Question

In part two you will submit a copy of the notes you have made for the following sections of the ARRT content specifications (5 points):

1. Radiation Physics and Radiobiology

A. Principles of Radiation Physics

1. x-ray production

a. source of free electrons (e.g., thermionic emission)

3. x-ray beam

​b. beam characteristics

​​1. quality

​​2. quantity

​​3. primary versus remnant (exit)

​​ 4. photon interactions with matter

​​​a. Compton effect

​​​b. photoelectric interaction

​​​d. attenuation by various tissues

​​​​1. thickness of body part

​​​​2. type of tissue (atomic number)

Answer 1

(1)

Radiation Physics and Radiobiology

(A) Principles of radiation physics

Electromagnetic radiation is energy transmitted at a fixed velocity through sinusoidally varying electric and magnetic fields. The frequency of variation of this energy, represented by the Greek letter ν, is the number of oscillations per second, measured in hertz (Hz). The wavelength, λ, is the distance in meters between two crests of the sine wave. The velocity of propagation of the radiation is the product of the wavelength and the frequency, which, in a vacuum, is equal to the speed of light, c = 3 × 10⁸ m/sec.

The range of wavelengths encountered in conventional physics is from 10⁵ m for AM radio waves, to 10^-7 m for visible light, to 10^-12 m for x-rays and cosmic rays. Although electromagnetic radiation is conventionally described as waves of energy, quantum physics tells us that it is equally valid to describe the radiation as particle-like packets of energy called photons. Experiments that scatter x-rays off particles have been used to validate this concept. In general, the shorter wavelength radiation is more “particle-like” than the longer wavelength radiation. The energy of a photon is directly proportional to the frequency of the radiation, with a constant of proportionality called Planck’s constant. That is, E = hν, where h = 6.626 × 10^-34 J/s and the energy is in Joules.

Henri Becquerel discovered natural radioactivity in 1896 when he observed the blackening of wrapped photographic plates when they were placed in contact with certain elements. This event was preceded by the discovery of an invisible form of energy, dubbed x-rays, by Röntgen, who observed the glowing of fluorescent material placed near a gas discharge tube.

We now understand that Becquerel observed the natural decay of radioactive nuclei into one of three types of radiation: positively charged alpha particles (α), which we now know are helium nuclei; negatively charged beta particles (β), which are electrons; and uncharged gamma-rays (γ), which are a type of electromagnetic radiation emitted from nuclei. It is entirely possible that Becquerel also observed other types of elementary particles that can exist in the nucleus, or can be produced in nuclear decay processes.

The explanation of nuclear decay depends on understanding the interplay between the very strong, attractive nuclear force that binds the neutrons and protons together in the nucleus, and the moderately strong repulsive electromagnetic force between the protons. As the protons and neutrons move about within the nucleus, there is some probability that one of them will acquire enough kinetic energy to escape from the nuclear potential energy “well.” Large nuclei with many neutrons and protons tend to be less stable because of the increasing repulsion of the protons. For large nuclei to be stable, an excess of neutrons is required to provide the nuclear attraction to keep the protons from escaping.

(2) ​​​​​​

X ray production

(A) Source of free electrons

Most radiotherapy is delivered with beams of x-rays that are produced as a result of the interactions of accelerated electrons with matter. This can be either through excitation or ionization of the atom via interactions of the accelerated electrons with target electrons leading to the emission of characteristic x-rays, mentioned earlier, or through direct interaction between the electrons and the electromagnetic field of the target nuclei, leading to bremsstrahlung x-rays.

When electrons are incident on target atoms, they can ionize those atoms by depositing sufficient energy to eject an inner shell electron. The inner shell vacancy is subsequently filled by an outer shell electron, causing the emission of a characteristic x-ray with energy equal to the difference between the binding energies of the inner and outer shells. This process is diagrammed in below figure. Although this process can take place in both low-Z and high-Z atoms, only for high-Z atoms are the binding energies sufficient to produce radiation in the x-ray portion of the electromagnetic spectrum. For example, the binding energy of K-shell electrons in tungsten, a common target for x-ray tubes and accelerators, is about 70 keV, whereas for aluminum, it is only about 1.5 keV. As mentioned earlier, the characteristic x-ray energy is occasionally transferred directly to an orbital electron, leading to the production of an Auger electron.When high-energy electrons interact directly with the electromagnetic field of a target nucleus, they are deflected and lose a portion of their energy due to deceleration. The energy lost is then emitted in the form of radiation called bremsstrahlung (literally, braking radiation).

