Basic Fluoroscopic Concepts and Applied Radiation Safety
Most vascular patients who require an intervention for the severity or refractory nature of their disease will eventually undergo some type of angiographic procedure in the endovascular suite or operating room. Fluoroscopy, which refers to the digital or photographic formation of images using electromagnetic radiation, is the main principle behind angiography, which pertains specifically to imaging of the blood vessels. With the aid of devices and contrast material within the vessel lumen (i.e., endovascular), fluoroscopy becomes angiography and allows the provider to see the contour of the vasculature to diagnose, quantify, and even treat disease. Simply put, electromagnetic radiation applied with the techniques of angiography allows the provider to have x-ray vision of the patient’s vascular system. While angiography has led to remarkable advances in the treatment of vascular disease, its expanded use must be viewed in balance, with recognition of the potentially harmful effects of radiation to both the patient and provider.
This point is relevant given that much of the recent enthusiasm for angiographic procedures has outpaced provider awareness of basic radiation terminology and radiation safety. In contrast to even a decade ago, when most angiographers had formal instruction in radiology and were dedicated to interventional fluoroscopic procedures, today’s endovascular specialist has a more diverse training and practice background. Although well versed in the natural history of vascular disease and the range of management options, today’s vascular specialist may not have had dedicated schooling in radiation science. In lieu of such formal training, basic concepts are often passed down to trainees by mentors who may not stress a basic understanding of a few key radiologic principles. While practical, this method may not be ideal in a time when the number and complexity of endovascular procedures is increasing along with the diversity of providers enlisting to perform them.
The intent of this chapter is to recognize this important aspect of vascular care and to present basic radiation terms and concepts including the effects of radiation on patients and providers. Basic steps toward radiation safety are presented, including steps to minimize radiation risk. To cover the entire field of interventional fluoroscopy is beyond the capacity of this chapter and therefore the reader is encouraged to use this as a primer to familiarize and to stimulate additional reading.
I. Basic radiation concepts.
A. X-rays are a form of electromagnetic radiation.
The main characteristics of x-rays are similar to those of visible light and radiation is frequently quantified in a unit called a photon. A single photon is a quantum of electromagnetic radiation containing a defined amount of energy, in this instance defined in terms of electron volts (eV) (Table 12.1). The stronger the radiation source, the more photons per second can be produced. It takes thousands of x-ray photons per square millimeter to form a single fluoroscopic frame that is thousands of times greater than the energy contained in a photon of visible light. X-ray photons used for imaging have energies that range from 10,000 to 150,000 eVs. The source of radiation or x-ray tube converts electrical energy into electromagnetic quanta and heat is generated as a side product. Radiation is the transport of this energy away from the x-ray tube by these electromagnetic quanta. It is important to remember that the intensity of an x-ray beam decreases inversely as the square of the distance between the source and the measuring point (e.g., the greater the distance, the less the beam intensity).
Table 12.1. Basic radiation terminology | ||||||||||||||||||
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B. Radiation units and quantities are found in the International System (SI) of Units and include the important unit of radiation dose, referred to as the gray (Gy).
Radiation dose is the amount of energy absorbed from the radiation source at a point divided by the mass of tissue at that point (Table 12.1). Surprisingly a very small amount of energy is actually absorbed by or deposited into tissue during medical procedures. In fact, the amount of energy absorbed by tissue is similar to the amount of radiation energy absorbed by the surrounding air. 1 gray (Gy) = 1 joule of energy absorbed per kilogram of material. A gray is a large unit of radiation. In relation, radiation therapy doses are in the 1-2 gray range meaning that 1-2 joules of energy are delivered per kilogram of radiated tissue. In contrast, a standard chest x-ray delivers a dose of approximately 100 µ Gy.
Table 12.2. Maximum permissible doses | |||||||||||||
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Taking into account all forms of radiation, including natural, industrial, nuclear, and others, different types of radiation produce vastly different biological effects for the same number of Gy or dose. So the quantity dose equivalent (H) was developed to account for this range for radiation protection purposes and is expressed in units called sieverts (Sv). The dose equivalent expresses on a common scale for all forms of radiation the amount of irradiation incurred by exposed persons and was established by means of an experimentally defined quality factor. While quality factors for some forms of radiation are as high as 20, conveniently the quality factor for medical x-ray energies is 1: 1 Sv = 1 Gy × quality factor.
Finally, the term exposure refers to the strength or quantity of the radiation field at a given point (Table 12.1). Most SI measurements of exposure are actually measurements of air dose, which, as was previously noted, has very similar absorption properties to human tissue. Therefore, our standard measures of “exposure”are actually measures of the x-ray beam’s ability to ionize air caused by the small amount of absorbed energy. The working unit of exposure is the kinetic energy released per unit mass of air acronym (KERMA), which is a measure of the energy extracted from the x-ray beam by air. The unit of measurement for exposure is also the gray. Note that the unit measure of exposure was formally referred to as the Roentgen (R) which was the measure of ionization produced per one gram of air. An air-KERMA of 1Gy corresponds to an exposure of 114 R.
C. Radiation measuring device.
A radiation measuring device called a dosimeter is necessary to determine the dose of radiation emitted by any given source at a particular location. Dosimeters come in different forms, depending upon the type and amount of radiation that will be monitored or measured. Some basic instruments directly measure the ionization produced in a defined volume of air (i.e., KERMA) within a cylindrical instrument called an ionization chamber. The Geiger counter is another type of radiation detection device, which uses a Geiger tube and associated electrical display components. While these devices provide an accurate representation of soft tissue dose, they are not designed to monitor lesser amounts of exposure to patients or providers on a routine basis.
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