High Resolution Rectal MRI

Role in Rectal Cancer

High-resolution rectal MRI is central to rectal cancer evaluation because of its exquisite depiction of the relationship between the primary tumor and pelvic structures. It enables:

1. Accurate T and N staging
2. Assessment of MRF involvement (predicting CRM status)
3. Identification of EMVI, tumor deposits, and LPLN metastasis
4. Surgical planning (TME vs APR, sphincter-saving feasibility)
5. Determining need for nCRT or TNT

Key Sequences

• High-resolution T2-weighted images are the backbone of staging
• Multiplanar acquisition (axial oblique perpendicular to tumor, sagittal, coronal)
• Axial oblique images perpendicular to the rectal lumen at the tumor level are essential for MRF assessment

What MRI Must Assess

Assessment	Key Landmark/Criterion
Tumor location	Relationship to APR, anorectal ring, STO
T stage	Depth of invasion through muscularis propria
MRF involvement	<1 mm from MRF at tumor base
Peritoneal involvement	T4a if tumor penetrates peritoneal surface
EMVI	Tubular intermediate signal in expanded vessels
LPLN	Short-axis size in internal iliac and obturator compartments
Sphincter involvement	Relationship to puborectalis, internal/external sphincter
Distance from anal verge	Plus landmark-based location

Impact on Outcomes

• MRI + TME + nCRT → LR rate <5%
• MERCURY study: NPV 94-98.1% for excluding MRF involvement
• PPV 86.5% for MRF involvement
• 1 mm cutoff for MRF: increasing to 5 mm reduced accuracy (more false positives)

Reporting Recommendations

Per SAR/ESGAR, MRI reports should include:

• Distance from anal verge
• Relationship to anorectal ring (puborectalis)
• Relationship to APR
• Relationship to STO
• MRF status at tumor base
• EMVI status
• LPLN assessment (for tumors below APR)
• Sphincter involvement (for low tumors)

Related

• Rectal Cancer Staging Overview
• Mesorectal Fascia
• Rectal Cancer T Staging
• Lateral Pelvic Lymph Nodes