What isotopes are used in PET imaging?

F-18 (t_half = 1.83 h) is the most widely used PET isotope, primarily as FDG for oncology, neurology, and cardiology. Ga-68 (t_half = 1.13 h) is used for neuroendocrine tumor imaging (DOTATATE) and prostate cancer (PSMA). Rb-82 (t_half = 1.27 min) is used for cardiac PET. Cu-64 (t_half = 12.7 h) and Zr-89 (t_half = 78.4 h) are used for immuno-PET with antibodies.

How does decay correction affect multi-patient PET sessions?

When a single F-18 FDG batch serves multiple patients, each patient receives a different fraction of the calibrated batch depending on when their injection occurs. The dose for patient 1 at calibration time requires no correction, while patient 4 injected 3 hours later needs a calibrated dose 1.96x higher. Activity planners use A_cal(patient n) = A_inj times e^(lambda times t_n) to account for this decay over the session.

What is the shelf life of a Ga-68 radiopharmaceutical?

Ga-68 has a half-life of 67.71 minutes. A dose calibrated at 300 MBq decays to about 150 MBq (50%) after 68 minutes and to 37.5 MBq (12.5%) after 204 minutes. In practice, Ga-68 preparations are typically discarded 2-3 hours after calibration when the activity falls below the minimum required for diagnostic imaging, usually around 100-150 MBq per patient dose.

How do you calculate how long before scan time to calibrate a dose?

Rearrange the decay formula: t = ln(A_cal / A_inj) / lambda. For example, if you need 370 MBq at injection and your calibrated batch is 1000 MBq, then t = ln(1000/370) / lambda. For Tc-99m (lambda = 0.1155/h), t = ln(2.703) / 0.1155 = 8.6 hours before injection. This tells you to calibrate the dose 8.6 hours before the scheduled injection.

PET/SPECT Isotope Activity Planner

Q: How do you calculate required calibration activity for a PET scan?

Use the formula A_cal = A_inj times e^(lambda times t), where lambda = ln(2)/t_half is the decay constant and t is the time in hours between calibration and injection. For example, for F-18 with t_half = 1.83 h and a 60-minute prep time, the decay factor is e^(-0.379 times 1) = 0.684, so you need A_cal = A_inj / 0.684, approximately 1.46x the desired injected activity.

Q: What is the difference between calibration activity and injected activity?

Calibration activity is the measured activity of a radiopharmaceutical dose at a specific reference time (the calibration time), typically when the dose leaves the radiopharmacy. Injected activity is the activity actually delivered to the patient at the time of injection. Because radioactive isotopes decay continuously, the injected activity is always less than the calibration activity unless injection occurs at the exact calibration time.

Q: How long is a Tc-99m dose usable after calibration?

Tc-99m has a half-life of 6.01 hours. A dose is generally considered usable while it retains at least 5-10% of its calibrated activity. At the 10% threshold that corresponds to roughly 19.9 hours, or about 3.3 half-lives. In practice, most nuclear medicine departments use Tc-99m doses within 12 hours of the calibration time to stay within practical activity ranges.

Calculate the required calibration activity for PET and SPECT radiopharmaceuticals, or find the remaining activity at any point after calibration.

Isotopet½ = 1.830 h

Custom Half-Life (hours)

Target Activity at Injection

MBq

Preparation + Transit Time

min

Isotopet½ = 1.830 h

Custom Half-Life (hours)

Calibration Activity

MBq

Elapsed Time Since Calibration

hours

Required Calibration Activity

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Decay Factor

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Activity Remaining at Injection

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Batch Shelf Life (to 5%)

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Isotope Half-Life

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Remaining Activity

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Fraction Remaining

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Time to 50% (1 half-life)

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Time to 25% (2 half-lives)

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Time to 10% (3.32 half-lives)

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Time to 5% (4.32 half-lives)

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Isotope Half-Life

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🏥 What is the PET/SPECT Isotope Activity Planner?

The PET/SPECT Isotope Activity Planner is a clinical nuclear medicine tool that calculates the activity of a radiopharmaceutical dose at any point in its lifecycle, from the moment it is calibrated in the hot lab to the moment it is injected into a patient. Because every radioactive isotope decays continuously, the activity present at calibration time is always greater than the activity delivered at injection time. Accurate decay correction is essential for both patient safety and diagnostic quality.

The planner addresses two complementary problems faced daily in nuclear medicine departments. The first problem is determining how much activity to prepare. If you need to inject 370 MBq of F-18 FDG into a patient 60 minutes after the dose is calibrated, you cannot simply order 370 MBq from the cyclotron facility. You must account for decay during transit and preparation, which means you actually need roughly 541 MBq at calibration time. The second problem is determining how much activity remains in a batch at any given moment, which governs whether a batch is still usable and how many patients it can serve.

