Companion DVD

The companion DVD-ROM contains a video playback application with more than 200 individual video clips corresponding to the chapters in this book. The application is compatible with most Mac and PC computers. You will need a computer with a DVD-ROM drive, as the DVD will not operate in a CD-ROM drive.

The individual video clips are cited in the text along with the figure to which they correspond by number. In addition, descriptive captions are provided in the DVD, and these will appear when you draw the cursor over the video selection listing.

PC Users

The application "Humana_EE.exe" should launch automatically on most Windows computers when the disc is inserted. If the application does not start after a few moments, simply double click the application "Humana_EE.exe" located on the root of the DVDROM.

Mac Users osx: Double click the application "Humana_EE OSX" after inserting the DVD-ROM. The Mac OSX operating system does not support an auto-start feature.

os9: The application "Humana_EE" should launch automatically on Mac computers running OS 9.2 when the disc is inserted. If the application does not start after a few moments, simply double click the application "Humana_EE" located on the root of the DVD-ROM.

System Requirements

PC/Windows

Intel Pentium II with 64 MB of available RAM running Windows 98, or

Intel Pentium III with 128 MB of available RAM running Windows 2000 or Windows XP.

Apple Macintosh OS X

Power Macintosh G3 with 128 MB of available RAM running Mac OS X 10.1.5, 10.2.6 or higher.

Apple Macintosh Classic

Power Macintosh G3 with 64 MB of available RAM running System 9.2.

Illustrations

All llustrations appearing in the book are also included on the Companion DVD. The image files are organized into folders by chapter number and are viewable in most Web browsers. The number following "f" at the end of the file name identifies the corresponding figure in the text.

I Introduction

Echocardiography Instrumentation and Principles of Doppler Echocardiography

Scott D. Solomon, MD

Contents

Introduction Principles of Ultrasound Principles of Doppler Ultrasound Summary

Suggested Reading introduction

Echocardiography has emerged as the principal tool for noninvasive assessment of the cardiovascular system. The basic principles of echocardiography, including the mechanical features of echocardiography equipment, are no different from diagnostic ultrasound in general. Nevertheless, there are aspects of echocardiography that set it apart from general ultrasonography. Because the heart is a moving organ, and because echocardiography must additionally capture that movement, an understanding of echocardiography requires an understanding of both cardiac anatomy and physiology. This chapter reviews the basic principles of echocardiography and serves as a basis for understanding the specific disease processes discussed in the remainder of this text.

principles of ultrasound ultrasound is simply high-frequency sound well outside the range of human hearing. Sound frequency is measured in hertz (cycles per second); humans can hear sounds between 20 and 20,000 Hz. Ultrasound begins in the range of 1 million hertz (MHz). Ultrasound waves share the same characteristics of all sound waves (Fig. 1): frequency (f, number of cycles per second, and similar to the pitch of a note), wavelength (X, the distance between sound waves), amplitude (equivalent to the loudness or magnitude of the sound waves), and propagation speed (c, the rate at which the sound waves travel through a particular medium). Ultrasound frequencies of 2.5-10 MHz are typically utilized for normal diagnostic work. Frequency and wavelength are inversely related, but this relationship is dependent on the propagation velocity (speed) of ultrasound. sound waves traverse different media—water, tissue, air, bone—at different speeds.

The following equation defines the relationship between frequency, wavelength, and propagation velocity:

The speed of sound (at any frequency) through water and through most bodily tissues is roughly 1540 m/s. Hence, ultrasound waves with a frequency of 2.5 MHz will have a wavelength of 0.616 mm. This relationship is important because the resolution of ultrasound—the ability to discern small structures—is dependent on the wavelength; the shorter the wavelength, the higher the resolution. The resolution of ultrasound is about half of the wavelength. For a frequency of 2.5 MHz, this translates to a resolution of approx 0.3 mm.

Ultrasound Image Generation

Ultrasound machines emit sound waves from a transducer; these waves bounce off internal structures within the body and generate reflections that return back to the transducer. Because sound travels at essentially a constant

From: Contemporary Cardiology: Essential Echocardiography: A Practical Handbook With DVD Edited by: S. D. Solomon © Humana Press, Totowa, NJ

Frequency Dvd
Fig. 1. Simple figure of a sine wave illustrating amplitude and wavelength. Frequency is inversely related to amplitude.

rate through tissue, the ultrasound machine can calculate the time required for the sound waves to make the round-trip between the transducer and the reflecting structure. From that transit time, the machine can calculate the depth within the body of the reflecting struc-ture(s). This information can be used to generate a scanline in which reflecting structures are depicted on a screen along the scanline based on the distance from the transducer and the amplitude of the reflected waves. Early ultrasound machines utilized a single directional beam of ultrasound that was manually directed toward different reflecting structures; this technique is called "M-Mode" echocardiography and is still used today (Fig. 2). In M-Mode echocardiography, the resulting scanline is displayed along a moving paper sheet (or on a screen) so that time is recorded on the x-axis and distance from the transducer on the y-axis. The amplitude of the reflection is recorded as the intensity of an individual point along the scanline. M-mode echocardiography is utilized as part of the standard echocardiographic examination (see Chapters 2 and 3).