Since the incident electron can lose any portion of its energy in this process, the energy of these bremsstrahlung x-rays can vary continuously from nearly zero to the full energy of the incident electron. The angle of the x-rays, relative to the beam direction, depends on the energy of the electron. At low energies typical of those used for diagnostic radiology, the x-ray has equal probability of being emitted in any direction, whereas at high energies typically used for radiotherapy, the x-rays are emitted preferentially in the forward direction. These facts explain the use of thick targets and 90-degree angles between electrons and x-rays for diagnostic applications and thin transmission targets for radiotherapy.

The probability of bremsstrahlung production varies with Z2 of the target material, whereas the efficiency of x-ray production depends on the product of Z and E, the energy of the electrons. At 100 keV in a tungsten target, only about 1% of the incident energy of the beam is converted into x-rays, the remainder ending up in heat.

In general, both bremsstrahlung and characteristic x-rays are present when electrons strike a target. Below figure shows x-ray spectra for a thick tungsten target for electron energies of from 65 keV to 200 keV. Note the appearance of the characteristic x-rays on top of the bremsstrahlung spectrum once the electron energy is in excess of the K-shell binding energy of 70 keV. Any material in the x-ray beam, including the target itself, will absorb some of the energy of the beam. The effect of such absorption is to preferentially filter out the low energy portion of the bremsstrahlung spectrum. The process of preferentially attenuating the low-energy component of the beam is referred to as “beam hardening.” When the beam is hardened, the average energy of the x-rays increases at the expense of reduced intensity.

(3) X ray beam

(A) Beam characteristics

Beam quality

Beam quality refers to the overall energy or wavelength of the beam and its penetrating power. A high-quality beam has short wavelength, high mean energy and high maximum energy. The beam quality is controlled by the kilovoltage. The kVp regulates the speed of electrons traveling from the cathode to the anode and determines the penetrating ability of the x-ray beam. When the kVp increases, the resulting x-ray beam is of higher energy and increased penetrating ability.

Beam quantity

The amperage and the exposure time determine the amount of electrons passing through the cathode filament. An increase in the amperage or in the exposure time will result in an increase in the number of photons generated in the x-ray tube. Since the amperage and the exposure time have a direct influence on the number of photons emitted, they form a common factor called the milliampere-seconds (mAs). When one of these two factors is increased, the other must be decreased to maintain the same beam quantity.

Primary versus remnant

The source of x-rays is the x-ray tube. X-rays are formed within a very small area inside the tube. From this point, the x-rays diverge into space. The x-ray tube is surrounded by a lead-lined tube housing. Some of the scattered x-rays are absorbed by the tube housing. X-rays that are created exit the housing through an opening called the tube port. These x-rays form the triangular-shaped x-ray beam. The radiation that leaves the tube is called primary radiation. The squared area of the x-ray beam that strikes the patient and x-ray table is called the radiation field. An imaginary line in the center of the x-ray beam and perpendicular to the long axis of the x-ray tube is called the central ray. The central ray is important in positioning the patient because this point is used to align the x-ray tube to the body part to be imaged.

During a radiographic exposure, x-rays from the tube are directed through the patient to the image receptor (IR). As the x-rays pass through the patient, some of them are absorbed by the patient and others are not. Anatomic structures that have greater tissue density (mass), such as bone, will absorb more radiation than less dense tissue, such as muscle. This results in a pattern of varying intensity in the x-ray beam that exits on the opposite side of the patient. This radiation, called remnant radiation or exit radiation, then passes through to the IR. The IR now contains an “unseen” image called a latent image. This image remains stored in the IR phosphors until it is processed. Processing will convert the latent image into a visible image.

Photon interaction with matter

X-rays can be considered as packets of energy, called photons, for purposes of considering their interactions with matter. There are a number of different interaction processes that the photons can experience, and each of these has a probability described by an attenuation coefficient.

where dN is the reduction in the number of photons due to interactions in a thickness dx of an absorber, N is the number of incident photons, and µi is the attenuation coefficient for the ith interaction process. After integration over the thickness, this equation becomes.

where N(x) is the number of photons left in the beam (without interaction) as a function of the thickness x of material traversed, and µtot is the sum of the individual attenuation coefficients. (µi/ρ) is called the mass-attenuation coefficient for process i, where ρ is the density of the material. Dividing the conventional attenuation coefficients by density removes the dependence on the physical density of materials and the resulting mass attenuation coefficients are approximately equal from material to material. The corresponding thickness, ρx, is in units of grams per square centimeter (g/cm2) . There are five different photon interaction processes, including coherent scattering, photoelectric effect, Compton scattering, pair production, and photodisintegration.

