Ultrasound unit

Definition

An ultrasound unit is a noninvasive medical device used to produce images of body tissues and organs from differential reflections of ultrasonic sound waves. The technique of diagnostic imaging performed by ultrasound units is called ultrasonography. Ultrasonic waves are sound waves of a higher frequency than the human ear can detect. The frequency of a sound wave is the number of times per second that it cycles, and the number of cycles is measured in hertz (Hz). For example, one kilohertz (kHz) is one thousand cycles per second. Human hearing can detect sound in the range between 20 hertz to about 20 kilohertz (20kHz), or 20,000 cycles per second. Ultrasound images are generally produced using sound waves in the range between 1.6 to 10 million megahertz (MHz). Body tissues of different density reflect, or echo, sound waves differently, allowing the sonographer to distinguish between the structures.

Purpose

The first account of diagnostic ultrasound was published in 1942 by Dr. Karl Dussik, an Austrian psychiatrist. Dr. Dussik used ultrasound to locate brain tumors. Although ultrasound is better known as a technique of diagnostic imaging, it is also used at present in a variety of therapeutic applications.

Diagnostic applications

Ultrasonographic imaging can be used to visualize most soft-tissue organs. Dr. Dussik used ultrasound to visualize the cerebral ventricles in his pioneering use of the technique. Ultrasound is now used routinely to examine the kidneys or liver for the presence of tumors or cysts. The gall bladder can be checked for gallstones. Ultrasonography can also be used to examine blood vessels in the abdomen, extremities, or neck for evidence of swelling or blockage. One of the best-known diagnostic applications of ultrasound is its use during pregnancy to monitor the development, position, sex, and number of babies present in the mother's uterus.

Diagnostic ultrasound units are used to guide instruments during such invasive treatments as needle biopsies. Intraoperative sonography is used during many other procedures and even in combination with other medical imaging techniques. For example, intraoperative ultrasound is used during neurosurgery to detect brain tissue movement that can compromise the use of other more detailed imaging systems, such as computed tomography (CT) or magnetic resonance imaging (MRI).

Therapeutic applications

Ultrasonography also has therapeutic applications, although the frequencies used for therapy are usually in different frequency ranges than those used in diagnostic ultrasound.

BODY FLUID SAMPLING. A technique developed at MIT uses ultrasound to draw samples of tissue fluid through the skin withut the use of needles. Ultrasound waves disorganize the fatty layers in the outer layer of human skin, thus increasing the skin's permeability sufficiently to allow molecules of tissue fluid to travel through into a vacuum cylinder. The researchers apply ultrasound to the skin at 20 kHz frequency for two minutes. This frequency is much lower than that used to visualize fetuses in the womb. The technique shows promise as a noninvasive way for diabetics to monitor their blood sugar levels.

PHYSIOTHERAPY. Ultrasound waves produce heat as well as sound echoes. The low levels of heat produced by ultrasound appear to speed up wound healing, first by facilitating the release of histamine, a chemical that attracts white blood cells to the injured tissue. Second, ultrasound stimulates fibroblasts to secrete collagen, a fibrous protein found in connective tissue that increases the strength of the healing tissue. Ultrasound can also be applied to the area around the wound to provide mild heat in order to stimulate blood circulation in the area. A frequency of 3 MHz is used for most skin wounds , 1 MHz for deeper wounds or the area around the wound.

A British study indicates that ultrasound therapy applied to the wrist provides good short-term relief of mildly to moderately severe carpal tunnel syndrome . The beneficial effects of the treatment last for at least six months. It is thought that the ultrasound waves relieve the symptoms of the syndrome by relieving inflammation.

TUMOR DETECTION AND TREATMENT. Ultrasound can be used to scan for endometrial cancer in post-menopausal women without the need for a surgical biopsy. The ultrasound captures a detailed image of the lining of the uterus, which allows not only for immediate evaluation of the results, but is also more accurate than a biopsy.

Focused high-intensity ultrasound is being tested as a technique for destroying cancerous tumors within the body. The high-intensity beam, which is about 10,000 times as powerful as the ultrasound beams used to monitor pregnancies, appears to work by heating the cancer cells to nearly the temperature of boiling water. The cancer cells die within seconds. In 1999, the FDA granted approval for the use of focused ultrasound to treat enlarged prostate glands in men.

