Examples of Normal Variants
In this section, several examples of frequent normal variants are presented.
Alteration of the number of vertebrae or ribs is one of the most frequent anomalies. Usually, human beings have seven cervical (C1-C7), 12 thoracic (T1- T12), and five lumbar (L1-L5) vertebrae, as well as 12 pairs of ribs. However, alterations in the total number of vertebrae is as frequent as 8.2%, according to a survey  (Fig. 2.34). Additionally, a transitional vertebra, which has an intermediate shape between a lumbar and a sacral vertebra, is sometimes found. These variants may cause difficulties in the identification and labeling of each vertebra.
There are also variations in position and number of structures, including:
- • Ureteropelvic duplication
- • Abberrant right hepatic artery
- • Pancreas divisum
- • Cavum septum pellucidum
Fig. 2.35 Ureteropelvic duplication (T1-weighted MR image, coronal cross- section)
The occurrence rates of these four variants are reported as 0.5-1% , 1126.5% , 8% , and 3% , respectively (Figs. 2.35,2.36,2.37, and 2.38). Because of these high prevalences, these variants should be considered in modeling of the corresponding organs. However, it is difficult to represent both normal and variant subjects with a single model, because these variants accompany alterations in topologies of the organs (i.e., the ureter, the arterial system, the pancreatic duct, and the brain ventricles).
The branching patterns of vessels may also vary widely. The larger arteries and veins may have normal variants with topology changes. Note that such normal variants sometimes become a problem in surgical operations; thus it may have some clinical impact.