Coherent scattering is also described as

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.

FIGURE 9-2 Classic coherent scatter interaction.

Photoelectric Interaction.

The second common interaction of x-rays with matter in the diagnostic range is the photoelectric effect [Fig. 9-3]. The photoelectric effect 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.

FIGURE 9-3 Photoelectric absorption interaction.

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 worker exposure to radiation.

FIGURE 9-4 Compton scatter interaction.

Pair Production.

The last two interactions that occur between ionizing radiation and matter require high-energy photons above 1 million electron volts [MeV]. They are less relevant to diagnostic radiography because the equipment used in the production of x-rays cannot produce photons that possess this energy, but they are of particular importance in radiation therapy.

For pair production to occur, an incoming x-ray photon must possess a minimum of 1.02 MeV of energy [Fig. 9-5]. This photon does not interact with the surrounding electron orbits; instead, it approaches the nucleus of the atom and interacts with its force field. The photon disappears, and two particles emerge to replace it: a positron and a negatron. A positron is a positively charged particle, and a negatron is negatively charged. Each particle possesses half the energy [minimum, 0.51 MeV] of the original x-ray photon. The particles continue to travel, causing ionization, until the positron interacts with another electron, annihilates it, and produces two photons moving in opposite directions. Because the energy level necessary for pair production is at least 1.02 MeV, it does not occur in the diagnostic x-ray range.

FIGURE 9-5 Pair production interaction.

Photodisintegration.

X-ray photons possessing a minimum of 10 MeV of energy can interact directly with the nucleus of the atom, causing a state of excitement within the nucleus, followed by the emission of a nuclear fragment [Fig. 9-6]. This process is referred to as photodisintegration. It does not occur in diagnostic radiography, but does occur in the nuclear industry.

FIGURE 9-6 Photodisintegration interaction.

Roentgen [Coulombs per Kilogram].

The roentgen [R] is the measure of ionization in air as a result of exposure to x-rays or gamma rays. It is defined as the quantity of x-radiation or gamma radiation that produces the quantity 2.08 × 109 ion pairs per cubic centimeter [cc] of air, for a total charge of 2.58 × 10−4 coulombs per kilogram [C/kg] [coulomb is a quantity of electric charge]. The roentgen is restricted to measuring photons with energy below 3 MeV and only exposure in air. It does not indicate actual exposure to individuals when absorbed. The roentgen has no equivalent in the SI units because exposure may be expressed directly as coulombs per kilogram, so the roentgen is being phased out as a unit of measure.

Radiation Absorbed Dose [Gray].

The need for discussing absorbed dose resulted in the development of the radiation absorbed dose [rad]. The rad measures the amount of energy absorbed in any medium, defined as 100 ergs of energy absorbed in 1 g of absorbing material. The rad has been replaced by the gray [Gy] in the SI system, which is defined as 1 joule [J] of energy absorbed in 1 kg of material. The Gy is 100 times larger than the rad; 1 Gy = 100 rad.

Radiation Equivalent Man [Sievert].

Not all types of radiation produce the same response in living tissue. Alpha particles, neutrons, and beta particles may produce a different degree of biologic damage than x-rays and gamma rays. To express accurately the biologic response of exposed individuals to the same quantity of differing radiations, the rem was developed. The radiation equivalent man [rem] is the unit of dose equivalence, expressed as the product of the absorbed dose in rad and a quality factor.

The quality factor varies, depending on the type of radiation being used. For example, the quality factor for x-rays is 1; therefore 1 rad of x-ray exposure equals 1 rem of dose equivalence [1 rad × 1 = 1 rem]. The quality factor for fast neutrons is 10; thus 1 rad of fast neutron exposure equals 10 rem of dose equivalence [1 rad × 10 = 10 rem], meaning that neutrons are 10 times as biologically damaging as x-rays when their dose equivalents are compared. The rem has been replaced by the sievert [Sv] in SI units, which is defined as the product of the Gy and the quality factor. The sievert is 100 times larger than the rem; 1 Sv = 100 rem.

Curie [Becquerel].

The measure of the rate at which a radionuclide decays is referred to as activity. The curie [Ci] is the unit of activity, equal to 3.7 × 1010 disintegrations per second [dps]. The SI unit of activity is the Becquerel [Bq], defined as 1 disintegration per second [dps]. Therefore 1 Ci = 3.7 × 1010 Bq. These units are commonly employed in nuclear medicine and radiotherapy.

The traditional and SI units are compared in Table 9-1.

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. 9-8]. 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.

FIGURE 9-8 Diagram of a typical animal cell.

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