“Don't walk behind me; I may not lead. Don't walk in front of me; I may not follow. Just walk beside me and be my friend.”
― Albert Camus

“Never go to bed mad. Stay up and fight.”
― Phyllis Diller

“Don't touch me, I'm full of snakes.”
― Jack Kerouac

“Be who you are and say what you feel, because those who mind don't matter, and those who matter don't mind.” 
 Bernard M. Baruch

Radiation Detectors

Radiation Detectors

Instruments used for radiation measurement fall into two broad categories:
   - rate measuring instruments and
   - personal dose measuring instruments.

Rate measuring instruments measure the rate at which exposure is received (more commonly called the radiation intensity). Survey meters, audible alarms and area monitors fall into this category. These instruments present a radiation intensity reading relative to time, such as R/hr or mR/hr. An analogy can be made between these instruments and the speedometer of a car because both are measuring units relative to time.

Dose measuring instruments are those that measure the total amount of exposure received during a measuring period. The dose measuring instruments, or dosimeters, that are commonly used in industrial radiography are small devices which are designed to be worn by an individual to measure the exposure received by the individual. An analogy can be made between these instruments and the odometer of a car because both are measuring accumulated units.

The radiation measuring instruments commonly used in industrial radiography are described in more detail in the following pages.

Radiosensitivity

Cell Radiosensitivity

Radiosensitivity is the relative susceptibility of cells, tissues, organs, organisms, or other substances to the injurious action of radiation. In general, it has been found that cell radiosensitivity is directly proportional to the rate of cell division and inversely proportional to the degree of cell differentiation. In short, this means that actively dividing cells or those not fully mature are most at risk from radiation. The most radio-sensitive cells are those which:

• have a high division rate
• have a high metabolic rate
• are of a non-specialized type
• are well nourished

Examples of various tissues and their relative radiosensitivities are listed below.

High Radiosensitivity

Lymphoid organs, bone marrow, blood, testes, ovaries, intestines

Fairly High Radiosensitivity

Skin and other organs with epithelial cell lining (cornea, oral cavity, esophagus, rectum, bladder, vagina, uterine cervix, ureters)

Moderate Radiosensitivity

Optic lens, stomach, growing cartilage, fine vasculature, growing bone

Fairly Low Radiosensitivity

Mature cartilage or bones, salivary glands, respiratory organs, kidneys, liver, pancreas, thyroid, adrenal and pituitary glands

Low Radiosensitivity

Muscle, brain, spinal cord

Reference: Rubin, P. and Casarett. G. W.: Clinical Radiation Pathology (Philadelphia: W. B. Saunders. 1968).

Nonstochastic Effects

Nonstochastic (Acute) Effects

Unlike stochastic effects, nonstochastic effects are characterized by a threshold dose below which they do not occur. In other words, nonstochastic effects have a clear relationship between the exposure and the effect. In addition, the magnitude of the effect is directly proportional to the size of the dose. Nonstochastic effects typically result when very large dosages of radiation are received in a short amount of time. These effects will often be evident within hours or days. Examples of nonstochastic effects include erythema (skin reddening), skin and tissue burns, cataract formation, sterility, radiation sickness and death. Each of these effects differs from the others in that both its threshold dose and the time over which the dose was received cause the effect (i.e. acute vs. chronic exposure).

There are a number of cases of radiation burns occurring to the hands or fingers. These cases occurred when a radiographer touched or came in close contact with a high intensity radiation emitter. Intensity on the surface of an 85 curie Ir-192 source capsule is approximately 1,768 R/s. Contact with the source for two seconds would expose the hand of an individual to 3,536 rems, and this does not consider any additional whole body dosage received when approaching the source.

More on Specific Nonstochastic Effects

Hemopoietic SyndromeThe hemopoietic syndrome encompasses the medical conditions that affect the blood. Hemopoietic syndrome conditions appear after a gamma dose of about 200 rads (2 Gy). This disease is characterized by depression or ablation of the bone marrow, and the physiological consequences of this damage. The onset of the disease is rather sudden, and is heralded by nausea and vomiting within several hours after the overexposure occurred. Malaise and fatigue are felt by the victim, but the degree of malaise does not seem to be correlated with the size of the dose. Loss of hair (epilation), which is almost always seen, appears between the second and third week after the exposure. Death may occur within one to two months after exposure. The chief effects to be noted, of course, are in the bone marrow and in the blood. Marrow depression is seen at 200 rads and at about 400 to 600 rads (4 to 6 Gy) complete ablation of the marrow occurs. In this case, however, spontaneous regrowth of the marrow is possible if the victim survives the physiological effects of the denuding of the marrow. An exposure of about 700 rads (7 Gy) or greater leads to irreversible ablation of the bone marrow.

Gastrointestinal SyndromeThe gastrointestinal syndrome encompasses the medical conditions that affect the stomach and the intestines. This medical condition follows a total body gamma dose of about 1000 rads (10 Gy) or greater, and is a consequence of the desquamation of the intestinal epithelium. All the signs and symptoms of hemopoietic syndrome are seen, with the addition of severe nausea, vomiting, and diarrhea which begin very soon after exposure. Death within one to two weeks after exposure is the most likely outcome.

Central Nervous SystemA total body gamma dose in excess of about 2000 rads (20 Gy) damages the central nervous system, as well as all the other organ systems in the body. Unconsciousness follows within minutes after exposure and death can result in a matter of hours to several days. The rapidity of the onset of unconsciousness is directly related to the dose received. In one instance in which a 200 msec burst of mixed neutrons and gamma rays delivered a mean total body dose of about 4400 rads (44 Gy), the victim was ataxic and disoriented within 30 seconds. In 10 minutes, he was unconscious and in shock. Vigorous symptomatic treatment kept the patient alive for 34 hours after the accident.

Other Acute Effects
Several other immediate effects of acute overexposure should be noted. Because of its physical location, the skin is subject to more radiation exposure, especially in the case of low energy x-rays and beta rays, than most other tissues. An exposure of about 300 R (77 mC/kg) of low energy (in the diagnostic range) x-rays results in erythema. Higher doses may cause changes in pigmentation, loss of hair, blistering, cell death, and ulceration. Radiation dermatitis of the hands and face was a relatively common occupational disease among radiologists who practiced during the early years of the twentieth century.

