直肠腺癌占所有结肠腺癌的约三分之一。直肠MRI已成为评估新诊断的直肠癌患者的必须，因为它可以帮助准确分期疾病，影响选择给予新辅助治疗或继续进行前沿手术，甚至指导手术切除面。更好地了解新辅助化疗放疗对直肠肿瘤的影响以及认识到30％的患者可能出现病理性完全缓解，已为用于直肠腺癌的无手术“观察等待”管理方法打开了大门。符合此器官保留方法的人应在新辅助治疗后的响应评估的所有三个组成部分（即数字直肠检查，内窥镜和直肠MRI）上都没有恶性证据。因此，直肠MRI再次在指导患者管理方面发挥了重要作用，并可能使患者免受不必要的手术发病率。在本文中，作者们讨论了新辅助治疗适应症，描述了完成新辅助治疗后直肠腺癌的预期影像外观，并概述了MRI肿瘤退缩分级系统。由于盆腔侧壁淋巴结清扫与永久性泌尿生殖功能障碍的高风险相关，因此仅为具有侧壁淋巴结影像学证据的患者进行。因此，作者回顾了盆腔的相关淋巴结区，并描述了确定局部区域淋巴结扩散的淋巴结标准。最后，作者讨论了直肠MRI的局限性，描述了新辅助治疗后几个潜在的诠释陷阱，并强调如何避免这些陷阱。© RSNA，2023本文的测验问题可在补充材料中获得。

Rectal adenocarcinoma constitutes about one-third of all colorectal adenocarcinoma cases. Rectal MRI has become mandatory for evaluation of patients newly diagnosed with rectal cancer because it can help accurately stage the disease, impact the choice to give neoadjuvant therapy or proceed with up-front surgery, and even direct surgical dissection planes. Better understanding of neoadjuvant chemoradiotherapy effects on rectal tumors and recognition that up to 30% of patients can have a pathologic complete response have opened the door for the nonsurgical "watch-and-wait" management approach for rectal adenocarcinoma. Candidates for this organ-preserving approach should have no evidence of malignancy on all three components of response assessment after neoadjuvant therapy (ie, digital rectal examination, endoscopy, and rectal MRI). Hence, rectal MRI again has a major role in directing patient management and possibly sparing patients from unnecessary surgical morbidity. In this article, the authors discuss the indications for neoadjuvant therapy in management of patients with rectal adenocarcinoma, describe expected imaging appearances of rectal adenocarcinoma after completion of neoadjuvant therapy, and outline the MRI tumor regression grading system. Since pelvic sidewall lymph node dissection is associated with a high risk of permanent genitourinary dysfunction, it is performed for only selected patients who have radiologic evidence of sidewall lymph node involvement. Therefore, the authors review the relevant lymphatic compartments of the pelvis and describe lymph node criteria for determining locoregional nodal spread. Finally, the authors discuss limitations of rectal MRI, describe several potential interpretation pitfalls after neoadjuvant therapy, and emphasize how these pitfalls may be avoided. © RSNA, 2023 Quiz questions for this article are available in the supplemental material.