## 1 Introduction  
Diffusion tensor imaging (DTI) is a diffusion-weighted magnetic resonance imaging (MRI) technique that quantifies the diffusivity, its direction, and anisotropy of water diffusion in voxels [1, 2, 3, 4, 5, 6]. DTI can therefore be used for the reconstruction of the primary fiber direction within the white matter [7, 8, 9, 10]. The calculation of parameters such as fractional anisotropy, axial, radial, and mean diffusivity [11, 12, 13, 14] enables further description of microstructural properties of white matter tracts within the voxel, implying their potential damage or reorganization [1, 15, 16, 17, 18, 13]. Fractional anisotropy (FA) quantifies the degree of directional dependence of water diffusion in biological tissue by measuring the variance of the diffusion tensor's eigenvalues normalized to its trace [5, 12, 14, 19]. Meandiffusivity (MD) is defined as the arithmetic mean of the diffusion tensor's eigenvalues, providing an index of the overall magnitude of water diffusion independent of its directional bias [12; 14; 19]. Building on these principles, tractography may be used to reconstruct entire white matter tracts [2; 19; 20; 21]. In deterministic tractography, the algorithm invariably follows paths of greatest diffusivity creating streamlines, theoretically reconstructing the underlying tracts [3; 11; 19; 20]. Conversely, probabilistic tractography randomly samples the direction according to a distribution function and generates a multitude of possible trajectories [3; 8; 19; 20; 21]. However, tractography has limitations. It cannot determine the direction of the connection (afferent or efferent) [12; 22; 23], it has certain restrictions when it comes to investigating white matter (WM) areas with various fiber orientations and numerous associative fiber tracts [24; 25], and can produce both false-positive and false-negative associations, complicating the interpretation of results [26; 27]. The substantia nigra (SN) and the ventral tegmental area (VTA) are two key dopaminergic midbrain structures that play a crucial role in basal ganglia (BG) circuits [28; 29; 30; 31; 32]. The SN primarily projects to the putamen via the nigrostriatal pathway (connections from the SN pars compacta to the dorsal striatum) [28; 33; 34; 35; 36] (see Figure 1A). The VTA, in contrast, has diverse neuronal populations that project to various forebrain regions, including the nucleus accumbens and amygdala [29; 37; 38; 39; 40; 41]. The SN has traditionally been divided into the dopaminergic part (the pars compacta (SNc)) and the GABAergic part (the pars reticulata (SNr)) [29; 31; 42; 43; 44; 45]. The SNe projects to the striatum in order to modulate movement, while the SNr receives inputs from the striatum and the subthalamic nucleus, and sends outputs to the thalamus (nucleus ventralis anterior and lateralis) [28; 34; 46; 47] (see Figure 1B). Tractography has facilitated non-invasive mapping of connections [48; 49; 50; 51; 52]. Recent studies have confirmed the presence of connections between the SN and various cortical areas, with the prefrontal, motor, and somatosensory cortices being of note [7; 34; 53]. The present findings suggest that the SN is not merely an isolated component of the basal ganglia but rather is involved in broader brain networks [7; 34; 36; 54; 55; 56]. This is of crucial importance for understanding the pathophysiology of neurological and psychiatric disorders involving the SN [7; 34]. However, despite the growing number of studies focusing on individual SN connections, a comprehensive review that systematically maps individual pathways, their anatomy, functional significance, and clinical implications is lacking. The extant literature is characterized by a preponderance of isolated descriptions of individual connections. The objective of this review is to provide an overview of the current knowledge on SN structural connectivity, with a particular emphasis on its functionally specialized projections.