Chapter 1: Fundamentals of Nuclear Cardiology Physics

A practical review of basic atomic and nuclear physics is essential to understand the origins of radiations, as well as their interactions with matter. The nature and type of emissions are determined by the structural character of the atom and nucleus. The ways in which radiation interacts with matter have a direct relationship with imaging and radiation safety. The types of radiation and how they interact with matter are the foundation of radionuclide imaging and radiation safety. This chapter will focus on atomic and nuclear structure and the interaction of radiations with matter as they relate to radionuclide imaging.

Matter is composed of atoms and the characteristics of a specific form of matter are determined by the number and type of atoms that make it up. How atoms combine is a function of their electron structure. The electron structure is determined by the nuclear architecture. As we have yet to image the atom, its structure is based on a "most-probable" model that fits physical behaviors we observe. The probabilistic approach is based on the model of the atom proposed by Niels Bohr in 1913. The Bohr atom proposed a positively charged nucleus, surrounded by negatively charged electrons. A neutral atom is one in which the positive and negative charges are matched. A mismatch in these charges determines the ionic character of the atom, which is the basis for its chemistry. The electron configuration is also a source for emissions used in radionuclide imaging.

These emissions, or radiations, may be in either of the two forms: particulate or electromagnetic. The origins of either type of radiation may be from the nucleus or the electron structure.

Electron Configuration

Electrons are arranged around the nucleus in shells. The number of shells is determined by the number of electrons, which is, in turn, determined by the number of protons in the nucleus. The force exerted on these shells, called binding energy, is determined by the proximity of the shell to the nucleus. Higher binding energies are exerted on shells closest to the nucleus and conversely, lower binding energy for those more distant from the nucleus. The innermost shell is named the "K" shell and electrons in this shell are subject to the highest binding energy. The magnitude of that energy is dependent on the positive forces, which is determined by the number of protons in the nucleus. The shells more distant from the nucleus are named L, M, N, and so on. Each of these shells has lower binding energies as a result of their distance from the nucleus (Fig. 1-1).

The radii of each of these shells increase as a function of their distance from the nucleus. An expression of this is given by assigning an integer value (1, 2, 3, …) to each shell. The lower values represent smaller radii. These integer values are called quantum numbers. Therefore, the K shell has a quantum number of 1, L = 2, M = 3, etc. This pattern continues until all available electrons are bound to a shell. The innermost shells are filled with electrons preferentially. The maximum number of electrons is specific to each shell and is calculated by 2n², where n is the quantum number. Therefore, the maximum number of electrons for each shell is:

These shells are further subdivided into substates. The number of substates for each shell can be calculated by 2n – 1; therefore:

Each substate for a given shell will have a unique binding energy. For instance, the L shell has 3 substates, L_I, L_II, and L_III.¹ Each of these has slightly different distances from the nucleus, and therefore slightly different binding energies (Fig. 1-2).²

Atomic Radiations

Electrons in inner shells being under high binding energy and thus tightly bound to the nucleus are in an inherently low-energy state. Outer shell and free electrons are in an inherently higher-energy state. Therefore, to move an inner shell electron to an outer shell requires energy. The amount of energy required is simply the difference between binding energies.

Example: Binding energy for a hypothetical "K" shell = 100 keV and "L" = 50 keV. K₁₀₀ – L₅₀ = 50 keV of energy input to move the electron.

Conversely, the movement of an electron from an outer shell to an inner shell, L → K, yields energy. This energy yield results in the emission of radiation. The energy of the radiation is equal to the differences in binding energies of the shells. The radiation may take on two different forms: characteristic x-ray or Auger (oh-zhay) effect.

Example: Binding energy for a hypothetical "K" shell = 100 keV and "L" = 50 keV. K₁₀₀ – L₅₀ = 50 keV of energy given to the electron.

Characteristic x-rays are electromagnetic radiations (photons) that are created when an outer shell electron moves to fill an inner shell vacancy. This vacancy may occur for several reasons—to be discussed later. The energy of this photon is equal to the difference between binding energies. Since binding energies are determined by, or characteristic of, the number of protons in the nucleus, and it is the number of protons that determines an element's identity, the characteristic x-ray energies are specific to each element and the electron shells from which they originate. X-radiation is defined as an electromagnetic radiation originating outside the nucleus, therefore the term characteristic x-ray.

