Centre for Anaesthesia Critical Care & Pain Medicine


Imaging the needle

The basic principle for imaging the needle is to ensure that the ultrasound waves are able to reflect back to the transducer so that an image can be generated. This requires an appreciation of where the needle is in respect to the sound waves generated.

There are two likely causes if you are unable to see your needle:

1) Your needle is not laterally within the narrow beam of the ultrasound, i.e you are not in the plane of the beam.

2) Your needle is in plane, but the angle is too obtuse in relation to the transducer, and so the waves are not being reflected. This occurs at increasing proportion past 40o until most needles are invisible at >50o (unless using echogenic technologies).

If your needle is not in plane, the easiest way to see is to look at you hands, if the needle entry point does not correlate with the centre point of the short axis of the transducer, you are not in plane. Move the transducer so that the needle is in the centre. Equally if the needle direction relative to the long axis of the transducer is not parallel, then your needle is not in plane. Twist the transducer on its central axis so that the needle is parallel, and the entry point is in the centre of the short axis. DO NOT MOVE THE NEEDLE WHILST REORIENTATING THE PROBE.

If your needle is steeply angled then you have two choices

1) move your transducer so that the target structure is at the lateral edge of the screen and reinsert your needle as far away from the transducer as your needle length will allow, this will give you the most shallow approach angle possible.

2) "Toe" the transducer, i.e press the farthest edge of the transducer into the skin, whilst lifting the nearest edge so it barely is in contact. This has the effect of creating a less obtuse needle angle relative to the probe, and improves visibility. The opposite of this maneuver is to "Heel" the transducer.

Other measures to locate your needle include hydrolocalisation, which is to inject a small amount of local anaesthetic and look for the signs of spread to indicate where the tip position must be. Or, tissue deformation, where the needle tip position is revealed by movement of tissue planes around it. Extreme caution must be used with both these techniques, as an unrecognized needle puncture of vessel or nerve is quite possible.

Technological solutions

Needle solutions

1) Needle guides can be used to fix the needle in position relative to the transducer. This has the effect of ensuring an in plane approach, but once the needle is inserted the transducer is fixed in position, so does not allow a "freehand" movement of the transducer and needle to optimize local anaesthetic spread. Needle guides can also be optical, avoiding this limitation.

2) Echogenic needle use special coatings or "cornerstone reflectors" to increase the reflection of sound back to the transducer. They are not required in shallow insertions, and only help with visibility problems due steep insertions, not with lateral positioning mistakes.

3) Advanced needle designs include magnetic stylets to interface with tracking systems, or piezoelectric crystals at the tip to generate a Doppler signal.

Machine Solutions

1) Compound spatial imaging is available on most modern machines, and uses beam steering to generate ultrasound waves which are not perpendicular to the transducer orientation, as the beam is steered, multiple images are created which are then compounded over a number of frames to give a composite picture which ideally has the best picture of target structure and needle. It also reduces speckle and noise. Examples are multibeam (MB) with Sonosite, or XBeam with GE.

2) User defined beam steering, such as Sonosite's MBe, where the angle of insertion is set by the user, and the ultrasound uses beam steering to ensure optimal angle to reflect from the needle

3) 3D/4D ultrasound is limited by usability and low resolution.