Incidental adrenal nodule — characterisation

ACR AC Adrenal Mass Evaluation 2021; ACR incidental adrenal white paper; ESE/ENSAT adrenal incidentaloma guideline

The unenhanced attenuation does most of the work and comes first. A homogeneous nodule at or below about 10 HU is a lipid-rich adenoma and the study can stop there; only above that threshold does the washout protocol have anything to add — and washout cannot be calculated at all without the unenhanced number.

Adrenal nodule found on imaging performed for another reason, or an adrenal nodule in a patient with known or suspected malignancy where adenoma versus metastasis changes management.

Referenced decision support — confirm against your local protocol.

Decision support, not a directive. Protocols and timings shown are typical published ones — your local protocol takes precedence, and the vetting radiologist decides.

The request in front of you

Everything is optional. Leave a field alone and the answer assumes nothing — the verdict updates as you go.

Study requested

What the referrer actually asked for. It is evidence of intent, not a constraint on the right answer.

Contrast as written

What the request form says, not what it should say.

Age

Pick a band, or type an exact age if it matters.

years

Pregnancy status

eGFR

Only needed where the pathway itself changes with renal function — not as a routine gate.

Renal risk factors

The question a vetter can actually answer from the request. An explicit “none known” is a real answer, and it removes checks rather than deferring them.

Previous contrast reaction

The pathway — tap anything already done

Marking a study complete moves the answer on. A patient arrives partway through a pathway far more often than at the start of one.

Accept as requested

CT Adrenal — Washout Protocol

CT Adrenal Washout

What we'd amend, and why

The protocol is sequential and the first acquisition is the decisive one. Intracytoplasmic lipid lowers unenhanced attenuation, so a homogeneous nodule at or below about 10 HU is diagnostic of a lipid-rich adenoma and the remaining acquisitions can be abandoned — dose saved by a decision, not by a compromise. Above that threshold, the discriminator becomes the rate at which contrast leaves the lesion: adenomas de-enhance rapidly and metastases hold contrast, and the percentage washout is computed from all three attenuations using the same region of interest. Without the unenhanced value the absolute calculation is impossible, which is why an enhanced-only study cannot be salvaged into an answer.

Nothing needs resolving before this goes ahead. Routine checks below are owned downstream.

How was this decided?

pathwaystandard — Adults — adrenal nodule characterisation
rulerule-contrast-reaction-premed — Prior contrast reaction and elective premedication; checked by Nurse before the scan
rulerule-metformin — Metformin and iodinated contrast; checked by Radiographer at the scanner
rulerule-paeds-dose — Child-sized technique and contrast dose; checked by Radiographer at the scanner
rulerule-pregnancy-ionising — Pregnancy status before an ionising exposure; checked by Radiographer at the scanner
rulerule-renal-iodinated — Kidney function and intravenous iodinated contrast; checked by Radiographer at the scanner
rulerule-iv-access — Intravenous access adequate for the planned injection; checked by Radiographer at the scanner

Decision support only. Local protocol takes precedence.

Handled at the scanner(4)nothing for you to do

Settled and owned downstream. Each returns to a radiologist only on the stated trigger.

Metformin and iodinated contrast
Confirm whether the patient takes metformin or a metformin-containing combination, and if so whether ACR Category II applies (eGFR below 30, known or suspected AKI, or an arterial catheter study likely to cause renal embolisation). If Category I — that is, no AKI and eGFR at or above 30 — no action of any kind is needed.
Radiographer at the scannerAt the scanner
Flags back if: The patient takes metformin AND meets ACR Category II — eGFR below 30 mL/min/1.73 m2, known or suspected acute kidney injury, or an arterial catheter procedure with likely renal arterial embolisation. Metformin plus a normal or mildly reduced eGFR is explicitly NOT a flag-back: there is no need to stop metformin before or after intravenous iodinated contrast in Category I patients, and no need to re-check creatinine afterwards.
Child-sized technique and contrast dose
Confirm that a size- or weight-based protocol is selected — child-sized kV and mAs against size-based diagnostic reference ranges — and that contrast volume is calculated by weight rather than taken from an adult default. Weight-based iodinated contrast volumes of roughly 1.5–2.0 mL/kg are widely used in paediatric CT.
Radiographer at the scannerAt the scanner
Flags back if: No paediatric or size-based protocol exists on the scanner for the requested examination, or the requested coverage or number of phases exceeds what the clinical question needs — for example a multiphase study where a single phase answers it, or whole-body coverage for a focal question.
Pregnancy status before an ionising exposure
Make the pregnancy enquiry immediately before the exposure and record the answer. In the UK this is a statutory operator duty discharged at the time of exposure under the employer’s written procedures required by IR(ME)R 2017 — it is not something the vetting radiologist can perform or pre-empt, and a request is complete without it.
Radiographer at the scannerAt the scanner
Flags back if: The patient states that she is, or may be, pregnant AND the uterus is in or near the primary beam. The exposure is then paused for re-justification by the IR(ME)R practitioner before it proceeds.
Intravenous access adequate for the planned injection
Site and test a cannula that supports the protocol flow rate, preferring an antecubital or large forearm vein, and observe the injection for extravasation. A 20-gauge or larger cannula is preferred for flow rates of 3 mL/s or more.
Radiographer at the scannerAt the scanner
Flags back if: No cannula can be sited that supports the protocol flow rate — for example only a 22-gauge hand or foot cannula for a CT angiogram needing 4–5 mL/s; or the only available access is a central line or port that is not labelled power-injectable; or an extravasation occurs.

