Radiation Concepts

Radiologic nomenclature describes the quantity of radiation in terms of exposure, dose, dose equivalent, and activity. Conventional terms are used in the United States, and an international system of units defined in 1960 by the General Conference of Weights and Measurements is primarily used in Europe. Each system has its unique terms (Table 5-1).¹

Table 5-1 Radiation Quantities and Units

Terminology

Like matter, energy can be transformed from one form to another. When ice (solid) melts and turns to H₂O (liquid) and then evaporates (gas), a transformation of matter has occurred. Similarly, x-rays transform electrical energy (electricity) into electromagnetic energy (x-rays), which then transforms into chemical energy (radiographic image). Electromagnetic energy emitted into and transferred through matter is called radiation. The spectrum of electromagnetic radiation extends more than 25 orders of magnitude and includes not only x-rays, but also the wavelengths responsible for visible light, magnetic resonance imaging (MRI), microwaves, radio, television, and cellular phone transmission (Fig. 5-1).10 Irradiation occurs when matter is exposed to radiation and absorbs all or part of it.

Figure 5-1 The electromagnetic spectrum extends over more than 25 orders of magnitude. This chart shows the values of energy, frequency, and wavelength and identifies some common values and regions of the spectrum.

(From Bushong S: Radiologic Science for Technologists: Physics, Biology, and Protection, 4th ed. St. Louis, Mosby, 1988, with permission.)

Ionizing Radiation

The two basic types of electromagnetic radiation are ionizing and nonionizing. A unique characteristic of ionizing radiation is the ability to alter the molecular structure of materials by removing bound orbital electrons from its atom to create an electrically charged positive ion. The ejected electron and the resulting positively charged atom are called an ion pair. Ionizing radiation gradually uses its energy as it collides with the atoms of the material through which it travels. This transfer of energy and the resulting electrically charged ions can induce molecular changes and potentially lead to somatic and genetic damage.

X-Rays and Gamma Rays

Ionizing radiation includes x-rays and gamma rays, which are emitted from x-ray machines, nuclear reactors, and radioactive materials. Gamma rays and x-rays are identical in their physical properties and biologic effects; the only difference is that gamma rays are natural products of radioactive atoms, whereas x-rays are produced in machines. In the production of x-rays, a high dose of voltage, measured in kilovolts (kVp), and a sufficient dose of electrical current, measured in milliamperes (mA), are required.

X-ray is a form of electromagnetic energy of very short wavelength (0.5 to 0.06 ångstrom), which allows it to readily penetrate matter. When an object or body is exposed to ionizing radiation, the total amount of exposure is a unit of measurement called the roentgen (R). The definition describes the electrical charge per unit mass of air (1 R = 2.58 × 10^-4 coulombs/kg of air). The output of x-ray machines usually is specified in roentgen (R) or milliroentgens (mR). Ionizing radiation exposed to a body interacts with the atoms of the material it comes in contact with in the form of transfer of energy. This dose of transferred energy is called absorption, and the quantity of absorbed energy in humans is referred to as the radiation absorbed dose (rad). By definition, 1 rad = 100 ergs/g where the erg (joule) is a unit of energy and the gram is a unit of mass. The gray (Gy) is a commonly used international unit of measurement to describe absorbed dosages and can be calculated by multiplying the rad by 0.01. Biologic effects usually are related to the rad, which is the unit most often used to describe the quantity of radiation received by a patient. The rad equivalent man (rem) is the unit of occupational radiation exposure and is used to monitor personnel exposure devices such as film badges.

Radiologic Procedures

Fluoroscopy

In general, there are two types of x-ray procedures: radiography and fluoroscopy. Conventional fluoroscopic procedures, such as myelography, barium enemas, upper gastrointestinal series, and swallowing studies, usually are conducted on a fluoroscopic table. The conventional fluoroscope consists of an x-ray tube located above a fixed examining table. The physician is provided with dynamic images that are portrayed on a fluoroscopic screen and the ability to hold and store (“freeze frame”) an image in memory for review or to print as a radiograph (“spot view”) for future reference. Conventional fluoroscopy is considered suboptimal for spinal interventional procedures because of the inability to manipulate the x-ray tube around the patient, and it has been virtually replaced by C-arm fluoroscopes with image intensification for use in spinal injection procedures. The C-arm permits the physician to rotate and angle the x-ray tube around the patient while the patient rests on a radiolucent support table (Fig. 5-2). Image intensification is achieved through the addition of an image-intensifier tube located opposite the x-ray tube. The intensifier receives remnant x-ray beams that have passed through the patient and converts them into light energy, thereby increasing the brightness of the displayed image and making it easier to interpret. In the current image-intensified fluoroscopy, the x-ray tube delivers currents between 1 and 8 mA. Federal regulations limit the maximum output for C-arm fluoroscopes to 10 R/min at 12 inches from the image intensifier.

Figure 5-2 The C-arm rotated to the anteroposterior projection (A), oblique projection (B), and lateral projection (C).

Factors Affecting Radiation Exposure

Exposure to ionizing radiation is an unavoidable event while performing fluoroscopic procedures. If one cannot avoid the radiation, then one must minimize its absorption by biologic tissues. The primary source of radiation to the physician during such procedures is from scatter reflected back from the patient. Of lesser concern is the small amount of radiation leakage from the equipment housing.

The cardinal principles of radiation protection are: (1) maximize distance from the radiation source; (2) use shielding materials; and (3) minimize exposure time. These principles are derived from protective measures that were adopted by individuals who worked on the atomic bomb in the Manhattan Project; such measures also may be instituted in the fluoroscopic suite. In addition, the concept of ALARA (as low as reasonably achievable) should be applied in all situations of radiation exposure.

Distance

Distance is the most effective means of minimizing exposure to a given source of ionizing radiation. According to the inverse square law, the intensity of the radiation is inversely proportional to the square of the distance. That is, when a given amount of radiation travels twice the distance, the covered area becomes four times as large and the intensity of exposure reduces to 1⁄4 (Fig. 5-3). Therefore, at four times the distance from the source, exposure is reduced to 1⁄16 the intensity.

Figure 5-3 When the distance from a point source of radiation is doubled, the radiation covers an area four times larger than the original area. However, the intensity at the new distance is only one fourth of the original intensity.

(From Statkiewicz MA, Ritenour ER: Radiation Protection for Student Radiographers. St. Louis, Mosby, 1983, with permission.)

A rough estimate of the physician’s exposure at a distance of 1 meter from the x-ray tube is 1/1000th of the patient’s exposure.6 It is therefore recommended that the technician and physician remain as far away from the examining table as practical during fluoroscopic procedures. The position of the physician’s body, especially the hands, should be closely monitored and his or her position should be kept at a maximum distance from the fluoroscope at all times.² For example, it is advisable that the physician deliberately step away from the patient before acquiring each image and also use extension tubing during contrast injection to maximize the physician’s distance from the beam.