Parathyroid Scintigraphy: Sestamibi & SPECT/CT

Parathyroid scintigraphy exists to answer one surgical question: where is the hyperfunctioning gland? When the biochemistry already proves primary hyperparathyroidism, accurate preoperative localization is what lets a surgeon perform a focused, minimally invasive parathyroidectomy through a small incision instead of exploring all four gland sites through a bilateral open neck exploration. The imaging physics — sestamibi kinetics, collimator choice, tomographic timing, and dosimetry — determines whether that map is trustworthy.¹²

Tc-99m sestamibi is the workhorse radiopharmaceutical, and it can be imaged two different ways: a dual-phase (single-tracer) washout technique, or a dual-tracer subtraction technique that pairs sestamibi with a thyroid-specific tracer. Adding SPECT/CT to either approach raises sensitivity and, just as importantly, pins the focus to an anatomic location the surgeon can act on.¹²³

Introduction

Primary hyperparathyroidism (pHPT) is usually caused by a single hyperfunctioning parathyroid adenoma, though multigland hyperplasia and, rarely, carcinoma also occur. Secondary hyperparathyroidism (sHPT), common in chronic kidney disease, typically involves multiple hyperplastic glands. The diagnosis of hyperparathyroidism is biochemical — elevated parathyroid hormone with hypercalcemia in pHPT — and imaging plays no role in making that diagnosis. Imaging exists solely to localize the culprit gland or glands so surgery can be planned.¹²

That distinction matters for how a nuclear medicine department should think about the study. The value of parathyroid scintigraphy is measured not by whether it "sees something" but by whether it correctly guides the operation. A false-positive focus from a thyroid nodule can send a surgeon to the wrong side of the neck; a missed ectopic gland in the mediastinum can turn a 20-minute focused procedure into a failed exploration and a second operation. The techniques below are all attempts to maximize true localization while suppressing the false ones.¹³

This article covers the tracer physics, the dual-phase and dual-tracer techniques, the role and timing of SPECT/CT, a worked dosimetry example, the collimator and QC considerations that decide image quality, and the regulatory framework for a byproduct-material study. DRPS supports these programs through its PET/CT and nuclear medicine physics and medical physics consulting services.

Topic Explanation

What is parathyroid scintigraphy?

Parathyroid scintigraphy is a functional nuclear medicine study that maps hyperfunctioning parathyroid tissue using a radiopharmaceutical that concentrates in metabolically active, mitochondria-rich cells. Tc-99m sestamibi (methoxyisobutylisonitrile, MIBI) accumulates in cells with high mitochondrial density and blood flow. Hyperfunctioning parathyroid adenomas are rich in mitochondria-laden oxyphil cells, so they take up and retain sestamibi avidly.¹²

The complication is that normal thyroid tissue, which sits immediately adjacent to the parathyroid glands, also takes up sestamibi. Every parathyroid imaging technique is therefore a strategy for separating parathyroid signal from thyroid background — either by exploiting a time difference (differential washout) or by using a second tracer that marks only the thyroid so it can be subtracted.¹²

The clinical setting: focused surgery

The modern surgical goal in pHPT is minimally invasive parathyroidectomy: a small, targeted incision guided by preoperative imaging, often combined with intraoperative parathyroid hormone monitoring to confirm the offending gland has been removed. This approach depends on confident preoperative localization. When imaging clearly identifies a single adenoma, the surgeon can plan a focused procedure with shorter operating time and lower morbidity than bilateral exploration.¹² For the physics of the emission tomography that makes anatomic localization possible, see our overview of SPECT/CT quality control.

