Practical Manual of Echocardiography in the Urgent Setting (eBook)

Vladimir Fridman, Mario Garcia (Herausgeber)

eBook Download: EPUB

2013
John Wiley & Sons (Verlag)
9781118613412 (ISBN)

Infos

Practical Manual of Echocardiography in the Urgent Setting In the acute care setting, medicine happens at full speed and with little margin for error. As echocardiography plays an ever more important role in the diagnosis of patients who present with symptoms that suggest a cardiovascular emergency, clinicians must learn to collect, process and act on echocardiographic information as quickly and effectively as possible. Practical Manual of Echocardiography in the Urgent Setting covers the essentials of echocardiography in the acute setting, from ultrasound basics to descriptions of all pertinent echocardiographic views to clear, stepwise advice on basic calculations and normal/abnormal ranges. This compact new reference: Provides step-by-step guidance to acquiring the correct views and making the necessary calculations to accurately diagnose cardiac conditions commonly encountered in urgent settings. Presents information organized by complaint/initial presentation so that readers can work from this first knowledge of the patient through the steps required to pinpoint a diagnosis. Covers echo basics, from sound wave characteristics/properties to common device settings to basic ultrasound formulas Includes diagnostic algorithms fitted to address the differential diagnosis in the most commonlyencountered clinical scenarios. Designed and written by frontline clinicians with extensive experience treating patients, Practical Manual of Echocardiography in the Urgent Setting is the perfect pocket-sized guide for residents in cardiology, emergency medicine, and hospital medicine; trainees in echocardiography; medical students on cardiology or emergency medicine rotations; technicians, nurses, attending physicians anyone who practices in the urgent setting and who needs reliable guidance on echocardiographic views, data and normal/abnormal ranges to aid rapid diagnosis and decision-making at the point of care. RELATED TITLES: Kacharava, et al: Pocket Guide to Echocardiography; ISBN: 978-0-470-67444-4 Sun, et al: Practical Handbook of Echocardiography: 101 Case Studies; ISBN: 978-1-4051-9556-0

Autor

Edited by: Vladimir Fridman MD Department of Cardiology Long Island College Hospital New York, NY, USA Mario J. Garcia MD Professor, Department of Medicine (Cardiology); Professor, Department of Radiology; Chief, Division of Cardiology; Co-Director, Montefiore Einstein Center for Heart and Vascular Care. New York, NY, USA

Rezensionen

"This is a very practical and comprehensive book on the
role of echocardiography in the diagnosis and management of
critically ill patients." (Doody's, 23
August 2013)

Textauszug

CHAPTER 1

Ultrasound physics

Vladimir Fridman

Cardiovascular Diseases, New York, NY, USA

Echocardiography is one of the most valuable diagnostic tests for the evaluation of patients with suspected cardiovascular disease in the acute setting. Even though echocardiography has become more widely available, its performance and interpretation require practice and knowledge of the principles of image formation. Although the physical principles and instrumentation of ultrasound can be quiet complex, there are a few basic concepts that every echocardiographer and interpreting physician must understand to maximize the diagnostic utility of this test and avoid misinterpretations. These key concepts are covered in this chapter.

The echocardiogram machine (Figure 1.1) is made up of few distinct components:

1 Monitor

2 CPU (central processing unit), responsible for all functions of the echocardiogram

3 Transducer

4 Keyboard/controls

5 Printer

The control panel of any echocardiogram looks similar to that shown in Figure 1.2a. The panel is shown in more detail in Figures 1.2b–d, with the important controls labeled. Although slight changes in control positions are noted between machines from different companies, all machines have the key controls that are shown in these images.

The panel from above image, is split into three frames, and the important controls are labeled below.

Figure 1.1 Echocardiogram machine.

Figure 1.2 Typical echocardiogram control panel.

Figure 1.3 Echocardiography settings.

The important echocardiographic settings as displayed on the monitor of most ultrasound machines are shown in Figure 1.3. These settings can be changed, as needed, to adjust the image quality.

The different echocardiographic modes that are available, which are described later in this book, are:

• M-mode: a graphic representation of a specific line of interest of a two-dimensional image (Figure 1.4).
• 2D: a two-dimensional view of cardiac structures that can be visualized as time progresses (Figure 1.5).
• Color Doppler: a color representation of blood flow velocities ­superimposed on a two-dimensional image (Figure 1.6).
• CW/PW Doppler: the representation of flow velocities as plotted with time on the x axis and velocity on the y axis (Figure 1.7).
• Tissue Doppler: the measurement of tissue velocities (Figure 1.8).

The controls, as shown in the figures, switch between the different modes of echocardiography. However, before moving on to performing and interpreting echocardiograms, it is necessary to be aware of the physics behind this imaging modality.

Figure 1.4 M-Mode: a graphic representation of a specific line of interest of a two-dimensional image.

Figure 1.5 2D: a two-dimensional view of cardiac structures that can be visualized as time progresses.

Figure 1.6 Color Doppler: a color representation of blood flow velocities superimposed on a two-dimensional image.

Figure 1.7 CW/PW Doppler: the representation of flow velocities as plotted with time on the x axis and velocity on the y axis.

Figure 1.8 Tissue Doppler: the measurement of tissue velocities.

Ultrasound generation

Ultrasound is a cyclic sound pressure waveform whose frequency is greater than the limit of human hearing. This number is generally considered to be 20 kHz, or 20 000 Hz (Hertz). Echocardiography usually relies on sound waves ranging from 2 to 8 MHz. The echocardiograph, or any other medical ultrasound machine, produces these high frequency sound waves using transducers that contain a piezoelectric crystal.

