Ultrasonography is a diagnostic technique that involves directing high frequency sound waves at tissues in the body to generate images of anatomical structures. Ultrasonography is also called sonography, diagnostic sonography, and echocardiography when it is used to image the heart.
Purpose
Ultrasonography has a variety of uses in medical diagnostics. It is most well suited for imaging soft tissues that are solid and uniform or filled with fluid. It does not perform well when imaging calcified objects such as bone or objects filled with air like the bowel. Some of the more common uses for ultrasonography include imaging fetus development during pregnancy, diagnosing gallbladder disease and some forms of cancer, and evaluating abnormalities in the scrotum and prostate, heart, and thyroid gland. Ultrasound can also be used to perform breast exams. A technique called Doppler imaging ultrasonography can also be used to view the movement of blood through blood vessels and to guide needles through anatomical structures for obtaining specimens for biopsy. Three-dimensional ultrasounds provide detailed images of fetuses in the uterus.
The majority of ultrasonic exams are performed externally by running a transducer over the surface of the skin. Usually a gel is applied to the skin on which the transducer will glide during the exam. The gel helps prevent the formation of air pockets between the transducer and the skin that interfere with the ultrasonic signal. Some ultrasound diagnostic tests require the insertion of a probe into a body orifice. For example, during a trans-esophageal echocardiogram a specialized transducer is placed in the esophagus to better image the heart. Trans-rectal exams require a transducer to be inserted into a man's rectum to obtain images of the prostate. Transvaginal ultrasounds are used to provide images of a woman's ovaries and uterus or of a fetus during the early weeks of pregnancy.
Ultrasound is generally a painless procedure. Some discomfort may be felt when the transducer is pressed against the skin or when the transducer is inserted in the body. Most ultrasonic procedures take less than half of an hour.
Cranial ultrasound
Cranial ultrasonography is most often used in infants to diagnose problems with the brain and the ventricles in the brain through which cerebrospinal fluid (the clear fluid that circulates through the brain and spinal cord) flows. These abnormalities are often associated with premature birth. Because ultrasound waves are poorly conducted through bones, cranial ultrasonography must be performed on infants before the fontanel (gaps between the bones of the cranium) have closed. Cranial ultrasonography is also performed on adults during brain surgery to help identify the location of brain tumors. In adults, the skull must be surgically opened in order to use ultrasonography.
In infants, cranial ultrasonography is most often used to diagnose two complications. Intraventricular hemorrhage (IVH) occurs when there is bleeding in the brain. This occurs more commonly in premature babies and is likely to happen within the first week of the infant's life. Periventricular leukomalacia (PVL) occurs when the tissue around the ventricles in the brain is damaged. This complication can occur within several weeks of birth. Both IVH and PVL are associated with mental disabilities and developmental delays. Cranial ultrasonography can also be used to evaluate brain abnormalities in babies, such as congenital hydrocephalus or tumors, or to detect infection.
Description
Ultrasonography relies on sound waves to create an image of the soft tissues in the body. Sound waves are a form of energy called longitudinal pressure waves that result when molecules are pushed together and then become rarified (less dense). The molecules through which the wave passes are not transported by the wave; rather, they vibrate back and forth around a neutral position. The number of times that a molecule moves through a compression and rarification cycle in one second is called the frequency of the wave. The unit of the frequency of a sound wave is the Hertz (Hz). Frequencies between about 20 Hz and 20,000 Hz are audible to the human ear and the greater the frequency, the higher a sound wave sounds. Frequencies above 20,000 Hz are called ultrasonic and the human ear cannot detect these sound waves. The frequencies of sound waves used in ultrasonography are between about one million and 15 million Hz (or one and 15 MHz).
An ultrasound machine typically consists of four parts: the transducer, which allows for the movement of the ultrasound machine over the body; the electronic signal processing unit, which controls the power to the transducer; the display unit, which is usually a computer screen; and a device for storing the images, which is usually a videotape or a camera.
The transducer is the most technologically interesting part of the ultrasonography machine. It is usually a hand-held device that can be pushed against the skin or inserted into an orifice. The transducer is made up of a plastic or ceramic material that has piezoelectric properties. This means that it is capable of generating and detecting ultrasound waves. If pulses of electric current are applied to the surface of a transducer, the piezoelectric surface will change in thickness in response to the pulses. This change in thickness causes a change in pressure in the molecules surrounding the piezoelectric surface, generating sound waves. If the pulses occur between one and 15 million times a second, then the result is a sound wave with an ultrasonic frequency. Similarly, the piezoelectric surface acts as a receptor for return waves. When sound waves collide with the piezoelectric surface, they cause a change in its thickness. This change in thickness is converted to a change in the electric current in the transducer, which is then interpreted as various shades of gray and used to form an image on the display unit. The electronics of the transducer are constructed so that ultrasound beams are generated, followed by a pause during which the return waves are detected; this cycle continues during the entire diagnostic procedure.
