Currently, fluoroscopic guidance is used routinely for many interventional pain management procedures to obtain more precise localization of anatomic target areas. Fluoroscopy is used in many procedures, including swallowing studies, urologic evaluations, peripheral joint injections, and, perhaps most commonly, interventional spine procedures. The ability to perform many spinal injections, including transforaminal epidurals, facet joint injections, medial branch blocks, sympathetic blocks, discograms, and sacroiliac joint injections, is entirely dependent on fluoroscopic imaging. This chapter reviews the basic concepts of radiation safety and their practical application in the fluoroscopy suite to minimize exposure risks for the patient and spinal interventionalist.
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 ).
| Quantity | Conventional Unit | SI Unit | Conversion |
|---|---|---|---|
| Exposure | Roentgen (R) | Coulomb/kg of air (C/kg) | 1 C/kg = 3876 R 1 R = 258 μC/kg 1 R = 2.58 × 10 -4 C/kg |
| Dose | Rad (100 ergs/g) | Gray (Gy) (joule/kg) | 1 Gy = 100 rad 0.01 Gy = 1 cGy = 1 rad 0.001 Gy = 1 mGy = 100 mrad |
| Dose equivalent | Rem (rad × Q) | Sievert (Sv) (Gy × Q) | 1 Sv = 100 rem 0.01 Sv = 1 cSv = 1 rem 0.001 Sv = 1 mSv = 100 mrem |
| Activity | Curie (Ci) | Becquerel (Bq) | 1 mCi = 37 MBq |
Terminology
Like matter, energy can be transformed from one form to another. When ice (solid) melts and turns to H 2 O (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 ). Irradiation occurs when matter is exposed to radiation and absorbs all or part of it.
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.
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 (a s l ow a s r easonably a chievable) 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.
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. 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. 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.
Shielding
Shielding factors include filtration, beam collimation, intensifying screens, protective apparel (e.g., leaded aprons, eyewear, and gloves), and protective barriers (e.g., leaded glass panels or drapes). Appropriate shielding of critical tissues (i.e., gonads, thyroid, lungs, breast, eyes, and bone marrow) from ionizing radiation is critical to the safe use of fluoroscopic equipment. In filtration, metal filters (usually aluminum) are inserted into the x-ray tube housing so that low energy x-rays emitted by the tube are absorbed before they reach the patient or medical staff. Beam collimation constricts the useful x-ray beam to the part of the body under examination, thereby sparing adjacent tissue from unnecessary exposure. It also serves to reduce scatter radiation and thus enhances imaging contrast. Protective apparel, such as a leaded apron ≥0.5 mm Pb, is mandatory to reduce exposure to the physician and technologist. Such shielding decreases radiation exposure by 90% to critical body areas. Lead-impregnated leather or vinyl aprons and gloves may be ordered in different thicknesses ranging from 0.55 mm Pb protection, which protects at 80 kVp, to 0.58 mm Pb, which protects at 120 kVp. The use of a leaded thyroid shield also is recommended because of the superficial location and sensitivity of the thyroid gland and to protect a limited amount of cervical bone marrow. Protective, flexible lead-lined gloves also may reduce exposure without sacrificing dexterity; however, their use is no substitute for vigilant avoidance of direct x-ray beam exposure. Leaded glasses or goggles will effectively eliminate approximately 90% of scatter radiation from frontal and side eye exposure. The leaded acrylic shields are made of clear lead equivalent to 0.3 mm Pb at 7-mm thickness. The lenses are leaded glass with a minimum thickness of 2.5 mm, which creates a lead shielding with more than 97% attenuation up to 120 kVp. Clear, leaded glass x-ray protective barriers are available in several styles and shapes. They may be height-adjustable or full-height, floor-rolling radiation barriers or suspendable on an overhead track. They weigh between 100 and 400 lbs with lead thicknesses of 0.5 to 1.0 mm. When it is necessary to remain near the x-ray beam during a procedure, additional shielding should be used.
Exposure Time
To minimize exposure time to ionizing radiation, the clinician and radiologic technician need to work as a team. The technologist assists by optimally orienting the C-arm around the patient before beginning any kind of interventional procedure. The technologist also should ensure that the orientation of the C-arm is such that the x-ray tube is positioned directly under the patient to minimize scatter to that which is attenuated through the patient. The operator should minimize exposure time through the judicious use of the “beam on” controls (i.e., a foot or hand switch). If the technologist is responsible for the controls, then communication with the physician is critical to avoid unintended exposure. Training and experience of all personnel in the intricacies of complex procedures help to reduce unnecessary exposure. Fluoroscopic equipment may have features such as high- and low-dose modes, pulsed fluoroscopy, hold-and-store image capability, and beam collimation—all of which can minimize exposure time. A high kilovolt-low milliamperage approach to imaging will minimize the absorption of x-ray by the patient and improve the contrast of the visualized image ( Fig. 5-4 ). Freeze-frame capabilities minimize repeated exposures and should be used to review the last image in preparation for needle adjustments during the procedure.