A piezoelectric crystal (such as quartz or titanate cyramics) is a special material that compresses and expands as electricity is applied to it. This compression and expansion generates the ultrasound wave. The rate (frequency) of compression and expansion is based on the current that the ultrasound machine applies to the piezoelectric signal, which in turn is based on the settings the operator has selected on the machine.

An ultrasound wave, as all sound waves, has some basic physical properties (Figure 1.9). These are:

• Cycle – the sum of one compression and one expansion of a sound wave.
• Frequency (f) – the number of cycles per second.
• Wavelength (λ) – the length of one complete cycle of sound.
• Period (p) – the time duration of one cycle.
• Amplitude – the maximum pressure change from baseline of a sound wave.
• Velocity (v) – speed at which sound moves through a specific medium.

Figure 1.9 A sound wave is made up of varying pressure cycles formed by repeating of compression and rarefaction. The distance between similar points in a wave is called the wavelength (λ) [1].

Figure 1.10 A pulse can consist of multiple wavelengths of a sound wave. In this figure, three pulses are shown, each the length of two wavelengths (Reproduced from Case [2], with permisison from Elsevier).

A basic property of all sound waves is: Velocity = Frequency (f) x Wavelength (λ). This formula shows that frequency and wavelength are inversely related, since the velocity of a sound wave depends on the density of the medium the wave is traveling in.

In an echocardiogram machine, current is applied to the piezoelectric crystal, which then emits ultrasound energy into human tissue. The ultrasound is emitted in pulses that usually consist of several consecutive cycles of a sound wave with the same frequency separated by a pause (Figure 1.10). An extremely important concept for ultrasound is the frequency of pulses that the ultrasound emits; this is called the Pulse Repetition Frequency (PRF). The inverse of PRF is the Pulse Repetition Period (PRP), which is the time from the beginning of one ultrasound pulse to the next:

The actual length of the pulse – the spatial pulse length (SPL) – is equal to the wavelength multiplied by the number of cycles in a pulse.

Once an ultrasound pulse is emitted from the transducer, the entire mechanism enters the “listening” phase. At this time, the ultrasound machine is waiting to receive back the pulse it emitted after it was reflected from distant structures. It is important to know that the ultrasound machine spends almost 99% of the time listening for, and 1% of the time generating, a signal.

Image formation

As the ultrasound wave exits the echocardiogram probe, it enters the human tissue. When the ultrasound waves encounter a change in tissue density, such as the endocardium–blood interphase, some of them will be reflected back while others will penetrate deeper into the tissue. Thus, ultrasound energy is greater near the transducer and is progressively lost as it penetrates into the tissue. The ultrasound systems typically compensate by amplifying more the signals that are received from the far field to make the image homogeneous. The interaction of ultrasound with human tissue is also very complex. However, it is important to know that within soft tissue the velocity of ultrasound is fairly constant at 1540 m/s. In fact, it is usually assumed that this is the velocity of sound in human tissue. However, it is not always the truth. The velocities of ultrasound in various human tissues are shown in Table 1.1.

This concept is extremely important, since the ultrasound machine is not able to recognize whether the ultrasound it receives back from the body traveled mainly through bone, through soft tissue, through air, or any combination of the above structures. As such, it computes the distance the ultrasound traveled based on a velocity of 1540 m/s. Therefore, objects can be misplaced on an ultrasound image because of this velocity assumption, which is built into the ultrasound machine. This explains why interposition of ribs or lung tissue between the transducer and the heart will produce severe imaging artifacts and make part of the image uninterpretable (Figure 1.11).

Table 1.1 Velocity of ultrasound in various human tissues.

Medium

Velocity (m/s)

Air (the slowest)

330

Soft tissue

1540

Blood

1570

Muscle

1580

Bone (the fastest)

4080

Figure 1.11 An apical four-chamber view of the same patient when the patient has exhaled(a), as the patient is inhaling(b), and as the patient is fully inhaled(c). As clearly seen, the quality of the myocardial image declines acutely as more air enters the lung of the patient, to a point where no myocardium is seen in full inhalation(c).

Another important point to remember is the behavior of the ultrasound beam as it emerges from the transducer (Figure 1.12). The ultrasound beam is initially parallel and cylindrical (near zone). However, after its narrowest point, the focal zone, it begins to diverge and acquires a cone shape (far zone). For reasons outside the scope of this book, the imaging is much...

Produkt-Details

• Erscheint lt. Verlag: 19.3.2013
• Sprache: englisch
• Themenwelt: Medizin / Pharmazie ► Allgemeines / Lexika
• Themenwelt: Medizinische Fachgebiete ► Innere Medizin ► Kardiologie / Angiologie
• Themenwelt: Medizin / Pharmazie ► Medizinische Fachgebiete ► Notfallmedizin
• Themenwelt: Medizinische Fachgebiete ► Radiologie / Bildgebende Verfahren ► Sonographie / Echokardiographie
• Schlagworte: acoustic windows • Approach • Bildgebende Verfahren f. Herz- u. Gefäßuntersuchung • Bildgebende Verfahren f. Herz- u. Gefäßuntersuchung • cardiovascular imaging • Comprehensive • Echocardiogram • Echokardiographie • Examination • Fridman • fridman echocardiographic • heart • Medical Science • Medizin • Patient • Physics • Pulmonary • systemic flow ratio • transducer • transthoracic • Valves • vasavada
• ISBN-13: 9781118613412 / 9781118613412