Bedside Ultrasound

Level 1 - Second Edition

Peter Steinmetz

• Éditeur: A-line Press
• Année: 2020
• ISBN:
  9781999247720
• Sujet: Medicine
• Sous-sujet: Medical Theory, Practice & Reference

The second edition of this textbook offers expanded chapters with additional ultrasound images, videos, and text. Thetextbook is a beginner book for healthcare workers starting to apply bedside ultrasound in their daily practice. The book has received outstanding reviews from medical students, residents, and physicians clinicians. Over 1300copies have been sold worldwide. Clear diagrams, ultrasound images, and instructional videos help to illustrate the concepts outlined in the text. No prior experience in bedside ultrasound is necessary. Chapters 1-3 address the basics of ultrasound. Chapters 4-11 address different applications for bedside ultrasound. Troubleshooting tips, false-positives and negatives, and documentation guidelines are provided at the end of the chapters. And index and full bibliographic references complete the book.

1.ULTRASOUND BASICS

1.1What is ultrasound?

1.2How do ultrasound probes send and receive ultrasound?

1.3How does ultrasound behave travelling through tissue?

1.4Gain

1.5Depth

1.6Resolution and penetration

1.7Doppler and flow

1.1What is ultrasound?

Ultrasound is a sound wave that oscillates with a frequency greater than 20,000 Hz (20,000 cycles per second or 20 kHz). The human ear can detect sound waves with frequencies between 0.02-20 kHz. Sound waves above 20 kHz, such as those generated by dog whistles, bat calls, and ultrasound machines, cannot be interpreted as sound by the human ear. These high frequency sound waves are called ultrasound.

It is useful to understand some basic concepts about ultrasound in order to correctly interpret the images generated by the bedside ultrasound machine.

Figure 1.1 Different sound wave frequencies.

The human ear can interpret sound waves with frequencies up to 20 kHz. Sound waves with frequencies above 20 kHz are termed ultrasound. Ultrasound probes emit frequencies from 2,000-20,000 kHz (2-20 MHz).

1.2How do ultrasound probes send and receive ultrasound?

Ultrasound probes have two functions: to send and receive ultrasound. An ultrasound probe spends 1% of its time sending sound waves and 99% of its time receiving sound waves.

The probe starts by sending out bursts of ultrasound. When ultrasound encounters a structure, it reflects off that structure and returns to the probe. The reflected ultrasound is interpreted and expressed as an image on the ultrasound machine monitor.

Figure 1.2 Ultrasound probes send and receive ultrasound.

A. Schematic demonstrating a probe sending sound waves (blue) and receiving the reflected returning sound waves (red).

B. The returning sound waves are detected by the probe and translated into an ultrasound image.

1.3How does ultrasound behave travelling through tissue?

Attenuation

Attenuation describes the dampening of the amplitude of ultrasound as it travels through tissue. Attenuation occurs as ultrasound energy is converted into heat, absorbed by a structure, reflected back to the probe, or scattered away from the probe.

Attenuation is directly proportional to the distance that the ultrasound travels and to the ultrasound frequency. Attenuation is also affected by the characteristics of the medium encountered.

Distance: Attenuation of ultrasound increases as the ultrasound propagates deeper into the body. When imaging the abdomen, the far-field (deeper) structures can appear darker due to attenuation of the ultrasound.

Frequency: High frequency ultrasound attenuates more rapidly than low frequency ultrasound. Thus, high frequency probes cannot be used to image deep structures.

Type of medium: Air and bone cause a high degree of attenuation. Fluid causes a low degree of attenuation.

Reflection

An image is generated by the ultrasound waves that are reflected from a structure and returned to the probe. The image appears brighter as the amount of reflection increases. In general, reflection is greatest when ultrasound encounters a structure of high-density or when it crosses an interface between structures of different densities.

Hyperechoic:	Bone appears hyperechoic (white) on the ultrasound image. Its high-density and the interface between surrounding structures of lower density cause a high degree of reflection.
Hypoechoic:	Muscle and liver appear hypoechoic (grey) on the ultrasound image due to their moderate density.
Anechoic:	Fluid (blood, ascites, pleural effusions) appears anechoic (black) on the ultrasound image. This is because ultrasound travels through low-density structures with minimal attenuation and reflection back to the ultrasound probe.

Table 1.1 Ultrasound image terminology

Figure 1.3 Appearance of structures with different densities.

The vertebral body (VB) is a bony, high-density structure that appears hyperechoic (white) on the ultrasound image. The aorta (Ao) and inferior vena cava (IVC) are low-density structures that appear anechoic (black). The tissues surrounding these structures are of moderate density and so appear hypoechoic (varying shades of grey).

1.4Gain

Due to the attenuation of the ultrasound returning to the probe, structures may appear dark and hard to identify on the image. Increasing the gain on the ultrasound machine increases the amplification of the reflected ultrasound. This adjustment makes structures appear hyperechoic (white) on the screen and easier to identify.

Figure 1.4 Gain adjustment.

A. Ultrasound...