What is ultrasonography?

Indications and Procedures

Sound waves are mechanical pressure waves that can propagate through liquids, solids, and, to some extent, gases. A sound wave is composed of cyclic variations that occur over time; one cycle per second is called 1 hertz (Hz). Ultrasound waves have a frequency of oscillation that is higher than 20,000 hertz, placing ultrasound above the audible range for humans. The useful frequency range for medical diagnostic ultrasound is between 1 and 10 megahertz (10 million hertz), although surgical instruments often use carrier frequencies greater than 20 megahertz.

The basic ultrasound system has two principal components. The first, and perhaps more important, component is the transducer, or probe. The transducer converts electrical pulses into mechanical pressure (sound) waves that are transmitted into the tissues. It then detects the echoes that are reflected from the tissues and transforms those echoes into electrical voltages. The second component is the audiovisual electronic component, which processes and displays the reflected echoes in the form of an image of internal organs and structures or an image of the movement of red blood cells.

Ultrasound waves are created when the crystalline material within the transducer is excited by an electrical voltage produced by the instrument’s oscillator. The application of an electrical charge causes the crystalline particles to expand and contract, producing mechanical waves and pulses. These pulses of sound pass from the face of the transducer into the body, where they strike the organs, bones, and blood vessels. The reflected echoes in turn strike the face of the transducer, again causing the crystalline particles to vibrate and produce an electrical charge. Such crystalline material is said to have piezoelectric (a combination of the Greek word piesis, meaning “pressure,” and the word “electric”) properties.

Ultrasound systems commonly employ sound in two modalities. The transducer uses sound waves to create an echo image of body structures. The audiovisual component uses the Doppler shift theory to analyze the range of velocities over which red blood cells are moving.

To create an echo image, millions of pulses of sound must be transmitted into the body each second. For each transmitted pulse, one line of echo information is received by the transducer crystal. To build up an image rapidly and depict the real-time motion of body structures, the pulses are sent into the body from many angles as the sound beam is moved over the body surface. The depth of the echoes is displayed as a function of time, and a two-dimensional image is created by relating the sound’s direction of propagation to the direction of the echo-image trace that appears on the instrument’s oscilloscope.

The time required for a sound pulse to travel from the transducer to its target within the body, reflect, and return to the transducer can be used to measure the distance to the target, as radar does. In body tissues, sound travels at a speed of 1,540 meters per second. It takes approximately thirteen microseconds for a sound pulse to travel 1 centimeter into the body and return to the transducer. The depth and orientation of echoes may be determined by using this information.

As a sound beam travels through tissues, it is attenuated, or reduced in amplitude and intensity. Attenuation occurs as the energy from the beam is absorbed by the tissues and transformed into heat. Additionally, a part of the beam may be reflected into the surrounding tissues at an angle away from the incident angle or backscattered as the long wavelength of the sound beam strikes the smaller red blood cells. Only a small fraction of the returning echoes reach the face of the transducer.

Because the attenuation of the sound beam increases as the depth of penetration increases, the echoes that return from the deepest part of the image field will be reduced in intensity when compared to the echoes that return from the structures nearer the skin surface. The echo intensity is dependent on the degree of change and impedance of each tissue through which the echo passes, the strength of the incident sound beam, and the degree of attenuation of the beam. To equalize the intensity of the echoes from all depths of the image field, the echoes that travel farthest, and therefore take the longest time to reach the transducer, are amplified over time by using time-gain compensation methods.

For medical imaging applications, the returning echoes may be displayed in several ways. The amplitude mode (A-mode) depicts the returning echoes as deflections on the instrument’s oscilloscope; the height of the deflection depends on the strength of the returned signal, and the distance between the deflections depends on the depth of the signal. The brightness mode (B-mode) depicts the strength of the echoes as shades of gray, with the strongest echoes appearing the brightest. The B-mode display makes it possible to differentiate tissue texture characteristics. The time-motion mode depicts movement over time by moving the B-mode trace across the face of a high-persistence oscilloscope, showing the depth, orientation, and strength of echoes with respect to time.

The transducer crystal determines the shape and focus of the sound beam and the frequency of the sound waves, features that are important in resolving echo information into complex images. The beam may be divided into three parts: the near field, the focal zone, and the far field. The beam width, close to the face of the transducer, is equal to the width of the transducer. The beam converges as it travels away from the transducer and then diverges at its narrow focal zone. In order for tissue targets to be resolved into discrete image points, both the lateral and the axial planes of the beam must be narrow. The focusing of the beam is facilitated by placing convex acoustic lenses in front of the transducer crystal to shorten the near field to a narrow focal point, thereby increasing the lateral resolution.

