In 1913 the German physicist Niels Bohr compared the atom with a miniature solar system. Current theories have evolved beyond the Bohr atom; however, Bohr’s theory works well for illustrating and understanding atomic forces. Basically, Bohr described the atom as having a dense core, or nucleus, made up of neutrons and positively charged protons. Negatively charged electrons, spinning on their axes, orbit the nucleus at fixed distances called quantum shells or energy levels. Quantum shells are assigned the letters K, L, M, N, and so on, with K being the innermost shell (Fig. 1-1). Electrons occupying shells farther away from the nucleus have greater potential energy than those found closer to the nucleus.

In a neutral atom the number of protons in the nucleus, or atomic number (Z), is equal to the number of orbiting electrons. The maximum number of electrons occupying a given shell is determined by the formula 2n², where n is the quantum shell number. The quantum shell number is obtained by counting the shells outward from the nucleus. The K shell (n = 1) can hold 2(1)², or two, electrons; the L shell (n = 2) can hold 2(2)², or eight, electrons; and so on. The outermost shell, or valence shell, never exceeds eight electrons at one time in a stable atom.

Positively charged nuclear protons exert an electrostatic attractive force that binds electrons to their orbit. Electron spin velocity counters the attractive force and keeps electrons at discrete distances from the nucleus. Electrons occupying shells closer to the nucleus are bound tighter. The amount of energy needed to remove an electron completely from its orbit is called electron binding energy. Electron binding energy is measured in electron volts (eV), the same units used to describe x-ray energy. A free electron is assumed to have zero binding energy; therefore a bound electron is in a negative energy state, because it takes positive energy to unbind or raise the binding energy to zero. Electrons occupying shells closer to the nucleus have greater binding energy than those found farther from the nucleus. The binding energy for any given electron increases as atomic number increases (Fig. 1-2).

Four units used to measure radiation are the roentgen (R), rad, rem, and curie. The roentgen is a measurement of radiation exposure, or intensity, which creates 2.08 × 10⁹ ion pairs in a cubic centimeter (cm³) of air. The SI system (Systéme Internationale d’Unités) is a modernized metric system based on meters-kilograms-seconds or centimeters-grams-seconds. The SI units are used more broadly in science and most countries of the world than are the British, or Customary, units of feet-pounds-seconds. The SI unit for radiation exposure is coulombs/kilogram (C/kg); 1 roentgen is equal to 2.58 × 10⁻⁴ C/kg. Radiation exposure emitted from x-ray machines is measured in roentgens or C/kg.

The rad is the unit that measures absorbed dose. It is a measure of energy (expressed in ergs) deposited into a mass of tissue (expressed in grams or kilograms) and is often related to the biologic effects of radiation. One rad is equal to 100 ergs in 1 gram of irradiated tissue. The SI unit for absorbed dose is the gray (Gy). One gray is equal to 100 rads. One rad is equal to 0.01 gray, or 1 centigray (cGy).

The unit that defines absorbed dose equivalent for humans is the rem (rad equivalent man). The rem is used exclusively for radiation protection reporting of occupational exposure. It is a measure of the biologic effectiveness of different types of radiation in humans. Compared with x-rays and gamma rays, radiation such as alpha and beta particles and fast neutrons produces different magnitudes of biologic effects, even at the same absorbed dose.

The National Council on Radiation Protection and Measurements (NCRP) is the governing body responsible for reporting guidelines for radiation protection and measurements. The most recent report, NCRP No. 116: Limitations of Exposure to Ionizing Radiation, which succeeds NCRP report No. 91, replaces the term dose equivalent (H) with the term equivalent dose (H_TR). The change goes beyond word semantics. Dose equivalent (H) is a measurement of absorbed dose at some certain location in tissue. Equivalent dose (H_TR)is a measurement of an average absorbed dose in tissues and organs. Equivalent dose is the product of the average absorbed dose (D_TR) of radiation (R) in a tissue (T) and a radiation weighting factor (W_R):

The curie (Ci) is a quantitative measure of radioactive material. It is defined as the amount of radioactive material in which 3.7 × 10¹⁰ atoms disintegrate every second. The radiation emitted from a curie of radioactive material is measured in roentgens, rads, and rems. The SI unit is the becquerel (Bq), defined as 1 disintegration per second. Millicurie (mCi) and microcurie (µCi) amounts are common in nuclear medicine procedures. The reporting units used in the radiologic sciences are listed in Table 1-1.

The tube housing is a grounded, lead-lined metal shelter that protects and supports the glass x-ray tube insert (Fig. 1-4). X-rays are emitted multidirectionally from the tube, but only those rays passing through an opening, or port window, in the housing expose the patient. All other rays are trapped in the housing wall, thereby decreasing unnecessary exposure to patients or x-ray personnel. Radiation emitted from the tube housing is called primary radiation.

