## 7 Implications for User Performance Studies  
The field of advanced three-dimensional visualization systems is large and growing. Different visualization approaches are suggested to meet the demand for the qualitative three-dimensional presentation of complex medical information, and these approaches offer benefits in terms of improved ergonomics for diagnostic radiology. Specific user performance studies allow the estimation of the usability of new visualization systems and define the requirements for the adoption and eventual transition to the new visualization paradigm. However, it is often difficult to recruit participants to be involved in the study, because professionals do not have time to participate as experimental observers. The previous user studies employed residents [39; 45] or analyzed the results of radiologists and residents together [2; 14; 18] to investigate visual search through medical images and test the application of new workstations for professional purposes. Similar to the case of performance assessment using multi-slice images [43], the results of our study highlighted that the performance accuracy can be similar, however, the image manipulation strategies differ significantly depending on the amount of radiologist experience. Because image manipulation strategies are important, to be carefully investigated, aiming to make appropriate navigation systems for three-dimensional visualization tools in diagnostic radiology, user performance assessment should be conducted, involving experienced radiologists in order to obtain reliable and acceptable results. It is important to note that, in comparison to multi-slice images traditionally presented on flat-panel displays, the three-dimensional visualization of information returns radiologists an opportunity to benefit from the global search. In our study, radiologists largely examined the entire three-dimensional images without image manipulation, however, they changed their search strategy and navigated actively through images when the target conspicuity was reduced. Active navigation through images is characteristic to the local search. In the human-centered design of a three-dimensional visualization system, one should consider the alternation of global and local search when radiologists read medical images [41], and ensure flexible and meaningful navigation through the rendered images, which will facilitate both modes of search. As an alternative to traditionally used navigation devices, a gaming controller has been proposed to facilitate visual search in the three-dimensional environment [1; 16]. The practicality of a new navigation device has not yet been reported and should be assessed using quantitative and qualitative measures. In addition to global measures, such as the time and accuracy of search performance, local measures, such as oculomotor behavior, should be further analyzed to describe search patterns and cognitive processes during visual search in detail. In terms of qualitative assessment, as professionals and novices are likely to verbalize less than intermediates [45], we consider a survey based on recent findings about main viewing preferences and manipulations to be the most appropriate method for collecting and analyzing user feedback in the future study. In addition to the ergonomic design of navigation, the high quality of rendered images is critical to ensure the usability and acceptance of new three-dimensional visualization methods. Accurate, high-quality rendering of multi-slice images is a challenge to work on when planning the implementation of three-dimensional visualization systems in radiology. Namely, approximately one hundred image slices should be condensed to be presented on the limited number of display planes which are located at different positions within the physical space of an optical element. This can be done by defining image boundaries to be visualized and then applying volume rendering [20].