special subjects
Ultrasonography
Variations of Ultrasonography
Ultrasound information can be displayed in several ways. A-mode is the simplest; signals are recorded as spikes on a graph. The vertical (Y) axis of the display shows the echo amplitude, and the horizontal (X) axis shows depth or distance into the patient. This type of ultrasonography is used for ophthalmologic scanning. B-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. M-mode is used to image moving structures; signals reflected by the moving structures are converted into waves that are displayed continuously across a vertical axis. M-mode is used primarily for assessment of fetal heartbeat and, in cardiac imaging, most notably to evaluate valvular disorders. Doppler ultrasonography is used to assess blood flow. It 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 To evaluate vascularity of tumors and organs To evaluate heart function (eg, as for Echocardiography ) To detect occlusion and stenosis of blood vessels To detect blood clots in blood vessels (eg, in Deep Venous Thrombosis (DVT) ) To detect synovitis in joints Spectral Doppler ultrasonographydisplays 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 ultrasonographycombines the graphic display of spectral ultrasonography with the images of B-mode. Color Doppler ultrasonographyconverts 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). 