PET/CT Shielding Calculations: TG-108 and NCRP 147

PET/CT facilities require shielding designs that address both 511 keV annihilation photons from positron-emitting radiopharmaceuticals and diagnostic X-ray output from the CT subsystem. Because PET sources are distributed across the patient, hot lab, uptake rooms, scanner room, post-scan areas, and waste pathway, shielding design has to follow the clinical workflow rather than the scanner footprint alone.¹²³

A defensible PET/CT shielding evaluation combines PET-specific methodology from AAPM Task Group 108 with CT shielding methodology from NCRP Report No. 147, then checks the result against applicable NRC or Agreement State requirements and facility-specific occupancy assumptions.¹²⁴⁵

Introduction

PET/CT shielding differs from conventional diagnostic shielding because the dominant PET source is often the injected patient, not the imaging equipment. The design process must account for isotropically emitted 511 keV photons from patients and handled radiopharmaceuticals, as well as CT primary, scatter, and leakage radiation calculated under diagnostic X-ray shielding methods.¹²

In practice, the PET contribution often governs wall, floor, and ceiling thicknesses near uptake rooms and scanner rooms. CT may still be limiting at the control-room barrier, viewing window, or a selected primary-beam orientation. A PET/CT shielding design is incomplete if it evaluates only one side of the system.¹²

This guide walks through the radiation sources, design goals, calculation methods, a worked example, clinical impact, optimization tips, regulatory context, and the verification steps that separate a defensible PET/CT shielding package from a back-of-envelope estimate.

Topic Explanation

What is PET/CT shielding?

PET/CT shielding is the structural barrier design that limits radiation dose in areas adjacent to a combined positron emission tomography and computed tomography suite. It is a mixed-modality problem because two physically different radiation sources must be evaluated together: high-energy 511 keV photons from positron annihilation, and the diagnostic-energy X-ray spectrum from the CT tube.

Key terms used throughout this guide:

• 511 keV annihilation photons — the paired gamma rays produced when a positron from an injected radiopharmaceutical annihilates with an electron. They are emitted in nearly opposite directions and are highly penetrating.
• Design goal (P) — the weekly dose limit a barrier is engineered to meet at an occupied point on the far side.
• Occupancy factor (T) — the fraction of time the adjacent area is occupied by the most exposed individual.
• Transmission factor (B) — the fraction of unshielded dose that must pass through the barrier to meet the design goal.

What radiation sources matter in PET/CT shielding?

PET imaging most commonly uses fluorine-18 fluorodeoxyglucose (F-18 FDG), which has a physical half-life of about 110 minutes, although other positron-emitting radionuclides may also be present depending on the clinical program. Positron annihilation produces two 511 keV photons emitted in nearly opposite directions. For shielding purposes, the resulting radiation field is treated as effectively isotropic around the source region.¹⁶

The principal PET shielding sources include:¹

• Unit doses, multidose vials, and waste containers in the hot lab.
• Injected patients during uptake.
• Injected patients in the scanner room during acquisition.
• Injected patients in post-scan holding or discharge areas.
• Calibration or quality-assurance sources when present.

Because the patient is frequently the dominant source, shielding calculations should be organized by room occupancy, source-to-barrier distance, time-integrated activity, and patient throughput rather than by scanner output alone. This is one of the central distinctions between PET/CT and conventional CT shielding design.¹ For a deeper look at the radionuclides involved, see our overview of common PET and radiopharmaceutical-therapy isotopes.

The CT component produces radiation in the same broad categories considered for diagnostic CT shielding: primary radiation, patient scatter, and tube leakage. These components are evaluated using workload, use factor, occupancy factor, distance, and attenuation methods consistent with NCRP Report No. 147.²

What design goals are commonly used?

Shielding calculations are usually performed to weekly design goals rather than annual limits directly. For uncontrolled areas, a common design objective is 0.02 mGy per week, corresponding approximately to 1 mSv per year for members of the public. For controlled areas, a common design objective is 0.1 mGy per week, corresponding approximately to 5 mGy per year in routinely occupied controlled locations.¹²

These design objectives are more restrictive than the absolute annual occupational dose limits in 10 CFR Part 20 and are intended to support ALARA and sustained occupancy in adjacent spaces.⁴ Accessible public areas must also satisfy applicable public-dose constraints, including the dose limit for individual members of the public and applicable hourly limits under NRC or Agreement State rules.⁴

The principal technical references commonly used in PET/CT shielding design are:

• AAPM Task Group 108, PET and PET/CT Shielding Requirements.¹
• NCRP Report No. 147, Structural Shielding Design for Medical X-Ray Imaging Facilities.²
• NRC regulations and licensing guidance, including 10 CFR Parts 20 and 35 and NUREG-1556 Volume 9, for radioactive material program expectations.⁴⁵⁷