Electrons strike the surface of the anode at a target site called the actual focal spot (Fig. 1-8). An increase in either filament length or anode angle results in an increase in actual focal spot size. As the focal spot size is increased, the electrical loading capacity of the anode is increased, which safely allows exposures that require higher electrical loads. Although the entire target area emits x-rays, only those rays traveling in the direction of the patient are useful. The line-focus principle is used to reduce the effective area of the actual focal spot to that portion of the x-ray beam that is useful. The effective target area, or effective focal spot, is a measure of the width of the actual focal spot projected along the central ray and perpendicular to the plane of the x-ray port. As with the actual focal spot, the size of the effective focal spot is determined by the size of the filament and angle of the anode (see Fig. 1-8). Tube specifications typically are reported in effective focal spot size, which essentially is a measurement of the resolution afforded by a particular x-ray tube. In general, the smaller the effective focal spot size, the better the resolution. Effective focal spot sizes commonly used in diagnostic radiology range from 0.6 to 1 mm for small focus and 1.5 to 2 mm for large focus.

The anode angle on a rotating anode tube also determines maximum field coverage when the tube is placed at specified tube distances (Fig. 1-9). A standard angle of 12° is used for sectional imaging to cover 17 inches² at a 40-inch focal-spot-to-film distance. A 14° angle is needed to cover 36 inches² at a 72-inch focal-spot-to-film distance for scoliosis screening and other full-spine imaging needs.

Focal spot selection is linked to specific mA stations on the control console. For example, the small focal spot is coupled with mA stations below 200 mA on most diagnostic x-ray machines. This limits the load on the anode and is best used with small body parts, such as the cervical spine and extremities. The large focal spot is coupled with the 200 mA and higher station to allow for greater electron loading of the anode. It is best used with large body parts, such as the lumbar spine. The small focus produces images with better detail than the large focus.

X-ray intensity emanating from the tube is not uniform across the span of the beam. Exposure intensity is greater on the cathode side of the beam than the anode side. This is called the anode heel effect. Electrons penetrate the target at various depths, causing x-rays to be emitted with equal intensity in all directions. Those x-rays traveling into the substance of the anode are immediately absorbed. The angle at which x-rays emerge from the target surface toward the patient varies and causes some x-rays to be absorbed in the heel of the anode. The heel attenuates a portion of the x-ray beam and leaves less exposure (intensity) on the anode side of the beam (Fig. 1-10).

When body parts of different thicknesses are imaged, the heel effect can be used to advantage if the cathode portion of the beam is placed over the thickest part of the patient. The heel effect is less noticeable with smaller film sizes (8 × 10 inch) and longer tube distances (72-inch), because more of the central, uniform portion of the beam is used.

Very-low-energy x-rays, or soft rays, add no diagnostic information to the image, because they are completely absorbed by the patient. To reduce patient dose from soft radiation, federal law requires a minimum of 2.5 mm equivalent of aluminum (equiv/Al) filtration for a beam greater than 70 kVp. Aluminum absorbs many of the low-energy photons while transmitting a large proportion of high-energy photons. The glass port window and oil within the tube housing provide inherent filtration of approximately 0.5 mm equiv/Al. The silver-coated mirror in the collimator is placed so that the x-ray beam must pass through it. It may supply another 1 mm equiv/Al of inherent filtration.

A three-phase generator uses a three-phase AC line source for the purpose of creating more pulsations per unit time. This effectively reduces voltage ripple and increases efficiency of x-ray production. Three-phase power is best understood by imagining three single-phase line sources electrically intertwined to create greater incoming power. A six-pulse generator delivers six pulsations per cycle, which reduces voltage ripple to 13% of kVp (see Fig. 1-11). Changing the wiring configuration produces 12 pulses per cycle (12-pulse generator), reducing voltage ripple to 3% of kVp. Using three-phase power results in greater photon output per unit of electrical input (mR/mAs) with higher average energy compared with single-phase units. Three-phase generators are expensive and rely on costly power line installations.

Medium- and high-frequency generators operate on either single-phase or three-phase power and virtually eliminate voltage ripple through rectification and smoothing circuits. Medium-frequency generators produce an electrical frequency in the 6- to 30-kHz range, and high-frequency generators produce an electrical frequency of approximately 100 kHz. Medium- and high-frequency generators allow more accurate control of tube voltages. Tube voltage regulates penetration of the x-ray beam. High-frequency generators deliver the greatest mR/mAs output, the lowest soft radiation dosage to patients, the highest average (effective) x-ray energy, and the shortest exposure time.

Every x-ray exposure requires a certain predetermined tube current (mA) for a specific period of time. Both the timer and timer circuit are located on the control console and are electrically connected to the exposure switch.

Timers are described as mechanical, synchronous, or electronic. A simple mechanical timer is spring-loaded and unwinds at a set rate. Mechanical timers are used with single-phase, low-output x-ray generators such as those seen in dental offices. They are not very accurate and are limited to exposure times longer than 1/4 second.

