Optimizing an Ultrasound Image

Optimizing an image produced by ultrasound is an essential skill for performance of an ultrasound-guided nerve block. Anatomically, peripheral nerves are often located in the vicinity of an artery or between muscle layers. The echo texture of normal peripheral nerves can have a hyperechoic, hypoechoic, or honeycomb pattern (Figure 1). Several scanning steps and techniques can be used to facilitate ade- quate nerve imaging, including the selection of sonographic modes, adjustment of function keys, needle visualization, and interpretation of image artifacts.

All in all, sonographic imaging modes used for ultrasound- guided regional anesthesia and medical diagnostics are conventional imaging, compound imaging, and tissue harmonic imaging (THI). The conventional imaging is generated by a single-element angle beam. The compound imaging is implemented by acquiring several (usually three to nine) overlapping frames from different frequencies or from different angles. The tissue harmonic imaging acquires the information from harmonic frequencies generated by ultrasound beam transmission through tissue, which improves tissue contrast by suppression of scattering signals. Compound imaging with The tissue harmonic imaging can provide images with better resolution, penetration, and interfaces and margin enhancement, compared with those obtained using conventional sonography. In Figure 2, compound imaging and conventional imaging was used to view the interscalene brachial plexus. There is a clear margin definition of two hypoechoic oval-shaped nerve structures in compound imaging. As an example, the contrast resolution between the anterior scalene muscle and the surrounding adipose tissue is enhanced in comparison with those made with conventional imaging techniques.

Five function keys on an ultrasound machine are of crucial importance to achieve an optimal image during the performance of peripheral nerve imaging (Figure 3A).

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Figure 1: Architecture of peripheral nerves

Figure 2: Examples of image quality typically obtained with conventional versus compound imaging.

Figure 3: Optimizing an ultrasound image using five key functional adjustments (A) and specific tips on adjusting the focus (B) and gain (C). Some ultrasound models are specifically optimized for regional anesthesia application and may not incorporate user-adjustable focus and/or time gain compensation (TGC).

Figure 3 (Continued)

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 Table 1: Suggested Optimal Imaging Depth for Common Peripheral Nerve Blocks

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Two needle insertion techniques with relevance to the needle-transducer relationship are commonly used in ultrasound-guided nerve block: the in-plane and out-of-plane techniques (Figure 4). In-plane technique means the needle is placed in the plane of ultrasound beam; as a result, the needle shaft and the tip can be observed in the longitudinal view real time as the needle is advanced toward the target nerve. When the needle is not visualized on the image, the needle advancement should be stopped. Tilting or rotating the transducer can bring the ultrasound beam into alignment with the needle and help with its visualization. Additionally, a subtle, fast needle shake and or injection of small amount of injectate may help depict the needle location. The out-of-plane technique involves needle insertion perpendicularly to the transducer. The needle shaft is imaged in a cross-section plane and can be identified as a bright dot in the image. Visualization of the tip of the needle, however, is difficult and unreliable. The method used to visualize the tip of the needle is as follows: Once a bright dot (shaft) is seen in the image, the needle can be shaken slightly and/or the transducer can be tilted toward the direction of needle insertion simultaneously until the dot disappears. Shaking the needle helps differentiate the echo as emanating from the needle or from the surrounding tissue. The last capture of the hyperechoic dot is its tip. A small amount of injectate can be used to confirm the location of needle tip. Whenever injectate is used to visualize the needle tip, attention must be paid to avoid resistance (pressure) to injection because when the needle–nerve interface is not well seen, there is a risk for an intraneural injection.

By definition, ultrasound artifact is any image aberration that does not represent the correct anatomic arrangement. The five artifacts often seen in regional anesthesia practice (Figure 5) are as follows.

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Figure 5: Common artifacts during ultrasound imaging. LA - local anesthetic, MSM - middle scalene muscle.