CT Adrenal — Washout Protocol
A limited block through the adrenal glands for all three acquisitions; the venous acquisition may be extended if there is a concurrent staging question.
Timings, volumes and delays here are representative values drawn from published guidance. Scanner generation, injector, cardiac output and local preference all move them. Confirm against your department's own protocol before you rely on a number.
When to use it
- Characterisation of an indeterminate adrenal nodule whose unenhanced attenuation exceeds the lipid-rich adenoma threshold.
- An adrenal nodule in a patient with known or suspected malignancy where adenoma versus metastasis changes management.
- Follow-up of a nodule previously called indeterminate.
Technique
- The three acquisitions are conventionally unenhanced, a venous acquisition at around 60 s, and a delayed acquisition at 15 minutes; absolute and relative percentage washout are calculated from the same region of interest across all three.
- The unenhanced series does most of the diagnostic work on its own: an attenuation at or below about 10 HU already indicates a lipid-rich adenoma and the enhanced series can be abandoned.
- Relative washout is the fallback when no unenhanced series exists.
- Regions of interest should sample the same part of the nodule each time and avoid the rim and adjacent fat.
Where it goes wrong
- Different breath-holds between phases move the nodule and cause a different part of it to be measured — the commonest source of a spurious washout value.
- Measuring a heterogeneous or haemorrhagic nodule as if it were homogeneous produces washout values that do not mean what the thresholds assume.
- Washout criteria were validated in specific populations; applying them to a nodule with imaging features of phaeochromocytoma or adrenocortical carcinoma is a category error, not a timing error.
- Running the full three-phase protocol on a nodule that was already sub-10 HU unenhanced is avoidable dose.
- Delaying the third acquisition to an arbitrary time other than the protocol delay invalidates the published thresholds.
Clinical questions that reach this study
Contrast
Typically 80-120 mL of non-ionic iodinated contrast at around 2-3 mL/s.
- No oral contrast.
Acquisition
- Breathing
- Identical breath-hold instruction for all three acquisitions.
- Reconstruction
- Thin axial reconstructions with matched slice position across the three series so that the same region of interest can be re-measured.
- Preparation
- No oral contrast. Identical breath-hold instruction for all three acquisitions so the same voxels are re-measured.
Phases
Each phase is authored once and shared across every protocol that uses it, so the physiology below is the same wherever you meet it.
- Non-contrast (unenhanced)No injection. Acquired before any contrast is given.
Shows intrinsic tissue attenuation, and nothing else. The transferable principle is that contrast is anti-signal for anything that is already dense: calcification, acute haemorrhage, urinary and biliary calculi, iodine-containing or haemorrhagic fluid, and intrinsic fat all lose conspicuity, or become uninterpretable, once surrounding tissue enhances. It is also the only baseline against which enhancement can be measured, so any protocol that quantifies enhancement or washout (a lesion "enhances by X HU", adrenal absolute washout, renal mass characterisation) is arithmetically impossible without it. Conversely, an unenhanced series adds dose and no information whenever the question is purely about vascularity or perfusion.
- Portal venous phaseTypically ~60–90 s after the start of injection. Confirm locally.
The portal vein and hepatic veins are opacified and the liver is at maximum parenchymal enhancement; bowel wall, mesentery, spleen and peritoneum are all well enhanced. The transferable principle is that HYPOVASCULAR lesions are most conspicuous when the BACKGROUND peaks, so this is the single most productive general-purpose abdominal phase and the correct default when the question is "what is wrong in this abdomen?". Its corollary is the classic error: a hypervascular lesion that was obvious 30 s earlier can become isodense and invisible here, so a normal portal venous study never excludes hypervascular disease.
- Delayed / equilibrium (washout) phaseQuestion-dependent: ~3–5 min for hepatic equilibrium/washout, ~15 min for adrenal absolute-washout calculations. Confirm locally.
Intravascular and interstitial compartments have equilibrated and contrast is being cleared, so most normal tissue is falling in attenuation. The transferable principle is that the diagnostic information is now in the RATE OF CHANGE rather than in the absolute density: tissues with rapid capillary exchange and a small interstitium wash out quickly, whereas fibrous, myxoid or otherwise expanded interstitial spaces retain contrast and become relatively dense. That single mechanism underlies delayed enhancement of scar and fibrosis, retained enhancement in cholangiocarcinoma and haemangioma fill-in, and the arithmetic of adrenal washout — all of which require a matched earlier acquisition to be interpretable at all.
Safety checks this protocol carries
Derived from the contrast agent and phases above, not authored here — which is why they cannot drift apart from what the protocol actually does.
- Prior contrast reaction and elective premedication· nurse pre scan
- Intravenous access adequate for the planned injection· radiographer at scan
- Metformin and iodinated contrast· radiographer at scan
- Child-sized technique and contrast dose· radiographer at scan
- Pregnancy status before an ionising exposure· radiographer at scan
- Kidney function and intravenous iodinated contrast· radiographer at scan