Physical Principles of Ionizing Radiations
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This radiology continuing education module explores physical principles of ionizing radiations. Basic scientific principles of ionizing radiations used in radiology and nuclear medicine are discussed. Some of the topics include: Bremsstrahlung and characteristic radiations, positron production, annihilation reaction, and more.
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Author: Nicholas Joseph Jr. (RT) R
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In nuclear medicine some beta emitters are useful because they are pure beta emitters and do not give off gamma radiation. These types are good therapeutic agents because beta does not leave the patient's body so only local tissues are exposed. Non pure beta emitters give off gamma radiation and it can be detected outside the body for diagnostic imaging. Positron and electron capture are below the theoretical line and are competing reactions among those radioisotopes that are used for positron scanning.
Table 1.4 illustrates the relative resultant energy to the beta particle from the conversion of a neutron to a proton causing beta emission. The total energy released from the nucleus is known to be 1.7 MeV, which is Emax. This energy is partitioned between the beta particle and the antineutrino during beta emission; and between the beta, antineutrino, and a gamma photon if the nucleon is not a pure beta emitter. Beta particles used in nuclear medicine generally have 25-30% of the maximum energy (0.6 MeV), called Eβ- ; the balance is imparted to the antineutrino and/or to the gamma photon. A unique characteristic of electromagnetic radiation is that it will travel in a vacuum, where other types of radiation energy will not.
The net energy produced by neutron conversion to a proton is partitioned between a beta particle and an antineutrino, with a concomitant ejection of them both from the nucleus. Because neither energy nor mass is created nor destroyed, but can be converted from one form to another (the first law of thermodynamics-conservation of energy and mass), we must account for all energy released in the disintegration pattern of a non pure beta emitter. According to Albert Einstein, whenever mass is lost, it can be accounted for by a transformation to pure energy and vice versa. For non-pure beta emitters, along with isobaric transition a portion of the energy goes to the production of a gamma ray, which is also released from the nucleus along with the beta particle and antineutrino. As we have stated, the beta particle is special to radiological science because of its therapeutic applications. Its properties are not useful in terms of nuclear medicine imaging because these particles lack sufficient energy to escape the patient's body and be detected. A closer look at the physical properties of β-particles explains its interactions with other atoms causing ionization. There are two interactions we will discuss related to beta initiated ionization: 1) the specific ionizations caused by a β-particle per centimeter traveled, 2) the energy losses by a beta particle to achieve a deionization state. Deionization is the process of a charged particle loosing all of its kinetic energy through ionization of adjacent atoms to achieve a rest state.
The ionization potential and distance of travel for any particulate radiation is related to its mass and its charge; the larger the particle's size and charge, the greater is its ionization potential although its distance of travel diminishes. A beta particle has the mass of an electron and carries a minus one negative charge. An alpha particle by comparison contains twice its charge and 7400 times its mass. Gamma radiation has no mass and no charge. What this means is that an alpha particle because of its mass and charge travels the shortest distance and has the least penetrability. An α-particle is easily absorbed from air by a sheet of paper. A beta particle having 3.4 MeV will cause about 100,000 ionizations in tissue before achieving its rest state of a free electron, or deionization. In soft tissue the beta particle can travel 1-3 cm, and in tissue a 1 MeV beta has a range of about 0.42 cm. Gamma photons because they have no mass or charge have unlimited range of distance, and have relatively few ionizations in tissue. Pure beta emitters are great therapeutic agents because a beta particle looses 3.4 eV per ionization event. This energy is harnessed to treat clinical states such as leukemia, ascites, polycythemia, and other diseases. Table 1.2 illustrates the ionization and penetrating potentials of some radiations.
Ionization of adjacent atoms by a beta particle is quite different from alpha or gamma radiation mechanisms. When a high speed beta particle passes through an atom's orbitals, it ionizes by repulsion force between it and the atom's electron(s) ejecting them from their orbital(s) causing ionization of the atom.
Electrons are required to maintain a maximum distance from each other because of their like charge (opposites attract, likes repel) according to what is known as VESPER Theory. This creates a strong repulsion force that ejects an orbital electron when the high speed β-particle collides with it.
