Although exposure fold characterization is easier (Fig. 9.11) when using the 3D display, the barriers to 3D evaluation as the primary strategy for review have been its time-consuming and labor-intensive nature (Macari et al. 2000; McFarland et al. 2001).
Two matters underlie its time consuming nature. First, additional processing time is needed in order to create a 3D rendered view. In order to create a 3D view the CT-data files need additional processing by a computer. The speed of this process is dependent on the calculation speed of the used computers, RAM capacity, 3D rendering-interval (increment) along the central path and the resolution of both the raw CT data and the display-method. Improvements in
processing speed have reduced the time needed for this process and in future this process will reduce even further.
Although this processing time is not part of the reviewing time, it results in waiting time before the examination can be evaluated. With 3D methods, processing may not be fully automated and require interaction, which is a disadvantage as compared to 2D methods. Recent data on processing time are sparse; Hoppe et al. (2004) published processing times for the dissected colon method varying from 4.5 to 42 min with an average of nearly 16 min.
Fenlon et al. (1999) reported an average time of 30 min for endoluminal reconstruction. Although the computer details were not specified, it is likely that at least part of this time difference with Hoppe et al. can be attributed to the improved increase of computer power in the more recent study. Introduction of real time image rendering will probably even further reduce this time.
The second cause of its time consuming nature is the extra time needed to examine a colon compared to the 2D method. The cause of this problem lies within the fact that to cover as much colonic surface as possible with conventional 3D methods, evaluation in both antegrade and retrograde direction is mandatory. Consequently, in comparative studies primary conventional 3D evaluation requires an extra 50 to 70% in interpretation time compared to 2D evaluation (McFarland et al. 2001; Iannaccone et al. 2004b).
Studies that used display modes that did not require bidirectional fly-through demanded less extra time or were even faster compared to 2D evaluation. Van Gelder et al. who used an unfolded cube display only used an additional 15% evaluation time compared to 2D evaluation. Hoppe et al. reported an examination time of 20.9 minutes for virtual dissection display, compared to a 2D examination time of 29.2 minutes. Although in this study virtual colon dissection was not feasible in both prone and supine position in 30% of segments, the method did not require bi-directional viewing.
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