Purpose <p>Accurate preoperative localization of parathyroid adenomas is crucial in managing primary hyperparathyroidism (pHPT). Although [¹⁸F]Fluorocholine PET/CT and PET/MR have shown superior sensitivity over conventional imaging, distinguishing upper from lower pole adenomas remains challenging due to the invisibility of the recurrent laryngeal nerve. This study introduces the “tracheal bisecting line” (TBL), a novel anatomical landmark on axial PET/MR images, to aid in polarity differentiation.</p> Methods <p>This retrospective study included 110 patients (128 adenomas) with biochemically confirmed pHPT who underwent [¹⁸F]Fluorocholine PET/MR at the University Hospital Zurich between December 2020 and October 2023. Adenomas were classified as upper (posterior to the TBL), lower (anterior), with mid-TBL cases (neither anterior nor posterior) treated as upper per default, according to evidence from surgical experience. Two board-certified readers independently assessed the images. Surgical findings served as the reference. Inter-reader agreement was evaluated using weighted kappa statistics, and diagnostic accuracy was calculated for all adenomas and subgroups: orthotopic, non-orthotopic, and intrathyroidal.</p> Results <p>Among the 128 adenomas, 109 were orthotopic and 19 non-orthotopic (6 intrathyroidal). The inter-reader agreement was nearly perfect (<i>κ</i> = 0.99). The overall accuracy using the TBL was 0.945. The accuracy improved to 0.982 for orthotopic adenomas but decreased to 0.789 for non-orthotopic and 0.667 for intrathyroidal cases. Excluding intrathyroidal adenomas raised accuracy to 1.0 in non-orthotopic cases and 0.959 overall for non-intrathyroidal adenomas.</p> Conclusions <p>The TBL is a reliable and reproducible landmark on [¹⁸F]Fluorocholine PET/MR for predicting parathyroid adenoma polarity, particularly in orthotopic and non-intrathyroidal cases. It may enhance surgical planning by improving anatomical clarity.</p>