The Physics of Ultrasound

Article: Physics of Ultrasound

Susannah J. Patey, James P. Corcoran. physics of ultrasound. Anaesthesia & Intensive Care Medicine. 2021. 22 (1). 58-63.

Basics

Ultrasound is a sound wave at frequency greater than 20,000 HZ
Sound waves are compressive (longitudinal) waves that require a medium to be transmitted
In the clinical setting, we use 1-20MHz, sometimes up to 75MHz

Ultrasound generation
Ultrasound is a “speaker” and “microphone”
Traditionally, probes use piezoelectric crystals, but newer ones use silicon chips
When a voltage is applied the crystal creates a mechanical wave
Conversely, when the crystal is mechanically moved a voltage and electrical signal are created.

In a diagnostic ultrasound, the probe sends 2-3 pulses, waits, and interprets the signal coming back
it then calculates time taken for the pulse to return which is interpreted on screen as the distance from the probe
the machine calculates amplitude which is interpreted on screen as brightness

Sound wave components:
1) wave length (distance between peaks), which is inversely proportional to frequency (cycles/sec)
2) amplitude (maximum deformation/compression from equilibrium)
3) acoustic impedance -- resistance of material to ultrasound propagation, calculated using tissue density x speed of sound wave in that tissue

           

Increasing impedance: air (0.0004) -> water (1.48) -> fat -> blood -> muscle -> bone (7.8)

At junctions of differing impedance, waves are reflected or refracted or scattered. The larger the difference in impedance, the larger the reflection.

  • boundaries will appear as bright lines or speckles

  • gel is used to eliminate air pockets between probe and skin

    • otherwise, air would reflect waves immediately back into probe

  • the biggest differences are soft tissue to bone, and tissue to air


4 main outcomes for ultrasound waves once in the tissue

  • reflection

    • sound waves are reflected back, with some waves transmitted through. The higher the impedance difference, the more is reflected back

  • scattering (uneven surfaces of organs, small diffuse reflector like RBC's)

    • small scattered waves interpreted by machine as speckles, grain

  • refraction

    • Snell's law bends an angled incident wave when passing through two mediums with differing impedance (waves propagate at different velocities through different mediums)

  • attenuation (loss of amplitude)

    • attenuation happens faster with higher frequency and longer distances

    • underlies the principle of the inverse relationship between probe depth and wave frequency (lower frequency probes can penetrate deeper into tissues)

Doppler Imaging

analyzes velocity of waves using doppler effect

  • when an object producing sound is moving toward the probe, sound waves are perceived as traveling the speed of the wave plus the speed of the object; therefore the waves are compressed which is perceived as higher frequency

    • when the object is moving away from probe, sound waves are perceived as traveling the speed of the wave minus the speed of the object; therefore the waves are spaced further apart, which is perceived as lower frequency

  • doppler equation is used by machine to infer flow and velocity of particles (like RBC’s) based on characteristics of ultrasound waves that bounce off of them

  • doppler modes

    • color flow

      • direction of flow and velocity of RBC in given area

    • pulsed wave (designate a depth and get flow through that area)

      • good depth precision, but not good for high frequency doppler shifts (from fast moving objects)

    • continuous wave

      • good for high frequency doppler shifts, but not good at depth (good for cardiac output monitoring)

Imaging Modes
A (amplitude): one-dimensional signal, not usually used much. Although has uses in ophthalmology in assessing ocular layers
B (brightness): two-dimensional signal: most common mode we use, gives us a sense of width, depth, and intensity.
M (motion): usually for echo, displays velocity/motion of the defined area over time

Common Artifacts:

  • shadowing at interfaces with large acoustic impedance mismatches; most commonly:

    • tissue to bone (ribs, spine, but also stones)

    • tissue to air (ultrasound with no gel, or commonly bowel gas)

      • air can be used in bubble echo, IV air, and visualize small septal defects in heart)

  • anisotropy

    • at a certain angle, the angle of beam hitting structure of interest can causes all waves to refract and reflect at an angle, with no waves returning to the probe

      • can be mistakenly interpreted as no signal

        • if looking at a tendon, one might interpret this as tendon rupture or fluid surrounding a tendon

Potential Risks

  • heating

    • absorption of the mechanical energy from ultrasound waves results in heating of the tissues

    • higher acoustic impedance correlates with a larger increase in temperature

    • most of concern in fetus in first 8 weeks.

  • mechanical

    • continuous ultrasound can potentially move small objects in direction of travel of waves. RBC's can be agglomerated by US in continuous doppler

  • cavitation

    • production, excitation and sometimes collapse of tiny gas bubbles in tissues or body fluids exposed to US

      • rare in diagnostic non continuous scanning modes

      • in lung tissue, existing gas bubbles can become excited and collapse, leading to collapse cavitation, mechanical damage and heat production

      • sometimes used therapeutically to break down adipose tissue

 

Author: Phil Delrosario, MS4