Ultrasound Physics Registry Review

Ultrasound Physics Registry Review: Study Guide

Table of Contents

• Physics Principles
  • Properties of Sound …………………………………………. 1
  • Properties of the Medium ………………………………. 3
  • Sound Propagation in Tissue …………………….. 4
  • Resolution ………………………………………………………. 8
• Transducers
  • Construction and Function …………………………… 11
  • Types ………………………………………………………………. 13
• Imaging Principles and Instrumentation
  • Pulse-echo Principle and Modes ……………………. 17
  • Image Processing and Instrumentation ………. 18
  • Harmonics ………………………………………………………. 24
  • Artifacts ………………………………………………………….. 25
• Doppler and Hemodynamics
  • Doppler Principles and Instrumentation ……….. 28
  • Hemodynamics ………………………………………………. 34
• Safety and Quality Assurance
  • Bioeffects and Safety ……………………………………. 39
  • Quality Assurance ………………………………………….. 41
  • New Technologies …………………………………………. 43

Physics Principles (15%)

Properties of Sound

• Sound Definition: Sound is a mechanical, longitudinal wave.

• Longitudinal Wave: Molecules vibrate in a direction parallel to the sound source.

• Mechanics: Moves through the medium by vibrating molecules or changing pressure, consisting of compressions (high pressure) and rarefactions (low pressure).

• Cycle Definition: One complete cycle comprises one compression and one rarefaction.

• Categories of Sound:

  • Infrasound: Frequencies below 20 Hz
  • Audible Sound: Frequencies from 20 Hz to 20,000 Hz
  • Ultrasound: Frequencies above 20,000 Hz
  • Diagnostic Ultrasound: Frequencies between 2-20 MHz

• Common Unit Prefixes:

  • kilo (k) = 1000
  • mega (M) = 1,000,000
  • centi (c) = 1/100
  • milli (m) = 1/1,000
  • micro (µ) = 1/1,000,000
  • Example: 20,000 Hz is expressed as 20 kHz

• Understanding Terms:

  • Frequency: The number of cycles occurring in one second (measured in Hertz).
  • Wavelength: The distance of one cycle (measured in millimeters).
  • Period: The time duration of one cycle (measured in microseconds).
  • Relationships:
    • Direct relationships: Both frequencies increase or decrease together.
    • Inverse relationships: As one increases, the other decreases.

• Wave Properties:

  • Wavelength Dependence:
    • Wavelength depends on frequency (determined by the sound source) and propagation speed (determined by medium).
    • Inverse relationship between wavelength and frequency; directly related to propagation speed.
    • Propagation Speed Equation: $ext{c} = ext{f} imes ext{λ}$
    • Where:
      • c = propagation speed
      • f = frequency
      • λ = wavelength

Measuring Energy

• Power: Rate of flow of energy, measured in Watts.

• Intensity: Calculated as power per unit area (Watts/cm²).

  • Example: A 40 Watt light bulb in a small vs. large room—smaller space gives greater intensity.

• Amplitude: Height of the pressure wave (measured in MPa or megapascals).

  • A hydrophone measures the pressure profiles of the ultrasound beam.
  • Not related to frequency, wavelength, or period.

• Decibels:

  • Used to describe relative intensity changes.
  • An increase in intensity by a factor of 2 corresponds to an increase of 3 dB.
  • Halving intensity results in a decrease of 3 dB.
  • Half-Value Layer: Sound reaches half its original intensity at -3 dB.
  • Example: A reduction of 6 dB corresponds to an intensity reduction to 1/4 of the original intensity.
  • Gain Adjustment: Adjusting from 25 dB to 28 dB doubles intensity due to a 3 dB increase.

Properties of the Medium

Propagation Speed

• Definition: Speed of sound in a medium, solely dependent on the medium's properties.

• Soft Tissue Speed: Fixed value of 1.54 mm/μs or 1540 m/s.

• Factors Influencing Propagation Speed:

  • Stiffness: Related to bulk modulus. Higher stiffness = higher speed.
  • Density: Generally, denser mediums are stiffer, leading to higher propagation speed.

• Implications:

  • Impedance: Measure of resistance, expressed in Rayls.
  • Impedance Formula: $Z = p imes c$
    • Where:
      • Z = impedance
      • p = density
      • c = propagation speed
  • Impedance is not affected by frequency or wavelength.

Effects of Sound Encountering Interfaces

• Reflection: Occurs at interfaces with impedance mismatch. No mismatch means no reflection.
• Typical Reflection Coefficients in Soft Tissue Imaging: 99% transmitted, < 0.1% reflected.
• Attenuation: Weakening of sound; average rate in soft tissue is 1/2 dB/cm/MHz.
• Frequency and Attenuation: Directly related; increased frequency leads to increased attenuation.
  • Attenuation Coefficient: Decrease the frequency to find how much attenuation occurs per cm (half the frequency).

