What is the procedure and preparation for a Metaiodobenzylguanidine (MIBG) scan?

MIBG Scan Procedure and Preparation

The MIBG scan procedure requires careful medication management, thyroid blockade with potassium iodide starting at least 24 hours before the scan, and intravenous administration of the radiotracer followed by imaging 20-24 hours later. 1

Patient Preparation

Medication Management

• Discontinue interfering medications:
  • Most medications should be withheld for 1-3 days before the scan 1
  • Labetalol must be stopped 10 days prior to MIBG administration 1
  • Depot forms of antipsychotics require 1 month withdrawal 1
  • Common interfering medications include:
    • Opioids
    • Tricyclic antidepressants
    • Sympathomimetics
    • Antipsychotics
    • Antihypertensive agents 1, 2

Thyroid Protection

• Thyroid blockade is essential to prevent uptake of free radioiodine:
  • For 123I-MIBG: Start potassium iodide (SSKI, 60mg twice daily) or Lugol's solution (1 drop three times daily) at least 24 hours before and continue for 2 days after administration 1, 3
  • For 131I-MIBG: Continue thyroid blockade for 5 days after administration 1
  • For iodine-allergic patients: Potassium perchlorate can be substituted, starting 4 hours before injection and continuing for 2 days (400-600 mg/day) 1

Administration Procedure

Radiopharmaceutical Administration

• Adult dose: 200-400 MBq for 123I-MIBG or 40-80 MBq for 131I-MIBG 1
• Pediatric dose: Calculated based on body weight according to EANM Paediatric Task Group schedule 1
• Administration method: Slow intravenous injection over at least 5 minutes to minimize adverse events 1

Potential Side Effects

• Rare adverse events include tachycardia, pallor, vomiting, and abdominal pain 1
• These can be minimized by slow injection 1
• No adverse allergic reactions have been reported 1

Imaging Protocol

Image Acquisition

• Timing: Images are typically obtained 20-24 hours after tracer injection 1
• Imaging fields: Anterior and posterior planar static images of:
  • Head and neck (including right and left lateral views)
  • Thorax
  • Abdomen
  • Pelvis 1
• Image parameters:
  • 10-15 minutes per image
  • 256×256 matrix
  • Large-field-of-view camera
  • Medium-energy collimators preferred (reduce septal penetration) 1

Additional Imaging

• SPECT/CT: Often performed over regions showing pathological tracer uptake on planar images
  • 360° orbit
  • 128×128 word matrix
  • 6° angle steps
  • 30-45 seconds per stop 1
• Optional early images: May be obtained 4-6 hours after injection 1
• Whole-body imaging: Can be performed with anterior and posterior images into 1,024×512 or 1,024×256 word matrix for at least 30 minutes 1

Special Considerations

Pregnancy and Breastfeeding

• Pregnancy: Clinical decision needed to weigh benefits against potential harm 1
• Breastfeeding:
  • For 123I-MIBG: Discontinue for at least 2 days after scan
  • For 131I-MIBG: Breastfeeding must be stopped completely 1

Renal Insufficiency

• Plasma clearance of 123I-MIBG is reduced in patients with renal insufficiency
• 123I-MIBG is not cleared by dialysis 1

Normal Distribution and Interpretation

Normal Physiological Uptake

• Myocardium
• Salivary glands
• Thyroid gland (if inadequate blockade)
• Liver
• Lungs
• Adrenal glands (mild uptake in up to 80% of patients)
• Bowel 1

Common Pitfalls and Caveats

• Medication interference: The most common cause of false-negative scans is failure to discontinue interfering medications 2, 4
• Inadequate thyroid blockade: Can result in excessive thyroid uptake, obscuring nearby structures 1
• Drug-induced false negatives: Documented cases where drug interactions have caused false-negative MIBG scans in patients with confirmed disease 5
• Slow injection: Failure to inject slowly may increase risk of adverse events 1

By following this standardized protocol for MIBG scanning, clinicians can optimize diagnostic accuracy while minimizing patient discomfort and radiation exposure.