Bedside Ultrasound Manual

Bedside Ultrasound Manual

Nagehan Ayakta, MD, Benjamin Supat, MD, MPH, Amir Aminlari, MD

Table of Contents

Knobology
Physics
More coming soon

Knobology:

  1. Video 1
  2. Every machine has Power, transducers, gain and depth
    • Power – total energy delivered by transducer. Normally fixed to avoid adverse biologic effects
    • Gain – degree of amplification of the returning signal, similar to volume on radio.
  3. Probes/transducers
    • Phased-array – lowest frequency (1-5Hz) and great for deep structures (e.g. heart)
    • Curvilinear – middle frequency (2-5 Hz) and great for abdomen, retroperitoneal, OBGYN studies 
    • Linear – highest frequency (5-15 Hz) and best for superficial structures. Great for lung sliding, vascular access, soft tissue and MSK, venous compression studies 
  4. Moving the transducer (cardinal movements)
    • Fanning - Tilting the probe along the short axis (like a domino falling)
    • Rocking - Tilting the probe along the long axis (like slicing a pizza with a knife)
    • Rotating/twisting
    • Sliding
    • 3

Physics:

  1. Video 1
  2. 5 parameters relevant to a pulsed ultrasound:
    • Pulse duration
    • Pulse repetition period
    • Pulse repetition frequency
    • Duty factor
    • Spatial pulse length
  3. Pulse duration- length of time from beginning to the end of a pulse
  4. Pulse repetition period – length of time from beginning of one pulse to the next so includes the pulse duration + listening time
  5. Pulse repetition frequency (PRF) – number of pulses per second
    • Similar to frequency but refers to pulses/sec rather than cycles/sec
    • Changed by adjusting the imaging depth – increase in depth, decreases PRF because listening time increases
    • Increase in frequency increases resolution but also increases attenuation by increasing scattering
  6. Duty factor – percentage of time spent producing a pulse
    • Affected by
      • Imaging depth – increasing depth, decreases DF
      • PRF – increasing PRF, increases DF
      • PD – increasing PD, increases DF
      • PRP – increasing PRP, decreases DF
      • DF (%) = PD(sec) / PRP(sec) * 100
  7. Spatial pulse length – length or distance of a pulse
  8. Picture 1 2
  9. The returning echo intensity of the image is proportional to the grayscale of the pixel of information
    • Stronger signal aka more echoes means brighter dot
  10. Attenuation – loss of energy as the sound waves move through a medium
    • Increase in frequency, increases attenuation
    • Increase in distance, increases attenuation
    • Picture 2 2
    • Absorption – conversion of soundwave to heat, leading to attenuation. occurs most with soft tissue 
    • Reflection – redirection of sound back to probe leads to attenuation
    • Scattering – wave reflected in different directions, occurs with media with irregular boundaries such as in lungs. Increase in frequency, increases scattering
    • Refraction – redirection of part of sound wave when it crosses from medium to another
    • Attenuation coefficient – amount of attenuation per centimeter of tissue, increases with increased frequency.
  11. Impedance – resistance to the propagation of sound, characteristic of medium and correlated with density and propagation speed
    • Relative impedance: bone>>muscle>fat>blood>water>>>>>air
  12. Angle of incidence or insonation – need to scan perpendicular to object of interest to maximize returning echoes and improve image quality
  13. Resolution:
    • Axial – distinguish between objects in a plane parallel to ultrasound beam, shorter pulses (lower spatial pulse length) leads to better images
    • Lateral – distinguish between objects in a plane perpendicular to ultrasound beam aka lying horizontal tissue. Closer array of crystals means better images as well as higher frequency and lower gain 
    • Temporal resolution aka frame rate – detect position of moving objects in a given time. More frames per second is higher resolution. Decreasing depth is also better
  14. Scanning modes:
    • B-mode – grayscale ultrasound, B=brightness
      • Picture 4 2
      • M-mode – shows movement of tissue over time
        • Vertical axis is depth and corresponds to B-mode image
        • Horizontal axis is time
        • Color doppler – measures mean velocity and direction of flow, shown over a B-mode image
