Ultrasonic Imaging

Summary

Ultrasonic imaging is a medical diagnostic tool that visualizes the 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 to visualize the developing fetus within the uterus. The amniotic fluid surrounding the fetus transmits the sound beam to the fetus and its internal structures.

Definition and Basic Principles

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

Ultrasound is relatively less expensive and more portable than other imaging modalities. 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 obstetrics to observe a fetus's growth and internal organs. Many abnormalities can be diagnosed through this modality. Cardiologists use it to image the heart in real-time. This technology is known as echocardiography. Ophthalmologists use it to visualize the internal structure of the eye.

Transducers come in various shapes and sizes, depending on their use. They also are designed to emit different frequencies. Higher frequencies produce a more detailed image but 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, 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 he termed “hyperphonography.” His equipment purportedly aided in diagnosing 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 using ultrasound to differentiate body tissues. This type of ultrasound, later termed “A-mode,” produced an echo spike on recording paper or an oscilloscope.

During the 1950s and 1960s, 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 and images of fetal motion. Also, at this time, Doppler ultrasound, which allowed visualization of blood flow, was being developed. In 1987, Olaf von Ramm and Stephen Smith of Duke University developed three-dimensional and four-dimensional ultrasounds for imaging fetuses. Both techniques are three-dimensional. However, the four-dimensional version adds real-time movement recording to the three-dimensional 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 involves 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. Several 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 detect a fetal heartbeat about two weeks later. Once a living fetus is visualized, various measurements can be made to determine its gestational age. In several 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 the top of the 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 to conduct a biophysical profile to 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. For an ultrasound performed early in a pregnancy, the patient may be asked to have a full bladder to provide an acoustic window for better visualization. This is not usually necessary for an ultrasound performed later in pregnancy. 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 to analyze the function of the fetus's heart both in the mother's 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. The procedure is painless other than mild discomfort from a full bladder or internal probe.

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 stating 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.” However, the vast majority of experts believe that ultrasound does not harm a mother or child, and the benefits of having an ultrasound far outweigh the risks. A greater risk may exist for three-dimensional ultrasound because the level of ultrasound energy is higher.

Ultrasound can identify many conditions that place the fetus in jeopardy and provide the opportunity to reduce that risk. The United States (US) 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 1970s, ultrasound has been an essential diagnostic tool for obstetricians. Many obstetrician-gynecologists have ultrasound machines in their offices. This allows for rapid assessment of a problem. For example, suppose a woman complains that she has not felt the baby move for a period of time or a fetal heartbeat cannot be heard. In that case, 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 produce a better image of areas near the chest wall. In cardiology, echocardiography is 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 various 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.

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 several 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, during which part of the coursework 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, 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 US, 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. The European Federation of Societies for Ultrasound in Medicine and Biology (EFSUMB) is a major European organization based in London, England. Its membership is open to physicians, nurses, and technicians.

Social Context and Future Prospects

Research and development in ultrasound technology are ongoing. Since ultrasound emerged in the 1970s, 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 is a significant component of 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 ultrasounds are very popular. In many instances, the technology does not aid in diagnosing an abnormality. However, the high-quality images are extreme patient pleasers. A four-dimensional ultrasound generates a moving picture of the fetus. This type of ultrasound is primarily used for entertainment. However, the FDA does not recommend that patients have ultrasounds for enjoyment. Most professionals encourage patients to get ultrasounds only when needed for diagnostic information, not for keepsake photographs of the fetus. Emerging technologies, such as artificial intelligence and machine learning, have also enhanced the medical community's ability to analyze the data provided by ultrasounds, and doctors can more easily recognize fetal abnormalities. 

Bibliography

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