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Genitourinary/Musculoskeletal

Pelvis Anatomy

The pelvis contains the bladder, reproductive organs, rectum, pelvic floor musculature, and iliac vasculature within a bony ring of ilium, ischium, pubis, and sacrum. MRI provides superior soft-tissue contrast for gynecologic, prostatic, and rectal disease, while CT surveys bone, vessels, and nodes. Recognizing peritoneal reflections, fascial planes, and pelvic floor anatomy guides staging and surgical planning.

Key Structures

  • Bony pelvis: ilium, ischium, pubis fused at the acetabulum; sacrum and coccyx; sacroiliac and pubic symphysis joints
  • Bladder and ureters: distensible bladder with trigone at the base; ureters entering posterolaterally at the ureterovesical junctions
  • Female organs: uterus (zonal anatomy on T2: endometrium, junctional zone, myometrium), cervix, vagina, ovaries with follicles
  • Male organs: prostate zonal anatomy (peripheral, transition, central zones), seminal vesicles, and vasa deferentia
  • Rectum and anal canal: mesorectum enclosed by mesorectal fascia; levator ani and the anal sphincter complex
  • Pelvic floor: levator ani (puborectalis, pubococcygeus, iliococcygeus) and the urogenital diaphragm supporting viscera
  • Vasculature and nodes: common iliac dividing into external/internal iliac vessels; obturator, external iliac, and internal iliac nodal chains

Imaging Planes & Modalities

  • MRI multiplanar T2 is the workhorse: sagittal for uterus/bladder/rectum, oblique planes tailored to the cervix, prostate, and rectal tumor axis
  • Prostate mpMRI: T2, DWI/ADC, and dynamic contrast per PI-RADS for cancer detection and localization
  • Rectal cancer MRI: high-resolution T2 perpendicular to the tumor for T-staging and mesorectal fascia relationship
  • CT with contrast for nodal/vascular survey, bony detail, and acute processes; CT urography for the collecting system
  • Ultrasound (transabdominal/transvaginal/transrectal) for ovaries, endometrium, and prostate guidance
  • Dynamic/defecography MRI for pelvic floor dysfunction and prolapse

Landmarks & Normal Values

  • Endometrial thickness thresholds guide postmenopausal bleeding workup (commonly ≤4-5 mm reassuring)
  • Normal junctional zone <5-8 mm on T2 (thickening suggests adenomyosis)
  • Pelvic lymph nodes generally considered suspicious above ~8-10 mm short axis (site-dependent)
  • Mesorectal fascia is the surgical circumferential resection margin; tumor distance to it drives staging
  • Prostate volume from ellipsoid formula; PSA density derived from volume
  • Pubococcygeal and H/M lines used to grade pelvic organ prolapse on dynamic imaging

Anatomical Variants

  • Mullerian duct anomalies (septate, bicornuate, unicornuate, didelphys uterus) with associated renal anomalies
  • Ovarian position variation and physiologic functional cysts/corpus luteum
  • Prostatic median lobe hypertrophy protruding into the bladder
  • Duplicated or ectopic ureters and ureteroceles
  • Variant internal iliac branching and gonadal vein drainage
  • Sacral variants (transitional segments, sacral meningeal/Tarlov cysts)

Pathology Correlation

  • Uterine zonal anatomy distinguishes adenomyosis, fibroid location, and depth of endometrial/cervical cancer invasion
  • Prostate zonal anatomy: most cancers arise in the peripheral zone, whereas BPH arises in the transition zone
  • Mesorectal fascia and sphincter anatomy determine rectal cancer resectability and the need for neoadjuvant therapy
  • Nodal chains follow drainage pathways relevant to gynecologic, prostatic, and rectal cancer staging
  • Pelvic floor anatomy underlies prolapse, incontinence, and obstructed defecation
  • Peritoneal reflections and spaces predict spread of endometriosis, abscess, and carcinomatosis

Pitfalls & Mimics

  • Underdistended bladder mimics wall thickening or masses; ensure adequate filling
  • Bowel peristalsis and gas artifact degrade pelvic MRI; antispasmodics and technique mitigate this
  • Physiologic ovarian cysts and hemorrhagic corpus luteum mistaken for neoplasm
  • Adenomyosis mimicking a fibroid and vice versa on ultrasound
  • Post-treatment fibrosis versus residual/recurrent tumor after chemoradiation
  • Free pelvic fluid is often physiologic in premenopausal women and should not be overcalled

References

  • Hamm, Forstner. MRI and CT of the Female Pelvis
  • Weissleder et al. Primer of Diagnostic Imaging
  • PI-RADS v2.1 Steering Committee Guidelines
  • Netter. Atlas of Human Anatomy

⚕️ Educational reference — correlate with clinical context and confirm with a qualified radiologist.