The reproductive organs are particularly radiosensitive. A single dose of only 30 rads (300 mGy) to the testes results in temporary sterility among men. For women, a 300 rad (3 Gy) dose to the ovaries produces temporary sterility. Higher doses increase the period of temporary sterility. In women, temporary sterility is evidenced by a cessation of menstruation for a period of one month or more, depending on the dose. Irregularities in the menstrual cycle, which suggest functional changes in the reproductive organs, may result from local irradiation of the ovaries with doses smaller than that required for temporary sterilization.

The eyes too, are relatively radiosensitive. A local dose of several hundred rads can result in acute conjunctivitis.

Stochastic Effects

Stochastic Effects

Stochastic effects are those that occur by chance and consist primarily of cancer and genetic effects. Stochastic effects often show up years after exposure. As the dose to an individual increases, the probability that cancer or a genetic effect will occur also increases. However, at no time, even for high doses, is it certain that cancer or genetic damage will result. Similarly, for stochastic effects, there is no threshold dose below which it is relatively certain that an adverse effect cannot occur. In addition, because stochastic effects can occur in individuals that have not been exposed to radiation above background levels, it can never be determined for certain that an occurrence of cancer or genetic damage was due to a specific exposure.

While it cannot be determined conclusively, it often possible to estimate the probability that radiation exposure will cause a stochastic effect. As mentioned previously, it is estimated that the probability of having a cancer in the US rises from 20% for non radiation workers to 21% for persons who work regularly with radiation. The probability for genetic defects is even less likely to increase for workers exposed to radiation. Studies conducted on Japanese atomic bomb survivors who were exposed to large doses of radiation found no more genetic defects than what would normally occur.

Radiation-induced hereditary effects have not been observed in human populations, yet they have been demonstrated in animals. If the germ cells that are present in the ovaries and testes and are responsible for reproduction were modified by radiation, hereditary effects could occur in the progeny of the individual. Exposure of the embryo or fetus to ionizing radiation could increase the risk of leukemia in infants and, during certain periods in early pregnancy, may lead to mental retardation and congenital malformations if the amount of radiation is sufficiently high.

More on Specific Stochastic Effects

Cancer

Leukemia

Genetic Effects

Cataracts

Interaction Radiation and Matters

Interaction of Electromagnetic Radiation and Matter

It is well known that all matter is comprised of atoms. But subatomically, matter is made up of mostly empty space. For example, consider the hydrogen atom with its one proton, one neutron, and one electron. The diameter of a single proton has been measured to be about 10^-15 meters. The diameter of a single hydrogen atom has been determined to be 10^-10 meters, therefore the ratio of the size of a hydrogen atom to the size of the proton is 100,000:1. Consider this in terms of something more easily pictured in your mind. If the nucleus of the atom could be enlarged to the size of a softball (about 10 cm), its electron would be approximately 10 kilometers away. Therefore, when electromagnetic waves pass through a material, they are primarily moving through free space, but may have a chance encounter with the nucleus or an electron of an atom.

Because the encounters of photons with atom particles are by chance, a given photon has a finite probability of passing completely through the medium it is traversing. The probability that a photon will pass completely through a medium depends on numerous factors including the photon’s energy and the medium’s composition and thickness. The more densely packed a medium’s atoms, the more likely the photon will encounter an atomic particle. In other words, the more subatomic particles in a material (higher Z number), the greater the likelihood that interactions will occur  Similarly, the more material a photon must cross through, the more likely the chance of an encounter.

When a photon does encounter an atomic particle, it transfers energy to the particle. The energy may be reemitted back the way it came (reflected), scattered in a different direction or transmitted forward into the material. Let us first consider the interaction of visible light. Reflection and transmission of light waves occur because the light waves transfer energy to the electrons of the material and cause them to vibrate. If the material is transparent, then the vibrations of the electrons are passed on to neighboring atoms through the bulk of the material and reemitted on the opposite side of the object. If the material is opaque, then the vibrations of the electrons are not passed from atom to atom through the bulk of the material, but rather the electrons vibrate for short periods of time and then reemit the energy as a reflected light wave. The light may be reemitted from the surface of the material at a different wavelength, thus changing its color.

X-Rays and Gamma RaysX-rays and gamma rays also transfer their energy to matter though chance encounters with electrons and atomic nuclei. However, X-rays and gamma rays have enough energy to do more than just make the electrons vibrate. When these high energy rays encounter an atom, the result is an ejection of energetic electrons from the atom or the excitation of electrons. The term "excitation" is used to describe an interaction where electrons acquire energy from a passing charged particle but are not removed completely from their atom. Excited electrons may subsequently emit energy in the form of x-rays during the process of returning to a lower energy state.

Each of the excited or liberated electrons goes on to transfer its energy to matter through thousands of events involving interactions between charged particles. With each interaction, the energy may be directed in a different direction. The higher the energy of a photon, the more likely the energy will continue traveling in the same direction. As the radiation moves from point to point in matter, it loses its energy through various interactions with the atoms it encounters. If the radiation has enough energy, it may eventually make it through the material.

Photon Interaction with Matter is KeyFrom the previous paragraph, it can be deduced that the energy of X- and Gamma ray photons is largely responsible for their penetrating power. Einstein linked the energy of a photon to its frequency and wavelength when he postulated that each photon carries an energy of the frequency of the wave times Planck’s constant (E = hƒ). The frequency of an EM wave equals the speed of light divided by the wavelength (ƒ =c/λ ). However, it should be understood that the wavelength or frequency of electromagnetic radiation does not in itself makes the EM wave more or less penetrating. The key is its interaction with matter, or more specifically, whether the photon's energy is right to excite some transition of a charged particle. For instance, microwaves penetrate glass very easily, but they are strongly absorbed by water. Move up to slightly higher frequency, and infrared is strongly absorbed by both glass and water, but both substances transmit visible light. Ultraviolet is stopped by glass, but not so readily by water.

Nature of Radiation

Nature of Radiation

Radiation is a form of energy. There are two basic types of radiation. One kind is particulate radiation, which involves tiny fast-moving particles that have both energy and mass. Particulate radiation is primarily produced by disintegration of an unstable atom and includes Alpha and Beta particles.

Alpha particles are high energy, large subatomic structures of protons and neutrons. They can travel only a short distance and are stopped by a piece of paper or skin. Beta particles are fast moving electrons. They are a fraction of the size of alpha particles, but can travel farther and are more penetrating.