Axial resolution is the ability to distinguish targets along the sound beam. If a single pulse is emitted from the transducer, echo sources lying close together in the axial path of the beam may not be separated. Multiple short bursts of sound are used to separate the echo sources; each echo is captured as a discrete burst. Because axial resolution is inversely proportional to the duration of the ultrasound pulse (and the resonant frequency of the crystal is inversely proportional to its diameter), small-diameter, high-frequency crystals are used to obtain maximum axial resolution.

Ultrasound may be used to determine the velocity of blood flow. This velocity is determined in relation to the frequency of the incident sound beam according to the Doppler theory. Several different techniques may be used to process and display the echoes from moving red blood cells. The ultrasound system’s computers may be programmed to perform fast Fourier transform analysis, a complex mathematical method for ranking the speed of the echoes returning over time. The signals may be displayed either as spectral tracings of the range of Doppler frequency shifts, represented in the returned echoes recorded throughout the cardiac cycle, or as color-coded Doppler-shifted signals from within the blood vessels, superimposed on a gray-scale image of the surrounding tissues.

Uses and Complications

High-resolution abdominal ultrasonography is a valuable technique for the visualization of intra-abdominal organs and disease processes. For example, liver conditions such as parenchymal abnormalities, abscesses, hematomas, cysts, and cancerous lesions can be identified easily by means of this technique. B-mode and Doppler color-flow imaging are particularly valuable technologies that can be used to evaluate the tissue characteristics and blood flow patterns of transplanted organs. An ultrasound examination of the
gallbladder may reveal gallstones, obstruction of the common bile duct, or inflammatory disease. Ultrasound imaging of the
pancreas is used to identify pancreatitis, pancreatic pseudocysts, and carcinoma of this organ. An ultrasound examination of the
spleen may reveal splenomegaly, or enlargement of the spleen in
response to disease or trauma. Additionally, ultrasonography can be used to evaluate splenic volume and to identify hematomas, congenital cysts, infarctions, and tumors within the organ. The technology is particularly well suited for the study of tumors and abscesses within the
abdomen. Ascites and other fluid collections may be recognized, and primary tumors and lymph node metastases within the abdominal cavity may be identified by means of pulse-echo imaging.

Ultrasound has certain characteristics that make it particularly valuable for examining the kidneys and the genitourinary tract. The ability to image both native and transplanted kidneys noninvasively from the longitudinal and transverse planes provides additional diagnostic information in uremic patients for whom the injection of contrast agents is undesirable or may fail to provide sufficient information. Urologic ultrasonography may be used to determine renal size and position or to identify cysts and masses, kidney or bladder stones, obstruction of the ureters, and bladder contour.

Transabdominal scanning of the pelvic organs, which is used to determine the presence or absence of suspected lesions, makes possible the precise localization and quantitative mapping of pelvic abdominal masses, facilitating the determination of disease stages and the positioning of radiation ports. The technology is used to differentiate cysts from solid tumors and to determine if pelvic tumors are of uterine, ovarian, or tubal origin.

The sonographic resolution of deep abdominal structures is achieved by internal scanning; endorectal or endovaginal approaches are used to reduce the distance between the transducer and the target organ. During these procedures, the transducer probe either is in direct contact with the genital organs or prostate gland or is separated from them by the thin walls of the bladder or rectum. The information obtained with these techniques is thought to be submacroscopic, observed at approximately twenty to thirty times light magnification.

Ultrasonography plays a major role in the evaluation of obstetrical cases. Ultrasonic imaging is used to study early
pregnancy and high-risk cases, as well as to confirm ectopic pregnancy (development of the fetus outside the uterus). In cases of spontaneous abortion, ultrasound procedures are used to indicate whether the fetus and placenta have been retained. Ultrasonography is often used to determine fetal growth rate and placental development and to confirm intrauterine fetal death, threatened abortion, and fetal abnormalities. It is the best method for guiding
amniocentesis (the sampling of placental fluids). A study published in the Journal of the American Medical Association
in 2013 confirmed that ultrasound is the best detector of ectopic pregnancies.

Echocardiography
, the ultrasound evaluation of the heart, is a reliable and useful tool for the study of patients with congenital and acquired
heart disease. The role of cardiac ultrasound in the investigation of cardiac dysfunction, tetralogy of Fallot, transposition of the great vessels, and atrial septal defect has been well defined. Echocardiology is used to detect pericardial effusion; is coupled with Doppler ultrasound to evaluate the pulmonic, mitral, tricuspid, and aortic valves; and is used to investigate primary myocardial disease and atrial tumors. Improved resolution of cardiac structures and patterns of blood flow can be achieved by using endoesophageal (transesophageal) imaging and Doppler color-flow technology.

The
vascular system of the body can be studied by combining pulse-echo imaging of the blood vessels and Doppler ultrasound detection of red blood cell movement. This combined technology, known as duplex scanning, not only offers information that is relevant to the anatomy and morphology of blood vessels but also—and this is most important—provides the opportunity to evaluate the dynamics of blood flow and the pathophysiology of vascular disease. Duplex technology is used to demonstrate the presence and characteristics of atherosclerotic disease and to define the severity of vascular compromise resulting from the progression of disease or the presence of blood clots in vessels (thrombosis).

