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Mesorectum

The mesorectum is a fatty connective tissue envelope that surrounds the rectum, enclosed by the mesorectal fascia. It is an important anatomical and oncological structure, as it contains lymph nodes, blood vessels, and connective tissue that are essential in the spread and staging of rectal cancer. Complete removal of the mesorectum during total mesorectal excision (TME) is the gold standard in rectal cancer surgery, making its anatomy crucial for surgeons and radiologists.

Synonyms

  • Perirectal fat and lymphatic envelope

  • Mesorectal tissue

  • Rectal mesentery

Boundaries

  • Anterior: Denonvilliers’ fascia (separating rectum from prostate/seminal vesicles in males, vagina in females)

  • Posterior: Presacral fascia and sacrum

  • Lateral: Mesorectal fascia on both sides

  • Superior: Continuous with sigmoid mesocolon

  • Inferior: Tapers and blends with pelvic floor musculature at the level of the anorectal junction

Contents

  • Perirectal fat

  • Mesorectal lymph nodes and lymphatic vessels

  • Branches of the superior rectal artery and vein

  • Nerve fibers from the pelvic autonomic plexus

  • Connective tissue framework surrounding the rectum

Relations

  • Lies within the pelvis, surrounding the rectum

  • Bordered laterally by pelvic sidewall and iliac vessels

  • In close relation to pelvic organs (prostate, seminal vesicles, uterus, vagina) through fascial planes

Function

  • Provides a vascular and lymphatic supply pathway to the rectum

  • Supports the rectum within the pelvis

  • Acts as a conduit for tumor spread, making it critical in staging and surgical oncology

  • Serves as a protective cushion between rectum and adjacent pelvic structures

Clinical Significance

  • Crucial in rectal cancer staging: tumor involvement of mesorectum or its fascia changes treatment and prognosis

  • Removal during total mesorectal excision (TME) reduces recurrence rates

  • Involved in infections, abscesses, and inflammatory processes (e.g., diverticulitis complications)

  • Important surgical landmark in pelvic dissections

MRI Appearance

T1-weighted images:

  • Mesorectal fat appears with high signal intensity

  • Lymph nodes appear as low-to-intermediate signal within fat

  • Mesorectal fascia appears as a thin low-signal line encasing the mesorectum

T2-weighted images:

  • Mesorectal fat shows high signal

  • Mesorectal fascia seen as a distinct low-signal linear boundary

  • Tumor extension into mesorectal fat appears as intermediate-to-high signal intensity disrupting normal fat

STIR (Short Tau Inversion Recovery):

  • Fat suppressed and becomes dark

  • Pathological changes such as edema, inflammation, or tumor infiltration appear bright within the mesorectal compartment

T1 Fat-Sat Post-Contrast:

  • Normal fat remains suppressed

  • Lymph nodes enhance homogeneously

  • Tumors show irregular or heterogeneous enhancement

  • Abscesses may show rim enhancement with central non-enhancing cavity

CT Appearance

Non-Contrast CT:

  • Mesorectum appears as fat-density tissue surrounding the rectum

  • Vessels and lymph nodes are seen as soft tissue densities within fat

  • Fascia may not always be visible without contrast

Post-Contrast CT:

  • Mesorectal vessels enhance normally

  • Lymph nodes enhance mildly and uniformly

  • Tumor infiltration or inflammatory changes appear as stranding or irregular soft tissue within mesorectal fat

  • Abscess shows low-density center with rim enhancement

MRI image

Mesorectum MRI axial image

MRI image

Mesorectum MRI sagittal image

CT image

Mesorectum ct axial anatomy image-img-00000-00000