Foundations of clinical sonography





Objectives


On completion of this chapter, you should be able to:




  • Describe the role of the sonographer and the career path



  • Know the historical developments in medical ultrasound



  • List the basic principles and terminology of medical ultrasound



  • Identify the transducers necessary for specific ultrasound applications



  • Explain the multiple display modes on ultrasound instrumentation



  • State the Doppler effect





The primary purpose of this chapter is to introduce the sonographer to the fascinating field of diagnostic medical ultrasound. Historians will tell us that we cannot know where we are going until we know where we have been. Therefore a brief background into the historical development of ultrasound will be presented to enable the sonographer to understand the progress that has been made with technology in the medical application of ultrasound. It is important for sonographers to understand their role in the health care field, as well as to have a global concept of anatomic reconstruction. An introduction into the terminology of the basic principles of ultrasound is critical for the student to understand how and why an anatomic image appears as it does on the ultrasound monitor.


The terms diagnostic medical ultrasound, ultrasound, and ultrasonography have all been used to describe the instrumentation used in ultrasound. Sonography is the term used to describe a specialized imaging technique to visualize soft tissue structures in the body. The term echocardiography, or simple “echo,” refers to an ultrasound examination of only the cardiac structures.


A sonographer is a member of the allied health professions who has received specialized education in diagnostic medical sonography and has successfully completed the national boards given by the American Registry of Diagnostic Medical Sonography. A sonologist is a physician who has received specialized training in ultrasound and has successfully completed the national boards granted by their respective specialty (radiology, cardiology, obstetrics, etc.).


The field of diagnostic medical ultrasound has grown to become a well-respected and valuable addition to diagnostic imaging by providing pertinent clinical information to the physician and to the patient. The applications of ultrasound are extensive; they include, but are not limited to, the following areas:



  • 1.

    General ultrasound (abdominal, renal, retroperitoneal, chest)


  • 2.

    Superficial ultrasound (breast, thyroid, scrotum)


  • 3.

    Neonatal and pediatric ultrasound (abdomen, renal, hips, brain, spine)


  • 4.

    Echocardiography (adult, pediatric, neonatal, fetal)


  • 5.

    Interventional and therapeutic-guided ultrasound


  • 6.

    Obstetric and gynecologic ultrasound


  • 7.

    Intraoperative ultrasound


  • 8.

    Musculoskeletal ultrasound


  • 9.

    Ophthalmologic ultrasound


  • 10.

    Point-of-care ultrasound



Extensive research has verified the safety of ultrasound as a diagnostic procedure. No harmful effects of ultrasound have been demonstrated at power levels used for diagnostic studies when performed by qualified and nationally certified sonographers, under the direction of qualified and board-certified sonologists, using appropriate equipment and techniques.


Diagnostic ultrasound has developed into a valuable imaging technique for many reasons. First is the lack of ionizing radiation for the ultrasound procedure compared with the various other imaging modalities, such as computed tomography (CT) or nuclear medicine. The second reason is the portability of the ultrasound equipment. Even the high-end ultrasound equipment may be moved into the intensive care unit, emergency department, operating room, cardiac catheter laboratory, or small doctor’s office. The low-end systems are now so portable, they can fit into the physician’s laboratory coat to be used at the bedside as an initial quick look evaluation of the patient physical examination.


Ultrasound is unique in other ways as well. The ultrasound image is presented in a real-time cine clip format, which makes it possible to see the image transition from one cardiac structure to another, or from one organ system to another. The flexible multiplanar imaging capability allows the sonographer to “follow” the path of a tortuous vessel, a moving cardiac structure, or a moving fetus to capture the necessary images. Moreover, Doppler techniques allow the qualitative and quantitative evaluation of blood flow hemodynamics within a vessel. Finally, the cost analysis of an ultrasound system is superior compared with the other imaging diagnostic systems.


Today nearly every hospital and medical clinic has some form of ultrasound instrumentation to provide the clinician with an inside look at the soft tissue structures within the body. Ultrasound manufacturers continue their research to improve image acquisition, develop efficient transducer functionality and design, and create software to improve computer assessment of the acquired information. Two-dimensional (2D) ultrasound information can be recreated in a three-dimensional (3D) or four-dimensional (4D) (real-time) format to provide an “en face” surface rendering of the specific area. Color Doppler, harmonics, tissue characterization, and spectral analysis have greatly expanded the utility of ultrasound imaging. The development of specialized contrast agents for use with ultrasound has enabled clinicians to make specific diagnoses with greater precision.