Description

An ultrasound unit includes a television monitor (cathode ray tube or CRT), a transducer for sending and receiving the ultrasonic waves, a transmitter, receiver, amplifier, and a strip chart recorder.

The transducer

The transducer, which is also called a probe, is a handheld instrument used to both generate the sound waves and receive the echoes. The transducer typically functions as a generator about 10% of the time and as a receiver the other 90%. The transducer includes an element, electrode connections to the transmitter and the receiver, backing material, a matching layer, and a protective face.

The element is the core of the transducer—the material that actually produces the sound waves. Elements are usually made of such ceramic materials as barium titanate or lead zirconate titanate. These materials change shape when electrical current is applied, which produces the ultrasonic waves. When ultrasonic waves are absorbed by the element, electrical energy is produced. The transducer of a real-time scanner typically contains over 300 crystals arranged in a row, each emitting and receiving an ultrasound beam in rapid succession.

The remaining parts of the transducer help to focus the sound waves for most effective function. The backing material directs the sound energy into the element, while the matching layer reduces reflection of the sound from the transducer surface. The protective face shields the internal components of the transducer.

There are three fundamental types of transducer used in medical applications: convex, linear, and phased array. Transducers come in different shapes and sizes for use in different scanning applications. Obstetrical scans often use a convex probe that is shaped like a curved soap bar. Probes for vaginal scans are long and slender. There are specially designed probes that couple biopsy needles with the transducer, so that the ultrasound can be easily used to guide the needle.

Remaining parts of the unit

The remaining parts of the ultrasound unit initiate or receive the signals collected by the transducer or are involved in reconstructing the electronic signals into an image. The transmitter creates the impulses sent to the transducer to generate the sound energy. The receiver accepts the electric current generated in the transducer by returning sound energy. Electrodes connect the transmitter and the receiver to the transducer. The amplifier boosts the returning signals and prepares them for display on the monitor (CRT).

One of the advantages of ultrasound is the compactness of the actual unit. Although most portable units are stored on a wheeled cart, completely portable ultrasounds weighing just over 5 lbs (2 kg) and carried by a handle built into the unit, are also available.

The compactness and portability of ultrasound has made it the diagnostic method of choice for isolated medical settings. Remote linkups can allow doctors to review ultrasound images taken many miles away. The space shuttle is equipped with an ultrasound, both for monitoring the effects of weightlessness on astronauts and experimental animals and for emergency use.

Ultrasound modes

The ultrasound monitor is used to display the images produced. Depending on the kind of transducer, the monitor has several basic modes of display, including A-mode, B-mode, M-mode, and Doppler. A-mode is the simplest form of ultrasound; it analyzes a single beam. The A-mode display is a series of peaks indicating the distance of the structure being scanned from the transducer as time elapses. Isolated use of A-scans is now rare, but this display mode can be used to ensure that the time-gain compensation is set correctly and to check the accuracy of the distance measurements between echoes.

B-mode is the image that results from converting the peaks of A-mode into dots whose brightness varies with the strength of the signal. Stronger signals appear more nearly white and weaker signals more nearly black. In a real-time system, B-mode scans repeat about 30 times a second, thus capturing such movements within the patient as the beating of the heart or a fetus sucking its thumb.

M-mode displays B-mode dots on a moving-time basis. Before the development of real-time systems, M-mode was used to monitor the opening and closing of heart valves. It is still useful in determining the precise timing of valve opening as well as coordinating valve motion with electrocardiography , phonocardiography (the study of heart sounds), and Doppler.

Doppler ultrasound depicts the movement of fluid, usually blood, within the body. The technique is based on the fact that sound waves change in frequency when bounced off a moving target, called a sample volume. If the sample volume is moving away from the transducer, the frequency of the bounced sound wave is increased after the echo, while the frequency is decreased if the sample volume is moving toward the transducer. There are two kinds of Doppler analysis, pulsed wave (PW) and continuous wave (CW). Pulsed wave has proved very useful for analyzing blood flow in a particular area of the heart or group of vessels, while continuous wave is better suited for evaluation of a single valve or vessel. Doppler output is often enhanced with a color display. With most of these systems, shades of red indicate that blood is flowing toward the transducer, while shades of blue represent flow away from the transducer.

