Photopenic (Cold) Lesions

Quick Answer

A photopenic (cold) lesion is a region of reduced or absent tracer uptake against a background that should be active. The reasoning trap is treating absence as nothing: a cold area is a positive finding whose differential depends entirely on which tracer, which organ, and whether an artifact could explain it. Across nuclear medicine the recurring causes are replacement of normal tissue (tumour, cyst, necrosis, infarct), interruption of blood supply or tracer delivery, and technical attenuation (metal, contrast, patient positioning). The discipline is to exclude artifact first, then localise the differential to the organ.

The general principle

Cold means one of four things: tissue that normally concentrates tracer has been replaced or destroyed; perfusion/delivery to that tissue is interrupted; the tracer's target is absent there; or something between the source and the detector is attenuating the signal. Working through those four in order — replacement, perfusion, target, artifact — turns "a cold spot" into a specific, defensible differential every time.

Organ-by-organ differentials

Bone (MDP/NaF). Most bone metastases are osteoblastic and hot, so a photopenic metastasis is a red flag for an aggressive, purely lytic, or rapidly destructive process — multiple myeloma, renal cell and thyroid metastases, anaplastic lesions, and early avascular necrosis before repair. Other cold causes: radiation ports (sharply geographic margins), surgical defects/prostheses, bone infarct, and attenuation from barium, pacemakers, or belt buckles. A cold lesion on bone scan can be more ominous than a hot one because it implies the osteoblastic response has been outstripped.

Thyroid (pertechnetate/I-123). A cold nodule is one that fails to concentrate tracer relative to normal parenchyma. Most thyroid nodules are cold; the majority are benign, but cold nodules carry the higher malignancy risk among functional categories, so they are triaged by ultrasound and cytology rather than dismissed. Hot (autonomous) nodules are almost always benign — the functional pattern is a risk stratifier, not a diagnosis.

Liver–spleen (sulfur colloid) and hepatobiliary. Cold areas reflect replacement of Kupffer-cell–bearing parenchyma — metastases, cysts, abscess, haemangioma, or focal fat. A classic pearl is focal nodular hyperplasia, which can appear iso- or hot (it contains Kupffer cells) while most other lesions are cold. On lung V/Q, a cold perfusion defect with preserved ventilation is the language of pulmonary embolism (mismatch).

Brain perfusion (HMPAO/ECD) and FDG. Cold regions denote hypoperfusion or hypometabolism — infarct, tumour necrosis, radiation change, or the disease-specific patterns of neurodegeneration (e.g. temporoparietal hypometabolism in Alzheimer disease). Here the shape and location of the cold region carry the diagnosis.

FDG oncology. A photopenic centre within an avid mass usually means central necrosis; diffuse cold in an expected structure can mean prior treatment, or a genuinely FDG-low tumour.

Artifact must be excluded first

Before assigning pathology to a cold defect, exclude attenuation and technical causes: metallic implants and jewelry, CT-contrast and barium, pacemakers/ports, arms-down positioning, breast and diaphragmatic attenuation on cardiac imaging, and injection/acquisition errors. Correlating with the CT (on hybrid imaging) or with non-attenuation-corrected images resolves most of these instantly.

On cardiac SPECT MPI specifically, fixed anterior (breast) and inferior (diaphragm) attenuation defects mimic infarct; gated wall motion (normal motion in an attenuation artifact, abnormal in true scar) and attenuation correction are the tools that separate them — covered in the attenuation-artifacts page.

Clinical response

Name the organ, exclude artifact, then apply the organ-specific differential and correlate with anatomic imaging. The key mental habit is refusing to let a cold region read as "unremarkable" — in bone and thyroid especially, the cold finding is often the important one.

Self-Check

Key References

• SNMMI/EANM procedure guidelines and normal-variant/artifact atlases across bone, thyroid, hepatic, and cardiac imaging.
• Organ-specific reviews of the photopenic-lesion differential in the nuclear medicine literature.