To obtain even more information from the ultrasound image, various medical centers and manufacturers have been working toward the development of effective contrast agents that may be ingested or administered intravenously into the bloodstream to facilitate the detection and diagnosis of specific pathologies. Early attempts at producing a contrast effect with ultrasound imaging involved administration of aerated saline or carbon dioxide. Research today is focused on the development of gas microspheres, which are injected into the patient to provide visual contrast during the ultrasound study. Specific applications of ultrasound contrast are found in Chapter 17 .




The role of the sonographer


The sonographer is an allied health professional who has received specific training in diagnostic medical sonography (general applications) or cardiovascular technology (cardiac and vascular applications). The sonographer performs ultrasound procedures and gathers diagnostic data under the direct or indirect supervision of a physician. Sonographers are known as “image makers” who have the ability to create images of soft tissue structures and organs inside the body, such as the liver, pancreas, biliary system, kidneys, heart, vascular system, musculoskeletal system, uterus, and fetus. In addition, sonographers can record hemodynamic information with velocity measurements through the use of color Doppler and spectral analysis to determine whether a vessel or cardiac valve is patent (open) or restricted.


Sonographers work directly with physicians and patients as a team member in a medical facility. They also interact with nurses and other medical staff as part of the health care team. The sonographer must be able to review the patient’s records to assess clinical history and clinical symptoms; to interpret laboratory values; and to understand other diagnostic examinations. The sonographer is required to understand and operate complex ultrasound instrumentation using the basic principles of ultrasound physics.


To produce the highest quality sonographic image for interpretation, the sonographer must possess an in-depth understanding of anatomy and pathophysiology and be able to evaluate a patient’s problem specific to the examination ordered. Sonographers use their knowledge and skills to provide physicians with clinical information such as the rapid focused assessment with sonography for trauma (FAST) scan evaluation of a trauma victim’s injury, visualization of detailed fetal anatomy, measurement and evaluation of fetal growth and progress, or even to evaluate the patient for cardiac abnormalities or injury. In addition to technical expertise and knowledge of anatomy and pathophysiology, several other qualities contribute to the sonographer’s success.



Qualities of a Sonographer


The sonographer must possess the following qualities and talents:




  • Intellectual curiosity to keep abreast of developments in the field



  • Perseverance to obtain high-quality images and the ability to differentiate an artifact from structural anatomy



  • Ability to conceptualize two-dimensional images into a three-dimensional format; ability to reconstruct a 2D image into a 3D format to product an “en face” image



  • Quick and analytical mind to continually analyze image quality while keeping the clinical situation in mind



  • Technical aptitude to produce diagnostic-quality images



  • Good physical health because continuous scanning may cause strain on back, shoulder, or arm; equipment is mobile, thus the sonographer must be able to manipulate equipment weighing greater than 250 pounds; Doppler is audible, thus sonographers must have adequate hearing to interpolate the returning Doppler audible sound



  • Independence and initiative to analyze the patient, the history, and the clinical findings and tailor the examination to answer the clinical question



  • Emotional stability to deal with patients in times of crisis; this means the ability to understand the patient’s concerns without losing objectivity



  • Communication skills for interactions with peers, clinicians, and patients; this includes the ability to clearly communicate ultrasound findings to physicians and the ability not to disclose or speculate on findings to the patient during the examination



  • Dedication because a willingness to go beyond the “call of duty” is often required of the sonographer




What makes the sonographer distinct from the other health care professionals? The sonographer has the following responsibilities:




  • Reviews the clinical chart and speaks directly with patients to identify symptoms that relate directly to the ultrasound examination



  • Explains the procedure to the patient and performs the examination using the protocol established by the department



  • Analyzes each image and correlates the information with patient information



  • Uses independent judgment in recognizing the need to make adjustments with the sonographic protocol to answer the clinical question



  • Reviews the previous sonograms and provides an oral or written summary of the technical findings to the physician for the medical diagnosis



  • Alerts the physician if critical findings or new changes are found on the sonographic examination



The sonography career


Advantages.


