A sound knowledge of the basic concepts of musculoskeletal ultrasound is essential to obtain optimal images during ultrasound-guided regional anesthesia (USGRA). This chapter briefly summarizes the ultrasound principles that the operator should be aware of when performing USGRA.
ULTRASOUND TRANSDUCER FREQUENCY
Spatial resolution is the ability to distinguish two closely situated objects as separate. Spatial resolution includes axial resolution (the ability to distinguish two objects at different depths along the path of the ultrasound beam) and lateral resolution (the ability to distinguish two objects that are side by side perpendicular to the ultrasound beam). Higher transducer frequencies increase spatial resolution but penetrate poorly into the tissues. Lower transducer frequencies penetrate deeper into the tissues at the expense of lower spatial resolution. Spatial resolution and beam penetration have to be balanced when choosing the transducer frequency.
Examples: A high-frequency (6–13 MHz) ultrasound transducer is used to image superficial structures such as the brachial plexus in the interscalene groove or supraclavicular fossa. A lower-frequency transducer (5–10 MHz) is suitable for slightly deeper structures such as the brachial plexus in the infraclavicular fossa, and a low-frequency transducer (2–5 MHz) is used to image deep structures such as the lumbar paravertebral region or the sciatic nerve. High-frequency (6–13 MHz) linear transducers with a small footprint (25–26 mm) are particularly suited for regional blocks in young children.
Scans can be performed in the transverse (axial) or longitudinal plane. During a transverse scan, the transducer is oriented at right angles to the long axis of the target, producing a cross-sectional display of the structures (Fig. 1–1A). During a longitudinal (sagittal) scan, the transducer is oriented parallel to the long axis of the target (eg, a blood vessel or nerve) (Fig. 1–1B). During USGRA, ultrasound scans are most commonly performed in the transverse plane in order to easily visualize the nerves, the adjacent structures, and the circumferential spread of the local anesthetic.
TRANSDUCER AND IMAGE ORIENTATION
The ultrasound image must be correctly oriented in order to accurately identify the anatomical relationships of the various structures on the display monitor. Ultrasound transducers have an orientation marker (eg, a groove or a ridge) on one side of the transducer, which corresponds to a marker on the monitor (eg, a dot or logo) (Fig. 1–2). There are no accepted standards on how to orient a transducer, but it is common to have the orientation marker on the transducer directed cephalad when performing a longitudinal scan, and directed towards the right side of the patient when performing a transverse scan (Fig. 1–3). In this way, the monitor “marker” should be at the upper-left corner of the screen representing the cephalad end during a ...