Particulate radiation is of secondary concern to industrial radiographers. Since these particles have weight and are relatively large, they are easily absorbed by a small amount of shielding.However, it should be noted that shielding materials, such as the depleted uranium used in many gamma radiography cameras, will be a source of Beta particles if the container should ever develop a leak. If a leak were to occur, the material could be transferred to the hands and other parts of a radiographer’s body, causing what is known as particulate contamination. This is the reason periodic “leak” and “wipe tests” are performed on equipment.

The second basic type of radiation is electromagnetic radiation. This kind of radiation is pure energy with no mass and is like vibrating or pulsating waves of electrical and magnetic energy. Electromagnetic waves are produced by a vibrating electric charge and as such, they consist of both an electric and a magnetic component. In addition to acting like waves, electromagnetic radiation acts like a stream of small "packets" of energy called photons. Another way that electromagnetic radiation has been described is in terms of a stream of photons. The massless photon particles each travel in a wave-like pattern. Each photon contains a certain amount (or bundle) of energy, and all electromagnetic radiation consists of these photons. The only difference between the various types of electromagnetic radiation is the amount of energy found in the photons. Electromagnetic radiation travels in a straight line at the speed of light (3 x 10⁸m/s).

Light waves, radio waves, microwaves, X-rays and Gamma rays are some examples of electromagnetic radiation. These waves differ in their wavelength as shown in the electromagnetic spectrum image above. Although all portions of the electromagnetic spectrum are governed by the same laws, their different wavelengths and different energies allow them to have different effects on matter. Radio waves, for example, have such a long wavelength and low energy that our eyes cannot detect them and they pass through our bodies. It takes a special antenna and electronics to capture and amplify radio waves.

The wavelength of visible light is on the order of 6,000 angstroms, while the wavelength of X-rays is in the range of one angstrom and that of Gamma rays is 0.0001 angstrom. This very short wavelength is what gives X-rays and Gamma rays their power to penetrate materials that light cannot. Unlike light, X- and gamma rays cannot be seen, felt, or heard. The fact that they cannot be detected with our normal human senses and can damage our cells is why they must be treated carefully.

Ionization and Cell Damage

Ionization and Cell Damage

As previously discussed, photons that interact with atomic particles can transfer their energy to the material and break chemical bonds in materials. This interaction is known as ionization and involves the dislodging of one or more electrons from an atom of a material. This creates electrons, which carry a negative charge, and atoms without electrons, which carry a positive charge. Ionization in industrial materials is usually not a big concern. In most cases, once the radiation ceases the electrons rejoin the atoms and no damage is done. However, ionization can disturb the atomic structure of some materials to a degree where the atoms enter into chemical reactions with each other. This is the reaction that takes place in the silver bromide of radiographic film to produce a latent image when the film is processed. Ionization may cause unwanted changes in some materials, such as semiconductors, so that they are no longer effective for their intended use.

Ionization in Living Tissue (Cell Damage) In living tissue, similar interactions occur and ionization can be very detrimental to cells. Ionization of living tissue causes molecules in the cells to be broken apart. This interaction can kill the cell or cause them to reproduce abnormally.

Damage to a cell can come from direct action orindirect action of the radiation. Cell damage due to direct action occurs when the radiation interacts directly with a cell's essential molecules (DNA). The radiation energy may damage cell components such as the cell walls or the deoxyribonucleic acid (DNA). DNA is found in every cell and consists of molecules that determine the function that each cell performs. When radiation interacts with a cell wall or DNA, the cell either dies or becomes a different kind of cell, possibly even a cancerous one.

Cell damage due to indirect action occurs when radiation interacts with the water molecules, which are roughly 80% of a cells composition. The energy absorbed by the water molecule can result in the formation of free radicals. Free radicals are molecules that are highly reactive due to the presence of unpaired electrons, which result when water molecules are split. Free radicals may form compounds, such as hydrogen peroxide, which may initiate harmful chemical reactions within the cells. As a result of these chemical changes, cells may undergo a variety of structural changes which lead to altered function or cell death.

Various possibilities exist for the fate of cells damaged by radiation. Damaged cells can:

• completely and perfectly repair themselves with the body's inherent repair mechanisms.
• die during their attempt to reproduce. Thus, tissues and organs in which there is substantial cell loss may become functionally impaired. There is a "threshold" dose for each organ and tissue above which functional impairment will manifest as a clinically observable adverse outcome. Exceeding the threshold dose increases the level of harm. Such outcomes are called deterministic effects and occur at high doses.
• repair themselves imperfectly and replicate this imperfect structure.  These cells, with the progression of time, may be transformed by external agents (e.g., chemicals, diet, radiation exposure, lifestyle habits, etc.). After a latency period of years, they may develop into leukemia or a solid tumor (cancer). Such latent effects are called stochastic (or random).

Exposure of Living Tissue to Non-ionizing RadiationA quick note of caution about non-ionizing radiation is probably also appropriate here. Non-ionizing radiation behaves exactly like ionizing radiation, but differs in that it has a much greater wavelength and, therefore, less energy. Although this non-ionizing radiation does not have the energy to create ion pairs, some of these waves can cause personal injury. Anyone who has received a sunburn knows that ultraviolet light can damage skin cells. Non-ionizing radiation sources include lasers, high-intensity sources of ultraviolet light, microwave transmitters and other devices that produce high intensity radio-frequency radiation.

