Discussions on basic ultrasound instrumentation have historically focused on proper handling of the transducer and adjustment of controls for image optimization. These will of course be addressed in this book, but more importantly in a text concentrating on interventional ultrasound one must also appreciate the dynamic nature of ultrasound as an instrument for achieving the procedure at hand. Special attention will be given to intraoperative and endoscopic techniques, which are solely the realm of the proceduralist. What follows is simply the fundamentals for performing an ultrasound examination and/or guided procedure; familiarity and experience cannot be overemphasized.
Any ultrasound system has three basic components: a transducer, or probe; the processing unit, including the controls; and the display. The transducer (Figure 2-1) consists of the piezoelectric material (active element) within a nonconductive housing, which may consist of one solitary element or several hundred elements (known as an array). A piezoelectric material has the property of converting mechanical energy (ultrasound vibrations) into electrical energy, and vice versa. In an ultrasound transducer, the electrical excitation impulse is transmitted to the piezoelectric element, and the returning echoes are then converted into electrical signals by the element and transmitted to the processing unit via a shielded cable for further image processing. The thickness of the piezoelectric element determines the center frequency of the transducer (multiple elements allow for a range of frequencies in a single transducer). Backing material absorbs excess vibration of the received signal (dampening) and its impedance should closely match that of the active element to optimize resolution. To minimize energy loss at the face of the transducer, a matching layer is incorporated of a material with an acoustic impedance somewhere in between that of the piezoelectric material and the tissue under investigation (in medical applications). The thickness of this matching layer should be half that of the element so that waves reflected within the matching layer remain in phase when they exit the layer.
Schematic of an ultrasound transducer.
Ultrasound transducers can be divided into two basic types: mechanical and electronic. Mechanical probes contain one or multiple piezoelectric elements that physically oscillate to scan a region of interest, resulting in the classic sector scan image. These are the least expensive type of transducer to manufacture, but scanning and processing options are limited. In the case of radial scanning, such as that employed for endocavity (endorectal, endoanal, and esophageal) applications, rigid mechanical transducers are utilized for a 360 degree view of the luminal structure. They are available in a variety of sizes, frequencies, and lengths (Figure 2-2).
Photograph of various ultrasound transducers, including laparoscopic, endorectal, transcutaneous and intraoperative types.
Electronic transducers, or arrays, employ groups of piezoelectric elements working in concert, and can further be divided into linear, ...