Nuclear Medicine Technology Exam

The exam asks you to apply the physics: activity at injection time, the right survey meter, a feature in a spectrum, or the counts needed for a reliable result. Work every example with a calculator, as the exam allows one.

This chapter reviews the parts of the atom, the numbers that describe a nuclide, why some nuclei are unstable, and the four "iso" terms that the exam likes to mix up.

An atom has a small, dense nucleus of protons (positive) and neutrons (no charge), together called nucleons, surrounded by negatively charged electrons in shells. Almost all of the mass is in the nucleus.

Three numbers describe a nucleus:

• Z, the atomic number, is the number of protons. Z defines the element. Every atom with 43 protons is technetium, and every atom with 53 protons is iodine.
• N is the number of neutrons.
• A, the mass number, is the total number of nucleons, so A = Z + N.

Nuclide notation writes the mass number before the element symbol, as in Tc-99m or I-131. Using technetium as an example, Tc-99m has Z = 43 and A = 99, so N = 99 - 43 = 56 neutrons. I-131 has Z = 53 and A = 131, so N = 78.

Electron shells are labeled K, L, M outward from the nucleus, and K-shell electrons are the most tightly bound. When an inner-shell vacancy is filled, the energy difference leaves as a characteristic x-ray or an Auger electron with an energy specific to the element, which is why Tl-201 decay always yields mercury x-rays at about 69-80 keV.

Inside the nucleus, the strong nuclear force holds nucleons together over very short distances, while the electrostatic force pushes protons apart. Neutrons add strong force without adding repulsion, so heavier nuclei need proportionally more neutrons to stay stable.

If you plot N against Z for all stable nuclides, they fall along a curved band called the line of stability. For light elements the stable neutron-to-proton (N:Z) ratio is about 1:1 (carbon-12 has 6 and 6). For heavy elements it rises to about 1.5:1 (lead-208 has 126 neutrons and 82 protons, a ratio of about 1.54).

A nuclide that sits off this line is radioactive and will decay in the direction that brings it closer to stability:

• Neutron-rich nuclides (too many neutrons, above the line) decay by beta minus emission, which converts a neutron into a proton. Reactor-produced nuclides such as Mo-99, I-131, Xe-133 and Lu-177 are typically neutron rich.
• Proton-rich nuclides (too few neutrons, below the line) decay by positron emission or electron capture, both of which convert a proton into a neutron. Cyclotron-produced nuclides such as F-18, Tl-201, Ga-67, In-111 and I-123 are typically proton rich.
• Very heavy nuclides (roughly Z above 82) often decay by alpha emission, which removes two protons and two neutrons at once. Ra-223 is the therapy example.

The exam often tests the four nomenclature terms by giving a pair of nuclides and asking what they have in common.

An isomer is a nucleus held in an excited state long enough to be measured; when that state has a measurable half-life it is called metastable and gets the letter m. Tc-99m releases its extra energy (half-life 6.01 h), mostly as a 140 keV gamma, and becomes Tc-99, whose half-life of about 211,000 years makes its activity negligible.

• Z is protons and defines the element; A is protons plus neutrons; N = A - Z.
• Stable nuclei follow the line of stability, with an N:Z ratio near 1 for light elements and near 1.5 for heavy elements.
• Neutron-rich nuclides undergo beta minus decay; proton-rich nuclides undergo positron emission or electron capture; very heavy nuclides often undergo alpha decay.
• Isotopes share Z, isobars share A, isotones share N, and isomers share both Z and A but differ in energy state.
• Tc-99m and Tc-99 are the classic isomer pair.