Survey Meters

Survey Meters The survey meter is the most important resource a radiographer has to determine the presence and intensity of radiation. A review of incident and overexposure reports indicate that a majority of these type of events occurred when a technician did not have or did not use a survey meter. There are many different models of survey meters available to measure radiation in the field. They all basically consist of a detector and a readout display. Analog and digital displays are available. Most of the survey meters used for industrial radiography use a gas filled detector. Gas filled detectors consists of a gas filled cylinder with two electrodes. Sometimes, the cylinder itself acts as one electrode, and a needle or thin taut wire along the axis of the cylinder acts as the other electrode. A voltage is applied to the device so that the central needle or wire become an anode (+ charge) and the other electrode or cylinder wall becomes the cathode (- charge). The gas becomes ionized whenever the counter is brought near radioactive substances. The electric field created by the potential difference between the anode and cathode causes the electrons of each ion pair to move to the anode while the positively charged gas atom is drawn to the cathode. This results in an electrical signal that is amplified, correlated to exposure and displayed as a value. Depending on the voltage applied between the anode and the cathode, the detector may be considered an ion chamber, a proportional counter, or a Geiger-Müller (GM) detector. Each of these types of detectors have their advantages and disadvantages. A brief summary of each of these detectors follows. Ion Chamber Counter Ion chambers have a relatively low voltage between the anode and cathode, which results in a collection of only the charges produced in the initial ionization event. This type of detector produces a weak output signal that corresponds to the number of ionization events. Higher energies and intensities of radiation will produce more ionization, which will result in a stronger output voltage. Collection of only primary ions provides information on true radiation exposure (energy and intensity). However, the meters require sensitive electronics to amplify the signal, which makes them fairly expensive and delicate. The additional expense and required care is justified when it is necessary to make accurate radiation exposure measurements over a range of radiation energies. This might be necessary when measuring the Bremsstrahlung radiation produced by an X-ray generator. An ion chamber survey meter is sometimes used in the field when performing gamma radiography because it will provide accurate exposure measurements regardless of the radioactive isotope being used. Proportional Counter Proportional counter detectors use a slightly higher voltage between the anode and cathode. Due to the strong electrical field, the charges produced in the initial ionization are accelerated fast enough to ionize other electrons in the gas. The electrons produced in these secondary ion pairs, along with the primary electrons, continue to gain energy as they move towards the anode, and as they do, they produce more and more ionizations. The result is that each electron from a primary ion pair produces a cascade of ion pairs. This effect is known as gas multiplication or amplification. In this voltage regime, the number of particles liberated by secondary interactions is proportional to the number of ions produced by the passing ionizing particle. Hence, these gas ionization detectors are called proportional counters. Like ion chamber detectors, proportional detectors discriminate between types of radiation. However, they require very stable electronics which are expensive and fragile. Proportional detectors are usually only used in a laboratory setting. Geiger-Müller (GM) CounterGeiger-Müller counters operate under even higher voltages between the anode and the cathode,usually in the 800 to 1200 volt range. Like the proportional counter, the high voltage accelerates the charges produced in the initial ionization to where they have enough energy to ionize other electrons in the gas. However, this cascading of ion pairs occurs to a much larger degree and continues until the counter is saturated with ions. This all happens in a fraction of a second and results in an electrical current pulse of constant voltage. The collection of the large number of secondary ions in the GM region is known as an avalanche and produces a large voltage pulse. In other words, the size of the current pulse is independent of the size of the ionization event that produced it. The GM counter was named for Hans Geiger who invented the device in 1908, and Walther Müller who collaborated with Geiger in developing it further in 1928. The electronic circuit of a GM counters counts and records the number of pulses and the information is often displayed in counts per minute. If the instrument has a speaker, the pulses can also produce an audible click. When the volume of gas in the chamber is completely ionized, ion collection stops until the electrical pulse discharges. Again, this only takes a fraction of a second, but this process slightly limits the rate at which individual events can be detected. Because they can display individual ionizing events, GM counters are generally more sensitive to low levels of radiation than ion chamber instruments. By means of calibration, the count rate can be displayed as the exposure rate over a specified energy range. When used for gamma radiography, GM meters are typically calibrated for the energy of the gamma radiation being used. Most often, gamma radiation from Cs-137 at 0.662 MeV provides the calibration. Only small errors occur when the radiographer uses Ir-192 (average energy about 0.34 MeV) or Co-60 (average energy about 1.25 MeV). Since the Geiger-Müller counter produces many more electrons than a ion chamber counter or a proportional counter, it does not require the same level of electronic sophistication as other survey meters. This results in a meter that is relatively low cost and rugged. The disadvantages of GM survey meters are the lack of ability to account for different amounts of ionization caused by different energy photons and noncontinuous measurement (need to discharge). Comparison of Gas Filled Detectors The graph to the right shows the relationship of ion collection in a gas filled detector versus the applied voltage. In the ion chamber region, the voltage between the anode and cathode is relatively low and only primary ions are collected. In the proportional region ,the voltage is higher, and primary ions and a number of secondary ions (proportional to the primary ions originally formed) are collected. In the GM region, a maximum number of secondary ions are collected when the gas around the anode is completely ionized. Note that discrimination between kinds of radiation (E1 and E2) is possible in the ion chamber and proportional regions. Radiation at different energy levels forms different numbers of primary ions in the detector. However in the GM region, the number of secondary ions collected per event remains the same no matter what the energy of the radiation that initiated the event. The GM counter gives up the ability to accurately measure the exposure due to different energies of radiation in exchange for a large signal pulse. This large signal pulse simplifies the electronics that are necessary for instruments such as survey meters.

Film Badge

Film Badges

Personnel dosimetry film badges are commonly used to measure and record radiation exposure due to gamma rays, X-rays and beta particles. The detector is, as the name implies, a piece of radiation sensitive film. The film is packaged in a light proof, vapor proof envelope preventing light, moisture or chemical vapors from affecting the film.

A special film is used which is coated with two different emulsions. One side is coated with a large grain, fast emulsion that is sensitive to low levels of exposure. The other side of the film is coated with a fine grain, slow emulsion that is less sensitive to exposure. If the radiation exposure causes the fast emulsion in the processed film to be darkened to a degree that it cannot be interpreted, the fast emulsion is removed and the dose is computed using the slow emulsion.

The film is contained inside a film holder or badge. The badge incorporates a series of filters to determine the quality of the radiation. Radiation of a given energy is attenuated to a different extent by various types of absorbers. Therefore, the same quantity of radiation incident on the badge will produce a different degree of darkening under each filter. By comparing these results, the energy of the radiation can be determined and the dose can be calculated knowing the film response for that energy. The badge holder also contains an open window to determine radiation exposure due to beta particles. Beta particles are effectively shielded by a thin amount of material.

The major advantages of a film badge as a personnel monitoring device are that it provides a permanent record, it is able to distinguish between different energies of photons, and it can measure doses due to different types of radiation. It is quite accurate for exposures greater than 100 millirem. The major disadvantages are that it must be developed and read by a processor (which is time consuming), prolonged heat exposure can affect the film, and exposures of less than 20 millirem of gamma radiation cannot be accurately measured.

