Week 2: Ultrasound

Key learning points below. For further details including use of ultrasound in IV access, see attached powerpoint.

1) What is ultrasound?

  • Sound travels as a mechanical longitudinal wave in which back-and-forth particle motion is parallel to the direction of wave travel. ​

  • Ultrasound frequencies commonly used for medical diagnosis are between 2 and 15 MHz. ​

  • For interest:

  • Human ears can hear sounds with frequencies between 20 Hz and 20 kHz. ​

  • Bats and dolphins produce sounds in the range of 20 to 100 kHz for precise navigation ​

2) What is the piezoelectric effect?

  • Mechanical deformation as a result of an electric field being applied to a piezoelectric material. Alternation of this field results in oscillation which produces sound.

3) Key principles:

  • Stiffness: the resistance of a material to compression.​

  • Acoustic velocity: the speed at which a sound wave travels through a medium​

  • Propagation speed increases if the stiffness is increased or the density is decreased.​

  • Acoustic impedance: the degree of difficulty demonstrated by a sound wave being transmitted through a medium​

  • Equal to density multiplied by acoustic velocity c (z = ρc)​

  • Attenuation coefficient: Estimate of the decrement of ultrasound amplitude in certain media as a function of ultrasound frequency. ​

  • Axial resolution is the minimum separation of planes along the beam axis.​

  • Axial resolution = Wavelength λ × Number of cycles per pulse n ÷ 2​

  • The axial resolution of current ultrasound systems is between 0.05 and 0.5 mm.​

The result of these principles is that higher-frequency ultrasound can detect smaller objects and provide an image with better resolution, however due to attenuation depth of vision will be decreased with higher frequencies

4) What are some common frequencies used by ultrasound probes?

  • 2.5 MHz: deep abdomen, obstetric and gynecological imaging​

  • 3.5 MHz: general abdomen, obstetric and gynecological imaging​

  • 5.0 MHz: vascular, breast, pelvic imaging​

  • 7.5 MHz: breast, thyroid​

  • 10.0 MHz: breast, thyroid, superficial veins, superficial masses, musculoskeletal imaging.​

  • 15.0 MHz: superficial structures, musculoskeletal imaging.​

5) What are different ultrasound types?

A-mode: the oldest ultrasound technique and was invented in 1930​

  • A one-dimensional ultrasound: peaks corresponding to the depth of structures the ultrasound encounters in different tissues.

B-Mode: the "standard" ultrasound view

  • The B-mode is a two-dimensional (2D) image of the area that is simultaneously scanned by a linear array of 100–300 piezoelectric elements rather than a single one as in A-mode

Doppler: Utilises the doppler effect to display motion towards and away from the probe

M-Mode: A single beam used to produce an image with a motion signal.

Used for ultrasound of the lungs

Download PPTX • 20.88MB

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