The Auger effect occurs under the same conditions as characteristic x-ray, that is, an inner shell vacancy being filled by an outer shell electron. The difference is that the excess energy from the cascading electron is radiated to another electron. This ejects that electron from its shell. This free electron will have kinetic energy equal to the difference in the binding energies less the binding energy of the shell of the free electron. The Auger effect is more common in elements with lower numbers of protons (Z number).^1–3

Nuclear Structure

The nucleus is composed mainly of neutrons and protons. Any particle contributing to the structure of the nucleus is called a nucleon. The conventional nomenclature to describe the nucleons is: .

The total mass of an atom is essentially the combined masses of the nucleons. Electrons contribute less than 1% to the total mass.¹

Nuclides having the same number of protons (Z number) are called isotopes. Isotopes may exhibit different atomic masses (A number) and therefore have different numbers of neutrons (N number). Nuclides with the same N are called isotones and will be different elements, since the Z numbers are different. Nuclides with the same A number are called isobars and different elements as well.

Isotopes having different N numbers are of particular interest to imagers because they have the same chemistry, since their Z numbers and, therefore, electron numbers are the same.^1–5 Some isotopes exhibit the emission of radiations, which is due to the differences in the number of neutrons. These isotopes are called unstable. If all the stable isotopes of all elements are plotted, comparing proton number to neutron number, a pattern emerges as illustrated in Figure 1-3.

Elements with low Z numbers have proton to neutron ratios that are 1:1. As Z numbers increase, this ratio increases to as high as 1.5. This distribution of stable elements is called the line of stability. By definition, an element with a proton to neutron ratio that falls to either the left or right of the line of line of stability is unstable. The unstable isotopes, radioisotopes, are unstable because their nuclear configurations are either proton rich or neutron rich relative to stable configurations. These radioactive elements seek stability by undergoing transformations in their nuclear configurations to a more stable P ↔ N ratio. The type of transformation will be a result of P ↔ N ratio, that is, proton rich versus neutron rich. This type of transition is called the mode of decay.^1–4

Modes of Decay

The goal of nuclear decay is to equate the balance of forces in the nucleus. The repelling forces originating from the positive charge (coulombic forces) of the protons, when matched by the attractive forces from within the nucleus (exchange forces), define stability. When these forces are mismatched, nuclear transformations (radioactive decay) result. The mode of decay will produce unique emissions and lead to a more stable nuclear configuration. In radionuclide imaging, the ideal mode of decay would result in a high yield of photons, at an energy that is efficiently detected by our imaging instrumentation. Photon emission is also desirable from the radiation safety and dosimetry perspectives, due to their lower probability of creating potentially damaging interactions as compared to particles. With these considerations, it is important to understand the modes of decay of ^99mTechnetium, ²⁰¹Thallium, and ⁸²Rubidium—the most commonly used radionuclides used in nuclear cardiology.^1,2,6

BETA⁻ Decay

In an unstable nuclear configuration where the nucleus is neutron rich, β⁻ decay occurs. To decrease the neutron–proton ratio, a neutron is converted to a proton and an energized electron is emitted. The expression of this nuclear transition is:

where n is the neutron, p the proton, e the electron, and ν the neutrino.

The neutrino (ν) behaves like a particle with no mass and is not critical to imaging considerations. The primary emission is the energized electron (e⁻). The nuclear configuration that results from β⁻ decay is a daughter with a stable or more stable energy state and an additional proton in its nucleus.

Example:

Since the number of protons is changed, the elemental identity changes. This is called a transmutation. The daughter atomic mass (X) remains the same as the parent nucleus, and the energy carried off by the ejected electron called transition energy. This leads to a more balanced relationship of coulombic force (repelling forces due to the protons) and exchange force (attractive nuclear forces). The resulting emission of the energized electron, a β⁻ particle, is of no use in imaging and contributes to an increase in radiation dose in a biologic system. This decay process may lead to a daughter that is not fully stable, but more stable than the parent.^1,2

BETA⁺ Decay

In nuclear configurations where the parent is proton rich, β⁺ decay may occur. In this mode of decay, a proton is converted to a neutron and the emission of an energized, positively charged electron (β⁺) results. The nuclear equation is:

The energy of the β⁺ particle contributes to resolving the transition energy between the unstable parent and more stable daughter, as in β⁻ decay.

