Rectal cancer MRI: protocols, signs and future perspectives radiologists should consider in everyday clinical practice

Magnetic resonance imaging (MRI) represents the first-choice exam for rectal cancer staging. Usually, clinicians require rectal MRI in case of positive colonoscopy for cancer and/or impossibility to endoscopically remove the lesion. In this context, the role of other imaging modalities, such as endorectal ultrasound and computed tomography, has some limitations. Endorectal ultrasound has the advantage, compared to MRI, of being able to differentiate T1-T2 tumours. However, in case of T3-T4 tumours, it is burdened by its limited field of view, which negatively impacts on the assessment of the mesorectal fat involvement and of tumour extension. Moreover, it is a highly operator-dependent method, requiring a not straightforward learning curve to reach an optimal diagnostic accuracy [ 1 ]. Computed tomography offers the advantage of a panoramic view of the rectum with the possibility to benefit from multiplanar reconstructions. It provides appreciable values of accuracy in the staging of high- and middle-rectum cancers [positive predictive value (PPV) and negative predictive value (NPV) of 86% and 94% respectively] but suffers from the low contrast resolution if the tumour is located in the low rectum (PPV and NPV of 53% and 73% respectively) [ 2 ].