Pearls, Pitfalls & Normal Variants

Quick Answer

Most avoidable errors in nuclear medicine come from a short list of physiologic variants, benign mimics, and technical artifacts. Knowing them cold — brown fat and muscle on FDG, celiac ganglion on PSMA, the uncinate process on DOTATATE, superscan on bone imaging, attenuation artifacts on cardiac SPECT, and labeling artifacts from free pertechnetate — prevents the majority of false positives.

FDG (oncology)

• Brown adipose tissue: symmetric supraclavicular/paraspinal/mediastinal uptake, more in cold/lean/young patients — not nodal disease. Warmth and patient prep reduce it.
• Muscle uptake: exertion, tension, insulin — diffuse and symmetric along muscle groups.
• Post-treatment inflammation: surgery, radiation, infection, and G-CSF marrow stimulation cause false positives; timing of imaging matters.
• Physiologic: brain, myocardium (diet-dependent), liver/spleen (reference regions), bowel, urinary tract.
• FDG-low malignancies: low-grade, mucinous, some neuroendocrine — a normal SUV does not exclude cancer.

PSMA

• Celiac ganglion: the classic mimic of a retrocrural node.
• Healing ribs/fractures, benign bone lesions (fibrous dysplasia, Paget) — bone uptake that is not metastasis.
• PSMA is not prostate-specific: uptake in other tumors, inflammation, and normal salivary/lacrimal glands, kidneys, and bowel.

DOTATATE (SSTR)

• Uncinate process of the pancreas: physiologic focal uptake mimicking a pancreatic-head tumor.
• Physiologic spleen (highest), adrenals, pituitary; accessory spleen mimicking a lesion.

Bone scintigraphy

• Superscan: diffusely intense skeletal uptake with faint kidneys ("absent kidney sign") — diffuse metastatic or metabolic disease.
• Flare phenomenon: transient increased uptake in healing metastases after effective therapy — not progression.
• Photopenic (cold) lesions: aggressive lytic disease can be missed on planar imaging.

Cardiac

• Attenuation artifacts: breast (anterior) and diaphragm (inferior) mimic defects — use attenuation correction, gating, prone/upright imaging.
• Balanced ischemia: multivessel disease may look uniform on relative SPECT — watch TID, LV dysfunction, and PET flow.
• CT–emission misregistration on hybrid systems (motion/respiration) creating cardiac defects.

Technical / radiopharmacy

• Free pertechnetate (poor labeling): thyroid, stomach, and salivary activity — a classic signature across Tc-99m studies.
• Injection-site extravasation and urinary/skin contamination mimicking pathology.
• Collimator/energy mismatch (e.g. low-energy collimator with I-131) causing septal-penetration artifacts.

The mimics that cause most false positives

A short list of variants and mimics causes most avoidable false positives — know them cold: brown fat and muscle on FDG, the celiac ganglion on PSMA (mimics a retrocrural node), the uncinate process of the pancreas on DOTATATE (mimics a head tumor), the superscan on bone imaging, breast/diaphragm attenuation artifacts on cardiac SPECT, and free pertechnetate (thyroid/stomach/salivary) from poor labeling. Symmetry plus a known anatomic location favors a physiologic variant over disease.

The traps that hide real disease

Some pitfalls cut the other way — they mask disease. On bone imaging, photopenic (cold) lesions from aggressive lytic disease can be missed on planar images, and the post-therapy flare phenomenon mimics progression. On cardiac SPECT, balanced (multivessel) ischemia can look uniform on relative perfusion — watch TID, LV dysfunction, and PET flow reserve. On PSMA, remember it is not prostate-specific (salivary/lacrimal glands, kidneys, bowel, other tumors, inflammation), and FDG-low histologies (mucinous, low-grade, some neuroendocrine) mean a normal SUV never excludes cancer. When in doubt: SPECT/CT, correlate with priors and anatomy, ask about recent surgery/radiation/G-CSF/iodine load, and consider interval repeat imaging.

High-Yield Pearls

• Symmetry and a known anatomic location usually favor a physiologic variant over disease.
• When in doubt, use SPECT/CT, correlate with anatomy and priors, and consider repeat/interval imaging.
• Ask about recent surgery, radiation, G-CSF, and iodine load before over-reading.

Related Pages

• Tracers: FDG, PSMA, DOTATATE; pitfalls: Incidental findings on FDG-PET; reporting: Structured reporting.

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