Key Technical Principles

Time-integrated PET activity

PET shielding calculations are typically based on average or time-integrated activity present in a given room over a defined interval. For an initial activity $A_0$, decay constant $\lambda$, and elapsed interval $\Delta t$, the average activity $\bar{A}$ may be written as:¹⁶

$$\bar{A}=A_0\cdot R_t$$

where the reduction factor is:

$$R_t=\frac{1-e^{-\lambda \Delta t}}{\lambda \Delta t}$$

This accounts for radioactive decay during uptake, scanning, or recovery. Weekly workload is then estimated by multiplying room-specific average activity by time per patient and the number of patients per week.¹

Unshielded PET dose rate

AAPM TG-108 provides an effective dose-rate constant for F-18 in the patient on the order of $\Gamma =0.092\mu \text{Sv}{\text{m}}^2{\text{MBq}}^{-1}{\text{h}}^{-1}$.¹ For a source with average activity $\bar{A}$ at distance $d$, the unshielded dose rate at a design point is estimated by:

$${\dot{D}}_0=\frac{\Gamma \cdot \bar{A}}{d^2}$$

The weekly unshielded dose contribution from that source location is then:

$$D_0={\dot{D}}_0\cdot t_{\text{per patient}}\cdot N_{\text{patients/week}}$$

The total unshielded weekly dose at a design point is the sum of contributions from all relevant PET source locations, including hot lab, uptake rooms, scanner room, recovery areas, and adjacent hot storage functions. This summation matters because a single barrier may receive meaningful contribution from more than one room during a typical clinical week.¹

Barrier transmission and thickness

For a receptor location with weekly design goal $P$, occupancy factor $T$, and total unshielded weekly dose $D_0$, the required barrier transmission factor $B$ may be expressed as:

$$B=\frac{P}{D_0\cdot T}$$

Once $B$ is known, the barrier thickness is derived from half-value-layer or tenth-value-layer data for the selected material. Using tenth-value-layer notation:

$$t={\text{TVL}}_{511}\cdot {\log }_{10}\left(\frac{1}{B}\right)$$

Lead is substantially more space-efficient than ordinary concrete for 511 keV photons, while concrete may still be advantageous when structural thickness is already available. Practical design must also account for density assumptions, seams, overlap details, door frames, ducts, conduit, and other penetrations.¹² The same material-selection logic underpins general lead shielding design principles in diagnostic imaging and nuclear medicine.

Worked PET wall example

The following simplified example illustrates a PET wall calculation for an uncontrolled office adjacent to a scanner room using TG-108 style methodology.¹

Assumptions:

• Radiopharmaceutical: F-18 FDG.
• Administered activity per patient: 370 MBq.
• Patients per week: 30.
• Time spent near the barrier in the scanner room: 1 hour per patient.
• Effective F-18 patient dose-rate constant: $\Gamma =0.092\mu \text{Sv}{\text{m}}^2{\text{MBq}}^{-1}{\text{h}}^{-1}$.¹
• Distance from patient to occupied point beyond the wall: 3 m.
• Occupancy factor for office: $T=1.0$.
• Weekly design goal for uncontrolled area: $P=20\mu \text{Sv/week}$.¹²

Approximating the average patient activity during the scanner interval as 370 MBq, the unshielded dose rate is:

$${\dot{D}}_0=\frac{0.092\times 370}{3^2}\approx 3.8\mu \text{Sv/h}$$

The weekly unshielded dose becomes:

$$D_0=3.8\times 30\approx 114\mu \text{Sv/week}$$

The required transmission factor is:

$$B=\frac{20}{114\times 1.0}\approx 0.18$$

If lead TVL at 511 keV is taken as approximately 1.6 cm for illustrative design purposes, the required lead thickness is:

$$t_{\text{Pb}}=1.6\cdot {\log }_{10}(1/0.18)\approx 1.2\text{cm}$$

This simplified result shows why PET barriers can quickly become space-limiting when occupied areas are close to uptake or scanning rooms. A complete design would also include contributions from other rooms, CT scatter and leakage, realistic occupancy, future throughput growth, construction details, and post-installation survey data before finalizing the shielding schedule.¹²

CT shielding methodology

The CT subsystem is evaluated using NCRP Report No. 147 methods for primary and secondary barriers.² A common expression for primary-barrier transmission is:

$$B=\frac{P\cdot d^2}{W\cdot U\cdot T}$$

where $P$ is the design goal, $d$ is the source-to-point distance, $W$ is workload, $U$ is use factor, and $T$ is occupancy factor. Secondary barriers are assessed using scatter and leakage assumptions appropriate to CT operation.²

In many PET/CT rooms, the CT contribution does not dominate the thickest barriers, but it must still be included because local design points such as the control booth window or beam-facing wall may be governed by CT conditions rather than PET alone. This is especially relevant when scanner orientation or room geometry brings a control area close to the CT gantry rotation plane.² Realistic CT workload assumptions also tie back to scanner technique; our guide to CT protocol optimization explains how dose-relevant settings are chosen in clinical practice.

