Wiii: Describe the physical principles of ultrasound and the Doppler Effect

Biological Variable

The attenuation of high frequency sound waves (5 – 15 MHz) at tissue interfaces with variable density

  • Sound = a form of mechanical E that travels in a longitudinal wave in a series of compressions (high P) & rarefractions (low P)
  • Sound energy is attenuated as it passes through tissue because of:
Biological Variable
  • ABSORPTION = tissue absorption of sound, responsible for most attenuation of ultrasound wave
  • REFRACTION = the change in direction (bends) of a sound wave, occurs when the sound strikes the boundary of two tissues at an oblique angle

Image turns out unclear i.e. ‘2 needles’ traversing a vessel

  •  REFLECTION = a reflection (echo) is how we identify structures

Reflection occurs when the sound wave contacts tissues with different densities (& part of the beam reflects)

  • SCATTERING = when the sound wave is bigger than the object it contacts (i.e. RBC) the wave does not reflect back to the probe, it scatters in all directions

Sensor

  • Probe containing a Piezoelectric transducer (PZT)
  • Piezoelectric Effect = electrical voltage is applied to quartz crystal & crystal changes dimension

∴ converts Electric E → sound E

  • Each PZT has ~250 elements
  • Electrodes either side of PZT
  • Voltage applied → crystals vibrate → sound emitted
  • These crystals spend:
    • 1% time emitting sound
    • 99% time listening back for sound
  • On the way back, use of Reciprocal Piezoelectric Effect calculates how much reflection is occurring
  • PROBES all emit at different frequencies

Probe

Frequency

Depth

Linear

Frequency

15 – 6 MHz

Depth

6cm

Curved

Frequency

8 – 3 MHz

Depth

15cm

Cardiac

Frequency

5 – 1 MHz

Depth

35cm

i.e. Higher frequency probes have better resolution but shallow depth

Integrator

  • Reflection returns to probe & is transduced by Reciprocal Piezoelectric Effect (Sound E → electrical current)
  • Central processor calculates the distance b/w transducer & object according to the speed of sound (1540m/sec) → ∴ uses this to calculate depth
  • Measures intensity of a signal → which is displayed as brightness

RESOLUTION = how fine a detail can be seen in an image

  • Axial Resolution = the ability to display small targets along beam path as separate entities
Axial Resolution
  • Lateral Resolution = the ability to separate targets perpendicular to the beam path
Lateral Resolution
  • Temporal Resolution = the ability to show changes in anatomy over time esp imp with echo

Output/Mode

  • B – Mode (brightness node)
    • 2D cross-section through body
  • M – Mode (motion mode)
    • Movement of structures over time
    • Cardiac scans can be timed with ECG

Doppler Effect

  • The change in sound pitch as the sound wave changes frequency

Think ∆ pitch as ambulance gets closer/further away from you → Replace yourself with the probe

  • Sound waves reflected off objects move away/towards transducer (usually blood)
  • Object moving towards transducer → ↑frequency sound wave, +ve Doppler shift
  • Object moving away from transducer → ↓frequency sound wave, – ve Doppler shift
  • Doppler shifts are calculated by the cosine of the angle

∴ Maximal Doppler shift at 0 degrees (cosine of 0 = 1) when flow is either directly toward/away from transducer

Sound Waves