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Genitourinary

Urinary System Anatomy

The urinary system comprises the retroperitoneal kidneys and adrenal glands, the ureters, and the pelvic bladder and urethra, evaluated primarily with multiphase CT urography, MRI, and ultrasound. The perirenal and pararenal spaces bounded by Gerota fascia govern the spread of hemorrhage, infection, and tumor. Understanding corticomedullary architecture, collecting-system anatomy, and vascular supply is fundamental to diagnosing obstruction, stones, and neoplasia.

Key Structures

  • Kidneys: cortex and medullary pyramids showing corticomedullary differentiation on the nephrographic sequence; renal sinus fat, calyces, infundibula, and renal pelvis
  • Collecting system: minor calyces coalesce into major calyces, renal pelvis, and ureter; opacified on the excretory phase
  • Ureters: cross the iliac vessels at the pelvic brim; three physiologic narrowings (ureteropelvic junction, pelvic brim, ureterovesical junction)
  • Bladder: dome, body, trigone (between ureteric orifices and internal urethral meatus), and neck; normal distended wall <3 mm
  • Urethra: male (prostatic, membranous, bulbar, penile) and shorter female urethra
  • Adrenal glands: inverted-Y/V shape, limbs <10 mm thick, suprarenal in the perirenal space
  • Renal vasculature: single renal artery/vein at the hilum; the left renal vein crosses anterior to the aorta beneath the SMA

Imaging Planes & Modalities

  • CT urography with unenhanced, nephrographic (~100 s), and excretory (delayed) phases evaluates stones, renal masses, and the urothelium; unenhanced phase detects calculi and characterizes hemorrhage
  • Multiphase CT (corticomedullary, nephrographic, excretory) characterizes renal masses and defines vascular anatomy for nephrectomy planning
  • Ultrasound assesses hydronephrosis, renal size/echogenicity, and bladder volume, and screens for cysts and stones; adds resistive index
  • MRI (T1/T2, chemical-shift for adrenal, dynamic post-gadolinium) characterizes indeterminate renal and adrenal masses and is used in renal impairment
  • CT dedicated adrenal protocol (unenhanced HU with washout) differentiates adenoma from metastasis
  • Non-contrast low-dose CT is the reference standard for urolithiasis; MR urography avoids radiation in young patients

Landmarks & Normal Values

  • Normal renal length 9-12 cm; adrenal limb thickness <10 mm (cross-sectional diameter <1 cm)
  • Adrenal adenoma: unenhanced attenuation <10 HU (lipid-rich) or absolute washout >60% / relative washout >40%
  • Perirenal space bounded by anterior and posterior Gerota (renal) fascia, between anterior and posterior pararenal spaces
  • Ureter narrows at the UPJ, pelvic brim (crossing iliac vessels), and UVJ—common sites of stone impaction
  • Bladder wall <3 mm when adequately distended; post-void residual normally minimal

Anatomical Variants

  • Horseshoe kidney with lower-pole fusion and isthmus anterior to the aorta, associated with UPJ obstruction and stones
  • Duplicated collecting system with the Weigert-Meyer rule (upper-pole ureter inserts inferomedially/ectopically, lower pole prone to reflux)
  • Malrotation, pelvic/ectopic kidney, and crossed fused ectopia
  • Accessory/multiple renal arteries and early branching—relevant before donor nephrectomy or endovascular repair
  • Retroaortic or circumaortic left renal vein; extrarenal pelvis mimicking hydronephrosis
  • Column of Bertin (hypertrophied cortical tissue) simulating a renal mass

Pathology Correlation

  • Obstructing ureterolithiasis: hydroureteronephrosis, perinephric stranding, and the soft-tissue rim sign around the calculus
  • Renal cell carcinoma: enhancing solid mass; assess venous invasion, perinephric spread, and nodal disease for staging
  • Pyelonephritis shows a striated nephrogram and wedge-shaped hypoenhancement; abscess and emphysematous infection are complications
  • Transitional (urothelial) carcinoma appears as an enhancing filling defect or focal urothelial thickening—evaluate the whole urothelium on the excretory phase
  • Adrenal incidentaloma workup: lipid-rich adenoma vs pheochromocytoma vs metastasis using HU and washout
  • Bladder cancer as focal wall thickening or a papillary enhancing mass; assess muscle invasion and perivesical spread

Pitfalls & Mimics

  • Column of Bertin, prominent renal hilar lip, and fetal lobulation mimic renal masses—confirm with enhancement pattern
  • Pseudoenhancement of renal cysts on CT (adjacent to enhancing parenchyma) can falsely suggest a solid lesion
  • Extrarenal pelvis and parapelvic cysts can be mistaken for hydronephrosis—look for calyceal dilatation
  • Phleboliths in the pelvis mimic distal ureteric stones; the comet-tail sign and lack of rim sign help distinguish
  • Do not overlook subtle urothelial thickening or a small TCC on the excretory phase; evaluate both kidneys and the entire ureter
  • Adrenal collision tumors and hemorrhage can confound washout characterization—correlate with unenhanced attenuation and history

References

  • Dyer RB, et al. Genitourinary Radiology. Springer
  • Zagoria RJ, et al. Genitourinary Imaging: The Requisites. Elsevier
  • Silverman SG, et al. ACR/Bosniak classification and renal mass reporting (ACR guidance)
  • Federle MP, et al. Diagnostic Imaging: Genitourinary. Elsevier

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