Principles of Ultrasound

Describe the physical principles of ultrasound and the Doppler Effect.

Ultrasound is an imaging technique where high-frequency sound waves (2-15MHz) are used to generate an image. An ultrasound wave is produced by a probe using the piezoelectric effect:

  • Certain crystalline structures will vibrate at a particular frequency when a certain voltage is applied across them
    The conversion of electrical energy to kinetic energy is how the ultrasound probe creates an ultrasound wave.
  • Similarly, they can generate a voltage when a vibration is induced in them
    This is how the probe interprets reflected waves.

Basic principles

  • Spatial resolution
    How close two separate objects can be to each other and still be distinguishable. It is divided into:
    • Axial resolution, how far apart two objects can be when one is above the other (in the direction of the beam)
    • Lateral resolution, how far apart two objects can be when side side-by-side
  • Contrast resolution is how similar two objects can appear (in echogenic appearance) and still be distinguishable

  • Higher frequency settings offer greater spatial resolution but decreased penentration

  • Lower frequency settings offer reduced spatial resolution but increased penetration
    They are used for visualising deep structures.

Affect of Tissues on Ultrasound

At tissue interfaces, the wave may be:

  • Absorbed
    Sound is lost as heat, and increases with decreased water content of tissues.
  • Reflected
    Sound bounces back from the tissue interface, and returns to the probe.
    • Reflection is dependent on the:
      • Difference in sound conduction between the two tissues
      • Angle of incidence (close to 90° improves reflection)
      • Smoothness of the tissue plane
    • The amplitude of sound returning to the probe determines echogenicity, or how white the object will be displayed
    • The time taken for the sound to return determines depth
      • The time taken for a wave to return is proportional to twice the distance of the object from the probe
      • Depth can be calculated using , where:
        • is Depth
        • is the speed of sound in tissue, and is assumed to be 1540 ms-1
        • t is Time
  • Transmitted
    Sound passes through the tissue, and may be reflected or absorbed at deeper tissues.
  • Scattered
    Sound is reflected from tissue but is not received by the probe.
  • Attenuated
    Attenuation describes the loss of sound wave with increasing depth, and is a function of the above fctors.
    • Attenuation is managed by increasing the gain
      Gain refers to amplification of returned signal.
    • Time-gain compensation refers to amplification of signals which have taken longer to return, which amplifies signals returned from deep tissues

Modes

Ultrasound modes include:

  • B-Mode (brightness mode)
    The standard 2D ultrasound mode, and plots the measured amplitude of reflected ultrasound waves by the calculated depth from which they were reflected.
  • M-Mode (movement mode)
    Selects a single vertical section of the image and displays changes over time (i.e. depth on the y-axis, and time on the x-axis).

Doppler Effect

The doppler effect is the change in observed frequency when a wave is reflected off (or emitted from) a moving object, relative to the position of the receiver. In medical ultrasound, this is the change in frequency of sound reflected from a moving tissue (e.g. an erythrocyte). It is given by the equation:

where:

= Velocity of object
= Frequency shift
= Speed of sound (in blood)
= Frequency of the emitted sound
= Angle between the sound wave and the object

Reflected frequencies are higher towards the probe and lower away.

Calculation of Cardiac Output:

  • Heart rate is measured
  • Stroke volume is calculated by:
    • Measuring the cross-sectional area of a vessel
      Obtained by measuring the diameter using ultrasound.
    • Measuring the stroke distance
      Obtained via integrating the flow-time waveform for distance across the left ventricular outflow tract (LVOT VTI)

References

  1. Cross ME, Plunkett EVE. Physics, Pharmacology, and Physiology for Anaesthetists: Key Concepts for the FRCA. 2nd Ed. Cambridge University Press. 2014.
  2. CICM July/September 2007.
Last updated 2018-01-16

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