Film badges need to be worn correctly so that the dose they receive accurately represents the dose the wearer receives. Whole body badges are worn on the body between the neck and the waist, often on the belt or a shirt pocket. The clip-on badge is worn most often when performing X-ray or gamma radiography. The film badge may also be worn when working around a low curie source. Ring badges are worn on a finger of the hand most likely to be exposed to ionizing radiation. A LIXI system with its culminated and directional beam would be one example where monitoring the hands would be more important than the whole body.

Thermoluminescent dosimeters (TLD)

Thermoluminescent Dosimeter

Thermoluminescent dosimeters (TLD) are often used instead of the film badge. Like a film badge, it is worn for a period of time (usually 3 months or less) and then must be processed to determine the dose received, if any. Thermoluminescent dosimeters can measure doses as low as 1 millirem, but under routine conditions their low-dose capability is approximately the same as for film badges. TLDs have a precision of approximately 15% for low doses. This precision improves to approximately 3% for high doses. The advantages of a TLD over other personnel monitors is its linearity of response to dose, its relative energy independence, and its sensitivity to low doses. It is also reusable, which is an advantage over film badges. However, no permanent record or re-readability is provided and an immediate, on the job readout is not possible.

How it works
A TLD is a phosphor, such as lithium fluoride (LiF) or calcium fluoride (CaF), in a solid crystal structure. When a TLD is exposed to ionizing radiation at ambient temperatures, the radiation interacts with the phosphor crystal and deposits all or part of the incident energy in that material. Some of the atoms in the material that absorb that energy become ionized, producing free electrons and areas lacking one or more electrons, called holes. Imperfections in the crystal lattice structure act as sites where free electrons can become trapped and locked into place.

Heating the crystal causes the crystal lattice to vibrate, releasing the trapped electrons in the process. Released electrons return to the original ground state, releasing the captured energy from ionization as light, hence the name thermoluminescent. Released light is counted using photomultiplier tubes and the number of photons counted is proportional to the quantity of radiation striking the phosphor.

Instead of reading the optical density (blackness) of a film, as is done with film badges, the amount of light released versus the heating of the individual pieces of thermoluminescent material is measured. The "glow curve" produced by this process is then related to the radiation exposure. The process can be repeated many times.

Biological Effect

Measures Relative to the Biological Effect
of Radiation Exposure

There are four measures of radiation that radiographers will commonly encounter when addressing the biological effects of working with X-rays or Gamma rays. These measures are: Exposure, Dose, Dose Equivalent, and Dose Rate A short summary of these measures and their units will be followed by more in depth information below.

• Exposure:  Exposure is a measure of the strength of a radiation field at some point in air. This is the measure made by a survey meter. The most commonly used unit of exposure is the roentgen (R).
• Dose or Absorbed Dose:  Absorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. In other words, the dose is the amount of radiation absorbed by and object. The SI unit for absorbed dose is the gray (Gy), but the “rad” (Radiation Absorbed Dose) is commonly used. 1 rad is equivalent to 0.01 Gy. Different materials that receive the same exposure may not absorb the same amount of radiation. In human tissue, one Roentgen of gamma radiation exposure results in about one rad of absorbed dose.
• Dose Equivalent:  The dose equivalent relates the absorbed dose to the biological effect of that dose. The absorbed dose of specific types of radiation is multiplied by a "quality factor" to arrive at the dose equivalent. The SI unit is the sievert (SV), but the rem is commonly used. Rem is an acronym for "roentgen equivalent in man." One rem is equivalent to 0.01 SV. When exposed to X- or Gamma radiation, the quality factor is 1.
• Dose Rate: The dose rate is a measure of how fast a radiation dose is being received. Dose rate is usually presented in terms of R/hour, mR/hour, rem/hour, mrem/hour, etc.

For the types of radiation used in industrial radiography, one roentgen equals one rad and since the quality factor for x- and gamma rays is one, radiographers can consider the Roentgen, rad, and rem to be equal in value.

More Information on Exposure, Dose, Dose Equivalent, and Dose Rate

Exposure
Exposure is a measure of the strength of a radiation field at some point. It is a measure of the ionization of the molecules in a mass of air. It is usually defined as the amount of charge (i.e. the sum of all ions of the same sign) produced in a unit mass of air when the interacting photons are completely absorbed in that mass. The most commonly used unit of exposure is the Roentgen (R). Specifically, a Roentgen is the amount of photon energy required to produce 1.610 x 10¹²ion pairs in one gram of dry air at 0°C. A radiation field of one Roentgen will deposit 2.58 x 10^-4 coulombs of charge in one kilogram of dry air. The main advantage of this unit is that it is easy to directly measure with a survey meter. The main limitation is that it is only valid for deposition in air.

Dose or Absorbed Dose
Whereas exposure is defined for air, theabsorbed dose is the amount of energy that ionizing radiation imparts to a given mass of matter. The absorbed dose is used to relate the amount of ionization that x-rays or gamma rays cause in air to the level of biological damage that would be caused in living tissue placed in the radiation field. The most commonly used unit for absorbed dose is the “rad” (Radiation Absorbed Dose). A rad is defined as a dose of 100 ergs of energy per gram of the given material. The SI unit for absorbed dose is the gray (Gy), which is defined as a dose of one joule per kilogram. Since one joule equals 10⁷ ergs, and since one kilogram equals 1000 grams, 1 Gray equals 100 rads.

The size of the absorbed dose is dependent upon the the intensity (or activity) of the radiation source, the distance from the source to the irradiated material, and the time over which the material is irradiated. The activity of the source will determine the dose rate which can be expressed in rad/hr, mr/hr, mGy/sec, etc.

Dose EquivalentWhen considering radiation interacting with living tissue, it is important to also consider the type of radiation. Although the biological effects of radiation are dependent upon the absorbed dose, some types of radiation produce greater effects than others for the same amount of energy imparted. For example, for equal absorbed doses, alpha particles may be 20 times as damaging as beta particles. In order to account for these variations when describing human health risks from radiation exposure, the quantity called “dose equivalent” is used. This is the absorbed dose multiplied by certain “quality” or “adjustment” factors indicative of the relative biological-damage potential of the particular type of radiation.

