First and second year — the floor first, then every step
Unenhanced ≤10 HU for a lipid-rich adenoma; negative HU for a myelolipoma. Contrast density is not a native test.
Orient first
Place the ROI on the NON-CONTRAST series, covering at least two-thirds of the lesion, away from the edge and away from any macroscopic-fat pocket if you are asking the adenoma question.
≤10 HU unenhanced is the conventional lipid-rich adenoma threshold. Negative HU is macroscopic fat and is a myelolipoma, not an adenoma.
If only a contrast-enhanced series exists, native density is not assessable — say so, and name unenhanced CT or chemical-shift MRI as the next test rather than inventing a washout.
Acquire the study
Unenhanced series first. If density is above 10 HU and the lesion is not already macroscopic fat, a dedicated adrenal washout protocol (unenhanced, portal venous, delayed) is the next CT step.
Do not calculate washout unless those phases were actually acquired.
The manoeuvre
Confirm both limbs of both adrenals and that the mass arises from one of them, not from the kidney or the tail of pancreas.
Place an ROI on the NON-CONTRAST series. If the mean is negative, the tissue is macroscopic fat — myelolipoma, not adenoma.
If the mean is 10 HU or less (and not negative), call a lipid-rich adenoma.
If the mean is above 10 HU, say that washout or chemical-shift MRI is the next characterisation, not that adenoma is excluded.
What confirms it
Unenhanced density of 10 HU or less, or opposed-phase signal drop with an India-ink outline, characterises a lipid-rich adenoma.
Macroscopic fat (negative HU, or T1-bright tissue that suppresses on fat saturation) in an adrenal mass characterises a myelolipoma.
What licenses you to exclude it
A density above 10 HU does NOT exclude adenoma — lipid-poor adenomas exist and need washout or chemical-shift imaging, not a "not adenoma" report.
Absence of visible macroscopic fat does not exclude a collision lesion or a myelolipoma whose fat is occult. Growth, heterogeneity or a non-fat soft-tissue nodule still needs a second look.
The classic misread
Applying 10 HU to a post-contrast number.
An edge ROI that averaged in retroperitoneal fat and manufactured a false adenoma or a false myelolipoma.
Reference values
Each value carries the caveat that keeps it from being misused. Normal limits and diagnostic criteria are kept apart on purpose: a disease cut-off read as a normal range is the more dangerous mistake.
Diagnostic criteria
Adrenal nodule · Absolute and relative washout
adenoma suggested by absolute washout above 60% or relative washout above 40%
⚠️ THE WASHOUT IS INVALID IF THE TIMING IS WRONG, and the delay differs between departments. A non-contrast attenuation of 10 HU or less already indicates a lipid-rich adenoma and makes washout unnecessary. Washout does not reliably separate adenoma from phaeochromocytoma.
an unenhanced attenuation of 10 HU or less characterises a lipid-rich adenoma
About 30% of adenomas are lipid-poor and measure above 10 HU — those need washout or chemical-shift MRI (see the washout entry). The threshold applies to a homogeneous nodule with a representative ROI, not to a heterogeneous or haemorrhagic lesion. Versioned criterion — verify against the current edition before clinical use.
CT
Adrenal myelolipoma · Macroscopic fat as the diagnostic feature
a named diagnosis: an adrenal mass containing macroscopic fat (see the fat-HU entry) is a myelolipoma until a collision tumour is suspected
Fat is the diagnosis, not a size cut-off. Haemorrhage can mask fat; a myelolipoma that is growing or atypical still needs a second look for a collision lesion. Versioned criterion — verify against the current edition before clinical use.
CT · MRI
See it on real cases
Direct links to Radiopaedia — the reference article and worked cases with their images. Each opens on Radiopaedia.