Ultrasound & Doppler

Ultrasonography Principles

• Definition
  • A sound wave with a frequency > 20 kHz → higher than frequency range audible by human ear
  • Used medically → typically involves frequency range of 2-15 MHz
• Piezoelectric and converse piezoelectric effect
  • Change of polarization of molecules in a quartz crystal in response to
    mechanical stress – Interconverts electrical and sound energy
  • Application of electrical field creates mechanical deformation in a crystal

• US Generation: Piezoelectric crystal within probe is stimulated by electrical current to vibrate → produce sound wave
• US Detection: Sound wave reflected by medium causes same crystals to vibrate → produce electrical signal
• Piezoelectric transducers in US
  • Electrical current converted into precise sound waves (1~20mHz)
  • Sound waves reaches interface of two mediums of differing density (or acoustic impedance).
• Acoustic impedance = tissue density x acoustic velocity
  • unique to tissue type (e.g. fat, bone, etc.)

• At interface:
  • Reflection
    • sound wave reflected directly back to transducer
    • Amount of reflection depends of ratio of acoustic impedance (or density)
    • Increased density ~ increased reflection
  • Refraction
    • sound wave is deflected within the medium
    • based on Snell’s law
• Attenuation: Loss of energy or strength of an ultrasound wave as it travels through a medium. Occurs through:
  • Absorption: The ultrasound wave energy is converted into heat as it interacts with the tissue.
  • Reflection: The sound waves bounce off the boundaries between tissues with different acoustic impedances.
    • sound wave reflected back to transducer
    • Amount of reflection depends of ratio of acoustic impedance
    • Increased density ~ increased reflection
  • Scattering: The sound waves are deflected in various directions by irregularities in the tissue and do not reach transducer
  • Refraction: bending of the sound wave as it travels from one tissue to another
    • results in a change in the wave’s direction
    • relationship between the angle of incidence (θi), the angle of refraction (θt), and the speeds of sound (c1 and c2) in the two media is described by Snell’s Law: sin(θi) / c1 = sin(θt) / c2

• Central Processor
  • Electrical current generated by piezoelectric crystal signaled to CPU
  • CPU calculates the distance between transducer and object according to
    • Speed of sound (1540m/sec)
    • Delay in echo return
  • Information relayed to display for visualization
  • Gain
    • Sensitivity of CPU to signals received from transducer
    • Time-Gain Compensation – selective sensitivity of CPU to different interval of sound delay

Resolution and Penetration

• Ultrasound Resolution:
  • Defined as the ability to differentiate b/t structures that are closely related
  • Resolution is ↑ with either:
    • (i) ↑ frequency (or ↓ wavelength) of sound wave → but this ↓ tissue penetration
    • (ii) ↑ amplitude of sound wave → but this ↑ artefact
    • (iii) ↑ gain → but this ↑ noise
  • Types of Resolution: Spatial (Axial, Lateral, Elevational), Temporal
• Spatial Resolution:
  • Axial Resolution: The ability to distinguish two structures that are side-by-side and parallel to the ultrasound beam.
    • Achieved with a higher frequency and shorter pulse length.
    • Mathematically, it’s half the spatial pulse length.
  • Lateral Resolution: The ability to distinguish two structures that are side-by-side to the ultrasound beam.
    • Achieved with a narrower ultrasound beam, which is related to the width of the beam.
    • Higher frequencies generally lead to narrower beams and better lateral resolution.
    • Lateral resolution is roughly three times worse than axial resolution at the focal region of the beam.
  • Elevational resolution (aka slice thickness resolution)
    • refers to the ability to distinguish structures that are close together in the direction perpendicular to the imaging plane. (similar to lateral resolution, but in perpendicular plane)
    • crucial for accurately visualizing the depth and thickness of tissues
  • Temporal Resolution:
    • The ability to distinguish between instantaneous events of rapidly moving structures.
    • Achieved with a high frame rate.
    • A higher frame rate means the ultrasound machine can capture and display more images per second, allowing for better visualization of movement.

• Ultrasound penetration:
  • defined as the depth to which ultrasound waves can travel into tissue
  • Primarily determined by the frequency of the ultrasound waves, with lower frequencies generally penetrating deeper than higher frequencies. 
• Trade-off between Resolution and Penetration:
  • There’s an inherent trade-off between image resolution and penetration depth. 
  • Higher resolution requires higher frequencies, which means less penetration, and vice versa. 

Modes of Ultrasound

• A (amplitude scan): Amplitude of U/S signal plotted against time → provides information about tissue depth (BUT is no longer used)
• B (brightness): Depth recorded as bright spot (rather than a spike as in A-mode) →
  amplitude of U/S signal is proportional to brightness
• M (motion): B-mode plotted against time (Ie. assess heart valve movement over time)
• 2-D: Sequential B-mode across 90° (most commonly used) → requires an array of crystals
• Doppler: Uses “Doppler shift” to establish velocity of moving object which is reflecting sound waves → superimposed on 2D mode with colours representing direction of movement (red = towards, blue = away)

Doppler effect

• Change in apparent frequency of sound for an observer moving relative to its source
• Use in ultrasound
  • Sound waves reflected off objects moving toward or away from transducer
    (usually blood)
    • If object moving toward transducer, frequency appears increased – displayed as red (however this is not standardized across machines)
    • If object moving away from transducer, frequency appears decreased –
      displayed as blue
  • Can be used to measure velocity of flow, as well as direction
    
    

Sakurai 2016

Answer by Sakurai