Basic Radiation Protection and Radiobiology

Human-made ionizing radiation, or x-rays as they are more commonly called, is a form of electromagnetic radiation that travels at the speed of light. Unlike particulate radiation, which is a liberated portion of a radioactive atom capable of traveling for short distances and reacting with matter, x-rays are bundles of energy moving as waves in space, depositing their energy randomly. For x-rays to be produced, three things must be present: (1) a source of electrons, (2) a means to rapidly accelerate the electrons, and (3) something to rapidly stop this movement. These conditions are all met by the x-ray tube and its electrical supply (Fig. 9.1). The tube itself is composed of a cathode, or negative terminal, and an anode, or positive terminal, enclosed in a special heat-resistant glass envelope to maintain the vacuum necessary for optimal x-ray production. The filament in the cathode assembly is composed of thoriated tungsten, which provides the source of electrons. When milliamperage (electric current) is applied to the filament, it responds by boiling off electrons through a process known as thermionic emission. Once kilovoltage (high voltage) is applied to the tube terminals, the electrons resulting from thermionic emission instantaneously accelerate toward the anode end of the tube. X-rays are produced when the electrons strike the anode, undergoing an energy conversion that produces both x-rays and heat. The resultant x-ray beam is heterogeneous—that is, it has many energies, measured in kiloelectron volts (keV). These x-rays, also known as the primary or useful beam, are directed toward the patient through a window in the tube. Once the x-rays strike matter, three possibilities exist: (1) they can be absorbed, (2) they can transfer some energy and then scatter, or (3) they can pass through unaffected.

Interactions of X-Rays With Matter

X-rays interact with matter in five ways: (1) classic coherent scattering, (2) photoelectric interaction, (3) Compton scattering, (4) pair production, and (5) photodisintegration. Classic coherent scattering, photoelectric absorption, and Compton scattering occur within the diagnostic range of x-ray energies, whereas pair production and photodisintegration occur in the therapeutic range of energies. Both Compton scattering and photoelectric interaction directly influence patient and occupational worker exposure. They are the way in which x-rays transfer their energy to living tissue. They constitute the basis for all patient exposure and the reason behind the need for protective measures.

Classic Coherent Scattering.

X-rays that possess energy levels below 10 keV can interact with matter through classic coherent scattering (Fig. 9.2). Also known as coherent, Thomson, or unmodified scattering, classic coherent scattering occurs when an incoming x-ray photon strikes an atom and is absorbed, causing the atom to become excited. The atom then releases the excess energy in the form of another x-ray photon possessing the same energy as the original photon, but proceeding in a different direction. This change in direction is known as scattering. Most of these scattered photons travel in a forward direction, stopping when they strike anything in their path. More importantly, classic coherent scattering results in no energy transfer to the patient.

Photoelectric Absorption.

The second common interaction of x-rays with matter in the diagnostic range is the photoelectric absorption (Fig. 9.3). Photoelectric absorption occurs when an incoming x-ray photon strikes an inner shell electron and ejects it from its orbit around the nucleus of the atom, creating an ion pair. The atom, having lost an electron, is positively charged, and the released electron, referred to as the photoelectron, continues to travel until it combines with other matter. All the energy from the photon is completely consumed in this interaction; it is said that the energy is absorbed by the atom. As outer shell electrons transition to fill vacancies in inner shells, they release excess energy in the form of x-rays. X-rays originating within the body as a result of the photoelectric effect are collectively known as characteristic radiation and are the source of secondary radiation. Because complete energy absorption takes place in photoelectric interactions, this constitutes the greatest hazard to patients in diagnostic radiography.

Compton Scattering.

The last interaction common to the diagnostic x-ray range is the Compton effect (Fig. 9.4). The Compton effect, also known as modified or Compton scattering, occurs when an incoming x-ray photon strikes a target atom and uses a portion of its energy to eject an outer shell electron. The remainder of the photon’s energy proceeds in a direction different from that of the incoming photon. This process results in a Compton or recoil electron ejected from the outer shell, which travels until it combines with matter, and a photon of less energy that can react with the patient through further Compton or photoelectric interactions, or that can exit the patient and reach imaging equipment or the occupational worker. This interaction is also referred to as modified scattering, because the original photon possesses less energy after the interaction. The Compton effect is extremely important because it is responsible for a majority of occupational workers exposure to radiation.

Units of Measure

To quantify the amount of radiation a patient or occupational worker receives, a system of units has been developed. The units most commonly used since the 1920s are listed in Table 9.1. In 1948, the International Committee for Weights and Measures developed a system of units based on metric measurement. The SI units system (Système International d’Unitès, or International System [SI] of Units) was officially adopted in 1985.