Operation

To perform an ultrasound scan, the patient is placed on an examination table with the area to be imaged uncovered. A gel, warmed for comfort, is applied to the skin to prevent air bubbles between the transducer and the body. The sonographer sweeps the transducer across the area of interest, keeping contact with the patient's skin.

In order to obtain the best possible images, the sound waves are sent within a particular area called the acoustic window. For example, a commonly used acoustic window for an echocardiograph (ultrasound of the heart) is the left parasternal approach, which allows visualization of all the valves and chambers. Acoustic windows avoid bone and such air-filled organs as the lungs , as both bone and air are poor media for ultrasonic waves.

The level of detail of the imaging process is known as resolution. Resolution is affected by the frequency of sound waves used. As frequency increases, resolution increases. Increase in frequency, however, reduces the sound waves' depth of penetration into body tissue. Because of this inverse relationship, sonographers usually focus on the structures of interest, maximizing the resolution by using the highest frequency that will penetrate the tissue to the required depth.

Settings

The settings of an ultrasound unit include the power output (gain or attenuation) and the time gain compensation (gain curve or swept gain), controlled by four variables: the slope rate (slope), the slope start (delay), and the near and far gain (initial gain). Each of these controls affects the way the echoes are sent or received. The power output alters the echoes throughout the ultrasonic field by varying the amount of sound sent (gain) or the strength of the signal after it comes back to the transducer (attenuation), measured in decibels. Excessive gain can produce too many echoes and differentiation between tissues can be lost.

One major source of interference is echoes from the skin and subcutaneous tissues. These can be eliminated by altering the slope delay value by 2–3 cm. Near gain alters the power of the echoes in the near field. This value is adjusted so that enough information about the near field is present in the image, but not so much as to swamp out signals from small structures farther from the transducer. Far gain governs the strength of the echoes from distant structures, and must be adjusted to ensure all parts of the organ or structure being studied are well represented in the image.

Preprocessing and postprocessing controls can be used to clarify an image for a more detailed look at a particular section. Preprocessing controls assign values to returning echoes before they are displayed and can help accentuate borders between structures. Postprocessing assigns values to echoes after they have been displayed, thus helping to accentuate low-level echoes.

Real-time scanners have such special controls as calipers (to measure distances); cineloop (a replay function to help select an image for a photo); frame rate; freeze frame; and record (to videotape the image).

Doppler ultrasound units have an additional set of controls. The range gate cursor is used to indicate the depth and area placement of the sample volume, overlaid on a B-scan. A second control is inversion, which allows flow away from the transducer to become a positive rather than negative value for easier viewing. Velocity scale allows changes for different rates of cardiac output; sweep speed changes the rate at which the information is displayed. Wall filter settings are used to eliminate signal noise and artifacts caused by the patient's movement. The angle correct bar aligns the blood flow and the ultrasound beam because the angle must be no greater than 60 degrees. The size of the angle is important because the smaller the angle between beam and flow, the greater the Doppler shift. Finally, Doppler gain adjustments ensure that the image of color flow is set so that variations are seen within the vessel and any artificial flow outside the vessel is eliminated.

Safety

Ultrasound appears to be one of the safest imaging technologies as well as one of the least expensive. As of 2001, there are no confirmed biological effects on patients or instrument operators with exposure to ultrasound for the time periods and frequencies used in diagnostic procedures. The current position of the American Institute of Ultrasound in Medicine (AIUM) is that the benefits to patients with diagnostic ultrasound outweigh any known risks—although the possibility always exists that adverse biological effects may be identified in the future.

Ultrasound during pregnancy is regarded as appropriate when performed to help the physician determine the baby's health and due date. Ultrasound examination can also determine the baby's position in the womb or the existence of a multiple pregnancy . Ultrasound also may be used to detect some birth defects. The AIUM, however, does not condone the use of ultrasound solely to determine fetal sex.

