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Ultrasonography

Ultrasonography

Definition

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.

Resources

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}>.

"Ultrasound." Medline Plus National Library of Medicine. December 3, 2003 (April 4, 2004). <http://www.nlm.nih.gov/medlineplus/tutorials/ultrasound/rd209101.html>.

Juli M. Berwald, PhD

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Ultrasonography

Ultrasonography

Definition

Ultrasonography is the study of internal organs or blood vessels using high-frequency sound waves. The actual test is called an ultrasound scan or sonogram. Duplex ultrasonography uses Doppler technology to study blood cells moving through major veins and arteries. There are several types of ultrasound. Each is used in diagnosing specific parts of the body.

Purpose

An ultrasound is a noninvasive, safe method of examining a patient's eyes, pelvic or abdominal organs, breast, heart, or arteries and veins. It is often used to diagnosis disease, locate the source of pain, or look for stones in the kidney or gallbladder. Ultrasound produces images in real time. Images appear on the screen instantly. It may also be used to guide doctors who are performing a needle biopsy to locate a mass. (Needle biopsies are often used to obtain a sample of breast tissue to test for cancer cells.) Duplex/Doppler ultrasound aids in diagnosing a blockage in or a malformation of the vessel. Different color flows aid in identifying problem areas in smaller vessels. Endoscopic ultrasound combines a visual endoscopic exam, during which a flexible tube called an endoscope is threaded down the throat, with an ultrasound test. The ultrasound probe is attached to the end of the endoscope. An endoscopic ultrasound is helpful in determining how deeply a tumor has grown into normal tissues or the gastrointestinal tract. During a transvaginal ultrasound , the ultrasound probe is inserted into the vagina to obtain better images of the ovaries and uterus. Color flow Doppler imaging, using a transvaginal probe, is being performed to detect abnormal blood flow patterns associated with ovarian cancer .

Precautions

Ultrasound is considered safe with no known risks or precautions. The exam uses no radiation. Under nor mal circumstances the exam is normally painless. However, if the patient has a full bladder, pressure exerted during the exam may feel uncomfortable. An ultrasound conducted in conjunction with an invasive exam carries the same risks as the invasive exam.

Description

The patient will be asked to lie still on an exam table in a darkened room. The darkness helps the technician see images on a screen, which is similar to a computer monitor. Sometimes the patients are positioned so they can watch the screen. The technician will apply a lubricating gel to the skin over the area to be studied. Ultrasound uses high-frequency sound waves to produce an image. A small wand-like device called a transducer produces sound waves that are sent into the body when the device is pressed against the skin. The gel helps transmit the sound waves, which do not travel through the air. Neither the patient nor the technician can hear the sound waves. The technician moves the device across the skin in the area to be studied. The sound waves bounce off the fluids and tissues inside the body. The transducer picks up the return echo and records any changes in the pitch or direction of the sound. The image is immediately visible on the screen. The technician may print a still picture of any significant images for later review by the radiologist.

Preparation

Depending on the type of ultrasound ordered, patients may not need to do anything prior to the test. Other ultrasound studies may require that the patient not eat or drink anything for up to 12 hours prior to the exam, in order to decrease the amount of gas in the bowel. Intestinal gas may interfere in obtaining accurate results. The patient must have a full bladder for some exams and an empty bladder for others.

Aftercare

Remove any gel still left on the skin. No other after-care is required following an ultrasound.

Risks

Standard, diagnostic ultrasound is considered risk-free. Risks may be associated with invasive tests conducted at the same time, such as an endoscopic ultrasound or an ultrasound-guided needle biopsy.

Normal results

An ultrasound scan is considered normal when the image depicts normally shaped organs or normal blood flow.

Abnormal results

Abnormal echo patterns may represent a condition requiring treatment. Any masses, tumors, enlarged organs or blockages in the blood vessel are considered abnormal. Additional testing may be ordered.

See Also Upper gastrointestinal endoscopy

Resources

BOOKS

Pfenninger, John L. Procedures for Primary Care Physicians, 2nd edition. St. Louis, MO: Mosby-Year Book, Inc, 2000.

Rosen, Peter. Emergency Medicine: Concepts and Clinical Practice. St. Louis, MO: Mosby-Year Book, Inc. 1999.

