Advancing the Science of Ultrasound Guided Regional Anesthesia and Pain Medicine

Basic Principles

Characteristics of Ultrasound

Ultrasound is a form of mechanical sound energy that travels through a conducting medium (e.g., body tissue) as a longitudinal wave producing alternating compression (high pressure) and rarefaction (low pressure). Sound propagation can be represented in a sinusoidal waveform with a characteristic pressure (P), wavelength (λ), frequency (f), period (T) and velocity (speed (c) + direction).

The frequency of an ultrasound wave is above 20,000 Hz (or 20 KHz) and medical ultrasound commonly is in the 2.5-15 MHz range. Human hearing is in the 20-20,000 Hz range.

The speed of sound varies for different biological media but the average value is assumed to be 1,540 m/sec (constant) for most human soft tissues. The speed of sound (c) can be calculated by multiplying wavelength (λ) x frequency (f). Thus sound with a high frequency has a short wavelength and vice versa. For example, the wavelength of a 2 MHz ultrasound wave = 0.77 mm and that of a 15 MHz wave = 0.10 mm.

Ultrasound Speed (m/sec)
Soft Tissue

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 (Gabriel Lippman 1881).

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 2 or 3 sound cycles of the same frequency.

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). 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 1-10 kHz. For example, if the PRF = 5 kHz and the time between pulses is 0.2 msec, it will take 0.1 msec to reach the target and 0.1 msec to return to the transducer. This means the pulse will travel 15.4 cm before the next pulse is emitted (1,540 m/sec x 0.1 msec = 0.154 m in 0.1 msec = 15.4 cm).

Generation of an Ultrasound Image

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.

A schematic diagram of direct and indirect piezoelectric effect, where electrical energy is converted into mechanical and then sound energy and vice versa.

The transducer waits to receive the returning wave (i.e. echo) after each pulsed wave. 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 (Pierre Currie 1880).

The image can be displayed in a number of modes:
1) amplitude (A) mode
2) brightness (B) mode
3) motion (M) mode

Among the 3 modes, the B mode is most commonly used for ultrasound guided regional anesthesia There are 5 basic components of an ultrasound scanner that are required for generation, display and storage of an ultrasound image.
1. Pulser - applies high amplitude voltage to energize the crystals
2. Transducer - converts electrical energy to mechanical (ultrasound) energy and vice versa
3. Receiver - detects and amplifies weak signals
4. Display - displays ultrasound signals in a variety of modes
5. Memory - stores video display

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. 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 (see table below). A reduction of 3 dB corresponds to diminution of the original intensity by half. The higher the attenuation coefficient, the more attenuated the ultrasound wave is by the specified tissue. For example, bone with a very high attenuation coefficient severely limits beam transmission.

The degree of attenuation also varies directly with the frequency of the ultrasound wave (see figure below) and the distance traveled. 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.

The Impact of Body Tissues on Attenuation

Body Tissue Attenuation Coefficient
(dB/cm at 1MHz)
Water 0.002
Blood 0.18
Fat 0.63
Liver 0.5 - 0.94
Kidney 1.0
Muscle 1.3 - 3.3
Bone 5.0

The Impact of Ultrasonic Frequency on Attenuation

↑ Frequency = ↑ Attenuation; ↑ Attenuation = ↓ 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.

Echo Reflection and Scattering

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).

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.

Variation of Acoustic Impedance with Body Tissues

Body Tissue Acoustic Impedance
(106 Rayls)
Air 0.0004
Lung 0.18
Fat 1.34
Liver 1.65
Blood 1.65
Kidney 1.63
Muscle 1.71
Bone 7.8

The degree of reflection is high for air because air has an extremely low acoustic impedance (0.0004) relative to other body tissues. 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.

The angle of the incidence is also a major determinant of reflection. An ultrasound wave hitting a smooth mirror like interface at a 90 degree angle will result in a perpendicular reflection. An incident wave hitting the interface at an angle less than 90 degrees 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. This explains why it is difficult to visualize a needle inserted at a steep angle (> 45 degrees to the skin surface).

