The Anatomy of the Medial Lemniscus within the Brainstem Demonstrated at 3 Tesla with High Resolution Fat Suppressed T1-Weighted Images and Diffusion Tensor Imaging

The medial lemniscus is part of the main somatosensory pathways ascending within the brainstem. It is formed by the heavily myelinated axons of the second order neurones of the dorsal column nuclei. This pathway ascends through the rostral medulla, pons and mesencephalon to finally terminate by synapsing with third order neurones in the ventral posterior nucleus of the thalamus. The medial lemniscus conveys proprioception and fine tactile discrimination as part of the somatosensory system. Conventional MRI studies of the brainstem have been relatively poor in demonstrating these fibre pathways. Diffusion tensor imaging and tractography may demostrated fibre pathways in the brainstem. These techniques do however suffer from relatively poor spatial resolution and some degree of image distortion – especially if based on echo planar imaging techniques. Knowledge of the anatomical relationships of the medial lemniscus is important for the understanding of clinical manifestations of disease processes affecting the somatosensory pathways and also to demonstrate important adjacent structures. Specifically, the pedunculopontine nucleus (PPN) lies in close anatomical relationship to the medial lemniscus and the decussation of the superior cerebellar peduncle. This nucleus is a promising target for deep brain stimulator placement for alleviation of non-dopamine responsive dystonias. Six healthy male volunteers (mean age 33 years) were imaged at 3 Tesla. Imaging protocols consisted of thin section, high resolution, fat suppressed T1-weighted sequences as well as thin section, high isotropic resolution diffusion tensor imaging (DTI), which was analysed to generate colour fractional anisotropy (FA) maps. These were correlated with the fat suppressed T1 weighted images. In all volunteers the medial lemniscus was seen as a pair of bands of low signal on axial, high resolution, fat suppressed T1-weighted images. They were indentified through the upper medulla, pons and mesencephalon. They correlated well with the head to foot orientated fibres on the colour FA maps generated from the DTI data. This study of normal volunteers has illustrated the value of high resolution, fat suppressed T1-weighted images in demonstrating the anatomy of the heavily myelinated medial lemniscus within the brainstem. These high resolution images with good spatial accuracy can potentially be used to aid the localisation of other nuclei, such as the PPN.

Focal Lesion in Splenium of Corpus Callosum on FLAIR MRI: Common Findings in Aged Patients

Focal high signal intensity in the splenium of the corpus callosum on fluid-attenuated inversion-recovery (FLAIR) images is generally considered an abnormal MR finding. The aim of this study was to determine the frequency of this finding in elderly patients and review the differentiation from other diseases with the similar findings. FLAIR images of 132 patients with suspect CNS disease were retrospectively reviewed. The changes in the splenium of corpus callosum, deep white matter lesions, periventricular matter lesions, infarcts, atrophy and age were analyzed, as well as history. Among the initial 132 patients, focal high signal intensity in the splenium was associated with aging, white matter changes, atrophy, and cognitive disorders. Focal high signal intensity in the splenium of the corpus callosum on FLAIR image is a common finding in elderly patients, especially in aged patients with cognitive disorders. The pathologic alterations were commonly described by the term of "leukoaraiosis". Knowledge of this finding and differentiation from other lesions focusing on the splenium of corpus callosum may help avoid unnecessary invasive diagnostic and therapeutic intervention.

Analysis of the brain-stem white-matter tracts with diffusion tensor imaging

The authors have reviewed the diffusion tensor imaging (DTI) of the brain stem in 19 subjects, consisting of 15 normal volunteers and four multi-system atrophy patients. The study was performed with 1.5 T MRI scanners. DTI was correlated with an automated program allowing superposition of the structural anatomy. Axial, sagittal, and coronal images demonstrated major white-matter fibers within the brain stem, including cortico-spinal tracts, transverse pontine fibers, and medial lemniscus. Smaller fibers, such as medial longitudinal fascicles and central tegmental tracts are difficult to visualize. To identify the anatomical orientation of the brain stem, white-matter fibers will help us understand the different functional disease processes, and DTI will play an important role for the evaluation of the different white matter fibers in the brain stem.