Orientation

Transducer orientation is an important, but a rarely discussed topic in ultrasound-assisted anesthesia procedures.  Proper orientation of the transducer in relation to patient position is paramount to obtaining an accurate image.  A common practice is to place the desired object (blood vessel or nerve) in the center of the screen to avoid inserting the needle into the wrong side.  However this can also be problematic when the transducer is not oriented properly, because any adjustments required in needle position will appear to be the opposite of what is intended.  To ensure accurate and consistent orientation, the transducer should be held so the external notch is facing the toward the anesthetist’s left in a cross-sectional view or toward the patient’s head in a longitudinal view.  When a transducer is correctly oriented, it is not only possible to predict where needle insertion will be displayed on the B-Mode image, but also to anticipate the direction of any adjustments that may be required during the procedure.  Consistent and accurate transducer orientation is fundamental to reducing potential complications during ultrasound-guided vascular access or regional anesthesia.

Proper transducer orientation is the first step to accurately interpreting an ultrasound image.  When the transducer is placed on the patient’s neck to produce a cross-section or short-axis view, a notch on the outer casing will correlate to an orientation icon on the B-Mode image (indicated by the yellow arrows).

Proper Orientation

The images below show needle localization in a phantom with the transducer oriented correctly.  Note how the needle appears on the same side of the screen where it is inserted.

Improper Orientation

The images below show needle localization in a phantom with transducer improperly orientated.  Note how the needle appears on the opposite side of the screen where it is inserted. 

Implications of Improper Orientation

Below are two supraclavicular images.  The image on the left is properly oriented, while the image on the right is not.  Consider the implications needle insertion could potentially present on the inverted image.  The white line indicates the desired path of the needle, while the yellow arrow indicates its actual course.  Additionally, any adjustments to the needle would require the practitioner to move it in the opposite direction of what is desired.  This example magnifies how ultrasound is not a replacement for anatomical knowledge.  Knowing the relationships between blood vessels, nerves and other key structures in the body is vital when evaluating whether an image is properly oriented.


References

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

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