The quality factor (Q) is a factor used in radiation protection to weigh the absorbed dose with regard to its presumed biological effectiveness. Radiation with higher Q factors will cause greater damage to tissue. The rem is a term used to describe a special unit of dose equivalent. Rem is an abbreviation for roentgen equivalent in man. The SI unit is the sievert (SV); one rem is equivalent to 0.01 SV. Doses of radiation received by workers are recorded in rems, however, sieverts are being required as the industry transitions to the SI unit system.

The table below presents the Q factors for several types of radiation.

Type of Radiation Rad Q Factor Rem X-Ray 1 1 1 Gamma Ray 1 1 1 Beta Particles 1 1 1 Thermal Neutrons 1 5 5 Fast Neutrons 1 10 10 Alpha Particles 1 20 20

Dose Rate The dose rate is a measure of how fast a radiation dose is being received. Knowing the dose rate, allows the dose to be calculated for a period of time. Fore example, if the dose rate is found to be 0.8rem/hour, then a person working in this field for two hours would receive a 1.6rem dose.

Exposure Limit

Exposure Limits

As discussed in the introduction, concern over the biological effect of ionizing radiation began shortly after the discovery of X-rays in 1895. Over the years, numerous recommendations regarding occupational exposure limits have been developed by the International Commission on Radiological Protection (ICRP) and other radiation protection groups. In general, the guidelines established for radiation exposure have had two principle objectives: 1) to prevent acute exposure; and 2) to limit chronic exposure to "acceptable" levels.

Current guidelines are based on the conservative assumption that there is no safe level of exposure. In other words, even the smallest exposure has some probability of causing a stochastic effect, such as cancer. This assumption has led to the general philosophy of not only keeping exposures below recommended levels or regulation limits but also maintaining all exposure "as low as reasonable achievable" (ALARA). ALARA is a basic requirement of current radiation safety practices. It means that every reasonable effort must be made to keep the dose to workers and the public as far below the required limits as possible.

Regulatory Limits for Occupational Exposure
Many of the recommendations from the ICRP and other groups have been incorporated into the regulatory requirements of countries around the world. In the United States, annual radiation exposure limits are found in Title 10, part 20 of the Code of Federal Regulations, and in equivalent state regulations. For industrial radiographers who generally are not concerned with an intake of radioactive material, the Code sets the annual limit of exposure at the following:
1) the more limiting of:

• A total effective dose equivalent of 5 rems (0.05 Sv)
  or
• The sum of the deep-dose equivalent to any individual organ or tissue other than the lens of the eye being equal to 50 rems (0.5 Sv).

2) The annual limits to the lens of the eye, to the skin, and to the extremities, which are:

• A lens dose equivalent of 15 rems (0.15 Sv)
• A shallow-dose equivalent of 50 rems (0.50 Sv) to the skin or to any extremity.

The shallow-dose equivalent is the external dose to the skin of the whole-body or extremities from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.007 cm averaged over and area of 10 cm².
The lens dose equivalent is the dose equivalent to the lens of the eye from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 0.3 cm.
The deep-dose equivalent is the whole-body dose from an external source of ionizing radiation. This value is the dose equivalent at a tissue depth of 1 cm.
The total effective dose equivalent is the dose equivalent to the whole-body.
Declared Pregnant Workers and Minors
Because of the increased health risks to the rapidly developing embryo and fetus, pregnant women can receive no more than 0.5 rem during the entire gestation period. This is 10% of the dose limit that normally applies to radiation workers. Persons under the age of 18 years are also limited to 0.5rem/year.
Non-radiation Workers and the PublicThe dose limit to non-radiation workers and members of the public are two percent of the annual occupational dose limit. Therefore, a non-radiation worker can receive a whole body dose of no more that 0.1 rem/year from industrial ionizing radiation. This exposure would be in addition to the 0.3 rem/year from natural background radiation and the 0.05 rem/year from man-made sources such as medical x-rays.

Dual Energy Radiography

Dual Energy Radiography Acquisition and Processing

A major limitation of projection radiography is the projection of the three-dimensional patient volume and anatomy on a two-dimensional image plane. In chest imaging, for example, the bony structure of the ribs, clavicle, etc. will often hide subtle soft-tissue lesions in the lung because of anatomical overlap caused by the projection process. Removing the bony structure, therefore, might aid in the visualization of otherwise undetectable lesions; similarly, removing the soft-tissue components and emphasizing the bony structures might allow the discrimination of soft versus calcified lesions. The higher differential attenuation of bones as a function of energy compared to soft tissue allows the ability to decompose two images taken at different x-ray energies into tissue-selective representation of the anatomy, namely “soft-tissue only” and “bone-only” images. 

As the above graph shows, two images of the same object acquired with two distinct x-ray energy beams will exhibit different attenuation characteristics. Note the much greater attenuation of bone at low energies, and similar attenuation of bone and soft tissue at higher energies. The x-axis represents mono-energetic photon energy. Effective energy of a spectrum is equal to the energy of a mono-energetic x-ray photon with the same overall attenuation. Shown are typical x-ray “effective energies” for spectra generated at 60 kVp (green) and 120 kVp (red). If two x-ray images are acquired at these energies, one can “weight” one image relative to the other that when subtracted, will null the signal due to either bone or soft tissues, depending on the weighting factors.

Illustrated in the figure above are two images acquired at low energy and high energy using a Computed Radiography dual-energy system (more about the technology subsequently). Note the higher bone-tissue contrast on the left, which represents the “low energy” image. Below each image is a stylized rendition of the relative attenuation of soft tissue and bone in the low energy image (left) and the high energy image (right), which illustrates the larger overall bone signal (8 units in the low energy image, 4 units in the high energy image), and less energy-dependent soft tissue signals of lower signal in the low and high energy images, respectively.

Dual energy processing involves weighting each image according to the desire to null the signal due to bone for a “soft-tissue only” image, or to null the signal due to soft tissue for a “bone only” image. In the stylized illustration shown below, to remove bone requires that the signal due to bone be zeroed out. This can be achieved by multiplying the high energy image by 2 and the low energy image by 1, subtracting the weighted high from the weighted low image, and scaling the residual tissue signal over a range to produce a tissue-only image, as shown in the illustration below.

Similarly, to remove soft tissue requires that the signal due to soft tissue be zeroed out. This can be achieved by multiplying the low energy image by 2 and the low energy image by 3, subtracting the weighted low energy image from the weighted high energy image, and scaling the residual bone signal over a range to produce a bone-only image, as shown in the illustration below.