The calculator covers 11 commonly used PET and SPECT isotopes: F-18, Tc-99m, Ga-68, I-123, Rb-82, Cu-64, Zr-89, Lu-177, Y-90, Tl-201, and In-111. Each isotope has a substantially different half-life, ranging from 76 seconds for Rb-82 to 6.7 days for Lu-177, which creates very different logistical challenges. A custom half-life entry supports any other isotope not in the preloaded list.

The Decay Calculator mode goes beyond a simple current-activity estimate by showing the times at which 50%, 25%, 10%, and 5% of the original calibration activity remain. These landmarks help radiopharmacists plan multi-patient sessions, decide when to discard a batch, and schedule quality control measurements. Together, the two modes replace the manual lookup tables and spreadsheet calculations that nuclear medicine technologists previously relied on for daily dose planning.

📐 Formula

A(t) = A₀ · e^−λt

A(t) = activity at time t after calibration (MBq)

A₀ = calibration activity at reference time t=0 (MBq)

λ = decay constant = ln(2) / t½ (h⁻¹)

t½ = physical half-life of the isotope (hours)

t = elapsed time since calibration (hours)

A_cal = A_inj ÷ e^−λt_prep = A_inj · e^λt_prep

A_cal = required calibration activity (MBq)

A_inj = target activity at time of injection (MBq)

t_prep = time from calibration to injection in hours

Example: F-18, A_inj = 370 MBq, t_prep = 60 min = 1 h

λ = ln(2)/1.8297 = 0.3788 h⁻¹

A_cal = 370 × e^0.3788×1 = 370 × 1.461 = 540.5 MBq

📖 How to Use This Calculator

Required Calibration Activity (Mode 1)

Select your isotope - Choose from the dropdown. The half-life updates automatically in the label. Select Custom to enter any half-life in hours.

Enter target injected activity - Type the prescribed patient dose in MBq. This is the activity you want at the moment of injection, not the batch activity.

Enter preparation and transit time - Type the total time in minutes from calibration to injection. Include packaging, transit, and any waiting time at the scanner.

Read results - The calculator shows the required calibration activity, the decay factor (fraction of activity remaining at injection), and the shelf life of the batch down to 5% of calibration activity.

Switch to Decay Calculator for batch planning - If you have a known batch activity and want to know how much remains at different times during the day, switch to Decay Calculator mode and enter the calibration activity and elapsed time.

💡 Example Calculations

Example 1 - F-18 FDG dose for a PET scan with 60-minute prep time

Target: 370 MBq F-18 FDG at injection, 60 minutes after calibration

F-18 half-life = 1.8297 h. Decay constant lambda = ln(2)/1.8297 = 0.3788 h^-1.

Prep time = 60 min = 1.0 h. Decay factor = e^{-0.3788 x 1.0} = 0.6847.

Required calibration activity = 370 / 0.6847 = 540.7 MBq.

Required calibration activity = 540.7 MBq (68.5% remains at injection)

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Example 2 - Tc-99m bone scan with 2-hour transit from commercial supplier

Target: 740 MBq Tc-99m at injection, 120 minutes after calibration

Tc-99m half-life = 6.0072 h. Decay constant lambda = ln(2)/6.0072 = 0.1154 h^-1.

Prep time = 120 min = 2.0 h. Decay factor = e^{-0.1154 x 2.0} = 0.7937.

Required calibration activity = 740 / 0.7937 = 932.4 MBq.

Required calibration activity = 932.4 MBq (79.4% remains at injection)

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Example 3 - Ga-68 DOTATATE batch: how much remains after 90 minutes?

Calibration: 500 MBq Ga-68, elapsed time: 1.5 hours (decay calculator mode)

Ga-68 half-life = 1.1285 h. Decay constant lambda = ln(2)/1.1285 = 0.6142 h^-1.

Remaining activity = 500 x e^{-0.6142 x 1.5} = 500 x 0.3969 = 198.5 MBq.

Time to 10% remaining = ln(10) / 0.6142 = 3.749 h = 225 minutes after calibration.

Remaining activity = 198.5 MBq (39.7% of original, about 1.33 half-lives elapsed)

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❓ Frequently Asked Questions

How do you calculate required calibration activity for a PET scan?+

Use A_cal = A_inj times e^(lambda times t), where lambda = ln(2)/t_half and t is preparation time in hours. For F-18 (t_half = 1.83 h) with 60 min prep, the decay factor is e^(-0.379) = 0.685, so you need A_cal = A_inj / 0.685, approximately 1.46x the target dose. This calculator performs this computation instantly for 11 common isotopes.

What is the half-life of F-18 used in FDG PET scans?+

F-18 has a physical half-life of 109.77 minutes (approximately 1.83 hours). This relatively short half-life requires same-day production at a nearby medical cyclotron and limits the usable shelf life of a batch to roughly 10 hours. A batch calibrated at 37,000 MBq decays to approximately 1,160 MBq after 5 hours (about 2.7 half-lives).

What is the difference between calibration activity and injected activity?+

Calibration activity is the measured activity at a specific reference time, typically when the dose leaves the radiopharmacy. Injected activity is the actual dose delivered to the patient at the moment of injection. Because radioactive isotopes decay continuously, injected activity is always less than calibration activity. The difference depends on the isotope's half-life and the time between calibration and injection.

