expended in constructing a three-dimensional image in the individual's mind and more effort is channeled into learning the anatomical relationships.

A second model is a rudimentary simulator for the abdomen created by Satava (1993b). With this simulator, one can travel from the esophagus throughout the intestine, taking side trips through the biliary system and the pancreas. It is a unique instructional tool that describes anatomy from the inside of the intestines rather than from the outside. It is of considerable benefit in training individuals to perform colonoscopy and esophago-gastroduodenoscopy, as well as teaching students the true anatomic relationships of intraabdominal structures. Basically, one is able to fly around various organs and experience their actual relationships—the model provides the learner with the ability to interdigitate between organs and behind them without destroying their relationships to one another in the process.

Another educational tool is an augmented-reality system that allows the user to see virtual information superimposed over real structures. See-through displays provide the user with a view of the surrounding environment, along with an image displayed on goggles. Investigators in Boston and at the University of North Carolina (UNC) have created see-through displays using computer-assisted tomography (CT) scan, magnetic resonance imaging (MRI), or ultrasound technology as imaging techniques (Bajura et al., 1992).

The work on augmented reality at UNC (Bajura et al., 1992) is based on the images from an ultrasound that delineates abdominal structures in three dimensions. Specifically, the investigators created a graphic of a three-dimensional model and projected it through the head-mounted display (HMD), as an overlay onto the user's view of the abdomen. This program, used on a pregnant woman, allows the operator to "open a window" into the abdomen and view the fetus in a three-dimensional manner without incising the skin. Although the application of such programs to view a developing fetus is limited, the technology raises the possibility of visualizing other intraabdominal structures.

See-through models can be used to teach surgeons where an organ is located and show its relation to surrounding tissues. Novice surgeons often have difficulty visualizing the location of the gallbladder and the cystic duct in relation to the common bile duct. Despite extensive anatomy instruction, the first few operations are difficult because structures in a living body appear different from the illustrations in an anatomical atlas. A see-through display gives surgeons in training an opportunity to develop their own internal three-dimensional map of living organs, rather than having to operate without one.

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