Compton effect

Compton scattering is a process in which the incident photon interacts with an orbital electron as if it were a free particle, since the binding energy is small compared to the photon energy. The dynamics of the interaction can be described as a typical particle–particle scattering interaction whereby the photon transfers some of its energy to the electron and is scattered at an angle φ relative to the incident direction. The electron is ejected at an angle θ relative to the forward direction. The energy of the outgoing Compton-scattered photon is equal to the difference between the incident photon energy and the energy transferred to the electron. A diagram of the Compton process is shown in below figure.

The mass attenuation coefficient for Compton scattering is independent of Z, decreases slowly with photon energy, and is directly proportional to the number of electrons per gram, which varies by only 20% from the lightest to the heaviest elements (with the exception of hydrogen). Therefore, in the energy region where Compton processes dominate, the attenuation of the beam will vary according to the integrated density of the material traversed. This fact is responsible for the relatively poor contrast observed in portal verification films exposed with megavoltage x-rays exiting from the irradiated patient.

photoelectric introduction

In the photoelectric effect, the energy of the photon is totally absorbed by the atom and subsequently transferred to an orbital electron, which is then ejected from the atom with an energy equal to the original energy of the photon minus the binding energy of the electron. The vacancy created by the ejection of the photoelectron from a shell gets filled by an electron from a shell with lower binding energy, followed by the emission of a characteristic x-ray with energy equal to the difference in binding energies of the two shells. Again, an Auger electron can be emitted in lieu of the x-ray in some cases.

The most likely angle of ejection of the photoelectron relative to the incident photon direction is 90 degrees for low-energy photons (50 keV or less), becoming smaller (more forward) as the photon energy increases. The mass attenuation coefficient for the photoelectric effect is proportional to Z³/E³, where Z is the atomic number of the target atom and E is the energy of the photon.

The photoelectric effect is important at diagnostic x-ray energies of 100 keV and is the basis for the radiographic contrast of bone versus soft tissue. However, because of the 1/E³ energy dependence, the photoelectric effect is relatively unimportant at typical radiotherapy energies of several MeV.

Thickness of body parts

A radiographic image is composed of a 'map' of X-rays that have either passed freely through the body or have been variably attenuated (absorbed or scattered) by anatomical structures. The denser the tissue, the more X-rays are attenuated. For example, X-rays are attenuated more by bone than by lung tissue.

attenuation by various tissues

When the x-ray passes through an absorber (e.g., oral tissues), it gets differentially absorbed by what constitutes the absorber and the thickness of each component. When the x-ray beam exits this absorber, it will have varying levels of intensities. This variation will be recorded on a radiographic receptor as different densities generating the radiographic contrast. The densities related to a thick absorber (i.e., aluminum) will be brighter than the densities of the thin absorbers.Contrast within the overall image depends on differences in both the density of structures in the body and the thickness of those structures. The greater the difference in either density or thickness of two adjacent structures leads to greater contrast between those structures within the image.For descriptive purposes there are five different densities that can be useful to determine the nature of an abnormality. If there is an unexpected increase or decrease in the density of a known anatomical structure then this may help determine the tissue structure of the abnormality.

Type of tissue

We mentioned earlier that the probability of photoelectric absorption is proportional to (Z/E)3. What we did not discuss, however, is the concept of the k-edge. Albert Einstein won the Nobel prize for explaining how photoelectric events require a minimum amount of energy (enough to dislodge the electron); it turns out that this is also the most likely energy for those events to occur. Higher energy photons are much less likely to cause photoelectric events. The k-edge represents the energy needed to eject a K-shell electron (the innermost and most strongly bound electrons). Outer shell electrons have absorption edges but these are much too low energy to be relevant.

In soft tissues, the dominant elements (e.g. C, H, O, and N) have very low K-edges, in the range of a few keV. While these elements do contribute to the photoelectric effect and attenuate low energy x-rays, there is no relevant k-edge with its substantial change in attenuation. However, the elements iodine and barium have K-edges around 30-40 keV, right in the middle of the x-ray beam spectrum. Thus, soft tissues with even a small amount of iodine will have a much stronger x-ray stopping power than those without.