An ultrasonic wave that is directed out of the transducer and into tissues of the body has one of four outcomes: it can be absorbed by the material, in which case the transducer will receive no return signal; it can be reflected back to the transducer, in which case the transducer will receive a strong return signal; it can be refracted so that it changes direction and only a part of the signal will return to the transducer; finally, the wave can be scattered, greatly reducing the signal received by the transducer. At various tissue interfaces, different amounts of the wave energy are returned to the transducer as a result of various combinations of absorption, reflection, refraction, and scattering. For example, at a fat-muscle interface, about 1% of the incident wave is returned to the transducer, while at a bone-muscle interface, about 40% of the incident wave is returned. At any interface that involves air, such as a gas bubble in the bowel, nearly 100% of the incident wave will be returned to the transducer. Similarly, bones and other calcified objects like kidney stones and gallstones result in very high reflection of the incident wave. Because air acts as such a strong reflector of an ultrasonic wave, gel or some other lubricant is usually placed between the transducer and the skin during an ultrasonic exam.
Some ultrasonic machines take advantage of the Doppler effect in order to display color images of the flow of blood or other fluids. When an ultrasound wave is directed at a stationary object, the return wave will remain the same frequency as the incident wave, although it will be attenuated depending upon the structures with which it interacts. On the other hand, when an ultrasound wave is directed at a moving object, the return wave will have a different frequency than the incident wave depending on whether the moving object is in the same direction as, or in the opposite direction from, the incident wave. This change in frequency can be interpreted, for example, as the speed of blood flow within a vessel.
The recent development of color Doppler sonography (CDS) has improved several diagnostic exams. In this technique, a black and white image of the anatomical structures resulting from traditional ultrasonography is overlaid with a color image showing the flow of a fluid within the tissues generated from a Doppler ultrasonograph. CDS has proven extremely useful for evaluating the blood flow to the placenta and uterus during pregnancy. It has also been used to quantify the blood flow to various tumors; malignant tumors tend to have greater rates of blood flow and longer residence times than benign ones.
Several other new technologies associated with ultrasonography are becoming available as diagnostic tools. Some physicians are using ultrasonography in conjunction with contrast agents that provide better resolution of internal structures. This is particularly useful for visualizing the heart and kidneys more effectively. Harmonic imaging is a technique that is used to improve the signal-to-noise ratio of an ultrasonic image. It is based on the idea that the tissues of the body resonate harmonically, similar to a musical instrument. Therefore, taking advantage of sound waves at two and three times the frequency of the incident wave should provide additional information about the internal structures of the body. For example, if the incident wave of the transducer is 4 MHz, then using return waves that are 8 MHz should improve the resolution of the image. Finally, three-dimensional sonography is available on some machines. In some cases, the three-dimensional image is reconstructed from several sweeps of the transducer at different levels through the body. In others, two transducers that are oriented perpendicular to each other are used to build a three-dimensional image. This technology has been used most frequently to visualize fetuses in the uterus.
Preparation
Preparation for ultrasonography differs depending on the type of exam being performed. For some exams, no preparation is necessary. For others, fasting and abstaining from drinking for up to 12 hours prior to the exam is required. Some exams, like the transabdominal ultrasound, require that the patient have a full bladder because the ultrasonic waves are best transmitted through fluid. If a biopsy is required, antibiotics may be administered prior to the test. The physician or technician performing the exam usually provides instructions on proper preparation prior to the exam.
Risks
Because ultrasonography uses high frequency sound waves, and not x rays or other forms of radiation, there are very few risks associated with its use. Sound waves are either reflected back to the transducer, or the tissues of the body absorb them and they dissipate as heat. There may be a slight increase in heat in the body as a result, but no negative effects of this heat have been documented.
Normal results
Results of ultrasonic tests are usually sent to a physician and possibly to a radiologist. They are usually made available to the patient within one to two days.
BOOKS
Fleischer, Arthur C., et al. Sonography in Obstetrics and Gynecology: Principles and Practice, 6th ed. New York: McGraw-Hill Companies, Inc. 2001.
Fleischer, Arthur C., and Donna M. Kepple. Diagnostic Sonography: Principles and Clinical Applications, 2nd ed. Philadelphia: W. B. Saunders Company, 1995.
OTHER
"Ultrasound Imaging." The Mayo Clinic. January 3, 2004 (April 4, 2004). <http://www.mayoclinic.com/invoke.cfm?objectid=2F0F9036-342F-4723-9DDBAB9FAEA73A77>.
Moyer, Paula. "3-D Ultrasound: Do You Really Need It?" WebMDHealth. February 2, 2004 (April 4, 2004). <http://my.webmd.com/content/article/21/1728_55256.htm?lastselectedguid={5FE84E90-BC77-4056-A91C-9531713CA348}>.
