Abstract: Objective To explore the diagnostic value of multimodal magnetic resonance imaging (MRI) combined with tumor markers ［carbohydrate Antigen 19-9 (CA19-9), cancer antigen 72-4 (CA72-4), carcinoembryonic antigen (CEA), alpha-fetoprotein (AFP)］ in the preoperative T staging and extramural venous invasion (EMVI) of rectal cancer. 
Methods A retrospective analysis was conducted on 136 patients with rectal cancer treated at the Third People′s Hospital of Chengdu City. Using postoperative pathological staging as the gold standard, all patients underwent multimodal MRI and levels of CA19-9, CA72-4, CEA, and AFP were detected. Based on results of pathological diagnosis, patients were divided into positive EMVI group (n=58) and negative EMVI group (n=78). MRI results and levels of CA19-9, CA72-4, CEA, and AFP in the two groups were compared, and the diagnostic efficacy of multimodal MRI combined with these tumor markers in preoperative T staging and EMVI was analyzed using receiver operating characteristic (ROC) curves. 
Results Preoperative T staging showed 12 patients in T1, 22 in T2, 68 in T3, and 34 in T4. By multimodal MRI, 8 patients were diagnosed as T1, 16 as T2, 59 as T3, and 29 as T4. The levels of CA19-9 (43.14±4.28） kU/L, CA72-4 (12.88±2.69） kU/L, CEA (23.11±2.89) μg/L, and AFP (21.25±2.64) μg/L in T3-T4 were higher than those in T1-T2 (10.33±2.03) kU/L, (6.12±1.47) kU/L, (13.15±3.24) μg/L, (16.25±3.27) μg/L (P＜0.05). Positive EMVI group had higher Ktrans, Kep, and lower ADC compared with the negative EMVI group; The levels of CA19-9 (55.87±5.63) kU/L, CA72-4 (14.92±3.12) kU/L, and CEA (14.89±2.14) μg/L were higher than those in the negative EMVI group (9.42±2.01) kU/L, (7.21±1.54) kU/L, (7.84±1.56) μg/L (P＜0.05). ROC analysis showed that multimodal MRI combined with tumor markers in EMVI diagnosis had a sensitivity of 93.1% and a specificity of 97.4%, with an area under the ROC curve (AUC) of 0.975 (95%CI: 0.946-1.000), which was higher than single-item detection. 
Conclusion Multimodal MRI combined with CA19-9, CA72-4, CEA, and AFP levels demonstrates high sensitivity and specificity in the diagnosis of EMVI of rectal cancer, significantly outperforming detection by each indicator alone. It provides a more accurate diagnostic basis for the preoperative assessment of rectal cancer. 

Key words: rectal neoplasms, magnetic resonance imaging, tumor markers