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. 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.

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.

After reflection and scattering, the remainder of the incident beam is refracted with a change in the direction of the transmitted beam. 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.

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., diaphragm, gallstone, bone, pericardium. Weaker diffuse reflections produce grey 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.

Tissue Ultrasound Image For Regional Anesthesia
Veins anechoic (compressible)
Arteries anechoic (pulsatile)
Fat hypoechoic with irregular hyperechoic lines
Muscles heterogeneous (mixture of hyperechoic lines within a hypoechoic tissue background)
Tendons predominantly hyperechoic technical artifact (hypoechoic)
Bone ++ hyperechoic lines with a hypoechoic shadow
Nerves hyperechoic / hypoechoic technical artifact (hypoechoic)

A sonogram showing a round anechoic artery (A) and an oval shaped anechoic vein (V); a vein is collapsible while an artery is not.

Fat has a hypoechoic background containing streaks of hyperechoic lines that are often irregular in texture and length; the fat layer is most superficial; note the difference in appearance between fat and muscle

Muscle also has a hypoechoic background containing short streaks of hyperechoic lines; the outline of a muscle layer (the fascial sheath) is highly hyperechoic. Bone often shows a hyperechoic outline (arrows) and a hypoechoic bony shadow underneath due to a lack of beam penetration.

A nerve (arrowhead, N) and a tendon (arrow, T) of the forearm in cross section; the nerve is oval and the tendon has an irregular shape; note that the tendon will merge into a muscle proximally while the nerve does not.

A nerve in longitudinal section (arrowheads) showing internal echotexture consisting of continuous hypoechoic longitudinal elements (fascicle groups) interspersed with hyperechoic perineural connective tissues.

A tendon in longitudinal section (arrows); a tendon appears very much like a nerve in this view although the tendon has a fibrillar internal echotexture and discontinuous hyperechoic speckles.

A hypoechoic nerve root (arrowhead) in the low interscalene region; nerves are generally hypoechoic in the interscalene and supraclavicular regions; the hypoechoic component represents the neural tissue.

Nerves below the clavicle and in the lower limbs are predominantly hyperechoic and have a honey comb appearance. For example, the median nerve in the elbow region (arrowhead) is predominantly hyperechoic. The degree of hyperechogenicity likely reflects the amount of connective tissue within the nerve.
Anisotropy refers to a change in echogenicity of tissues (e.g., tendon and nerve) as a result of transducer angle. A hyperechoic nerve structure can appear hypoechoic when the angle of incidence is changed from 90 degrees to 45 degrees (see Transducer Movement).

Image Resolution

Spatial resolution determines the degree of image clarity. Resolution is the ability of the ultrasound machine to distinguish two structures (reflectors or scatterers) that are close together as separate. Spatial resolution is influenced by axial and lateral resolution, both of which are closely related to ultrasound frequency. Axial resolution refers to the ability to distinguish two structures that lie along the axis (i.e. parallel) of the ultrasound beam as separate and distinct. Axial resolution is determined by the pulse length. A high frequency wave with a short pulse length will yield better axial resolution than a low frequency wave.

In figure A, a 5 MHz transducer generates ultrasound waves that travel 0.3 mm per cycle (wavelength = 0.3 mm = speed of sound / frequency = 1,540 m/sec divided by 5 x 106 cycles/sec). The pulse length is the distance traveled by one echo (3 cycles in this case). As seen in figure A, a 5 MHz transducer (wavelength = 0.3 mm and pulse length = 0.9 mm; 3 cycles), the axial resolution is sufficient to distinguish the 2 target objects as separate because the incident wave hits target # 1 (brown) before hitting target # 2 (green).

