Basic Principles and Physics of Ultrasound

Abstract

This chapter discussed the basic knowledge that one needs to acquire before they start the use of ultrasound scanning. These include the physics of sound/ultrasound and image generation by an ultrasound machine, how the image is acquired and processing in the ultrasound machine, the basic knowledge in operating an ultrasound machine (knobology), the basic sonoanatomy of tissue and common artifacts encountered in scanning, and the necessary terms and ergonomics for the performance of ultrasound scanning. A glossary is also included for various terms that are commonly encountered in ultrasound literature.

Understanding the Physics of Ultrasound and Image Generation

Characteristic of Sound Wave

Audible sound wave lies within the range of 20–20,000 Hz. Ultrasound is a sound wave beyond audible range (>20,000 Hz). Ultrasound system commonly used in clinical settings incorporates transducers generating frequencies between 2 and 17 MHz. Some special ultrasound system even generates frequencies between 20 and 55 MHz. Sound waves do not exist in a vacuum, and propagation in gases is poor because the molecules are too widely spaced, which explains the use of gel couplant between the skin of the subject and the transducer interface to eliminate the air-filled gap.

Fig. 1.1

Comparison of high-frequency and low-frequency waveform. (Reprinted with permission from Philip Peng Educational Series)

Fig. 1.2

A longitudinal wave showing alternating compression and rarefaction. (Reprinted with permission from Philip Peng Educational Series)

Sound wave is a form of mechanical energy that travels through a conducting medium (e.g., body tissue) as a longitudinal wave producing alternating compression (high pressure) and rarefaction (low pressure) (Figs. 1.1 and 1.2). Sound propagation can be represented in a sinusoidal waveform with a characteristic pressure (P), wavelength (λ), frequency (f), period (T), and velocity. See Table 1.1 for details.

Table 1.1 Basic terminology

The speed of sound varies for different biological media, but the average value is assumed to be 1540 m/s for most human soft tissues. It can vary greatly, being as low as 330 m/s in air and as high as 4000 m/s through bone.

The wavelength (λ) is inversely related to the frequency (f). Thus, sound with a high frequency has a short wavelength and vice versa.

Generation of an Ultrasound Wave

An ultrasound wave is generated when an electric field is applied to an array of piezoelectric crystals located on the transducer surface. Electrical stimulation causes mechanical distortion of the crystals resulting in vibration and production of sound waves (i.e., mechanical energy). The conversion of electrical to mechanical (sound) energy is called the converse piezoelectric effect. Each piezoelectric crystal produces an ultrasound wave. The summation of all waves generated by the piezoelectric crystals forms the ultrasound beam. Ultrasound waves are generated in pulses (intermittent trains of pressure waves), and each pulse commonly consists of two or three sound cycles of the same frequency.

Fig. 1.3

Pulse repetition frequency. (Reprinted with permission from Philip Peng Educational Series)

The pulse length (PL) is the distance traveled per pulse. Waves of short pulse lengths improve axial resolution for ultrasound imaging. The PL cannot be reduced to less than 2 or 3 sound cycles by the damping materials within the transducer.

Pulse repetition frequency (PRF) is the rate of pulses emitted by the transducer (number of pulses per unit time) (Fig. 1.3). Ultrasound pulses must be spaced with enough time between pulses to permit the sound to reach the target of interest and return to the transducer before the next pulse is generated. The PRF for medical imaging ranges from 1 to 10 kHz. For example, if the PRF = 5 kHz and the time between pulses is 0.2 ms, it will take 0.1 ms to reach the target and 0.1 ms to return to the transducer. This means the pulse will travel 15.4 cm before the next pulse is emitted (1540 m/s × 0.1 ms = 0.154 m in 0.1 ms = 15.4 cm).

