Image properly: Diagnostic reference levels and optimization in oral and maxillofacial radiology

Abstract

Earlier this year, a health physicist from the Radiation Safety Section of our state's Occupational Safety and Health Administration conducted an inspection of the 76 x-ray devices in our institution. Radiation output from each unit was recorded for the exposure parameters that are used for average-sized adult patients. Within 2 weeks, a letter certifying compliance with regulations was issued. As written in the section's mission statement, the goals of these inspections are “to reduce unnecessary exposure to ionizing radiation from x-ray machines and other radiation machines” and “to protect and improve the health of (the) population through the development and enforcement of appropriate regulatory requirements pertaining to the use of radiation machines.”1www.michigan.gov/leo/0,5863,7-336-94422_11407_35791_ 35792_35799-46422–,00.html.Accessed 10 December 2021.Google Scholar The goals are laudable, but what standards of radiation exposure have been established by the state to serve as the benchmark for the use of x-rays in diagnostic medical and dental procedures? Regulatory bodies around the world have long used the diagnostic reference level (DRL) as the basis for controlling exposure to ionizing radiation in health care, a fact that is well known to the medical profession. Most dentists are probably unaware of DRLs. However, considering the great numbers of radiographs acquired in dental practice and the large range of exposures from intraoral to cone beam computed tomography (CBCT) images, this concept should be more widely understood. One of the principles of diagnostic radiology is optimization, which requires that the highest quality images be produced with the smallest radiation exposure consistent with the diagnostic goals. The concept of the DRL was first published by the International Commission on Radiological Protection (ICRP) as a metric that would bring attention to situations where optimization is in question.2International Commission on Radiological ProtectionDiagnostic reference levels in medical imaging. ICRP Publication 135.Ann ICRP. 2017; 46: 13-70Google Scholar The DRL is set at the 75th percentile of the distribution of the median values of exposure levels obtained from a sample of imaging centers in a specific region.2International Commission on Radiological ProtectionDiagnostic reference levels in medical imaging. ICRP Publication 135.Ann ICRP. 2017; 46: 13-70Google Scholar This means that 75% of imaging centers keep doses below the DRL; the 25% that exceed it are in need of investigation and a review of radiographic practices. DRL values come from patient doses gathered through surveys of imaging facilities or through phantom exposure data.2International Commission on Radiological ProtectionDiagnostic reference levels in medical imaging. ICRP Publication 135.Ann ICRP. 2017; 46: 13-70Google Scholar,3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar A related measurement, the achievable dose (AD), is defined as the median dose, or the 50th percentile, of that distribution.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar DRL can be calculated in milligrays (mGy) on the basis of the air kerma measurement at the site where the photon beam encounters the patient as the entrance surface dose (ESD). It can also be calculated as the dose area product (DAP), also called the kerma area product, defined as the product of air kerma at a reference point and the area of the x-ray beam at that point and measured in mGy. cm2. DAP is therefore not dependent on the site of measurement. It is, however, affected by changes in the area of exposure and can indicate the beneficial effects of tighter collimation of the beam. DRL in dental radiography should be derived from the standard exposures used by dentists; because settings should be different for adults and children, DRL values should be established for both.2International Commission on Radiological ProtectionDiagnostic reference levels in medical imaging. ICRP Publication 135.Ann ICRP. 2017; 46: 13-70Google Scholar ICRP recommends that measurements be made at standard settings for adults and children with a calibrated detector at the open end of the x-ray source.2International Commission on Radiological ProtectionDiagnostic reference levels in medical imaging. ICRP Publication 135.Ann ICRP. 2017; 46: 13-70Google Scholar There is a paucity of data in the United States with which to calculate DRLs for images acquired in oral and maxillofacial radiology.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar One source of information is the Nationwide Evaluation of X-Ray Trends (NEXT) survey program, created by the Food and Drug Administration and the Conference of Radiation Control Program Directors (CRCPD). The goal of the Conference is to “protect the public, radiation workers, and patients from unnecessary radiation exposure” by collecting data in the NEXT surveys.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar The most recent NEXT survey of dental procedures dates back to 2015, in which exposure data were collected from 199 dental offices in 25 states.