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.

Pelvis Anatomy

🧩 Key Structures

🎞️ Imaging Planes & Modalities

📏 Landmarks & Normal Values

🔀 Anatomical Variants

🧠 Pathology Correlation

💡 Pitfalls & Mimics