Diffusion Tensor Imaging of the Stria Medullaris and Habenula

What is it about?

The Stria Medullaris (SM) is a white-matter tract that contains afferent fibers that connect the cognitive-emotional areas in the forebrain to the Habenula (Hb). The Hb plays an important role in behavioral responses to reward, stress, anxiety, pain, and sleep through its action on neuromodulator systems. The Fasciculus Retroflexus (FR) forms the primary output of the Hb to the midbrain. The SM, Hb, and FR are part of a special pathway between the forebrain and the midbrain known as the Dorsal Diencephalic Conduction system (DDC). Hb dysfunction is accompanied by different types of neuropsychiatric disorders, such as schizophrenia, depression, and Treatment-Resistant Depression (TRD). Due to difficulties in the imaging assessment of the SM and HB in vivo, they had not been a focus of clinical studies until the invention of Diffusion Tensor Imaging (DTI), which has revolutionized the imaging and investigation of the SM and Hb. DTI has facilitated the imaging of the SM and Hb and has provided insights into their properties through the investigation of their monoamine dysregulation. DTI is a well-established technique for mapping brain microstructure and white matter tracts; it provides indirect information about the microstructural architecture and integrity of white matter in vivo, based on water diffusion properties in the intra- and extracellular space, such as Axial Diffusivity (AD), Radial Diffusivity (RD), mean diffusivity, and Fractional Anisotropy (FA). Neurosurgeons have recognized the potential value of DTI in the direct anatomical targeting of the SM and Hb before Deep Brain Stimulation (DBS) surgery for the treatment of certain neuropsychiatric conditions, such as TRD. DTI is the only non-invasive method that offers the possibility of visualization in vivo of the white-matter tracts and nuclei in the human brain. This review study summarizes the use of DTI as a promising new imaging method for accurate identification of the SM and Hb, with special emphasis on direct anatomical targeting of the SM and Hb before DBS surgery.

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Why is it important?

This study has described DTI as a promising imaging method for accurate localization of the SM and Hb. DTI facilitates imaging of the SM and Hb and provides insights into their properties through the investigation of their monoamine dysregulation. DTI has received heightened interest for the stimulation of the limbic system’s white-matter tracts, and a single probabilistic seed point with a local approach is a reliable probabilistic tractography technique for defining the SM and Hb before DBS. Due to the promising results of the application of DTI indirect anatomical targeting, broader targeting of the lateral Hb via the SM in the future could provide a more anatomically favourable target for DBS. DTI represents a new light in the dawn of neuropsychiatric management and research. Enhancement of DTI acquisition techniques by decreasing scanning time, increasing the signal-to-noise ratio, and the implementation of high spatial resolution will assure the utilization of DTI as an accurate diagnostic tool, and it could even be applied in clinical research.

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