MRI anatomical variants of mammillary bodies

The mammillary bodies (MBs) are classically defined as a pair of small round structures located on the undersurface of the diencephalon. The systematic observation of MR brain images of patients with neurological diseases, but also of healthy subjects enrolled in research protocols, reveals, however, a greater anatomical variability. The aim of the present study was to define the spectrum of such variability using spatial normalized 3D TFE T1-weighted MR images in a group of 151 healthy right-handed young subjects (78 females, age range 16-39 years). The MBs were identified on reformatted coronal and axial images and classified according to morphological, positional and numerical criteria. On the basis of coronal images, MBs were first divided into symmetrical (86.1 \%) and asymmetrical (13.9 \%), depending on their respective height. Symmetrical MBs were further subdivided into three variants [type A (2.7 \%), B (76.2 \%), C (7.3 \%)] according to the depth of the intermammillary sulcus. Two morphological variants were defined on axial images, depending on whether the MBs were circular (63.6 \%) or elliptic (36.4 \%). This latter group was further divided in two subgroups, depending on whether the MBs were parallel (21.9 \%) or convergent (14.6 \%). Finally, two subjects (1.3 \%) presented a supernumeral MB. The transverse size of the third ventricle was greater in the type A compared to the type B and C groups. Gender did not significantly affect the frequency of MBs variants, except for the three symmetrical subgroups in which the variants A and C were more frequent in males than in females. These findings suggest the presence of an anatomical variability of the MBs, in contrast to their classical definition. Therefore, atypical presentation of MBs can be the expression of this variability rather than a marker of neurological disorders (i.e. cerebral malformation, mesial temporal sclerosis, Wernicke-Korsakoff syndrome).