Fields of Study

Medical imaging; echocardiography; obstetrics and gynecology; radiology; reproductive endocrinology.

Summary

Ultrasonic imaging is a medical diagnostic tool that visualizes internal structures of the body with a high-frequency sound beam. In contrast to X-rays, which produce harmful, ionizing radiation, ultrasound has no known harmful effects. The heart of the device is the transducer, which transmits the sound beam to and receives an echo from internal structures. Structures within the body reflect the sound beam to different degrees. Sound passes through liquid readily, is reflected to some degree by muscle, and is strongly reflected by bone. Ultrasound is commonly used for visualization of the developing fetus within the uterus. The amniotic fluid surrounding the fetus readily transmits the sound beam to the fetus and its internal structures.

Key Terms and Concepts

Acoustic Window: Fluid-filled structure, such as the urinary bladder, which aids in visualization of structures beneath it.

Coupling Gel: Gel placed on the skin to facilitate contact of the transducer with underlying structures.

Doppler Technique: Technique that visualizes motion in blood vessels; color Doppler measures the degree and direction of blood flow and displays it by color variations; flow away from the transducer is in blue tones while flow toward the transducer is in red tones.

Echocardiogram: Visualization of the internal structure of the heart via ultrasound.

Echogenicity: Refers to the degree of reflection from a structure. A structure may be termed hyperechoic (high echogenicity), hypoechoic (low echogenicity), or anechoic (no echogenicity).

Piezoelectric Effect: Production of electric voltage when pressure is applied to certain crystals; when placed in an electric field, these crystals become compressed.

Ultrasonic Transducer: Device that transmits and receives high-frequency sound waves (above 20,000 hertz); also known as transceivers because they both transmit and receive sound.

Definition and Basic Principles

Ultrasound works on a principle similar to that of radar, which transmits then receives radio waves and converts them into an image. Structures within the body are differentiated by the varying degrees that they reflect a focused sound beam. Ultrasound is one of the most widely used diagnostic medical tools.

Compared with other imaging modalities, ultrasound is relatively less expensive and more portable. It can image many internal organs to visualize their size, structure, and any abnormal (pathological) lesions, such as a cancerous tumor, within them. It is used extensively in the field of obstetrics to observe the growth and internal organs of a fetus. Many abnormalities can be diagnosed through this modality. It is used by cardiologists to image the heart in real time; this technology is known as echocardiography. It is used by ophthalmologists to visualize the internal structure of the eye.

Transducers come in a variety of shapes and sizes, depending on their use. They also are designed to emit different frequencies. Higher frequencies produce a more detailed image; however, they do not penetrate as deeply. In addition, transducers are designed to focus at different depths, depending on their intended use. Transducers can be placed over the skin or within a body cavity, such as the vagina or rectum.

Ultrasound can be used to guide instruments passed into the body. For example, an obstetrician can use ultrasound to guide a needle within the amniotic cavity (sac around the fetus) for an amniocentesis (withdrawal of amniotic fluid for analysis). Reproductive endocrinologists use ultrasound to guide needles passed through the vaginal wall and into the ovary for aspiration of ova (eggs) from the ovary.

Background and History

In 1841, Swiss physicist Jean-Daniel Colladon conducted experiments regarding sound transmission in Lake Geneva; he determined that sound traveled more than four times faster in water than in air. In 1881, French physicist Pierre Curie, who is well known for his work regarding ionizing radiation, discovered the piezoelectric effect, which later made the development of the ultrasound transducer possible.

In 1937, Karl Dussik, an Austrian physician, developed a technique that he termed “hyperphonography.” His equipment purportedly aided in the diagnosis of brain tumors using heat-sensitive paper that recorded extremely rudimentary images of sound echoes generated from quartz crystals. Over the next decade, Dussik continued research on the use of ultrasound for differentiating body tissues. This type of ultrasound, later termed “A-mode,” produced an echo spike on recording paper or an oscilloscope.

During the 1950’s and 1960’s, B-mode scanners were developed and improved. Using a linear array of transducers, these scanners produced a static, two-dimensional image of internal structures. Subsequently, real-time sonography was developed. The image was two-dimensional; however, it was continually updated in real time. Real-time ultrasound can display a beating heart and its internal structure; it also images fetal motion. Also, at this time, Doppler ultrasound, which allowed visualization of blood flow, was being devloped. In 1987, Olaf von Ramm and Stephen Smith of Duke University developed three-dimensional and four-dimensional ultrasound for imaging fetuses. Both techniques are three dimensional; however, the four-dimensional version adds real-time recording of movement to the three-dimensionsl image.

How It Works

Many internal structures can be visualized with ultrasound. Compared with X rays, its main limitation is its inability to penetrate bone; structures behind a bone are obscured. An ultrasound examination consists of manipulating a transducer over a portion of the body. It is moved or angled over an area, and images of interest are recorded through film, a printer, or a videotape. Continuous recording of an ultrasound examination is often done for later review. A number of different transducers can be attached to an ultrasound machine. An examination might involve the use of more than one transducer.

