Nuclear Medicine Physics


Definition/Introduction

Nuclear medicine uses radioactive materials and their emitted radiation from the body to diagnose and treat disease. Unstable atoms (radionuclides) are typically administered orally or intravenously and, less commonly, intra-arterially, directly into the CSF spaces, peritoneum, or joint space. These radionuclides are often chelated (labeled or tagged) with other molecules that provide them their physiologic properties, forming a radiopharmaceutical and allowing the combination to preferentially localize to organs of interest. Specialized cameras are used to record the radiation emission from these unstable atoms to localize pathology and guide treatment.

In the field of medicine, nuclear physics helps us understand:

  • Which atoms and their tagged molecules can be used in diagnostics.
  • Which radioactive atoms can be used in therapeutics
  • Precautions necessary to protect the general public from radiation
  • Precautions necessary to minimize the patient radiation
  • The type of shielding and other protective measures necessary in the handling of certain radioactive products
  • The proper disposal of radioactive substances in the nuclear medicine department

The nucleus of an atom consists of protons and neutrons held together by a strong nuclear force. Atoms are stable when this strong nuclear force can keep its constituent protons and neutrons within the nucleus. When the number of neutrons and protons are plotted on a graph, stable nuclei have been found to fall within a narrow region called the “band of stability” that closely approximates a 1:1 ratio of neutrons to protons in low atomic number atoms. As the atomic weight of an atom increases, this ratio skews towards a higher ratio of neutrons when compared to protons. This band of stability helps us understand the basis of nuclear physics, as atoms that are located outside of this band undergo reactions (decay) in an attempt to “reach” this band of stability.

Types of decay – Depending on the imbalance between protons and neutrons, different types of nuclear decay occur.[1][2] These include:

Beta positive decay – occurring with an excess of protons.

Electron capture – occurring with an excess of protons.

Beta minus decay – occurring with an excess of neutrons.

Alpha decay – occurring with atoms with high atomic weights that are unstable.

Beta positive decay – Beta positive decay occurs when an atom has an excess of protons (and a low number of neutrons). In this process, a positively charged proton is converted into a neutron, which is neutral in charge. This process results in the emission of a positron and a neutrino (a particle without charge and whose mass is essentially zero). A positron is akin to a positively charged version of an electron. Depending on its energy, this positron traverses a short distance until its energy decreases to 1.02 MeV and it encounters an electron, resulting in an annihilation event. The result of an annihilation event is the emission of two photons emitted 180 degrees opposite in direction from each other, each measuring 511 keV in energy. This emission of two photons is the basis of PET imaging.

Electron capture – Electron capture results from the “capture” of a negatively charged electron by a proton heavy nucleus resulting in a neutron. This electron is usually located in the K shell of an atom. The process of electron capture is an isobaric transition, considering the atomic mass (total number of protons and neutrons) does not change. The atomic number, based upon the number of protons in the nucleus, decreases since a proton is now transformed into a neutron. The electron capture process is also an isomeric transition, resulting in the emission of photons that can be used for imaging.

Beta minus decayBeta minus decay occurs when an atom contains an excess of neutrons.[1] In this process, a neutron is transformed into a proton, resulting in a beta particle release and an anti-neutrino. A beta particle is akin to an electron. However, it is distinguished from an electron due to its origin from the nucleus. The release of an anti-neutrino and a beta particle balances the change of charge in the process. The atomic number also increases by one, considering the neutron is now transformed into a proton. This process is an isobaric transition, as the mass of the atom has not changed. Beta minus imaging is not useful for imaging but is useful in therapy.

Alpha decay – Alpha decay occurs when the nucleus emits an alpha particle. An alpha particle is essentially a helium-4 atom and consists of two protons and two neutrons. Alpha particles have a charge of +2 and are emitted by heavy nuclides.

Isomeric transition – After the isobaric transition, excess energy is emitted in the form of isomeric transition, mostly by emitting gamma photons. These gamma photons' energy is variable and depends on the intermediate and final states of the nucleus undergoing isomeric transition. These packets of released energy in the form of gamma photons often provide a unique fingerprint for different radionuclides, as each releases a predictable amount of energy to reach its stable state. The intermediate states can be short in duration, or as in the case of “metastable states” can belong, as is the case with technetium 99-metastable radionuclides used in imaging. Technetium 99-metastable radionuclides release 140 keV photons before reaching their stable ground state.

Production of radionuclides – Atoms with an excess of neutrons and protons can be generated for medical imaging and therapy. Radioactive material not occurring naturally is generated by bombardment and fission, which results in unstable isotopes (a nucleus with an “unfavorable” neutron: proton ratio). The bombardment occurs via the irradiation of a nucleus with either neutrons (in a nuclear reactor) or other charged particles from a cyclotron. Charged particles in a cyclotron used to irradiate neutrons can include alpha particles, deuterons, or protons. The daughter isotopes resulting from bombardment are separated easily in cyclotrons where transmutation occurs (a change in atomic number) and less easily in processes involving neutron bombardment (when atomic numbers are not changed). Neutron bombardment and nuclear fission generally result in nuclei with a neutron excess, and cyclotron-produced isotopes are neutron deficient.

