- 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