Basic Cell Structure

The cell is the simplest unit of organic protoplasm capable of independent existence. Simple organisms are composed of one or two cells; complex organisms are multicellular—that is, made of many cells. Although cells may differ from one another depending on their primary function, their structures are similar. Most cells are divided into two parts: (1) the nucleus and (2) the cytoplasm (Fig. 9.6). The nucleus is separated from the rest of the cell by a double-walled membrane called the nuclear envelope. This membrane has openings, or pores, that permit other molecules to pass back and forth between the nucleus and the cytoplasm. Most important, the nucleus contains the chromosomes, which are made up of genes. Genes are the units of hereditary information, composed of DNA, which is a double-stranded structure coiled around itself as a spiral staircase. It is one of the molecules at risk when a cell is exposed to ionizing radiation.

The cytoplasm of the cell is separated from its environment by the cell membrane. It contains several organelles responsible for the metabolic function of the cell. The cytoplasm itself is primarily water, which can undergo changes when struck by ionizing radiation.

Cell Types

Cells are of two types: (1) somatic cells and (2) germ cells. Somatic cells perform all the body’s functions. They possess two of every gene on two different chromosomes. Their chromosomes are paired, but each pair is different. Somatic cells possess a total of 46 chromosomes, or 23 pairs. They divide through the process of mitosis. Germ cells are the reproductive cells of an organism; they possess half the number of chromosomes as the somatic cells, for a total of 23. Germ cells reproduce through the process of meiosis.

Theories for Cellular Absorption of Ionizing Radiation

When a cell absorbs ionizing radiation, two basic theories exist to explain this interaction. The first theory, known as the direct hit theory, proposes that any type of radiation transfers its energy directly to the key molecule it has struck, resulting in the formation of ion pairs or elevation to an increased, excited energy state. Although any important structure can be hit by radiation, serious consequences arise when radiation interacts with DNA. Breaks in the bases or phosphate bonds can result in rearrangement or loss of genetic information, which can injure or kill the cell as it continues through its life cycle.

The other interaction with ionizing radiation is by indirect hit. Key molecules are affected by radiation depositing its energy elsewhere in the cell. Because cells are approximately 80% water, indirect action occurs when water molecules are ionized. This action produces chemical changes within the cell that alter the internal environment, injuring the cell, which can result in eventual cell death. With x-radiation and gamma radiation, the vast majority of cellular damage is the result of indirect hits.

Target Theory of Absorption of Ionizing Radiation

Both direct and indirect interactions with ionizing radiation apply to the target theory of absorption of ionizing radiation. Simply stated, certain molecules existing within a cell are key to the continued viability or life of that cell. Some of these molecules exist in great number; others in limited supply. If damage occurs to a molecule in abundant supply, the effect on the cell may not be as detrimental because others exist to maintain the function of the cell. Injury to a molecule in limited supply, however, can be life threatening, because no immediate replacement is available. The term target is used to describe any critical molecule, in this case the nucleus of a cell, that has undergone some interaction with ionizing radiation, either directly or indirectly. The target whose damage has serious consequences to the life of the cell is DNA.

Radiosensitivity of Cells

To study the cell’s response to radiation, a method of classification according to sensitivity was developed by Bergonie and Tribondeau in 1906. These researchers determined that mitotic activity and specific characteristics of each cell affected how the cell exhibited radiation damage. Cells are most sensitive to radiation during active division, when they are primitive in structure and function. Examples of radiosensitive cells include the basal cells of the skin, crypt cells of the small intestine, and germ cells. Cells resistant to radiation, being more specialized in structure and function, do not undergo repeated mitosis. These cells include nerve, muscle, and brain cells.

Ancel and Vitemberger, who modified this theory, stated that all cells possess the same sensitivity to radiation; the time of expression of injury is the factor that differs. This factor depends on mitosis and the external conditions in which the cell is placed. Therefore, rapidly dividing cells demonstrate the injury sooner and only appear as though they are more sensitive to radiation than those whose mitotic rate is slower. Organs composed of parenchymal cells that rapidly divide, such as skin or the small intestine, exhibit injury sooner than the esophagus or muscle, whose cells divide more slowly.

Response of Cells to Radiation

Cells respond to radiation in many ways. They die before beginning mitosis, delay entering mitosis, or fail to divide at their normal rate. Fortunately, cells also try to repair the damage sustained through absorption of ionizing radiation. This possibility depends on how sensitive the cell is to radiation, the type of damage sustained, the kind of radiation (particulate or electromagnetic), the exposure rate, and the total dose given. Incomplete repair can result in adverse biologic effects occurring after time has elapsed.

Total Body Response to Irradiation

The total body response of any organism to radiation depends on the effect on all the systems of the body. Because every system is different in its sensitivity or resistance, the total body response at a particular dose is defined by the system most affected. This response, known as acute radiation syndrome, occurs only when the organism is exposed fully (total body) to an external source of radiation given in a few minutes. Only then does the organism develop the full set of signs and symptoms that define each syndrome, which depends on the dose received.

