Nuclear Medicine Differential Diagnosis
Pattern first, diagnosis second. Pick a study, recognise the pattern, and work the differential with the discriminators that separate the look-alikes. A thinking aid, never a substitute for the full clinical picture and correlation.
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Bone scanFDG PETThyroidV/Q lungCardiac MPIRenogramHow this pairs with the rest of the siteSourcesBone scan patterns
Multifocal random uptake
- Skeletal metastases: random axial > appendicular, varying size/intensity.
- Multifocal degenerative/trauma: juxta-articular, endplate, rib line.
- Polyostotic Paget / fibrous dysplasia: expansile, whole-bone.
Discriminator: mets are random + asymmetric; degenerative uptake sits at joints/endplates; Paget follows a whole bone with cortical expansion. SPECT/CT resolves most.
Superscan (diffuse high skeletal uptake, faint kidneys)
- Diffuse metastatic disease (prostate, breast).
- Metabolic bone disease: renal osteodystrophy, hyperparathyroidism, osteomalacia.
Discriminator: metastatic superscan is heterogeneous with focal accentuation; metabolic is more uniform with beading of costochondral junctions + “tie” sternal sign. Absent/faint kidneys (“absent kidney sign”) is the giveaway of a superscan.
Photopenic (cold) lesion
- Lytic/aggressive metastasis (e.g. renal, thyroid, myeloma).
- Avascular necrosis (early), radiotherapy field, prosthesis/artefact.
Discriminator: sharp geometric cold area = external cause (attenuation, RT port, hardware); irregular cold-with-rim = aggressive lysis or infarct. Correlate CT.
Three-phase: focal delayed uptake
- Osteomyelitis: increased on all three phases (flow, pool, delayed).
- Cellulitis: increased flow/pool but not focal delayed bone uptake.
- CRPS, fracture, arthropathy: patterns vary.
Discriminator: osteomyelitis = focal bone uptake on the delayed phase; cellulitis stays soft-tissue. Add labelled WBC ± marrow subtraction for prosthesis/diabetic foot.
FDG PET patterns
Focal intense uptake
- Malignancy: primary or metastasis.
- Active infection/abscess or focal inflammation.
- Physiological/benign: brown fat, muscle, bowel, tonsil.
Discriminator: anatomical correlate on CT + clinical context; symmetric = physiological; new focal avidity in a known cancer = suspicious. Inflammation can be as avid as tumour; biopsy if it changes management.
Symmetric neck/supraclavicular uptake
- Brown fat: bilateral, follows fat density on CT.
- Muscle: talking/tension.
- Nodal disease (usually asymmetric).
Discriminator: brown fat maps onto fat on CT + is symmetric; keep patient warm/rested to avoid it. Nodes have soft-tissue correlate.
Diffuse marrow / splenic uptake
- Reactive: G-CSF, anaemia, recent chemo.
- Marrow infiltration: lymphoma/leukaemia.
Discriminator: recent G-CSF gives diffuse homogeneous marrow + spleen uptake (know the treatment history); focal marrow lesions suggest infiltration.
Non-FDG-avid tumour
- Low-grade / mucinous / lepidic tumours, well-differentiated NET, some prostate + RCC, lobular breast.
Discriminator: match tracer to tumour biology: NET → DOTATATE; prostate → PSMA; a “negative” FDG scan may just be the wrong tracer.
Thyroid uptake patterns
Diffuse increased uptake
- Graves disease: diffuse high uptake, raised uptake %.
- Hashitoxicosis (early).
Discriminator: Graves = smooth diffuse, high uptake %, ± pyramidal lobe; correlate TRAb + clinical.
Focal hot nodule
- Autonomous (toxic) adenoma: hot with suppressed background.
Discriminator: a hot nodule is almost always benign (low malignancy risk); the suppressed rest of the gland confirms autonomy.
Cold nodule
- Benign colloid/cyst (majority) or malignancy (~10–15%).
Discriminator: a cold nodule needs ultrasound ± cytology; scintigraphy can’t exclude cancer. Beware discordant nodules (hot on Tc, cold on I-123).
Low/absent uptake
- Thyroiditis (subacute/silent), iodine load (contrast, amiodarone), factitious thyrotoxicosis, exogenous thyroxine.
Discriminator: low uptake + thyrotoxic = thyroiditis or exogenous/iodine; check recent contrast/amiodarone and thyroglobulin.
V/Q lung patterns
Perfusion defect, normal ventilation (mismatch)
- Pulmonary embolism (segmental/subsegmental wedge mismatch).
- Prior PE, vasculitis, tumour compressing vessels.
Discriminator: multiple segmental mismatches = high probability of PE (PIOPED II / modified / EANM). Correlate with CXR + clinical probability.
Matched V and Q defect
- Airway/parenchymal disease: COPD, pneumonia, effusion.
Discriminator: matched defect + CXR opacity argues against PE (triple-match in lower zone is intermediate; use SPECT/CT). “Stripe sign” (preserved perfusion at pleura) argues against PE.
Multiple non-segmental patchy defects
- COPD, prior clumped MAA, technical.
Discriminator: COPD reduces specificity → favour SPECT or CTPA. Focal hot spots on perfusion = clumped MAA (technique), not pathology.
Cardiac MPI defect patterns
Reversible defect (stress only)
- Inducible ischaemia in the corresponding coronary territory.
Discriminator: normal rest, defect on stress = ischaemia. Grade extent/severity; TID + lung uptake flag severe/multivessel disease.
Fixed defect (stress + rest)
- Infarct/scar or attenuation artifact.
Discriminator: check gated wall motion; normal motion over a “fixed defect” = attenuation (breast/diaphragm), not scar. Prone/upright + CT correction help.
Normal-looking scan, high-risk patient
- Balanced three-vessel disease (globally reduced flow looks uniform).
Discriminator: look for TID, reduced EF reserve, lung uptake; quantitative PET flow reserve unmasks balanced disease.
Renogram curve patterns
Rising curve, no washout
- Obstruction vs non-obstructed dilatation.
Discriminator: give furosemide: washout = dilated but not obstructed; no response = obstruction. Ensure hydration + empty bladder + adequate function first.
Delayed uptake, reduced function
- Chronic obstruction, renovascular disease, poor overall function, acute injury.
Discriminator: captopril challenge for renovascular hypertension (post-captopril deterioration); split function quantifies the affected side.
Prompt uptake + rapid washout
- Normal, or high-flow states.
Discriminator: symmetric prompt peak (~3–5 min) then decline with ~50:50 split = normal renogram.
How this pairs with the rest of the site
Recognise the pattern here, confirm the protocol in the Study & Protocol Explorer, check the tracer in the Radiopharmaceutical Reference, and phrase the report with the Reporting Templates. Rule out mimics with the Artifact Atlas.
Sources
- SNMMI / EANM procedure guidelines (bone, V/Q, renal, thyroid, cardiac). eanm.org
- PIOPED II / modified PISAPED & EANM V/Q interpretation criteria.
- Ziessman, O’Malley & Thrall, Nuclear Medicine: The Requisites (pattern recognition).
High Yield Imaging · Scan-Pattern Differential Guide. Educational reasoning aid. Correlate with the full clinical picture, prior imaging and hybrid CT/MR. Not a diagnostic algorithm.
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Educational reference for imaging professionals. It does not replace local policy, manufacturer labelling or clinical judgement.