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.
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.
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:
extdistance=extspeedimesexttime$$ ext{distance} = ext{speed} imes ext{time}$$
To measure depth, the formula is applied as:
extdistance=racextspeedimesexttime2$$ ext{distance} = rac{ ext{speed} imes ext{time}}{2}$$
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.
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 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.
Definition: Ability to differentiate between tissue types based on greyscale variation.
Factors: Frequency, machine design, and user settings (like gain).
Definition: Visualizing moving structures in real-time.
Influencing Factors:
Frame rate, pulse repetition frequency, scan line numbers, depth, field of view, focal zones.
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:
Absorption
Reflection
Scattering
Refraction
Divergence
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).
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).
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.
Definition: Images that contain structures not present or misrepresented due to machine assumptions or operator errors.
Types of Artefacts:
Helpful artefacts and hindrances.
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.
Principles of Ultrasound by James Harcus