CHAPTER 7 Principles of ultrasound-guided regional anesthesia
The ability to use ultrasound guidance for regional anesthesia is achieved by systematic learning and maintained by regular practice. Knowledge of anatomy is paramount for the successful practice of regional anesthesia. In order to visualize this anatomy, one has to know how to use an ultrasound machine. We hope this chapter will give the reader the knowledge and impetus to make better use of their ultrasound equipment, but it takes learning from more experienced colleagues, practice, self-discipline and reflection to get one’s skills to an adequate level.
Ultrasound is a mechanical wave with frequencies over 20 000 Hz. Ultrasound used in medicine is generated and sensed by piezoelectric crystals. The ultrasound transducer incorporates a battery of piezoelectric crystals. When scanning, the transducer switches quickly between transmitter and receiver modes. When in transmitting mode, the piezoelectric crystals are stimulated by electrical energy, vibrate and emit ultrasound waves. In the receiver mode the crystals are hit by the ultrasound waves reflected from the tissues (Fig 7.1). The resultant mechanical stimulation of the crystals is converted to electrical signals, which are processed and ultimately create the image we see on the screen.
Figure 7.1 (A) Stimulated by alternating electric current, the piezoelectric crystals vibrate and emit ultrasound waves. (B) The reflected waves hit back the crystals, which undergo conformational changes and generate electric current.
Figure 7.2 (A) Impact angle 90°: the reflected beam reaches the transducer and generates good image. (B) Impact angle 80°: there is partial loss of signal and the image is degraded. (C) Impact angle 30°: the reflected signal does not reach the transducer and the image is lost.
Figure 7.3 (A). High frequency transducer: deep structures do not visualize due to the absorbtion of the ultrasound. Note the good resolution at superficial level. (B) Low frequency transducer: deep structures visualized due to the better penetration of the ultrasound. Note the image has lower resolution, compared with Fig. 7.3A.
Figure 7.5 (A) When the object is moving towards the transducer, the reflected waves have increased frequency. (B) When the object is moving away from the transducer, the reflected waves have lower frequency.
Figure 7.6 (A) Insufficient gain. Note the deep structures are not visualized well. (B) Optimal gain. Note the overall good resolution and the improved visualization of the deeper structures. (C) Excessive gain. Note the blurred image resulting in poor resolution.
|Tissue||Acoustic impedance (g/cm2 sec × 100)|
The ultrasound machine consists of a transducer (acting as transmitter and receiver), main unit (generating pulses for the transmitter, processing the impulses from the receiver, control unit, memory) and a display.
Transducers vary in size, shape, frequency range and number of piezoelectric crystals. For superficial blocks, a high frequency transducer (7–15 MHz) will provide better axial resolution (i.e. better ability to distinguish as separate structures dots lying along the path of the ultrasound beam) (Fig. 7.3). The more piezoelectric crystal elements, the better the resolution. A lower frequency transducer (1–5 MHz) is more appropriate for deeper blocks as there is less absorption and thus better signal from the deeper structures (Fig. 7.3). Transducers with a small footprint (i.e. hockey stick transducers) are useful in children or where space is limiting (Fig. 7.8). Wider (with large footprint) and curvilinear transducers (sector) allow for visualization of a bigger area and thus may be helpful in visualizing landmark structures at the same time as the nerves of interest (Fig. 7.8).