Transducer Selection

Transducer selection is ultimately determined by two factors; frequency and “footprint”.  When using ultrasound for regional anesthesia, the goal is to create a high resolution image that creates minimal artifacts.  Selection is often determined by the depth of the structures to be imaged.  Compare the two images of the same neck below.  The image on the left was generated with a high frequency (9MHz) linear array transducer, while the image on the right was taken with a low frequency (4MHz) curved array transducer.  Note how the high frequency image differentiates the homogeneous tissue of the thyroid from the heterogeneous tissue of the fascia and muscle, whereas in the low frequency image it is much more difficult to separate them.  With the exception of the sciatic nerve at the sub-gluteal level (which is an advanced block), almost all nerves and major blood vessels can be located at less than 4cm of depth, making the high frequency transducer the preferred probe for performing regional anesthesia and vascular access.

The footprint of the transducer refers to the area of skin that must be contacted to create an image. Transducers used in ultrasound-guided regional anesthesia (UGRA) or vascular access can either be linear or curved array transducers.  Array transducers are electronic, and consist of multiple elements aligned side-by-side forming the assembly.  Linear transducers, as the name implies, aligns the elements in a straight line that forms a rectangular image.  Curved array transducers were developed for abdominal scanning as an alternative to linear and sector* transducers.  It offers the depth and penetration of a sector transducer, but with better near field resolution.

Linear Array Transducers

Linear array transducers tend to be high frequency, and have a small footprint, making them a good choice for many of the peripheral nerve blocks where the surface area is limited and the nerves are not located deep in the tissue.  Because the footprint is linear, the distant between the ultrasound waves remains constant, and near field resolution is good.

 A “hockey stick”, shown below, is a type of linear array transducer which has an even smaller footprint that makes it ideal for ankle and elbow blocks.

Curved Array Transducers

Curved array transducers are low frequency transducers that require a large surface area.  Because these transducers are low frequency, they offer good depth penetration.  However, because they are curved the density of the scan lines decreases as depth increases reducing image resolution.  Additionally, the transducer must be compressed against the skin to ensure complete contact.  This may occlude small blood vessels creating the potential for complications such as inadvertent vascular puncture.

Sector Transducers

Sector transducers produce a fan-like image that is narrow as it leaves the transducer and increases with depth.  It is ideal for imaging through small “windows” such as the cranial window in the temple, or between the ribs.  They are commonly used for transcranial and cardiac imaging.  These transducers provide good depth penetration, but have very poor near field resolution.  In general, they are not useful for peripheral nerve blockade.


References

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:11-15.

Gray AT. Atlas of ultrasound-guided regional anesthesia. Philadelphia, PA. Saunders, Elsevier; 2010:21.

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:18-20.

    Legal Disclaimer

  • Provision of education and research information only - always seek professional advice

    The VAULT website was developed by Christian R. Falyar, CRNA, DNAP, to share information related to ultrasound-guided regional anesthesia with other anesthesia providers. It is not intended to be a substitute for accredited regional anesthesia training. Christian R. Falyar CRNA, DNAP does not accept any liability for any injury, loss or damage that results from the use of or reliance on information contained in this website.
  • Quality of information - always check the information

    Christian R. Falyar, CRNA, DNAP has made every effort to ensure the quality of the information presented on this website is current and checks it regularly. However, before relying on the material on this website, users should carefully evaluate its accuracy, currency, completeness and relevance to their practice. Christian R. Falyar, CRNA, DNAP, cannot guarantee and assumes no legal liability or responsibility for the accuracy, currency or completeness of the information.
  • Links to external websites

    This website may contain links to other websites which are external to the VAULT website. Christian R. Falyar, CRNA, DNAP takes reasonable care in selecting linking websites. It is the responsibility of the user to make their own decisions about the accuracy, currency, reliability and correctness of information contained in linked external websites. Linkage to external websites should not be taken to be an endorsement or a recommendation of any third party products or services offered by virtue of any information, material or content linked from or to this website. Users of links provided by this website are responsible for being aware of which organization is hosting the website they visit. Views or recommendations provided in linked websites do not necessarily reflect those of Christian R. Falyar, CRNA, DNAP.