In multidimensional imaging, there are, and will continue to be, situations wherein automatic image segmentation methods fail and extensive user assistance in the process is needed. For such situations, we introduce a novel user-steered image boundary segmentation paradigm under two new methods, live-wire and live-lane. The methods are designed to reduce the time spent by the user in the segmentation process providing tight user control while the process is being executed. The strategy to reach this goal is to exploit the synergy between the superior abilities of human observers (compared to computer algorithms) in boundary recognition and of computer algorithms (compared to human observers) in boundary delineation. We describe evaluation studied to compare the utility of the new methods with that of manual tracing based on speed and repeatability of tracing and on data taken from a large on-going application. We conclude that the new methods are more repeatable and on the average two timers faster than manual tracing. Live-wire and live-lane operate slice-by-slice in their present form. Their 3D and 4D extensions, which we are currently developing, can further reduce the total segmentation time significantly.