Schull, Patricia. Illustrated Guide to Diagnostic Tests. Spring-house, PA: Springhouse Corporation, 1997.

ORGANIZATION

American Cancer Society (National Headquarters). 1599Clifton Road, NE Atlanta, Georgia 30329. (800) 227-2345. <http://www.cancer.org>

National Cancer Institute. 9000 Rockville Pike, Building 31, Room 10A31, Bethesda, Maryland, 20892. (800) 422-6237. <http://wwwicic.nci.nih.gov>

Radiological Society of North America (RSNA). 820 Jorie Boulevard, Oak Brook, IL 60523-2251. <http:www.radiologyinfo.org>

Debra Wood, R.N.

KEY TERMS

Biopsy

Removal of a tissue sample for examination under a microscope to check for cancer cells.

Endoscopy

Examination of the upper gastrointestinal tract using a thin, flexible instrument called an endoscope.

Radiologist

Doctor who has received special training and is experienced in performing and analyzing ultrasounds and other radiology exams.

QUESTIONS TO ASK THE DOCTOR

  • Did you see any abnormalities?
  • What future care will I need?

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ultrasound

ultrasound or sonography, in medicine, technique that uses sound waves to study and treat hard-to-reach body areas. In scanning with ultrasound, high-frequency sound waves are transmitted to the area of interest and the returning echoes recorded (for more detail, see ultrasonics). First developed in World War II to locate submerged objects, the technique is now widely used in virtually every branch of medicine. In obstetrics it is used to study the age, sex, and level of development of the fetus and to determine the presence of birth defects or other potential problems. Its use to determine fetal sex has led to the widespread abortion of female fetuses in some countries, such as China and India, where male offspring are more highly valued. Ultrasound is used in cardiology to detect heart damage and in ophthalmology to detect retinal problems. It is also used to heat joints, relieving arthritic joint pain, and for such procedures as lithotripsy, in which shock waves break up kidney stones, eliminating the need for surgery. Ultrasound is noninvasive, involves no radiation, and avoids the possible hazards—such as bleeding, infection, or reactions to chemicals—of other diagnostic methods.

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ultrasonography

ultrasonography (sonography) (ultră-sonn-og-răfi) n. the use of ultrasound to produce images of structures in the human body. The ultrasound probe sends out a short pulse of high-frequency sound and detects the reflected waves (echoes) occurring at interfaces within the organs. The direction of the pulse can then be moved across the area of interest with each pulse to build up a complete image. As far as is known, there are no adverse effects from the use of ultrasound at diagnostic energies. Ultrasonography is used extensively in obstetrics and also to examine the abdominal organs, urinary tract, blood vessels, muscles, and tendons. transrectal u. (TRUS) ultrasonography for examination of the prostate gland and seminal vesicle, in which the ultrasound probe is placed through the anus to lie directly behind these structures. transvaginal u. ultrasonography using a vaginal probe, which provides a detailed anatomy of the female pelvis and early accurate identification of fetal structures.
www.cancerhelp.org.uk/help/default.asp?page=150 Further explanation of the procedure from CancerHelp UK

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ultrasound

ultrasound Waves of higher frequency than audible sound waves. Reflection of ultrasound waves was applied to underwater detection during World War II, and subsequently to imaging the body. The harmless waves (>100 Mhz) are aimed at the part to be examined, and reflections are detected from tissue components in proportion to their acoustic impedance. These signals can be processed to create two- or three-dimensional images. The most common use is for viewing the fetus in early pregnancy, but there are also many other diagnostic applications.

Treatment by ultrasound (ultrasonics) is widely used by physiotherapists, particularly for soft tissue injuries but also for a variety of more chronic conditions, with a view to promoting healing and relieving pain. Most commonly frequencies of 1–3 MHz are used. There are differences of opinion and practice, related to whether the intended action should be primarily thermal or non-thermal, but there is a lack of controlled trials on the efficacy of the different methods used.

J. K. Davidson, and Stuart Judge


See imaging techniques; sonogram.

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ultrasound

ultrasound (ultrasonic waves) (ul-tră-sownd) n. sound waves of high frequency (above 20 kHz), inaudible to the human ear. Ultrasound in the range 2–20 MHz can be used to produce images of the interior of the body as the waves reflect off structures back to the probe (see ultrasonography). The vibratory effect of ultrasound can also be used to break up stones (see lithotripsy) and cataracts (see phacoemulsification), to remove calculus from the teeth, to destroy tumours (high-intensity focused u.; HIFU), and in the treatment of rheumatic conditions.

