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.

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.

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

Here are some examples:

Why don’t I see my needle?

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).

Why cannot I see the deeper structures?

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).

Why is the artery blue?

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).

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 7.4 (A) Transducer tilted against the direction of the flow: the artery is visualized in red. (B) Transducer tilted in the direction of the flow: the artery is visualized in blue. (C) Transducer perpendicular to the flow: there is no Doppler shift and the artery is visualized in black.

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.

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.

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

Figure 7.7 Scan of the axilla. L: fat tissue; B: bone; M: muscle; F: fascia; V: vessel; N: nerve.

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).