Generation of an Ultrasound Image

Fig. 1.4

Ultrasound wave interaction with body tissue. (Reprinted with permission from Philip Peng Educational Series)

An ultrasound image is generated when the pulse wave emitted from the transducer is transmitted into the body, reflected off the tissue interface, and returned to the transducer. The schematic diagram above showed the transducer waits to receive the returning wave (i.e., echo) after each pulsed wave (Fig. 1.4). The transducer transforms the echo (mechanical energy) into an electrical signal which is processed and displayed as an image on the screen. The conversion of sound to electrical energy is called the piezoelectric effect.

The image can be displayed in a number of modes (Fig. 1.5):

Fig. 1.5

Three different modes of ultrasound. (Reprinted with permission from Philip Peng Educational Series)

• Amplitude (A) mode is the display of amplitude spikes in the vertical axis and the time required for the return of the echo in the horizontal axis.

• Brightness (B) displays a two-dimensional map of the data acquired and is most commonly used for ultrasound guided intervention.

• Motion (M) mode, also called time motion or TM mode, displays a one-dimensional image usually used for analyzing moving body parts. This mode records the amplitude and rate of motion in real time and is commonly used in cardiovascular imaging.

Ultrasound Tissue Interaction

As the ultrasound beam travels through tissue layers, the amplitude of the original signal becomes attenuated as the depth of penetration increases (Fig. 1.6).

Fig. 1.6

Different types of attenuation. (Reprinted with permission from Philip Peng Educational Series)

Attenuation (energy loss) is due to:

• 1. Absorption (conversion of acoustic energy to heat)

• 2. Reflection

• 3. Scattering at interfaces

In soft tissue, 80% of the attenuation of the sound wave is caused by absorption resulting in heat production. Attenuation is measured in decibels per centimeter of tissue and is represented by the attenuation coefficient of the specific tissue type. The higher the attenuation coefficient, the more attenuated the ultrasound wave is by the specified tissue.

Absorption

Absorption is the process of transfer of the ultrasound beam’s energy to the medium through which it travels through heat generation and it accounts for most of the wave attenuation. The quality of the returning sound waves depends on the attenuation coefficient of different tissue.

Fig. 1.7

Variation of attenuation with frequency in different organs. (Reprinted with permission from Philip Peng Educational Series)

Table 1.2 Attenuation coefficient of various tissues

The degree of attenuation also varies directly with the frequency of the ultrasound wave and the distance traveled (Fig. 1.7 and Table 1.2). Generally speaking, a high-frequency wave is associated with high attenuation, thus limiting tissue penetration, whereas a low-frequency wave is associated with low tissue attenuation and deep tissue penetration.

To compensate for attenuation, it is possible to amplify the signal intensity of the returning echo. The degree of receiver amplification is called the gain. Increasing the gain will amplify only the returning signal and not the transmit signal. An increase in the overall gain will increase brightness of the entire image, including the background noise. Preferably, the time gain compensation (TGC) is adjusted to selectively amplify the weaker signals returning from deeper structures.

Reflection

Attenuation also results from reflection and scattering of the ultrasound wave. The extent of reflection is determined by the difference in acoustic impedances of the two tissues at the interface, i.e., the degree of impedance mismatch.

Table 1.3 Acoustic impendence of various tissues

Acoustic impedance is the resistance of a tissue to the passage of ultrasound. The higher the degree of impedance mismatch, the greater the amount of reflection (Table 1.3). The degree of reflection is high for air because air has an extremely low acoustic impedance (0.0004) relative to other body tissues. The bone also produces a strong reflection because its acoustic impedance is extremely high (7.8) relative to other body tissues. For this reason, it is clinically important to apply sufficient conducting gel (an acoustic coupling medium) on the transducer surface to eliminate any air pockets between the transducer and skin surface. Otherwise, much of the ultrasound waves will be reflected limiting tissue penetration.

