target="_blank" rel="nofollow" href="#u2c27f127-9302-5a80-b334-85982f41e29a">Chapter 9.
Topics in physics that are significant to radiobiology include atomic structure, the nature and properties of x‐rays, ionization, excitation, linear energy transfer (LET), and relative biologic effectiveness (RBE). Additionally, the essential chemistry of the interactions of radiation and patient which occur with water (since the body contains 70–85% water) is the radiolysis of water. Such chemical interactions result in ionization of water, forming ion pairs and free radicals of which the latter can react further to form other molecules that are toxic to the cell.
Two significant and important topics in radiobiology are stochastic and deterministic effects. Stochastic effects are those for which the probability of the effect increases as the dose increases and for which there is no threshold dose. For stochastic effects, there is no risk‐free dose. These effects can occur at the local tissue level and can cause life‐span shortening, radiation‐induced malignancy, and hereditary effects (late effects that occur in the offspring of the irradiated individual). Stochastic effects are classified as late effects, since they occur years after the exposure of the individual. Deterministic effects on the other hand are those effects for which the severity of the effect depends on the dose. These effects have a threshold dose and increase with increasing dose. These effects are called early effects since they can occur within minutes, hours, days, weeks, and months after the exposure.
Radiation protection in diagnostic radiography
The overall objectives of radiation protection are to prevent deterministic effects and minimize the probability of stochastic effects. The current standards for radiation protection of patients in medicine are established by authoritative radiation protection organizations responsible for providing guidelines and recommendations on radiation protection. While some of these address radiation risks, others are devoted to radiation protection based on the radiation risks data. Three such organizations are the International Commission on Radiologic Protection (ICRP), the National Council on Radiation Protection and Measurements (NCRP), and the Food and Drug Administration (FDA), the latter two are in the United States.
Radiation protection criteria and standards are guided by two triads, namely radiation protection principles and radiation protection actions. While the former deals with the ICRP's principles of justification, optimization, and dose limitation, the latter addresses the triad of time, shielding, and distance. The principle of justification is intended for physicians ordering x‐ray examinations; it addresses the notion that there should be a net benefit associated with each and every exposure a patient receives. Optimization implies that radiation workers should work within the ALARA philosophy; that is, to obtain the best possible image with the lowest radiation dose and not compromise the image quality. Finally, the dose limits principle deals with the legal limits on the radiation dose received per year or accumulated over a working lifetime for persons who are occupationally exposed and for others as well, such as students in training and members of the public.
The triad of radiation protection actions include time, shielding, and distance. To be effective, it is necessary to keep the time of exposure to radiation as short as possible, since the relationship between time and exposure is proportional; that is, if the time of exposure doubles, the exposure doubles. Shielding ensures protection of patients and workers and members of the public through the use of lead shields and aprons for patients and workers, respectively. Furthermore, the walls of x‐ray rooms are also shielded using concrete or lead, for example, to prevent exposure of members of the public who are in a waiting room, waiting for patients having x‐rays. Finally, exposure is inversely proportional to the distance; that is, the further away individuals are from the source of the radiation, the less exposure they will receive.
The notion of dose limits is vital to radiation protection of occupationally exposed individuals such as technologists, for example, and non‐occupationally exposed individuals (members of the public). Essentially, these limits are established by organizations such as the ICRP, and national organizations, to minimize the risks of the stochastic effects of radiation. The ICRP recommended dose limit for occupational exposure, for example, is 20 mSv/year averaged over defined periods of five years.
The diagnostic reference level (DRL) is a concept used to address the limits of exposure for patients. DRLs are not equivalent to dose limit s for occupationally and non‐occupationally exposed individuals. The DRL has been defined by various radiation protection organizations. One such definition is that of the American College of Radiology (ACR) as “an investigation level to identify unusually high radiation dose or exposure levels for common diagnostic medical x‐ray procedures.” DRLs are tools that radiology departments can use to measure and assess radiation doses to patients for a defined set of procedures. If the doses delivered are consistently greater than established DRLs for that facility's country or region, then the department should be concerned about its radiation protection procedures, investigate why exposures are beyond the established DRLs, and take corrective action.
Technical factors affecting dose in radiographic imaging
Radiographic imaging includes modalities such as DR, DF, and CT. There are several technical factors including operator‐selectable (under the control of the technologist and/or radiologist) factors that affect the dose to the patient and operator. The major factors affecting dose to the patient in DR include beam energy and filtration, exposure technique factors, beam collimation, the size of the x‐ray field, the size of the patient, SID, grids, image receptor sensitivity or speed, and DQE. Major technical factors affecting the dose to the patient in fluoroscopy include beam energy or filtration, x‐ray tube current, beam‐on time, automatic dose‐rate control, collimation, source‐to‐skin distance, patient‐to‐image intensifier distance, patient size, anti‐scatter grids, image magnification, last image hold, image recording method, and pulsed fluoroscopy.
In CT, there are numerous technical factors affecting the dose to patients. Major factors include exposure technique factors, pitch, effective mAs, collimation and slices, overbeaming and overranging, automatic tube current modulation, automatic voltage selection, and IR algorithms.
Radiation protection regulations
Radiation protection regulations and guidelines address equipment specifications, procedures for minimizing the dose to patients and personnel, and shielding, outlined in various reports, and are issued by major agencies in respective countries. In the United States (US), for example, while the NCRP deals with medical x‐ray, electron beam, and gamma ray protection for energies up to 50 MeV in NCRP Report No. 102: Equipment Design and Use; the Code of Federal Regulations (CFR) Title 21 (US Department of Health and Human Services, Food and Drug Administration [FDA]) deals with the Performance Standards for Ionizing Radiation Emitting Products.
Equipment specifications for radiography, fluoroscopy, and CT are intended for manufacturers. Specifications for radiographic equipment relate to the x‐ray control panel, leakage radiation from the x‐ray tube, filtration, collimation, SID, source‐to‐skin distance, and the exposure switch for fixed and mobile radiographic systems. An example of one such recommendation for the exposure switch is that for fixed radiographic equipment, the exposure switch must be on the control panel to ensure that the operator remains in the control booth during the exposure. Furthermore, the switch must be a “dead man” switch; that is, pressure must be applied to the switch for the exposure to occur.
For fluoroscopy, these specifications include specifications for filtration, collimation, source‐to‐skin distance, exposure switch, cumulative timer, protective curtain, table and Bucky‐slot shielding, and accessory protective clothing. The recommendation for protective aprons worn during fluoroscopy is that they shall have at least a 0.5 mm lead equivalent.
Guidelines and regulations also focus on practices and procedures for reducing the dose to patients and personnel and to keep exposures