In ultrasonography, a signal generator is combined with a transducer. Piezoelectric crystals in the signal generator convert electricity into high-frequency sound waves, which are sent into tissues. The tissues scatter, reflect, and absorb the sound waves to various degrees. The sound waves that are reflected back (echoes) are converted into electric signals. A computer analyzes the signals and displays an anatomic image on a screen.
Ultrasonography is portable, widely available, relatively inexpensive, and safe. No radiation is used.
Ultrasonography can identify superficial growths and foreign bodies (eg, in the thyroid gland, breasts, testes, limbs, and some lymph nodes). With deeper structures, other tissues and densities (eg, bone, gas) can interfere with images.
Ultrasonography is commonly used to evaluate the following:
Heart (echocardiography): For example, to detect valvular and chamber size abnormalities and to estimate ejection fraction and myocardial strain (see Echocardiography)
Gallbladder and biliary tract: For example, to detect gallstones and biliary tract obstruction (see Imaging Test of the Liver and Gallbladder: Ultrasonography)
Urinary tract: For example, to distinguish cysts (usually benign) from solid masses (often malignant) in the kidneys or to detect obstruction such as calculi or other structural abnormalities in the kidneys, ureters, or bladder (see Genitourinary Imaging Tests: Ultrasonography)
Female reproductive organs: For example, to detect tumors and inflammation in the ovaries, fallopian tubes, or uterus (see Introduction to Gynecologic Tumors)
Pregnancy: For example, to evaluate the growth and development of the fetus and to detect abnormalities of the placenta (eg, placenta previa—see Evaluation of the Obstetric Patient: Ultrasonography).
Musculoskeletal: To evaluate muscles, tendons, and nerves.
Ultrasonography can also be used to guide biopsy sampling and place intravenous catheters.
Ultrasonography is sometimes done internally, using a small transducer on the tip of an endoscope or vascular catheter.
Ultrasound information can be displayed in several ways.
This mode is most often used in diagnostic imaging; signals are displayed as a 2-dimensional anatomic image.
B-mode is commonly used to evaluate the developing fetus and to evaluate organs, including the liver, spleen, kidneys, thyroid gland, testes, breasts, uterus, ovaries, and prostate gland.
B-mode ultrasonography is fast enough to show real-time motion, such as the motion of the beating heart or pulsating blood vessels. Real-time imaging provides anatomic and functional information.
This type of ultrasonography is used to assess blood flow. Doppler ultrasonography uses the Doppler effect (alteration of sound frequency by reflection off a moving object). The moving objects are red blood cells in blood.
Direction and velocity of blood flow can be determined by analyzing changes in the frequency of sound waves:
If a reflected sound wave is lower in frequency than the transmitted sound wave, blood flow is away from the transducer.
If a reflected sound wave is higher in frequency than the transmitted sound wave, blood flow is toward the transducer.
The magnitude of the change in frequency is proportional to blood flow velocity.
Changes in frequency of the reflected sound waves are converted into images showing blood flow direction and velocity.
Doppler ultrasonography is also used
Spectral Doppler ultrasonography displays blood flow information as a graph with velocity on the vertical axis and time on the horizontal axis. Specific velocities can be measured if the Doppler angle (the angle between the direction of the ultrasound beam and the direction of blood flow) can be determined. Velocity measurements and the appearance of the spectral Doppler tracing can indicate the severity of vascular stenoses.
Duplex Doppler ultrasonography combines the graphic display of spectral ultrasonography with the images of B-mode.
Color Doppler ultrasonography converts the Doppler blood flow information into a color image with blood flow in color; it is displayed on a gray-scale anatomic ultrasound image. Direction of blood flow is indicated by the shade of color (eg, red for blood flow toward the transducer, blue for blood flow away from the transducer). Average blood flow velocity is indicated by the brightness of the color (eg, bright red indicates high-velocity flow toward the transducer; dark blue indicates low-velocity flow away from the transducer).