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar The survey discovered that 85.6% of practices used digital intraoral systems. Film imaging was most commonly performed with D-speed film (63.3%). Circular collimation was employed in the large majority of clinics (99.4%). The average ESD for intraoral radiography in adults was 1.0 mGy; for children it was 0.7 mGy.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar The median dose for intraoral exposures overall was 0.86 mGy for adults (75th percentile: 1.48 mGy) and 0.60 for children (75th percentile: 0.95).4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar Because 14.4% of the NEXT data came from film imaging, mostly with low-speed film and short circular beam collimation that require exposure doses larger than those for digital systems, it is almost certain that data from digital systems alone would be lower. However, the National Council on Radiation Protection and Measurements (NCRP), in its 2019 publication No. 177, “Radiation Protection in Dentistry and Oral &Maxillofacial Imaging,” recommends DRL values for intraoral radiography of 1.5 mGy for adults and 1.0 mGy for children, similar to the NEXT dose data.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar For panoramic radiography, most of the data in the NEXT report (78.9%) came from digital units. Based on settings for adult patients, the pre-exposure mean DAP was 103.4 mGy.cm2, with a median of 104 mGy.cm2 and 75th percentile of 123 mGy.cm2. For children, the doses were 58.7, 55.6, and 67 mGy.cm2, respectively.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar CBCT survey data revealed that the mean, median, and 75th percentile doses for adults were 827, 624, and 727 mGy.cm2, respectively; for children they were 575, 384, and 624 mGy.cm2, respectively.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar It is important to note that the NEXT report states that “These survey results are NOT recommended performance levels or diagnostic reference levels, but rather are statistical indicators of the state of practice at the time of survey.”5www.fda.gov/radiation-emitting-products/radiation-safety. Nationwide Evaluation of X-Ray Trends (NEXT). Accessed 10 December 2021.Google Scholar This may be because of the mixture of data from film and digital intraoral and panoramic imaging. As pointed out in NCRP No. 177, DRL and AD values derived from analog film radiography become irrelevant when more dose-efficient digital techniques allow optimization of exposures at much lower doses.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Another problem is that the report is 7 years old. Changing technology has undoubtedly made some results obsolete. Maybe the most important limiting factor is the very small sample size from which the NEXT data were taken. NCRP suggests in its 2019 report that DRL data from other nations might be helpful in formulating U.S. exposure guidelines regarding procedures for which information is not available. The doses reported in the NEXT survey are comparable to the DRL data published around the world, but some of these investigations date back almost a decade.6Kim EK Han WJ Choi JW Jung YH Yoon SJ Lee JS. Diagnostic reference levels in intraoral dental radiography in Korea.Imaging Sci Dent. 2012; 42: 237-242Google Scholar, 7Jose A Kumar AS Govindarajan KN Manimaran P. Assessment of regional pediatric diagnostic reference levels for panoramic radiography using dose area product.J Med Phys. 2020; 45: 182-186Google Scholar, 8SEDENTEXCT. Radiation protection: cone beam CT for dental and maxillofacial radiology. Evidence based guidelines 2011. Available at: https://www.sedentexct.eu/files/guidelines_final.pdf. Accessed December 20, 2021.Google Scholar, 9Hodolli G Kadiri S Nafezi G Bahtijari M Syla N. Diagnostic reference levels at intraoral an dental panoramic examinations.Int J Rad Res. 2019; 17: 147-150Google Scholar, 10Han S Lee B Shin G et al.Dose area product measurement for diagnostic reference levels and analysis of patient dose in dental radiography.Rad Protect Dosimetry. 2012; 150: 523-531Google Scholar, 11Deleu M Dagassan D Berg I et al.Establishment of national diagnostic reference levels in dental cone beam computed tomography in Switzerland.Dentomaxillofac Radiol. 2020; 4920190468Google Scholar NCRP No. 177 suggests that organizations including NCRP, the Food and Drug Administration, CRCPD (working through the NEXT program), the American Academy of Oral and Maxillofacial Radiology, and the American Dental Association make efforts to publish interim DRLs and ADs on the basis of data obtained from reputable institutions. The council recommends that DRLs and ADs “be developed and regularly updated” for intraoral, panoramic, and cephalometric radiography as well as dental CBCT imaging and that these values “be used by all dental facilities.”3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Establishment of DRL must consider the fact that exposure doses employed by dentists vary greatly, not merely between different radiographic techniques but within the same types of projections. This is due in part to the need to alter exposure settings based on patient size. However, it is known that overexposure is a common problem. A major offender in this regard is the continued use of film imaging. Although in decline among dentists around the world, film-based radiography requires exposure doses that are generally higher than those needed for digital techniques, especially with solid-state sensors. NCRP No. 177 includes a table of ESD for film and digital bitewing radiographs with optimal quality. The exposures for digital sensors range from 0.35 to 1.04 mGy. Doses range from 0.87 to 1.09 mGy for E/F or F-speed film, but D-speed film requires 1.52 to 1.95 mGy, which is 2 to 5 times larger than needed for solid state sensors.