Maintenance

Maintenance of ultrasound equipment is performed by specially trained technicians who may belong to the hospital engineering staff or an outsource company. Maintenance includes visual inspections, periodic cleaning, and system performance checks. Performance checks ensure that all power supply voltages are within tolerance and image performance is maintained.

Health care team roles

Ultrasound units are operated by specially trained ultrasound technologists. Nurses assist with patient preparation and education about the procedure. A physician, who may be a radiologist, surgeon, internist, or gynecologist performs the final review and diagnosis based on the results of the ultrasound. The physician may be present during the examination or may make the final review and diagnosis based on saved images.

Training

Ultrasound technologists have usually completed a training program in a two-year college or vocational program. A typical course list includes:

• elementary principles of ultrasound
• ultrasound transducers
• pulse-echo principles and instrumentation
• ultrasound image storage and display
• artifacts (erroneous results)
• quality assurance
• bioeffects and safety

The American Registry of Diagnostic Medical Sonographers certifies ultrasound technologists as registered diagnostic medical sonographers (RDMS); registered diagnostic cardiac sonographers (RDCS); registered vascular technologists (RVT); and registered ophthalmic ultrasound biometrists. Specialty areas within these credentials include abdominal sonography, neurosonology, obstetrics and gynecology, and ophthalmology (RDMS); adult and pediatric echocardiography (RDCS); and noninvasive vascular technology (RVT).

Resources

BOOKS

Allan, Paul L. et al. Clinical Doppler Ultrasound. London: Churchill Livingstone, 2000.

Rumach, Carol M. et al., eds. Diagnostic Ultrasound. St. Louis, MO: Mosby, 1998.

Sanders, Roger C. Clinical Sonography: A Practical Guide. Boston: Little, Brown and Company, 1998.

Segan, Joseph C., and Joseph Stauffer. The Patient's Guide to Medical Tests. New York: Facts On File, Inc., 1998.

PERIODICALS

"Drawing Blood Could Become History with MIT Ultrasound Technique." MIT News (February 28, 2000).

Ebenbichler, GR, et al. "Ultrasound treatment for treating the carpal tunnel syndrome: Randomised 'sham' controlled trial." British Medical Journal 316 (March 7, 1998): 731-735.

Galen, Barbara A. "Diagnostic Imaging: An Overview." Primary Care Practice 3 (September/October 1999).

ORGANIZATIONS

American Institute of Ultrasound in Medicine, 14750 Sweiter Lane, Suite 100, Laurel, MD 20707-5906. (301) 498-4100 or (800) 638-5352. <www.aium.org>.

American Registry of Diagnostic Medical Sonographers (ARDMS), 600 Jefferson Plaza, Suite 360, Rockville, MD 20852-1150. (301) 738-8401 or (800) 541-9754. <http://www.ardms.org>.

OTHER

McCulloch, Joseph, PhD. Ultrasound in Wound Healing. <http://www.medicaledu.com/ultrasnd.htm>.

KEY TERMS

Acoustic window —The area through which ultrasound waves move freely.

Attenuation —Reduction of the strength of the sound signal as it travels through body tissues.

Decibel —A unit used to express differences in acoustic power. A decibel (dB) is equal to 10 times the common logarithm of the ratio of two sound signals.

Frequency —The number of cycles per second of a sound wave, measured in hertz. One thousand cycles per second is equal to one kilohertz (kHz).

Gain —The strength of the ultrasound signal as it leaves the transducer.

Hertz —A unit of measurement of the frequency of sound waves, equal to one cycle per second. It is named for Heinrich R. Hertz (1857-1894), a German physicist.

Real-time —A type of ultrasound that involves computerized images that respond immediately to user input, in order to record movement.

Sample volume —The area of blood flow analyzed by Doppler ultrasound.

Sonographer —A technician who operates ultrasound units.

Transducer —The handheld part of the ultrasound unit that produces the ultrasound waves and receives the echoes.

Ultrasonography —A diagnostic imaging technique that utilizes reflected ultrasonic waves to delineate, examine, or measure internal body tissues or organs.

Ultrasound —Sound above what can be heard by the human ear, generally above 20,000 Hz (cycles per second).

Michelle L. Johnson, M.S., J.D.