Nuclear Medicine and Molecular Imaging
FDG PET/CT
FDG uptake reflects glucose metabolism and is not tumour-specific. Quantify with SUVmax but interpret against physiological uptake and inflammation. Normal high uptake occurs in brain, myocardium (variable), liver, kidneys, bladder and lymphoid tissue in children. Pitfalls include brown fat (supraclavicular, symmetric), infection, sarcoidosis, postoperative change and urinary activity.
Lung
Ventilation/perfusion imaging: a mismatched defect (normal ventilation, absent perfusion) in a segment or larger indicates high probability of pulmonary embolism; matched defects usually reflect parenchymal disease. SPECT/CT improves specificity.
Bone and other studies
Bone scintigraphy: three-phase technique for osteomyelitis and complex regional pain syndrome; a superscan with uniformly increased uptake and faint kidneys indicates diffuse metastatic disease. Sentinel node mapping with radiolabelled colloid is standard in breast cancer and melanoma. Thyroid radioiodine uptake distinguishes Graves disease from thyroiditis; a cold nodule requires cytology.
Therapy
Radioiodine for hyperthyroidism and differentiated thyroid cancer; lutetium-177 dotatate for neuroendocrine tumours; radium-223 for bone metastases in prostate cancer.
Traps
Hot clot artefact in the syringe or vein, free pertechnetate and urine contamination can mimic disease. A high SUV does not equal malignancy.
Rule of thumb
V/Q: mismatched defects mean pulmonary embolism until proven otherwise; matched defects do not.