Critical Care Ultrasound Manual

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TABLE OF CONTENTS

1 Basic2Dultrasoundphysics 000Anthony McLean & Stephen Huang

2 Knobology 000Anthony McLean & Stephen Huang

3 Commonultrasoundartefactsin2Dimaging 000Anthony McLean & Stephen Huang

4 Transthoracicechocardiogram–viewsandanatomy 000Anthony McLean & Stephen Huang

5 RACEoutline 000Anthony McLean & Stephen Huang

6 Leftheartassessment 000Anthony McLean & Stephen Huang

7 Rightheartassessment 000Anthony McLean & Stephen Huang

8 Pericardialassessment 000Anthony McLean & Stephen Huang

9 Intravascularvolume(preloadassessment) 000Anthony McLean & Stephen Huang

10 Otherpathologies 000Anthony McLean & Stephen Huang

11 Vascularultrasoundrapiddeepveinthrombosis(DVT)assessment 000Anthony McLean & Stephen Huang

12 Ultrasound-guidedvascularaccess 000Anthony McLean & Stephen Huang

13 Lungandpleuralultrasound 000Marek Nalos

14 ThestandardFASTprotocol 000Arvind Rajamani

15 Criticalcareabdominalultrasound 000Arvind Rajamani

AppendixA BasicDopplerPhysics 000Anthony McLean & Stephen Huang

AppendixB Normalreferencevaluesfortransthoracic echocardiography 000

Anthony McLean & Stephen Huang

AppendixC RACEstudychecklistandsamplereport 000Anthony McLean & Stephen Huang

AppendixD Criticalcareultrasoundlevelofcompetency 000Anthony McLean & Stephen Huang

MCQsforonlineancillary 000Anthony McLean & Stephen Huang

Index 000

McLean & Huang_Table of contents_main.indd 5 2/17/2012 3:29:25 PM

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IntroductionBeginners in echocardiography are often tempted to skip the basic principles of ultrasound physics and just concentrate on the practice of echocardiography. Although this is deemed acceptable, beginners will soon find it difficult to cope with the different technical terms as well as the processes of image optimisation. Without an understanding of basic ultrasound physics, image optimisation becomes a daunting, random trial-and-error exercise.

Properties of sound waves

• Sound is a mechanical wave produced by vibration of the medium (e.g. air, water).

• Sound waves are longitudinal pressure waves in which the particles of the medium vibrate (oscillate) in the direction of the sound wave (Fig. 1.1). As sound waves propagate, alternate regions of compression (high pressure) and rarefaction (low pressure) are created. These variations in pressure can be represented by a sinusoidal wave (Fig. 1.1).

• Sound waves are characterised by the following properties:» Wavelength (λ) is the distance between two consecutive points of the same

phase, or the distance over which the wave repeats (Fig. 1.1).» Frequency (f ) is the number of cycles (or oscillations) per second. The units are

Hertz (Hz) (Fig. 1.2).» Speed (c) is the distance travelled by the sound wave per unit time. It is the

product of wavelength and frequency (c = λf ). Sound travels at slightly different speeds in different biological tissues (Table 1.1). The average speed is taken as 1540 m/s for the purposes of diagnostic ultrasound.

» Amplitude is the maximal distance covered by each particle during an oscillation. It accounts for the maximal change in pressure and is represented by the peak-to-peak distance of the sinusoidal wave. Amplitude is related to the strength (or energy) of the signal. Large amplitude yields a stronger (brighter) signal (Fig. 1.1).

• Sound with a frequency above 20,000 Hz is termed ultrasound. Diagnostic ultrasound utilises a frequency range of 2–20 MHz (1 MHz = 106 Hz). Echocardiography typically employs frequencies of 2–4 MHz.

1Basic 2D ultrasound physics

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Critical Care Ultrasound Manual2

FIGURE 1.1 A sound wave is a longitudinal wave but it can be represented by a sinusoidal

(transverse) waveform.

