Background <p>To evaluate MRI-based anatomical differences between symptomatic and asymptomatic sides in patients with unilateral vascular thoracic outlet syndrome (TOS), and to contribute to the literature on these anatomical changes.</p> Methods <p>This retrospective single-center study included 32 patients (27 women, 5 men) with unilateral vascular TOS supported by clinical and supportive imaging findings. MRI examinations were performed in neutral and hyperabduction positions. Measured parameters included the interscalene angle, interscalene distance, anterior scalene muscle thickness, subclavius muscle thickness, first rib angle, costoclavicular distance, pectoralis minor muscle thickness, and pectoralis minor-anterior chest wall angle. The changes after hyperabduction in the angles and distances were calculated. All the measurements were performed by two radiologists and intra- and interobserver reliability was assessed. The measurements were compared between symptomatic and asymptomatic side and between neutral and hyperabduction positions.</p> Results <p>The interscalene distance was significantly shorter on the symptomatic side (<i>p</i> &lt; 0.001). ROC analysis showed the interscalene distance had the best discriminatory ability (AUC, 0.692; <i>p</i> = 0.008). With hyperabduction, a significantly greater reduction in the first rib angle and costoclavicular distance was noted on the symptomatic side; however, the change ratio of the pectoralis minor–chest wall angle was not significantly different. Intra- and interobserver reliability was good to excellent for most parameters.</p> Conclusions <p>MRI-based measurements revealed significant anatomical differences between symptomatic and asymptomatic sides in unilateral vascular TOS. Interscalene distance showed the highest discriminatory value. Side-to-side and positional MRI evaluation may contribute to the assessment of TOS, although larger multicenter studies are needed for further validation.</p>