Three-dimensional display is an integral part of reviewing of CT-colonography examinations and most likely a prerequisite for good sensitivity. The method can be used either primary or as adjunct to 2D evaluation. In populations with a high polyp prevalence the use of primary 3D does not seem advantageous compared to primary 2D, although comparative data are sparse and no optimized 3D methods have been used. The use of the later methods might prove to be valuable. The question which review method should be used as a screening method for colorectal cancer (low polyp prevalence) is a major issue. At the time of writing of this chapter no strong evidence for either primary 2D or primary 3D is available. Though, a higher sensitivity in polyp detection with primary 3D may make this review technique for the purpose of screening more appropriate. This question will be one of the main topics of research in CT colonography in the next couple of years.
An important topic in CT colonography is the reduction of ionizing radiation in CT colonography. This topic becomes of particular interest when CT
colonography is used as a potential screening tool in the prevention of colorectal cancer. Although the imaging of structures with a high contrast difference (polyps vs air or tagged stool) allows a higher noise level (resulting from lower radiation exposure), noise related artifacts arise. This noise in the data can be counteracted by smoothing the images by using a smooth reconstruction filter. Although the benefit of these filters is reduction of the noise level, this is at the expense of image resolution.
Two-dimensional images seem less affected by this noise than 3D images since noise on a 3D endo-luminal image appears as floating endoluminal debris that obscures the intraluminal anatomy and as coarsened mucosal texture that makes the detection of small or flat polyps difficult (Johnson and Dachman, 2000). However, in a experimental setting the radiation dose in a 3D setting can be reduced to very low levels without negative effect on sensitivity for large polyps (van Gelder et al. 2004d).
An important matter that may influence the discussion of 2D vs 3D is the use of computer aided diagnosis (CAD), as discussed in the following chapter. Currently, CAD is still in an experimental stage, but it most likely becomes implemented in the coming years. CAD has the potential to increase diagnostic performance, to reduce inter-reader variability and/ or reduce reader time. At present it is not known which role CAD will play, e.g. as first reader or as second reader. The place of the CAD algorithm in the reading of CT-colonography examinations most likely will influence the use of either primary 2D or primary 3D. One can envision that a CAD algorithm will present polyp candidates in a 3D display with either 2D (axial or MPR) or attenuation display in color for verification of heterogeneity.
Frans M. Vos is acknowledged for his comments.
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