Pathways

Big forks are separate pathways; the first whose conditions match is the one used.

eGFR under 30 or on dialysis — the contrast-free route answers the same question

Role	Study & protocol	Why this answers the question
First line	MRI Adrenal MRI Adrenal — chemical shift usually appropriate	Adrenal characterisation is one of the few problems where the unenhanced study is not a compromise. Chemical-shift imaging finds the same intracytoplasmic lipid as the unenhanced attenuation measurement, by a different physical mechanism: fat and water protons within a voxel cancel on the opposed-phase echo, so an adenoma loses signal relative to the in-phase image. It uses no iodine and no ionising radiation, so severe renal impairment reorders the ladder here rather than merely annotating it, and this is the study to book first — a request naming it in this patient is a correct request and should be accepted as written rather than redirected to a three-acquisition iodinated washout. Both echoes must come from a single breath-hold or the regions of interest no longer correspond; echo times and the quantitative thresholds are field-strength specific and do not transfer between 1.5 T and 3 T. The honest limit is that it identifies the lipid-RICH adenoma, so a nodule that does not lose signal remains indeterminate — and at that point the washout CT is a legitimate study, weighed against the renal risk as a considered decision rather than taken as the opening move. Gadolinium is not part of this protocol and its absence is not a limitation.
Second line	CT Adrenal Washout CT Adrenal — Washout Protocol	The washout protocol remains available and is the better-validated of the two, so this is a second-line study rather than a forbidden one — the point is the order, not a prohibition. It is the right answer for the nodule that did not lose signal on opposed-phase imaging, and for the patient in whom MRI is undeliverable. Where it is done, the ACR–NKF position applies: the association between modern contrast media and acute kidney injury is far weaker than was once assumed, the decision is a considered one rather than a refusal, and the unenhanced acquisition is still the decisive one — a contrast-only study cannot be salvaged into an absolute washout value at any eGFR.
Problem solving	FDG PET-CT FDG PET-CT — skull base to mid-thigh	Reserved for the nodule that remains indeterminate after dedicated characterisation in a patient with known malignancy, where the practical question is whether this single lesion converts the disease to metastatic and changes treatment intent.

Adults — adrenal nodule characterisation

Matches your inputsDefault

Role	Study & protocol	Why this answers the question
First line	CT Adrenal Washout CT Adrenal — Washout Protocol usually appropriate	The protocol is sequential and the first acquisition is the decisive one. Intracytoplasmic lipid lowers unenhanced attenuation, so a homogeneous nodule at or below about 10 HU is diagnostic of a lipid-rich adenoma and the remaining acquisitions can be abandoned — dose saved by a decision, not by a compromise. Above that threshold, the discriminator becomes the rate at which contrast leaves the lesion: adenomas de-enhance rapidly and metastases hold contrast, and the percentage washout is computed from all three attenuations using the same region of interest. Without the unenhanced value the absolute calculation is impossible, which is why an enhanced-only study cannot be salvaged into an answer.
First line	MRI Adrenal MRI Adrenal — chemical shift	Chemical-shift imaging detects the same intracytoplasmic lipid by a different physical mechanism: fat and water protons within a voxel cancel on the opposed-phase echo, so an adenoma loses signal relative to the in-phase image. It requires no contrast at all, which is why it is a co-equal opening study rather than a fallback, and why it is the better one in renal impairment, in pregnancy, in a prior severe iodinated reaction, and in younger patients facing repeated follow-up — the same answer with no iodine and no dose. A request for it in a patient with a low eGFR is a correct request and should be accepted as written rather than redirected to a three-acquisition iodinated washout. Both echoes must come from a single breath-hold or the regions of interest no longer correspond and the calculated index is meaningless; echo times and the quantitative thresholds are field-strength specific and do not transfer between 1.5 T and 3 T. The honest limit is that this identifies the lipid-RICH adenoma, so a nodule that does not lose signal is still indeterminate and the washout study is what follows.
Problem solving	FDG PET-CT FDG PET-CT — skull base to mid-thigh	Reserved for the nodule that remains indeterminate after dedicated characterisation in a patient with known malignancy, where the practical question is whether this single lesion converts the disease to metastatic and changes treatment intent.