Distance is the best measure of protection for an occupational worker. The principle of the inverse square law states that the intensity of radiation varies inversely with the square of the distance. Simply stated, increasing the distance from the source of the x-ray beam greatly reduces the quantity of radiation that reaches the radiographer (Fig. 9.7). This reduction occurs because the x-rays leaving the tube spread out (diverge) and cover a much larger area, which in turn lessens their intensity. The following formula can be used to determine the exact exposure reaching the worker:

For example, if the intensity of radiation received by the radiographer were 20 Gy at a distance of 1 m from the tube, what then would the intensity be at a distance of 2 m from the tube, all other factors remaining the same? Solving for the new intensity, we get:

New intensity20/gy=1222

New intensity=1×20Gy4−5Gy

Doubling the distance between the radiologic and imaging sciences professional and the source of radiation reduces the exposure by a factor of 4 (now 1/4 the dose at the original distance). Conversely, if the distance from the source of radiation is reduced by 1/2, the exposure will increase to 4 times the original. Therefore distance is an extremely important factor in radiation protection.

The imaging sciences professional should not make a practice of holding a patient who cannot cooperate during a radiographic procedure. This circumstance places the radiologic technologist closer to the beam and to the patient, who is a source of scatter radiation from Compton interaction, and increases the time a radiologic and imaging sciences professional is near the source of radiation. Whenever possible, immobilization devices such as sandbags or restraint bands should be used. If these devices are ineffective or unavailable, assistance should be obtained from a nonoccupational worker such as a nurse, physician, or relative of the patient. The person who assists the patient must wear shielding devices to minimize his or her exposure.

Optically Stimulated Luminescence Dosimeters

The Luxel OSL dosimeter (Landauer) is the most common method used to monitor personnel exposure (Fig. 9.8). This type of dosimeter consists of a strip of aluminum oxide, a copper filter, an open window, a tin filter, and an imaging filter. The device is heat-sealed within a laminated, light-tight paper wrapper. The entire package is then sealed in a tamper-proof plastic blister pack. The front of the dosimeter provides information identifying the person wearing it, the name of the department, and a badge placement icon. To determine the individual’s exposure, the aluminum oxide is exposed to a laser light, which stimulates the aluminum oxide after use, causing it to become luminescent in proportion to the amount of radiation exposure, which determines the occupational worker’s exposure.

The OSL dosimeter can detect x-rays and gamma radiation in the range of 5 keV to in excess of 40 MeV. The dose measurement range is from .01 mSv to 1000 Sv. Doses less than .01 mSv are not detectable and reported as M, or minimal. Typically, the OSL dosimeter is worn for 2 months, but may be exchanged on a monthly basis. The holder should be worn between the collar and waist, on the front of the occupational worker. An advantage of this personal radiation monitoring device is that, because of the blister packaging, it is unaffected by heat, moisture, and pressure. The main disadvantage of this device is the inability to provide an immediate reading of the worker’s exposure; the dose can be determined only when the aluminum oxide is analyzed by the dosimetry company.

Thermoluminescent Dosimeters

Another device used to monitor personnel exposure is the TLD. It consists of a plastic holder containing crystals that absorb a portion of the energy they receive from a radiation exposure. When the device is exposed, the absorbed energy causes the outer valence electrons to be trapped in the forbidden zone, the region immediately past their resting orbit. The number of electrons elevated to this state depends directly on the amount of radiation received. When the time comes to determine the dose, these crystals are heated so that the trapped electrons return to their original resting state. This process results in a release of the extra energy in the form of a light photon. The light is collected and analyzed to determine the quantity of the dose received by the TLD.

The crystal most commonly used in TLDs is lithium fluoride. Once the lithium fluoride crystals have been heated (annealed), they can then be reused. The TLD provides readings as low as 0.05 mSv (Table 9.5).

Pocket Dosimeters

The last dosimetry device is the pocket direct ion storage (DIS) dosimeter, which is a small ionization gas filled dosimeter connected to a “solid state” device, with electrically erasable programmable read-only memory (EEPROM). EEPROMs are used in microprocessors and various consumer products to store small amounts of data will allowing some memory to be erased and reprogrammed to store more data. In the personnel dosimeter (Fig. 9.9), when radiation ionizes the gas in the ionization chamber, the cumulative electric charge is stored in the EEPROM and will remain in the device until “read out” by the introduction of a small control signal. The amount of charge stored in the device is directly proportional to the amount of radiation exposure that produced ionization in the chamber. The DIS dosimeter is read out through a physical connecting device such as a universal serial bus (USB) and the data can be stored electronically. The DIS dosimeter provides instant access to data and no need for the institution to collect and mail dosimeters to the manufacturer for the read out.

Field Survey Instruments

Other types of instruments are used to detect the presence of radiation and provide the user an indication of the intensity of the source. These devices are known as field survey instruments. A common instrument used to detect x-radiation, gamma radiation, and beta radiation is the Geiger-Müller counter, which is an ionization chamber constructed of an electrode housed within a chamber. The walls of the chamber are negatively charged, and the electrode is positive. When x-rays pass through the chamber and interact with the air, ionization occurs. Free electrons are attracted to the positively charged electrode, where they can be measured. The number of free electrons is directly proportional to the radiation exposure and can be displayed on a special meter that interprets this information and determines the exposure in coulombs per kilogram.