Fig. 1.8

Angle of incidence. (Reprinted with permission from Philip Peng Educational Series)

The angle of the incidence is also a major determinant of reflection. An ultrasound wave hitting a smooth mirror-like interface at a 90° angle will result in a perpendicular reflection. An incident wave hitting the interface at an angle <90° will result in the wave being deflected away from the transducer at an angle equal to the angle of incidence but in the opposite direction (angle of reflection). When this happens, the signal of the returning echo is weakened, and a darker image is displayed (Fig. 1.8). This explains why it is difficult to visualize a needle inserted at a steep angle (>45° to the skin surface).

Fig. 1.9

Different types of reflection. (Reprinted with permission from Philip Peng Educational Series)

Specular reflection occurs at flat, smooth interfaces where the transmitted wave is reflected in a single direction depending on the angle of incidence. Examples of specular reflectors are fascial sheaths, the diaphragm, and walls of major vessels (Fig. 1.9). Block needles are also strong specular reflectors. For specular reflection to occur, the wavelength of the ultrasound wave must be smaller than the reflective structure. Otherwise, scattering will occur.

Scattering

Reflection in biological tissues is not always specular. Scattering (diffuse reflection) occurs when the incident wave encounters an interface that is not perfectly smooth (e.g., surface of visceral organs). Echoes from diffuse reflectors are generally weaker than those returning from specular reflectors. Scattering also occurs when the wavelength of the ultrasound wave is larger than the dimensions of the reflective structure (e.g., red blood cells). The reflected echo scatters in many different directions resulting in echoes of similar weak amplitudes. Ultrasonic scattering gives rise to much of the diagnostic information we observe in medical ultrasound imaging.

Transmission

Fig. 1.10

Transmission of beam. (Reprinted with permission from Philip Peng Educational Series)

After reflection and scattering, the remainder of the incident beam is refracted with a change in the direction of the transmitted beam (Fig. 1.10). Refraction occurs only when the speeds of sound are different on each side of the tissue interface. The degree of beam change (bending) is dependent on the change in the speed of sound traveling from one medium on the incident side to another medium on the transmitted side (Snell’s Law). With medical imaging, fat causes considerable refraction and image distortion, which contributes to some of the difficulties encountered in obese patients. Refraction encountered with bone imaging is even more significant leading to a major change in the direction of the incident beam and image distortion.

The final image on the screen of an ultrasound machine is the result of the interaction of ultrasound waves with the tissues being examined. As the ultrasound wave travels through the tissues, it loses amplitude, and hence energy (attenuation), which is the summative effect of absorption, reflection, and refraction of ultrasound waves.

Image Acquisition and Processing

Transducer Basic

An ultrasound transducer has a dual functionality. It is responsible for both the production of ultrasound waves and, after a set period of time, the reception of waves reflected from the tissues. This is called pulsed ultrasound. The pulse repetition frequency (PRF) is the number of pulses emitted by the transducer per unit of time. The PRF for medical imaging devices ranges from 1 to 10 kHz

Fig. 1.11

Anatomy of a transducer. (Reprinted with permission from Philip Peng Educational Series)

The ultrasound transducer has the following layers (Fig. 1.11):

• (a) Backing material: located behind the piezoelectric element, it serves to prevent excessive vibration. This causes the element to generate ultrasonic waves with a shorter pulse length, improving axial resolution in images.

• (b) Piezoelectric elements: they generate ultrasonic waves and also generate images. Piezoelectric ceramic (PZT: lead zirconate titanate) is most commonly used because of their high conversion efficiency.

• (c) Acoustic matching layer: this reduces the acoustic impedance mismatch between the transducer and the object and thus minimizes reflection off the interface. This is usually made up of a resin.

• (d) Acoustic lens: the acoustic lens prevents the ultrasonic waves from spreading and focuses them in the slice direction to improve the resolution.

Transducer Selection

The choice of which transducer should be used depends on the depth of the structure being imaged. The higher the frequency of the transducer crystal, the less penetration it has but the better the resolution. Therefore, if more penetration is required, you need to use a lower-frequency transducer with the sacrifice of some resolution.