Sonographers with specialized education in ultrasound obtained from a nationally accredited diagnostic medical sonography or cardiovascular technology program have demonstrated their ability to analyze the clinical situation and to produce high-quality sonographic images, thereby earning the respect of other allied health professionals and clinicians. Every day, sonographers are faced with varied human interactions and opportunities to solve problems. These experiences give sonographers an outlet for their creativity by requiring them to come up with innovative ways to meet the challenges of performing quality sonographic examinations on difficult patients. Sonographers must have the creative ability to alter their normal protocol as difficult situations arise (e.g., trauma patient, immobile patient, postoperative surgical patient with multiple bandages). New applications in ultrasound and improvements in instrumentation create a continual challenge for the sonographer. Flexible schedules and variety in examinations and equipment, not to mention patient personalities, make each day interesting and unique. Certified sonographers find that employment opportunities are abundant, schedule flexibility is high, and salaries are attractive.


Disadvantages.


On the other hand, some sonographers find their position to be stressful and demanding, with the constant changes in medical care and decreased staffing causing increased workloads. Hours of continual scanning may lead to tendinitis, arm and shoulder pain, and back strain. ( Chapter 3 focuses on ergonomics and musculoskeletal issues in sonography.)


Sonographers may become frustrated when dealing with terminally ill patients, which can lead to fatigue and depression.


Employment.


The field of sonography continues to expand. The demand for certified sonographers exceeds the supply nationwide. Sonographers may find employment in the traditional setting of a hospital or medical clinic. Staffing positions within the hospital or medical setting may include the following: Director of Imaging, Technical Director, Supervisor, Chief Sonographer, Sonographer Educator, Clinical Staff Sonographer, Research Sonographer, or Clinical Instructor. Clinical research opportunities may be found in the major medical centers throughout the country. Sonographers with advanced degrees (i.e., BS, MS, or PhD) may serve as faculty in diagnostic medical sonography programs as Program Director, Department Head, or Dean of Allied Health. Many sonographers have entered the commercial world as Clinical Application Specialists or Director of Education/Continuing Education Director, marketing specialist, product design/engineering, sales, service, or quality control. Other sonographers have become independent business partners in medicine by offering mobile ultrasound services to smaller community hospitals.


Resource organizations.


Specific organizations are devoted to developing standards and guidelines for ultrasound:




  • American Institute of Ultrasound in Medicine (AIUM), www.aium.org . This organization represents all facets of ultrasound to include physicians, sonographers, biomedical engineers, scientists, and commercial researchers.



  • American Society of Echocardiography (ASE), www.asecho.org . This very active organization represents physicians, sonographers, and scientists involved with cardiovascular applications of sonography.



  • Society of Diagnostic Medical Sonography (SDMS), www.sdms.org . This is the principal organization for more than 25,000 sonographers. The website contains information regarding the SDMS position statement on the code of ethics for the profession of diagnostic medical ultrasound; the nondiagnostic use of ultrasound; the scope of practice for the diagnostic ultrasound professional; and diagnostic ultrasound clinical practice standards.



  • Society for Vascular Ultrasound (SVU), www.svunet.org . This is the principal organization representing physicians, sonographers, and scientists in vascular sonography.



Certification.


The National Certification Examination for Ultrasound is provided by the following organization:




  • American Registry for Diagnostic Medical Sonography (ARDMS), www.ardms.org . This is the primary organization offering international credentials for sonographers once their training has been completed.



Joint review committee.


The national review boards for educational programs in sonography are provided by two groups:




  • Joint Review Committee on Education in Diagnostic Medical Sonography (includes general ultrasound, echocardiography, and vascular technology) (JRC-DMS), www.jrcdms.org



  • Joint Review Committee on Education in Cardiovascular Technology (includes noninvasive cardiology [echocardiography], invasive cardiology [cardiac catheterization], and vascular technology) (JRC-CVT), www.jrccvt.org





Historical overview of sound theory and medical ultrasound


A complete history of sound theory and the development of medical ultrasound is beyond the scope of this textbook. The following is a brief overview, designed to provide readers a sense of the extensive history and exciting developments in this area of study. For a more detailed outline of historical data, the reader is referred to Dr. Joseph Woo’s excellent online article titled “A Short History of the Development of Ultrasound in Obstetrics and Gynecology” and other resources listed in the Bibliography at the end of this chapter.


