CHAPTER 7 Principles of ultrasound-guided regional anesthesia
Introduction to ultrasound
Why understanding ultrasound physics and how to use an ultrasound machine is important
Ultrasound physics
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 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).
The control unit
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