Ultrasound Physics Registry Review

Author

• Melessa Cizik RDMS, RVT
• Website: myultrasoundtutor.com
• Edition: 2020

Table of Contents

1. Physics Principles
   • Properties of Sound
   • Properties of the Medium
   • Sound Propagation in Tissue
   • Resolution
2. Transducers
   • Construction and Function
   • Types
3. Imaging Principles and Instrumentation
   • Pulse-Echo Principle and Modes
   • Image Processing and Instrumentation
   • Harmonics
   • Artifacts
4. Doppler and Hemodynamics
   • Doppler Principles and Instrumentation
   • Hemodynamics
5. Safety and Quality Assurance
   • Bioeffects and Safety
   • Quality Assurance
   • New Technologies

Physics Principles

Properties of Sound

• Sound is a mechanical, longitudinal wave.
  • Infrasound: < 20 Hz
  • Audible sound: 20-20,000 Hz
  • Ultrasound: >20,000 Hz
  • Diagnostic US: 2-20 MHz
• Common Unit Prefixes:
  • kila (k) = thousand
  • mega (M) = million
  • centi (c) = 1/100
  • milli (m) = 1/1,000
  • micro = 1/1,000,000
• Frequency: # cycles per second (Hz)
• Wavelength: length of one cycle (mm)
• Period: time of one cycle (s)
• Relationships:
  • Direct: Increase in one = increase in other (e.g., wavelength & propagation speed)
  • Inverse: Increase in one = decrease in other (e.g., wavelength & frequency)

Measuring Energy

• Power: Rate of flow of energy (Watts)
• Intensity: Power/area (Watts/cm²)
• Amplitude: Height of pressure wave (MPa)
• Decibels: Used to describe relative intensity changes
  • Doubling intensity = +3 dB
  • Halving intensity = -3 dB

Properties of the Medium

• Propagation Speed:
  • Speed of sound in a medium, dependent only on the medium.
  • In soft tissue: 1540 m/s
  • Determined by stiffness and density
• Impedance: Resistance measure (Rayls), dependent on medium only

Sound Propagation in Tissue

• Reflection and Transmission
  • Occurs at interfaces with impedance mismatch
  • Greater mismatch = greater reflection
• Attenuation: Weakening of sound
  • Directly related to frequency

Pulsed Ultrasound Parameters

• PRF (Pulse Repetition Frequency): Number of pulses in 1 second
• PRP (Pulse Repetition Period): Time from start of one pulse to the next
• SPL (Spatial Pulse Length): Length of one pulse

Resolution

• Spatial Resolution: Ability to distinguish closely spaced objects
  • Axial: Vertical, determined by SPL
  • Lateral: Horizontal, determined by beam width
• Temporal Resolution: Frame rate, important for visualizing motion

Transducers

• Converts electrical energy to mechanical and vice versa
• Crystal material: PZT or Quartz
• Impedance Matching Layer: Reduces reflection at transducer face
• Types:
  • Mechanical: Moving parts, fixed focus
  • Array: Electronic, multiple crystals, capable of electronic focusing and steering

Imaging Principles and Instrumentation

• Pulse-Echo Principle: Reflections at interfaces help determine distance
• Modes:
  • A-mode: Amplitude mode, displays reflections as spikes
  • B-mode: Brightness mode, converts spikes to dots on screen
  • M-mode: Motion mode, displays motion over time
• Image Processing
  • Pre-Processing: Compensation, Amplification, Gain
  • Post-Processing: Image Memory, Dynamic Range

Doppler and Hemodynamics

• Doppler Effect: Frequency shift due to movement
  • Positive Shift: Moving towards source
  • Negative Shift: Moving away
• Doppler Equation: Velocity, frequency, and angle affect Doppler shift
• Color Doppler: Uses color to represent flow direction and velocity
• Pulsed Doppler: Evaluates flow at a specific location, subject to aliasing

Hemodynamics

• Blood Flow Factors: Pressure gradient and resistance
• Poiseuille's Law: Relationship between flow rate, resistance, and pressure gradient
• Bernoulli Effect: Inverse relationship between pressure and velocity at stenosis

Safety and Quality Assurance

• ALARA Principle: Minimize exposure to reduce bioeffects
• Bioeffects Indices: Measure of risk likelihood, not actual occurrence
• Quality Assurance: Routine checks to ensure equipment performance

New Technologies

• Elastography: Assesses tissue stiffness
• 3D and 4D Imaging: Volumetric and real-time 3D imaging
• Contrast Agents: Enhances visualization with microbubbles
• Tissue Doppler: Measures myocardial motion for cardiac assessment

These notes provide a comprehensive overview of the key concepts in ultrasound physics, including sound properties, imaging principles, transducer types, and safety measures. They serve as a study guide for understanding how ultrasound technology functions and its clinical applications.