Beta radiation may also produce electromagnetic radiation consequential to its interaction with atoms in its path. Ionization by particulate radiation can produce bremsstrahlung and characteristic radiations, but not gamma radiation, in the orbitals of atoms. Electromagnetic radiation we stated earlier differs from particulate radiation in several ways, primarily in that it is composed of small discrete packets of pure energy, termed photons that have no mass or charge, and travels at the speed of light. The energy and wave form of electromagnetic radiation can be represented with an oscilloscope.
The energy of a photon is measured in electron volts (eV) and multiples thereof, keV and MeV. A unique property of energy is that it determines the penetrating ability of a photon. Low energy photons do not penetrate a patient's body and are absorbed in bodily, whereas high energy photons will penetrate the patient and contribute to the radiographic image. Any photon with greater than 20 eV is classified as ionizing radiation since any element on the periodic table can be ionized by photon energies ranging from as little as 5 to 20 eV.
Section III: Basic properties of electromagnetic radiation
Unlike particulate radiation, electromagnetic radiation energy possesses a waveform that is sinusoidal, having amplitude, frequency, and wavelength. The wavelength (γ) may be described as the distance the radiation travels through space in a single oscillation. Radio waves and television wavelengths are often expressed in meters, but light energy such as gamma and x-rays are expressed in shorter units such as the angstrom or nanometer (nm). Photon energy and wavelength are inversely related by the formula:
The frequency of a photon describes its rate of oscillations, which is cycles of its wavelength per second; its units are cycles per second (cps) or Hertz (Hz). The Hertz is equal to one cycle per second. This description is almost exclusively used to describe photons at the lower end of the spectrum, called the RF portion, which includes radio waves, television broadcasting, even microwaves, and MRI are all located at the lower end of this spectrum. Photon frequency is not a particularly useful way of describing gamma or x-rays because of their short wavelength. Wavelength and frequency are inversely related so that as the frequency increases so does the photon's energy and its wavelength decreases inversely. X-rays and gamma rays have such short wavelength and high frequency that to describe them in hertz would be incomprehensible. Photon energy (E) and frequency (f) are directly proportional according Max Planck's quantum theory from which the following formula is given:
The relationship between energy, frequency, and wavelength is illustrated below by the electromagnetic spectrum. Some energy radiations are strong enough to eject an electron from an atom creating an ion pair.
Electromagnetic Spectrum Chart from: Berkeley Lab. Berkeley, CA. with Permission
Frequency, wavelength, and the corresponding energy associated with the photon are what give it its penetrating ability. A photon has great penetrability because of its wave like properties, which is something that particulate radiation does not possess. Frequency and wavelength cannot be used to describe particulate radiations such as alpha and beta radiations because they do not have a waveform. A particle's energy is due to its kinetic or potential energy it may contain, as well as its charge, and mass.
Frequency is useful in describing magnetic resonance because during MRI, protons emit a signal frequency of 42.58 MHz when placed in a 1 tesla magnetic field. Proton frequency is important to MRI imaging; however, frequency is not an especially a useful term to describe the application of photon energy in diagnostic imaging. A further discussion on the application of frequency in MR imaging is discussed in module 6 of this issue.
Section IV: Bremsstrahlung and characteristic radiation produced by â-radiation
Bremsstrahlung and characteristic radiation can be produced as a result of a beta particles release from the nucleus of a decaying atom. Let's look at the mechanism of production of bremsstrahlung radiation by a beta particle first. Bremsstrahlung radiation is generated by projectile beta particles released from a decaying nucleus. This type of radiation is also called "breaking radiation" because its origin is from a partial deceleration of the high speed β-particle as it passes close to the highly positive charges in the nucleus of an adjacent atom, causing the particle to undergo a slight change direction. But because the incident beta particle is moving at a high velocity it is not sequestered by the nucleus, instead, it is deflected enough to give off energy. The vectorial change results in a loss of kinetic energy by the β-particle, which is released as a photon of electromagnetic energy, termed bremsstrahlung radiation. Bremsstrahlung radiation produced by a β-particle is a two edged sword, first because it is not especially useful for therapeutic purposes because of its low ionization potential; secondly, bremsstrahlung produced by a beta electron is more harmful to the technologist than the beta particle that produces it, because of the penetrability of an electromagnetic ray. For beta particles the amount of bremsstrahlung increases as the density (high Z number) of the material through which it passes increases.
