tailieunhanh - Gastrointestinal Endoscopy - part 9

Những nguyên tắc cơ bản của hình ảnh siêu âm. Hình ảnh siêu âm dựa trên cùng nguyên tắc như SONAR RADAR, và định vị bằng tiếng vang (được sử dụng bởi loài dơi và cá heo). Tốc độ của âm thanh tương đối ổn định trong bất kỳ phương tiện thông tin nhất định. | Chapter 24 Endoscopic Ultrasonography EUS of the Upper Abdomen Shawn Mallery Background Basic principles of ultrasound imaging. Ultrasound imaging is based upon the same principles as SONAR RADAR and echolocation used by bats and dolphins . The speed of sound is relatively constant in any given media. By measuring the time required for sound to reflect off a distant object and return to the observer the distance to that object can be determined Distance Velocity x Time . For the purposes of imaging each echo is displayed visually as a bright spot at the measured distance from the source of sound the ultrasound transducer . Ultrasound characteristics of various tissues. Structures containing fluid produce few if any echoes and are thus displayed as dark black anechoic . Solid structures with a high water content appear dark but contain scattered internal echoes hypoechoic . Solid structures with a high fat content appear relatively white hyperechoic . Shadowing. When structures are dense or highly reflective and do not transmit sound only the surface facing the transducer can be visualized. As little sound energy is transmitted through to deeper structures these are hidden. Acoustic enhancement. Entirely fluid-filled structures may transmit sound more readily than surrounding solid tissue. Structures deeper than the fluid are therefore exposed to more energy and appear brighter reflect more energy than adjacent structures that are deep to solid tissue. Limitations of traditional ultrasonography - Tissue-related limitations. Sound waves are transmitted more efficiently through fluid than solids or gases. Ultrasound imaging is thus not possible through dense solids . bone or gases . lungs bowels . For this reason standard ultrasound imaging of the mediastinum distal common bile duct which passes posterior to the duodenum and pancreas has been of limited value. - Distance-related limitations. As sound frequency measured in megahertz MHz increases improved imaging