Rarefaction

Compression (+)

Amplitude

Rarefaction (–)

Pre

ssur

e

Wavelength (λ)

Compression

FIGURE 1.2 Frequency is the number of oscillations per second (Hz). λ is the wavelength.

1 second

Low frequency (2 Hz)

l l

High frequency (8 Hz)

1 second

TABLE 1.1 Velocities of ultrasound in different tissues and media

Tissue Velocity (m/s)

Blood 1570

Bone 3500

Fat 1450

Kidney 1560

Liver 1550

Muscle 1580

Water 1480

Air 330



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CHAPTER 1 Basic 2D ultrasound physics 3

The ultrasound transducer

• Modern ultrasound transducers utilise piezoelectric elements (crystals) to generate ultrasound. These elements are closely spaced and packed together.

• The piezoelectric elements change shapes (vibrate) in response to a changing electrical field (Fig. 1.3). Each of the elements has its own electrical connection; hence it can be activated individually or as part of a group to produce ultrasound beams.

• A constantly changing electrical field causes the piezoelectric elements to vibrate, thus emitting ultrasound. On the other hand, deformations of these elements by sound waves generate voltages that are detected as echo signals.

Beam formation

• In 2D-imaging, ultrasound is emitted as short pulses (usually less than 5 μs in modern machines) at regular intervals. These pulses are reflected back to the transducer by various reflectors.

• Pulse duration (PD) or pulse length is the length of the pulse measured in time. Short PD results in better axial resolution – the ability to resolve two closely spaced objects along and parallel to the ultrasound path.

• PD is determined by the frequency and the number of cycles in a pulse. Higher frequency or less number of cycles results in shorter pulse duration (Fig. 1.4).

FIGURE 1.3 The piezoelectric element changes shape in response to a change in the direction

of the electrical field.

+

–

+

–

FIGURE 1.4 Pulse duration (PD) is the length of the pulse and can be reduced by increasing the

frequency or by reducing the number of cycles of the ultrasound.

Long pulse duration

Short pulse duration

Increased frequency

Reduced number of cycles



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• For technical reasons, the pulse is composed of a range of frequencies instead of a single pure frequency. For example, a 2.5-MHz pulse may contain frequencies ranging from 1.5 MHz to 3.5 MHz although the main component (central frequency) is 2.5 MHz.

• In medical imaging, 2000–4000 pulses are emitted per second (pulse repetition frequency) (PRF) (Fig. 1.5). The time interval between two consecutive pulses is known as the pulse repetition period (PRP).

• The transducer ‘listens’ for returning ultrasounds (echoes) between two emitted pulses. Typically, a transducer spends more than 99% of the time listening.

• During a scan, the piezoelectric elements in the transducer are activated in a specific sequence and, as a result, a wavefront is formed. A typical wavefront converges at a certain point (the focus) and diverges afterward (Fig. 1.6; Videos 1.1 and 1.2).

• At the focus, the beam width is at its narrowest and has its highest intensity, yielding the best lateral resolution – the ability to resolve two objects spaced closely together and perpendicular to the ultrasound beam.

Reflection versus transmission

• Based on the time it takes for the echoes to return to the transducer, the computer calculates the depth (distance from the transducer surface) of the reflector, and plots the information on the screen.

• Although a proportion of the emitted ultrasound signals are reflected back to the transducer at each tissue boundary, most ultrasound signals are transmitted through the various tissue layers, allowing visualisation of deeper structures.

DVD

FIGURE 1.5 Ultrasound is emitted in pulses. The number of pulses per second is the pulse repetition frequency (PRF). The duration of each pulse is known as the ‘pulse duration’ (PD) or ‘pulse length’. Note that the PRF is not the same as the ultrasound frequency.

Pulse repetition period(PRP)

Time spenton ‘listening’PD

1 second

(PRF = 3 pulses/second)

FIGURE 1.6 Characteristics of a typical ultrasound beam. By activating the piezoelectric elements in specific sequence, the transducer focuses the beam at a certain distance (the focus). The zone before the focus is known as the near field, whereas the zone beyond is the far field.

