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Ultrasound Basics

Imaging

Imaging of a target nerve or vessel may be performed in one of two directions -

Short axis (SAX) image of the left internal jugular vein and carotid artery with plaque
  • Short axis imaging (SAX) - the imaging is essentially a cross section of the structure with the probe held at right angles to the direction of the target structure
  • Long axis imaging (LAX) - the probe and target are aligned producing an image of the structure in its longitudinal axis

Needle imaging -

  • In plane imaging - this refers to a needle which is directed to the target in the same plane as the ultrasound beam. The needle can be seen in its entire length if this is performed accurately
  • Out of plane imaging - the needle is directed to the target perpendicular to the ultrasound beam. In this method the needle will be visible as a bright spot on the screen with some deeper artifact. When using this method it is important to appreciate that the bright spot represents that some part of the needle shaft is crossing the US beam. It does NOT indicate the position of the needle tip. To ascertain tip position requires the probe to be angled or shifted along the in the direction of the needle tip

Visibility of Needles

The visibility of needles with ultrasound is dependent on a number of factors. Since the ultrasound beam has a thickness of only 2mm, larger diameter needles offer greater reflection and visibility. The approach angle favours needle insertion parallel to the transducer (long axis view). The Hustead (Tuohy) needle tip exhibits the best visibility, and the use of a metal stylet increases the tip visibility. Water, air or insulation does not increase needle visibility. Newer technologies are appearing in which the needle shaft is rough or coated in polymers with multiple microscopic air bubbles which increase the visibility of the needle shaft although these are not as yet commercially available. Finally, for long axis needle insertion, shallow angles of about 30 degrees are optimum whilst for short axis insertion steeper angles (>60°) improve visibility.

The Ultrasound Appearance of Tissues

Structure
Appearance
Artery
Hypoechoic, pulsatile, non-compressible. Doppler - pulsatile flow
Vein
Hypoechoic, non-pulsatile, compressible. Valsalva effect, doppler - continuous flow
Muscle
Hypoechoic with multiple hyperechoic lines
Tendon
Hyperechoic with anisotropy - bright lines longitudinally or bright dots at right angles fibrillary pattern
Nerve
Variable hypo- or hyperechoic with anisotropy fascicular pattern
Bone
Hyperechoic

Reference

Schafhalter-Zoppoth I, McCulloch CE, Gray AT. Ultrasound visibility of needles used for Regional Nerve Block: An in vitro study. Reg Anaes Pain Med, 2004; 29: 480-488