Bremsstrahlung radiation is made as a beta particle passes close to the nucleus of an atom causing the beta particle to be deflected from its original course. The β-particle looses energy as it is slowed down in a "braking" deceleration that creates a photon of energy.
It is for this reason that shielding is an important consideration when using beta emitters. Plastic or Lucite are effective shielding materials for a beta emitter since these materials have a low Z number and therefore produce less bremsstrahlung than lead shielding.
The other type of electromagnetic radiation the beta particle can produce is characteristic radiation. If a high speed beta particle approaches an electron in an inner orbital, it can cause it to be ejected from the atom by the strong repulsive force between it and the electron because of their same charge. When an inner orbital electron is ejected the atom rearranges its electrons to fill the vacancy by moving outer orbital electrons inward. When an electron is moved to a different orbital it must give up some of its energy during the transition; this energy is released in the form of a characteristic ray. These are generally low energy photons depending on the atom type and orbital from which the photon originates. Characteristic radiation produced by beta particles is not useful to diagnostic imaging or to therapy. The production of characteristic radiation by a high speed beta particle is minimal in comparison to bremsstrahlung produced; bremsstrahlung production is the dominant mechanism responsible for bringing the beta particle to a rest state.
Section V: Positron production by proton to neutron transition
By definition, a positron is a positive charged electron emitted from the nucleus of an atom during disintegration. A positron is identical to the electron or beta particle with the exception that it carries a positive charge. It is the plus one (+1) charge of the positron along with its mass and speed, and interaction outside the nucleus that gives it its properties. Its relative size is that of an electron which is approximately 1/2000 the size of a proton, yet it has the charge of a proton.
The underlying influence driving the production and emission of positrons is an unstable nucleus suffering a low n:p ratio. The ratio is raised by increasing the number of neutrons while simultaneously diminishing the number of protons in the nucleus during transitions that produces positrons. A positron is a type of particulate radiation emitted from the nucleus of a decaying atom when a proton is converted into a neutron. In this type of disintegration a positive electron called a positron (e+ or β+) is created, along with a neutrino, and excess energy. Positron production is therefore the opposite of beta production. With beta decay a neutron is converted to a proton with the release of a beta particle and an antineutrino; in positron decay a proton is converted into a neutron with a positron and a neutrino ejected from the nucleus.
Section VI: Positron interactions and annihilation reaction
Three important physical interactions occur once a positron is ejected from the nucleus: Bremsstrahlung radiation is produced, ionization of the atom occurs, and the annihilation reaction. Bremsstrahlung radiation production is by a different mechanism than that produced by a beta particle or by a high speed electron as it is deflected from its course toward an atom's nucleus. With positron production there is repulsion between the positively charged nucleus and the positively charged positron. This repulsion causes a "negative" acceleration away from the nucleus, which is nearly opposite the mechanism of deceleration we discussed earlier. The net result is the production of electromagnetic radiation energy which is lost from the atom.
Two interactions are caused by the positron as it traverses matter, ionization and annihilation. Ionization interactions also occur by intrinsic mechanisms that alter the parent atom's stability becoming a cation. Ionization is a conditional reaction to the positron's positive charge as it is ejected from the nucleus and traverses its own orbitals to leave the atom. Along its ejection track it may draw orbital electron(s) towards it and away from the atom, causing the atom to be ionized. Ionization does cost the positron some of its energy which concomitantly cost it some of its speed. This manner is very different from beta ionization in that the positron's positivity attracts orbital electrons from the atom during its ejection flight, causing self ionization. It differs from beta ionization in that a negative beta particle ionizes atoms by repulsion between itself and orbital electrons during its expulsion. When a positron looses all of its kinetic energy it attains a rest state (0 keV) followed by interaction with an oppositely charged rest state electron, which is a highly favorable condition for their annihilation.
