## 4 Discussion  
This paper provides a synthesis of the current knowledge on the DTI and tractography-derived structural connectivity of the SN and its relevance to the understanding of neurological and neuropsychiatric disorders. The tractographic reconstructions confirm that the SN is not merely part of the classical motor circuits but rather represents a complex integrative node that links motor, cognitive, and affective brain regions. This perspective extends conventional neuroanatomical frameworks and possesses extensive clinical ramifications, which are elaborated upon in subsequent sections. The nigrostriatal pathway has received the most research attention of all SN connections. The degeneration of this pathway is the central pathophysiological mechanism of PD. As [PERSON] et al. (2015) [62] confirmed, an analysis of the available data indicates a close correlation between the diffusion parameters of the pathway in question and the motor deficits in question. A number of studies have indicated the possibility that DTI may assist in the early detection of [PERSON]'s disease and the subsequent monitoring of its progression [12]. The utilization of DTI markers (e.g., increased diffusivity in SN) is also being explored to distinguish idiopathic PN from atypical Parkinsonian syndromes [12]. [PERSON] and [PERSON] (2022) [76] measured two key parameters from diffusion tensor tractography: fractional anisotropy and tract volume for both the ipsilesional nigrostriatal tract and the corticospinal tract. The results showed that the tract volume of the ipsilesional nigrostriatal tract was strongly positively correlated with the Motricity Index score, which is a measure of motor function in patients who suffered putaminal hemorrhage. Similarly, both the fractional anisotropy and tract volume of the ipsilesional corticospinal tract had strong positive correlations with the Motricity Index score. This means that as the integrity and fiber amount (volume) of these tracts increased, the motor function of the patients improved. [PERSON] et al. (2021) [77] used a tractography technique that extracts directional diffusion information in brain white matter using diffusion tensor imaging (DTI). Tractography reconstructs the corticospinal tract (CST), a major nerve fiber bundle responsible for motion control. This study used an automated workflow to extract the FA (fractional anisotropy) values within the reconstructed CST volume while ensuring that anatomically unrealistic fibers and noise were minimized. The study found that the mean FA in the CST was around 0.4406 and identified a lower cutoff of 0.15 as a reliable threshold for neurosurgical tractography, with infiltrated or compressed CSTs requiring consideration of an even lower cutoff (around 0.1) to accurately depict affected fibers [77]. In addition to diagnostics, structural connectivity of the SN is also studied in the context of therapy; for example, changes in diffusion parameters following the deployment of dopaminergic therapy or in the context of deep brain stimulation may reflect circuit remodeling. The capacity to quantify degenerative changes in the SN-striatum circuit furnishes objective markers for clinical assessment of patient status and treatment efficacy. This renders DTI and other advanced methods a valuable tool not only for research but potentially for clinical use. While degeneration of the nigrostriatal pathway leads to the classical motor symptoms of PN, abnormalities in other SN pathways, particularly in SNr-thalamic wiring, may underlie the complex motor phenotypes of other disorders. For instance, in cases of dystonia, creation of a lesion (i.e., pallidotomy) or high-frequency stimulation of the GPi have been observed to result in the alleviation of symptoms, which may be attributable to the reorganization of the connections between the GPi, SNr, and thalamus.