The India ink artefact on opposed-phase MRI
Why thin black lines outline organs on opposed-phase MRI, why they vanish in phase, and what the line means at the edge of a mass.
Key points
- The India ink (etching) artefact is a thin black line at fat-water interfaces on opposed-phase gradient-echo images.
- It is a chemical shift artefact of the second kind.
- It arises in voxels that hold both fat and water: on the opposed-phase echo their signals cancel.
- The line is absent on the in-phase echo of the same dual-echo sequence.
- A line within a mass or at its edge shows macroscopic fat; diffuse signal drop across a lesion is a different finding (microscopic or intracellular fat).
A line that is not anatomy
Look at the left half of the image above. Thin black lines run around the liver and around the mass beside it, as if someone had traced them with a pen. Now look at the right half, the same slice at a different echo: the lines are gone. They were never anatomy. They are physics.
Water, fat and phase
A dual-echo gradient-echo sequence acquires the same slice twice. Inside each voxel, hydrogen in water and hydrogen in fat precess at slightly different frequencies, so their signals drift in and out of step. At the in-phase echo they point the same way and add up. At the opposed-phase echo they point opposite ways.
Most voxels are mainly water or mainly fat, so they stay bright or grey on both echoes. But a voxel that sits exactly on a fat-water border holds both. On the opposed-phase echo its two signals cancel, and the voxel turns black. Line up all such voxels along an interface and you get the India ink, or etching, artefact: a chemical shift artefact of the second kind.
In phase: gone
Because the cancellation only happens when the two signals are opposed, the line is absent on the in-phase echo of the same sequence. Comparing the two echoes side by side is the quickest way to recognise it.
When it matters: at a mass
A black line inside a mass, or at its edge where fat meets water, tells you the mass contains macroscopic fat. This is a different finding from a diffuse drop in signal across a whole lesion on the opposed-phase image, which reflects microscopic or intracellular fat. Keeping the two apart is part of describing fat-containing masses clearly.
How to practise
Put the in-phase and opposed-phase images side by side. Find every black outline on the opposed-phase image and check that it disappears in phase. Then look inside any mass for the same kind of line.
Check yourself
A thin black line outlines the edge of a mass on opposed-phase MRI and is gone on the in-phase image. What does it show?
Sources
- Schieda N, Davenport MS, Pedrosa I, et al. Renal and adrenal masses containing fat at MRI: proposed nomenclature by the Society of Abdominal Radiology disease-focused panel on renal cell carcinoma. J Magn Reson Imaging 2019;49(4):917-926. doi:10.1002/jmri.26542 · PMID 30693607
- Merkle EM, Nelson RC. Dual gradient-echo in-phase and opposed-phase hepatic MR imaging: a useful tool for evaluating more than fatty infiltration or fatty sparing. RadioGraphics 2006;26(5):1409-1418. doi:10.1148/rg.265055711 · PMID 16973772
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For educational purposes only. Not medical advice.