On 99mTc-MIBI (sestamibi) parathyroid scintigraphy, the principle of differential washout is exploited. If there is persistent uptake on the 2-hour delayed image that was absent on the early image, this indicates:

• A Ectopic thyroid tissue with delayed iodine uptake
• B Hyperfunctioning parathyroid tissue (adenoma) with delayed tracer retention ✓
• C Lymph node metastasis from medullary thyroid carcinoma
• D Artifact due to patient motion between acquisitions

Explanation

Parathyroid scintigraphy with 99mTc-MIBI exploits differential washout: both thyroid and parathyroid tissue take up the tracer early, but normal thyroid washes out rapidly (early phase only), while hyperfunctioning parathyroid tissue (adenoma or hyperplasia) retains the tracer longer (shows on 2-4 hour delayed image). The technique involves subtracting the delayed image from the early image or using dual-phase imaging to identify the persistent parathyroid focus. SPECT-CT allows anatomical localization, particularly for ectopic adenomas. The subtraction technique compares early (thyroid + parathyroid) minus delayed (thyroid only) to highlight parathyroid foci.

Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.