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Ureteropelvic junction

The ureteropelvic junction (UPJ) is the anatomical region where the renal pelvis transitions into the proximal ureter. Located at the hilum of the kidney, it represents the first natural narrowing of the ureter. The UPJ is approximately 1–2 cm long and is surrounded by smooth muscle fibers that regulate urine flow from the renal pelvis into the ureter.

The UPJ is clinically significant because it is the most common site of congenital ureteral obstruction and also a frequent location for impacted kidney stones. Congenital UPJ obstruction is often due to abnormal muscular development, high insertion of the ureter, or crossing vessels compressing the junction. Acquired causes include scarring, strictures, stones, and tumors.

Synonyms

  • Renal pelvis–ureter junction

  • UPJ

  • Pyeloureteric junction

Function

  • Acts as the funnel between renal pelvis and ureter, ensuring unidirectional urine flow

  • Functions as a physiological narrowing that prevents retrograde flow

  • Plays a regulatory role in renal drainage dynamics

  • Clinical landmark for stone impaction, obstruction, and surgical interventions (e.g., pyeloplasty)

Associated Conditions

  • UPJ obstruction (congenital or acquired): causes hydronephrosis and impaired renal drainage

  • Urolithiasis: kidney stones frequently lodge at the UPJ

  • Crossing vessels: accessory lower pole renal arteries or veins can externally compress the UPJ

  • Tumors and strictures: less common but important causes of narrowing

MRI Appearance

T1-weighted images:

  • UPJ appears as a low-to-intermediate signal tubular structure continuous with renal pelvis and proximal ureter

  • Distended renal pelvis in obstruction may appear hypointense due to urine signal

T2-weighted images:

  • Urine-filled renal pelvis and ureter appear bright hyperintense, sharply outlining the UPJ

  • Narrowing, stricture, or obstructed UPJ appears as a transition zone between dilated pelvis and collapsed ureter

STIR:

  • Suppresses fat, making the UPJ and surrounding edema or inflammation stand out as hyperintense areas

  • Useful in evaluating periureteric inflammation or fibrosis

T1 Fat-Saturated (Pre-contrast):

  • UPJ lumen shows intermediate signal against suppressed perirenal fat

  • Helps in delineating ureteral wall from adjacent tissues

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Normal ureteral wall enhances thinly and uniformly

  • In obstruction or stricture, there may be delayed or irregular enhancement

  • Demonstrates vascular crossing vessels compressing the UPJ

MR Urography (3D):

  • Non-contrast MR urography using heavily T2-weighted sequences shows dilated renal pelvis and ureter with narrowed UPJ

  • Contrast-enhanced MR urography provides functional assessment of urine flow and excretion

  • Highly valuable in pediatric imaging where radiation avoidance is crucial

CT Appearance

CT Pre-Contrast:

  • UPJ seen as a soft-tissue density between renal pelvis and ureter

  • Non-contrast CT is most useful for detecting stones lodged at the UPJ

CT Post-Contrast:

  • Urographic phase highlights the UPJ as contrast flows from renal pelvis into ureter

  • Narrowing, obstruction, or delayed excretion indicates pathology

CT Urography (CTV):

  • Gold standard for structural evaluation

  • Provides high-resolution multiplanar and 3D reconstructions showing renal pelvis, UPJ, and ureter course

  • Detects UPJ obstruction, strictures, crossing vessels, or anomalies

  • Essential for preoperative planning in pyeloplasty, stone removal, or tumor resection

MRI images

ureteropelvic junction MRI coronal image anatomy image -img-00000-00000

MRI images

ureteropelvic junction MRI sag image anatomy image -img-00000-00000

CT images

ureteropelvic junction axial ct image

CT images

ureteropelvic junction CT coronal image