The story of acoustics began with the Greek philosopher Pythagoras (sixth century bc ), whose experiments on the properties of vibrating strings led to the invention of the sonometer, an instrument used to study musical sounds. Two thousand years later, in 1500 ad, Leonardo da Vinci (1452–1519) discovered that sound traveled in waves and discovered that the angle of reflection is equal to the angle of incidence. Galileo Galilei (1564–1642) is said to have started modern studies of acoustics by elevating the study of vibrations to scientific standards. In 1638 he demonstrated that the frequency of sound waves determined the pitch. Sir Isaac Newton (1643–1727) studied the speed of sound in air and provided the first analytical determination of the speed of sound. Robert Boyle (1627–1691), an Irish natural philosopher, chemist, physicist, and inventor, demonstrated the physical characteristics of air, showing that it is necessary in combustion, respiration, and sound transmission. Lazzaro Spallanzani (1729–1799), an Italian biologist and physiologist, essentially discovered echolocation. Spallanzani is famous for extensive experiments on bat navigation, from which he concluded that bats use sound and their ears for navigation in total darkness. Augustin Fresnel (1788–1827) was a French physicist who contributed significantly to the establishment of the theory of wave optics, forming the theory of wave diffraction named after him. Sir Francis Galton (1822–1911) was an English Victorian scholar, explorer, and inventor. One of his numerous inventions was the Galton whistle used for testing differential hearing ability. This is an ultrasonic whistle, which is also known as a dog whistle or a silent whistle. Christian Johann Doppler (1803–1853) was an Austrian mathematician and physicist. He is most famous for what is now called the Doppler effect, which is the apparent change in frequency and wavelength of a wave as perceived by an observer moving relative to the wave’s source. In 1880 Paul-Jacques Curie (1856–1941) and his brother Pierre Curie (1859–1906) discovered piezoelectricity, whereby physical pressure applied to a crystal resulted in the creation of an electric potential. John William Strutt (Lord Rayleigh) (1842–1919) wrote The Theory of Sound. The first volume, on the mechanics of a vibrating medium which produces sound, was published in 1877; the second volume on acoustic wave propagation was published the following year. Paul Langevin (1872–1946) was a French physicist noted for his work on paramagnetism and diamagnetism. He devised the modern interpretation of this phenomenon in terms of spins of electrons within atoms. His most famous work was on the use of ultrasound using Pierre and Jacques Curie’s piezoelectric effect. During World War I, he began working on the use of these sounds to detect submarines through echolocation.


SONAR is an acronym for SOund Navigation and Ranging. Sonar is a technique that uses sound propagation, usually underwater, to navigate, communicate with other vessels, or detect other vessels. Sonar may be used as a means of acoustic location and measurement of the echo characteristics of “targets” in the water. The term sonar is also used for the equipment necessary to generate and receive the sound. The acoustic frequencies used in sonar systems vary from very low (infrasonic) to extremely high (ultrasonic). World War II brought sonar equipment to the forefront of military defense, and medical ultrasound was influenced by the advances in sonar instrumentation.


In the 1940s, Dr. Karl Dussik (1908–1968) made one of the earliest applications of ultrasound to medical diagnosis when he used two transducers positioned on opposite sides of the head to measure ultrasound transmission profiles. He discovered that tumors and other intracranial lesions could be detected by this technique. Dr. William Fry, an electrical engineer whose primary research was in the field of ultrasound, is credited with being the first to introduce the use of computers in diagnostic ultrasound. Around this same time, he and Dr. Russell Meyers performed craniotomies and used ultrasound to destroy parts of the basal ganglia in patients with parkinsonism.


Between 1948 and 1950, three investigators, Drs. Douglass Howry, a radiologist, John Wild, a clinician interested in tissue characterization, and George Ludwig, who was interested in reflections from gallstones, each demonstrated independently that when ultrasound waves generated by a piezoelectric crystal transducer are transmitted into the body, ultrasound waves of different acoustic impedances are returned to the transducer.