How is dual-energy radiography performed?

Currently there are two clinical systems available for dual-energy radiography

• One specialized radiography system employs “passive” photostimulable storage phosphor imaging plates to acquire two images simultaneously. The imaging plates are stacked, geometrically aligned, and separated by a copper filter, which preferentially absorbs lower x-ray energies. A low energy image (front imaging plate) and high energy image (back imaging plate) are acquired with a single kVp x-ray beam. This is illustrated below.

With this technology, the low and high energy images are acquired simultaneously, essentially eliminating any artifacts due to patient motion, but the energy separation between the two image pairs is small, which results in a relatively low SNR for the tissue and bone images at typical patient exposures.

• Another dual-energy capable radiography system uses an “active” flat-panel detector, where a low energy image (~60 kVp) is initially acquired, rapidly read out and detector reset, followed by a high energy image (~120 kVp) immediately afterward. This is illustrated below.

This dual-energy method uses a flat-panel detector with a fast readout capability, enabling the use of two separate x-ray beams producing large differences in the effective energy of the beams. The first acquisition occurs with the high (120 kVp) energy beam, then by image readout of the TFT flat-panel array, followed immediately by the low (60 kVp) energy beam acquisition, then by image readout. Energy separation is large, allowing for a relatively high SNR for a given patient exposure. However, because of the delay time required for acquiring two images and the readout time of the flat-panel array, the difference in time between the images often results in motion artifacts due to involuntary and voluntary patient motion over the ~230 ms acquisition time for both images and the readout time between images.

X-ray beam spectra for the dual-energy approaches
Depicted below are the typical x-ray beam spectra used for the two dual energy approaches described above.
On the left is the single x-ray beam acquisition at 120 kVp using the CR dual detector / filter sandwich. Advantages include simpler x-ray operation and no patient motion. Disadvantages include relatively poor energy separation and lower detection efficiency (compared to the flat-panel detector).

On the right is the dual x-ray beam acquisition at 60 kVp and 120 kVp using the single, fast readout thin-film-transistor array detector. Advantages include better energy separation and better image quality at the same dose compared to the CR detector sandwich. Disadvantages include potential for patient motion artifacts, and the need for a more complicated system interface and more costly system.

Example dual-energy images

Dual energy radiography can assist in the differential diagnosis of
soft versus calcified lesions. In the dual-energy image acquisition using a CR sandwich detector pair shown in the figure below, it is clear that the lesions on the composite image (left) are calcified as seen in the bone-only image. For the flat-panel dual energy image acquisition, note the clearly visible soft tissue lesion in the tissue-only image. In retrospect, the lesion is reasonably easy to detect in the conventional composite image, but clearly the ribs project anatomical “noise” that interferes with the conspicuity of the relatively large lesion in the pulmonary tissues. Also of note is the cardiac motion visible in the bone-only image, where soft-tissue .

Dual energy image gallery

Dual energy images can be manipulated with different grayscale presentations like any other digital image. The next several sets of images demonstrate a variety of composite, tissue-only and bone-only images of the postero-anterior chest projection. Many of the images contain soft-tissue and calcified pulmonary lesions, and there are examples of energy-sensitive elements that project specific signals in either the tissue-only or bone-only images.

Example image sets illustrating the value of tissue-only and bone-only image presentations:

On the left is the conventional single-energy image; in the middle is the bone-subtracted “soft-tissue only” image; on the right is the soft-tissue subtracted “bone only” image. In the upper image set, the soft tissue pulmonary lesions are clearly visible in both the composite and soft-tissue only images, although there are other lesions in the heart region in the bone-only image that indicates the presence of calcium-containing lesions. These might be due to calcium deposits in the vasculature.   In the lower image set, there are no readily apparent lesions, but surgical clips are readily visualized on the bone only image.

Image examples demonstrating grayscale manipulation. Of interest in these images is the presence of silicone in the breasts of this patient, and a soft-tissue lesion in the left upper quadrant of the lung.

For flat-panel dual energy detector systems, motion can be a problem when the x-ray system is energized during the rapid contraction period of the heart (end systole). Most motion artifacts appear in and around the cardiac anatomy, and often in the pulmonary architecture and diaphragm area.

Dual energy radiography can often improve the diagnostic information content and sensitivity of projection radiography in many situations by removing the anatomic shadows that can mask soft tissue lesions. In the example below, the composite image (left) does not show evidence of a pulmonary lesion, which is hidden by the overlying rib signal. By selectively removing the bone signal, a soft tissue lesion is clearly visible in the “tissue only” image (middle). A subsequent CT scan for needle biopsy illustrates the cross-sectional volume of the lesion and the correlation to the dual-energy image. The value of cross-sectional imaging is nicely demonstrated, although with higher radiation dose and much higher costs.

Summary, Dual Energy Imaging

Dual energy imaging provides the capability of selectively imaging two clinically relevant materials, namely soft tissue and bone tissue. Energy dependent differences of bone versus soft tissue are used to eliminate one tissue or the other, determined by energy spectra differences used for acquiring independent images. Elimination of structured anatomy (noise) is the major benefit of the technique. 

Two major methods include a CR sandwich (passive detector) with inter-detector filter (copper) to achieve low (front) and high (back) image pairs. Attributes of the CR method is single-shot, no motion, but poor energy separation, resulting in noisy images for low dose typical of a chest x-ray examination. DR (using a fast flat-panel readout detector) acquires images with different kVp (usually ~60 kVp and ~120 kVp) to produce two distinct images with good energy separation but poor temporal response time, allowing motion artifacts to sometimes be a significant problem with image quality. Characteristics of the DR dual energy images are the possibility of involuntary patient motion (particularly the heart), but good energy separation and superior image noise properties for a given patient dose, resulting in images with excellent signal to noise ratio.

Scatter and Collimation

The set of images of the pelvis phantom shown above were all obtained using the same radiographic technique (70 kV and 3 mAs), and obtained using a computed radiography cassette with no scatter removal grid. The four successive images of the pelvis phantom, used lateral dimensions of 43 cm (upper left), 20 cm (upper right), 10 cm (lower left), and 5 cm (lower right). As the collimation is narrowed, the visibility of the spine markedly improves because scatter generated in the phantom has an increasingly likely chance of missing the exposed image region. This example illustrates that any narrow irradiation geometry, such as that used in CT, will be effective in removing scatter. It also shows that scatter is very important issue when large areas are exposed, and for conventional radiography, it is essential to take steps to minimize the amount of scatter reaching the image receptor.