Key Technical Principles

The radiopharmaceutical and its dose

Tc-99m sestamibi emits 140 keV gamma photons, ideal for a modern gamma camera with a low-energy collimator. A typical administered activity for parathyroid imaging is 740–1110 MBq (20–30 mCi) given intravenously.¹ Using the ICRP Publication 128 effective-dose coefficient for Tc-99m sestamibi (rest protocol) of approximately $9.0\times 10^{-3}\text{mSv/MBq}$, the effective dose from the radiopharmaceutical alone is:⁹

and at the upper end of the activity range:

A SPECT/CT study adds the CT component, which varies widely with technique — a low-dose localization CT contributes far less than a diagnostic-quality CT, and that trade-off between anatomic detail and dose is a legitimate protocol-optimization decision for the medical physicist and nuclear medicine physician.²

Dual-phase (single-tracer) technique

The dual-phase technique uses sestamibi alone and exploits differential washout. Early images are acquired roughly 10–15 minutes after injection, when both thyroid and parathyroid tissue are visualized. Delayed images are acquired at approximately 1.5–3 hours, by which time sestamibi has largely washed out of normal thyroid but is retained in the hyperfunctioning parathyroid gland.¹²

The differential can be expressed as a washout or retention index. If $C_{\text{early}}$ and $C_{\text{delayed}}$ are the counts (or count densities) in a region of interest at the two time points, the washout for that region is:

Normal thyroid shows a high washout (the signal fades), while a parathyroid adenoma shows a low washout (the signal persists). A focus that remains bright on delayed images while the surrounding thyroid dims is the classic dual-phase signature of an adenoma. The limitation is biological variability: some thyroid tissue washes out slowly, and some adenomas wash out quickly, which produces false positives and false negatives that no timing choice fully eliminates.¹⁷

Dual-tracer (subtraction) technique

The dual-tracer technique adds a thyroid-specific tracer — iodine-123 (as sodium iodide) or Tc-99m pertechnetate — that is taken up by thyroid but not by parathyroid tissue. The thyroid image is then subtracted from the sestamibi image, and what remains is parathyroid signal. Because subtraction removes the thyroid contribution directly rather than waiting for it to wash out, the dual-tracer approach does not depend on washout kinetics and is generally regarded as more sensitive, particularly when thyroid washout is atypical or nodular disease is present.²⁷

The trade-off is technical: subtraction imaging requires accurate co-registration of the two acquisitions, so patient motion between images degrades it, and using two tracers adds workflow complexity. The EANM guidelines note that the sestamibi–iodine-123 dual-tracer method is more efficient than the dual-phase scan, while SPECT/CT improves the sensitivity and specificity of either.²

Collimator and tomographic considerations

For the 140 keV photons of Tc-99m, a low-energy high-resolution (LEHR) parallel-hole collimator is standard. Many protocols add a pinhole collimator for high-resolution planar neck views: the pinhole magnifies the small, superficial neck structures and improves effective spatial resolution for small glands, at the cost of sensitivity and field of view. The choice between collimators is the same resolution-versus-sensitivity trade-off that governs all gamma-camera imaging — discussed in general terms in our guide to gamma-camera collimator selection.¹

SPECT/CT adds cross-sectional localization and CT-based attenuation correction, and it is decisive for ectopic glands and for distinguishing a parathyroid focus from a thyroid nodule. Acquisition timing matters: at least one study found that a single late-phase SPECT/CT was significantly superior to early SPECT/CT for identifying adenomas, and that late SPECT/CT improved accuracy over planar imaging — suggesting early SPECT/CT can often be eliminated to shorten the protocol.⁵ The accuracy of that fused image depends on the same tomographic QC — uniformity, center of rotation, and SPECT/CT registration — that underlies every hybrid study; see SPECT center of rotation QC.

Comparison of imaging approaches

The table summarizes the main techniques and representative per-patient sensitivities reported in the literature. Sensitivities vary widely with patient population, gland size, multigland disease, and reader experience, so these are orientation values, not guarantees.³⁴⁶

Across techniques, the rate of ectopic parathyroid adenomas is reported at roughly 4–20 percent, and SPECT/CT is consistently superior to planar and SPECT imaging for localizing these ectopic sites — the situation where hybrid imaging changes the operation the most.³

Clinical Impact

The clinical payoff of good parathyroid localization is a smaller, faster, lower-morbidity operation. When imaging confidently identifies a single adenoma, the surgeon can perform a focused parathyroidectomy; when imaging is negative or discordant, bilateral exploration remains the fallback. In one single-institution series, dual-phase sestamibi with SPECT/CT identified an adenoma in about two-thirds of pHPT patients and reached a sensitivity of roughly 92 percent among operated patients, allowing the surgeon to plan the appropriate approach.⁶

