I thank Dr. Andreae for his thoughtful comments on the recent Clinical Concepts and Commentary article.1He raises two critical issues regarding ultrasound-guided regional block: the method of approach and needle tip visibility. With the in-plane approach, the needle tip and shaft are contained within the plane of imaging. Positioning of the needle parallel to the skin surface (perpendicular to the sound beam) will yield strong specular reflections. Although parallel needle positioning is ideal from a needle visibility standpoint, it is not essential to the in-plane approach because backscatter echoes are still received by the transducer (nonparallel approach is illustrated in fig. 1B of the article).
With the out-of-plane approach, the needle tip crosses the plane of imaging. Skilled operators can often maintain the needle tip within the plane of imaging by sliding and tilting the transducer so as to follow the needle tip as it is advanced. In the nomenclature put forth in the article, the approach described by Dr. Andreae would be considered an extreme example of the out-of-plane approach. One disadvantage of needle entry close to the transducer is that skin displacement by the needle can disrupt the acoustic coupling between skin and transducer (contact artifact). Another problem is the discrimination of tip and shaft echoes.
We have made controlled measurements of needle tip visibility for regional block needles in phantoms that mimic tissue.2Over a range of angles from 0 to 65°, needle tip visibility is reduced at steep angles away from the surface. As Dr. Andreae suggests, the out-of-plane approach is less susceptible to this effect than the in-plane approach. The use of compound imaging technology (steering of the beam to different angles to produce a composite image) will reduce (but not eliminate) the influence of needle angle.3
The dawn of three-dimensional imaging of nerves is now upon us.4Although it has been used to guide other interventions and can improve needle pass efficiency,5it may not ultimately be embraced by our specialty. The principal imitations include the time for acquisition and rendering of reconstructed images, as well as the interpretation of the display in a setting where the acoustic interfaces do not have marked contrast. This could detract from the dynamic nature of ultrasound guidance for regional block that so many anesthesiologists have found appealing.
University of California, San Francisco, San Francisco General Hospital, San Francisco, California. email@example.com