One of the pioneers in the clinical investigation and development of ultrasound was Dr. Joseph Holmes (1902–1982). A nephrologist by training, Dr. Holmes’ initial interest in ultrasound involved its ability to detect bubbles in hemodialysis tubing. Holmes began work in ultrasound at the University of Colorado Medical Center in 1950, in collaboration with a group headed by Douglass Howry. In 1951, supported by Joseph H. Holmes, Douglass Howry, along with William Roderic Bliss and Gerald J. Posakony, both engineers, produced the “immersion tank ultrasound system,” the first 2D B-mode (or plan position indicator [PPI] mode) linear compound scanner. Two-dimensional cross-sectional images, published in 1952, demonstrated that interpretable 2D images of internal organ structures and pathologies could be obtained with ultrasound. The Pan Scanner, put together by the Holmes, Howry, Posakony, and Richard Cushman team in 1957, was a landmark invention in the history of B-mode ultrasonography. With the Pan Scanner, the patient sat on a modified dental chair strapped against a plastic window of a semicircular pan filled with saline solution, while the transducer rotated through the solution in a semicircular arc ( Figure 1-1 , A ).










FIGURE 1-1


A, One of the early ultrasound scanning systems used a B-52 gun turret tank with the transducer carriage moved in a 360-degree path around the patient. B, Dr. Lehman used a water path system to scan his obstetric patients. C, The Octoson used eight transducers mounted in a 180-degree semicircle and completely covered with water. The patient would lie on top of the covered waterbed, and the transducers would automatically scan the patient. D, Real-time image of the neonatal head. TV, Third ventricle.


In 1954 echocardiographic ultrasound applications were developed in Sweden by Drs. Hellmuth Hertz and Inge Edler, who first described the M-mode (motion) display.


An early obstetric contact compound scanner was built by Tom Brown and Dr. Ian Donald (1910–1987) in Scotland in 1957. Dr. Donald went on to discover many fascinating image patterns in the obstetric patient; his work is still referred to today. Meanwhile, in the early 1960s in Philadelphia, Dr. J Stauffer Lehman designed a real-time obstetric ultrasound system ( Figure 1-1 , B ).


In 1959 the Ultrasonic Institute (UI) was formed at the National Acoustic Laboratory in Sydney, Australia. George Kossoff and his team, including Dr. William Garrett and David Robinson, developed diagnostic B-scanners with the use of a water bath to improve resolution of the image ( Figure 1-1 , C and D ). This group was also responsible for introducing gray-scale imaging in 1972. Kossoff and his colleagues were pioneers in the development of large-aperture, multitransducer technology in which the transducers were automatically programmed to operate independently or as a whole to provide high-quality images without operator intervention, as was required with the contact static scanner that had been developed in 1962 at the University of Colorado.


The advent of real-time scanners changed the face of ultrasound scanning. The first real-time scanner (initially known as a fast B-scanner) was developed by Walter Krause and Richard Soldner. It was manufactured as the Vidoscan by Siemens Medical Systems of Germany in 1965. The Vidoscan used three rotating transducers housed in front of a parabolic mirror in a water coupling system and produced 15 images per second. The image was made up of 120 lines with basic gray-scale imaging. The use of fixed-focus, large-face transducers produced a narrow beam to ensure good resolution and a good image. Fetal life and motions could be demonstrated clearly. In 1973 James Griffith and Walter Henry at the National Institutes of Health produced a mechanical oscillating real-time scanning device that could produce clear 30-degree sector real-time cardiac images with good resolution. The phased-array scanning mechanism was first described by Jan Somer at the University of Limberg in the Netherlands and was in use from 1968, several years before the appearance of linear-array systems.


Medical applications of ultrasonic Doppler techniques were first implemented by Shigeo Satomura and Yasuhara Nimura at the Institute of Scientific and Industrial Research in Osaka, Japan, in 1955 for the study of cardiac valve motion and pulsations of peripheral blood vessels. The Satomura team pioneered transcutaneous Doppler flow measurements in 1959. In 1966 Kato and T. Izumi pioneered the directional flow-meter using the local oscillation method whereby flow directions were detected and displayed. This was a breakthrough in Doppler instrumentation because reverse flow in blood vessels could now be documented. In the United States Robert Rushmer and his team did groundbreaking work in Doppler instrumentation, beginning in 1958. They pioneered transcutaneous continuous-wave flow measurements and spectral analysis in 1963. Donald Baker, a member of Rushmer’s team, introduced a pulsed-Doppler system in 1970. In 1974 Baker, along with John Reid and Frank Barber and others, developed the first duplex pulsed-Doppler scanner, which allowed 2D scale imaging to be used to guide placement of the ultrasound beam for Doppler signal acquisition. In 1985 a work titled “Real-Time Two-Dimensional Blood Flow Imaging Using an Autocorrelation Technique” by Chihiro Kasai, Koroku Namekawa, and Ryozo Omoto was published in English translation. The autocorrelation technique described in this publication could be applied to estimating blood velocity and turbulence in color flow imaging. The autocorrelation technique is a method for estimating the dominating frequency in a complex signal, as well as its variance. The algorithm is both computationally faster and significantly more accurate compared with the Fourier transform, because the resolution is not limited by the number of samples used. This provided the rapid means of frequency estimation to be performed in real-time that is still used today.


