Principles of Ultrasound by James Harcus

Module Learning Outcomes

• Physical Principles: Describe and explain the fundamental physical principles behind cross-sectional imaging modalities and how different parameter selections can impact outcomes.
• Quality Assurance: Assess equipment operation's acceptability using Quality Assurance, Quality Control, and radiation dosimetry tests.
• Hazards and Errors: Evaluate potential hazards and errors (both system and human) within cross-sectional imaging environments, identify their causes, and propose solutions.
• Patient Communication: Emphasize the importance of effective verbal and written communication for obtaining informed patient consent.
• Risk Communication: Identify, explain, and communicate the risks associated with cross-sectional imaging technologies to patients, caregivers, healthcare teams, and the general public.

Introduction to Ultrasound

• Objectives: By the end of the session, you will be able to:
  • Describe the basic process of ultrasound image formation.
  • Explain and evaluate ultrasound image resolution.
  • Understand how ultrasound signals are attenuated in the body.
  • Explain ultrasound safety and associated risks.
  • Describe the Doppler effect in ultrasound.
  • Recognize common ultrasound artefacts.

Image Formation in Ultrasound

• Soft Tissue Interfaces:
  • Reflections occur at soft tissue interfaces due to changes in acoustic impedance (Z).
  • Greater echoes result in brighter pixel values in the ultrasound image.
• Depth Measurement:
  • Images are formed by understanding the relationship between speed, distance, and time.
  • Transducer Formula:
    ext{distance} = ext{speed} imes ext{time}
  • To measure depth, the formula is applied as:
    ext{distance} = rac{ ext{speed} imes ext{time}}{2}

Transducer Design

• Construction:
  • Transducer consists of an array of PZT crystals, usually between 128-256 elements, arranged in a line.
  • Each element can be fired sequentially to create a series of image lines.

Image Resolution

• General Resolution:
  • The ability to distinguish between separate entities in the image.
  • Types include:
  • Spatial Resolution: Distance between entities.
  • Contrast Resolution: Differences in shades of grey.
  • Temporal Resolution: Ability to see moving structures.
• Spatial Resolution:
  • Determined by wavelength; shorter wavelengths (higher frequencies) yield better spatial resolution.
  • Includes:
  • Axial Resolution: Along the beam's long axis, influenced by pulse length.
  • Lateral Resolution: Across the beam's width, relies on beam width and focusing.

Axial and Lateral Resolution

• Axial Resolution:
  • Dependent on pulse length, generally smaller than the wavelength for better resolution.
  • Longer pulses reduce axial resolution.
• Lateral Resolution:
  • Depends on beam width and element size; higher frequency and smaller elements improve resolution.

Contrast Resolution

• Definition: Ability to differentiate between tissue types based on greyscale variation.
• Factors: Frequency, machine design, and user settings (like gain).

Temporal Resolution

• Definition: Visualizing moving structures in real-time.
• Influencing Factors:
  • Frame rate, pulse repetition frequency, scan line numbers, depth, field of view, focal zones.

Attenuation of the Beam

• Concept: Ultrasound beams lose energy and intensity as they propagate through tissues, affecting signal strength and echo detail.
• Key Factors:
  • Tissue type (density relationship), frequency (higher frequencies result in more attenuation).
• Five Main Processes:
  1. Absorption
  2. Reflection
  3. Scattering
  4. Refraction
  5. Divergence

Absorption in Tissues

• Effect of Absorption:
  • The primary cause of attenuation, significant at higher frequencies.
  • Higher density tissues absorb sound more effectively, converting energy to heat (both a hazard and potential benefit).

Safety in Ultrasound

• General Safety:
  • Ultrasound is considered 'safe', but increased use and power raise concern for heating, cellular function alteration, and tissue structural changes.
  • The ALARA principle (As Low As Reasonably Achievable) applies to minimize risks.
• Heating Effects:
  • Heat produced due to absorption monitored by the Thermal Index (TI).
• Cavitation:
  • Formation of gas microbubbles from oscillating waves, relevant mostly in gas-containing tissues, monitored by the Mechanical Index (MI).

The Doppler Effect

• Concept:
  • Used to assess blood flow within structures; identified by changes in frequency based on motion.
  • Notable examples include the sound variation of an ambulance.
• Practical Application:
  • Identifying blood flow presence, direction, speed, and power.

Artefacts in Ultrasound

• Definition: Images that contain structures not present or misrepresented due to machine assumptions or operator errors.
• Types of Artefacts:
  • Helpful artefacts and hindrances.

Summary

• Key Learning Points:
  • Describe ultrasound image formation.
  • Explain resolution techniques in ultrasound imaging.
  • Understand signal attenuation in the body and its implications.
  • Discuss ultrasound risks and safety practices.
  • Describe the Doppler effect's role in imaging.