Ultrasound is routinely incorporated into the work-up of patients with a variety of trauma, critical care, or emergency surgical disorders.1,2,3,4,5,6 Despite advances in other superior imaging modalities such as multidetector-row computed tomography (MDCT), the ability to acquire real-time data with ultrasound allows the surgeon to make critical decisions expeditiously with minimal risk of complications. Surgeons are personally capable of obtaining good quality images and interpreting the examination independently.7,8,9 Presently, medical students, surgical residents and fellows have become familiar with the management of trauma patients and those with emergency general surgery problems using ultrasound as they have multiple opportunities to learn the technique of bedside ultrasound examination.10 Similarly, the management of critically ill patients including resuscitation, hemodynamic monitoring, and procedural guidance has drastically changed in the past few decades as the ultrasound examination is more commonly performed.11,12,13,14,15 These critical care ultrasound studies can be completed at the bedside and avoid transporting patients from the intensive care unit (ICU). In addition to basic techniques to identify free fluid in the abdominal cavity, surgical intensivists have successfully applied the advanced echocardiographic examination to guide the resuscitation of critically ill patients.16,17,18 This chapter reviews the indications, techniques, and currently available data for ultrasound examinations in trauma, emergency general surgery, and critical care patients.
PHYSICS OF ULTRASOUND
Ultrasonography is operator-dependent. Therefore, an understanding of select principles of ultrasound physics is necessary so that images may be acquired rapidly and interpreted correctly. Knowledge of these basic principles enables the acute care surgeon to select the appropriate transducer, optimize resolution of the image, and recognize artifacts. Some basic terms and principles of physics relative to ultrasound imaging in the acute setting are defined in Tables 16-1, 16-2, 16-3.
Table Graphic Jump Location TABLE 16-1Ultrasound Physics Terminology Relevant to Ultrasound Imaging ||Download (.pdf) TABLE 16-1 Ultrasound Physics Terminology Relevant to Ultrasound Imaging
|Term ||Definition ||Significance |
|Ultrasound ||High-frequency (>20 kHz) mechanical radiant energy transmitted through a medium || |
Diagnostic ultrasound: 1–30 MHz
Medical diagnostic ultrasound: 2.5–10 MHz
|Frequency ||Number of cycles/s (106 cycles/s = 1 MHz) || |
Increasing frequency improves resolution
Higher-frequency transducers (eg, 7.5 MHz) provide better resolution of tissues
|Propagation speed ||Speed with which wave travels through soft tissue (1540 m/s). Propagation speed (determined by density and stiffness of medium) is greater in solids than in liquids and greater in liquids than in gases || |
To image an organ, the ultrasound wave must be emitted from the transducer, travel through a medium (soft tissue or liquid), strike the organ, and bounce back to the transducer. It is the reflected wave that forms the ultrasound image. Ultrasound waves travel better through solids and liquids (molecules are more compact, less interference) than through ...