An important secondary emission will result from the formation of the β⁺ particle. Since there is an abundance of negatively charged electrons in nature, the resulting positively charged electron (β⁺) will be attracted to, and collide with, a free negatively charged electron. This collision results in the annihilation of both particles. The annihilation leads to the conversion of the mass of these particles to their equivalent energy state. This is expressed by Einstein's equation E = mc², where E is energy, m the mass, and c the speed of light. This essentially states that energy and mass are simply two physical forms of the same thing. Therefore, two photons (E) are emitted, each with the energy equivalent to the mass (m) of an electron, which is 511 keV. Unique to this annihilation is that these photons are emitted in a 180-degree trajectory from each other. It is these photons that are detected and registered into an image in positron imaging. The change in nuclear configuration is a decrease in Z and increase in N.

Example:

Electron Capture

An alternative to β⁺ decay in proton-rich nuclear configuration, is electron capture. This mode of decay is defined as the capture of a K-shell electron by the nucleus, the subsequent combination with a proton, and creation of a neutron. The nuclear expression is therefore:

The vacancy left by the captured electron would then be filled by an outer shell electron. A cascade of an electron, filling subsequent vacancies, creates secondary emissions called, characteristic x-rays and Auger electrons. The energies of these emissions will be characteristic of the binding energy of the daughter, since the nuclear transition occurred prior to the production of the x-rays and Auger electrons. It is the characteristic x-rays that are imaged in ²⁰¹Tl myocardial perfusion imaging. The energy of the x-rays is determined by the binding energy of ²⁰¹Hg, the daughter of the decay of ²⁰¹Tl. Electron capture decreases proton–neutron ratio.¹

Example:

Isometric Transitions and Internal Conversions

The daughter of the decay of a radioactive parent will ideally be in its most stable energy configuration or ground state. This does not always occur, leading to either of two unstable states; excited state or metastable state. Excited states are very unstable and exist for very short time periods, usually less than 10⁻¹² seconds. Metastable states, however, may exist for several hours. These metastable states lead to the release of energy in the form of radiation, without changing the proton–neutron ratios. The parent nucleus has the same nuclear structure as the daughter, but in a more stable energy configuration. This form of decay is called an isometric transition and results in electromagnetic emissions called γ-rays. These radiations are the same as x-rays, differing only by their location of origin, that is, the nucleus. As noted with the production of characteristic x-rays, there is a competing process, resulting in a particulate radiation. This process is called internal conversion. For any given metastable state, there is a specific ratio of isometric transitions to internal conversions. In imaging, the higher percentage of isometric transitions compared to internal conversions is preferred due to the resulting higher yield of photons. The decay of ^99mTc to ⁹⁹Tc is an example of an isometric transition of the metastable state (^99mTc). The percent occurrence of isometric transitions of a population of ^99mTc nuclei is approximately 87%. For example, for every 100 decays of ^99mTc nuclei, there is a yield of 87 γ photons and 13 internal conversion electrons.

For any given mode of decay, should the daughter be metastable, there will be the emission of γ-photons and internal conversion electrons as secondary emissions. This will be indicated as [B⁻, γ], [B⁺, γ], [EC, γ], and so forth. The internal conversion electron yield, in ratio to γ-photon yield, is specific to a given radionuclide.^1,2,5

Alpha Decay

In unstable nuclei with very high atomic masses, the most probable mode of decay is α decay. An alpha particle consists of two protons and two neurons, which is essentially a helium nucleus. Alpha decay results in a daughter with a Z number of 2 less than the parent and an atomic mass less by 4 relative to the parent.

Example:

Due to its high charges and heavy mass, the alpha particle has a very short travel distance in matter and deposits its energy very quickly. It has no application in diagnostic imaging and induces significant potential for biologic damage.^1,3

Decay Schemes

The modes of decay may be expressed graphically, called decay schemes. Decay schemes graphically illustrate all possible nuclear transitions that unstable nuclei undergo. They are often accompanied by tables with detailed information about the transitions such as the percentage occurrence, isomeric transitions, internal conversions, characteristic x-rays, Auger electrons, and biologic dose information.

In decay schemes, the nuclear energy levels are expressed as horizontal lines. The space between these lines represents the transition energy (Q).

The types of emissions are depicted by a unique direction of a line (Fig. 1-4).

Note the arrows may be angled to either the right or left. In neutron-rich parents, the mode of decay "shifts" the daughter to the right, corresponding to the shift on to the line of stability graph. Conversely, a mode of decay for a proton-rich parent moves to the left, toward the line of stability.

The tables that accompany decay schemes provide additional detail including the secondary emissions, as mentioned earlier. Since many of the secondary emissions are particulate, that is, electrons, these data are of particular interest in radiation dosimetry. In the decay scheme for ²⁰¹Tl, the data regarding the characteristic x-rays of ²⁰¹Hg are in these tables.

