What is the procedure for performing thyroid scintigraphy with I-123 (iodine-123)?

Thyroid Scintigraphy with I-123: Procedural Protocol

Pre-Procedure Preparation

Thyroid blockade is essential to prevent unnecessary radiation exposure to the thyroid gland from free radioiodide. 1

• Administer thyroid blockade starting 24 hours before I-123 injection:

  • Potassium iodide or Lugol's solution (standard approach) 2
  • Continue for 2 days after injection 2
  • Alternative: Potassium perchlorate 400-600 mg/day for iodine-allergic patients, starting 4 hours before injection 2

• Medication review and discontinuation:

  • Withhold interfering medications for 1-3 days (opioids, tricyclic antidepressants, sympathomimetics, antipsychotics, antihypertensives) 2
  • Exception: Labetalol requires 10 days discontinuation 2
  • Depot antipsychotics require 1 month withdrawal 2

• Avoid IV iodinated contrast:

  • Free iodide from contrast media directly interferes with thyroid iodide uptake 1
  • Careful timing is essential if contrast CT is needed 1

• Pregnancy and breastfeeding screening:

  • Weigh benefits against risks in pregnant patients 2
  • Discontinue breastfeeding for at least 2 days after I-123 administration 2

Radiopharmaceutical Administration

Administer 200-400 MBq (5.4-10.8 mCi) of I-123 sodium iodide intravenously in adults. 2

• Injection technique:

  • Slow IV injection over at least 5 minutes to minimize adverse events 2
  • Rare side effects include tachycardia, pallor, vomiting, and abdominal pain 2

• Pediatric dosing:

  • Calculate based on body weight using EANM Paediatric Task Group schedule 2
  • Range: 80-400 MBq 2
  • Never decay adult capsules to obtain pediatric doses due to radiocontaminant accumulation 3

• Patient hydration:

  • Ensure adequate hydration before and for at least 1 day after injection to reduce radiation exposure 4

Image Acquisition Timing

Perform imaging at 4-5 hours post-injection for thyroid uptake and scan, which provides equivalent or superior diagnostic information compared to traditional 24-hour imaging. 5

• 4-5 hour imaging protocol:

  • Provides equal or better discrimination between euthyroid and hyperthyroid patients 5
  • Offers significant logistical advantages by completing the exam same-day 5
  • No discrepancy in image quality compared to 24-hour scans in most cases 5

• 24-hour imaging considerations:

  • Higher target-to-background ratio if SPECT/CT is performed 4
  • May reveal additional foci not seen at 4 hours, particularly in thyroid cancer patients 6
  • Recommended when searching for metastatic lesions or remnant tissue in differentiated thyroid cancer 6

Image Acquisition Protocol

Acquire anterior and posterior planar static images of the neck for 10-15 minutes per view using a 256×256 matrix. 2

• Camera setup:

  • Large-field-of-view gamma camera 2
  • Low-energy or medium-energy collimator 2
  • Medium-energy collimators reduce septal penetration from high-energy photons in I-123 decay 2
  • 20% energy window centered at 159 keV photopeak 2

• Standard views:

  • Anterior and posterior neck images 2
  • Optional: Right and left lateral views for better anatomic localization 2

SPECT/CT Acquisition (When Indicated)

SPECT/CT provides attenuation correction and precise anatomic localization, particularly valuable for thyroid cancer evaluation. 4

• SPECT parameters:

  • 360° orbit acquisition 2, 4
  • 128×128 word matrix 2, 4
  • 6° angle steps 2, 4
  • 30-45 seconds per stop 2, 4

• CT parameters:

  • 100-130 kV 2, 4
  • mAs modulation recommended 2, 4
  • Enables attenuation correction and precise lesion localization 4

• Radiation considerations:

  • Additional CT dose: 3-5 mGy (volume CT dose index) 2

Uptake Measurement

Calculate thyroid uptake as a percentage of administered dose, not as SUV. 1

• Measure uptake at the same time as imaging (4-5 hours or 24 hours) 5
• Use region of interest over thyroid with appropriate background correction
• Compare to reference standards for euthyroid, hyperthyroid, and hypothyroid states 5

Image Interpretation Pitfalls

Be aware of physiologic uptake patterns that can cause false positives. 4

• Normal physiologic uptake sites:

  • Salivary glands 4
  • Thymus 4
  • Liver 4
  • Gastrointestinal tract 4

• I-123 advantages over I-131:

  • Superior image quality with lower administered activity 7
  • I-123 at 50 MBq (1.5 mCi) provides better images than I-131 at 111 MBq (3 mCi) 7
  • Pure gamma emission without beta particles 7
  • Lower radiation dose: 0.013 mSv/MBq for I-123 vs 0.14 mSv/MBq for I-131 2

Special Populations

• Renal insufficiency:

  • Plasma clearance of I-123 is reduced 2
  • I-123 is not cleared by dialysis 2

• Postpartum women:

  • Diagnostic I-123 scintigraphy can assess potential breast uptake before therapeutic I-131 ablation 8
  • Lactation-inhibiting medications (bromocriptine, cabergoline) reduce breast uptake within 3 weeks 8
  • Without medication, breast uptake may persist up to 6 months post-lactation 8