Ultrasound Physics Part Two Study Notes

Introduction to Ultrasound Physics Part Two

Interaction of Sound Waves with Tissue

• Sound waves sent by the transducer interact with various organs and structures in the body.

  • Some sound waves return to the transducer for image formation.

  • Some sound waves penetrate deeper tissues.

  • Some sound waves are lost during interaction.

• Different structures and surfaces react differently to the sound wave.

Speed of Sound in Tissue

• Varies among different types of tissues and impacts sound wave behavior.

  • The speed of sound affects the quantity of original sound wave energy retrieved by the transducer or the amount capable of further propagation into the body.

Attenuation of Sound Waves

• Defined as the loss of amplitude and intensity of sound waves as they travel through tissue.

• Types of attenuation:

  1. Absorption

  2. Reflection

  3. Scattering

  4. Refraction

• Each type leads to reduced energy available for deeper tissue imaging.

Absorption

• Accounts for the greatest reduction in sound wave energy, up to 80%.

• Caused by friction during interaction with body tissues, resulting in heat generation.

• Factors affecting absorption:

  • Frequency:

  • Higher frequencies lead to greater absorption and reduced energy for wave propagation.

  • Example: A transducer sending a sound wave at 5 MHz attenuates more slowly than at 10 MHz.

  • Depth:

  • Greater depth results in increased absorption as sound waves lose energy as they penetrate.

  • Viscosity:

  • More viscous tissues result in more energy loss (
    Example: Energy expended increases when moving through higher viscous substances like honey versus water.

Reflection

• Occurs when sound waves meet an interface between tissues, causing a portion of energy to be reflected back to transducer.

• Important for imaging formation as returning signals are essential for generating images.

  • Energy not reflected is transmitted into deeper tissues.

• Example scenario:

  • A sound wave traveling from the wall of the left ventricle encounters the ventricular chamber, leading to amplitude differences (initial, transmitted, and reflected waves).

• When sound reflects off angles, directional energy is lost to the transducer.

Scattering

• Occurs when sound waves encounter a ragged or irregular surface.

• Results in sound beam reflection in various directions, causing energy loss toward both propagation and reflection back to the transducer.

• Speckled imaging appearance in ultrasound stems from scattered echoes within adjacent tissues.

Refraction

• Involves changes in the direction of sound waves due to varying propagation speeds in different tissues.

  • If sound encounters an interface at 90 degrees, it will transmit and reflect at the same angle.

  • Refraction occurs with varying propagation speeds and angled incidence:

  • If two media have the same propagation speed, the angle transmitted equals that of the incident angle.

  • Under different speeds, angles become distinct, leading to refracted transmission.

• Snell's Law:

  • $\frac{V<em>1}{V</em>2} = \frac{sin(\theta<em>1)}{sin(\theta</em>2)}$

  • Used to determine angles of transmission when encountering varied speeds.

Example Calculation

• Case: Fat to Muscle transition

  • Given:

  • V1 (Fat) = 1450 m/s; V2 (Muscle) = 1580 m/s

  • Incident angle = 45 degrees

  • To calculate transmission angle:

  • Apply Snell’s Law: 45 degrees to $\theta$ resulting in $\theta = 49$ degrees.

  • Reflected angle equals incident angle.

  • The angle of transmission reflects movement away from normal incidence increasing as speed increases in the second medium.

Image Formation in Ultrasound

• Electrical pulses to the transducer result in bending of crystals emitting sound waves.

• Reflected waves cause similar bending of crystals which translates into images by the ultrasound machine.

Importance of Physics in Ultrasound

• Understanding ultrasound physics is essential for:

  1. Passing the SPI registry exam (Sonography Principles and Instrumentation).

  2. Controlling settings affecting image quality and the ability to obtain images.

  3. Identifying and correcting for potential artifacts in imaging.

Artifacts in Ultrasound

• Defined as discrepancies caused by incorrect assumptions programmed into ultrasound machines.

• Examples: False appearances like mirror images or shadowing that obstructs real pathology viewing.

• Common assumptions made by ultrasound units:

  • Average sound speed applies uniformly (which it doesn’t).

  • Assumed structures encountered at 90 degrees which is often not the case.

• Artifacts can misrepresent actual conditions, leading to misguided diagnosis.

Summary and Study Recommendations

• Important skills and knowledge prerequisites include:

  • Unit conversions and problem-solving techniques (cross multiplication, logs, etc.).

  • Understanding wave mechanics, particularly longitudinal mechanical waves.

  • Hints for studying: Revisit pre-requisite material to ensure a solid foundation before commencing the sonography program.