Synchronous timers operate from a synchronous motor running at 60 Hz. The time increments are in multiples of 1/60 second, such as 1/30, 1/20, and 1/10. The shortest exposure time is 1/120 second. Synchronous timers are not as accurate as electronic timers and cannot be used with serial exposures because they require a short recycling time.

An electronic timer is the most common timer in use today. It is accurate to less than 1 millisecond (msec) and to greater than 1 second. Electronic timers operate on sophisticated electronic circuitry by energizing a silicon-controlled rectifier (SCR), which activates the exposure. The combination of constant potential generators and very fast imaging systems (rare earth) require a timer accurate in milliseconds.

The product of mA and time, milliampere-seconds (mAs), determines the number of x-ray photons emitted and hence the relative darkness of the film. Patient part size and density govern how many mAs are needed to produce a diagnostic image. Calculating exposure by using units of mAs is common in a clinical setting. A mAs timer accurately provides the highest safe tube current with the shortest exposure time for a given mAs. Independent control of mA and time may not be possible with some mAs timers.

Automatic exposure control (AEC), often called phototiming, terminates the exposure when a predetermined amount of film density (darkness) is reached by x-rays passing through the patient to the image receptor. The radiographer selects the appropriate beam penetration (kVp) and desired tube current (mA) for the part under examination. The phototiming device senses the exposure and, in response, creates an electronic signal that breaks the timer circuit.

Automatic exposure devices include the earlier photomultiplier tube and the more common ionization chamber. The photomultiplier tube converts a light signal from a fluorescent screen exposed to x-rays to an electronic signal that feeds back to terminate the exposure. The radiation-sensitive ionization chamber creates an electronic signal proportional to the number of ions produced by radiation exposure and feeds back to terminate the exposure.

The phototiming circuit usually has one, two, or three photocells of different shapes and positions in relation to the image receptor. From the control console, the operator may choose to use one or more photocells to determine exposure. When more than one photocell is used, the exposure is averaged between them. A manual backup timer is set to approximately 1.5 times the anticipated exposure to prevent tube overload and excessive patient dose. Setting the backup time too short may result in a risk of underexposure.

AEC decreases the need for repeat radiographs by adjusting for patient density and reducing human error in exposure calculation. However, positioning the body part under examination precisely over the photocell(s) is paramount to produce adequate exposure. Malposition results in overexposure or underexposure. Exposure area also affects the photosensor readings, especially considering scatter radiation production; therefore proper beam collimation is necessary to produce accurate exposure time. The calibration of the phototimer must be matched with the sensitivity of the image receptor (film/screen).

Bremsstrahlung is the German word for braking, or slowing down. When a high-speed projectile electron from the cathode passes the nucleus of a tungsten atom in the target, the positively charged nucleus exerts an attractive force on the electron. A strong nuclear electric field inhibits penetration of the electron into the nucleus but causes the electron to decelerate and change direction (Fig. 1-15). This deceleration results in a loss of kinetic energy, which is converted into electromagnetic radiation (x-rays). The quality (or energy) of radiation released is contingent on the amount of deceleration and kinetic energy possessed by the incoming electron (measured in kVp). Deceleration is affected by the proximity in which electrons randomly approach the nucleus and size of the nucleus. Electrons directly striking the nucleus and giving up 100% of their kinetic energy generate the highest-energy x-rays.

When a high-speed electron from the cathode interacts with a lower orbit (inner shell) electron in the tungsten target and the kinetic energy of the projectile electron exceeds the binding energy of the electron it interacts with, the orbital electron is ejected, leaving a vacancy in the inner shell (Fig. 1-16). Immediately, an upper orbit electron fills the vacancy, resulting in a release of a discrete amount of electromagnetic radiation (x-ray). The amount of energy released is equal to the difference in the binding energies of the orbital shells involved. For tungsten, atomic number 74, the following applies:

A cascade effect causes all inner shell vacancies to be filled and lower energy photons to be released. The total amount of energy released equals the input energy necessary to remove the inner shell electron.

Binding energies are unique to each element. The radiation released is “characteristic” of the atom it is generated from, hence the name characteristic radiation. A K-characteristic x-ray of tungsten is 57.4 keV. A K-characteristic x-ray of molybdenum is 17.4 keV. It takes at least 70 keV (kVp) of input energy to release a K-shell electron in tungsten. Kilovoltage has no effect on the quality (energy) of characteristic radiation. However, the energy of characteristic x-rays increases as atomic number increases. Characteristic x-rays make up approximately 10% of the radiation emitted in the 80- to 100-kVp range. Figure 1-17 illustrates a filtered beam of bremsstrahlung and characteristic x-rays at 80 kVp.

The photoelectric effect is a total absorption reaction in which x-ray photons interact with inner shell electrons of an absorbing medium to cause ionizations (Fig. 1-18). This is most likely to occur when the energy of the incident photon slightly exceeds the binding energy of an electron at the K- or L-shell level. The photon gives up all of its energy in overcoming the binding energy of an inner shell electron, and the photon ceases to exist. The photoelectron

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