Adrenal Imaging

Adrenal Gland 101 - Anatomy & Scan Slam

• Anatomy: Retroperitoneal. Right: pyramidal/triangular. Left: crescentic/semilunar.
  • Outer Cortex (steroids), inner Medulla (catecholamines).
• Imaging Essentials:
  • CT: Primary tool. Non-contrast (NCCT) for Hounsfield Unit (HU) density assessment.
  • MRI: Problem-solving. Chemical Shift Imaging (CSI) detects intracellular lipid in adenomas (signal drop on out-of-phase sequences).
  • USG: Useful in neonates; limited utility in adults.

⭐ An adrenal mass with ≤ 10 HU on unenhanced CT strongly suggests a benign lipid-rich adenoma.## Adrenal Gland 101 - Anatomy & Scan Slam

• Anatomy: Retroperitoneal. Right: pyramidal/triangular. Left: crescentic/semilunar.
  • Outer Cortex (steroids), inner Medulla (catecholamines).
• Imaging Essentials:
  • CT: Primary tool. Non-contrast (NCCT) for Hounsfield Unit (HU) density assessment.
  • MRI: Problem-solving. Chemical Shift Imaging (CSI) detects intracellular lipid in adenomas (signal drop on out-of-phase sequences).
  • USG: Useful in neonates; limited utility in adults. (image)[c7343c32-63da-460f-bf96-e938ecb1d778]

⭐ An adrenal mass with ≤ 10 HU on unenhanced CT strongly suggests a benign lipid-rich adenoma.

Incidentaloma Insights - Spotting the Suspects

Adrenal mass >1 cm, found incidentally. Goal: Benign vs. Malignant/Functional.

• Benign Adenoma (Lipid-Rich):
  • NCCT: Homogeneous, ≤10 HU.
  • CECT Washout: APW >60%, RPW >40%.
    • $ \text{APW} = \frac{(\text{PV} - \text{D})}{(\text{PV} - \text{U})} \times 100 $
    • $ \text{RPW} = \frac{(\text{PV} - \text{D})}{\text{PV}} \times 100 $ (U: Unenhanced, PV: Portal Venous, D: Delayed)
  • MRI (Chemical Shift): Signal drop on opposed-phase (intracellular lipid).
• Suspicious Features (Malignancy/Pheochromocytoma - PCC):
  • Size >4 cm (↑ ACC risk).
  • NCCT: >20 HU (PCC/Mets often >30-40 HU).
  • Heterogeneous, irregular, necrosis, calcifications (coarse/central).
  • Poor washout. Invasion. Rapid growth.
  • PCC: T2 MRI "light bulb" sign.

⭐ In cancer patients, new adrenal mass = metastasis until proven otherwise.

Biochemical screen (PCC, Cushing's, Conn's) vital.

Benign Buddies - Common Adrenal Finds

• Adrenal Adenoma: Most common benign adrenal tumor.

  • NCCT: Homogeneous, round/oval, attenuation ≤10 HU (lipid-rich).
  • Contrast CT Washout (15 min delay):
    • Absolute (APW): $((HU_{peak} - HU_{delayed}) / (HU_{peak} - HU_{unenhanced})) \times 100%$. Threshold: >60%.
    • Relative (RPW): $((HU_{peak} - HU_{delayed}) / HU_{peak}) \times 100%$. Threshold: >40%.
  • MRI: Signal drop on out-of-phase chemical shift imaging (intracellular lipid).

• Adrenal Myelolipoma: Contains mature fat & hematopoietic tissue.

  • CT: Macroscopic fat (attenuation -30 to -100 HU). Often heterogeneous.

• Adrenal Cysts: Typically simple; endothelial, pseudocysts, or epithelial.

  • CT/US: Well-defined, thin-walled, anechoic/water attenuation. No enhancement.

• Adrenal Hemorrhage: Causes include trauma, stress, anticoagulation.

  • CT: Acute phase: hyperdense. Chronic phase: may become hypodense, cystic, or calcify.

⭐ On non-contrast CT, an adrenal mass with attenuation ≤10 HU is highly suggestive of a benign lipid-rich adenoma, a key differentiator from most malignant lesions or pheochromocytomas (which are typically >10 HU).

Malignant Mayhem & Mimics - Critical Adrenal Calls

• Adrenocortical Carcinoma (ACC):
  • Typically large (>4-6 cm), irregular margins, heterogeneous.
  • Central necrosis, hemorrhage, calcifications common.
  • Risk of local invasion (e.g., kidney, liver), IVC thrombus.
  • CT: Poor absolute washout (<60%), relative washout (<40%).

  ⭐ ACCs are typically large (>4 cm, often >6 cm) at diagnosis, with heterogeneous enhancement and central necrosis/hemorrhage.

• Metastases:
  • Most common adrenal malignancy; often bilateral.
  • Primaries: Lung, breast, kidney, melanoma, lymphoma.
  • Typically round/oval, variable enhancement; PET avid.
• Pheochromocytoma (Malignant potential ~10-15%):
  • MRI: Classic T2 "lightbulb" sign (very hyperintense).
  • Avid, often heterogeneous enhancement; cystic changes common.
  • MIBG scan positive (SPECT/CT for localization).
  • 📌 Rule of 10s (simplified): 10% bilateral, 10% extra-adrenal, 10-15% malignant.
• Adrenal Lymphoma (Primary):
  • Rare; often bilateral, diffuse enlargement, homogenous.
  • May encase vessels without significant compression.

High‑Yield Points - ⚡ Biggest Takeaways

• Adrenal adenomas: Typically <10 HU (unenhanced CT); signal drop on out-of-phase MRI; washout >60% (absolute).
• Pheochromocytoma: "10% tumor"; classically T2 hyperintense ("light bulb sign") on MRI.
• Adrenocortical carcinoma (ACC): Large, heterogeneous, invasive, with necrosis, calcification, hemorrhage.
• Adrenal metastases: Often bilateral; common primaries: lung, breast, melanoma, kidney.
• Myelolipoma: Benign tumor with macroscopic fat (negative HU values on CT).
• Waterhouse-Friderichsen syndrome: Bilateral adrenal hemorrhage, due to meningococcal sepsis.