Interpreting and reporting parathyroid 4D CT

Why 4D CT matters

PHPT is the most common cause of hypercalcemia and most often results from a single parathyroid adenoma (85%–90%). Less common causes are multiglandular hyperplasia (~6%), double adenomas (~4%), and carcinoma (<1%). Definitive treatment is surgical removal of abnormal parathyroid tissue.

Preoperative imaging helps select the surgical approach:

If imaging does not localize a gland, the patient still remains a surgical candidate, but BNE is more likely required. Localization is particularly important in persistent or recurrent PHPT, where guidelines regard preoperative localization as mandatory because reoperation carries higher risk.

Anatomy that drives interpretation

Most individuals have four glands—two superior and two inferior—although fewer than four occur in 3%–6% and supernumerary glands in 5%–13%. The key embryologic point is that a gland’s cranio-caudal position does not reliably establish whether it is a superior or inferior gland.

• Superior glands arise from the fourth pharyngeal pouch and are relatively constant in location. About 80% are posterior to the upper thyroid pole, between the cricoid and thyroid cartilages.
• Inferior glands arise from the third pharyngeal pouch along with the thymus, explaining their more variable locations—from the mandibular angle to the pericardium. About half are within 1 cm of the lower thyroid pole.
• An enlarged superior gland can descend into the superior mediastinum—“acquired ectopia”—and may lie lower than an inferior gland.
• The more dependable discriminator of superior versus inferior origin is the tracheoesophageal groove/RLN plane: superior glands are posterior (dorsal) to the recurrent laryngeal nerve (RLN), whereas inferior glands are anterior (ventral).

Up to 16% of glands are ectopic, more often inferior glands. Important ectopic locations include:

• Superior gland: retropharyngeal or retroesophageal.
• Inferior gland: thyrothymic ligament, mediastinum, carotid sheath, or intrathyroidal location.
• Less common sites for either group include carotid sheath, intrathyroidal tissue, or scalene fat.

Operative landmarks and RLN safety

The authors recommend reporting lesion location relative to the cricoid cartilage and the tracheoesophageal groove, rather than primarily by cervical vertebral level. These landmarks are visible or usable in the operative field and are especially valuable in reoperative necks. Also useful are the upper/lower poles and isthmus of the thyroid and the suprasternal notch.

A critical additional responsibility is identifying arterial anatomy associated with a nonrecurrent laryngeal nerve:

• An aberrant right subclavian artery can produce a nonrecurrent right RLN.
• A right aortic arch with aberrant left subclavian artery can produce a nonrecurrent left RLN.
• A nonrecurrent nerve has a substantially increased operative injury risk—reported up to 13%—because usual landmarks for identifying the nerve are unreliable. These vascular variants should therefore be explicitly reported.

Recommended 4D CT technique

The article uses a three-phase protocol:

• Coverage: maxilla through carina.
• Noncontrast phase.
• Arterial phase: 30 seconds after contrast injection begins.
• Venous phase: 60 seconds after injection begins.
• Contrast: 100 mL of 370 mg iodine/mL at 4 mL/s, followed by 40 mL saline.
• Slice thickness: 1.25 mm; interval: 1 mm.
• Tube voltage: 140 kVp, chosen to reduce beam-hardening in the lower neck and upper mediastinum.
• Typical CTDIvol: 19–24 mGy per phase; typical DLP: 400–600 mGy·cm per phase.

The authors favor three phases because the true noncontrast acquisition helps distinguish parathyroid lesions from thyroid tissue. Dual-energy CT with virtual noncontrast imaging could reduce dose by eliminating true noncontrast imaging, but it also removes intrinsic thyroid iodine contrast and is therefore an imperfect substitute.

Practical quality measures include shoulder-lowering positioning and, when feasible, right-arm contrast injection to decrease streak/scatter artifact in the central neck and superior mediastinum.