How long is a Tc-99m dose usable after calibration?+

Tc-99m has a half-life of 6.01 hours. Most nuclear medicine departments use Tc-99m doses within 8 to 12 hours of calibration. At 12 hours, approximately 25% of the original activity remains (about 2 half-lives). The practical usable window depends on whether sufficient activity remains to achieve the desired diagnostic image quality for the specific scan type and patient weight.

What isotopes are used in SPECT imaging?+

The most common SPECT isotope is Tc-99m (t_half = 6.01 h), used in bone scans, renal studies, lung perfusion, and brain SPECT. Others include I-123 (t_half = 13.2 h) for thyroid imaging, Tl-201 (t_half = 72.9 h) for cardiac perfusion, In-111 (t_half = 67.3 h) for white blood cell labeling, and Lu-177 (t_half = 161 h) for PRRT therapy with SPECT dosimetry verification.

Why does Rb-82 require an on-site generator for cardiac PET?+

Rubidium-82 has an extremely short half-life of just 76.4 seconds (1.27 minutes). 99% of the activity decays within 8.5 minutes, making external transport completely impractical. Rb-82 is therefore produced continuously from a Sr-82/Rb-82 generator that is permanently installed in the cardiac PET suite and eluted directly into the patient injection line immediately before administration.

What activity is typically injected for F-18 FDG PET?+

SNMMI guidelines recommend 185 to 370 MBq (5 to 10 mCi) for standard adult F-18 FDG whole-body PET. Weight-based dosing of 3.7 to 5.2 MBq/kg is commonly used for pediatric patients and in protocols optimized for newer high-sensitivity scanners. The effective radiation dose is approximately 5 to 7 mSv per examination at the 370 MBq dose level.

How does Ga-68 compare to F-18 for PET imaging logistics?+

Ga-68 (t_half = 67.7 min) is slightly shorter-lived than F-18 (t_half = 109.8 min), making same-day logistics even tighter. However, Ga-68 can be produced from a Ge-68/Ga-68 generator (Ge-68 half-life = 270.9 days) without requiring an on-site cyclotron. This makes Ga-68 accessible to smaller centers. The trade-off is a narrower scanning window of about 2 to 3 hours per elution compared to F-18's 6 to 8 hour window.

What is the shelf life of a Lu-177 PRRT therapy dose?+

Lu-177 has a half-life of 6.71 days (161 hours). A dose calibrated at 7.4 GBq (200 mCi) decays to approximately 3.7 GBq (100 mCi) after one half-life of 6.71 days. The extended half-life allows therapeutic doses to be shipped from centralized production facilities, but also means that decay correction over a multi-day transit window can shift the delivered dose by 5 to 15% depending on transit time.

How do you plan activities for multiple patients in a single PET session?+

For a multi-patient F-18 FDG session, each patient's dose must be decay-corrected to their individual injection time. If Patient 1 is injected at calibration time and Patient 3 is injected 2 hours later, Patient 3's calibration activity must be A_inj times e^(lambda times 2) = A_inj times 2.14. The total batch activity ordered from the cyclotron facility is the sum of all individual decay-corrected doses plus a quality control aliquot.

What is the decay constant and how is it related to half-life?+

The decay constant (lambda) is the probability per unit time that a nucleus will decay, measured in inverse time units (h^-1, min^-1, or s^-1). It relates to half-life by lambda = ln(2) / t_half = 0.6931 / t_half. For F-18 with t_half = 1.8297 h, lambda = 0.3788 h^-1, meaning 37.88% of nuclei decay per hour. The decay equation A(t) = A_0 times e^(-lambda times t) uses lambda directly.

Can this calculator be used for radioiodine I-131 therapy planning?+

I-131 is not in the preloaded list because it is primarily a therapy isotope rather than a PET or SPECT imaging agent, and its dosimetry involves biological clearance modeling beyond simple physical decay. However, you can use the Custom isotope option and enter t_half = 192.5 hours (8.02 days) to perform purely physical decay corrections for transit and shelf-life purposes. For full I-131 therapy dosimetry, use the Radiopharmaceutical Dosimetry (MIRD) calculator on this site.

🔗 Related Calculators

📌 Quick Tips

💡Always account for both preparation time and transit time from the hot lab to the scanner when computing required calibration activity.

💡For F-18 FDG with a typical 60-minute prep window, calibration activity should be about 1.96x the desired injected dose.

💡Rb-82 has an extremely short half-life of 1.27 minutes, so calibration and injection are essentially simultaneous - use it only with an on-site generator.

💡Ga-68 DOTATATE has a half-life of 68 minutes, making it ideal for same-day production from a Ge-68/Ga-68 generator with a 1-2 hour shelf life.

💡For Lu-177 PRRT therapy doses, the longer 6.7-day half-life means decay correction over a multi-day shipping window is critical to accurate dosing.