  Immersive Colonography allows medical professionals to navigate inside the
intricate tubular geometries of subject-specific 3D colon images using Virtual
Reality displays. Typically, camera travel is performed via Fly-Through or
Fly-Over techniques that enable semi-automatic traveling through a constrained,
well-defined path at user controlled speeds. However, Fly-Through is known to
limit the visibility of lesions located behind or inside haustral folds, while
Fly-Over requires splitting the entire colon visualization into two specific
halves. In this paper, we study the effect of immersive Fly-Through and
Fly-Over techniques on lesion detection, and introduce a camera travel
technique that maintains a fixed camera orientation throughout the entire
medial axis path. While these techniques have been studied in non-VR desktop
environments, their performance is yet not well understood in VR setups. We
performed a comparative study to ascertain which camera travel technique is
more appropriate for constrained path navigation in Immersive Colonography. To
this end, we asked 18 participants to navigate inside a 3D colon to find
specific marks. Our results suggest that the Fly-Over technique may lead to
enhanced lesion detection at the cost of higher task completion times, while
the Fly-Through method may offer a more balanced trade-off between both speed
and effectiveness, whereas the fixed camera orientation technique provided
seemingly inferior performance results. Our study further provides design
guidelines and informs future work.
