Optimization of the ultrasound (US) image is important to improve the visualization of the relevant anatomy during US-guided peripheral nerve blocks (PNBs). Because a nerve block is an injection of local anesthetic (LA) into a tissue space that contains the nerve, it is often more practical and easier to identify the interfascial tissue space containing the nerve than the nerve(s) to be blocked. Optimizing the US image requires knowledge of how the US machine operates and adequate training in image acquisition. In this chapter, we describe standardized scanning steps, including the selection of sonographic modes, adjustment of function keys, essential transducer maneuvers, and interpretation of artifacts to optimize the US view and guide the needle toward the target.
ULTRASOUND MACHINE SETTINGS
Conventional, compound, and tissue harmonic imaging (THI) are common sonographic imaging and signal processing modes used for medical diagnostics, which can all be utilized to visualize the relevant anatomy during regional anesthesia.
Conventional imaging is generated from a single angle beam at a primary frequency designated by the transducer.
Compound imaging acquires several overlapping frames from different frequencies or angles. Figure 5-1 demonstrates the difference between conventional and compound imaging applied to visualize the radial nerve proximal to the elbow. The contrast resolution between the muscle and the nerve is increased in comparison with conventional imaging. Compound imaging is automatically deactivated with the use of color Doppler, which cannot be applied with multiple angles of insonation.
Ultrasound image of the radial nerve at the elbow with (A) conventional and (B) compound imaging.
Tissue harmonic imaging (THI) combines the information from harmonic frequencies, which are multiples of the primary frequency, generated by US beam transmission through tissue. As such, THI suppresses scattering signals from tissue interfaces, thereby improving the axial resolution and boundary detection. An example of its advantage is in obese patients where the anatomical structures may be situated deeper. All modern US manufacturers incorporate THI as the default mode, because of the better resolution images, improved tissue penetration, better detection of tissue interfaces, and margin enhancement compared with conventional sonography.
The following function keys on an US machine are essential to achieve the best possible resolution during the performance of PNBs:
Transducer frequency: US frequency determines the axial resolution or the ability of the US to distinguish two separate points along the axis of the US beam. Frequency and depth, however, are interdependent and important in decision-making. The first step is to select the transducer with the optimal frequency range to visualize the target nerves at a certain depth. The second step is to adjust the best frequency within each transducer range according to the depth of the nerve. Some US machines display the full range of the transducer frequency, while ...