Half-life – The physical half-life of a radionuclide denotes the amount of time it takes for its activity to be reduced to half of its existing activity. The biological half-life of a radionuclide denotes the amount of time it takes for the radionuclide to reduce to half of its concentration within the human body (often through excretion or exhalation). The effective half-life describes the combination of these two aspects of a radionuclide.

Issues of Concern

Safety: Plastic and lead are commonly used to shield radiation workers from gamma photons and beta radiation. In the setting of beta particle emission, plastic as an initial absorber of beta emission shielding is important. Higher atomic materials such as lead will result in radiative interactions, which can actually increase the radiation exposure dose. However, the use of lead shielding is important in the protection of gamma photon emission and X-rays.

Clinical Significance

Various radionuclides are used in clinical imaging and treatment.

Common radionuclides used in imaging through the emission of gamma photons include:

Technetium-99m: Technetium is used for a wide range of clinical applications when tagged to various other compounds.[3] Clinical applications include evaluation for acute cholecystitis, bile leak, neonatal jaundice (Tc-99m mebrofenin), renal function (Tc-99m MAG3), and structure evaluation (Tc-99m DMSA), evaluation for pulmonary embolism (Tc-99m MAA), bone disorders (Tc-99m MDP), evaluation of cardiac function (Tc-99m sestamibi), parathyroid adenoma imaging (also Tc-99m sestamibi), gastrointestinal bleeding (Tc-99m tagged RBCs) and gastrointestinal transit studies (Tc-99m sulfur colloid).

Iodine-123: Iodine-123 is used in the evaluation of Parkinson's disease (I-123 ioflupane), the evaluation of pheochromocytoma, paraganglioma, neuroblastoma (I-123 MIBG, a norepinephrine analog), and in the evaluation of thyroid disease such as Graves disease (I-123).[4]

Iodine-131: Iodine-131 is mainly used for the therapy of benign and malignant thyroid diseases.

Indium-111: Indium-111 CSF leak (In-111 DTPA), evaluation of neuroendocrine tumors (In-111 pentreotide), and infection (In-111 labeled WBCs).[5]

Xenon-133: Xenon-133 is used for the evaluation of lung ventilation, often in the setting of the workup for pulmonary embolism.[6]

Commonly radionuclides used in imaging through positron emission include:

Fluorine-18: Fluorine-18 is most well known for its use in the evaluation of tumor, however, is also used in the evaluation of cardiac viability, brain metabolism, infection, dementia, (in the form of F-18-FDG, a radionuclide glucose analog) and prostate cancer (F-18 fluciclovine, an amino acid analog).[7]

Nitrogen-13: Nitrogen-13 is used in the evaluation of cardiac perfusion and is preferred for the quantification of myocardial blood flow.[8]

Rubidium-82: Rubidium-82 is used in the evaluation of myocardial perfusion.

Radionuclides used in therapy include:

Yttrium-90: Yttrium-90 is a beta emitter and is used commonly in the treatment of liver malignancy via selective intra-arterial injection.[9]

Radium-223: An alpha emitter used for the treatment of bone metastases in patients with prostate cancer that is resistant to other treatments.

Lutetium-177: Lutetium-177 is a beta emitter used for the treatment of gastrointestinal and pancreatic neuroendocrine tumors that are refractory to other treatments.

Strontium-89 and Samarium-153: Beta emitters used as a palliative treatment measure for bony metastasis.[10]

Nursing, Allied Health, and Interprofessional Team Interventions

The use of nuclear medicine radiotracers in medicine requires close coordination between various professionals, including:

  • The medical physicist - who is often involved in calculating hypothetical radiation doses to a patient and the public
  • The nuclear pharmacist - who plays a role in radiotracer preparation
  • The NM technician - who plays an important role in screening, preparing, and imaging the patient
  • The nuclear medicine physician - who supervises all aspects of nuclear medicine care as well as directing therapy and performing image interpretation
  • Inpatient nurses and the inpatient team - who sometimes play a role in inpatient admissions, such as in the case of admission during treatments and/or complications thereof
  • The referring provider - who often plays a central role in the patient selection and their initial education regarding the role of nuclear medicine in diagnosis and treatment

The involvement of multiple professionals is critical, considering the radiation dose often incurred on the public during the administration of certain radiopharmaceuticals. Close communication between members of the treatment team is critical in relating the amount of radioactivity to be expected, the way to shield workers and the public from radiation exposure, and the way to optimize patient treatment either in the setting of the diagnostic evaluation of disease or in the setting of therapy.



(Click Image to Enlarge)
The band of stability has a close relationship to a 1:1 ratio when the number of neutrons and protons are plotted on a graph.
The band of stability has a close relationship to a 1:1 ratio when the number of neutrons and protons are plotted on a graph.
Contributed by Dr.Dawood Tafti, MD.

(Click Image to Enlarge)
Types of decay when plotted on a graph. The x-axis corresponds to the atomic number and the y-axis corresponds to energy.
Types of decay when plotted on a graph. The x-axis corresponds to the atomic number and the y-axis corresponds to energy.
Contributed by Dr.Dawood Tafti, MD.
Article Details

Article Author

Dawood Tafti

Article Editor:

Kevin P. Banks

Updated:

2/17/2022 11:06:40 PM

References

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