Essentials of Ultrasound Imaging

By Thomas L. Szabo and Peter Kaczkowski

Essentials of Ultrasound Imaging offers a fast track introduction to the science, physics and technology of ultrasound imaging systems. Uniquely, principles are revealed by examples from software simulation programs, thus allowing the reader to engage with the concepts having minimal mathematical background. The material is organized around a functional block diagram which is, in turn, related to physical processes and implementations of the functional concepts on commercial and research imaging systems. Examples from a Verasonics Vantage Research Ultrasound System provide unparalleled insight into each step of ultrasound image creation including signal processing, transducer operation, different types of beamforming, and image formation.

The last chapter examines the potential and capabilities of ultrasound imaging and measurement for future applications. With a thorough grounding of the physics and methods of ultrasound imaging, this book is suitable for students learning about ultrasound and researchers involved, or starting out in, ultrasound research development who might not have the background to understand the latest developments.

• Gives an understanding of wave propagation, piezoelectric transducers, beam focusing, Doppler imaging of fluid flow, types of ultrasound systems, and real-time image formation and resolution
• Explains basic mathematical and scientific concepts underlying ultrasound imaging and physics
• Follows the passage of pulse-echo waveforms through the changes made by wave propagation, array beam formation, absorption, and system processing to image formation
• Describes the concepts written in MATLAB® that are illustrated by numerous examples from unique simulations of physics, processing, and imaging and from experiments and signals within an ultrasound research system
• Presents an accompanying simulator software package, in executable form, designed to demonstrate concepts with minimal mathematical background, together with a curriculum of hands-on experiments using an ultrasound research system, both available from Verasonics

• Language: English
• Publisher: Academic Press
• Release date: Nov 28, 2023
• ISBN: 9780323953726

Thomas L. Szabo

Professor Szabo has contributed to the fundamental understanding and design of surface acoustic wave signal processing devices, to novel means of transduction and measurement for nondestructive evaluation using ultrasound, to seismic signal processing, and to the research and development of state-of-the-art diagnostic ultrasound imaging systems for over fifty years. He is the author of the widely used textbook, Diagnostic Ultrasound Imaging: Inside Out, over 100 papers and twelve book chapters, and holds four patents and several patent applications. His wide range of interests include ultrasound tissue and spine characterization, wave equations, novel imaging systems, brain imaging, therapeutic ultrasound, nonlinear phenomena and geophysical exploration. Dr. Szabo is a Fellow of the American Institute of Ultrasound in Medicine, Acoustical Society of America, and a Life Senior member of the IEEE. He is a U.S. delegate to the International Electrotechnical Commission (IEC), Technical Committee 87 and a Convenor of Working Group 6 on high intensity therapeutic ultrasound and focusing. He was a recipient of a 1973 U. S. Meritorious Service Medal, a Hewlett Packard Fellowship and the 1974 best paper award in the IEEE Transactions on Sonics and Ultrasonics.

Book preview

Chapter 1

Introduction

Abstract

Those wishing to investigate ultrasound imaging for educational, work-related applications, or research and development need to learn about the underlying physical principles, signal processing, and the operation of imaging systems. An approach in using simulators and laboratory experiments accelerates the learning process. A new framework for understanding and comparing different types of imaging systems has led to an imaging scorecard approach. Different imaging modalities employing electromagnetic waves from X-rays to millimeter waves are compared on a consistent basis with the scorecard as well as ultrasound and magnetic resonance imaging. Our human visual system is analyzed in detail as an introductory prototype imaging system. A journey to deeper exploration of imaging concepts begins with interactive, real-time simulators operated from graphical user interfaces. These simulators constitute a rapid learning paradigm over conventional educational methods for introducing imaging systems and the physics behind them. Two simulators are introduced: one based on our human visual system, and another on three-dimensional (3D) object visualization. The concepts are brought to life through a real-time ultrasound imaging research system. The essential aspects of the Vantage ultrasound system are explained briefly, and the results of several experiments demonstrate two-dimensional imaging of several 3D objects.

Keywords

A-line; acoustic window; B-mode; block diagram; C-mode; computed tomography; diagnostic ultrasound; electromagnetic spectrum; graphical user interfaces; infrared; lens; magnetic resonance imaging; millimeter wave; positron emission tomography; radio frequency; simulators; three-dimensional imaging; ultrasound phantom; X-ray

1.1 Overview

1.1.1 Prelude

Ultrasound imaging, the fastest growing type of medical imaging, has also found applications in nondestructive testing, sound navigation and ranging (SONAR), and geophysics. Because of recent advances in electronics and digital processing and computation, ultrasound imaging systems have taken many forms from large hospital systems embedded in surgical procedures and ubiquitous diagnostic systems to pocket ultrasound devices. Innovations are continually opening new opportunities in ultrasound.