When a positron and an electron collide, they will together undergo an annihilation reaction in which their combined mass is converted to pure electromagnetic energy that is partitioned equally to two gamma photons. Annihilation occurs because the positron in addition to being opposite in charge to the electron, it is composed of what is sometimes called antimatter, which causes a reaction quite different than that of a proton with a plus one charge. Eventually, the accumulation of attractive interactions and decelerations of the positron will bring it to a rest state. A rest state positron will undergo its final interaction, annihilation of itself and matter (a negative electron). The process yields two high energy photons of gamma radiation that are expelled from the collision at 180 degrees to each other.
The two particles will together undergo annihilation of their masses, and a simultaneous conversion to pure energy. The equation is an Einsteinium conversion in which the energy produced is equal to the total mass times the speed of light squared. What is interesting here is that the energy produced from the annihilation reaction is divided equally between two gamma photons whose vector is 180 degrees to each other, and each photon will possess 511 keV of energy. The positron is important to radiology because it is the basis of PET scanning which is diagnostic imaging's newest and most promising technology.
Section VII: Electron capture decreases the proton number and raises the neutron count.
The final reaction important to atomic nuclear decay that we will discuss is electron capture. Electron capture is another option for an atom having too many protons or too few neutrons. Currently there are several theories about why this reaction occurs, but what is known about it is that the k-shell electron, and possibly the L-shell electron also, may pass through the strong influence of its own nucleus as a part of its normal orbital path. If the atom contains too many protons and too few neutrons for nuclear stability under certain conditions it may capture the K-shell or even an L-shell electron. When this happens there is sufficient nuclear instability to cause transformation of one of its protons into a neutron. As with all atoms in which an inner shell electron is lost, the atom will fill the vacancy by shifting electrons from its outer shells to fill inner vacant orbitals. Successive shifting of electrons inward results in the release of photons corresponding to the orbital difference between the initial and final orbital in which the electron comes to reside. Radiation released in this manner is termed characteristic radiation because the energy of the photon produced is characteristic of the orbital from which its electron was shifted. Finally, the vacancy that was created by the loss of the K-shell electron is expressed in the outermost valance shell. Here the atom has its best change of obtaining a free rest state electron and returning to a stable state. What important to us about electron capture is that with this type of transition, radiation is not emitted from the nucleus of the atom, instead, radiation is produced as a result of refilling electron vacancies from successive orbitals creating characteristic radiation. Characteristic radiation is the primary type of radiation produced as a result of electron capture. Positron emission and electron capture are somewhat competing reactions, for example, a nuclide that is a positron emitter some atoms of the same series will undergo positron emission and others electron capture. It is characteristic that for some nuclear medicine studies characteristic radiation generated by electron capture reactions is ideal as an in-vivo study and is perhaps one of the strongest rationales for using these nuclides.
The above diagram represents the conversion of a proton to a neutron as a result of electron capture. A neutron is produced, along with characteristic radiation or an Auger electron, which is not demonstrated.
Some radionuclides produce Auger electrons as well as characteristic radiation. An Auger electron is produced when the energy given off by the electron filling the K-shell is transferred to another electron ejecting it from the atom. The ejected electron is called an Auger electron because of the mechanism of its release. Electron capture is a competitive reaction with positron emission. Nuclides that are positron emitters may also undergo reactions that emit Auger electrons; the production ratio between positrons and Auger electrons is specific to the type of nuclide.
Section VIII: Manmade electromagnetic Radiation
Thus far we have discussed some of the properties of radioactive atoms and the ionizations they can cause. This section reviews some of the properties of electromagnetic radiation produced by x-ray tubes found in different types of x-ray apparatus, such as fluoroscopy units, CT, angiographic, and general diagnostic equipment. We will look at man-made radiation which is either bremsstrahlung or characteristic radiation; gamma radiation is also electromagnetic energy, however, it is not produced artificially in x-ray tubes. There are two basic classes of ionizing electromagnetic radiation: gamma rays which have their origin within the nucleus of decaying atoms, and x-rays which are man-made radiations produced in x-ray tubes. X-rays are heterogeneous photons composed of two types of radiations named according to how they are produced. X-rays are generated by two different processes: bremsstrahlung interactions which produce continuous photon energies, and characteristic radiation, which produce defined peaks of photon energy.
