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Renal medulla

The renal medulla is the innermost part of the kidney, lying between the renal cortex and the renal sinus. It consists of 8–18 renal pyramids, each tapering to an apex known as the renal papilla, which projects into a minor calyx. The medulla contains the loops of Henle, vasa recta, and collecting ducts, making it the functional core of the kidney’s concentrating and diluting mechanisms.

The medulla is separated into an outer medulla (containing proximal straight tubules and thick ascending limbs) and an inner medulla (containing thin limbs of Henle’s loops, collecting ducts, and papillae). Its unique architecture, countercurrent exchange system, and high interstitial osmolality are critical for water reabsorption and urine concentration.

Clinically, the renal medulla is a central site for conditions such as renal papillary necrosis, medullary sponge kidney, medullary cystic disease, renal medullary carcinoma, and ischemic injury. Its high metabolic demand and relatively low oxygen supply make it vulnerable to hypoxia and ischemia.

Synonyms

  • Medulla renalis

  • Inner kidney zone

  • Renal pyramids

Function

  • Concentration of urine: via countercurrent multiplier in Henle’s loop and countercurrent exchange in vasa recta

  • Water and electrolyte balance: collecting ducts regulate reabsorption under influence of ADH and aldosterone

  • Acid-base balance: intercalated cells in collecting ducts secrete hydrogen and bicarbonate

  • Excretion of solutes: fine-tunes sodium, potassium, and urea handling

  • Urine collection: funnels urine toward papillae → calyces → renal pelvis → ureter

MRI Appearance

T1-weighted images:

  • Renal medulla appears hypointense relative to renal cortex, reflecting lower protein and fat content

  • Differentiation between cortex and medulla is best in neonates, reduced in adults

T2-weighted images:

  • Medulla appears hyperintense relative to cortex because of higher water content

  • Renal pyramids stand out as bright triangular structures

STIR:

  • Suppresses fat and highlights medullary water content

  • Useful in detecting edema, inflammation, or papillary necrosis

T1 Fat-Saturated (Pre-contrast):

  • Medulla remains hypointense relative to cortex, contrast with surrounding fat enhanced by suppression

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • Corticomedullary differentiation is accentuated in the corticomedullary phase (30–70 sec post-injection)

  • Medulla enhances later than cortex, becoming more visible in the nephrographic phase

  • Delayed phase shows excretion of contrast into collecting system

MRI Non-Contrast 3D Imaging:

  • Provides volumetric analysis of medullary pyramids

  • Useful for detecting cysts, tumors, and medullary sponge kidney

CT Appearance

CT Pre-Contrast:

  • Medulla shows slightly lower attenuation than cortex

  • Pyramids appear hypodense, especially in excretory phase

CT Post-Contrast:

  • In corticomedullary phase, cortex enhances intensely, medulla lags (creating corticomedullary differentiation)

  • In nephrographic phase, cortex and medulla equilibrate, medulla becomes clearly defined

  • Excretory phase shows contrast outlining collecting ducts and calyces

CT Urography:

  • Demonstrates medullary anatomy and papillary impressions

  • Useful in diagnosing papillary necrosis, stones, or medullary cysts

CT image

Renal medulla CT coronal image

MRI image

Renal medulla MRI coronal image anatomy image -img-00000-00000