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ultrasound

ul·tra·sound / ˈəltrəˌsound/ • n. sound or other vibrations having an ultrasonic frequency, particularly as used in medical imaging. ∎  an ultrasound scan, esp. one of a pregnant woman to examine the fetus.

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ultrasound

ultrasoundabound, aground, around, astound, bound, compound, confound, dumbfound, expound, found, ground, hound, impound, interwound, mound, pound, profound, propound, redound, round, sound, stoneground, surround, theatre-in-the-round (US theater-in-the-round), underground, wound •spellbound • westbound • casebound •eastbound • windbound • hidebound •fogbound • stormbound •northbound • housebound •outbound • southbound • snowbound •weatherbound • earthbound •hellhound • greyhound • foxhound •newshound • wolfhound •bloodhound • background •battleground • campground •fairground • playground •whip-round • foreground •showground • merry-go-round •runaround • turnaround • ultrasound •pre-owned, unowned •unchaperoned • poind • untuned •Lund

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Ultrasound

Ultrasound

Definition
Purpose
Precautions
Description
Preparation
Aftercare
Risks

Definition

Medical ultrasound imaging involves the use of high frequency sound waves to produce pictures of different parts of the inside of the body. This medical procedure is painless, safe, and non-invasive. Ultrasound imaging is not an X-ray as it uses sound waves and not ionizing radiation. Ultrasound images are unlike x-rays also in that they are done in “real time” and not just a picture taken at a single moment. Therefore, ultrasound imaging can help to show movement inside of body organs as well as the structure of the organs. Most people are familiar with ultrasound imaging being used during pregnancy to look safely and carefully at the developing fetus. There are also many other uses in medicine for ultrasound imaging.

The following are some other uses for medical ultrasound imaging:

  • Cardiac ultrasound is used to diagnose problems with the heart and major blood vessels surrounding the heart.
  • Ultrasound imaging in gynecology is used to diagnose problems with the female reproductive tract including being used to diagnose problems associated with infertility. Ultrasound is also used to monitor infertility treatments.
  • Ultrasound imaging is used to look for problems with other internal organs, such as the gallbladder, bladder, testicles, liver, spleen, kidneys, and pancreas.
  • Ultrasound imaging is also used to look for problems with glands, such as the thyroid.
  • Vascular ultrasound imaging is used to watch the blood flow in blood vessels or blood flow to tumors. Ultrasound doppler imaging and color flow mapping can show the flow of blood.
  • Ultrasound imaging is also used during medical procedures such as needle biopsies or egg retrieval during in vitro fertilization.

Purpose

The purpose of ultrasound imaging in medicine is to help the physician diagnose, monitor and treat medical conditions.

Precautions

The greatest precaution that should be taken when using ultrasound imaging to diagnose medical problems is the over and under diagnosing of problems by staff that is not properly trained, using poor equipment or not adequately supervised. This is especially true in the obstetrical setting where it is important to have properly trained ultrasound technicians (sonographers) to perform routine and advanced diagnostic ultrasound on a pregnant woman. The Society of Diagnostic Medical pregnant and the American Institute for Ultrasound in Medicine are great resources for helping to find certified sonographers from accredited programs.

Description

Ultrasound imaging is performed by using a transducer, which is a small device that the technician holds in his/her hand and is attached to a cord that connects to the ultrasound machine. The ultrasound machine has a keyboard, a computer, and a display screen. The patient is usually lying down on an examination table and clear gel (cold or warm) is applied to the area of the body that is to be imaged or scanned so that the transducer makes easy contact with the body and can easily be slid back and forth during the ultrasound. As the transducer is moved over that part of the body, it sends out high frequency sound waves, looks for the returning echo and instantly puts that image up onto the screen. An ultrasound examination is usually painless, however, on occasion, discomfort from the pressure being pressed on the body may occur, especially if the patient’s bladder is full, or if the area being scanned is injured or tender. Sometimes ultrasound maging is performed by using an ultrasound probe that is inserted into an area of the body, such as the vagina. Vaginal ultrasounds are used to scan for early pregnancy or to look carefully at the ovaries or guide the physician during procedures such as egg retrieval for in vitro fertilization. This ultrasound is not usually painful, but to some may be uncomfortable.

