How Far Have We Come With Ultrasound Miniprobes?

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Abstract
Endoluminal ultrasonography incorporates the basic principles of physics, the technical accomplishments of modern engineering, the behavior of biological tissue, and the art of medicine. One of the inescapable physical properties of ultrasound is that while a higher frequency produces higher image resolution, it is at the cost of reduced tissue penetration. Correspondingly, medical applications depend on the frequency of the imaging instrument. Low frequencies are required for imaging large or distant lesions, whereas high frequencies are better suited for identifying subtle changes in small nearby lesions. The physical properties of the crystals used for transducers are such that small transducers can only produce high frequencies. Miniprobes are ultrasound catheters with diameters of approximately 2 - 3 mm, designed to pass through the accessory channel of a diagnostic endoscope. They generally incorporate transducers with frequencies of 12 MHz or higher. In contrast, echo endoscopes are usually operated at a frequency of 7.5 MHz. Not only is the imaging depth and resolution of these two types of endoluminal instrument different, but the focal length of miniprobes is typically in the 5 - 10 mm range, whereas the focal length of echo endoscopes is in the 20 - 30 mm range. Thus, although there is overlap, the medical applications of miniprobes differ from those of their counterpart, the echo endoscope. Endoscopic ultrasonography (EUS) using echo endoscopes is a well-established part of gastrointestinal endoscopic practice. However, EUS is one of the most difficult and cumbersome endoscopic procedures, since the instruments are relatively heavy and their tip is long and rigid. When small lesions are scanned with echo endoscopes, it is difficult to ascertain whether the ultrasound transducer is in the plane of the lesion. Perhaps the most significant limitation of these instruments is the fact that the examination of stenotic malignancies is often incomplete. The performance of dilation in order to complete the examination is not without risk [[ 1 ]]. Miniprobes are an attractive and technically easy to use alternative to echo endoscopes. Miniprobes can be positioned near small lesions, and the lesions can be imaged with great confidence. The probes can also be used to traverse and image stenotic lesions. Mechanical as well as phased-array scanning probes that scan in either the linear or the radial direction have been developed. Most reports have used rotational mechanical sector-scanning miniprobes. Applications of miniprobes can be placed in three categories: a) unique applications; b) applications in which miniprobes are preferable to echo endoscopes; and c) applications in which miniprobes are an alternative to echo endoscopes.

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