Concordance between scintigraphy and neck ultrasound is a particularly strong predictor of surgical success, which is why the two are treated as complementary first-line studies rather than competitors. When both are inconclusive, SPECT/CT provides meaningful incremental value — in one cohort of patients with inconclusive ultrasound, sestamibi SPECT/CT was positive in 80 percent with a sensitivity of 97 percent, versus 63 percent for planar imaging alone.⁴

Secondary hyperparathyroidism is a harder problem because it is usually multigland disease. Studies in sHPT patients on dialysis show that SPECT/CT outperforms planar dual-phase scintigraphy and ultrasound for detecting hyperplastic glands, including ectopic ones, though no single modality reliably maps all four glands.⁸ And when scintigraphy fails altogether, current guidelines increasingly point to F-18 fluorocholine PET/CT as a more sensitive second-line study, reflecting a broader shift toward PET tracers in parathyroid imaging.²⁷

Practical Optimization Tips

Protocol choices that matter

• Match the technique to the population. Dual-tracer subtraction is worth the added complexity where nodular thyroid disease is common or dual-phase washout is unreliable; dual-phase is simpler where thyroid anatomy is straightforward.²⁷
• Prefer late-phase SPECT/CT. Evidence supports late SPECT/CT over early SPECT/CT for adenoma detection, which can also streamline the protocol and improve throughput.⁵
• Add a pinhole planar view for small, superficial glands where magnification improves resolution.¹
• Co-register carefully in subtraction imaging. Minimize patient motion between the two acquisitions; misregistration creates subtraction artifacts that mimic or hide lesions.²

Quality control that protects the result

Parathyroid SPECT/CT is only as good as the underlying tomographic QC. The essentials are the same as for any hybrid study:

• Daily gamma-camera uniformity and energy-peak checks at the 140 keV Tc-99m photopeak.
• Center-of-rotation and SPECT/CT registration verification, so the emission focus lands on the correct CT voxel.
• Dose-calibrator constancy, accuracy, and linearity so the administered activity — and therefore the dosimetry above — is correct; see dose calibrator quality control.
• CT dose optimization for the localization CT, choosing the lowest technique consistent with reliable anatomic localization.

Pitfalls to avoid

• Thyroid nodules mimicking adenomas. A hyperfunctioning or sestamibi-avid thyroid nodule is the classic false positive; SPECT/CT and dual-tracer subtraction both help resolve it.³⁷
• Assuming a negative scan excludes disease. Small adenomas, hyperplasia, and multigland disease can be occult. The biochemical diagnosis governs; a negative scan means localization failed, not that the patient is well.²
• Neglecting ectopic sites. Always extend imaging to include the mediastinum when the neck is negative, given the 4–20 percent ectopic rate.³

Regulatory Considerations

Because Tc-99m sestamibi is a reactor- and generator-derived byproduct material, parathyroid scintigraphy falls under NRC (or Agreement State) medical-use regulation — not the FDA/state X-ray framework that governs the CT tube in the same scanner. The gamma-camera and radiopharmaceutical side is governed by 10 CFR Part 35; the CT subsystem of a SPECT/CT is separately regulated as a radiation-producing machine by the state.¹⁰

Key points for a compliant program:

• 10 CFR Part 35 — Medical Use of Byproduct Material authorizes the diagnostic use of Tc-99m radiopharmaceuticals under an authorized user and the radiation safety officer's program. Diagnostic administrations like sestamibi do not require a written directive (that requirement applies to therapy and to specified quantities of I-131), but they do require proper dosage determination and records.¹⁰
• 10 CFR Part 20 — Standards for Protection Against Radiation sets the occupational and public dose limits and the ALARA framework that shape technologist practice, waste handling, and patient-release considerations.¹¹
• Guideline conformance. The SNMMI parathyroid scintigraphy practice guideline and the EANM parathyroid imaging guidelines define acceptable acquisition, processing, and reporting practice, and the ACR–ACNM–SNMMI–SPR practice parameter provides parallel U.S. guidance. Documented conformance supports both accreditation and defensible clinical quality.¹²¹²
• Agreement States. Of the states DRPS serves, Florida, Maryland, Virginia, California, Nevada, Pennsylvania, New York, and New Jersey are NRC Agreement States administering their own medical-use programs, while Washington DC and Delaware are regulated directly by the NRC. Confirm which authority issues your license and which reporting and QC requirements apply.