Preparation

Preparation for an ultrasound examination depends upon the area of the body that is to be scanned. For example during pregnancy, a patient may be instructed to drink water and not to empty her bladder prior to the ultrasound examination to help with visualization during the ultrasound. Other procedures may require no eating or drinking prior to the ultrasound examination. Comfortable, loose clothing should be worn, although a gown may be provided to be worn for the ultrasound examination. It is important to ask for and follow the instructions that are given prior to the ultrasound examination so that the procedure does not need to be rescheduled.

Aftercare

After the ultrasound, the gel is wiped off and the patient is usually able to return to normal activity. Usually, after the examination, the technician has the images reviewed by the physician and the physician may then speak to the patient at that time. Otherwise, the results are called to the patient or discussed at a later visit. On occasion, especially during pregnancy, the technician or physician will discuss the results of the ultrasound while the ultrasound is being performed. If an ultrasound examination shows abnormal results, those results may need to be followed-up with other tests or consultations to discuss possible treatment.

Risks

For routine diagnostic ultrasound imaging, there are no known risks to humans and therefore, if necessary it is safe to repeat the procedure as often as needed to monitor a particular health concern or treatment. For over 30 years, diagnostic ultrasound imaging has been used on pregnant women. A multitude of studies on the effects of ultrasound use during pregnancy have been reported and although there have been a number of small studies citing possible hearing problems, low birth-weight and left handedness, these studies have not been verified by larger studies. Overall, there has been no evidence that ultrasound is harmful to a developing fetus, however, the medical community should be diligent about preventing unnecessary use of ultrasound in pregnancy.

Resources

BOOKS

Nyberg, David A., JP McGahan, D. Pretorius, G. Pilu. Diagnostic Imaging of Fetal Anomalies. 2 Sub ed.: Lippincott Williams and Wilkins, 2002.

ORGANIZATIONS

American Institute of Ultrasound in Medicine, 14750 Sweitzer Lane, Suite 100, Laurel, MD 20707. (301)498-4100. http://www.aium.org/.

Society of Diagnostic Medical Sonography, 2745 Dallas Parkway Suite 350, Plano, TX 75093. (214)-473-8057. http://www.sdms.org/.

OTHER

Medline Plus, a service of the National Library of Medicine and the National Institutes of Health, 8600 Rockville Pike, Bethesda, MD 20894. http://www.nlm.nih.gov/medlineplus/ency/article/003336.htm

Renee Laux, M.S.

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

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Ultrasound

ULTRASOUND

Ultrasound is a method of assessing the fetus using low-frequency sound waves to reflect off fetal tissue. The ultrasound transducer produces ultrasound waves, which bounce off tissue at different speeds depending on its density. Most commercial ultrasound equipment emits energy that is much lower than the determined maximum safety standard. There are no known reports of fetal damage from conventional diagnostic ultrasound.

There is no uniform agreement as to when ultra-sound should be performed during pregnancy. Nevertheless, ultrasound has become the predominant method for determining fetal age, assessing fetal anatomy, and monitoring fetal growth. The American Institute of Ultrasound in Medicine recommends that ultrasound be used in the first trimester to determine fetal age, number, and viability (via visualization of fetal heart activity). In the second and third trimesters, the fetus can be scanned for anatomic abnormalities, fetal growth, amniotic fluid volume, and placental location.

See also:AMNIOCENTESIS; BIRTH; BIRTH DEFECTS; PREGNANCY

Bibliography

Creasy, Robert K., and Robert Resnik. Maternal-Fetal Medicine. Philadelphia: Saunders, 1999.

Gabbe, Steven, Jennifer R. Neibyl, and Joseph L. Simpson. Obstetrics: Normal and Problem Pregnancies. New York: Churchill Livingstone, 1997.

GarrettLam

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Encyclopedia.com gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, Encyclopedia.com cannot guarantee each citation it generates. Therefore, it’s best to use Encyclopedia.com citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

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American Psychological Association

http://apastyle.apa.org/

Notes:
  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most Encyclopedia.com content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.