Deep Dive: Nuclear Medicine

• Basic concept: using radioactive (element that remits radiation as it decays) substances coupled with biologically active chemicals to visualize structures (some organs love certain substances- like thyroid and iodine)

• Important physics concepts to know:
  • Isotope = element on the periodic table with different # of neutrons but same atomic # 
    • Radioisotopes: technetium-99m, iodine-131

• How nuclear medicine actually works: patient is made radioactive, they emit gamma rays, a gamma camera has a detector made of crystal that scintillates in response, computer creates an image, multiple types of scans that utilize this principles

• Types of scans:
  • Positron emission tomography (PET) = uses radioisotope that produces positively charged electrons (positrons) that are attached to pharmaceuticals (glucose analog fluorodeoxyglucose [FDG] for example) to image based on metabolic activity 
  • Single photon emission computed tomography (SPECT) = uses gamma camera to take 2D pictures circling around the patient to create a 3D projection

• Bone scans:
  • Screening for metastatic disease, diagnosing early fractures 
  • Tracer = Technetium-99m (Tc99m) methylene diphosphonate (MDP)
  • Deposits best where there is bone turnover
  • Metastases, superscan, triple-phase

• Ventilation/perfusion (V/Q) scans:
  • Used to diagnose pulmonary embolism when patients cannot undergo CT angiography 
  • Tracer = Tc99m macroaggregated albumin (MAA)
  • Pulmonary embolism = segmental mismatch with normal ventilation but abnormal perfusion scans; probability can be categorized into normal, low, intermediate, high

• Cardiac scans:
  • Heart cells with decreased perfusion or viability will take up less tracer
  • Usually perform a resting and stress (adenosine, treadmill) scan
  • Decreased uptake on stress that corrects on rest suggests ischemia rather than infarct (reversible vs irreversible)

• GI bleed scans:
  • Couple Tech-99m to RBCs and scan abdomen
  • Bleeds show up as increased uptake of radiotracer in bowel lumen that increases in amount and moves through the bowel over time

• Thyroid scintigraphy:
  • Used to assess nodules, Grave’s disease, cancer
  • Hyperyhyroid patients will show increased uptake if there is truly an increased amount of thyroid hormone being actively produced 
  • Performed using radioactive iodine or T99m pertechnetate which both go to the thyroid 
  • 95% of hot nodules are benign, cold nodules are more concerning for malignancy

• Biliary scans:
  • Hepatobiliary iminodiacetic acid (HIDA) scan = couples Tc99m to iminodiacetic acid to assess the hepatobiliary system
  • Often used to diagnose acute cholecystitis (very sensitive and specific) or biliary leaks after surgery 
  • Lack of filling of the gallbladder (photopenic area) suggests obstruction of cystic duct and can diagnose acute cholecystitis 
  • Can scan the abdomen to see if tracer is outside of biliary system to suggest leak

References: Herring's Learning Radiology, Nuclear Medicine: The Requisites, Radiopaedia, Mandell's CORE Radiology