How to identify adenomas

A parathyroid adenoma is usually lower in attenuation than the normal iodine-rich thyroid on noncontrast CT. The classic postcontrast behavior is arterial hyperenhancement relative to thyroid followed by venous washout, but this classic “type A” pattern occurs in only about 20% of adenomas.

Other enhancement patterns are common:

• Type B: isoattenuating to thyroid in the arterial phase, then hypoattenuating in venous phase.
• Type C: isoattenuating to thyroid in both arterial and venous phases.
• Variant internal appearances include cystic change, fat, and calcification.

Therefore, the noncontrast images are important. In Hashimoto thyroiditis or chronic hypothyroidism, low thyroid iodine content can make thyroid tissue hypoattenuating, reducing the usefulness of the usual noncontrast distinction.

A polar vessel entering or exiting a lesion supports parathyroid origin; it differs from the hilar vascular pattern of a lymph node.

Important mimics

• Exophytic thyroid nodule: Can mimic an adenoma on enhanced images. On true noncontrast CT, thyroid tissue/nodules often remain relatively hyperattenuating due to iodine content. If uncertainty remains, ultrasound can help: adenomas are typically hypoechoic, whereas thyroid nodules are often iso- or hyperechoic to thyroid tissue.
• Lymph node: Usually hypoattenuating relative to thyroid on noncontrast CT, like an adenoma. However, lymph nodes usually demonstrate progressive enhancement from arterial to venous phase; this is unusual for parathyroid lesions. A fatty hilum also supports nodal origin.

The authors advise being suspicious of any structure that cannot confidently be classified as thyroid tissue, lymph node, or vessel, while also correlating with ultrasound and nuclear medicine studies when available. Correlation can improve confidence, especially in difficult or reoperative cases.

Recognizing multigland disease

Multiple candidate parathyroid lesions favor multiglandular disease (MGD), but a key counterintuitive point is that no visible lesion also favors MGD, with reported 88% specificity. MGD lesions are often small.

Features that should raise concern for MGD include:

• More than one candidate lesion.
• No convincing candidate lesion.
• Largest detected candidate lesion measuring less than 7 mm—79% specific for MGD, even when only one lesion is seen.
• In contrast, a lesion larger than 13 mm favors a single adenoma.

These distinctions matter directly because likely MGD shifts surgical planning toward BNE rather than a focused MIP.

Parathyroid carcinoma is rare and difficult to diagnose preoperatively. Large lesions with calcification, very high PTH levels, and younger patient age should increase suspicion.

Suggested search pattern

The authors stress completing a systematic search even after finding an apparently obvious adenoma, because additional abnormal or supernumerary glands may coexist.

For superior glands

• Posterior upper thyroid lobes: 80%–95%.
• Retropharyngeal/retroesophageal locations: about 3%.
• Rarely carotid sheath, intrathyroidal, or scalene fat pad: each <1%.

For inferior glands

• Adjacent to lower thyroid lobe: 50%–60%.
• Along thyrothymic ligament: about 25%.
• Mediastinum: about 3%.
• Rarely carotid sheath or intrathyroidal: each <1%.

What the report should tell surgeons

A clinically useful report should provide a surgical roadmap, not merely state “probable adenoma.” For every candidate lesion, report:

Practical synthesis

Parathyroid 4D CT is a high-value preoperative localization study when interpreted as a detailed surgical map. The radiologist should use embryology, RLN-based anatomy, systematic ectopic-gland search patterns, multiphasic enhancement assessment, and awareness of mimics to identify every credible candidate lesion. The greatest practical value comes from a concise structured report that states lesion number, size, exact surgically meaningful location, diagnostic confidence, ectopic/supernumerary tissue, thyroid disease, and arterial variants that increase RLN risk. Regular correlation with operative findings is the authors’ recommended method for improving future localization accuracy.

Related local reference

Parathyroid 4D CT reference PDF · Parathyroid reference PDF

The supplied article-derived percentages, size thresholds and acquisition parameters are preserved as source statements. The listed protocol is an example from the supplied material.