Adrenal Chemical Shift Calculator for MRI

Evaluate an adrenal nodule's behavior on MRI using chemical shift calculator

Adrenal MRI
What is the field strength of the MRI?
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ASII > 16.5% at 1.5T is considered a lipid-rich adenoma
ASII > 1.7% at 3T is considered a lipid-rich adenoma
ASII ≤ 16.5% at 1.5T is considered indeterminant: Recommend follow-up with CT washout
ASII ≤ 1.7% at 3T is considered indeterminant: Recommend follow-up with CT washout
ASR ≤ -35.9% at 1.5T is considered a lipid-rich adenoma
ASR ≤ -17.2% at 3T is considered a lipid-rich adenoma
ASR > -17.2% at 3T is considered indeterminant: Recommend follow-up with CT washout.
ASR > -35.9% at 1.5T is considered indeterminant: Recommend follow-up with CT washout
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Reference:
  1. Adam, S. Z., Nikolaidis, P., Horowitz, J. M., Gabriel, H., Hammond, N. A., Patel, T., Yaghmai, V., & Miller, F. H. (2016). Chemical Shift MR Imaging of the Adrenal Gland: Principles, Pitfalls, and Applications. Radiographics : a review publication of the Radiological Society of North America, Inc, 36(2), 414–432. https://doi.org/10.1148/rg.2016150139

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Adrenal chemical shift MRI is a specialized imaging technique used to differentiate between benign and malignant adrenal lesions by detecting microscopic fat within adrenal nodules.

Technique

Chemical shift MRI utilizes differences in the precession frequencies of fat and water protons. This difference creates signal variations that can be captured using in-phase and out-of-phase sequences. Key steps include:

  • Image Acquisition:
    • Obtain both in-phase and out-of-phase images.
    • Use the shortest echo times to minimize T2* decay, which can artificially reduce T1 signal intensity and lead to misinterpretation.
  • Reference Tissue:
    • Compare adrenal lesion signal intensity to a reference tissue, typically the spleen, as it is less likely to contain fat or hemosiderin deposits. This helps to ensure accurate signal intensity measurements.
Interpretation
  • Signal Loss: On out-of-phase images, lipid-rich adenomas will show significant signal loss compared to in-phase images. This loss is quantitatively evaluated:
    • Signal Intensity Index: Percentage of signal loss on out-of-phase images (greater than 16.5% indicates lipid-rich adenomas).
    • Adrenal-to-Spleen Signal Intensity Ratio: A ratio less than 0.71 suggests a lipid-rich adenoma.
Pitfalls
  • Microscopic Fat Detection: Chemical shift MRI identifies microscopic fat. If a lesion does not show fat content on nonenhanced CT (attenuation >10 HU), it is unlikely to be a lipid-rich adenoma on chemical shift MRI. Nodules with greater attenuation values (>20-30 HU) are better evaluated using CT enhancement kinetics rather than chemical shift MRI.
  • Technical Factors: Ensure the shortest echo times and correct echo order to avoid T2* decay effects. For 1.5-T MR imagers, optimal echo times are typically 2.2 ms (out-of-phase) and 4.4 ms (in-phase), while for 3-T imagers, they are 1.1 ms and 2.2 ms.

By employing these techniques and considerations, adrenal chemical shift MRI can provide crucial information in the characterization of adrenal masses, helping to distinguish benign adenomas from other pathologies.