CTMRedit (affectionately known as MRCAT, or 'Mr. Cat") is a software tool for displaying, editing, and three-dimensional reconstruction of MR and CT images. This paper describes the incremental evolution of MRCAT during a period in which it was used regularly by the two software designers, and by two medical experts naive with respect to the software design. Three methods were found to be useful for suggesting changes to the software: use of the software, and conversations with other users, were helpful for identifying useful new functionality, while direct observation of naive users was most useful for identifying problems with the existing user interface.

As expected, naive users are generally most efficient when presented with a familiar-looking user interface, e.g. radiologists are familiar with film, so images on screen should be smoothly interpolated so that they look like film. An unexpected finding is that not all of the software implementation details should be hidden. Implementation details which the user must know, e.g. organization of an image database, should be displayed prominently in the user interface, so that they will quickly become familiar to the user.