Adrenal Gland Anatomy
Overview
The adrenal (suprarenal) glands are paired endocrine organs residing in the superomedial perirenal space, enveloped by Gerota's fascia. They are divided into an outer cortex (steroid hormones) and inner medulla (catecholamines).
Gross Anatomy and Relations
- Right Adrenal: Inverted Y-shape. Wedged between the liver (anterolaterally), IVC (anteromedially), and right diaphragmatic crus (posteriorly).
- Left Adrenal: Crescent-shaped. Lies medial to the left kidney upper pole, behind the splenic artery and body of the pancreas.
- Both glands appear as inverted Y or V shape on axial CT, with a body and two limbs. Normal limb thickness <9-10 mm.
Cortical Zones and Hormones
- Zona Glomerulosa (GFR mnemonic - outermost first): Mineralocorticoids (Aldosterone). Regulated by RAAS.
- Zona Fasciculata (middle): Glucocorticoids (Cortisol). Regulated by ACTH.
- Zona Reticularis (innermost): Sex steroids (DHEA, androgens). Regulated by ACTH.
- Medulla (core): Catecholamines (Adrenaline 80%, Noradrenaline 20%). From chromaffin cells.
WarningAsymmetric Venous Drainage: Surgical Hazard
The right adrenal vein is very short and drains DIRECTLY into the IVC, making right adrenalectomy technically challenging. The left adrenal vein drains into the left renal vein (usually with the inferior phrenic vein). This asymmetry is crucial for adrenal vein sampling in primary hyperaldosteronism.
CT/MRI Imaging Features
- Adenoma: Well-defined, <10 HU on unenhanced CT (lipid-rich). Absolute washout >60% or relative washout >40% on delayed phase.
- Metastasis: Variable enhancement, usually >10 HU unenhanced. Poor washout on delayed imaging.
- Phaeochromocytoma: Characteristically 'lights up' on T2 MRI (T2 hyperintense, 'lightbulb sign'). High uptake on MIBG scan.
High Yield Facts
LightbulbFRCR / MD Prep Pearl
The '10 HU rule': unenhanced CT density <10 HU = adenoma (lipid-rich) with 71% sensitivity, 98% specificity. Chemical-shift MRI (in-phase vs. out-of-phase T1): signal drop on out-of-phase = intracellular fat = adenoma. This is complementary to washout CT.