Gamma Camera Testing with NEMA NU-1

NEMA NU 1 is the standard that defines how gamma camera performance is measured and reported. It specifies uniform methods for intrinsic spatial resolution, intrinsic spatial linearity, energy resolution, flood-field uniformity, planar sensitivity, count-rate performance, and SPECT characteristics including reconstructed resolution and center of rotation. The current edition is NEMA NU 1-2023, which supersedes the 2018 and 2012 editions.¹ A medical physicist uses these definitions to perform acceptance testing, set facility baselines, and design a routine quality control program that catches detector drift before it affects clinical images.

Gamma camera performance testing answers a practical question: is this camera producing accurate, uniform, well-resolved images today, and how does that compare to when it was installed? This guide walks through the NEMA NU 1 parameters, the math behind the most important ones, how they map to acceptance versus routine QC, and how they tie into accreditation. DRPS provides this work as part of its PET/CT and nuclear medicine physics and accreditation support services across Florida, Maryland, Virginia, Washington DC, California, and Nevada.

Introduction

A gamma camera is only as trustworthy as its weakest performance parameter. A camera can peak correctly and still produce nonuniform floods; it can show good uniformity and still have a center-of-rotation error that ruins SPECT reconstructions. Because these failure modes are independent, performance testing has to measure each one explicitly rather than rely on a single overall impression.¹²

NEMA NU 1 exists so that every manufacturer, physicist, and accreditation body measures these parameters the same way. The standard establishes definitions and measurement geometry for intrinsic and system performance, but — importantly — it does not set minimum performance levels. Its purpose is uniform measurement and reporting, so that a stated specification means the same thing across vendors.¹ Clinical acceptance criteria come from the manufacturer's specification sheet, from accreditation programs such as the ACR Nuclear Medicine Accreditation Program, and from the facility's own baselines.

That distinction matters in practice. A physicist performing acceptance testing measures the NEMA parameters and compares them against the purchase specification. A physicist designing routine QC selects the subset of those parameters that drift over time — uniformity, energy peak, resolution, COR — and sets a cadence to detect change early.²³

Topic Explanation

Intrinsic versus system measurements

NEMA NU 1 distinguishes two measurement conditions:

• Intrinsic — the collimator is removed and a point source of the imaging radionuclide is placed at a defined distance (commonly five times the largest field-of-view dimension) to approximate a uniform flux. Intrinsic tests characterize the detector crystal, photomultiplier tubes, and electronics alone.
• System (extrinsic) — the collimator is in place, as in clinical imaging, usually using a flood source. System tests reflect the complete imaging chain, including collimator condition.

Intrinsic measurements isolate the detector; system measurements describe what the patient actually gets. Both are part of a complete evaluation, and uniformity in particular is measured both ways.¹²

The useful and central fields of view

Most planar parameters are reported over two regions: the useful field of view (UFOV), essentially the full usable detector area, and the central field of view (CFOV), defined by scaling the UFOV linear dimensions by 0.75. Edge effects degrade performance near detector boundaries, so the CFOV typically shows better numbers than the UFOV. Reporting both prevents a camera with poor edges from hiding behind a good central value.¹²

For the parameters that change with time, see how they fit alongside the rest of a nuclear medicine QC program in our guides on SPECT/CT quality control and collimator selection.

Key Technical Principles

Flood-field uniformity: integral and differential

Flood-field uniformity is the most frequently performed gamma camera QC test because nonuniformity is both common and clinically consequential. NEMA NU 1 reports two measures, each over the UFOV and CFOV.

Integral uniformity (IU) captures the worst-case deviation across the field:

$$\mathrm{IU}=100\times \frac{C_{\max }-C_{\min }}{C_{\max }+C_{\min }}$$

where $C_{\max }$ and $C_{\min }$ are the maximum and minimum counts in any pixel of the smoothed flood image within the region.

Differential uniformity (DU) captures localized, high-frequency nonuniformity by finding the largest change over a small sliding set of contiguous pixels (typically five) along rows and columns:

$$\mathrm{DU}=100\times \max \left(\frac{C_{\mathrm{hi}}-C_{\mathrm{lo}}}{C_{\mathrm{hi}}+C_{\mathrm{lo}}}\right)$$

evaluated over each five-pixel window. A camera can have acceptable integral uniformity but poor differential uniformity if it has a sharp localized defect such as a failing PMT.¹⁴

Worked example. Suppose a smoothed intrinsic flood gives $C_{\max }=105$ and $C_{\min }=97$ counts in the UFOV. Then:

$$\mathrm{IU}=100\times \frac{105-97}{105+97}=100\times \frac{8}{202}\approx 4.0\%$$