Frequently Asked Questions (FAQs)

What is parathyroid scintigraphy used for?

Parathyroid scintigraphy localizes hyperfunctioning parathyroid glands before surgery, most often in primary hyperparathyroidism. Accurate preoperative localization is what allows a focused, minimally invasive parathyroidectomy instead of a bilateral open neck exploration.

How does Tc-99m sestamibi show a parathyroid adenoma?

Sestamibi concentrates in mitochondria-rich cells and is taken up by both thyroid and hyperfunctioning parathyroid tissue. In the dual-phase technique it washes out of normal thyroid faster than from a parathyroid adenoma, so the adenoma stands out on delayed images; in the dual-tracer technique a separate thyroid tracer is subtracted to reveal the parathyroid focus.

Why does adding SPECT/CT improve parathyroid imaging?

SPECT/CT adds three-dimensional and anatomic information to planar imaging. Meta-analytic data show pooled per-patient sensitivity around 86 percent for dual-phase sestamibi SPECT/CT, higher than SPECT or planar imaging alone, and it is particularly valuable for ectopic glands and for planning a minimally invasive approach.

What is the difference between the dual-phase and dual-tracer techniques?

The dual-phase (single-tracer) technique uses sestamibi alone and relies on differential washout between thyroid and parathyroid tissue over early and delayed images. The dual-tracer (subtraction) technique adds a thyroid-specific tracer — iodine-123 or Tc-99m pertechnetate — and subtracts the thyroid image from the sestamibi image. The dual-tracer approach can be more sensitive, especially when thyroid washout is atypical.

What radiation dose does a parathyroid sestamibi scan involve?

A typical administered activity of Tc-99m sestamibi is about 740 to 1110 MBq (20 to 30 mCi). Using the ICRP Publication 128 effective-dose coefficient of roughly 9.0 x 10^-3 mSv/MBq, that corresponds to an effective dose on the order of 7 to 10 mSv from the radiopharmaceutical, before any CT contribution from a SPECT/CT study.

Which collimator is used for parathyroid scintigraphy?

A low-energy high-resolution (LEHR) parallel-hole collimator is standard for the 140 keV photons of Tc-99m. A pinhole collimator is often added for high-resolution planar neck views because its magnification improves spatial resolution for small superficial glands.

Can imaging be normal when a patient truly has hyperparathyroidism?

Yes. Small adenomas, multigland disease, and hyperplasia can produce a negative or equivocal scan. When scintigraphy and ultrasound are inconclusive, guidelines increasingly point to F-18 fluorocholine PET/CT as a more sensitive second-line study, and the biochemical diagnosis of hyperparathyroidism still stands regardless of imaging.

Key Takeaways

• The study localizes, it does not diagnose. Hyperparathyroidism is a biochemical diagnosis; scintigraphy exists to guide surgery.¹²
• Two sestamibi techniques. Dual-phase relies on differential washout; dual-tracer subtracts a thyroid tracer and is generally more sensitive, especially with nodular thyroid disease.²⁷
• SPECT/CT raises sensitivity and localizes anatomically, with pooled dual-phase SPECT/CT sensitivity around 86 percent and the largest benefit for ectopic glands (4–20 percent of cases).³
• Late-phase SPECT/CT is often preferred and can shorten the protocol without losing adenomas.⁵
• Dosimetry is modest but real — roughly 7–10 mSv from 740–1110 MBq of sestamibi by ICRP 128, plus the CT contribution.⁹
• Jurisdiction is split — byproduct material under 10 CFR Part 35 (or Agreement State), CT under state machine rules.¹⁰

Conclusion

Parathyroid scintigraphy is a compact case study in how nuclear medicine physics serves surgery. The tracer kinetics of sestamibi, the choice between washout and subtraction, the collimator and tomographic timing, and the SPECT/CT registration all converge on a single deliverable: a reliable preoperative map that lets the surgeon operate through a small, targeted incision. When the physics and QC are sound, the study earns its place in the workup; when they are neglected, a false focus or a missed ectopic gland can send the operation astray.

