Adrenal Nodule MRI Calculator

Adrenal Nodule MRI Calculator for Endocrinology. An SI index greater than approximately 16.5% or an ASR less than 0.71 indicates a lipid-rich adenoma; values near these thresholds are indeterminate. In practice the SI index is the more discriminating single metric: validation cohorts found essentially no overlap between adenomas and metastases across a cutoff range of roughly 11 to 16.5%, and the SI index can characterize a substantial fraction of adenomas that remain indeterminate on CT. A clearly positive result (high SI index, low ASR) in a nodule under 4 cm with benign morphology supports stopping imaging surveillance; borderline or negative values in an at-risk patient warrant PET, biopsy, or short-interval follow-up rather than reassurance.

How this calculator works

This tool quantifies intracellular lipid in an adrenal nodule from chemical-shift MRI by comparing signal on in-phase (IP) and opposed-phase (OP) gradient-echo images, where fat and water within the same voxel cancel on OP acquisitions. It computes two metrics: the adrenal signal-intensity (SI) index = [(SI_IP - SI_OP) / SI_IP] x 100%, and the adrenal-to-spleen chemical-shift ratio (ASR) = (SI_adrenal/SI_spleen)_OP / (SI_adrenal/SI_spleen)_IP, using spleen as an internal fat-free reference. A lipid-rich adenoma drops in signal on OP images, producing a high SI index and a low ASR.

When to use this calculator

Use it to characterize an incidentally discovered adrenal nodule as a benign lipid-rich adenoma versus an indeterminate or possibly malignant lesion, particularly when unenhanced CT attenuation is indeterminate (>10 HU but <~30 HU), which is where chemical-shift MRI adds the most value. It is not a substitute for biochemical workup: every incidentaloma still needs hormonal screening (1-mg dexamethasone suppression, plasma metanephrines, and aldosterone/renin if hypertensive). It performs poorly for lipid-poor lesions and should not be relied on to exclude malignancy in nodules >4 cm, in known extra-adrenal malignancy with a growing lesion, or in suspected pheochromocytoma.

Inputs used

  • In-phase signal
  • Out-of-phase signal
  • Signal intensity index or adrenal-spleen ratio

Clinical interpretation

An SI index greater than approximately 16.5% or an ASR less than 0.71 indicates a lipid-rich adenoma; values near these thresholds are indeterminate. In practice the SI index is the more discriminating single metric: validation cohorts found essentially no overlap between adenomas and metastases across a cutoff range of roughly 11 to 16.5%, and the SI index can characterize a substantial fraction of adenomas that remain indeterminate on CT. A clearly positive result (high SI index, low ASR) in a nodule under 4 cm with benign morphology supports stopping imaging surveillance; borderline or negative values in an at-risk patient warrant PET, biopsy, or short-interval follow-up rather than reassurance.

Worked example

A 3-cm right adrenal nodule (28 HU on unenhanced CT, indeterminate) is imaged. Mean ROI signal is IP 480, OP 150 for the nodule; spleen is IP 520, OP 500. SI index = (480 - 150)/480 x 100% = 68.8%, far above the 16.5% threshold. ASR = (150/500)/(480/520) = 0.30/0.923 = 0.33, well below 0.71. Both metrics indicate a lipid-rich adenoma; no further imaging follow-up is needed once biochemical screening is negative.

Limitations and safety notes

Chemical-shift MRI detects intracellular lipid, so it cannot distinguish lipid-poor adenomas (about 10-30% of adenomas) from malignancy; a low SI index is not evidence of cancer. Pheochromocytomas and some clear-cell renal or hepatocellular metastases can contain lipid and produce false-positive signal drop, and collision tumors or hemorrhage distort ROI measurements. Cutoffs are sensitive to field strength, echo times, and ROI placement, and the ASR falsely elevates when the spleen contains iron (hemosiderosis) or fatty change. It gives no functional information, so a lipid-rich adenoma can still be hormonally active.

Frequently asked questions

Which is better, the SI index or the adrenal-to-spleen ratio?

The SI index is generally the more reliable single metric because it uses the lesion as its own reference and avoids error introduced by an abnormal spleen. Validation studies found little to no overlap between adenomas and metastases with the SI index, whereas the adrenal-to-spleen ratio misclassified some adenomas, particularly those indeterminate on CT. Reporting both is reasonable, but weight the SI index when they disagree.

Do I still need chemical-shift MRI if CT already shows low attenuation?

No. If unenhanced CT attenuation is 10 HU or less, the nodule is already a lipid-rich adenoma and MRI adds nothing. Chemical-shift MRI is most useful for the indeterminate CT range above 10 HU, where it can definitively characterize a meaningful proportion of adenomas.

Can a positive result rule out cancer?

Not by itself. A high SI index confirms intracellular lipid, which is characteristic of adenoma, but pheochromocytoma and occasional lipid-containing metastases can also drop signal. Interpret results alongside lesion size, growth, morphology, oncologic history, and biochemical testing.

Does a low SI index mean the lesion is malignant?

No. Lipid-poor adenomas are common and also show little or no signal drop. A low SI index makes the lesion indeterminate rather than malignant, and further evaluation (washout CT, PET, biopsy, or follow-up) is guided by the clinical context.

References

  • Israel GM, Korobkin M, Wang C, Hecht EN, Krinsky GA. Comparison of unenhanced CT and chemical shift MRI in evaluating lipid-rich adrenal adenomas. AJR Am J Roentgenol. 2004. PMID: 15208141.
  • Fujiyoshi F, Nakajo M, Fukukura Y, Tsuchimochi S. Characterization of adrenal tumors by chemical shift fast low-angle shot MR imaging: comparison of four methods of quantitative evaluation. AJR Am J Roentgenol. 2003. PMID: 12760936.
  • Namimoto T, Yamashita Y, Mitsuzaki K, Nakayama Y, Makita O, Kadota M, Takahashi M. Adrenal masses: quantification of fat content with double-echo chemical shift in-phase and opposed-phase FLASH MR images for differentiation of adrenal adenomas. Radiology. 2001. PMID: 11230633.
  • Halefoglu AM, Yasar A, Bas N, Ozel A, Erturk SM, Basak M. Comparison of computed tomography histogram analysis and chemical-shift magnetic resonance imaging for adrenal mass characterization. Acta Radiol. 2009. PMID: 19863419.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: June 5, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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