Focus

FocalzoneNear field Far field



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• The percentage of reflection depends on the difference in the acoustic impedance (Z) (or resistance) of the adjacent tissues. Z is proportional to tissue density (ρ). Small differences in Z or ρ favour transmission, whereas large mismatches favour reflection. For example, approximately 10% of the ultrasound (energy) is reflected at a fat–muscle interface (leaving 90% for transmission), whereas 99.95% is reflected at an air–tissue interface. Typical values of Z for common tissues are shown in Table 1.2.

• Tissues with a large Z mismatch are usually referred to as ‘strong reflectors’. Echoes from strong reflectors have larger amplitudes (energies) and appear brighter on the display (Fig. 1.7).

Forming an imageAfter the pulse repetition period for each pulse has lapsed, another pulse is sent out but at a slightly different angle (‘steering’). Each pulse therefore forms one scan-line. Together, these scan-lines form a sector of a 2D image (Fig. 1.8 and Video 1.3).

DVD

TABLE 1.2 Acoustic impedances of selected tissues and materials

Tissue or material Acoustic impedance (x106 rayls)

Air 0.0004

Water 1.48

Fat 1.36

Muscle 1.71

Liver 1.65

Kidney 1.63

Blood 1.63

Bone 7.80

FIGURE 1.7 Ultrasound signals are reflected at tissue interfaces due to the differences in

acoustic impedances (Z). A large Z mismatch results in reflection of echoes with large amplitudes (strong reflectors). Most ultrasound energy will be transmitted between tissues of similar densities (weak reflectors), and the amplitude of the reflected echo is smaller.

Emitted wave Reflected waveTransmitted wave

Strong reflector

Weak reflector

Tissueinterface



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FIGURE 1.8 Formation of a 2D image. The transducer sends out ultrasound pulses, each at a slightly different angle. Together, the echoes from these pulses form a sector of a 2D image.

FIGURE 1.9 The three main types of transducers for general ultrasound. The linear and curvilinear probes are not suitable for transthoracic echocardiography due to their large footprints. The ribs cast acoustic shadows in these two probes, partially blocking the views.

Linear

Cross-sectionof ribs

Acousticshadows

Curvilinear Phased-array

Types of transducers

• There are three main types of transducers, namely linear array, curvilinear array and phased array (Fig. 1.9).

• These transducers have different properties and difference uses (Table 1.3).



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TABLE 1.3 Comparisons of the three types of transducers

Linear array Curvilinear array Phased array

Frequency range (Hz)

5–12 2–5 1.5–4

Footprint (cm2) Large (~5 × 1) Large (~6 × 1.5) Small (~2.5 × 1.5)

Axial resolution Very good Good to average Good to average

Penetration Poor Good Good

Coverage Small Large Large

Uses • Vascular ultrasound

• Ultrasound-guided vascular access

• Abdominal ultrasound

• Echocardiography• Lung ultrasound• Pleural ultrasound



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MCQs CHAPTER 1 MCQS1 Which of the following statements is true about pulse duration?

a It is also known as the pulse length.b Short pulse duration results in better lateral resolution.c Decreasing the frequency reduces the pulse duration.d Increasing the number of cycles reduces the pulse duration.

2 Which of the following statements is true about medical ultrasound?a It travels at a speed of 1540 m/s in all biological tissues.b Ultrasound is a transverse wave.c The wavelength in biological tissues is in the centimeter range.d It utilises frequencies in the 106 Hz range.

3 Which of the following determines lateral resolution?a Pulse durationb Focusc Frame rated Frequency

4 Which of the following determines axial resolution?a Frame rateb Focusc Frequencyd Velocity

5 Which of the following statements about acoustic impedance (Z) is false?a Z is proportional to tissue density.b Z represents the tissue resistance to ultrasound.c A large difference in Z values between two tissues results in strong reflection.d Z is proportional to tissue thickness.

Answers to Chapter 1 MCQs

1 a2 d3 b4 c5 d



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