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Rectangular position–indicating devices in place of circular collimators also significantly reduce x-ray exposure to patients12Ludlow JB Davies-Ludlow LE White SC. Patient risk related to common dental radiographic examinations: the impact of 2007 International Commission on Radiological Protection recommendations regarding dose calculation.J Am Dent Assoc. 2008; 139: 1237-1243Google Scholar and improve radiographic quality by diminishing scatter radiation on the sensor surface.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Patient exposure dose in panoramic radiography is significantly lower when using digital systems instead of film13Visser H Hermann KP Bredemeier S Kohler B. Dose measurements comparing conventional and digital panoramic radiography.Mund Kiefer Gesichtschir. 2000; 4: 213-216Google Scholar and with reduced beam size for children.14Davis AT Safi H Maddison SM. The reduction of dose in paediatric panoramic radiography: the impact of collimator height and programme selection.Dentomaxillofac Radiol. 2015; 4420140223Google Scholar Implementation of these changes would make it very easy for dentists to comply with DRLs for intraoral and panoramic radiographs. However, employing digital sensors in place of film for intraoral and extraoral radiography does not necessarily lead to a reduction in dose to the patient. Overexposure can occur because digital system software can compensate for excessive exposure by adjusting the grayscale. The dentist will not be aware of the problem because the computer has “fixed it” before the radiograph appears on the screen.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Doses for CBCT can vary extensively depending on the model15Rottke D Patzelt S Poxleitner P Schulze D. Effective dose span of ten different cone beam CT devices.Dentomaxillofac Radiol. 2013; 4220120417Google Scholar but can be excessive if operators fail to appropriately limit the field of view, frame rate, milliampere-second setting, and exposure arc, or properly adjust voxel size.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar It is important to remember, however, that despite the emphasis on dose reduction in oral and maxillofacial radiology, the goal of determining DRLs is to optimize the resulting radiographic images to ensure that they have “sufficient detail and information to assure the practitioner that subtle pathology can be detected. Optimization is the balancing of image quality and patient dose.”3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar Furthermore, NCRP cautions that setting exposures below the 75th percentile may not guarantee that the x-ray system is functioning optimally or that radiographs are acceptable, emphasizing the need for adherence to quality assurance and quality control procedures.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar It is inappropriate simply to decrease dose to the patient without concurrently evaluating image quality; these 2 parameters are bound together.3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar The use of a phantom to determine the exposure dose required to maximize quantitative evaluation of spatial resolution, contrast, and dynamic range is an excellent method for ensuring optimization of image quality with the smallest x-ray exposure,3National Council on Radiation Protection and Measurements. Radiation protection in dentistry and oral & maxillofacial imaging. 2019. NCRP Report No. 177.Google Scholar as the NEXT survey did.4Conference of Radiation Control Program Directors, Inc. Frankfort, KY.Nationwide Evaluation of X-ray Trends (NEXT) Tabulation and Graphical Summary of the 2014-2015 Survey of Dental Facilities. 2019.Google Scholar To balance reduction of exposure dose without losing sight of the need for optimization of diagnostic outcome, dentists should take several steps. First, abandon the use of film and adopt digital imaging systems, use rectangular instead of circular beam–limiting devices for intraoral radiology, use small beam height for pediatric panoramic radiographs, and use the smallest field of view and exposure parameters consistent with the diagnostic goal in CBCT imaging. Second, avoid unnecessary exposure by adhering to the principle of justification as manifest by the use of selection criteria; that is, expose radiographs only when the expected benefit outweighs the risk of exposure based on the individual patient's needs. In addition, radiology education in dental schools should emphasize the importance of the DRL as a way to ensure optimization of radiographic examinations. Perhaps the best way to summarize this approach lies in the context of the valuable Image Gently and Image Wisely initiatives: In the words of the eminent radiation physicist Dr. Roberto Molteni, we should endeavor to Image Properly.