Nuclear Medicine · Conventional Scintigraphy
Conventional Scintigraphy One-Liners
High-yield, rapid-revision one-liners covering conventional nuclear medicine scintigraphy, including bone, renal, hepatobiliary, thyroid, and bleeding scans.
A rapid-revision collection of high-yield discriminators, classic signs, and radiotracer selection principles for conventional planar and SPECT scintigraphy.
- Triple-phase bone scan
- Differentiates cellulitis (increased flow and blood pool, normal delayed) from osteomyelitis (increased radiotracer uptake in all three phases).
- Superscan
- Diffuse, symmetrically increased skeletal uptake with absent renal and soft-tissue activity, classically indicating widespread osteoblastic metastases (e.g., prostate cancer).
- Shin splints
- Linear, corticomedullary uptake along the posteromedial tibial cortex on delayed images, distinct from the focal, transverse uptake of a stress fracture.
- Tc-99m MAG3 vs DTPA
- MAG3 is preferred for diuretic renography in renal impairment as it relies on tubular secretion, whereas DTPA depends on glomerular filtration.
- Diuretic renography T½
- A washout half-time (T½) of <10 minutes indicates no obstruction; >20 minutes suggests obstruction; 10–20 minutes is equivocal.
- Tc-99m DMSA scan
- The gold standard for detecting renal cortical scarring and assessing differential renal function, optimally performed 3–6 months post-acute pyelonephritis.
- HIDA scan for acute cholecystitis
- Non-visualisation of the gallbladder at 60 minutes (or after morphine augmentation) is highly sensitive for acute calculous cholecystitis.
- HIDA scan in biliary atresia
- Absent bowel excretion at 24 hours despite hepatic uptake and phenobarbital pretreatment strongly suggests biliary atresia in neonates.
- Thyroid 'cold' nodule
- A solitary photopenic nodule on scintigraphy carries a 15–20% risk of malignancy, whereas a 'hot' nodule is almost invariably benign.
- Graves' disease scintigraphy
- Diffusely increased, homogeneous radiotracer uptake in an enlarged thyroid gland with a high 24-hour radioiodine uptake value.
- Subacute (De Quervain) thyroiditis
- Markedly reduced or absent radiotracer uptake during the thyrotoxic phase due to follicular destruction and preformed hormone leakage.
- Meckel's scan
- Tc-99m pertechnetate detects ectopic gastric mucosa; sensitivity is increased by pretreatment with H2-receptor antagonists or pentagastrin.
- Tc-99m labelled RBC GI bleed scan
- Can detect bleeding rates as low as 0.1 mL/min, offering higher sensitivity than angiography (0.5 mL/min) but with lower spatial resolution.
- V/Q mismatch
- A segmental perfusion defect with normal ventilation is the scintigraphic hallmark of acute pulmonary embolism.
- Tc-99m MAA particle size
- Particles are 10–90 µm; use is relatively contraindicated in severe pulmonary hypertension or right-to-left shunts due to systemic embolic risk.
- Parathyroid sestamibi scan
- A persistent focus of uptake on delayed images (e.g., 2 hours) after thyroid washout indicates a parathyroid adenoma.
- Sentinel lymph node mapping
- Tc-99m sulphur colloid or tilmanocept identifies the first draining lymph node(s), crucial for staging in breast cancer and melanoma.
- CSF cisternography (In-111 DTPA)
- Ventricular reflux with delayed clearance over the cerebral convexities is abnormal and supports a diagnosis of normal pressure hydrocephalus.
- Radionuclide dacryocystography
- Evaluates nasolacrimal duct patency; delayed or absent activity in the inferior meatus indicates obstruction.
- Gastric emptying scintigraphy
- The gold standard for diagnosing gastroparesis; delayed emptying of a radiolabelled solid meal (e.g., Tc-99m sulphur colloid eggs) at 2 and 4 hours is diagnostic.
Viva questions
- How does a triple-phase bone scan differentiate osteomyelitis from cellulitis?
- Osteomyelitis shows increased uptake in flow, blood pool, and delayed phases, whereas cellulitis shows increased flow and blood pool but normal delayed uptake.
- What are the diuretic renography (MAG3) T½ thresholds for interpreting urinary tract obstruction?
- T½ less than 10 minutes indicates no obstruction, greater than 20 minutes suggests obstruction, and 10 to 20 minutes is equivocal.
- What is a 'superscan' on bone scintigraphy and what is a classic clinical cause?
- Diffuse, symmetrically increased skeletal uptake with absent renal and soft-tissue activity, classically caused by widespread osteoblastic metastases such as prostate cancer.
- Why is Tc-99m MAG3 preferred over DTPA for renography in patients with renal impairment?
- MAG3 is cleared by tubular secretion, providing adequate image quality and clearance data when GFR is low, unlike DTPA which relies on glomerular filtration.