Transducer characteristics, such as frequency and shape, determine ultrasound image quality. For simplicity, there are three types of transducers.

Fig. 1.12

Different types of transducer. (Reprinted with permission from Philip Peng Educational Series)

The linear transducer frequencies used for superficial structures and peripheral nerve blocks range from 6 to 15 MHz (Fig. 1.12). Curvilinear (or curved) transducers are most useful for deeper structures or imaging requiring a wide field of view (e.g., spine). Microconvex is commonly used for small acoustic window such as cardiac scanning.

Fig. 1.13

Frequency, resolution, and penetration. (Reprinted with permission from Philip Peng Educational Series)

For superficial structures (e.g., nerves in the interscalene region), it is ideal to use high-frequency transducers in the range of 10–15 MHz, but depth of penetration is often limited to 2–3 cm below the skin surface (Fig. 1.13). For visualization of deeper structures (e.g., in the gluteal region) or when a wider field of view is required, it may be necessary to use a lower-frequency transducer (2–5 MHz) because it offers ultrasound penetration of 4–5 cm or more below the skin surface. However, the image resolution is often inferior to that obtained with a higher-frequency transducer.

Basic Operation of Ultrasound Machine

To obtain a good ultrasound picture, clinician should be familiar with the basic function and buttons in the ultrasound machine: gain, depth, color (Doppler), and focus (Fig. 1.14).

Fig. 1.14

Basic button of ultrasound machine. (Reprinted with permission from Philip Peng Educational Series)

Gain

The echo signals returning from the tissues reach the crystals and produce an electric current (piezoelectric effect). This is then converted to a pixel on the image, with the energy of the returning wave being proportional to the brightness of the pixel dot on the image. The amplitude of the returning echo signals is very small to be properly displayed on a screen. Hence, it needs amplification. Amplification of signal can be adjusted using the GAIN button, but the use of it adds to “background noise.”

Fig. 1.15

Time gain compensation. (Reprinted with permission from Philip Peng Educational Series)

Time gain compensation

It is the preferential enhancement of signals at different depths returning from deeper tissues (Fig. 1.15). Thus, the echoes returning from similar reflectors can be represented by the same shade of gray regardless of their depth.

Depth

Fig. 1.16

Depth and image acquired. (Reprinted with permission from Philip Peng Educational Series)

The depth of the field should be adjusted to the area of interest (Fig. 1.16). Too shallow may miss the important information from the background in the deep field, and too deep will diminish the quality of the image in the superficial field.

Focus

Fig. 1.17

Focus of a beam. (Reprinted with permission from Philip Peng Educational Series)

Fig. 1.18

Effect of focus setting and image acquired. (Reprinted with permission from usra.ca)

The shape of the beam varies and is different for each transducer frequency. There is a fixed focused region of the ultrasound beam which is indicated on the system with a small triangle to the right of the image. This indicates the focal zone of that transducer and is where the best resolution can be achieved with that particular transducer (Fig. 1.17). Effort should be taken to position the object of interest in the subject to within that focused area to obtain the best detail by adjusting the FOCUS button (Fig. 1.18).

Doppler

The Doppler information is displayed graphically using spectral Doppler, or as an image using color Doppler (directional Doppler) or power Doppler (non-directional Doppler).

All modern ultrasound scanners use pulsed Doppler to measure velocity. Pulsed wave machines transmit and receive series of pulses.

Fig. 1.19

Color Doppler. (Reprinted with permission from Philip Peng Educational Series)

In color Doppler, echoes are displayed with colors corresponding to the direction of flow that their positive or negative Doppler shifts represent (toward or away from the transducer). The brightness of the color represents the intensity of the echoes, and sometimes other colors are added to indicate the extent of spectral broadening (Fig. 1.19).

