

Typical Values in Soft Tissue = 1.25 – 1.75 MRayls

Impedance Formula

Z = Density × Propagation Speed

Tissue Impedance

◦ Dense tissues resist sound more

◦ Stiff tissues transmit sound faster & have higher impedance

Tissue Impedance is measured not calculated

Higher Density + Higher Speed = Higher Impedance

Why is Impedance Important?

Reflection occurs when sound encounters a boundary between tissues with different impedances.

Small Difference:

◦ Little reflection

◦ Most sound transmitted

Large Difference:

◦ Strong reflection

◦ Less transmission