What is a sestamibi (Technetium-99m-sestamibi)?

What is Sestamibi?

Sestamibi (Technetium-99m-sestamibi) is a radiopharmaceutical imaging agent used primarily for cardiac perfusion imaging and parathyroid adenoma localization, with superior imaging characteristics compared to older agents like thallium-201 due to its higher photon energy and lower radiation exposure to patients. 1

Composition and Properties

Sestamibi consists of:

• Technetium-99m (Tc-99m): A radioactive isotope
• Sestamibi: A carrier molecule (hexakis-2-methoxyisobutyl isonitrile)

Key properties:

• Emits gamma radiation that can be detected by specialized cameras
• Has favorable imaging characteristics with higher photon energy than older agents
• Delivers lower radiation dose to patients compared to thallium-201 2
• Can be used in both planar and tomographic (SPECT) imaging formats

Clinical Applications

1. Cardiac Imaging

• Used for detection of coronary artery disease by localizing myocardial ischemia and infarction
• Helps assess myocardial viability in patients with mechanical dysfunction
• Aids in risk stratification for patients with unstable angina 1
• Protocol typically involves:
  • Injection during peak stress or at rest
  • Imaging 15-60 minutes after injection

2. Parathyroid Imaging

• Primary tool for localizing parathyroid adenomas in hyperparathyroidism
• Can be used in two main protocols:
  • Dual-phase imaging: Takes advantage of differential washout from thyroid and parathyroid tissue 3
  • Subtraction imaging: Combined with iodine-123 or pertechnetate to distinguish parathyroid from thyroid tissue 2
• Particularly valuable for:
  • Detecting ectopic parathyroid glands
  • Cases of recurrent disease
  • Pre-operative planning for parathyroid surgery 1

3. Breast Cancer Imaging

• Used in molecular breast imaging (MBI) as recommended by the American College of Radiology 1

Imaging Techniques

SPECT Imaging

• Single Photon Emission Computed Tomography
• Provides 3D visualization of radiotracer distribution
• Improves contrast resolution and anatomic localization compared to planar imaging 2
• Can be combined with CT (SPECT/CT) for even better anatomical correlation

Dual-Phase Protocol

• Based on differential washout kinetics between thyroid and parathyroid tissue
• Early (15 min) and delayed (2-3 hours) images are acquired
• Sensitivity ranges from 43% to 91%, with pooled sensitivity of 73% 3

Subtraction Protocol

• Uses either iodine-123 or technetium-99m pertechnetate to visualize thyroid
• Thyroid images are subtracted from sestamibi images to isolate parathyroid tissue
• Pooled sensitivity of 87% for parathyroid adenoma detection 3

Radiation Exposure

• Effective dose varies by patient size and age:
  • Adult: 2.9 mSv (for a 10 mCi dose at exercise)
  • Children receive proportionally higher doses (up to 16.7 mSv for 1-year-olds) 1
• A typical 1-day Tc-99m rest-stress cardiac imaging protocol delivers 9.8-16.3 mSv 2
• Significantly lower radiation exposure compared to thallium-201 2

Clinical Performance

• For parathyroid adenoma detection:
  • Sensitivity of 96-97% for single gland disease 4
  • Less accurate for multiglandular disease (79-82% accuracy) 4
  • Superior to conventional imaging techniques like ultrasound, CT, and MRI 3
• For cardiac imaging:
  • Has largely replaced thallium-201 due to superior imaging characteristics 2

Pitfalls and Limitations

• Less sensitive for detecting multiglandular parathyroid disease 2
• Sensitivity decreased in patients with concomitant thyroid nodular disease 2
• Small adenomas may be missed 2
• Interpretation requires experienced nuclear medicine specialists

Sestamibi imaging has revolutionized both cardiac and parathyroid imaging, providing high diagnostic accuracy with relatively low radiation exposure compared to previous radiopharmaceuticals.