Pitfalls

Requesting the washout protocol when the nodule was already under 10 HU unenhanced. That is a completed diagnosis and the extra acquisitions are pure dose.
Attempting washout on a study that had no unenhanced series. Only the relative calculation is then available and it is the weaker of the two.
Different breath-holds between the three acquisitions, so a different part of the nodule is measured each time — the commonest source of a spurious washout value.
Applying adenoma washout thresholds to a lesion that is haemorrhagic, heterogeneous, or has features of phaeochromocytoma or adrenocortical carcinoma. A nodule of 4 cm or more, or one that has grown, is a size-and-growth problem for the endocrine and surgical team rather than a washout problem.
Sending a patient with impaired renal function through a three-acquisition iodinated washout protocol when unenhanced chemical-shift MRI answers the same question. This is one of the few characterisation problems that has a genuinely contrast-free equivalent, and not using it is a contrast load bought for nothing.
Doing chemical-shift MRI and CT washout on the same nodule. They interrogate the same intracytoplasmic lipid by two mechanisms; the second study adds a cost and almost never an answer.
Treating an adrenal nodule in a patient with widespread metastatic disease as a diagnostic puzzle when it will not change management, and equally, dismissing a solitary adrenal lesion in an otherwise operable patient without characterising it.
Assuming imaging addresses hormonal function. It does not; that is a biochemical question.

Priors — what to pull first

Look for the nodule on any older scan. Stability over more than a year in a patient without known malignancy is itself strong evidence of a benign lesion and removes the need for a washout study.
Check whether a previous unenhanced series already recorded an attenuation at or below 10 HU — the question may already have been answered and forgotten.

What makes a good request

Absolute percentage washout uses the unenhanced, venous and 15-minute delayed attenuations, and a value at or above 60% indicates an adenoma. Relative percentage washout, at or above 40%, is the fallback where no unenhanced series exists — it is a salvage calculation, not the intended one.
The thresholds were validated in specific populations. Applying them to a lesion with the imaging features of phaeochromocytoma or adrenocortical carcinoma is a category error rather than a measurement problem.
Biochemical assessment for functional excess runs in parallel with imaging and is not answered by it. A request that asks imaging to exclude a functioning adenoma is asking the wrong test.
In a patient with no cancer history, a small homogeneous nodule with benign features needs no washout study at all, and running one is avoidable dose.
Size and growth belong to a different conversation from washout. A nodule of 4 cm or more, or one that has grown appreciably on interval imaging, raises adrenocortical carcinoma, and washout thresholds were never derived for that lesion — the answer is endocrine and surgical assessment, not another measurement.
This is one of the few characterisation problems with a genuinely contrast-free equivalent, and that changes what renal impairment means here. Chemical-shift MRI detects the same intracytoplasmic lipid with no contrast at all, so it is co-equal with the washout CT rather than a fallback from it — a request for unenhanced chemical-shift MRI in a patient with an eGFR of 28 is a correct request and must not be redirected to a three-acquisition iodinated study.

Scoring this once it is done

The classification and risk tools this question ends in.

Adrenal Washout Calculator (CT)Adrenal MRI Chemical Shift (Signal Intensity Index & Adrenal-to-Spleen Ratio)AAST Adrenal Injury Scale

How these studies are acquired

Contrast, phases and timing for every study on the pathways above.

MRI Adrenal — chemical shift CT Adrenal — Washout Protocol FDG PET-CT — skull base to mid-thigh

Confirm locally

CT Adrenal — Washout Protocol: timings are typical — confirm against local protocol.
Timings, contrast volumes and rates above are typical published values. Your department's protocol, scanner and patient population decide the actual numbers.
Safety thresholds and premedication policy follow local policy where it differs from the cited guidance.

References

Implemented from the cited published sources. Educational and workflow support only; confirm against current guidelines and local policy before clinical use.