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}$$$ ext{distance} = ext{speed} imes ext{time}$$

  • To measure depth, the formula is applied as:
    $ext{distance} = rac{ ext{speed} imes ext{time}}{2}$$$ ext{distance} = rac{ ext{speed} imes ext{time}}{2}$$

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.