Ultrasound cannot penetrate the airfilled lung parenchyma, because of the large difference in acoustic impedance between soft tissues and air.
The apparent lung parenchyma looks hypoechoic (greyish) but it is a reverberation artefact (see the next module about artefacts).
Fluid is anechoic (black) in ultrasound images.
M (motion or time-motion) mode displays real-time ultrasound data derived from one single stationary ultrasound beam.
It displays depth along the vertical axis and time along the horizontal axis.
M-mode is primarily used in cardiac and lung ultrasound.
Usually M-mode is displayed together with a 2D greyscale ultrasound image to allow spatial reference.
The 2D image has a graphical M-line superimposed to indicate the position of the M-mode beam that samples the data.
Colour Doppler is typically activated by pushing a button.
Colour Doppler is used to confirm whether there is flow in a vessel.
Colour Doppler displays a real-time 2D cross-section of blood flow.
When the colour Doppler is activated, the cross-section is displayed as a frame on the monitor. The shape of the frame depends on the probe. The position of the frame can be moved around using a touch pad.
Spatial orientation is obtained by overlaying the colour Doppler cross-section on top of a 2D greyscale ultrasound image displaying the soft tissues.
Conventionally, red colour is used for blood moving towards the probe, and blue is used for blood moving away from the probe.
The operator can adjust the intensity of the colour Doppler signal by using the gain user control.
In colour Doppler mode the function of the gain control is switched to colour Doppler.
Colour Power Doppler (CPD) like standard colour Doppler detects blood flow through arteries and veins.
CPD does not indicate the velocity or direction of blood flow.
CPD is based on the echo amplitude received from moving cells – not the frequency shifts.
CPD is up to five times more sensitive than colour Doppler in order to detect the velocity of blood flow and can detect low velocity blood flow in veins and within organs. The strong flow sensitivity is the real benefit of CPD.
The goal of depth adjustment is to align the target structure to the center of the monitor and to visualise the entire target structure inside the visual field.
The size of the displayed image can be adjusted using the depth controls.
When the depth of the field-of-view (FOV) is increased, it becomes possible to see deeper structures.
When the depth is decreased, the FOV is narrowed around structures closer to the probe, thereby omitting the deeper structures.
Reduction of the depth means that the time from emitted to received signal is reduced. This allows a higher frame rate. Higher frame rate can be used to increase the spatial or temporal resolution or the sector width.
