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Endocrine Imaging
Demonstration data

Adrenal Chemical-Shift (SII)

Quantifies signal drop between in-phase and opposed-phase MRI to characterise a lipid-rich adrenal adenoma using the adrenal signal intensity index.

Chemical-shift MRI exploits the different precession of fat and water protons: on opposed-phase images, voxels containing both fat and water lose signal. A lipid-rich adrenal adenoma therefore drops in signal from in-phase to opposed-phase imaging. The adrenal signal intensity index quantifies this drop; an adrenal-to-spleen ratio, using the spleen (which contains no microscopic fat) as an internal reference, provides an alternative index.

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📐 Formula

SII = (in-phase SI − out-of-phase SI) / in-phase SI × 100; ≥16.5% = lipid-rich adenoma

When to Use

  • Characterising an indeterminate adrenal nodule, particularly one that is not diagnostically lipid-rich on unenhanced CT (10–30 HU).
  • When ionising radiation or iodinated contrast is best avoided.
  • Problem-solving after equivocal CT washout.

How It Works

  • Place an ROI over the adrenal lesion on co-registered in-phase and opposed-phase images.
  • Compute the signal intensity index: SII = (in-phase − opposed-phase) / in-phase × 100.
  • Optionally compute the adrenal-to-spleen chemical-shift ratio using the spleen as a reference tissue.

Interpreting the Result

  • SII ≥16.5% indicates a lipid-rich adenoma (high specificity); higher thresholds (e.g. ≥20%) increase specificity further.
  • Adrenal-to-spleen ratio <0.71 supports an adenoma; a ratio near or above 1 suggests a non-adenoma.
  • Little or no signal loss (low SII) indicates a lipid-poor lesion requiring further work-up.

Limitations & Pitfalls

  • Lipid-poor adenomas (up to ~30%) may not lose signal and can be misclassified as non-adenomas.
  • Some clear-cell renal metastases, hepatocellular metastases and adrenal carcinomas contain intracellular lipid and can drop signal, mimicking adenoma.
  • Adequate opposed-phase signal loss requires correct echo times; haemorrhage and calcification confound the ratio.

References

  • Israel GM, Korobkin M, Wang C, et al. Comparison of unenhanced CT and chemical shift MRI in evaluating lipid-rich adrenal adenomas. AJR Am J Roentgenol. 2004;183(1):215-219.
  • Fujiyoshi F, Nakajo M, Fukukura Y, Tsuchimochi S. Characterization of adrenal tumors by chemical shift fast low-angle shot MR imaging. AJR Am J Roentgenol. 2003;180(6):1649-1657.

⚕️ For clinical decision support only. Always verify calculations and apply clinical judgment.