GREAT OF GRID

Scatter Removal Grids

The antiscatter grid plays an important role for enhancing image quality in projection radiography by transmitting a majority of primary radiation and selectively rejecting scattered radiation. This device is comprised of a series of thin lead strips separated by radiolucent interspaces in a form-factor that matches the detector size. Most grids have a linear geometry in one direction (usually along the long axis of the detector). Parallel grids have lead strips that are focused to infinity (i.e. the primary x-rays have a parallel trajectory). Focused grids have lead strips that are oriented parallel at the center (along the x-ray central axis) and progressively slanted to the periphery to match the beam divergence from the focal spot .to the detector at a specific source to detector distance.

The anti-scatter grid is typically manufactured with lead strips oriented along one dimension separated by a low attenuating interspace material such as carbon fiber or aluminum. For specialized applications, there are cross-hatched grids (lead strips in both directions, perpendicular to each other) for specialized applications such as dedicated chest imaging, and in mammography where a "cellular" grid design made of copper with air interspaces is used clinically by one manufacturer. By selectively allowing primary x-rays to be transmitted and scattered x-rays to be absorbed in the grid, image contrast is significantly enhanced; however, the grid attenuates some of the desired primary x-rays that are incident directly on the lead strips and allows transmission of some scattered radiation photons that have a small scattering angle, or scatter in a direction parallel to the lead strips, or are multiply scattered with an exit angle from the patient that can be transmitted through the grid.

Grids are chiefly characterized by the grid ratio, grid frequency, and focal distance. The grid ratio is a measure of the height of the lead strip to the interspace distance, and is a good measure of the selectivity of primary to scatter transmission. In general, a grid with a higher grid ratio will reject scatter better than a lower grid ratio, due to the limited angle that is allowed by the grid structure. However, a higher ratio grid typically has a higher dose penalty for its use (for screen-film imaging this is known as the "Bucky Factor" which represents the increased dose to the patient when using a grid compared to not using a grid when the film optical density is matched). With digital imaging, there is also a dose penalty when using a grid is used, and the benchmark is the signal to noise ratio (as opposed to film optical density). The grid frequency is a measure of the number of grid lines per unit distance (inches or centimeters), and is in the range of 40 - 50 lines/cm (100-120 lines/inch) for low frequency grids, 50-60 lines/cm (120 - 150 lines/inch) for medium frequency grids, and 60 - 70+ lines/cm (150-170+ lines/inch). Low frequency grids are used with systems having a moving grid assembly (known as a Bucky device) that oscillates during the exposure to blur the grid lines. Medium and high frequency grids are typically used with stationary grid holders (e.g., portable radiography and many digital radiography systems). High frequency grid use is particularly important for digital radiography systems to avoid aliasing artifacts (see section on radiography artifacts) that arise from insufficient sampling of high frequency patterns that are interpreted in the output signal as low frequency (aliased) signals. The grid focal distance is determined by the angle of the lead strip geometry that is progressively increased from the center of the grid to the periphery, to account for the diverging primary x-ray beam emanating from the focal spot. Typical focal distances are 100 cm (40 inches) and 180 cm (72 inches), although there are many specialized grid focal distances. Focal range is an indicator of the flexibility of grid positioning distance from the focal spot, and is a function of the grid ratio and frequency. General purpose grids for portable radiography have a fairly large range (e.g., 80 to 130 cm) while special purpose grids have a much narrower focal range. Grid artifacts arise from improper positioning of the grid device, such as tilting the grid at a non-perpendicular direction to the incident x-ray beam, not centering the grid to the x-ray beam central axis, using a focused grid outside the specified focal range, and placing the grid upside down (converging geometry is directed opposite of the focal spot).

 

The two images of the AP projection of the knee phantom were obtained at 60 kV at the table top (left) and using the scatter removal grid (Bucky) (right).  The final S numbers of both images were ~350, indicating an air kerma incident on the computed radiography imaging plate of ~6 uGy (0.6 mR) in both cases. The table top image on the left, however, required a technique of 3 mAs whereas the one on the right required 10 mAs, since the scatter removal grid removes most of the scattered photons that emerge from the phantom. The Bucky factor is thus 3.3 (i.e., 10 mAs/3 mAs), and this is a quantitative measure of the increase in patient dose resulting from the use of the scatter removal grid. Note the improvement in image quality achieved by removal of most of the scatter radiation.

 

The two lateral projection images of the skull phantom were obtained at 75 kV at the table top (left) and using the scatter removal grid (i.e., Bucky) (right).  The final S numbers of both images were ~100, indicating an air kerma incident on the computed radiography imaging plate of ~20 uGy (2 mR) in both cases. The table top image on the left, however, required a technique of 4 mAs whereas the one on the right required 20 mAs, since the scatter removal grid removes most of the scattered photons that emerge from the phantom. The Bucky factor is thus 5 (i.e., 20 mAs/4 mAs), and this is a quantitative measure of the increase in patient dose resulting from the use of the scatter removal grid. Note that there is a dramatic increase of image quality achieved by removal of most of the scatter radiation, and well worth the “cost” in additional radiation dose to the patient.

 

The two AP projection images of the pelvis phantom were obtained at 75 kV at the table top (left) and using the scatter removal grid (Bucky) (right).  The final S numbers of both images were ~240, indicating an air kerma incident on the computed radiography imaging plate of ~8 uGy (0.8 mR) in both cases. The table top image on the left, however, required a technique of 3 mAs whereas the one on the right required 25 mAs, since the scatter removal grid removes most of the scattered photons that emerge from the phantom. The Bucky factor is thus 8 (i.e., 25 mAs/3 mAs), and this is a quantitative measure of the increase in patient dose resulting from the use of the scatter removal grid. Note that there is a dramatic increase of image quality achieved by removal of most of the scatter radiation, and well worth the “cost” in additional radiation dose to the patient.

Also note that the Bucky factor for the abdomen is substantially higher than those for the knee and the skull radiographs; The reason for this is that scatter is reduced with decreasing kV, as well as when imaging predominantly bony structures where most interactions are through the photoelectric effect (Compton scatter dominates for radiographs of soft tissue structures).