Ultrasound uses high-frequency sound waves to produce an “echo map” that characterizes the position, size, form, and nature of soft tissue organs. Echoes of varying strength are produced by different types of tissues and are displayed as a visual pattern after computer processing of the echo information. The capability of acquiring real-time images means that ultrasound can readily demonstrate motion, as in the fetus or the heart. Ultrasound, however, cannot appropriately image air-filled structures such as the lungs.
A phenomenon that accompanies movement, the Doppler effect, can be combined with diagnostic ultrasound imaging to produce duplex scans. Duplex scans provide anatomic visualization of blood vessels and a graphic representation of blood flow characteristics. Flow direction, velocity, and the presence of flow disturbances can readily be assessed. Certain equipment is capable of advanced Doppler imaging techniques, such as:
Color Doppler imaging provides a color-coded depiction of selected blood flow parameters.
Doppler energy, power Doppler, or color angio is sensitive to very low blood velocity states and is often used to evaluate blood flow through solid organs.
B-flow Doppler images the blood itself, producing images that resemble an angiogram.
These techniques establish the patency of a given blood vessel and are useful in investigating perfusion to an organ or mass or clots within a blood vessel. They are also helpful in evaluating complications in transplanted organs.
General Procedure
A couplant is a nontoxic gel, paste, or liquid that is used to transmit sound energy between the body and the transducer. The couplant is applied to the skin over the area to be examined in order to conduct the sound waves.
An operator, known as an ultrasonographer or sonographer, holds a microphone-like device called a transducer. As the sonographer moves the transducer over a specific body part, a display of that area can be viewed on the monitor.
Sonography of structures in the abdominal region often requires that the patient control breathing patterns. Deep inspiration and exhalation may be used.
Selected images are recorded for documentation purposes.
The examination causes no physical pain. However, in certain applications, pressure may be applied to the transducer, causing some degree of discomfort. Long examinations may leave the patient feeling tired.
Tests usually take 20–45 minutes. This is the actual procedure time and does not include waiting and preparation times.
Some examinations require the patient to fast or to have a filled urinary bladder. Each examining department determines its own guidelines for patient preparation.
Advances in technology have allowed the development of very small high-resolution transducers. Catheter-sized transducers are used to visualize blood vessels “from the inside out” during angiographic procedures. Endoscopic ultrasound is used to evaluate gastrointestinal lesions and may be used to visualize pancreatic biliary structures. Small transducers passed through the esophagus permit exquisite visualization of the heart during transesophageal echocardiography (TEE). Slim transducers are introduced into the vagina to visualize gynecologic anatomy. Transrectal visualization of the prostate gland is an accepted method of screening for disease in the organ.