Ultrasound is commonly used by obstetricians and gynecologists. Ultrasound is an excellent modality for imaging the fetus and charting its growth and development. It can visualize a gestational sac at about four weeks of gestation and can detect a fetal heart beat about two weeks later. Once a living fetus is visualized, various measurements can be made to determine its gestational age. In a number of instances, the fetal age does not coincide with the age calculated from the last menstrual period. Up to twelve weeks of gestation, the crown-rump length (distance from top of head to the buttocks) is used to calculate the gestational age. Later, the head diameters, abdominal diameters, and femur length can be used. Sequential ultrasounds can determine if the fetus is growing properly. Many fetal anomalies can be detected with ultrasound. Ultrasound can be used for conducting a biophysical profile, which can assess fetal well-being. It is a valuable diagnostic tool for imaging abnormalities of the ovaries and uterus. Obstetrical ultrasound employs two methodologies. For early pregnancy (twelve weeks or less), a vaginal transducer is covered with a condom containing conductive gel and placed in the vagina; it is then pressed against the upper vaginal wall. For the remainder of the pregnancy, conductive gel is placed on the abdomen and an abdominal transducer is manipulated to view the uterine cavity and its contents. The patient is asked to have a full bladder to provide an acoustic window for better visualization. Gynecologic ultrasounds are conducted with a vaginal or abdominal transducer.

Other Uses. The breast is scanned by placing the transducer over it. The liver and gallbladder are scanned by an abdominal transducer placed just under the ribs. The kidneys are imaged between the ribs on the back, and the heart is imaged between the ribs on the chest. In addition to the standard two-dimensional real-time image of the heart, M-mode is employed. This mode displays the motion of the heart in a linear display somewhat like an electrocardiogram. M-mode is used for analyzing the function of the heart both in the uterus and after birth. Imaging of the hearts of adults and children is known as echocardiography. The vaginal transducer can also be inserted in the rectum for imaging of the prostate. Other than mild discomfort from a full bladder or internal probe, the procedure is painless.

Safety. Although, ultrasound is a far safer diagnostic modality than X-rays, it might have a slight risk; therefore, studies are ongoing to evaluate this possibility. In 2008, the American Institute of Ultrasound in Medicine published a report in which they stated that potential risks to an ultrasound exam might exist. These potential risks include “postnatal thermal effects, fetal thermal effects, postnatal mechanical effects, fetal mechanical effects, and bio-effects considerations for ultrasound contrast agents.” Animal studies with long-term, high-intensity administration of ultrasound to cattle reported that it caused decreases in the diameter of red blood cells. To date, any harmful effect from diagnostic ultrasound is unknown. If ultrasound can be harmful, the following factors would come into play: duration of exposure, intensity of ultrasound waves, and the number of exams. A greater risk exists for three-dimensional ultrasound because the level of ultrasound energy is higher.

In any event, in most cases, the information gained from the procedure far outweighs any possible harmful effect. Ultrasound can identify many conditions that place the fetus in jeopardy and provide the opportunity for reduction of that risk. The U.S. Food and Drug Administration (FDA) limits the amount of ultrasound energy for obstetrics and gynecology. The limit (94 milliwatts per square centimeter) is the same regardless of the type of ultrasound being applied.

Applications and Products

Since the 1970’s, ultrasound has been an essential diagnostic tool for obstetricians; in fact, many obstetrician-gynecologists have ultrasound machines in their offices. This allows for rapid assessment of a problem. For example, if a woman complains that she has not felt the baby move for a period of time or a fetal heart beat cannot be heard, an ultrasound examination can either provide reassurance or identify a fetal death. Ultrasound is used to guide a needle for an amniocentesis to check for genetic defects. The condition of the placenta and fetal well-being at later stages of pregnancy can be evaluated. Ultrasound can image ovarian cysts; the internal structure of the cyst can help the gynecologist determine whether the cyst is malignant. Abnormalities of the uterus, such as fibroid tumors, can be imaged.

Ultrasound is used in many other medical applications. It is sometimes used as a supplement to mammography to image the breast because it can differentiate between solid and fluid-filled cysts and can produce a better image of areas near the chest wall. In cardiology, echocardiography serves as an essential diagnostic tool to evaluate cardiac abnormalities. It can also examine venous clots and arterial blockage or narrowing indicative of vascular disease. Neurologists can examine the carotid arteries in the neck for stenosis (narrowing) and can confirm the diagnosis of brain death. The aqueous humor (in the front of the eyeball) and the vitreous humor (filling most of the eyeball) readily conduct ultrasound for imaging inner structures. Trauma-induced bleeding into the abdominal cavity, chest cavity, or other regions can be promptly diagnosed. Ultrasound can also be used to image the pancreas, liver, gallbladder, bile ducts, and kidney.