There is a need for those entering this field of research and development to learn about the underlying physical principles, signal processing, and systems. This book will be particularly useful for those involved or starting out in ultrasound from different backgrounds and skill levels including graduate students, scientists and engineers from other disciplines, physicians and medical professionals conducting ultrasound research, managers of research groups, and those curious about ultrasound science or who are considering or entering the field. A situation frequently arising in industry is that a company will hire employees from other disciplines, and they need to learn about ultrasound imaging to carry out their work. While the content here is primarily focused on graduate students and engineers in the medical ultrasound industry, the material can inform a wider circle of students, instructors, and professionals as well as those involved in application of ultrasound imaging to new areas. As explained later, those who may not have advanced scientific backgrounds can also benefit from this book because of its unique graduated approach.

Essentials of ultrasound imaging offers a fast-track introduction to the science, physics, and technology of ultrasound imaging systems in 10 chapters. It emphasizes the underlying physics that makes ultrasound imaging possible as well as its practical constraints. The interaction of these physical processes with the signal processing and system architectures necessary for image formation are explained in detail. Presentation of the material is unique in two revolutionary ways. First, principles are revealed by examples from software simulation programs which allow students to engage with the concepts with minimal mathematical background. Second, concepts are illustrated with actual data and examples using a Verasonics Vantage Research Ultrasound System.

The format of the material accommodates different types of readers on four levels. On the first level, the book is a standalone independent source of new introductory material which is drawn from examples using simulation programs which will be explained later in this chapter. On the second level, more advanced material is presented in each chapter in a graduated way including equations for the simulation programs and referrals to more specific detailed explanations available in Diagnostic Ultrasound Imaging: Inside Out (Szabo, 2014a). For the third level, names of specific simulators in the text lead to 25 different interactive simulation programs, which readers may access for a nominal fee through Verasonics' website (Verasonics, 2023). Most of the simulators provide quantitative outputs so they can serve as virtual laboratories for homework problems. At the fourth level, the book may be used in combination with the Verasonics Essentials of Ultrasound Curriculum, which includes lectures, homework exercises using the simulation programs, and hands-on labs using a Verasonics Vantage Research Ultrasound System. More information can be found in Sections 1.8.3 and 1.9.1. Part of this material was well received in an abbreviated format as a 4-hour short course at three IEEE International Ultrasonics Symposia.

1.1.2 In this chapter you will learn

The primary goal of most imaging systems is to make visible the internal structure of opaque material and bodies. How this is done using electromagnetic and acoustic waves is explained in depth. You will be introduced to your own advanced mobile adaptive imaging system based on your eye–brain visual processor. The role of simulators and the first simulators are presented. Medical imaging systems utilizing different physics are compared. How ultrasound images materials noninvasively and nondestructively is revealed. The main types of imaging modes employed to depict three-dimensional (3D) objects are introduced.

1.2 Waves

Waves are disturbances which propagate in a material (gas, liquid, or solid) without changing it. A classic type of wave shape, the sine wave moving in time at a fixed location z, is shown in Fig. 1.1. One cycle of length T=0.1 μs and amplitude A=5 is shown. Because this cycle is a snapshot of an unending sequence of identical cycles, it has a frequency f0 given by f0=1/T. This wave moves with a speed of sound, c0, and an equation describing this simple wave W having an amplitude A is

Equation (1.1)

Figure 1.1 One cycle of a propagating sine wave at a fixed value of z. Time scale in microseconds.

This equation shows that a wave traveling a distance z incurs a delay, t=z/c0. For a time scale in microseconds and a speed of sound, c0=1.5 mm/μs, the delay is at the center corresponding to z=0.075 mm. A sinusoidal wave has a wavelength, λ=c0/f0 which shows that the higher the frequency, the shorter the wavelength. What are f0 and λ? This plot displays a cycle of what is called a radio frequency (rf) wave which is a general term used to describe a high-frequency wave.

The primary goal of imaging in this book is to reveal invisible structures hidden in opaque materials. By visible we usually mean something seen; but in the context of our discussion, imaging is a picture of hidden structures or features obtained by a process which involves an imaging system. The imaging system acquires data about an object and translates it into an image that we can see. Later in this chapter, we will compare different types of imaging systems which provide very different images of the same object because different physical processes are involved in acquiring the data.

For now, we can stick to ordinary vision for describing how waves interact with different materials. A transparent material, such as glass, allows us to see through it, but an opaque material completely blocks waves from getting through by totally scattering or reflecting them. An intermediate situation is which some waves are scattered; others are absorbed, and the rest are let through. For ultrasound waves in the body, similar processes of scattering, reflection, and through transmission are at work.

1.3 Your very own imaging system

1.3.1 Electromagnetic spectrum

The waves we are most familiar with are electromagnetic. Fig. 1.2 is an illustration of characteristics of the electromagnetic frequencies or spectra. The visible spectrum extends from red at 430 THz (with a red wavelength of 700 nm) to blue at 750 THz (400 nm). Later, imaging at other frequencies will be described and compared.

