Adrenal MRI Chemical Shift (Signal Intensity Index & Adrenal-to-Spleen Ratio)

Two quantitative indices are computed. The adrenal signal intensity index (ASII) is the percentage signal loss of the lesion on opposed-phase images and needs no reference organ. The adrenal-to-spleen ratio (ASR) normalizes the lesion signal to the spleen and is only reported when both splenic measurements are supplied; it is unreliable when splenic signal is abnormal (e.g. iron overload). Thresholds are field-strength dependent: at 1.5 T an ASII above 16.5% or an ASR of -35.9% or less favors a lipid-rich adenoma; at 3 T the corresponding cutoffs are 1.7% and -17.2%. Results below threshold leave the lesion indeterminate and usually prompt CT washout or follow-up. Lipid-poor adenomas and occasional lipid-containing metastases (clear cell RCC, HCC) can cross these boundaries, so quantitative results must be integrated with morphology and clinical context.