The US Machine

The basic components of a system are:-

  • A  transmitter - pulsed ultrasound  in brief bursts
  • A receiver -  converts returning ultrasound waves into an electrical signal
  • A computer  - amplifies the electrical signal and converts  the signal into an meaningful picture
  • A display which displays the signal as a graphical image
  • A memory -stores pictures and video clips

A large number of probes are available.

  • Linear
  • Curved (curvilinear)

Linear probes are most useful for vascular access generally being of higher frequency (and therefore higher resolution) Different probe sizes and frequencies are available depending on target depth and space on the skin surface for the probe. Generally the “standard probe”  HFL38 or the shorter L25 are the best choice for vascular access as they offer the best compromise of good field of view and high resolution.
Curved probes usually have lower frequencies but greater tissue penetration. They are more useful for deeper regional anaesthesia techniques. The picture is displayed as an arch representing the curved field of view.

The US probes have a mark on one end which corresponds to a marker dot on the US display. Confusingly the mark on most probes is a bar and there is a dot on the opposite end!

If in doubt touch one end of the probe and check which part of the image changes.

Some machines allow separate control of the probe frequency. Others offer a more simple automatic optimisation with changing depth.
Remember increasing frequency increases resolution. Decreasing frequency improves tissue penetration.

The amplification of the received ultrasound information. Noise and artifacts are also amplified so probably best kept at the lowest level that allows clear distinction of the desired structures.

Increasing the gain will lighten the whole picture, so best adjust the image such that structures which are known to be dark (muscle, lungs and vessels) and the remainder are graduations of grey/white.
The latest machines offer complex computer controlled auto- gain to give excellent pictures with minimal input from the user.

Variable gain dependant upon the time taken for the signal to return to the probe (ie depth)

This allows more amplification of distant structures and compensates for tissue signal attenuation. Often represented on an US machine as a bank of sliders each representing a depth range.

Most machines have a control of the scanning depth. This changes the image on the display and allows optimal magnification of the image.

Modern machines  have automatic frequency changes with scan depth to change the frequency (and therefore resolution) as the depth changes.

Depth is usually best set such that the structures of interest are between the middle and two thirds of the way down the picture.

Adjusting the focus changes the depth at which the beam width is at its narrowest. Optimising focus improves lateral resolution. See Physics.

Compound imaging electronically compiles multiple images from different angles to form one image. This reduces image noise and can enhance edge definition.

Colour Doppler
Gives information regarding direction of flow towards or away from the probe. Usually configured to show flow away from the probe as blue and flow towards the probe as red. (ie not necessarily corresponding to arterial and venous)
Colour Power Doppler
Gives information about the flow velocity. Increasing velocity produces the greatest colour change. Does not give any directional information but is more sensitive to low flow rates than Colour Doppler..

(see Physics section)










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