Types of Reflection

• Specular Reflector: Large, smooth interface—good for normal incidence.
• Diffuse Reflector: Large, rough interface—reflects in all directions; less clear images.
• Scattering: Occurs at small interfaces equal to one wavelength.
• Rayleigh's Scattering: When structures are smaller than one wavelength, scattering occurs uniformly.

Refraction and Other Effects

• Refraction: Changes in direction of the sound beam when moving between mediums.
  • Snell's Law: Governs the relationship between angles of incidence and refraction.
  • Critical Angle: When the angle of refraction matches the medium's angle, preventing transmission.
• Divergence: Spreading of energy over a larger area, leading to intensity loss.

Pulsed Ultrasound Parameters

Key Terminology and Concepts

• Pulse Repetition Frequency (PRF): Number of pulses emitted per second, inversely related to depth. Higher depth leads to lower PRF.
• Pulse Repetition Period (PRP): Time from the start of one pulse to the start of the next, directly related to depth.
• Spatial Pulse Length (SPL): Length of one pulse, depends on frequency.
• Pulse Duration (PD): The time taken for a pulse to occur, also depends on SPL.

Duty Factor

• Definition: Fraction of time the machine is actively working and transmitting pulses. A typical duty factor for pulsed ultrasound is between 0.1% and 1.0%.

Resolution

Types of Resolution

• Spatial Resolution: Ability to distinguish between two closely spaced objects horizontally or vertically.
  • Axial Resolution: Measured in mm, determined by the SPL. Minimum separation = 1/2 SPL.
    • Example: SPL of 1.8 mm gives axial resolution of 0.9 mm.
  • Lateral Resolution: Determined by beam width. Smaller beam widths lead to improved lateral resolution.

Improving Resolution

• Axial: Increase frequency to decrease wavelength, improving axial resolution.
• Lateral: Techniques include focusing, increasing scan line density, and decreasing sector angle.

Transducers (16%)

Function and Design

• Transducer Elements: Convert electrical energy into mechanical (sound) and vice versa, typically using PZT (lead zirconate titanate) or quartz.
  • Curie Temperature: Above 400°C, the piezoelectric properties are lost.

Transducer Types

• Mechanical Transducers: Have moving parts and fixed focus.
• Array Transducers: Electronic, utilizing multiple crystals, capable of electronic focusing and steering.

Beam Characteristics

• Fresnel Zone: Region of converging beams prior to divergence.
• Fraunhofer Zone: Divergent beams region.

Imaging Principles and Instrumentation (28%)

Pulse-Echo Principle

• Fundamental Concept: Sound waves encounter interfaces with varying impedances, leading to partial reflection and transmission.
• Range Equation: Allows calculations of distance using the echo time. As a rule, every 13 µs corresponds to a depth of 1 cm.

Imaging Modes

• A-mode: Displays reflections as a series of spikes.
• B-mode: Converts A-mode information into dots on the screen, with brightness related to the amplitude of echoes.
• M-mode: Monitors motion along a single scan line over time.

Processing Steps

• Pulser: Controls the transmitted voltage to the transducer.
• Beam Former: Determines beam shape and timing.
• Receiver: Amplifies and processes returning echoes, compensates for attenuation.
• Signal Processor: Rectifies and converts signals for digital display.

Dynamic Range

• The range of gray levels displayed in the image. Higher dynamic range leads to softer images with lesser contrast.

Image Compounding

• Frequency Compounding: Enhances contrast resolution by identifying various signals.
• Spatial Compounding: Aims to reduce artifacts by gathering data from multiple angles.

Doppler Instrumentation and Hemodynamics (31%)

Doppler Principles

• Doppler Effect: Frequency of echoes changes with relative motion between the sound source and the reflector.
  • Positive Shift: Indicates motion towards the source.
  • Negative Shift: Indicates motion away from the source.
• Doppler Shift Equation: $ext{Doppler Shift} = 2 imes ext{F} imes ext{V} imes ext{cos}( heta)/c$
  • Where:
    • V = velocity of the reflector,
    • F = transmitted frequency,
    • θ = angle of the Doppler beam with respect to the flow direction.

Color Doppler Imaging

• Incorporates Doppler shift information into a color superimposed on the B-mode image.

Pulsed Wave Doppler

• Samples flow in a small, specific area known as a sample volume.

Aliasing

• Occurs when the Doppler shift exceeds the Nyquist limit.
• Solutions: Increase the scale or decrease the frequency to eliminate aliasing.

Safety and Quality Assurance

Bioeffects and Safety

• ALARA Principle: Maintain exposure to minimum levels necessary for diagnostic quality.
• Measure intensities (mechanical index, thermal index, etc.) to ensure patient safety.

Quality Assurance (QA)

• Regular testing to ensure equipment performs at standards, including mechanical and electrical tests.

New Technologies

• Elastography: Measures tissue elasticity to distinguish between tumor types.
• Hybrid Imaging: Combines ultrasound with CT or MRI for enhanced identification.
• Contrast Agents: Enhances imaging of blood flow through highly reflective microbubbles.