Brian Locke

Critical Care Echo

Physics

Parameters controlled by settings on the machine

  • power output (higher = higher amplitude reflected signals. Limited by safety. Bone heats faster than fat)
  • gain (displayed amplitude of received signals, analogous to turning up the volume)
  • Time-gain compensation (differential adjustment of gain along length of ultrasound beam; allows for correction of loss to attenuation in certain tissues)
  • Depth (how long the machine waits before sending the next signal; increase depth decreases frame rate and maximal display area)
  • Dynamic range and compression (amplitude range; larger dynamic range = more shades of grey)

In addition to these settings, there are post processing and preprocessing settings to adjust the display.

Artifacts

  • suboptimal image quality - usually poor tissue penetration (either poor contact, habits, tissue char)
  • Acoustic shadowing - decreased info behind specular reflector
  • reverberations - between two strong parallel reflectors
  • beam width - superimposition of structures within the beam profile
  • lateral resolution - the apparent width increases as depth of the object increases due to refractions in the tissue
  • range ambiguoity - echo from the previous pulse reaches transducer on the next cycle (results in superimposed second, deeper image)

Doppler

Intercept angle = angle between US beam and direction of travel. Cos(theta) = amount understimate

PRF = pulse <what?> frequency - determines maximum velocity (get nyquist effect if things are moving faster = signal aliasing)

  • Continuous wave doppler: continuous transmission, receive info from length of the beam so all you get is the distribution of velocities along the beam and no depth resolution.
  • Pulsed doppler - pulses are timed, so that the movement at a specific depth is recorded.

Spectral analysis = display doppler velocity data vs time (scale = amplitude). This is the usual presentation in e.g. as in M-mode.

(Note: M-mode has much higher sampling rates - 1800 per second - so can image fast moving structures better)

Color Doppler Flow imaging: on 2d mode, doppler interspersed over the area of interest. This means that the frame rate has to drop (drops more for a larger area of color). Things to adjust on color doppler to optimize:

  • color scale (represent the range of velocities across the maximum number of colors)
  • velocity range (within nyquist limit at that depth)
  • zero the baseline position on the color scale and set variance.

Tissue Doppler: compared to flow of fluids, all moves in the same direction (so spectral analysis does not have a filled waveform).

Normal Anatomy

3 types of movement:

  • translation (movement of an aspect of the heart)
  • rotation (circular motion around long axis)
  • torsion (unequal rotational motion around Apex vs base.

Nomenclature:

  • windows = where the probe is placed on the body
  • view = the image plane (either long axis, short axis, four chamber, or two chamber.

Flows

LV Outflow

Obtained from apical view

RV Outflow

Parasternal short or "RV outflow view"

LV Inflow

  • E = early diastolic peak velocity
  • A = late diastolic peak as a result of atrial kick.

Typically assessed from apical window.