A result near 4% would typically be acceptable against a common manufacturer specification of roughly 5% or better, but the value must be compared to the camera's own specification and baseline rather than a universal limit.¹² Independent analysis software can yield slightly different uniformity numbers than the vendor console because of differences in pixel size and applied corrections, which is why physicists often verify with a second tool.⁴⁵

Energy resolution

Energy resolution describes how precisely the camera measures the energy of detected photons. It is the full width at half maximum (FWHM) of the photopeak, expressed as a percentage of the photopeak energy:

$$\%\mathrm{ER}=\frac{\mathrm{FWHM}}{E_{\mathrm{photopeak}}}\times 100$$

Worked example. For technetium-99m (photopeak 140.5 keV), a measured photopeak FWHM of 14.0 keV gives:

$$\%\mathrm{ER}=\frac{14.0}{140.5}\times 100\approx 10.0\%$$

Modern sodium iodide cameras commonly achieve energy resolution on the order of about 9–10% at 140 keV; better energy resolution allows tighter energy windows, improving scatter rejection and image contrast.¹²

Intrinsic spatial resolution and linearity

Intrinsic spatial resolution is measured from line sources (or a slit/bar phantom) as the FWHM of the line spread function, in millimeters. Smaller FWHM means finer detail. Spatial linearity quantifies geometric distortion — how straight a line image remains — and is closely tied to uniformity, because linearity correction maps feed the uniformity correction.¹²

Sensitivity and count-rate performance

System planar sensitivity is the detected count rate per unit activity, typically reported in counts per minute per megabecquerel (cpm/MBq), measured with a defined source and collimator. Count-rate performance characterizes how the camera behaves at high count rates, where dead-time losses cause the observed rate to fall below the true rate. These parameters matter for quantitative and dynamic studies.¹²

SPECT center of rotation

For tomographic imaging, the center of rotation (COR) is the alignment between the mechanical rotation axis and the electronic center of the projection matrix. A COR offset shifts projections relative to one another, producing ring or "tuning-fork" artifacts and blurring reconstructed resolution. COR is verified at acceptance, after relevant service, and periodically, and is a required acceptance and reference test in international QC guidance.²³

NEMA NU 1 parameters at a glance

Specific numeric tolerances are not part of NEMA NU 1 itself; they come from the manufacturer specification, the accreditation program, and the facility baseline.¹²³

Relationship to NEMA NU 2 for PET

Facilities that operate both gamma cameras and PET scanners encounter two parallel standards. NEMA NU 1 governs gamma cameras (SPECT and planar); NEMA NU 2 governs PET scanners. They share the same philosophy — uniform measurement and reporting without mandated pass/fail limits — but address different physics. For the PET counterpart, see our guide on PET/CT NEMA NU-2 performance testing.

Clinical Impact

Uniformity defects and artifacts

Flood-field nonuniformity propagates directly into clinical images. In planar imaging, a nonuniform detector can mimic or mask focal uptake. In SPECT, even modest nonuniformity is amplified into ring artifacts centered on the rotation axis, which can be mistaken for pathology or can obscure it. This is why daily uniformity QC, with correction maps kept current, is the backbone of gamma camera quality control.²³

Energy windowing and quantification

Energy resolution sets the floor on how tightly the energy window can be placed. Poor energy resolution forces wider windows that admit more scatter, lowering contrast and degrading quantitative accuracy. As nuclear medicine moves toward quantitative SPECT/CT — where standardized uptake values and absolute activity concentrations are reported — stable energy resolution, sensitivity calibration, and uniformity become prerequisites for trustworthy numbers.⁶

Reliable comparisons over time

According to PubMed, an EANM guideline lays out routine QC recommendations for nuclear medicine instrumentation precisely so that performance can be tracked consistently and degradation detected early (Busemann Sokole et al., 2010, DOI).⁷ Comparative studies also show that uniformity values from different analysis packages agree well when methods are standardized, but can differ from console software because of pixel-size and correction differences — reinforcing the value of an independent physicist review (Edam et al., 2018, DOI; Supramaniam et al., 2024, DOI).⁴⁵

Practical Optimization Tips

A practical gamma camera QC program layers tests by how quickly each parameter can drift.

1. Peak and flood daily

Start each clinical day by setting the energy peak for the radionuclide in use and acquiring a uniformity flood. Review integral and differential uniformity against baseline before the first patient.

2. Keep correction maps current

Uniformity, linearity, and energy correction maps degrade as detectors age. Refresh high-count uniformity correction floods on the manufacturer's recommended schedule; a stale correction map is a common cause of creeping nonuniformity.