Parent–Daughter Equilibrium

Not all nuclear transitions lead to a stable daughter. The β⁻ decay of ⁹⁹Mo yields ^99mTc, which then decays to ^99mTc by isometric transitions and internal conversions. ^99mTc decays to ⁹⁹Tc with an 87% frequency through isometric transitions. Therefore, for every 100 decays of ^99mTc, we observe 87 γ-rays and 13 internal conversion electrons, as stated earlier. This higher yield of photons makes ^99mTc a very desirable radionuclide for imaging. A sample of ⁹⁹Mo would always contain some proportion of ^99mTc and ⁹⁹Tc. Since both parent and daughter are decaying, the relative activities would reach equilibrium, based on their half-lives. These states of equilibrium are employed when using both technetium and rubidium generators. When the parent half-life is marginally longer than that of the daughter, the amount of the daughter in the mixture will reach a maximum over a period of time. That elapsed time will be a multiple of half-lives of the daughter. If the daughter radionuclide is removed from the mixture, the same multiple of half-lives will have to occur, before the maximum amount of the daughter is subsequently reached. This equilibrium state is called transient equilibrium.^2,4 It is this transient state that is the basis of ^99mTc production from ⁹⁹Mo–^99mTc generators (Fig. 1-5).

In parent–daughter mixtures where the half-life of the parent is markedly longer than that of the daughter, secular equilibrium is reached. In this state of equilibrium, the concentration of the parent is decreasing so slowly relative to the daughter that the mixture appears to have the half-life of the parent. It is this equilibrium that is the basis for the ⁸²Sr–⁸²Rb generators used in ⁸²Rb PET imaging (Fig. 1-6).¹

The specific time that an unstable nucleus will undergo a transition cannot be determined, only predicted. Nuclear transitions are spontaneous and random, so the mathematics of radioactive are based on probabilities and rates, not specific nuclear events. If a population of radioactive atoms, N, is considered, the rate of nuclear transitions would be expressed as ΔN/Δt. The rate implies that a constant would express the average number of transitions that occur per unit time. This constant is called the decay constant; which is specific to a given radionuclide and is expressed as λ. The mathematical relationship is: ΔN/Δt = –λN, where N is the total number of radioactive nuclei and λ the decay constant. Since the total N decreases with time (t), the decay constant (λ) is a negative value.^1,2,4 The number of transitions per unit time (ΔN/Δt) is called activity. Activity is measured in curies (Ci), which is defined as 3.7 × 10¹⁰ disintegrations per second (dps). The International System of Units (SI) unit equivalent is the becquerel (Bq), which is defined as Bq = 1 dps. So 1 Ci = 3.7 × 10¹⁰ Bq. The most commonly used units are in the mCi (MBq) range for nuclear cardiology procedures.

In nuclear decay, the number of radioactive nuclei (N) is always decreasing as time passes at an average rate defined by the decay constant (λ). Decay is expressed, therefore, as exponential function; that is, the number of radioactive nuclei available is affected by both the number of unstable nuclei and its average rate of decay. To calculate the specific number of decays for a given time, we have the following expression:

where N_(t) is the number of unstable nuclei remaining after elapsed time (t) can be used. The expression e^–λt is called the decay factor (DF) and is unique to the time (t) and the decay constant (λ). The "e" is the base of natural logs, which is 2.718, so, when raised to the power of –λt, it determines the specific fraction of remaining radioactive nuclei after time (t) has elapsed.

When the elapsed time (t) is the time required for half of the total number of radioactive nuclei to decay, it is termed the half-life. Half-life (T_1/2) and the decay constant (λ) are related as:

Since ln 2 is equal to 0.693, we have

The activity (Ci) is dependent on the number of unstable nuclei and the decay constant. Therefore, units of activity (A) can be substituted for the number of radioactive nuclei (N), in the decay equation, yielding the following expression:

where A_(t) is the activity at elapsed time t, A₍₀₎ the activity at t = 0, and e^–λt the decay factor for the specific time (t).

As an example of practical use, consider a 30-mCi syringe of a ^99mTc-labeled myocardial perfusion agent. The calibration time for the 30 mCi is 8:00 am. What will be the activity at 9:45 am?