Figure 1.2 Energies (electron volts), frequencies (Hertz) and wavelengths of the electromagnetic spectrum. The visible spectrum is shown as colorbar.

The most remarkable imaging system is the one to which we have immediate access: our personal eye–brain visual system. Though we usually take this system for granted, it has all the basic elements of a complete imaging system in addition to being portable and adaptive. Before an examination of its components, the overall process, in terms of waves, is illustrated in Fig. 1.3. Here waves from a broadband transmitter, the sun, are sent to an absorbing target which reflects a certain range of colors that are then imaged by our stereoscopic imaging system. This process will be the basis of the first simulator described in the next section.

Figure 1.3 Basic components of the human visual imaging system.

1.3.2 Digital camera imaging system

First our imaging system will be compared to one with which we may be familiar: a digital camera depicted in Fig. 1.4. The major components are the lens, aperture (controlled by an iris diaphragm), and digital photoelectric array of m by n elements. The lens is chosen to focus the object onto the image plane which is composed of a 2D color mosaic filter to transform individual elements in the array to sense either red, blue, or green colors. The array then converts incoming light into electrical signals that are assembled into an m by n matrix of interpolated color values which are then displayed and stored digitally in a selected format. Present high end digital cameras have about 50 million elements in their arrays. Additional features include exposure control. Since the array has a dynamic range or range of light sensitivity, the amount of incident light for an exposure is ideally selected to be at a midrange level to utilize most of the available range. Exposure adjustment comes from changing the shutter speed and altering the size of the aperture. The aperture adjusts the amount of light passing through its iris diaphragm or F-stop. The length of exposure is controlled by two focal plane shutters: one which opens to let light pass through and a second shutter which closes, stopping the exposure.

Figure 1.4 Digital camera imaging system consisting of a lens, a light adjusting iris, two focal plane shutters which adjust the time of exposure, a mosaic of red, green, and blue (rgb) filters which convert individual elements in the m × n light sensor array to detect colors, analog to digital (A/D) converters and an image processor unit which creates a displayed image and stores the image data in as selected format.

Automatic focusing can be achieved by either active or passive means. One of the first active ranging methods was employed on Polaroid Sonar One Step and some SX-70 cameras; they utilized an ultrasound pulse which determined the focusing distance based on the speed of sound in air and the measured roundtrip delay time: z=tc0/2. This approach was superseded by an infrared sensor and later, a passive image analysis method.

1.3.3 Our analog imaging system

Our analog eye–brain visual system is far more complicated than a digital camera, yet some of its features are recognizable. The irises shown in Fig. 1.5 adaptively respond to the amount of incoming light and change the diameter of the pupils, the apertures of the eyes. The focal length of the lens is also adaptive through reshaping by ciliary muscles. Compared to a camera, the eye is much faster and more precise. Because of our stereoscopic vision, the eyes are synchronized and their 3D focusing movement converges on the same depth and horizontal and vertical position.

Figure 1.5 Eye–brain imaging system showing two eyeballs with adaptive lenses converged on rabbit object, the two retinal image acquisition two-dimensional sensor arrays, brain image processor producing final three-dimensional image. Eyes courtesy of Epicstessie. (n.d.). Own work, CC BY-SA 3.0. https://commons.wikimedia.org/w/index.php?curid=16442072; and Fischer, H. (n.d.). Eye: By artwork Holly Fischer. http://open.umich.edu/education/med/resources/second-look-series/materials—Eye Slide 3, CC BY 3.0. https://commons.wikimedia.org/w/index.php?curid=24367145, Wikipedia.

Images are focused on the retina, a 3D (slightly curved) array of photosensors, the rods and cones. There are about 250 million of these sensors (an order of magnitude more than current digital cameras) which convert light into electrical signals; they are adaptive: cones allow us to see in bright sunlight and rods in starlight (daylight or night vision) to sense a ratio of light intensity levels of 1000 or 30 dB (a logarithmic scale=10 LOG10 [level/reference]). Cones are separated into groups having different color photo sensitivities, each one acting as an individual spectral filter peaking at the electromagnetic light frequency corresponding to either red, green, or blue colors. Six sets of muscles move each eyeball in a coordinated way to change the direction of the vision in the outer landscape; the head can also be moved to increase angular range.

The outputs from these sensors are hardwired into two optical nerve bundles to several parts of the brain including the visual cortex for a considerable amount of image processing. For binocular vision, inputs from both eyes are combined to create high-resolution depth perception. Where is the display located? Complementary and redundant information are seamlessly processed so that the different locations where the optical nerves exit in each eye, blind spots in the retina, are automatically compensated for. The brain also has a nose filter which removes the nose from ordinary vision. If you close each eye, one at a time, you will see your nose. Simultaneous with vision is pattern recognition, our ability to recognize people and objects by their features and textures (Fig. 1.5). The eye–brain system also detects movement and can track objects in motion corresponding to about 30 frames per second. Your system is portable by crawling, walking, or running. Finally, your system was delivered free at