Patient Radioactivity After PET Imaging

Ever wondered how long you’re actually radioactive after a PET scan? Well… the answer isn’t as simple as you might think. This article provides straight forward answers to patient radioactivity after PET scans. Our simplified charts show radiation clearance times for the most common PET contrast agents or use the simple formula to calculate patient radioactivity yourself.

Table of Contents

• How Long are Patients Radioactive after a PET Scan?
  • Most Commonly Used PET Contrast Agents
    • 1. Fluorodeoxyglucose (18F-FDG)
    • 2. Sodium Fluoride (18F-NaF)
    • 3. Ammonia (13N-NH3)
    • 4. Rubidium Chloride (82Rb-RbCl)
    • 5. Choline (11C-Choline)
    • 6. Methionine (11C-Methionine)
    • 7. DOPA (18F-DOPA)
    • 8. PSMA Ligands (18F or 68Ga-PSMA)
    • 9. DOTATATE (68Ga-DOTATATE)
• What is a Half Life?
  • Calculating Effective Half-Life
    • PET Contrast 6 Half-Life Rule Simplified Chart
  • Additional Considerations
    • Tracer Selection
    • On-Site Production
• Key Takeaways
  • Related Resources

How Long are Patients Radioactive after a PET Scan?

When you get a PET scan, a small amount of radioactive material—called a radiotracer or contrast agent—is injected into your body. These radiotracers emit positrons that help generate detailed images of how your tissues and organs are functioning.

After a PET scan, patients remain radioactive for a short duration. The period of concern depends on the tracer’s half-life and biological clearance.

Most Commonly Used PET Contrast Agents

1. Fluorodeoxyglucose (18F-FDG)

• Isotope: Fluorine-18
• Half-life: Approximately 109.8 minutes
• Used for: Oncology (tumor detection), neurology, and cardiology
• Details: FDG mimics glucose and accumulates in areas of high metabolic activity. It is the most frequently used PET tracer worldwide.

2. Sodium Fluoride (18F-NaF)

• Isotope: Fluorine-18
• Half-life: Approximately 109.8 minutes
• Used for: Bone imaging
• Details: NaF provides excellent bone detail. It is absorbed rapidly into bone and is used for detecting skeletal metastases.

3. Ammonia (13N-NH3)

• Isotope: Nitrogen-13
• Half-life: Approximately 9.96 minutes
• Used for: Myocardial perfusion imaging
• Details: NH3 is useful in cardiac assessments, especially for identifying areas of decreased blood flow.

4. Rubidium Chloride (82Rb-RbCl)

• Isotope: Rubidium-82
• Half-life: Approximately 1.25 minutes
• Used for: Cardiac perfusion studies
• Details: Due to its short half-life, 82Rb is produced on-site using a generator. It mimics potassium and is taken up by myocardial tissue.

5. Choline (11C-Choline)

• Isotope: Carbon-11
• Half-life: Approximately 20.3 minutes
• Used for: Prostate cancer imaging
• Details: Choline is a marker for cell membrane synthesis and is taken up in proliferating tumor cells.

6. Methionine (11C-Methionine)

• Isotope: Carbon-11
• Half-life: Approximately 20.3 minutes
• Used for: Brain tumor imaging
• Details: It helps differentiate between tumor recurrence and radiation necrosis.

7. DOPA (18F-DOPA)

• Isotope: Fluorine-18
• Half-life: Approximately 109.8 minutes
• Used for: Parkinson’s disease and neuroendocrine tumors
• Details: DOPA is a precursor to dopamine and is useful in neurological and endocrine imaging.

8. PSMA Ligands (18F or 68Ga-PSMA)

• Isotope: Fluorine-18
• Half-life: Approximately 68 minutes
• Used for: Prostate cancer detection
• Details: These tracers bind to Prostate-Specific Membrane Antigen (PSMA), a protein overexpressed in prostate cancer cells.

9. DOTATATE (68Ga-DOTATATE)

• Isotope: Gallium-68
• Half-life: Approximately 68 minutes
• Used for: Neuroendocrine tumors
• Details: DOTATATE binds to somatostatin receptors (commonly overexpressed in certain tumors).

What is a Half Life?

The half-life (T½) is the time it takes for half of the radioactive atoms in a sample to decay. Most PET tracers lose radioactivity quickly due to their short half-lives.

Calculating Effective Half-Life

The effective half-life can be calculated with the formula:

1/Te = 1/Tp + 1/Tb

Where the time a person remains radioactive is influenced by:

• Physical half-life (Tp): The decay rate of the isotope
• Biological half-life (Tb): The rate at which the body eliminates the substance
• Effective half-life (Te): The actual duration of radiation in the body, accounting for both Tp and Tb

This formula gives an accurate estimate of how long the patient will emit radiation, however there is a simpler, safer, and more widely accepted calculation that simplifies the radioactivity of patients after PET scans.

PET Contrast 6 Half-Life Rule Simplified Chart

But here’s a simple shortcut used in clinical practice: the 6 half-life rule. After 6 half-lives, over 98% of the radioactivity is gone. That’s when you’re considered effectively non-radioactive. For example: a tracer with 110-minute half life (FDG) would be cleared in 11 hours (660 minutes). For more examples, see the chart below:

Isotope	Half-Life	Clearance Time (6 Half-Lives)
Fluorodeoxyglucose (18F-FDG)	110 minutes	11 Hours (660 minutes)
Sodium Fluoride (18F-NaF)	110 minutes	11 Hours (660 minutes)
Ammonia (13N-NH3)	9.96 minutes	0.996 Hours (59.76 minutes)
Rubidium Chloride (82Rb-RbCl)	1.25 minutes	0.125 Hours (7.5 minutes)
Choline (11C-Choline)	20.3 minutes	2.01 Hours (121 minutes)
Methionine (11C-Methionine)	20.3 minutes	2.01 Hours (121 minutes)
DOPA (18F-DOPA)	110 minutes	11 Hours (660 minutes)
PSMA Ligands (18F)	110 minutes	11 Hours (660 minutes)
DOTATATE (68Ga-DOTATATE)	68 minutes	6.8 Hours (408 minutes)

Additional Considerations

Tracer Selection

Each tracer is chosen based on its ability to target specific tissues or functions. Selection depends on the clinical question being investigated.

On-Site Production

Some isotopes with shorter half lives require cyclotrons or generators for on-site production. For example:

• 11C and 13N require a cyclotron due to short half-lives
• Additionally, 82Rb requires an on-site generator system for immediate production

This requirement can affect availability in smaller or rural imaging centers.

Key Takeaways

You can use the 6 half life rule to estimate the amount of time patients are radioactive after a PET contrast injection.

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