The same principles apply in the situation where an activity determination is required for a time (t) in the past, the precalibrated value. In this case, the DF would be the reciprocal of the elapsed time decay factor^1,2:

So, we have:

As discussed earlier, there are distinct types of radiations; particulate and electromagnetic. These radiations have distinct interactions with matter. Interactions of radiations with matter are processes that absorb or degrade the energy of the radiation and, in some cases, change its fundamental characteristics. The energy of the radiation and the Z number of the matter determine the type of interaction or multiple interactions that occur. Depending on the energy, radiations may be nonionizing; that is, the energy is not sufficient to free electrons from their orbit. If the energy is sufficient to free electrons from their binding energy, the radiation is ionizing. The energy of the ultrasound radiations in echocardiography does not cause ionization in tissues, and thus are labeled "nonionizing." The photons used in cardiac catheterization and nuclear cardiology are classified as ionizing, implying that their interaction with tissue creates ion pairs. This interaction in tissue forms the basis for radiation biology as applied in radiation safety practices. Further, these interactions form the physical bases for detecting radiation and creating images in our instrumentation.

Particle Interactions with Matter

As we have stated, radionuclide images are created through the detection of photons, γ-rays for ^99mTc, characteristic x-rays for ²⁰¹Tl, and annihilation photons for ⁸²Rb and other PET radiopharmaceuticals. When photons undergo interactions in matter, either tissue or imaging detectors, energized charged particles, that is, electrons, are produced. Particles, either β or α, interact with matter through electrical collisions. These collisions result in excitation, ionization, or bremsstrahlung. In excitation, energy from the incident particle is transferred to an outer shell electron. The electron is energized but does not exceed its binding energy. That increased energy is dissipated generally as heat radiation. Excitation is a low-energy interaction.

A higher-energy interaction occurs when the incident particle transfers enough energy to exceed the binding energy of the electron. An ion pair, an energized free electron and a positive ion, results. These ionizing interactions represent higher-energy level interactions and contribute to the specific exposure rate when applied to tissue, discussed in detail in a later chapter.

The third type of particulate interaction results when the incident particle penetrates the electron cloud and interacts with the charge field of the nucleus. The trajectory of the incident particle is markedly changed, resulting in a decrease in velocity. The decrease in velocity represents an energy loss. That energy loss produces photons of x-rays called bremsstrahlung, German for breaking radiation. Bremsstrahlung interactions are high-energy interactions more typical of high-energy particles interacting with high Z number matter.

Since any of these interactions may lead to partial dissipation of the energy of the incident particle, multiple interactions are required for the full energy of the particle to be absorbed in matter. These multiple interactions occur along a path that is determined by the energy, charge and mass of the particle, as well as the Z number of the matter. A highly charged massive particle will undergo many high-energy interactions over a very short path length. Alternately, a lower energy, less massive particle will undergo several low-energy interactions over a long path length. The difference in the degree of penetrability of the particle is called linear energy transfer (LET). A heavy highly charged particle would have a high LET, since the density of interactions is high over a short path length. An alpha particle because of its high charge (two protons) and heavy mass (Z = 2; A = 4) has high LET where dense levels of excitation, ionization, and bremsstrahlung are created in a very short distance. In contrast, a β particle, having little charge and mass, will lose its energy over a longer path length, and is thus characterized as low-LET particle. The LET may also be expressed in terms of the relative number of ionizations that are produced. This is termed specific ionization. Particles with high LET have high SI values; particles with low LET have low SI values. LET and SI characteristics have profound impact on the type of damage done to tissue.

Photon Interactions with Matter

The interactions of photons in matter are energy-degrading processes, just as in particle interactions. There are three mechanisms of photon interaction in matter; photoelectric absorption, Compton scatter, and pair production. The probability of occurrence for each of these mechanisms is a function of the energy of the photon and the atomic number of the matter (Fig. 1-7).

Photoelectric absorption occurs when the photon transfers all of its energy to an inner shell electron. The photon is completely absorbed and the electron, called a photoelectron, is ejected from its shell. The energy of the photoelectron is equal to the incident photon energy, minus the binding energy of its shell. It is key to remember that the photon no longer exists (it has been absorbed) and its energy is converted from electromagnetic energy to kinetic energy of the photoelectron. The kinetic energy of the photoelectron is dissipated by the mechanisms previously discussed: excitation, ionization, and bremsstrahlung.

The vacancy left by the ejected electron is quickly filled by an outer shell electron. Characteristic x-rays and Auger electrons are then emitted, as described earlier. Photoelectric absorption is most probable in interactions of low- to medium-energy photons in matter with high atomic numbers.

