MRI Renal — mass characterisation

Contrast

Macrocyclic gadolinium agent at a typical single dose of about 0.1 mmol/kg by power injector with saline chaser, with acquisitions in corticomedullary, nephrographic and delayed timings.

Acquisition

Breathing

End-expiratory breath-holds of roughly 15-20 seconds for the dynamic series.

Reconstruction

Subtraction of the unenhanced from each enhanced phase, and percentage enhancement measured from identically placed regions of interest.

Preparation

Renal impairment is common in this population — establish the gadolinium plan early rather than at the scanner. Provide prior CT so the same lesion can be matched; a mass characterised on CT rarely needs the whole MRI repertoire. Breath-hold ability determines whether the dynamic series will be diagnostic.

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.

1. 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.

2. Late arterial phase (arterial-dominant)Typically ~30–40 s after the start of injection. Confirm locally.

   Arteries are still dense and solid organs supplied by arteries have begun to enhance, but the portal vein has not yet delivered its load to the liver. The transferable principle is that HYPERVASCULAR lesions peak here and are maximally conspicuous against a still-dark background: this is where arterially hyperenhancing tumours, hypervascular metastases and vascular malformations declare themselves. The window is narrow and cardiac-output dependent — mistiming by ten seconds in either direction converts a diagnostic study into a non-diagnostic one, which is why bolus tracking rather than a fixed delay is preferred whenever the clinical question is hypervascularity.

3. 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.

4. 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
• Kidney function and gadolinium-based contrast· radiographer at scan
• Intravenous access adequate for the planned injection· radiographer at scan
• Metformin and iodinated contrast· radiographer at scan
• MR safety screening for implants and foreign bodies· radiographer at scan
• Kidney function and intravenous iodinated contrast· radiographer at scan