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Renal pyramids

The renal pyramids (also called medullary pyramids) are cone-shaped structures located in the renal medulla of each kidney. Typically, each kidney contains 8–18 pyramids, depending on size and anatomy. The base of each pyramid faces outward toward the renal cortex, while the apex (renal papilla) points inward toward the renal sinus, projecting into a minor calyx.

Each renal pyramid is composed primarily of straight tubules and collecting ducts, which converge at the papilla to deliver urine into the minor calyces. Between the pyramids are extensions of cortical tissue called renal columns, which separate the medullary regions and contain interlobar vessels.

The pyramids are crucial in the urine concentrating mechanism. Their parallel arrangement of tubules and vasa recta creates the countercurrent exchange system, essential for water and solute reabsorption. The papillae are often perforated by multiple tiny openings (area cribrosa), through which urine drips into the calyceal system.

Synonyms

  • Medullary pyramids

  • Kidney pyramids

  • Pyramids of Malpighi

Function

  • Concentrates and transports urine from nephrons to collecting ducts and renal papillae

  • Forms the countercurrent multiplier system via loops of Henle and vasa recta

  • Drains into minor calyces → major calyces → renal pelvis → ureter

  • Essential in maintaining fluid balance, electrolytes, and osmolality regulation

MRI Appearance

T1-weighted images:

  • Renal pyramids appear as low to intermediate signal intensity compared to the brighter renal cortex

  • Distinction is best in younger patients; cortical-medullary differentiation may reduce with age or pathology

T2-weighted images:

  • Pyramids appear hyperintense relative to cortex, especially when filled with fluid in collecting ducts

  • Useful in detecting hydronephrosis, cystic changes, or medullary sponge kidney

STIR:

  • Suppresses fat signal, making renal pyramids stand out as hyperintense structures relative to surrounding fat planes

  • Accentuates edema, inflammation, or cystic dilatation

T1 Fat-Saturated (Pre-contrast):

  • Pyramids appear as intermediate signal areas, distinguishable from surrounding fat-suppressed cortex

  • Enhances corticomedullary differentiation

T1 Fat-Saturated Post-Contrast (Gadolinium):

  • During corticomedullary phase: cortex enhances rapidly while pyramids remain relatively hypointense

  • During nephrographic phase: pyramids enhance more uniformly, highlighting collecting system anatomy

  • Useful in detecting ischemia, tumors, and scarring

MRI Non-Contrast 3D Imaging:

  • Provides volumetric view of renal architecture and allows segmentation of cortex vs medulla

  • Useful in congenital anomalies, surgical planning, and volumetric renal assessment

CT Appearance

CT Pre-Contrast:

  • Pyramids show slightly higher attenuation than cortex due to higher protein content in medullary tissue

  • Distinct corticomedullary differentiation may be seen

CT Post-Contrast:

  • In corticomedullary phase: cortex enhances earlier and more strongly than pyramids

  • In nephrographic phase: pyramids enhance more evenly, becoming similar in density to cortex

  • In excretory phase: pyramids drain into calyces, and collecting ducts may be outlined by contrast excretion

Clinical Importance

  • Medullary sponge kidney: pyramids show cystic dilatation of collecting ducts, detectable on CT urography

  • Papillary necrosis: ischemic damage at the pyramid tips (papillae), associated with diabetes, analgesic nephropathy, or infection

  • Hydronephrosis: pyramids may be distorted or compressed by dilated calyces

  • Renal tumors: pyramids help differentiate between cortical vs medullary origin of masses

MRI image

Renal pyramids MRI sag image anatomy image -img-00000-00000

CT image

Renal pyramids ct coronal image