ABSTRACT
Principles of duplex ultrasound Duplex ultrasound (DU) is one of the most commonly used non-invasive diagnostic modalities for the screening of stenoses in many vascular territories including the mesenteric arteries. The application of this technique is based on the physical principle of using sound waves to create images as well as deriving information from blood flow and flow velocities. The term ultrasound denotes sound-wave frequencies above 20 kHz which are out of the audible range. Ultrasound produces images of anatomic structures by using pulse-echo techniques. An ultrasonic transducer is placed in contact with the skin, often coupled with liquid gel to maximize sound transmission. The transducer then emits brief pulses of sound at a fixed repetition rate (pulse-wave technique). After the sound is generated, its transmission into the tissue may be absorbed, scattered, or reflected. Only the reflected sound waves can be caught by the transducer in between the brief intermittent periods of pulse-wave emission. These reflected sound waves are ultimately amplified and processed into a format suitable for display. Therefore, the images in B mode or brightness mode, are created with varying degrees of intensity which are proportional to the reflected echo signal amplitude and the depth (distance between the transducer the studied anatomical structure) (Figure 62.1).