We have already talked about these types as products of particulate radiation interactions, but we should review their origin from the x-ray tube as well. X-rays and gamma rays have the same physical properties, but differ in their origins and energy. Gamma rays have a slightly shorter wavelength than x-rays, and can be easily detected from outside the body because of their high penetrability; x-rays are manmade radiation produced from electrons that are accelerated and rapidly decelerated inside a vacuum tube. The origins of x-rays are in the energy shells that surround the nucleus, they contain no gamma radiation.
An understanding of x-ray production is important because unlike particulate radiation, x-rays are produce during a short period of time called the exposure time, whereas gamma radiation is produced continuously in a source as part of the decay process of a nuclide. X-ray production requires a vacuum tube, source of free electrons (filament), a rapid acceleration of the electrons (high kilovoltage), and a rapid deceleration of the electrons in the dense anode material (tungsten anode). Electromagnetic radiations are produced in several types of x-ray equipment: CT scanners, angiography machines, C-arm fluoroscopy units, portable x-ray equipment and the like, but MRI and ultrasound machines do not produce x-rays; instead, there are other concerns when working around these energies. It should be understood that during occupational work no employee should be exposed to bremsstrahlung or characteristic radiation directly because they should not be in the primary beam. But as we shall see, radiation may change direction during the brief exposure of the patient in an interaction called the Compton Effect, which produces Compton scatter. Most of occupational radiation exposure dose is from Compton scatter. Bremsstrahlung and characteristic radiations are the two components of the useful radiation beam that produces radiographic images.
The most important type of ionizing radiation useful for diagnostic imaging is bremsstrahlung radiation. It is produced in the anode of an x-ray tube when electrons from the cathode are accelerated towards the anode under the influence of high kilovoltage. These electrons penetrate the anode where they interact with tungsten atoms that make up the target. The subatomic picture is that these electrons pass close to the nucleus of tungsten atoms, and with each pass strong positive charges in the nucleus exerts a strong pull on the electron changing its course and decreasing its speed. These rapid deflection decelerations of the electron releases energy in the form of a photon, which is termed bremsstrahlung radiation, meaning "breaking radiation". Tungsten atoms are especially good producers of synthetic made radiation because their high proton density attracts projectile electrons better than would a lower proton dense atom. The speed of the incident electron when it interacts with tungsten atoms determines the energy range of the bremsstrahlung photons produced. A high speed projectile electron will continue to undergo many bremsstrahlung interactions each of which will reduce its speed slightly until the electron achieves rest mass state. Because tungsten atoms are tightly packed in the anode the electron will come to rest in a few milliseconds. Bremsstrahlung photons can have energy values up to the maximum kilovoltage of the electrons that produces them. As we can see the mechanism of bremsstrahlung production in the x-ray tube is identical to the mechanism of its production by a beta particle.
Bremsstrahlung radiation is part of what is called the continuous ejection spectrum, because these photons are emitted with energies from near zero up to the maximum kVp selected. For example, if 100 kVp is selected, then in order to produce a photon with 100 keV energy, an electron moving at 100 keV would have to come to a complete rapid deceleration giving off all of its energy, 100 keV, to a newly created photon. Instead, what usually happens is that the electron gives off many photons with energies less than its kinetic energy as it undergoes many bremsstrahlung interactions with atoms until it looses all of its energy and comes to rest. The sum of the photons produced is equal to the kinetic energy of the electron that produced them in keeping with the law of energy conservation.