Power Doppler depicts the amplitude or power of Doppler signals rather than the frequency shift. This allows detection of a larger range of Doppler shifts and therefore better visualization of the smaller vessels, but at the expense of directional and velocity information.

Understanding Sonoanatomy and Artifact

Tissue Echogenicity

When an echo returns to the transducer, its amplitude is represented by the degree of brightness (i.e., echogenicity) of a dot on the display. Combination of all the dots forms the final image. Strong specular reflections give rise to bright dots (hyperechoic), e.g., the diaphragm, gallstone, bone, and pericardium. Weaker diffuse reflections produce gray dots (hypoechoic), e.g., solid organs. No reflection produces dark dots (anechoic), e.g., fluid- and blood-filled structures, because the beam passes easily through these structures without significant reflection. Also, deep structures often appear hypoechoic because attenuation limits beam transmission to reach the structures, resulting in a weak returning echo.

Sonoanatomic Features of Tissue

Fig. 1.20

Sonoanatomy of vessels. (Reprinted with permission from Philip Peng Educational Series)

Fig. 1.21

Vessel with color Doppler on. (Reprinted with permission from Philip Peng Educational Series)

Vessels

Left sonogram showed a round anechoic artery (A) and an oval-shaped anechoic vein (V) (Fig. 1.20). An artery is pulsatile and not easily compressible. A vein is collapsible and non-pulsatile. Figure 1.21 showed the long axis of vessels with color Doppler on. As discussed in the above section, red color indicates the direction of flow and does not confirm the presence of an artery.

Fig. 1.22

Sonoanatomy of the muscle, adipose tissue, and bone. (Reprinted with permission from Philip Peng Educational Series)

The Adipose, Muscle, and Bone

The left sonogram showed the typical appearance of adipose tissue, which is hypoechoic background with streaks of hyperechoic lines that are often irregular in texture and length (Fig. 1.22). Appearance of muscle is generally hypoechoic. The structure deep to the adipose tissue is the rectus muscle in transverse plane, which has a speckled appearance. The sonogram on the right displayed the muscle in long axis, and the perimysial connective tissue is apparent in linear pattern. The bone appears as an hyperechoic line with an anechoic shadow due to lack of beam penetration.

Fig. 1.23

Sonoanatomy of nerves. (Reprinted with permission from Philip Peng Educational Series)

Nerve

The appearance of the nerve (yellow arrows) can be hyperechoic, hypoechoic, or mixed, depending on the location, size, amount of connective tissue, and the angle of the transducer (Fig. 1.23). Above the clavicle, the nerve roots typically appear as hypoechoic (left upper sonogram). As the peripheral nerve picks up more connective tissues, it appears as hyperechoic (femoral nerve in the right upper sonogram). A peripheral nerve can also appear as mixed hyper- and hypoechoic (honeycomb appearance) as shown in the left lower sonogram. In long axis, the nerve appears hypoechoic with fibrillar pattern in the right lower sonogram. However, compared with the tendon in long axis (right sonogram in Fig. 1.24 below), the nerve is more hypoechoic, and the echotexture is coarse.

Fig. 1.24

Sonoanatomy of a tendon. (Reprinted with permission from Philip Peng Educational Series)

Tendon

The left sonogram showed the tendon in short axis, which showed densely fibrillar pattern (Fig. 1.24). Comparison of the long axis view of the tendon in the right sonogram with that of the nerve in Fig. 1.23 (right lower sonogram) shows that the tendon is more echogenic and fibrillar in appearance.

Artifacts

Reverberation

Fig. 1.25

Reverberation artifact. (Reprinted with permission from Philip Peng Educational Series)

This is the production of false echoes due to repeated reflections between two interfaces with high acoustic impedance mismatch. The echo from the interface is received by the transducer and displayed on the image. Some of the energy in the returned echo is reflected at the transducer crystals and returns to the reflecting interface as if it was a weak transmitted pulse, returning as a second wave (Fig. 1.25). The time taken for the second wave to arrive is twice that taken by the depth.