Modern ultrasound machines utilize multiple scanlines (up to 512) to generate a two-dimensional (2D)

image (Fig. 3). Early 2D echocardiography machines generated multiple scanlines by utilizing a mechanically rotating transducer. Modern equipment, however, use electronically steered phased array transducers to generate multiple scanlines (Fig. 4). Most ultrasound machines use between 128 and 512 phased array elements to generate pulses of ultrasound in an orderly sequence, with the result being similar to that which can be achieved with a mechanically rotating transducer, but with better spatial resolution.

For standard imaging, ultrasonic transducers emit sound waves in pulses rather than continuously. The frequency of these pulses, called the pulse repetition frequency (PRF), is designed to allow sound waves to reflect from structures within the body and return to the transducer before emitting another pulse. If the PRF were too fast, then the ultrasound machine would not be able to determine which pulse was returning and would, therefore, not be able to accurately determine the depth of the reflecting structure. The PRF is determined by the velocity of ultrasound and by the greatest depth that is being interrogated (the depth setting on the ultrasound machine). For example, as many as 7700 pulses

Mode Examples Echocardiogram

Fig. 2. M-mode echocardiogram. An M-mode image represents a single scan line on the y-axis with time on the x-axis. In this illustration, we can see the movement of the interventricular septum (IVS) and posterior wall during ventricular contraction. The small 2D image in the upper right-hand corner shows where the M-mode "slice" is made.

Fig. 2. M-mode echocardiogram. An M-mode image represents a single scan line on the y-axis with time on the x-axis. In this illustration, we can see the movement of the interventricular septum (IVS) and posterior wall during ventricular contraction. The small 2D image in the upper right-hand corner shows where the M-mode "slice" is made.

Phased Array For Echocardiology

Fig. 3. How an ultrasound image is generated. (A) The phased array ultrasound transducer generates multiple scan lines. (B) Illustrates the resulting image on the screen. Although scanlines were visible in early ultrasound images, modern ultrasound equipment performs interpolation between scanlines to generate a smooth image without the appearance of scanlines.

Fig. 3. How an ultrasound image is generated. (A) The phased array ultrasound transducer generates multiple scan lines. (B) Illustrates the resulting image on the screen. Although scanlines were visible in early ultrasound images, modern ultrasound equipment performs interpolation between scanlines to generate a smooth image without the appearance of scanlines.

Piezo Array
Piezo-electlic electrode Fig. 4. Generation of scanlines with a phased-array transducer.

could make the return trip between the transducer and the edge of the 10-cm interrogation area.

Interaction of Ultrasound With Tissues

Ultrasonic images are produced because of the unique interaction of ultrasound with different tissues and fluids in the body. Reflections occur primarily at tissue interfaces (e.g., at the interface of blood and tissue). These reflections form the clearest boundaries on ultrasonic images and are termed specular reflections in which a significant proportion of the ultrasound energy is reflected back to the transducer. In contrast, reflections that occur from within relatively homogeneous tissues, such as myocardium, tend to be scattered in a variety of directions. These types of reflections are termed backscatter. Although some of the scattering ultrasound returns back to the transducer, the energy associated with these reflections is significantly lower than that emitted by the transducer. All sound waves are attenuated when they travel through tissue or fluid. Some tissues attenuate ultrasound to a greater extent than others. Finally, refraction occurs when ultrasound is reflected at an angle from the original ultrasound beam. All of these interactions with tissue are important in the ultimate image that is generated.

Although ultrasound can easily traverse through bodily fluids, including water and blood, as well as most soft tissues, ultrasound does not pass easily through bone or air. This limitation represents a major problem in cardiac imaging because the heart is surrounded by the thorax (bone) and the lungs (air). Ribs can cause significant artifacts. Air in the lungs can make imaging difficult. Indeed, patients with emphysema, who have overexpanded lungs, can be extremely challenging to image.

Ultrasound Resolution: Trade-Off

With Penetration

Resolution, the ability to discern detail, in ultrasound images is dependent on the wavelength (inversely related to frequency) of the ultrasound. The limit of the resolution of ultrasound is approximately one-half of the wavelength. With a standard imaging frequency of 2.5 MHz, this translates to a wavelength of approximately one 0.6 mm, which suggests a resolution of 0.3 mm. Although resolution can be increased by increasing the frequency of the ultrasound used, higher frequency ultrasound is less able to penetrate through tissues. Therefore, although higher frequency ultrasound can be used for highresolution imaging, its use will be limited because of decreased penetration. For this reason, high-frequency transducers only image well at short distances. Pediatric imaging, which requires less penetration, is often carried out at a frequency is of 5 MHz or higher. Because of decreased penetration, image quality can drop off dramatically when using higher frequencies in adults. In contrast, adults who have larger chest cavities will often require lower frequency probes. Indeed, 2.5-3 MHz resolution probes have become the standard for adult imaging.

Harmonic Imaging

Modern ultrasound machines tend to use two different imaging modes: fundamental imaging and harmonic imaging. In fundamental imaging, the ultrasound transducer listens for the returning ultrasound at the same frequency at which it was emitted. However, ultrasound can cause tissues to vibrate at frequencies that are multiples of the frequency of the original ultrasound pulse. The transducer can thus be set to listen at a frequency that is higher (by a multiple) than the original frequency. Harmonic imaging allows for improved

Fig. 5. Two side-by-side images from the same patient with fundamental imaging (left) and second harmonic imaging (right). Notice the improved endocardial resolution and ability to distinguish tissue from cavity utilizing harmonic imaging. (Please see companion DVD for corresponding video.)

Ultrasound Harmonic Imaging

Harmonic Imaging

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