The programs that get the most from parathyroid imaging treat it as a hybrid-imaging discipline — matching technique to population, favoring late SPECT/CT, guarding tomographic QC, and reading the scan in concordance with ultrasound and biochemistry rather than in isolation.

How DRPS Can Help

Diagnostic Radiation Physics Services supports nuclear medicine and SPECT/CT programs with acquisition-protocol optimization, gamma-camera and SPECT/CT quality control, dose-calibrator testing, radiopharmaceutical dosimetry review, and radiation safety program support aligned with SNMMI and EANM guidance and NRC or Agreement State requirements. Our PET/CT and nuclear medicine physics and medical physics consulting teams help facilities keep hybrid imaging accurate, low-dose, and inspection-ready.

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

Related Resources

• Gamma-camera collimator selection
• SPECT/CT quality control
• SPECT center of rotation QC
• Thyroid uptake measurement
• Dose calibrator quality control
• PET/CT and nuclear medicine physics services
• Medical physicist consulting

References

1. Greenspan BS, Dillehay G, Intenzo C, et al. SNM practice guideline for parathyroid scintigraphy 4.0. Journal of Nuclear Medicine Technology. 2012;40(2):111-118. doi:10.2967/jnmt.112.105122. doi.org
2. Petranović Ovčariček P, Giovanella L, Carrió Gasset I, et al. The EANM practice guidelines for parathyroid imaging. European Journal of Nuclear Medicine and Molecular Imaging. 2021;48(9):2801-2822. doi:10.1007/s00259-021-05334-y. doi.org
3. Wong KK, Fig LM, Gross MD, Dwamena BA. Parathyroid adenoma localization with 99mTc-sestamibi SPECT/CT: a meta-analysis. Nuclear Medicine Communications. 2015;36(4):363-375. doi:10.1097/MNM.0000000000000262. doi.org
4. Assante R, Zampella E, Nicolai E, et al. Incremental value of sestamibi SPECT/CT over dual-phase planar scintigraphy in patients with primary hyperparathyroidism and inconclusive ultrasound. Frontiers in Medicine. 2019;6:164. doi:10.3389/fmed.2019.00164. doi.org
5. Hunter K, Gavin N, McQuade C, Hogan B, Feeney J. Optimal timing of SPECT/CT to demonstrate parathyroid adenomas in 99mTc-sestamibi scintigraphy. Nuclear Medicine Review. 2022;25(2):89-94. doi:10.5603/NMR.a2022.0020. doi.org
6. Ciappuccini R, Morera J, Pascal P, et al. Dual-phase 99mTc sestamibi scintigraphy with neck and thorax SPECT/CT in primary hyperparathyroidism: a single-institution experience. Clinical Nuclear Medicine. 2012;37(3):223-228. doi:10.1097/RLU.0b013e31823362e5. doi.org
7. Huglo D. Functional imaging for hyperparathyroidism. Presse Médicale. 2022;51(2):104120. doi:10.1016/j.lpm.2022.104120. doi.org
8. Zhang R, Zhang Z, Huang P, et al. Diagnostic performance of ultrasonography, dual-phase 99mTc-MIBI scintigraphy, early and delayed 99mTc-MIBI SPECT/CT in preoperative parathyroid gland localization in secondary hyperparathyroidism. BMC Medical Imaging. 2020;20(1):91. doi:10.1186/s12880-020-00490-3. doi.org
9. International Commission on Radiological Protection. Radiation Dose to Patients from Radiopharmaceuticals: A Compendium of Current Information Related to Frequently Used Substances. ICRP Publication 128. Annals of the ICRP. 2015;44(2 Suppl). icrp.org
10. U.S. Nuclear Regulatory Commission. 10 CFR Part 35: Medical Use of Byproduct Material. ecfr.gov
11. U.S. Nuclear Regulatory Commission. 10 CFR Part 20: Standards for Protection Against Radiation. ecfr.gov
12. American College of Radiology. ACR–ACNM–SNMMI–SPR Practice Parameter for the Performance of Scintigraphy for Parathyroid Localization. acr.org