This sequence of transmission and reflection can occur many times, with the third wave taking three times as long to return to the transducer and being displayed at three times the depth. These reverberation echoes will be strong because of the high acoustic mismatch.

There are few tricks to reduce reverberation artifact:

• 1. Increase the amount of gel used

• 2. Use a standoff pad.

• 3. Reduce the gain.

• 4. Adjust the position of the transducer.

Acoustic Shadowing

Fig. 1.26

Acoustic shadowing. (Reprinted with permission from Philip Peng Educational Series)

This appears as an echo-free area behind structures of strongly attenuating tissue. Because of the very high attenuation of the beam at an interface, the transmission is substantially reduced. This artifact can also appear at an interface with large acoustic mismatch such as soft tissue and gas, also at a soft tissue and bone (Fig. 1.26).

Acoustic Enhancement Artifact

Fig. 1.27

Acoustic enhancement. (Reprinted with permission from Philip Peng Educational Series)

This appears as a localized area of increased echo amplitude behind an area of low attenuation. On a scan, it will appear as an area of increased brightness and can commonly be seen distal to fluid-filled structures such as a cyst (Fig. 1.27). This arises due to the application of the time gain compensation (TGC) to areas of low attenuation structure such as fluid. It is caused by the low level of attenuation of the beam as it passes through fluid relative to the greater attenuation of the beam in the adjacent more solid tissue.

Edge Shadowing

Fig. 1.28

Edge shadowing. (Reprinted with permission from Philip Peng Educational Series)

This appears as a narrow hypoechoic shadow line extending down from the edge of a curved reflector (Fig. 1.28). It arises due to refraction of the beam caused by both the curvatures of the rounded edges. When the ultrasound beam races the rounded edge of the structure, reflection will occur, with an angle of incidence equal to the angle of reflection. The outer part of the beam will be totally reflected, but the reminder of the beam passes through the rounded structure and is refracted.

Mirror Imaging Artifact

Fig. 1.29

Mirror imaging artifact. (Reprinted with permission from Philip Peng Educational Series)

This artifact results in a mirror image of the structure in an ultrasound display. They arise due to specular reflection of the beam at large smooth interface (Fig. 1.29). An area close to specular reflector will be imaged twice, once by the original ultrasound beam and once by the beam after it has reflected off the specular reflector. Mirror image artifacts are commonly seen where there is a large acoustic mismatch, such as fluid-air interface. As an example, typically this artifact can occur during the scanning of the subclavian artery on top of the lung (fluid-air interface). It will then have the appearance of a mirror image of the artery within the lung.

Air Artifact

Fig. 1.30

Air artifact. (Reprinted with permission from Philip Peng Educational Series)

At the transducer skin interface, a large dropout artifact can occur due to a lack of conductive gel and poor transducer to skin contact (Fig. 1.30).

Anisotropy

Fig. 1.31a

Angle of the ultrasound and the target structure. (Reprinted with permission from Philip Peng Educational Series)

Anisotropy refers to the angle dependence appearance of a structure (such as tendon) on an image (Fig. 1.31a). When the transducer is perpendicular to the long axis of the object, it results in maximum reception of the signal and optimal image. Tilting the transducing affects the signal and thus the image produced.

Fig. 1.31b

Anisotropy of median nerve. (Reprinted with permission from Philip Peng Educational Series)

This affects the quality of the image because the reflection of sound wave is maximum when the transducer (and the incident waves) is perpendicular to the structure (median nerve in this case) under examination (left panel) (Fig. 1.31b). Any change on this incidence angle dramatically reduces the returning echo, causing the structure to “disappear” out of the image (right panel). Manipulation of the transducer (so as to direct the incident beam perpendicular to the structure of interest) and beam-steering can help address anisotropy.