Applications of the technology have been extended to the evaluation of arteries and veins of the extremities, the abdomen, and the brain. Advances in computer technology have made it possible to color-code the Doppler-shifted signals returning from moving red blood cells within the vessels. Doppler color-flow imaging has facilitated the investigation of vascular disorders that result in slow or reduced blood flow (venous thrombosis or preocclusive narrowing of vessels) or that affect the vascularity of organs and tissues (tumors or transplanted organs). Therefore, vascular ultrasonography plays a major role in the evaluation of patients with arterial occlusive disease and those suspected of having thrombosis of the deep or superficial venous systems.

Perspective and Prospects

Ultrasonic techniques have assumed a preferred role in the diagnosis of many diseases and have become an essential component of quality medical care. In contrast to the rapid development and use of x-ray technology in medical diagnosis, the application of diagnostic ultrasound has been relatively slow. Progress depended in large part on the development of high-resolution electronic devices and transducers. Early research into medical applications involved the adaptation of instruments that had been designed for industrial or military purposes.

The first attempts to locate objects with ultrasound probably occurred following the sinking of the Titanic in 1912. Improvements in the technology led to the widespread industrial and military use of ultrasound for the detection of flaws in metals, for the determination of range and depth information, and for navigation. The first application of ultrasound to medical diagnosis occurred in 1937, when K. T. Dussik attempted to image the cerebral ventricles by measuring the attenuation of a sound beam transmitted through the head. In 1947, Douglas H. Howry pioneered the ultrasonic imaging of soft tissues and constructed a pulse-echo system that utilized a transducer submerged in water. The system utilized surplus Navy sonar equipment, a high-fidelity recorder power supply, and a metal cattle-watering trough in which the patient and the transducer were immersed.

In the 1960s, Howard Thompson and Kenneth Gottesfeld performed obstetric and gynecologic examinations using the first contact scanner, which had been produced in 1958 by Tom Brown, an engineer, and Ian Donald, a professor of midwifery, at Glasgow University in Scotland. The first commercial scanner marketed in the United States was designed by William L. Wright, an engineer at the University of Colorado.

The two-dimensional scanning system was developed in 1953 by John Reid, an engineer, in cooperation with John Wild, a physician who demonstrated that ultrasound could detect differences between normal tissues, benign tumors, and cancers. The collaboration between medicine and engineering has propelled diagnostic ultrasonography forward at a phenomenal rate of development since that time.

The field of echocardiography was pioneered by Inge Edler, who discovered in the 1950s that echoes from the moving heart could be received and displayed by using a time-motion ultrasonic flow detector. Using this technology, Edler diagnosed mitral stenosis, pericardial effusion, and thrombus in the left atrium.

The use of ultrasound to evaluate blood flow was first described by S. Satomura in 1959. This investigator observed that ultrasound could be transmitted through the skin to derive information about the velocity of blood flow by using the Doppler effect to analyze the reflected signals from the moving blood cells. The first transcutaneous continuous-wave Doppler system was developed at the University of Washington in the 1960s. The instrument was first used to detect fetal life by demonstrating the fetal heartbeat. This application of Doppler ultrasound spurred research under the guidance of Eugene Strandness Jr. that ultimately led to the development of duplex scanners, instruments that combine pulse-echo imaging with analysis of blood flow patterns derived from the Doppler effect. As a result of the efforts of these early investigators and others, diagnostic medical ultrasonography has evolved into a highly useful tool with diverse clinical applications.

Bibliography

Bates, Jane A. Abdominal Ultrasound: How, Why, and When. 3d ed. New York: Churchill Livingstone/Elsevier, 2011.

Bernstein, Eugene F., ed. Vascular Diagnosis. 4th ed. St. Louis, Mo.: Mosby, 1993.

Griffith, H. Winter. Complete Guide to Symptoms, Illness, and Surgery. Revised and updated by Stephen Moore and Kenneth Yoder. 6th ed. [N. p.]: Perigee/Penguin Group, 2012.

Hagan, Arthur D., and Anthony N. DeMaria. Clinical Applications of Two-Dimensional Echocardiography and Cardiac Doppler. 2d ed. Boston: Little, Brown, 1989.

Kremkau, Frederick W. Diagnostic Ultrasound: Principles and Instruments. 7th ed. St. Louis, Mo.: Saunders/Elsevier, 2006.

MedlinePlus. "Ultrasound." MedlinePlus, May 28, 2013.

Pagana, Kathleen Deska, and Timothy J. Pagana. Mosby’s Diagnostic and Laboratory Test Reference. 11th ed. St. Louis, Mo.: Mosby/Elsevier, 2013.

Voyatzis, Diane. "Doppler Ultrasound." Health Library, November 19, 2012.