In figure B, with a 2.5 MHz transducer (wavelength = 0.6 mm and pulse length = 1.8 mm; 3 cycles), the axial resolution is no longer adequate. Because both target # 1(brown) and target # 2 (green) are hit by the same wave, both target objects are seen as one.

Lateral resolution refers to resolution of objects lying side by side (i.e., perpendicular to the beam axis). Lateral resolution is directly related to the transducer beam width, which in turn is inversely related to the ultrasound frequency. A high frequency transducer emits a wave with a short wavelength and a small beam width. Lateral resolution is poor when the 2 structures lying side by side are located within the same beam width. Because the returning echoes overlap with each other side by side, the 2 structures (1 and 2 in figure) will appear as one on the display. It is therefore clinically important to choose the highest frequency transducer possible to keep the beam width as narrow as possible in order to provide the best possible lateral resolution. However, attenuation also increases with frequency thus one must strike a balance between resolution and attenuation.

The beam width can be further reduced by adjusting the focal zone (FZ). Lateral resolution is the best at the FZ, where the beam is narrowest. It is therefore clinically useful to focus the target structure within the focal zone to yield the best possible lateral resolution. The beam is known to diverge (increased beam width) as it propagates deep into the far field.

A high frequency beam has a narrower beam width.

Color Doppler

Color Doppler is an instrument to characterize blood flow. The Doppler effect occurs when there is a moving source (blood flow of red blood cells, RBC) and a stationary listener (ultrasound transducer). There is an apparent change in the returning echoes due to the relative motion between the sound source and the receiver. If the source (RBC) is moving towards the receiver (transducer), the perceived frequency is HIGHER (display in RED) and when the source (RBC) is moving away from the receiver, the perceived frequency is LOWER than the actual (display in BLUE).

It is important to note that Color Doppler detection of flow and flow direction is worst when the transducer is perpendicular (90 degrees) to the vessel and best when the transducer is parallel (0 degrees) to the blood flow.

Transducer perpendicular to radial artery (no flow is detected)

Transducer aiming away from artery (flow in BLUE)

Transducer aiming towards artery (flow in RED)

For ultrasound guided regional anesthesia, Color Power Doppler (CPD) is useful for differentiating vascular from non vascular structures. CPD is more sensitive than Color Doppler in flow detection but does not indicate flow direction.

Transducer perpendicular to radial artery (weak flow is detected)

Transducer aiming towards or away from artery (strong flow is detected)

Image Artifacts

Imaging artifacts are display distortions or errors that may adversely affect image interpretation or acquisition.

An acoustic enhancement artifact (hyperechoic region, arrow) deep to a fluid filled structure (e.g., a vessel, A = artery) is the result of beam penetration through an area of low attenuation coefficient to an area of higher attenuation coefficient.

An acoustic shadow artifact (hypoechoic region = bone shadow) deep to a hyperechoic bone outline (arrows) is the result of beam attenuation when the beam encounters bone with a high attenuation coefficient. Ultrasound beam penetration is severely impeded.

Tissue reverberation artifacts are generated by strong specular reflectors e.g., the pleura (yellow arrowheads). Air artifact in the lung results in "comet tail" acoustic signals (arrows) going from the hyperechoic pleural line into the lung parenchyma.

Reverberation artifacts (white arrows) can be seen during needle (yellow arrows) advancement in the infraclavicular region using a curved transducer.

The air artifact (arrows) at the transducer skin interface is due to a lack of conductive gel and poor transducer to skin contact. This results in a large dropout artifact.

Ultrasound vs Nerve Stimulation
Understanding Nerve Stimulation with Ultrasound
Colin McCartney, MD, FRCPC Associate Professor Sunnybrook Health Sciences Centre Toronto ON, CAN

Regional Anesthesia - When to Use Dual Ultrasound and Nerve Stimulation Guidance?
Thomas Grau, MD, PhD, MA Professor and Chair Klinikum Gutersloh gGmbH Department of Anaesthesia Intensive Care, Emergency and Pain Medicine Gutersloh, Germany

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