Rectal and anal endosonography.

Rectal and anal endosonography Conventional ultrasound using an external transducer on the body wall has an important limitation. A balance has to be struck between the better resolution of a high frequency transducer and its focal length. To obtain really good images the transducer has to be as close to the organ of interest as possible. The idea of using an ultrasound transducer inside a body cavity, now known as endosonography, was first proposed and explored by Wild nearly 30 years ago.' Wild and Foderick were the first to image bowel wall and a rectal cancer using this technique. Prostatic endosonography was developed first, followed in the early 1980s by rectal wall scanning.2 Although there has been increasing interest over the past decade, with the publication ofoutstanding images of normal anatomy, tumours, lymph nodes, and local recurrences and a series of excellent reports comparing the accuracy of endosonography with computed tomography, it remains the preserve of a minority of enthusiasts. The equipment for anorectal endosonography is less expensive than the flexible endoscope mounted systems used in the upper gastrointestinal tract to image oesophagus, stomach, and pancreas.3 Most of the pioneering work in the field has been carried out using a rigid rotating 7 mHz transducer which provides a continuous series of cross-sections through the rectum and pelvis.4 The transducer is surrounded by a water filled balloon to give good acoustic contact with the gut wall, although occasionally soft villous polyps are better imaged by filling the rectum with fluid so that they are not compressed by the balloon. The transducer is best placed in position through a rectoscope, and a good rectal preparation with a disposable enema is essential. The examination takes only 10 to 15 minutes and sedation is not usually necessary, though for some patients with painful tumours intravenous pethidine and midazolam is a good substitute for an examination under anaesthesia. The normal ultrasound anatomy of the rectal wall has been established by laboratory and clinical studies,5 and the appearances are very similar to those observed in the stomach and duodenum.3 The principal technical problems are tight stenotic tumours, small cancers, and tumours at 15 cm or higher. A plastic cone placed over the transducer can be used for strictures and imaging the anal canal, but a view of only the lower end of a bulky stenotic cancer may be all that is possible. The major applications of …