Clinical Impact

PET/CT shielding decisions affect more than regulatory plan review. They shape room layout, patient flow, staff work areas, hot lab placement, uptake-room capacity, construction cost, future expansion, and post-installation survey results.

High-activity spaces such as uptake rooms, hot labs, and scanner rooms should preferentially border low-occupancy areas such as storage rooms, mechanical spaces, or circulation corridors when feasible. This zoning strategy can reduce required barrier thickness more economically than adding more lead after the layout is fixed.¹

Vertical occupancy must also be evaluated. Rooms above and below the PET/CT suite may receive nontrivial exposure from patient sources, especially in multistory medical office buildings where upper or lower occupancies are full-time offices. The effective source-to-receptor distance should therefore be assessed in three dimensions rather than by plan view alone.¹

Practical Optimization Tips

Coordinate shielding with architecture early

Lead, structural concrete, and proprietary composite shielding products are all used in PET/CT facility design. Lead is often favored where space is limited because of its attenuation efficiency per unit thickness at both CT energies and 511 keV. Concrete may be preferable when structural walls or slabs already provide meaningful attenuation or when architectural constraints favor thicker, non-metallic barriers.¹²

Regardless of material, final barrier performance depends on more than nominal thickness. The design must address:

• joints and sheet overlap;
• door frames and hardware;
• lead glass and control-room windows;
• service penetrations;
• ducts, conduit, and cable chases;
• floor and ceiling transitions;
• field changes made after the shielding plan is issued.

Shielding schedules should be coordinated directly with architectural drawings and reviewed during construction, not treated as a standalone calculation package.¹²

Verify the installed condition

A shielding design is not complete until post-construction verification has been performed. After the PET/CT system is installed and clinical operation can be simulated or observed, radiation surveys should be conducted in adjacent controlled and uncontrolled areas to confirm that measured values are consistent with calculated predictions and design goals.¹² Choosing an appropriately calibrated, energy-suitable instrument matters here; see our guidance on choosing the right radiation survey meter.

The final documentation package should typically include:

• calculation assumptions;
• radionuclides considered;
• patient throughput and administered activity assumptions;
• occupancy factors;
• barrier materials and thicknesses;
• architectural drawing references;
• survey results;
• a physicist certification letter where required by the state, license condition, accrediting body, or project reviewer.

Thorough documentation supports regulatory review, future renovation planning, and defense of the original design basis if clinical practice changes over time.⁴⁵⁷

Avoid common PET/CT shielding errors

Several errors recur in PET/CT shielding projects:

1. Underestimating PET workload. PET barriers are sensitive to patient volume, administered activity, uptake time, and scan time.
2. Ignoring uptake or recovery areas. The patient may spend more time outside the scanner than inside it.
3. Using overly optimistic occupancy factors. A room labeled "storage" may become a full-time office later.
4. Neglecting CT primary-beam evaluation. PET may control many barriers, but CT can still control selected local design points.
5. Failing to assess floors and ceilings. PET sources are not limited to plan-view adjacencies.
6. Relying on nominal material equivalence. Installed shielding can be compromised by seams, penetrations, density assumptions, and field changes.
7. Skipping post-construction verification. Calculations and drawings need to be checked against the installed and operating suite.

A conservative but realistic design philosophy combines documented assumptions, appropriate safety margin, full workflow modeling, and post-installation survey confirmation rather than depending on a minimal theoretical barrier thickness.¹²

Regulatory Considerations

PET/CT shielding sits at the intersection of NRC or Agreement State materials regulation and state radiation-control rules for radiation-producing machines. Because a PET/CT suite combines byproduct material (the positron emitters) with an X-ray-producing device (the CT), both regulatory frameworks usually apply.

• Byproduct material (PET side). Possession and medical use of F-18 and other positron emitters fall under 10 CFR Part 35 (or the equivalent Agreement State program), with public- and occupational-dose limits set by 10 CFR Part 20. NRC NUREG-1556 Volume 9 provides program-specific licensing guidance, including expectations for facility and shielding adequacy.⁴⁵⁷
• CT side. The CT subsystem is regulated as a radiation-producing machine under state radiation-control programs, which generally adopt NCRP Report No. 147 shielding methodology for plan review.²
• State-specific rules. Many states require a qualified or board-certified medical physicist's shielding report before a PET/CT suite is approved for clinical use. In Florida, radiation-machine and materials requirements are administered under Florida Administrative Code Chapter 64E-5; DRPS also serves Maryland, Virginia, Washington DC, California, and Nevada, where Agreement State or state radiation-control authorities impose parallel shielding and survey expectations. Always confirm requirements with the authority having jurisdiction.

