Nuclear Medicine Physics: Production of Radionuclides

Production of Radionuclides

• There are over 2700 known radionuclides, but only some are used in medical imaging.
• Radionuclides are produced artificially in two ways:
  • Neutron excess: adding a neutron to a stable nucleus, resulting in a neutron excess (e.g., 98Mo + n → 99Mo).
  • Neutron deficit: adding a proton to a stable nucleus, resulting in a neutron deficit (e.g., 18O + p → 18F + n).

Radionuclides with a Neutron Excess

• Radionuclides with a neutron excess undergo β- decay.
• Example: 99mTc (half-life = 6 hours) decays to 99Tc (unstable, half-life = 211,000 years), which decays to ruthenium-99 (stable).

Radionuclides with a Neutron Deficit

• Radionuclides with a neutron deficit undergo β+ decay or K-electron capture.
• Example: 18F (fluorine-18) with atomic number 9 transforms into 18O (oxygen-18) with atomic number 8, resulting in a decrease in atomic number but no change in mass number.

Technetium Generator

• A technetium generator is used to produce technetium-99m (99mTc).
• 99mTc is washed off the column (eluted) with sterile saline solution, leaving behind the molybdenum (99Mo).
• Elution takes a few minutes and can be repeated daily.

Uses of Technetium-99m

• Sodium pertechnetate-99mTc is used for imaging tissues, such as:
  • Thyroid, gastric mucosa, and salivary glands
  • Cerebral blood flow and testicular imaging (with potassium perchlorate)
  • Gastric emptying studies (with bran porridge)
• 99mTc can be labeled to various compounds for:
  • Bone imaging (methylene diphosphonate, MDP)
  • Cerebral imaging (hexamethyl propylene amine oxime, HMPAO)
  • Renal studies (dimercaptosuccinic acid, DMSA, and mercaptoacetyltriglycine, MAG3)
  • Biliary studies (iminodiacetic acid, HIDA)
  • Liver, spleen, and red bone marrow imaging (human serum albumin, HSA)
  • Lung ventilation studies (diethylene triamine pentacetic acid, DTPA, aerosol)
  • Cardiac function and perfusion imaging (autologous red cells, sestamibi, or tetrofosmin)

Other Radionuclides and Their Uses

• Iodine-131 (131I): used for thyroid imaging and therapy, but largely replaced by 123I due to its long half-life and beta emissions.
• Iodine-123 (123I): used for thyroid imaging, more expensive than 131I, but with a shorter half-life and fewer beta emissions.
• Iodine-125 (125I): used for therapy, particularly brachytherapy seeds.
• Xenon-133 (133Xe): used for lung ventilation imaging, produced in a nuclear reactor, and has a half-life of 5.2 days.
• Krypton-81m (81mKr): used for pulmonary ventilation studies, produced in a generator, and has a half-life of 13 seconds.