          • Color scale : superior colors represent flow toward the probe and inferior colors away from the probe
        • Power doppler – averages flow over several frames, has more sensitivity for eval of low flow states (testicular or ovarian flow) but no info on flow direction
        • Spectral doppler – Uses pulsed or continuous waves to quantitatively assess flow velocity. Pulsed wave doppler details velocity over time in the region of the sampling gate, usually demarcated by a || symbol. Continuous wave doppler, while it does typically display a ♢ symbol, actually details the velocities along the entire course of the sampling vector (the whole line on the screen).
    • B-mode – grayscale ultrasound, B=brightness
      • Picture 4 2
      • M-mode – shows movement of tissue over time
        • Vertical axis is depth and corresponds to B-mode image
        • Horizontal axis is time
        • Color doppler – measures mean velocity and direction of flow, shown over a B-mode image
          • Color scale : superior colors represent flow toward the probe and inferior colors away from the probe
        • Power doppler – averages flow over several frames, has more sensitivity for eval of low flow states (testicular or ovarian flow) but no info on flow direction
        • Spectral doppler – Uses pulsed or continuous waves to quantitatively assess flow velocity. Pulsed wave doppler details velocity over time in the region of the sampling gate, usually demarcated by a || symbol. Continuous wave doppler, while it does typically display a ♢ symbol, actually details the velocities along the entire course of the sampling vector (the whole line on the screen).
    • B-mode – grayscale ultrasound, B=brightness
      • Picture 4 2
      • M-mode – shows movement of tissue over time
        • Vertical axis is depth and corresponds to B-mode image
        • Horizontal axis is time
        • Color doppler – measures mean velocity and direction of flow, shown over a B-mode image
          • Color scale : superior colors represent flow toward the probe and inferior colors away from the probe
        • Power doppler – averages flow over several frames, has more sensitivity for eval of low flow states (testicular or ovarian flow) but no info on flow direction
        • Spectral doppler – Uses pulsed or continuous waves to quantitatively assess flow velocity. Pulsed wave doppler details velocity over time in the region of the sampling gate, usually demarcated by a || symbol. Continuous wave doppler, while it does typically display a ♢ symbol, actually details the velocities along the entire course of the sampling vector (the whole line on the screen).
  15. Artifacts: 
    • Video
    • Acoustic shadowing – failure of US bream to pass through an object b/c of attenuation or reflection. occurs behind a stone or bone
    • Gain artifact – excessive amplification of returning echo leading to invisibility of anechoic structures aka fluid
    • Posterior acoustic enhancement – brighter signal behind a fluid filled structure due to relatively less attenuation of the sound waves that passed through the fluid as compared to the sound waves that did not pass through fluid
      • The software in the machine automatically increases the gain in the far field to give a nice looking image (otherwise the far field would be darker than the near field due to attenuation)
      • However, sound waves returning along the trajectory passing through fluid have a higher baseline energy/gain
      • Picture 10 2
    • Reverberation – occurs when two reflectors line parallel to each other and perpendicular to the US wave so that sound gets trapped between the two highly echogenic structures and bounces back and forth like a ping-pong. Ex: A-lines on lungs
    • Mirror artifact – image appears on both sides of a strong reflector such as the diaphragm
    • Lateral cystic shadowing/edge artifact – occurs when sound waves hit a rounded structure and some of the waves don’t return to the probe causing a shadow along the edge
    • Picture 13 2

References:

1. Avila, J. (2018). 5 min sono. Core Ultrasound. https://www.coreultrasound.com/5ms/
2. Schoenfeld, E. (2013). Introduction to Bedside Ultrasound, Volumes 1 and 2. Society for Academic Emergency Medicine.
3. Ransingh. Teaching Point-of-Care Ultrasound (POCUS) to the Perioperative Physician. 2018. https://doi.org/10.1017/9781316822548.01

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