X-rays are produced when electrons are produced by the filament in a process called thermionic emission; these electrons are accelerated towards the target on the anode under the influence of high kilovoltage. Their rapid deceleration through interactions with the anode produces x-ray photons. This process of producing x-rays is not very energy efficient, 99% of the energy of the electrons is converted to heat and approximately 1% is converted to x-ray photon energy. The target upon which the electrons bombard is made of tungsten-rhenium for mechanical strength, and molybdenum or graphite layered beneath the tungsten to conduct heat away from the anode. Tungsten has a high melting point (3400oC) and can withstand high tube current, additionally; the anode rotates at speeds of 3400-10,000 rpm, a modification that increases the surface area over which electrons bombard.
Bremsstrahlung radiation is especially important to radiographic imaging because it is responsible for the heterogeneity of the useful beam and therefore subject contrast. If photons all had the same energy, such as a homogeneous x-ray beam, then they would all have the same fate of either being completely absorbed or being completely transmitted through the patient depending on their energy value. Complete absorption would mean there would be no radiographic density, and complete transmission would yield a single radiographic density and no contrast. What is desirable is that there is radiographic balance that creates subject contrast which demonstrates all four radiographic densities: bone, muscle, fat, and air. This is achieved only by a heterogeneous beam whose energies are selectively absorbed and transmitted by different tissue types, which is an advantage of bremsstrahlung radiation. Bone absorbs more photons than muscle and is radiopaque, whereas, air absorbs few photons and is radiolucent. Selective photon absorption requires heterogeneous x-ray beam energies to produce diagnostic radiographic images.
Photon absorption occurs by a mechanism known as the photoelectric effect (P.E.). The P.E. effect occurs when a photon strikes an inner shell electron and is completely absorbed. The collision caused the ejection of the electron from the atom. At absorption, the photon disappears transferring all of its energy and speed to the ejected electron which is then called a photoelectron because it has characteristics of the photon and the electron. The photoelectron is important to radiology because it is the mediator of biological damage capable of causing ionization of matter. In a biological system photoelectrons interact with biomolecules including the ionization of water, macromolecules, and ultimately can damage the DNA in a cell inactivating it. The P.E. effect is also responsible for radiographic contrast. Differential absorption of photons by different types of tissues in the subject is what allows us to visualize subject contrast for radiographic interpretation. The photoelectric effect is related to the energy (keV) of the photon, at low kVp the P.E. effect increases while at high kVp the P.E. effect diminishes.
Bremsstrahlung radiation is a type of man-made radiation produced in the anode of the x-ray tube. Electrons are accelerated towards the anode under the influence of high kilovoltage. At the anode they are rapidly decelerated by the dense packed tungsten atoms composing it. Their rapid deceleration is a process of many close deflections as these electrons pass close to the nucleus of tungsten atoms. Strong positive nuclear charges exert an electromagnetic pull on the electrons. However, because the velocity of the electrons outweighs the attractive forces, the electron(s) is not sequestered by the atom, but its direction is changed. The process results in the release of energy in this braking process in the form of photons, termed bremsstrahlung radiation. This process continues until the electron looses sufficient energy to obtain a rest state and be absorbed by an atom needing an electron(s). Bremsstrahlung radiation is approximately 90-98% of the primary beam that is used to make a radiographic image.
Characteristic radiation is produced in the x-ray tube when inner shell electrons are ejected from tungsten atoms by high speed projectile electrons, creating a hole or vacancy in those orbitals. Inner shell electron vacancy is a highly unstable state for an atom so it immediately shifts its electrons inward successively filling these inner shell electron vacancies. When an electron is moved to a different energy orbital it is required to release some of its energy, and it does so in the form of a photon which is termed a characteristic ray. The process of electron shifting continues until the vacancy is expressed on the outer valance shell where there is a high probability for the atom to acquire a rest state electron returning it to a stable state. This is because the atom does not give up any energy in the process of filling a valance shell electron. All atoms regardless of the species will respond in this manner to the loss of an inner shell electron. The energy given off by electrons shifting to an inner orbital is characteristic for each atomic species and specific for the orbital from which the photon originates. The energy of a newly created photon is the difference between the binding energy of the ejected electron, and the binding energy of the electron that replaces it. The binding energy of an electron is defined as the energy required removing an electron from its orbital. This is the reason why only K-shell characteristic photons produced in tungsten atoms within the anode are useful to diagnostic radiographic imaging. The K-shell of tungsten has a binding energy of 69.5 keV so that an electron of less than 70 keV cannot remove the k-shell electron of a tungsten atom. Because the k-shell is usually filled by an electron from the L-shell which has a binding energy of 10.2 keV, a photon from this internal electron shift would have a discrete energy of 59.3 keV and no other. It is important to realize that the energy of a characteristic photon can only be a discrete energy and no other. If an electron or photon does not possess at least 70 keV it cannot eject a k-shell electron from tungsten. Therefore, in the tungsten anode of an x-ray tube all photons of the useful beam produced at less than 70 kVp are bremsstrahlung radiations. At greater than 70 kVp, for example 140 kVp used in many CT studies, most of the useful radiation is bremsstrahlung, and only a fraction of the useful beam that contains photons of 69.5 keV is from characteristic radiation.