The second mode of interaction is Compton scatter. It occurs when a photon interacts with an outer shell electron, that is, an electron with low binding energy. Contrary to photoelectric absorption, the photon is not completely absorbed. It transfers a portion of its energy to the electron, which is subsequently ejected, and called a Compton electron. The resultant Compton scattered photon is in an energy-degraded state and in an altered trajectory relative to the incident photon. The angle of the scattered photon is related to the amount of energy transferred to the electron. The greater the amount of energy transferred to the electron, the greater the angle of scatter. Compton scatter is most probable in low- to medium-energy photons, interacting in matter with low atomic masses. This is of particular interest in imaging, since it is the most probable interaction of ²⁰¹Tl (Hg x-rays) and ^99mTc γ-rays in tissues. As the photons are scattered, not absorbed, they still exist, but have lost their association with the point of origin through the scattering process. These photons are the source of image degradation, but can be identified by their lower-energy values. The identification of these photons and their rejection from an image is a critical function of imaging equipment.^1–5

The third interaction of photons in matter is pair production. High-energy photons may completely avoid interacting with orbital electrons and interact in the magnetic field of the nucleus. This interaction results in the creation of a pair of electrons, one positive and one negative. The positive electron immediately combines with a negative electron, creating two 511-keV annihilation photons. The energy of the incident photon must be at least two times the mass energy equivalency of an electron (511 keV) or 1.022 MeV. Since energies in this range are not used in imaging, pair production is not relevant to this discussion.²

In all three mechanisms, there are probabilities of occurrence based on photon energy and atomic number. The distribution of the probabilities is shown in Figure 1-8.

Attenuation

These interactive processes, in addition to photon energy and atomic number, are affected by the thickness of the absorber. For a given thickness, a number of photons will not interact and therefore be transmitted. As the thickness of the absorber increases, the fraction of transmitted photons will decrease. The fraction of absorbed photons is a function of photon energy and atomic number, but the total number of photons absorbed is a function of the thickness of the absorber.¹ For any given relationship of photon energy and atomic number, there is an average rate of absorption. The rate is called the linear attenuation coefficient and is symbolized by μ. Since the linear attenuation coefficient is a fixed rate, the rate of transmitted photons is also fixed. If the thickness of a given absorber is doubled, those transmitted photons will be subject to further absorption. The mathematical relationship of incident beam intensity, transmitted beam intensity, and linear attenuation is exponential.

The mathematical expression is:

where I_(x) is the beam intensity after interacting with an absorber of thickness x and a linear attenuation coefficient of μ, from an initial intensity I₍₀₎.⁷ This mathematical expression is the same as used in predicting the reduction in activity over time. Therefore, the expression e^-μx is the specific fraction of absorbed photons by a specific thickness of matter. The thickness of a given absorber that results in a reduction of initial beam intensity of 50% is called the half-value thickness (HVT) or half-value layer (HVL). The HVT can be calculated by,

where μ is the linear attenuation coefficient and 0.693 is the ln 2. Note that this is the same equation as used in calculating the half-life of a radionuclide. The absorption characteristics of tissue are essentially the same as water, since the linear attenuation values are similar. Table 1-1 illustrates the differences in HVTs for lead, a common material used in shielding, water (tissue), and NaI, a common detector material in imaging equipment.

Note that the HVT for Tl (80 keV) and Tc (140 keV) are 39.6 and 46.2 mm, respectively. It is typical to have these thicknesses of tissue overlaying the heart. If these thicknesses reduce the beam intensity by 50%, the imaging effects would be a reduction in count density of 50%, as well. It is critical, therefore, to differentiate these tissue attenuation effects, from reductions in biodistributions of tracers in interpreting radionuclide images.

In summary, the photons we use to construct cardiac radionuclide images come from both nuclear and atomic sources, as demonstrated by the emissions from ^99mTc and ²⁰¹Tl, respectively. The emissions used may also be the result of processes that are secondary to the primary emission, as seen with the annihilation photons from the decay of ⁸²Rb. The principles of the detectors used in imaging equipment, as well as those used in radiation safety, are based on characteristics of photons and the matter in which they interact. Those characteristics are defined by the physics of nuclear structure, mass–energy relationships, and radiation interactions in matter.

As a practical consideration, it is important to note that radionuclide image quality is directly related to the type, energy, and the behavior in matter of photons. The three mainstream radionuclides used in nuclear cardiology, ^99mTc, ²⁰¹Tl, and ⁸²Rb, exhibit all of these physical differences, therefore familiarity with the physical basis of radiation physics is essential.