Anal canal cancer protocol (MRI)

MRI protocol for anal canal cancer is a group of MRI sequences put together to asses extension and stage anal canal tumors.

Note: This article is intended to outline some general principles of protocol design. The specifics will vary depending on MRI hardware and software, radiologist's and referrer's preference, institutional protocols, patient factors (e.g. allergy) and time constraints. 

No special patient preparation is usually needed for the anal canal MRI (cf. other bowel MRI protocols). An endorectal coil should not be used as it can be really painful due to friable tumor mucosa ⁵. 

Sequences

A good protocol for this purpose would include:

• T1 weighted
  • plane: axial whole pelvis
  • sequence: fast-spin echo (T1 FSE) or gradient (T1 MPRAGE)
  • purpose: anatomical overview (some institutions replace this sequence for T2 weighted images of the whole pelvis)
• T2 weighted
  • plane: 
    • axial whole pelvis
    • anal canal centered high-spatial-resolution images (small FOV) on three planes ²
      • axial: perpendicular to the long axis of the anal canal
      • coronal: parallel to the long axis of the anal canal
      • sagittal
  • sequence: T2 FSE, high-resolution T2-weighted sequences with the small FOV images
  • purpose:
    • ​whole pelvis: anatomical overview 
    • small FOV: delineation of anal canal layers allowing evaluation of tumor depth (hyperintense submucosa, hypointense muscularis propria, and hypointense mesorectal fascia) ²
      • coronal images aid in assessing for extension along with the internal and external sphincters
• diffusion-weighted imaging (DWI)
  • plane: axial
  • sequence: DWI: B=0, B=1000 and ADC 
  • purpose: evaluation of the tumor cellularity; assessment of tumor recurrence after treatment ⁴ 
• postcontrast sequences
  • most institutions do not include post-contrast images on the assessment of anal canal tumors as it is unlikely to give additional information to the high-resolution T2 images ^2-5