Section IX: Basic Interactions of X-rays with Matter
Scatter occurs when photons interacts with valance shell electrons of atoms in matter. Scattering is one of the multiparts of interactions of radiation in matter. Almost all scatter in diagnostic radiology imaging is a result of Compton scatter.
There are five basic interactions of x-rays with matter depending on their energy:
The importance of these interactions is that sometimes the photon is absorbed, while at other times they are merely scattered. When a photon is absorbed it is completely removed from the x-ray beam and ceases to exist, whereas when it is scattered it is deflected into a random direction and is said to no longer carry useful imaging information. Preventing scattered radiation from reaching the image receptor will reduce film fog and image noise.
One of the characteristics of ionizing electromagnetic radiation is that it travels in a straight line vector away from its source. The only way its energy, wavelength, frequency, or direction will change is through interaction with matter. One of the basic types of interaction of ionizing radiation with matter is one in which it is deflected so that its direction is changed. Scatter radiation can be defined as a primary photon that has undergone a change in direction due to interaction with matter. Coherent scatter is a name given to those interactions in which there is a change in the direction of the photon but the wavelength of the photon does not change. This is why coherent scatter is sometimes called unmodified scatter. There are two types of coherent scattering: Thomson scattering and Rayleigh scattering. Essentially, with Rayleigh interaction a very low energy photon interacts with a tightly bound orbital photon and being unable to eject it, the electron is merely set it into vibration at the frequency of the photon. The vibrating electron cast off the excess energy by emitting an electromagnetic photon identical in wavelength to that of the incident photon, but in a different direction. It can therefore be stated that the initial photon is scattered in a different direction without a change in its energy, wavelength, or frequency. Unmodified scatter is not a concern in diagnostic imaging because it occurs with x-ray energies well below the range used in clinical radiology practice. Unmodified scatter does occur nevertheless in diagnostic imaging; however, the photon energy is low enough to be considered insignificant to imaging information. The point with coherent scatter is that there is only a change in the direction of the photon with no detectable change in the photon’s energy, wavelength, or frequency.
Coherent Scatter Production
In coherent scattering there is no loss of energy by the photon and no ionization of the atom as a result of the interaction.
Photoelectric Effect (P.E.)
The photoelectric effect is the mechanism by which photons are absorbed by matter. An incident photon with slightly more energy than the binding energy of the orbital electron it collides with strikes that electron and ejects it from its orbital. In the interaction the photon cease to exist imparting all of its energy to the orbital electron. Most of the energy is required to overcome the binding energy of the orbital electron and the remainder imparted to the electron upon its ejection. The ejected free electron is called a photoelectron which travels a short distance in tissue and is absorbed (charged particles have little penetrability). The atom’s resultant charge is +1 due to the electron void. Subsequently, electrons from outer orbitals drop inward to fill the void, such as from the L-shell to the k-shell and from the m-shell to the L-shell, etc. As electrons drop into inner shells they give up energy in the form of an x-ray photon called a characteristic ray. The three products of the P.E. effect: 1) a negative ion (photoelectron), 2) characteristic radiation, and 3) a positive ion (atom deficient one electron).