Things to Remember for Optimal Ultrasound Scanning and Performance

Scanning and Positioning Terminology

Fig. 1.32

The three different planes. (Reprinted with permission from usra.ca)

Fig. 1.33

(a) Short and long axis . (b) In-plane and out-of-plane. (Reprinted with permission from Philip Peng Educational Series)

View and Needle Insertion

Short and long axis is based on the position of the probe with reference to the axis of structure (Fig. 1.32). If the probe is place along the axis of the target structure to be scanned, it is showing the long axis view (Fig. 1.33a).

The approach to insert the needle is described according to the orientation of needle to the plane of ultrasound beam (Fig. 1.33b). If the needle tip and the shaft are inserted in a way such that both are in the ultrasound beam, it is called in-plane. If the axis of needle insertion is right angle to the ultrasound beam, it is out-of-plane.

Transducer Handling

There are four basic transducer handling (mnemonic = “PART”) (Fig. 1.34).

Fig. 1.34

Pressure, alignment, rotation, and tilt. (Reprinted with permission from Philip Peng Educational Series)

• P = Pressure, adjust according to the depth of target for visualization

• A = Alignment, sliding the transducer longitudinally

• R = Rotation, rotating the transducer (clockwise/counterclockwise)

• T = Tilting, tilting or angling the transducer

Ergonomics

Ergonomics is the study (or science) of the interaction between humans and their working environment. Recent years have witnessed an increasing application of optimal procedural ergonomics in regional anesthesia in an effort to improve outcomes. Poor ergonomics may not only lead to suboptimal performance of a procedure but may contribute to work-related musculoskeletal discomfort.

A sound application of the principles of ergonomics to regional anesthesia and pain management includes the consideration of the following factors:

Positioning and Care of the Patient

Position of patient is important for comfort of patient and for maximal exposure. This position may vary with the type of block being performed. For example, the patient may need a supine position for an upper limb procedure, a prone position for a lower limb procedure, and a sitting position for a neuraxial procedure. Additionally, the position of the limb may be adjusted to assist the procedure.

Positioning of the Physician

Fig. 1.35

Correct and incorrect position of the physician. (Reprinted with permission from Philip Peng Educational Series)

Maintaining a good position with respect to the patient helps to ensure operator comfort and allows optimal procedure performance (Fig. 1.35). This includes the following:

• (a) Adjusting the height of patient bed to an appropriate level for the operator.

• (b) Assuming a good posture, by choosing to stand or sit down (on a chair).

• (c) Performing the procedure from the same target side to avoid reaching over the patient.

Positioning the Equipment

Fig. 1.36

Correct and incorrect position between the operator, patient, and ultrasound machine. (Reprinted with permission from Philip Peng Educational Series)

The ultrasound machine is placed in a position so that the operator can view the site of injection and the machine without turning the head. Similarly, the screen of the monitoring equipment must be turned to face the operator during the block to allow a prompt recognition of any significant change of vital signs (Fig. 1.36).

Position of the Assistant

An assistant may be needed both to operate the ultrasound machine and to inject the local anesthetic solution. This may be achieved by standing opposite to the operator and near the ultrasound machine.

Holding the Probe and Needle Insertion

Fig. 1.37

Correct and incorrect position of holding the ultrasound probe. (Reprinted with permission from Philip Peng Educational Series)

It is important to have a steady control of the probe position. The probe can be held with 2–3 fingers, and the hand can rest on the body of the patient gently to gain support (Fig. 1.37).

Fig. 1.38

(a) Correct angle of insertion of needle. (b) Incorrect angle of entry of needle. (Reprinted with permission from Philip Peng Educational Series)

When inserting the needle, make sure your needle is within the field of view of the ultrasound scan, no matter it is in-plane or out-of-plane approach (Fig. 1.38a). Otherwise, the needle is inserted in the wrong plane with reference to the ultrasound field of view and may result in inadvertent tissue puncture (Fig. 1.38b).

Glossary (Table 1.4)

Table 1.4 Dictionary of terms used in the chapter