Criteria for Correct Measurement of Polyps
In general, polyps found on CT colonography are divided according to size into small (<6 mm), medium-size (6–9 mm), and large (≥10 mm). A polypoid lesion larger than 3 cm is defined as a colonic mass.
Correct measurement of colorectal polyps on CT colonography is important, because the size of a lesion is related to the risk of cancer as well as many other factors including patient management. This is especially relevant in the Colonography Reporting and Data System (CRADS), where inaccurate measurement near a threshold value, i.e., in a polyp with a diameter of 5–6mm or 9–10 mm, could result in incorrect size classification of the polyp.
Sessile polyps: The greatest diameter of the polyp is measured. This is often at the base of the lesion (Fig. 3.34).
Pedunculated polyps: In a pedunculated polyp, the greatest diameter of the polyp head is measured. The stalk of the polyp is not included in the measurement (Fig. 3.35).
Flat polyps: In flat polyps, along with the greatest diameter, the height of the lesion should also be measured (Fig. 3.36).
Techniques for Measuring Polyps
Depending on the software options available on the workstation, colorectal lesions may be measured manually on 2D or 3D views. In addition to manual measurement, software for automated polyp measurement is also available.
2D Measurement. Two-dimensional axial images and multiplanar reformatting should be used to determine which is the greatest diameter of the polyp. The measurement should then be done on the 2D plane that shows the largest polyp diameter.
If the polyp size is evaluated in only one plane (though it can also happen with multidimensional evaluation), there is a risk that the maximum diameter will not be shown in that plane. This is especially the case for ovoid polyps. There would thus be a risk in 2D measurement of underestimating the size of the polyp (Fig. 3.37).
Window setting is an important factor in measuring polyp size on 2D views. Wide window settings make the lesion appear larger while narrow window settings make it look smaller. The ESGAR consensus statement (Neri et al., 2012) recommends that measurement of polyps be performed on 2D views using wide window settings (e.g., width: 1500, level: –150) (Fig. 3.38).
3D Measurement. Along with 2D measurements, many workstations also allow direct linear measurement of a lesion in 3D views. As in 2D, the greatest diameter is measured (Fig. 3.39).
The data on the accuracy of linear polyp measurements on endoluminal 3D views are contradictory. According to some studies, linear measurement on 3D views is superior to manual 2D measurement, while other authors have reported greater measurement error when using a 3D display to estimate polyp diameter. It may be easier to determine the morphology and the greatest dimension to measure on 3D than on 2D views. However, additional 2D views are required to identify any fecal particles adhering to the polyp, or any lesions that are partially submerged in residual fluid. For such lesions, a 3D measurement would be incorrect. In addition, 3D measurements can be affected by perspective errors related to the position of the virtual camera vis-à-vis the polyp, and for this reason 3D measurements may occasionally be unreliable.