In 1987 the Center for Emerging Cardiovascular Technologies at Duke University started a project to develop a real-time volumetric scanner for cardiac imaging. In 1991 they produced a matrix array scanner that could image cardiac structures in real-time and in 3D. By the second half of the 1990s, many other centers throughout the world were working on laboratory and clinical research into 3D ultrasound. Today 3D ultrasound has developed into a clinically effective diagnostic imaging technique.




Introduction to basic ultrasound principles


To produce high-quality images that are free of artifacts, the sonographer must have a firm understanding of the basic principles of ultrasound. This section introduces the basic principles of acoustics, measurement units, instrumentation, real-time sonography, 3D ultrasound, harmonic imaging, and optimization of gray-scale and Doppler ultrasound to reinforce the sonographer’s understanding of scanning techniques. The student sonographer is introduced to a new language and terminology with ultrasound physics. This serves as a brief overview of the material that will be covered in depth in a dedicated ultrasound physics textbook.


Acoustics


Acoustics is the branch of physics that deals with sound and sound waves. It is the study of generating, propagating, and receiving sound waves. Within the field of acoustics, ultrasound is defined as sound frequencies that are beyond (ultra-) the range of normal human hearing. Most human hearing ranges between 20 hertz (Hz) and 20 kilohertz (kHz). Thus ultrasound refers to sound frequencies greater than 20 kHz.


Sound is the result of mechanical energy that produces alternating compression and rarefaction of the conducting medium as it travels as a wave ( Figure 1-2 ). (A wave is a propagation of energy that moves back and forth or vibrates at a steady rate.) Diagnostic ultrasound uses short sound pulses at frequencies of 1 to 20 million cycles /sec ( megahertz [MHz] ) that are transmitted into the body to examine soft tissue anatomic structures ( Table 1-1 ). In medical ultrasound, the piezoelectric vibrating source within the transducer is a ceramic element that vibrates in response to an electrical signal. The vibrating motion of the ceramic element in the transducer causes the particles in the surrounding tissue to vibrate. In this way the ultrasound transducer converts electrical energy into mechanical energy as the sonographic imaging is produced. As the sound beam is directed into the body by the transducer at various angles to the organs, reflection, absorption, and scatter cause the returning signal to be weaker than the initial impulse. Over a short period of time, multiple anatomic images are acquired in a real-time format.




FIGURE 1-2


As the transducer element vibrates, waves undergo compression and expansion, or rarefaction, by which the molecules are pulled apart.


TABLE 1-1

Applications of Sound Frequency Ranges


































Frequency Range Manner of Production Application
Infrasound
0–25 Hz Electromagnetic vibrators Vibration analysis of structures
Audible
20 Hz–20 kHz Electromagnetic vibrators, musical instruments Communications, signaling
Ultrasound
20–100 kHz Air whistles, electric devices Biology, sonar
100 kHz–1 MHz Electric devices Flaw detection, biology
1–20 MHz Electric devices Diagnostic ultrasound

Hz, Hertz; kHz, kilohertz; MHz, megahertz.


The velocity of propagation is constant for a given tissue and is not affected by the frequency or wavelength of the pulse. In soft tissues, the assumed average propagation velocity is 1540 m/sec ( Table 1-2 ). It is the stiffness and the density of a medium that determine how fast sound waves will travel through the structure. The more closely packed the molecules, the faster is the speed of sound.


May 29, 2019 | Posted by in ULTRASONOGRAPHY | Comments Off on Foundations of clinical sonography
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