New morphologic variants of the hand motor cortex as seen with MR imaging in a large study population

Abstract

Background and purpose: The hand motor cortex (HMC) has been classically described as having an omega or epsilon shape in axial-plane images obtained with CT and MR imaging. The aim of this study was to use MR imaging and Talairach normalization in a large sample population that was homogeneous for age and handedness to evaluate in a sex model a new classification with 5 morphologic variants of the HMC in the axial plane (omega, medially asymmetric epsilon, epsilon, laterally asymmetric epsilon, and null).

Materials and methods: Structural brain MR images were obtained from 257 right-handed healthy subjects (143 men and 114 women; mean age, 23.1 +/- 1.1 years) via a Talairach space transformed 3D magnetization-prepared rapid acquisition of gradient echo sequence. The frequencies of the different HMC variants were reported for hemisphere and sex.

Results: The new variants of the HMC (medially asymmetric epsilon, laterally asymmetric epsilon, and null) were observed in 2.9%, 7.0%, and 1.8% of the hemispheres, respectively. Statistically significant sex differences were observed: The epsilon variant was twice as frequent in men, and an interhemispheric concordance for morphologic variants was observed only for women.

Conclusion: The large study population permitted the description of a new morphologic classification that included 3 new variants of the HMC. This new morphologic classification should facilitate the identification of the precentral gyrus in subsequent studies and in everyday practice.

Fig 1.

Schematic representation of the criteria used for morphologic classification and corresponding MR image of a typical example. The position of the third fissure, which segments the knob, modifying its appearance from an omega to an epsilon in the lateral, central, or medial part of the hand knob indicates a medially asymmetric epsilon, epsilon, and laterally asymmetric epsilon, respectively. To distinguish an omega from a null, the height of the knob must be greater than the thickness of the precentral gyrus measured at the base of the knob. If the height is smaller than the thickness, the HMC is classified as null. Multiplanar reformatted MR imaging axial sections were obtained from left hemispheres 50 ± 2 mm above the Talairach ACPC plane. The red area in the MR images highlights the morphologic variant.

Fig 2.

Multiplanar reformatted axial sections obtained 50 ± 2 mm above the Talairach anterior AC-PC plane. Images are shown using the right-left radiologic convention. The following are representative examples of the different combinations of HMC morphologic variants (red highlight) in the 2 hemispheres: A, Omega-omega. B, Epsilon-epsilon. C, Medially asymmetric epsilon-medially asymmetric epsilon. D, Laterally asymmetric epsilon-laterally asymmetric epsilon. E, Null-null. F, Epsilon-omega. G, Omega-laterally asymmetric epsilon. H, Omega-epsilon. I, Omega-medially asymmetric epsilon. J, Medially asymmetric epsilon-omega. The first 5 indicate concordant combinations, and the latter 5, the most common disconcordant combinations.