There are three simple rules governing the probability of the photoelectric effect occurring:
Almost all of the scatter radiation that we encounter in diagnostic radiology is a result of Compton scatter. The basic process is that an incident photon with relatively high energy strikes a "free" valance shell electron of an atom, ejecting it from its orbit. Both the incident photon and the ejected electron are deflected in a new direction as part of the photon’s energy is imparted to the electron. An analogy of the reaction is a billiard ball (orbital electron) being hit by a cue ball (incident photon). The products of the interaction are a positive atom and a free electron called a "recoil" electron to distinguish it from a photoelectron.
Compton Scatter occurs when a primary beam photon strikes an outer valance shell electron of an atom ejecting it, with a resulting change in the direction of the incident photon. The amount of energy retained by the photon is determined by its angle of change. The initial energy of the photon is split between it and the ejected electron; however, the photon will retain most of the energy. For example, at 110 kev the photon will retain 91 kev or more provided the angle of change is 90 degrees or less.
Scatter is produced in the patient's body during radiation exposure sequences that generate diagnostic images for interpretation; most of which is absorbed in the patient's tissues. When a high kVp is selected in order to penetrate a part or to achieve low radiographic contrast, scattered photons may have sufficient energy to escape the patient’s body and reach the occupational worker present in the exam room. The amount of scatter generated within the patient's body is a function of the kVp used, field size and the amount of tissue exposed. As the kVp increases the number of x-rays that undergo Compton scattering increases along with a simultaneous decrease in the relative number of photoelectric interactions. Because scatter radiation from the patient is the primary source of radiation exposure to imaging personnel, a significant amount of radiation protection practices involve reducing patient exposure, which is beneficial to both the patient and the health care team.
Let’s consider why scatter radiation is so important to imaging professionals. Notice that a primary gamma photon with 140 keV of energy interacting with an atom in the patient's body in a Compton interaction will have a final energy that is dependent on the angle at which it scatters. Forward scatter will have the greatest energy being only slightly reduced from the initial energy. Backscatter originating from a 140 keV photon will have retained about 90-100 keV of energy, which may become significant taken as cumulative occupational radiation dose. Such a dose to an unprotected bystander is unacceptable.
Scatter radiation is really primary beam photons that can be extended beyond the patient in random directions from its original straight line path. Because it is radiation that has undergone a change in direction other areas of the radiographic room can be potentially exposed. Compton scatter is a particularly important problem for radiation workers and other health care personnel monitoring patients during x-ray procedures. This is because depending on how close to its origin one is, it could be very nearly like being in the primary beam. What one should remember is that it is really a primary beam photon that has undergone a slight change in direction and therefore retains most of its initial energy. So, under no circumstances should personnel be allowed to remain in a position where exposure can occur without wearing protective apparel. In addition, the technologist should not assume that because they are wearing "protective" shielding it is appropriate to be exposed to scatter, when it can be avoided altogether by remaining at a safe distance. The radiographer should remember that lower energy photons tend to scatter back at 180o, and high energy photons tend to scatter forward. These are constant observations of photon energies throughout the diagnostic energy range. To calculate the change in wavelength of a scattered photon, the following formula is used:
Energy of Compton-Scattered Photons
for various Angles of Deflection
Pair Production and Photodisintegration
Pair production and photodisintegration have no significance in diagnostic imaging at this time; therefore we will keep their discussion brief. In pair production a high energy photon strikes the nucleus of an atom and is completely absorbed. The photon's energy is converted into matter with the production of an electron and a positron. It should be remembered that the mass of an electron is equivalent to 0.511 meV, therefore, pair production does require minimum photon energy of 1.02 meV in order to occur. This energy range is well outside the diagnostic imaging range of energies.
Photodisintegration is a term referring to the process of a high energy photon colliding with the nucleus of an atom causing a partial ejection of nuclear contents. The ejected portion can be a neutron, proton, alpha particle, or a cluster of particles. Again this does not occur in diagnostic radiology imaging because to overcome nuclear binding energies photons must contain 7-15 MeV.
SummaryPoints to Ponder!