Documented assumptions, a physicist certification letter, and post-construction survey results are what make a shielding package defensible during plan review and inspection. For the broader compliance picture, see our guides to common radiation safety violations and how to avoid them and the Florida radiation safety requirements for imaging centers.

Frequently Asked Questions (FAQs)

Why can PET shielding dominate over CT shielding?

PET emissions are 511 keV annihilation photons emitted from the injected patient and other PET source locations. Those photons are more penetrating than diagnostic CT X-rays and may be present for extended uptake, scan, and recovery periods.¹

Does PET/CT shielding only apply to the scanner room?

No. PET/CT shielding should include the hot lab, uptake rooms, scanner room, post-scan holding areas, waste locations, and adjacent spaces above and below the suite. The patient and handled activity often matter as much as the scanner room itself.¹

Should PET/CT shielding be rechecked if patient volume increases?

Yes. Shielding assumptions depend on workload, administered activity, time per patient, occupancy, and workflow. If patient volume, radiopharmaceutical mix, uptake-room use, or adjacent occupancy changes materially, the original design basis should be reviewed.

Who can perform a PET/CT shielding design?

A PET/CT shielding design is typically prepared or reviewed by a qualified or board-certified medical physicist. Many states and license conditions require a physicist's shielding report and certification before a PET/CT suite is approved for clinical use.

Key Takeaways

• PET/CT shielding is a mixed-modality problem: PET patient sources and CT X-ray sources both need explicit evaluation.
• AAPM TG-108 is the core PET/PET-CT shielding reference; NCRP Report No. 147 is the core diagnostic X-ray shielding reference for the CT subsystem.¹²
• The injected patient — emitting 511 keV photons from a physical half-life of about 110 minutes for F-18 FDG — is frequently the dominant PET source, not the scanner.¹⁶
• Uptake rooms, hot labs, recovery areas, and waste locations can contribute meaningfully to adjacent-area dose, including floors and ceilings.
• Layout decisions can reduce shielding burden before construction; late shielding fixes are usually more expensive.
• Post-construction surveys and complete documentation are part of the shielding process, not optional afterthoughts.

How DRPS Can Help

Diagnostic Radiation Physics Services (DRPS) supports PET/CT and nuclear medicine facilities across Florida, Maryland, Virginia, Washington DC, California, and Nevada with radiation shielding design, architectural plan review, PET/CT workload modeling, hot lab and uptake-area evaluation, post-construction shielding surveys, and radiation safety documentation prepared by board-certified medical physicists.

A strong PET/CT shielding program is not just about passing plan review. It is about making sure the built facility matches the clinical workflow, protects adjacent occupied areas, and remains defensible as patient volume and service lines change.

Conclusion

PET/CT shielding design requires explicit treatment of both patient-based PET sources and equipment-based CT sources. A technically sound design integrates TG-108 PET methods, NCRP 147 CT methods, realistic occupancy and workload assumptions, construction coordination, and post-construction verification. When these elements are addressed systematically, the resulting facility can meet applicable radiation protection requirements while remaining efficient, practical, and adaptable to future growth.¹²⁴⁵

Related Resources

• Common PET & RPT isotopes
• Lead shielding design principles
• CT protocol optimization
• Choosing the right radiation survey meter
• Common radiation safety violations
• Radiation shielding design

References

1. Madsen MT, Anderson JA, Halama JR, et al. AAPM Task Group 108: PET and PET/CT shielding requirements. Medical Physics. 2006;33(1):4-15. doi:10.1118/1.2135911. aapm.onlinelibrary.wiley.com
2. National Council on Radiation Protection and Measurements. Structural Shielding Design for Medical X-Ray Imaging Facilities. NCRP Report No. 147. Bethesda, MD: NCRP; 2004. aapm.org
3. National Council on Radiation Protection and Measurements. Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities. NCRP Report No. 151. Bethesda, MD: NCRP; 2005. aapm.org
4. U.S. Nuclear Regulatory Commission. 10 CFR Part 20, Standards for Protection Against Radiation. nrc.gov
5. U.S. Nuclear Regulatory Commission. 10 CFR Part 35, Medical Use of Byproduct Material. nrc.gov
6. National Institute of Standards and Technology. Radionuclide Half-Life Measurements and Decay Data resources for fluorine-18 and other radionuclides. nist.gov
7. U.S. Nuclear Regulatory Commission. Consolidated Guidance About Materials Licenses: Program-Specific Guidance About Medical Use Licenses. NUREG-1556, Volume 9, Revision 3. nrc.gov