An ultrasound machine can be connected to a wide variety of transducers, depending on the intended use. For example, in the field of obstetrics and gynecology, the machine is usually equipped with a vaginal transducer and one or more abdominal transducers. The head of a vaginal transducer is about an inch long, and the abdominal probes are several inches long. Transducers can be focused for optimum imaging at different depths. Ultrasound machines, in general, are portable; thus, they can readily be moved from one location to another. In many locations, mobile ultrasound is available. A sonographer travels between locations in a van containing an ultrasound machine. Small machines—about the size of a shoebox—also are available. Although not as full-featured as larger machines, they have the advantage of extreme portability.

Impact on Industry

Because of the widespread usage of ultrasound equipment and the variety of specialized manufacturers of ultrasound equipment, manufactures of ultrasound equipment and associated products have a significant share of the medical equipment market. In 2009, the global market for ultrasound imaging equipment was estimated to be $4.9 billion, which was a 6 percent decrease from 2008. Analysts cite health care spending cuts and postponed equipment purchasing as the major reasons for the decline in Western Europe and North America. They predicted that sales would be slow for a few years and pick up when the economic situation improved. However, the need for ultrasound equipment is ongoing for many medical fields. Over time, equipment is subject to failure, which necessitates repair or replacement. Also, continued improvement in technology creates a need for a superior, newer model.

Subindustries. Ultrasound supports a number of subindustries, including maintenance and repair. Ultrasound equipment is connected to devices such as printers, photographic equipment, and video recorders. Consumables must be replenished. These include coupling gel, film, videotapes, and printer paper. Needles and syringes are necessary for performing an amniocentesis. A number of specialized instruments are manufactured for tissue biopsy or ovum extraction. Textbooks and other educational resources are purchased by practitioners. Other educational material is marketed for the public.

Careers and Course Work

Ultrasound is performed by physicians in many specialties as well as ultrasound technicians. Physicians who perform ultrasound must first graduate from college and then complete a four-year course of medical training. Initial specialty training can be in a number of fields, including radiology, obstetrics and gynecology, cardiology, nephrology, and ophthalmology. This is typically a three- or four-year residency program. The physician usually will receive training in ultrasound as part of the residency program. Many will receive additional training following their residency through continuing education courses. Others will take a fellowship of one or more years in which part of the course work is in ultrasound. An example is reproductive endocrinology (infertility), which is a heavy user of ultrasound for ovum (egg) harvesting. Physicians may use ultrasound to varying degrees, ranging from occasional to daily use.

For individuals desiring a career as an ultrasound technician, courses are available through sources such as city colleges, technical schools, and hospitals. The minimum requirement for enrollment is a high school diploma. Physicians and nonphysicians interested in ultrasound may belong to one or more professional organizations. In the United tates, physicians and registered nurses can join the American Institute of Ultrasound in Medicine. Another professional organization, the Society of Diagnostic Medical Sonography, has a broader membership base; it includes physicians, nurses, technicians, hospital administrators, and researchers. It is also international in scope. A major European organization is the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB), based in London, England. Its membership is open to physicians, nurses, and technicians.

Social Context and Future Prospects

Research and development is ongoing in ultrasound technology. Since ultrasound first emerged in the 1970’s, the imaging quality has improved tremendously. It has progressed from blurry images lacking in detail to stunning three-dimensional images. Imaging quality is expected to continue to improve in the foreseeable future. Ultrasound has and will continue to be a significant component to the field of obstetrics. Seeing a living fetus within the uterus has a profound effect on the parents. It transforms a vague entity into a recognizable, living creature. Three-dimensional and four-dimensional ultrasound are very popular. In many instances, the technology does not aid in diagnosis of an abnormality; however, the high-quality images are extreme patient pleasers. such as city colleges, technical schools, and hospitals. The minimum requirement for enrollment is a high school diploma. Physicians and nonphysicians interested in ultrasound may belong to one or more professional organizations. In the United States, physicians and registered nurses can join the American Institute of Ultrasound in Medicine. Another professional organization, the Society of Diagnostic Medical Sonography, has a broader membership base; it includes physicians, nurses, technicians, hospital administrators, and researchers. It is also international in scope. A major European organization is the European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB), based in London, England. Its membership is open to physicians, nurses, and technicians.

Social Context and Future Prospects

Research and development is ongoing in ultrasound technology. Since ultrasound first emerged in the 1970’s, the imaging quality has improved tremendously. It has progressed from blurry images lacking in detail to stunning three-dimensional images. Imaging quality is expected to continue to improve in the foreseeable future. Ultrasound has and will continue to be a significant component to the field of obstetrics. Seeing a living fetus within the uterus has a profound effect on the parents. It transforms a vague entity into a recognizable, living creature. Three-dimensional and four-dimensional ultrasound are very popular. In many instances, the technology does not aid in diagnosis of an abnormality; however, the high-quality images are extreme patient pleasers. A number of companies advertise production of three-dimensional and four-dimensional ultrasound images of fetuses for the public. Opponents of this practice state that this essentially nonmedical use of the technology should be discouraged until it can be proven that the fetus can suffer no harm from the procedure.

Further Reading

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