CHAPTER 7 Principles of ultrasound-guided regional anesthesia
Introduction to ultrasound
Why understanding ultrasound physics and how to use an ultrasound machine is important
When a sound wave hits a reflective surface, the resultant reflection is at an angle corresponding to the incidence angle. When the ultrasound beam hits a needle at an inappropriate angle, the reflected wave will not reach the transducer and there will be no image of the needle (Fig 7.2).
Tissues absorb ultrasound waves. The higher the frequency, the higher the absorption. Thus high frequency transducers are less suitable for visualization of deeper structures (Fig 7.3).
The color Doppler allows us to visualize moving particles. When the particles move towards the ultrasound transducer by convention they are visualized in red, when they move away from the ultrasound transducer, in blue. Thus, if we orient the ultrasound transducer against the flow, we get red image of the respective vessel. The same vessel will visualize blue if we turn the ultrasound transducer in the direction of the flow (Fig 7.4).
Ultrasound physics
Imaging modes. There are multiple imaging modes (A, B and M-mode, various Doppler modes – color, pulsed wave, continuous wave, etc.). B-mode is the imaging mode most commonly used in regional anesthesia. B-mode or ‘brightness’ imaging produces a two-dimensional image with a different grey scale between black and white. Structures appear hypoechoic (dark) or hyperechoic (bright) and shades in between.
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).
Absorption and gain. Ultrasound waves are partially absorbed by tissues and their energy converted to heat. This results in a reduction of the signal subsequently reflected and available for detection by the ultrasound transducer. In order to compensate for the absorbed waves, ultrasound machines amplify the reflected signals (gain) (Fig. 7.6). The operator can control gain. Gain can be adjusted at different depths of the scanned area, optimizing the resultant image.
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.
The angle of reflected waves depends on the angle of the incident waves. Ideally, the incident wave will hit the area of interest at a 90° angle and thus the reflected waves will return to the transducer to be processed and visualized. Any angle different from 90° will result in a suboptimal image (Fig. 7.2).
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 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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