3. Check resolution and linearity weekly

Acquire a bar-phantom image weekly to track intrinsic spatial resolution and linearity qualitatively, escalating to quantitative measurement if degradation is suspected.

4. Verify COR and sensitivity on schedule

Confirm center of rotation and system sensitivity at acceptance, after any service that could disturb mechanical alignment or detector tuning, and at the periodic interval defined in your QC program.

5. Use an independent analysis path

Because console software and third-party tools can compute uniformity differently, periodically analyze the same flood with an independent package (for example, a standards-based QC toolkit) to confirm the console is reporting correctly.⁴⁵

Common pitfalls to avoid

• Treating NEMA numbers as pass/fail limits. NEMA NU 1 standardizes measurement, not acceptance; use manufacturer and accreditation criteria.
• Reporting only CFOV. Edge defects hide in the UFOV; report both regions.
• Skipping differential uniformity. A camera can pass integral uniformity yet have a localized PMT defect.
• Ignoring COR after service. Mechanical work can shift alignment and ruin SPECT without affecting planar floods.
• Letting correction maps go stale. Old uniformity maps are a frequent source of artifacts.
• Assuming console software is authoritative. Verify with an independent analysis when results look borderline.

Regulatory Considerations

Gamma camera performance testing sits at the intersection of accreditation requirements, manufacturer specifications, and the radioactive material license that authorizes nuclear medicine. A qualified medical physicist performs acceptance testing and the comprehensive annual evaluation, while trained technologists perform daily and weekly QC under the physicist's program.²³

Key frameworks to reference:

• NEMA NU 1-2023 — the current standard defining how gamma camera performance parameters are measured and reported.¹
• IAEA Human Health Series No. 6, Quality Assurance for SPECT Systems — detailed acceptance, reference, and routine test procedures for scintillation cameras in planar, whole-body, and SPECT modes.²
• IAEA Quality Control Atlas for Scintillation Camera Systems — a visual reference for recognizing and diagnosing camera artifacts.³
• ACR Nuclear Medicine Accreditation Program — clinical accreditation criteria, including the requirement for an annual medical physicist or nuclear medicine scientist evaluation.⁸
• 10 CFR Part 35 — the NRC's Medical Use of Byproduct Material rule, which governs the radioactive material used in nuclear medicine and the responsibilities of the radiation safety officer and authorized users.⁹

The radiopharmaceuticals imaged by a gamma camera are byproduct material regulated by the NRC under 10 CFR Parts 20 and 35, or by an Agreement State program. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States that license medical use under their own rules, while Washington, DC and Delaware are regulated directly by the NRC. Performance testing should be documented so it supports both accreditation and license requirements; for the regulatory side, see our guide on the radiation safety officer role.

Frequently Asked Questions (FAQs)

What is NEMA NU 1?

NEMA NU 1 is the National Electrical Manufacturers Association standard titled Performance Measurements of Gamma Cameras. It defines uniform methods for measuring and reporting gamma camera performance parameters such as intrinsic spatial resolution, energy resolution, flood-field uniformity, sensitivity, count-rate performance, and SPECT characteristics, so that systems can be specified and compared consistently. The current edition is NEMA NU 1-2023.

What is the difference between intrinsic and system (extrinsic) measurements?

Intrinsic measurements are made with the collimator removed, using a point source, and characterize the detector and electronics alone. System or extrinsic measurements are made with the collimator in place, which is how the camera is used clinically. Intrinsic tests isolate detector performance; system tests reflect the full imaging chain including the collimator.

What are integral and differential flood-field uniformity?

Integral uniformity is the maximum deviation in counts across the field of view, expressed as a percentage, and reflects the worst-case nonuniformity. Differential uniformity measures the largest rate of change over a small group of contiguous pixels (typically five), reflecting localized or high-frequency nonuniformity. Both are reported separately for the useful field of view and the central field of view.

What is the center of rotation (COR) and why does it matter for SPECT?

The center of rotation is the alignment between the mechanical axis the camera rotates about and the electronic center of the projection data. A COR error causes misregistered projections, producing ring or tuning-fork artifacts and degraded reconstructed resolution in SPECT. COR is verified during acceptance and reference testing and after service that could affect alignment.

How often should gamma camera QC be performed?

Daily QC typically includes energy-peak setting and flood-field uniformity. Spatial resolution and linearity with a bar phantom are commonly weekly. Center of rotation and sensitivity are often checked at acceptance, after relevant service, and periodically thereafter, with a comprehensive performance evaluation by a medical physicist at least annually and at acceptance, consistent with accreditation requirements.

Does NEMA NU 1 set pass/fail limits for clinical cameras?

