Endocrine, Gastrointestinal, Genitourinary, and Pulmonary Nuclear Medicine
Endocrine, GI, GU, and pulmonary studies
Thyroid: Tc-99m pertechnetate images trapping while I-123 images trapping and organification; Graves disease shows diffusely increased uptake, subacute thyroiditis shows low uptake, and a hot nodule suppressing the remainder suggests autonomy. Parathyroid localization uses early and delayed sestamibi (or I-123 subtraction), exploiting washout from thyroid but not from adenoma.
GI: hepatobiliary scintigraphy with Tc-99m mebrofenin assesses cystic duct patency - nonvisualization of the gallbladder by 4 h (with morphine augmentation if needed) supports acute cholecystitis, but prolonged fasting, TPN, and chronic cholecystitis cause false positives. GI bleeding uses Tc-99m-labeled RBCs (sensitivity near 0.1 mL/min). A Meckel scan uses pertechnetate with H2 blockade or pentagastrin to enhance ectopic gastric mucosa uptake.
GU: MAG3 measures effective renal plasma flow and is preferred for obstruction and transplant evaluation; DTPA measures GFR; DMSA binds cortical tubules to show scarring. Diuretic renography with furosemide assesses obstruction, and ACE-inhibitor renography sensitizes detection of renovascular hypertension.
Pulmonary: V/Q scintigraphy is interpreted by mismatch. A perfusion defect with normal ventilation in at least two segments is high probability; matched defects suggest pneumonia or COPD. CTPA has largely displaced V/Q for acute PE, but V/Q SPECT/CT remains valuable in renal failure, contrast allergy, and pregnancy.
Traps: fasting beyond 24 h produces false-positive HIDA; a dilated but unobstructed system can mimic obstruction; and a matched defect should not be called PE.
Worked example: a MAG3 renogram after furosemide with a half-time of 25 minutes indicates obstruction, whereas under 10 minutes is normal.