Principles of ultrasound-guided regional anesthesia

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.

Introduction to ultrasound

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.

Why understanding ultrasound physics and how to use an ultrasound machine is important

Here are some examples:

Ultrasound physics

Frequency. Ultrasound waves can be of different frequency. The higher frequency waves produce better spatial resolution and thus allow us to see anatomic structures in greater detail (Fig 7.3). As a downside, the high frequency waves have low tissue penetration due to their higher tissue absorption. Thus, when visualising deeper structures, lower frequency waves are more useful (Fig 7.3). Ultrasound transducers have either fixed or adjustable frequencies. It is important to use an ultrasound transducer appropriate for the depth of the block.

The Doppler shift effect. When a sound wave hits a stationary object, the reflected wave has the same frequency. When a sound wave hits an object that is moving, the reflected wave changes its frequency. This is depicted in Figure 7.5. When the object (for example, a red blood cell) is moving towards the ultrasound transducer, the reflected waves will have a higher frequency than the original. When using Color Doppler mode imaging, this increase in frequency is visualized on the screen by convention in red. When the object is moving away from the transducer, the reflected waves have a lower frequency than the original, and this change is depicted in blue. If the ultrasound beam is perpendicular to the moving object, the reflected waves will not show any shift in frequency and there will be no color depicted on the screen. Thus, depending on the orientation of the transducer, the same blood vessel can appear red, blue, or black (Fig 7.4).

Reflection. When an ultrasound wave hits a tissue interface, a proportion is reflected. The extent of reflection depends on the extent of difference between the acoustic impedances at the interface. Tissue impedances are shown in Table 7.1. We want to achieve minimal reflection at the transducer-skin interface (thus the importance of using coupling gel to eliminate the air under the ultrasound transducer) and optimal reflection from the structures we are interested in.

Tissue echogenicity. Highly reflective tissues (e.g. bone, fascias) produce bright images (hyperechoic); moderately reflective tissues (e.g. muscle) produce moderate intensity images (hypoechoic), while fluid filled spaces (vessels) visualize as dark images (anechoic) (Fig. 7.7). When assessing tissue echogenicity one has to take into account depth. Due to absorption, the deeper the structure, the more hypoechoic it may appear (despite not being hypoechoic strictly speaking) unless depth-adjusted gain is used to compensate for the absorption.

Table 7.1 The acoustic impedances of selected body tissues

Tissue Acoustic impedance (g/cm2 sec × 100)
Air 0.0004
Fat 1.3
Water 1.5
Blood 1.6
Muscle 1.7
Bone 7

The ultrasound machine

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.

The transducer

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).

Jul 28, 2016 | Posted by in ANESTHESIA | Comments Off on Principles of ultrasound-guided regional anesthesia

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