No. NEMA NU 1 standardizes how parameters are measured and reported but does not establish minimum performance levels or clinical pass/fail criteria. Acceptance criteria come from the manufacturer's specifications, accreditation programs such as ACR, and the facility's own baselines established at acceptance testing.

Why measure energy resolution on a gamma camera?

Energy resolution determines how well the camera can separate the photopeak of the imaging radionuclide from scatter and from other photon energies. Better energy resolution allows tighter, more accurate energy windowing, which improves image contrast and is essential for dual-isotope and scatter-correction techniques.

Key Takeaways

• NEMA NU 1-2023 standardizes measurement, not acceptance. It defines how to measure and report gamma camera performance; clinical limits come from the manufacturer, accreditation, and baselines.
• Uniformity is the daily workhorse. Integral and differential uniformity, over both UFOV and CFOV, catch the defects that most often reach clinical images.
• Report intrinsic and system results. Intrinsic tests isolate the detector; system tests reflect clinical use with the collimator.
• COR is a SPECT-specific must. Center-of-rotation errors cause ring artifacts and must be verified after any service affecting alignment.
• Energy resolution enables tight windowing. It underpins scatter rejection, contrast, and quantitative accuracy.
• Verify with an independent path. Console and third-party software can differ; a physicist's independent analysis protects against silent drift.

Conclusion

Gamma camera performance testing is what keeps planar and SPECT images quantitatively trustworthy. NEMA NU 1-2023 gives the field a common language for spatial resolution, energy resolution, uniformity, sensitivity, count-rate behavior, and SPECT alignment, while leaving clinical acceptance to manufacturer specifications, accreditation programs, and facility baselines. The medical physicist's job is to translate the standard into a layered program — daily uniformity, weekly resolution, periodic COR and sensitivity, and a comprehensive annual evaluation — that detects degradation before it changes a patient's images.

Facilities that treat gamma camera QC as a structured, documented program rather than a daily formality are better prepared for accreditation, better protected against artifact-driven errors, and better positioned for quantitative nuclear medicine.

How DRPS Can Help

Diagnostic Radiation Physics Services performs acceptance testing and annual performance evaluation of gamma cameras and SPECT/CT systems using NEMA NU 1 methodology, establishes facility baselines, and designs routine QC programs for technologist staff. Our PET/CT and nuclear medicine physics services include uniformity, resolution, energy resolution, sensitivity, and center-of-rotation testing, and our accreditation support helps facilities meet ACR Nuclear Medicine requirements. We also provide radiation safety officer and broader medical physics consulting support.

DRPS supports facilities across our service locations, including Florida, Maryland, Virginia, Washington DC, California, Nevada, New York, Pennsylvania, New Jersey, and Delaware.

A well-run gamma camera program is invisible when it works — and that is exactly the point.

Related Resources

• PET/CT NEMA NU-2 performance testing
• SPECT/CT quality control
• Gamma camera collimator selection
• Cardiac SPECT MPI quality control
• Dose calibrator quality control
• PET/CT and nuclear medicine physics
• Accreditation support

References

1. National Electrical Manufacturers Association. NEMA NU 1-2023: Performance Measurements of Gamma Cameras. 2023. nema.org
2. International Atomic Energy Agency. Quality Assurance for SPECT Systems (IAEA Human Health Series No. 6). 2009. iaea.org
3. International Atomic Energy Agency. Quality Control Atlas for Scintillation Camera Systems. 2003. iaea.org
4. Edam AN, Fornasier MR, De Denaro M, Sulieman A, Alkhorayef M, Bradley DA. Quality control in dual head gamma-cameras: Comparison between methods and software used for image analysis. Appl Radiat Isot. 2018;141:288-291. doi:10.1016/j.apradiso.2018.07.027. PubMed
5. Supramaniam TT, Udin MY, Musarudin M. Comparative Assessment of Agreement in Uniformity Analyses across Quality Control Software Platforms. World J Nucl Med. 2024;24(1):47-56. doi:10.1055/s-0044-1795102. PubMed
6. Kurkowska S, Birkenfeld B, Piwowarska-Bilska H. Physical quantities useful for quality control of quantitative SPECT/CT imaging. Nucl Med Rev Cent East Eur. 2021;24(2):93-98. doi:10.5603/NMR.2021.0020. PubMed
7. Busemann Sokole E, Płachcínska A, Britten A, et al. Routine quality control recommendations for nuclear medicine instrumentation. Eur J Nucl Med Mol Imaging. 2010;37(3):662-671. doi:10.1007/s00259-009-1347-y. PubMed
8. American College of Radiology. Nuclear Medicine and PET Accreditation Program Requirements. acraccreditation.org
9. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov